Page | 1

Chapter 1

Synthesis and Characterization of Novel Low Band Gap

Semiconducting Polymers for Organic Photovoltaic and Organic Field Effect Transistor Applications

Page | 2

1.0 Synthesis and Characterization of Novel Low Band Gap Semiconducting Polymers for Organic Photovoltaic and Organic Field

Effect Transistor Applications

1.1 Introduction and Research Motivation

The development of clean and sustainable energy sources is one of the greatest

recurrent challenges we face in the 21st century. Many experts agree that generating energy

by the burning of fossil fuels is unsustainable in the foreseeable future, as finite resources are

fast depleting. In addition, the burning of fossil fuels generates carbon dioxide and other

noxious pollutants, and is widely suspected to be a major contributory factor to global

warning. In fact, the United States (US) environmental protection agency (EPA) recently

branded carbon dioxide as unhealthy to both current and future inhabitants of the earth.1

Amongst the many technological choices available that promises a reduction in carbon

emissions, solar energy is an attractive option. Indeed, the total solar energy absorbed by

earth's atmosphere, oceans, and land masses is approximately 3850 ZJ per year (while

worldwide energy consumption is around 0.5 ZJ), thus offering the theoretical capacity to

meet all the worlds‟ energy needs, without obvious environmental detriment. There are many

competing approaches and technologies for the direct conversion of sunlight into electricity

(i.e solar photovoltaics). One attractive route to enable the fabrication of large area of solar

cells, required for large scale energy generation, is to print the solar cells via a high-

throughput printing technique such as roll-to-roll gravure coating. Solar cells fabricated from

printable organic materials, so-called “organic photovoltaics (OPV)”, are therefore an

interesting area of potential research. As we shall see, there are many challenges however to

developing a viable technology. Chief amongst this is to improve the energy efficiency of the

OPV device. Here the development of new materials which can harvest more of the solar

spectrum is an important goal, and in order to achieve this, one of the aims of this thesis is to

develop an array of novel low-energy gap conjugated polymers.

1.1.1 The Sun and the Concept of Low Band Gap in Organic Semiconducting Polymers

The energy reserve of the sun is expected to last for many billion years to come.

Moreover, it is estimated that 5 million tons of energy in the form of γ-rays is released from

the sun every second (i.e. 3.86 x 1026

Joules in energy terms).2 These rays are absorbed and

Page | 3

then re-emitted at relatively low temperatures until it reaches the surface of the earth as the

visible light we observe. Nevertheless, as the emitted visible light migrates through the

various spectral regions of the atmosphere, some of the energy is irreversibly lost in relation

to the air mass (AM) it encounters. In space, the energy just outside the atmosphere is

estimated at approximately 1366 Wm-2

.2 In this region, the air mass is 0 (i.e. AM 0) which

implies that the AM encountered by the visible light is virtually negligible. Consequently, the

amount of energy dissipated in this region is nil. On the other hand, at an air mass of AM 1

and under ideal conditions, the loss in absorption equates to 28 % resulting to ca. 1000 W m-2

at the surface of the earth (at the equator). While an even greater absorption loss is

encountered at the latitudes of northern Europe and northern America, thus resulting to the

AM 1.5 spectrum (figure 4, vide infra). The difference between the AM 0, AM 1 and AM 1.5

spectral regions of the sun is as depicted figure 1.3

Figure 1. Schematics of the different AM spectral regions of the solar spectrum (reproduced from ref 4).4

The standard light spectra commonly used for outdoor organic photovoltaic cells

(OPV) are those based on the AM-solar spectra. In other instances, a reference spectrum

which combines a direct AM 1.5 spectrum and that of the standard spectrum of scattered light

is used, and is referred to as the global AM 1.5 spectrum (AM 1.5G) (see figure 2, vide

infra).5-7

Since, organic solar cells, based on conjugated semiconducting polymers, hereafter,

referred to as OPVs unless otherwise stated, optimally convert 1 photon into 1 electron, it is

imperative to consider the amount of photons absorbed rather than the energy dissipated at

the different regions of the solar spectra. According to the information presented in figure 2

and 3 below, the solar spectrum can be suitably represented as photon flux with respect to

Page | 4

wavelength. This presents a better understanding of the population of photons that are

available for conversion into electrons under ideal conditions.

Figure 2. The global AM 1.5 spectrum (reproduced from ref 4).3

Figure 3, depicts the distribution of the spectral photons in the AM 0 and AM 1.5 G

spectra, respectively.

Figure 3. The distribution of the AM 0 and AM 1.5 sun light spectral photon flux (reproduced from ref 3).3

It is recommended that the AM 1.5G spectrum be utilized in the determination of the

power conversion efficiency of OPV cells, since it defines a precise spectrum.3 Figure 4,

Page | 5

shows the number of photons and UV irradiance attainable at a particular wavelength in the

solar spectrum.

Figure 4. The number of photons (black) and UV irradiance (red) with respect to wavelength (reproduced from

ref 3).3

As indicated in figure 4, it is important to also harvest the low energy photons in the

longer wavelength direction of the sun‟s spectrum. For OPV cells, many of the initially used

semiconducting polymers (such as, poly(3-hexylthiophene), P3HT) only absorbed the short

wavelength part of the spectra (to around 650 nm), which limited the cell efficiency. In order

to improve the efficiencies of the cells, polymers which absorb further into the red are

required.8,9

In other words, conjugated polymers possessing narrow band gaps are crucial to

maximising the photon harvesting potential of OPVs, as will be expanded upon later. Table 1,

shows the hypothetically harvestable percentage of photons at specific wavelengths (nm) and

the corresponding current density (mA cm-2

) for a semiconducting polymer. In the case of

P3HT with a band gap of 1.9 eV (650 nm), the data in table suggests that only 22.4 % of the

available photons can be harnessed. This translates into a maximum theoretical current

density of 14.3 mA cm-2

(which may increase when blended with PCBM, due to contribution

from absorption of the latter). However, if the absorption can be extended from 650 nm to

1000 nm, about 54 % of the available photons can be harvested, thus giving a maximum

theoretical current density of 33.9 mA cm-2

.2 Nonetheless, these figures may be difficult to

attain in practical devices, since complete absorption is difficult, together with the fact that

the incident-to-photon current efficiency (IPCE) is not always 100 % (or unity).2 However,

the data presented in table 1 is to be used as a guide when designing new polymers structures

for application in organic solar cells.

Page | 6

Wavelength

(nm)

Maximum percentage (%) harvested

(280 nm onwards)

Max current density

(mA cm-2

)

500 8.0 5.1

600 17.3 11.1

650 22.4 14.3

700 27.6 17.6

750 35.6 20.8

800 37.3 23.8

900 46.7 29.8

1000 53.0 33.9

1250 68.7 43.9

1500 75.0 47.9

Table 1. The values of the maximum current density and integrated photon flux of a photovoltaic device with

light harvesting potential beyond 280 nm. It is assumed that every photon is converted into 1 electron in the

external circuit.2,10

The lowering of the band gap of conjugated polymer is crucial to the optimization of

efficiency in OPVs as predicted by Scharber et al.,11

and Koster et al.,12

. However, as

observed in P3HT, the maximum theoretical current density is said to depend on the

thickness of the active layer (that is, the polymer blend film thickness). The correlation

between the proportions of incident light absorbed by P3HT and the corresponding film

thicknesses, with respect to wavelength is shown in figure 5.

Figure 5. Proportion of incident light absorbed for various thickness of P3HT films with respect to wavelength

(reproduced from ref 2).2

It is immediately apparent from the graph in figure 5 that as the thickness of the P3HT

film increases so does the portion of incident light harvested. Consequently, an impressive

95% of the incident light is absorbed with a P3HT film thickness of just 240 nm at a

wavelength range of 450 – 600 nm. However, since the photon flux of the AM 1.5G solar

spectrum (this relates to the solar irradiance when the sun is at an angle of 45o above the

Page | 7

horizon2,9

) peaks around 700 nm, there is a disparity between the absorption spectrum of the

conjugated polymer and the solar spectrum. Therefore, in actual fact, only 21% of the sun‟s

photons are absorbed by the P3HT film thickness of 240 nm (see, figure 3). Further extension

of the absorption maxima (λmax) from 650 to 1000 nm (see, table 1), by decreasing the

HOMO and LUMO energy gap of the conjugated polymer may increase the absorbance up to

53% (or a current density of 33.9 mA cm-2

) of the available photons from the solar spectrum.2

In addition to polymer band gap, there are several other factors that may directly or

indirectly influence the magnitude of the current density generated including for example, the

mobility of the charge carriers, the polymer morphology, and the lifetime of the charge

carriers. The open circuit voltage (Voc) is related to the difference between the frontier

orbitals (that is, highest unoccupied molecular orbital (HOMO) and the lowest unoccupied

molecular orbital (LUMO)).11-13

1.2 The Emergence of Organic Photovoltaic Devices: A Concise Review

Generally, OPV cells can simply be described as semiconductors under illumination.

On absorbing visible (or incident) light from the sun, the organic semiconducting material

(usually a π-conjugated polymer) generates excitons (known as excited bound electron-hole

pairs), in a process called photoexcitation.12

During photoexcitation, an electron is excited

from the HOMO of the donor to the LUMO orbital of the donor, which results in the

generation of excitons. Any excess energy above the band gap of the polymer is lost by the

relaxation of the exciton back to the band edge, typically by vibronic interactions.12

Unlike

inorganic cells, the excitons in organic materials are strongly bound and do not readily

dissociate into free charge carriers.8,14-16

Thus, single layer OPV cells have very low device

efficiencies (typically <0.1%).

The solution to this problem was to introduce a second semiconductor material to the

device, which had a lower lying LUMO level. It therefore becomes energetically favourable

for the exciton to split into charges at the interface between the two organic semiconductors,

thus generating free holes in the donor polymer, and electrons in the acceptor material, which

can migrate to the collecting electrodes. For such a device to work, it is important that the

excitons are sufficiently long lived to be able to diffuse to the interface with an acceptor

material. If the exciton does not find an acceptor during its lifetime, it can relax back to the

ground state, and the photon energy is lost in terms of solar cell electrical generation. A

Page | 8

summary and detailed explanation of the operations (including the donor and acceptor energy

diagrams) of the OPV device will be presented in the coming sections. Figure 6 shows a

schematic representation of a typical OPV or OSC device architecture.

Figure 6. Polymer-based OPV architecture.

The parameters which govern the operation and power conversion efficiency (ηp) of

the OPV device includes; the (I) short-circuit current (JSC) in A m-2

, (II) open-circuit voltage

(VOC) in V, and fill factor (FF), as expressed by the equation below;4,17,18

)(

)(

light

OCSC

in

OCSC

in

outp

I

VJFF

p

VJFF

P

P

(1)

Here, p is expressed as the ratio of the power output ( outP ) in W m-2

(or maximum

power) and the power input ( inP ) (or incident-light intensity ( )(lightI ) also measured in W

m-2

under simulated AM 1.5 solar irradiation. Over the years, organic solar cells have

evolved from a single active layer, bilayer and bulk-heterojunction configuration (vide infra).

The JSC of the device can be approximated from equation 2 below;18-20

JSC = neμE (2)

Where n represents the charge carrier density, e is the elementary charge, μ is the

charge carrier mobility, and E is the electric field. This can also be ascertained from the

population of absorbed photons and internal conversion efficiency.18

Page | 9

On the other hand, the VOC of the OPV device comprising of both a donor and

acceptor (fullerene) as the photoactive component, can be calculated theoretically from the

expression below;18

1

O

SCOC

J

JIn

q

kTV

(3)

In the above equation, kT represents the thermal energy, where k is the Boltzmann

constant and T is the temperature, q stands for the elementary charge, JSC is the injection

current driving the charge transfer (CT) electroluminescence, JO signifies the dark current

obtained from the photoconversion experiments and the experimental

electroluminescence: )()()( EEqEQEEEQEJ BBPVELO .18

According to Chochos and

Choulis,18

the above model gives a precise relationship between the VOC and onset of the CT

state21

or the peak of the CT emission22,23

. Simplistically, the open-circuit voltage of the

device can also be estimated from the HOMO of the donor and LUMO of the acceptor, as

depicted in equation 4.

eVEEeV LUMOAcceptor

HOMODonor

OC 3.0/1 (4)

Where the elementary charge is represented by e, and 0.3 is an empirical potential

factor which depends on the difference between the electrochemical potential where charges

are harvested from the cell and the HONO/LUMO energy levels.11,19,20

In another study,

Janssen et al.18,24

made a prediction in which it was claimed that the highest attainable Voc of

the OPV device can be deduced from the lowest energy gap (Eg) of either the donor or

acceptor materials used according to VOC = Eg – 0.6 eV.24

Mathematically, the FF of a typical

OPV cell is defined as shown in equation 5.17,25-27

))((

))(( maxmax

ocsc VJ

VJFF (5)

Where, scJ is the short-circuit current density (A cm-2

), maxJ and maxV represent the current

and voltage at the maximum power point of the 4th

quadrant on the current-voltage (J-V)

curve (figure 7).

Page | 10

Figure 7. Current-voltage curves of a typical OPV device.

The FF delineates the quality of the diode in the fourth quadrant, and is dependent on

the lifetime of the charge carriers, the series resistance (RS) and the shunt resistance (RSH) of

the device.19

Another parameter that can be deduced experimentally is referred to as the

incident light-to-photon conversion efficiency (IPCE) or equilibrium quantum efficiency

(EQE). This is simply the number of electrons generated under short-circuit conditions

divided by the number of incident photons.28

This is expressed in the following equation;3,6

in

SCSC

P

J

eN

JIPCE

1240(%)

0

(6)

Where, 0N and λ represent is the incident photon flux density and incident photon

wavelength (nm). Furthermore, the spectral distribution of the IPCE determines the

maximum obtainable power conversion efficiency: inP PJV /maxmax .19

Page | 11

1.2.1 Single Layer Organic Photovoltaic Devices

Photovoltaic (PV) devices consisting of single layers of pure organic semiconducting

polymers with PCEs ranging from 10-3

to 10-1

% have been reported.29-31

The measured PCEs

values are much too small to be practically useful. Figure 8, depicts one of the so called, “first

generation” OPV device with a top electrode/organic semiconductor/transparent electrode

configuration.32

Figure 8. Single layer conjugated polymer PV device.

The OPV shown in figure 8 was manufactured by spin-casting and thermally

converting an undoped thin layer of a pure conjugated polymer (poly(p-phenylenevinylene) -

PPV) on top of a transparent indium-tin oxide (ITO) electrode, followed by evaporation of a

low-work function top contact.33

An open circuit potential (Voc) of between 1.2 and 1.7 V

(metal dependent) was recorded upon illumination with a 10 mW cm-2

light intensity. It was

discovered that in most cases the distinction between the work function of the top and bottom

electrodes divided by the charge of an electron was equal to the Voc. This suggests that in

such single layer OPV device there may be a partial correlation between the differences in the

work function of an electrode and the magnitude/sign of the Voc. Other contributing factors

include, Femi level pinning,34

chemical potential gradient,26,35

and non-negligible dark

current.36

Notwithstanding, the reasonable Voc produced in this ITO-PPV-metal OPV cell,

there is still a huge shortfall in their photocurrent generating capability. Moreover, the IPCE

or EQE recorded for the above PV device was generally low (0.1 - 1.0 %), regardless of the

high Voc produced by many single layer OPVs.

The anomaly between the high proportions of photons absorbed (˃ 60 %) by an

organic semiconducting polymer film of thickness in excess of 120 nm and the poor IPCE of

the resulting single layer OPV cell can be explained by the difficulty in splitting the exciton

Page | 12

in the absence of an acceptor material, as highlighted in the preceding section. For single

layer devices most of the excitons formed by absorption of a photon simply relax back to the

ground state without generating any free charges. The mechanism of charge generation in

single layer devices is not fully understood, and may be related to uncontrolled defects in the

conjugated material (for example, backbone defects or catalytic residues).

1.2.2 Dual-Layer Organic Photovoltaic Cells

A strategy proposed in a seminal report published by Tang in 1986 stipulates that by

constructing a bilayer PV device architecture with organic polymer semiconductors

possessing asymmetric HOMO/HOMO energy levels, the EQE may be significantly

improved.17,27

In this particular study, a dual-layer or bilayer PV cell consisting of a bottom

transparent ITO-coated glass anode upon which a layer of copper phthalocyanine (CuPc - 1)

was deposited, a second layer made of perylenedibenzimidazole (PDBI - 2), and a third layer

of opaque silver (Ag) cathode. Figure 9, depicts the structures of the active components (1

and 2) of the active bilayer OPV cell investigated.

Figure 9. Structural of the CuPc and PV semiconducting materials.

The EQE of the bilayer PV cell was estimated at 0.95% under AM 2G light

illumination (light intensity of 75 mW cm-2

) and a high EQE or IPCE of 15% at 620 nm. This

was a significant improvement to that reported for a single layer organic semiconductor based

OPV cell. Moreover, such enhanced performance can be attributed to the dissociation of the

excitons at the interface between the two layers of semiconducting materials. On the other

hand, for dissociation to occur, the photogenerated excitons are required to be within a few

nanometres to the donor/acceptor polymer interface, since the exciton diffusion length is

small. Once the exciton is split at the interface, the resulting electron and hole are still

coulombically bound. Provided this attraction can be overcome, the free holes and electrons

Page | 13

can migrate through the p and n-type semiconductor to their respective electrodes. The

bilayer geometry facilitates the transfer of electrons and holes to their respective electrodes,

and subsequently, precludes the recombination phenomenon by aiding the spatial separation

of electrons and holes.16

Recently, Maksudul and Jeneke at the University of Washington published one of the

highest energy conversion efficiencies (approximately 5.4%), under AM1.5G white light

illumination at 0.5 mW cm-2

(or 3.2% at 10 mW cm-2

) intensity, ever attained for a bilayer

OPV cell with an impressive EQE of nearly 62%.37

When compared to that of the single layer

ITO/MEH-PPV/Al OPV cell, an increase of over 800-folds in energy conversion efficiency at

10 mW cm-2

is noticeable. The bilayer OPV device architecture consisted of layered

nanostructures of poly(benzimidazobenzophenanthroline ladder) (BBL) acceptor

and PPV donor in the cell configuration; ITO/PPV(60 nm)/BBL(60 nm)/Al (figure 10).

Figure 10. Cross-section of the bilayer OPV cell (left) and (right) the donor and acceptor semiconductors.37

The fill factor (FF) obtained for the above bilayer PV cell was 48%, while that based

on a single layer of the PPV organic semiconductor was only 20%. This implies that more

excitons are likely to be generated and dissociated in the PPV/BBL bilayer PV cell.

Interestingly, an inverse correlation between the film thickness and the rate of diffusion of

excitons was established. It was observed that as the thickness of the donor polymer film

increases the amount of excitons reaching the D/A interface were reduced as a result of the

limitations posed by the exciton diffusion length (Ld). The estimated Ld of the PPV film was

about 8 ± 1 nm which is in good agreement with those deduced in similar studies.28,32,38-40

Page | 14

The reported Ld for other types of conjugated polymers ranged from 4 to 20 nm.3,41-44

The limited Ld, amongst other factors, has been long identified as one of the key limiting

factors to achieving high efficiencies in bilayer OPVs for many years.

Further research to improve the power conversion efficiency of the bilayer OPV cells

was undertaken by Meissner et al.,3,45-47

who adopted the concept of sensitization of organic

solar cells initially developed in the 1990s.48,49

In their study the OPV cell was sensitized

with fullerenes. Figure 11, shows the configuration of the Zinc-phthalocyanine (ZnPc) (50

nm) containing C60 [1:1]/N,N’-dimethylperylene-3,4,9,10-tetracarboxylic diimide (MPP ) (20

nm) bilayer excitonic cell utilized.

Figure 11. Illustration of the fullerene sensitized bilayer OPV cell.43

The bilayer OPV cell in figure 11 is very unique, in that it comprises of a composite

acceptor semiconducting (ZnPc-C60) material layer. This approach resulted in a power

conversion efficiency of 1.05% under an AM 1.5G light illumination of 860 W m-2

(equivalent to a SCJ of 5.2 mA cm-2

) and an IPCE of 37.5%, as shown in figure 12 and 13.43

Figure 12. The resulting IPCE of the substrate/ITO (30 nm)/MPP (20 nm)/ZnPc-C60 [1:1] (30 nm)/ZnPc (50

nm)/Au (40 nm) bilayer OPV cell (reproduced from ref 3).3

Page | 15

Figure 13. The current-voltage characteristics of the substrate/ITO (30 nm)/MPP (20 nm)/ZnPc-C60 [1:1] (30

nm)/ZnPc (50 nm)/Au (40 nm) bilayer OPV cell (reproduced from ref 3).3

The FF obtained was 45%. The results of the bilayer OPV cell performance studied

so far are very encouraging. However, it further highlights the fact that energetically efficient

bilayer layer OPV devices may still be realized by exploring novel structural modification (or

optimization) routes and device architectures. However it should be noted that these devices

were fabricated by vacuum deposition, and it is not clear that this is commensurate with low

cost, large area device manufacturing routes.

Page | 16

1.2.3 Bulk-Heterojunction Organic Photovoltaic Cell: From an Architectural Standpoint

The study of the design configuration and composition of the polymer solar cell

architecture is crucial in devising various optimization measures, which may in turn lead to

improved photoconversion efficiencies. This led to the development of the bulk-

heterojunction (BHJ) concept for optimal performance in organic solar cells. Figure 14

illustrates the structure of a BHJ organic solar cell (OSC).50

Aluminium (Al)

Calcium (Ca)

Polymer:Fullerene

PEDOT:PSS

Indium Tin Oxide (ITO)

Transparent Glass Substrate

Transp

arent A

node

Cathode

Electron transport layer (ETL)

Donor:Acceptor BHJ active layer

Hole transport layer (HTL)

Sunlight

Figure 14, Illustration of a conventional BHJ polymer solar cell.

The nanostructured photoactive BHJ layer is commonly embedded between two

highly selective modified electrodes; at the top, an electron-accepting low work-function

metal (usually, aluminium (Al), copper (Cu) or Magnesium (Mg)) with a lithium fluoride

(LiF) underlayer, and at the bottom, a transparent and high work-function cathode indium tin

oxide (ITO) glass electrode coated with a uniform layer of poly(3,4-

ethylenedioxylenethiophene):polystyrene sulphuric acid (PEDOT:PSS), for enhanced hole

extraction.50,51

The unsymmetrical work function of both electrodes also creates an electric

field across the device, which facilitates the effective transport of unbound charges and holes

to their respective electrodes (cathode and anode).

The efficiency of the device has been shown to depend strongly on the morphology of

the photoactive blended layer. Excitons must be generated within a diffusion length of the

donor-acceptor interface to ensure they are efficiently harvested. At the interface the excitons

Page | 17

are split into Coulombically-bound electron-hole pairs (sometimes referred to as geminate

pairs), and ideally these should not recombine but separate into holes and electrons which

then migrate through the percolating networks of the p-type and/or n-type semiconductor to

the electrodes. Due to the limited exciton diffusion length (Ld) in many conjugated polymers,

this requires phase segregation between the donor and acceptor material on the 10 - 20 nm

length scale (typical Ld is 5 - 10 nm).3,41-44

Figure 15. Illustration of the dual layer donor-acceptor architecture.43

The adoption of the BHJ concept was vital in mitigating the operational drawbacks

encountered in the dual-layer strategy, where efficiency was limited by the small exciton

diffusion length.52-54

Ideally, the intimately mixed composite photoactive layer would consist

of a percolated and bicontinuous or interpenetrating network of D and A sites, enabling a

viable path for the effective transport of unbound charges.55,56

As a testament of this

advancement, BHJ OPVs have drawn extensive investigation following the development and

implementation of various π-conjugated polymeric materials, including; the ubiquitously

studied poly(3-hexylthiophene) – P3HT (3 - 6.5 % PCE),57-62

as electron-rich donor

components in the photoactive layer. Additionally, PCEs as higher as 7.0 % to 8.1 % have

been realised with significant device enhancement strategies.63-66

Therefore, the process of

charge separation can proceed at almost any point in the entire active layer.

1.2.3.1 Bulk-Heterojunction Organic Photovoltaic Cell: Mechanistic, Operational and Electronic Considerations

In order to develop better performing BHJ devices, it is important to comprehend the

complex processes occurring in the cell. A simplistic and generally accepted mechanism

Page | 18

involved in the conversion of photogenerated excitons (or excited Coulombically-bound

electron-hole pairs21

) into electrical energy is depicted in scheme 1.38

Scheme 1. Representation of the mechanism involved in the conversion of photogenerated excitons to

electrical energy (reproduced from ref 38).38

.

The processes described in scheme 1 can be divided into four consecutive steps, as

follows;

I. The absorption of incident light from the visible region of the solar spectrum

by the donor semiconducting polymer effects the generation of excitons (or

bound electron-hole pairs).

II. The diffusion of the photogenerated excitons to the donor/acceptor interface.

III. Dissociation of the excitons into Coulombically-bound charge transfer states

(sometimes referred to as a bound radical pairs) at the interface. These can

either recombine (called geminate recombination), which is a loss

mechanism, or dissociate into free charge carriers.

IV. The transport and collection of oppositely charged species at the respective

electrodes. During transport to the electrodes, electrons and holes can also

Page | 19

recombine, known as non-geminate recombination. This is another loss

mechanism.

The fundamental operating principles of various OPV architectures is based on the

fact that the intimate mixing of a p-type (D) and an n-type (A) semiconducting polymer

possessing offset HOMO and LUMO energy levels forms a photoactive layer (D-A

composite blend). Moreover, the difference in electron affinity establishes an internal

downhill energetic driving force at the D/A heterojunction.

At the interface the photogenerated excitons are separated into free carriers (electrons

and positive holes) by a downhill energetic driving force, provided that the latter is

sufficiently greater than the binding energy of the excitons.16,38

In the donor (or acceptor)

material the coulombic attraction of the photogenerated bound electron-hole pair (exciton) is

referred to as the binding energy, and is estimated to be in the range of 0.1 to 1.0 eV.67-69

Electrons are transferred into the LUMO of the acceptor material, at the D/A interface,

leaving a positive hole in the HOMO of the donor. To ensure an efficient charge transfer

between the donor and acceptor materials an offset in LUMO energy must be present. The

energetic driving force required is the difference between the ionization energy of the donor

conjugated polymer in the excited state ( DI ), the electron affinity of the acceptor ( AE ) and

the Coulomb energy of the dissociated germinate pairs, whose value falls below zero, as

shown in the expression below;50,70

0 CADUEI (7)

If the excitons do not reach the interface, they usually decay back to the ground state

via thermal decay (that is, the dissipation of energy in the form of heat - “non-radiative

recombination”) or by emission of photons (radiative recombination), provided that the

radiative transition is permitted.70

Figure 16 depicts the probable decay paths of the excitons

generated upon photoexcitation of the conjugated polymer backbone structure.

Page | 20

Nanoseconds (ns) Microseconds (s)

(a - Ultrafast charge transfer in femsoseconds (fs))

Vacuum Level

Vacuum Level

Fluorescence

(b - Photoluminescence) (c - Back charge recombination)

Recombination

E

E

--

-

-

-

Charge Transfer

Charge Transfer

LUMO ( )LUMO ( )

LUMO ()

HOMO () HOMO ()

HOMO ()Donor

Acceptor

Excitation

Excitation

Excitation

+ +

+

Nanoseconds (ns) Microseconds (s)

(a - Ultrafast charge transfer in femsoseconds (fs))

Vacuum Level

Vacuum Level

Fluorescence

(b - Photoluminescence) (c - Back charge recombination)

Recombination

E

E

--

-

-

-

Charge Transfer

Charge Transfer

LUMO ( )LUMO ( )

LUMO ()

HOMO () HOMO ()

HOMO ()Donor

Acceptor

Excitation

Excitation

Excitation

+ +

+

Figure 16. Representation of the exciton decay processes (a) charge transfer upon photoexcitation (b)

photoluminescence and (c) charge recombination.

Pump-probe experiments performed on a blend of a conjugated polymer, poly(2-

methoxy,5-(3‟,7‟dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV), and a C60

derivative, PC61BM, (1:3) describe the photoinduced charge transfer between these materials

as an ultrafast process (~45 fs 77

), in contrast to the slower competing photoluminescence (

~ns) and back charge recombination (or germinate recombination) (~μs) pathways depicted

in figure 16.71-73

Thus, the photogeneration of free charges (free holes and electrons) between

the semiconducting materials is both a metastable and highly efficient process, which results

in a 100 % quantum yield. It is important to state that, unlike simple charge recombination,

photoluminescence involves the symmetry allowed relaxation of the excitons to the HOMO

of the conjugated polymer via emission of photons (fluorescence). This is also known as

radiative recombination.

In a publication by Yu et al.,74

it was revealed that by blending MEH-PPV donor and

the soluble PC61BM acceptor the total photoluminescence (PL) of the usually luminescent

polymer semiconductor was quenched, thereby leading to an optimized charge transport

regime. In other words, the photogenerated excitons were able to reach the donor-acceptor

Page | 21

heterojunction and effectively split before the process of radiative recombination could

proceed. However although all of the excitons were harvested, these did not all result in the

successful generation of charges. In this charge recombination of the separated charges

(figure 16c) was occurring. This may have been occurring immediately at the interface after

the exciton was split (generally known as geminate recombination), or during diffusion of

charge to the electrode (non-geminate recombination). The fact that photocurrent generation

efficiency reduced with film thickness suggested non-geminate recombination was a factor.16

Conceptually, the idea of a nanostructured composite film can play an important role

in ensuring the even distribution of the donor-acceptor interface throughout the photoactive

layer of the OPV cell. The exploration of such strategy may likely increase the probability of

excitons being able to locate the donor-acceptor interface within a shorter timescale and

subsequently split prior to the inception of any performance inhibiting processes. In a study

by Mihailetchi et al.,53,75

on conjugated polymer-fullerene blends, germinate polaron-pairs

were proposed as probable photoinduced intermediates. It was discovered that the electron-

hole pairs retained their Coulombically-bound state across the donor-acceptor interface, and

therefore, can only dissociate either via an applied electric field and/or a secondary process

mediated by elevated temperatures.

Nevertheless, on complete charge separation, the positive holes are transported by the

donor polymer, while, electrons diffuse through the acceptor backbone structure to the

corresponding electrode (cathode). This eventually results in the generation of electrical

energy. Since, the process of non-germinate recombination may occur at any point in time

during charge migration (due to inhibition in dead ends of inaccessible regions), it is

imperative to ensure the presence of percolated aggregates or homogeneous interpenetrating

networks of the donor and acceptor regions in the bulk heterojunction composite films.76,77

This will allow for effective migration of electrons and holes through separate domains

without ever coming in contact with one another. By adopting such approach, the prospect of

non-germinate charge recombination occurring is greatly minimised and the “lifetime” of the

charge carriers hereby extended. A sustained lifetime is crucial in ensuring the complete

extraction of charge carriers from the composite active layer in OPV devices, since the

charge carrier mobility in conjugated polymers is relatively low.50

Moreover, the internal

electric fields (F) across the cell‟s active layer essentially control the extraction of charge

carriers, induced by distinctions in electrode work functions for electrons and holes,

Page | 22

respectively. The distance (d) travelled by a typical charge carrier is related to the mobility of

the charge carriers according to the equation below;

Fd (8)

Where, τ is the charge carrier lifetime, and μ represents the charge carrier mobility,

respectively.

1.2.4 The Photocurrent Generation Mechanisms in BHJ OPV Cells

The photoconversion processes occurring in most organic photovoltaic (OPV) cells or

solar cells (OSCs) studied to date relies on the basic mechanistic pathways described in

scheme 5 (vide supra). However, due to the expansion of knowledge in the theoretical aspects

of the OPV technology, a variety of alternative and highly probable photocurrent generation

mechanisms have been proposed. In 2011, a review by Chochos and Choulis presented the

current understanding of the charge generation mechanism (figure 17) which occurs in

conjugated polymer:fullerene BHJ solar cells.18

Figure 17. Representation of the charge transfer, dissociation, recombination and photocurrent generation

regimes observed in the BHJ Solar Cells.

According to photogeneration mechanism in figure 17, upon photoexcitation (a) of the

active layer (polymer/PCBM), an exciton is generated. This excited Coulombically-bound

singlet electron-hole pair (exciton) migrates to the BHJ or D-A interface, where it is

undergoes dissociation into the charge transfer state (CT) via energy transfer (b). This occurs

provided that the optical band gap (Eg) of the polymer is greater than the energy of the CT

Page | 23

state (ECT): Eg ˃ ECT.78

79

At this stage, the hot CT state can either decay by thermalization

(i.e. thermal relaxation) (1) or split into the fully charge-separated state (CS) (4) and diffuse

to the electrodes, since it is initially generated with an excess of energy.80-82

If the CT state

undergo thermal relaxation, the Coulombically-bound polaron-pair generated may recombine

via geminate recombination into either the triplet state – T1 (2) on the polymer donor or to the

ground state (3). The latter usually occurs by back electron transfer, so long as the energy of

T1 (ET) is lower than that of the CT state (ECT) or ECT – ET ≥ 0.1 eV.18

On the other hand, it

can be dissociated to form unbound charge carriers by overcoming the Coulomb binding

energy of the germinate pair (4‟). These processes delineated above together with other

competing relaxation processes (see scheme figure 16) are known to affect the

photogeneration of charges in OPVs. Therefore, they must be taken into consideration when

designing new conjugated polymers and device architectures.

Recently, an investigation into the ultrafast electron transfer and carrier recombination

pathways in liquid-crystalline poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene)

(pBTTT)/PC61BM blends relative to pure pBTTT was investigated.83

The photoinduced

absorption (PIA) decay profiles of both neat pBTTT and the pBTTT/PC61BM BHJ composite

film inferred the existence of short-lived singlet state excitons at ca. 400 and 200 ps, and

long-lived triplet state excitons that persisted well beyond 1.5 ns. In an analogous study, the

formation of triplet energy states were detected in absorption studies involving individual

blends of two polythiophene copolymers, p(T8T8T0) (lowest lying HOMO at 5.6 eV) and

P(T12NpT12) (HOMO at 5.4 eV), with PC61BM (LUMO at 3.7 eV) against P3HT/PC61BM.84

These observations can be based on the notion that upon donor excitation an intermediate

charge-separated state (CSS) is generated due to charge migration to the HOMO of the

acceptor polymer. The CSS could exist as an “exciplex”, defined as a Coulombically-bound

polaron-pair capable of undergoing relaxation to form a heterogeneous, radiatively active,

excited state with a partial charge transfer character.38,85-87

Moreover, the CSS is also the

energy difference between the donor HOMO and the acceptor LUMO. Figure 18 illustrates

the positions of the CSS energy relative to those of the singlet (SA, SB, SC and SD) and triplet

(TA, TB TC and TD) states of the poly(thiophene) derivatives and PC61BM.17,88

Page | 24

eV

3.0

2.5

2.0

1.5

1.0

0.5

0.0

TA

TB TC TD

SASB

SC

SD

CSS

CSS

P(T8T8T0)

P(T12NpT12)

PC61BM

P3HT

eV

3.0

2.5

2.0

1.5

1.0

0.5

0.0

TA

TB TC TD

SASB

SC

SD

CSS

CSS

P(T8T8T0)

P(T12NpT12)

PC61BM

P3HT

Figure 18. Representation of the Energy levels diagram for the polythiophene-PC61BM composites, showing the

effects of the CSS and triplet states on the probability of free charge(s) generation. Note that, the CSS is only an

approximate value and the values of TA and TB also varies.

As depicted in figure 18, the position of the CSS energy relative to that of the triplet

states of the polymers determines the generation of free charges. Where the energy of the

CSS exceeds (or above) that of TA, TB and TC, triplet states are generated due to intersystem

crossing (ISC) from single excitons within the CSS, accompanied by energy transfer. This

eventually results in poor charge carrier generation yield. On the other hand, if the reverse

scenario proceeds, the lower the CSS energy relative to that of both TC and TD lead to the

creation of free charge carriers since transfer to the triplet is not longer possible.17

In spite of

the effect on the generation of free charge carriers, triplet state excitons, due to their long

lifetime (10-6

s) compared to singlet state excitons with relatively short lifetime (10-9

s), may

be central to the power conversion efficiency of OPVs. In other words, the exciton diffusion

length (Ld) of conjugated polymers could possibly be extended by the generation of triplet

state excitons since the longer lifetime leads to enhanced Ld. However one must find an

effective way to harvest the triplet excitons without losing voltage, due to lower lying triplet

state compared to the singlet.

A report published by Benson-Smith et al. presented evidence of a charge transfer

complex (CTC) in the photothermal deflection (PD) (studies absorption) and

photoluminescent (PL) (emission detection) spectra of a blend film consisting of poly(9,9-

dioctylfluorene-co-N-(4-methoxyphenyl)diphenylamine) polymer (TFMO) and PC61BM.79

Similar to the CSS, the CTC energy ( CTCE ) is equal to the energy difference

( )()( donorHOMOacceptorLUMOCTC EEE ) between the HOMO of the donor polymer (TFMO) and

the LUMO of the acceptor material (PC61BM). However, the distinction between the CTC

Page | 25

and CSS lies on the fact that, the latter is optically coupled to the ground state and is able to

absorb and re-emit photons. Therefore, the CTC can be best described as a Coulombically-

bound ground state donor-acceptor couple (sometimes, referred to as polaron-pair), which

differs greatly from an exciplex.38

On the contrary, upon photoexcitation the CTC becomes

an exciplex and thus facilitates the generation of photocurrents.79

According to Benson-Smith

and her colleagues, the proposed photoconversion mechanism involves the transfer of energy

from the polymer singlet state to the CTC in competition with ISC to the triplet state of the

donor polymer, accompanied by energy transfer to the PCBM singlet state. Subsequently, the

CTC may decompose by photon emission (radiative decay) or transformed into a weakly

bound polaron-pair state.79

Figure 19 depicts the influence of the displacements of the CTC

and polaron-pair energy levels relative to that of the triplet states of the donor and acceptor

semiconductors on the photoconversion mechanism.

1.0

1.5

2.0

2.5

3.0

0.0

0.5

3.5

4.0

eV

CTC

Polaron-pair state

SE

SETE

TE

Energy transfer

Charger separation

Radiative decay

(TFMO)

(PC61BM)

(TFMO-PC61BM blend)

(At reduced PC61BM content)

(a)

1.0

1.5

2.0

2.5

3.0

0.0

0.5

3.5

4.0

eV

CTC

Polaron-pair state

SE

SETE

TE

Energy transfer

Charger separation

Radiative decay

(TFMO)

(PC61BM)

(TFMO-PC61BM blend)

(At reduced PC61BM content)

(a)

ISC1.0

1.5

2.0

2.5

3.0

0.0

0.5

3.5

4.0

eV

CTC

Polaron-pair state

SF

SETF

TE

Energy transfer

Radiative decay

(PFO) (PC61BM) (PFO-PC61BM blend)

(b)

ISC1.0

1.5

2.0

2.5

3.0

0.0

0.5

3.5

4.0

eV

CTC

Polaron-pair state

SF

SETF

TE

Energy transfer

Radiative decay

(PFO) (PC61BM) (PFO-PC61BM blend)

(b)

Figure 19. Energy level diagrams illustrating relative positions of the energies of the CTC, polaron-pair (a)

above and (b) below, to that of the donor polymer and the acceptor PC61BM .

Page | 26

In Figure 19, the reduction in energy of the CTC ( CTCE ), caused by the low

ionization potential (5.2 eV) of the donor polymer (TFMO), relative to that of the triplet (TE)

and singlet (SE) states of PC61BM results in an energetically favourable formation of both

CTC and the polaron-pair (or germinate pair) states, respectively. However, in the case of the

poly(fluorene) polymer (PFO)/PC61BM composite film also studied, the high ionization

potential of PFO (5.8 eV) resulted to an increase in CTCE relative to TF and SE. This has the

effect of reducing the photocurrent generation efficiency, which could be related to the fact

that the radiative recombination through acceptor (PC61BM) SE and the ISC to TE become

energetically favourable. It should be noted that, the positioning of the weakly bound

polaron-pair state above that of the CTC signifies the fact that the formation of the former

requires overcoming the Columbic interaction energy present in the latter. Albeit that, the

overall energy of the CTC may still be greater than that of the final relaxed polaron-pair

state.38,79

In essence, the energy of the latter could still lie below that of the CTC. The

mechanism depicted in figure 19 is supported by various studies on polymer-fullerene

blends.89-91

Nonetheless, more investigation is needed to reveal more information about the

veracity of this mechanism.

Scheme 2. Schematics of the Förster energy transfer (FRET) photoconversion mechanism (reproduced from

ref 38).38

Another mechanism has recently been proposed for the photogeneration mechanism

(scheme 2). The mechanism suggests that instead of the diffusion of the excitons to the

donor-acceptor heterojunction, a Forster or resonance energy transfer from the donor to the

Page | 27

acceptor occurs, thus resulting in the generation of an exciton in the acceptor domain. Since

FRET (that is Förster resonance energy transfer) occurs by Förster transfer, this can occur

over a larger length scale than exciton diffusion. Following energy transfer (step II) to the

acceptor, electron transfer (step III) from the HOMO of the donor to the acceptor, via the

oxidation of the donor by the excited-state acceptor, leads to the creation of free hole and free

electron (provided that the offset between the HOMOs of the donor and acceptor is large

enough to drive the charge transfer).38

Evidence of the FRET mechanism was discovered in a study by Liu et al.,84

on an

intimate blend of a red-emitting organic chromophore (a Nile Red) dye and the solution-

soluble (6,6)-phenyC61 butyric acid methyl ester (PC61BM) derivative of C60 in an inert

polystyrene matrix. Since, Nile Red possesses a similar emission spectrum to that of

commonly used conjugated polymers in organic solar cells; an estimation of the emission

patterns of a hypothetical polymer was made possible. Moreover, the influence of any

intramolecular exciton diffusion is also circumvented by the inert matrix. The emission

spectrum of Nile Red showed an overlap with the weak absorption of the fullerene (PCBM)

in the region raging from 500 to 720 nm (which is a requisite for Forster resonance energy

transfer-FRET), as shown in figure 20.

Figure 20. Electronic spectra of the absorption of PCBM (solid line), the emissions of Nile red (dotted line) and

that of MDMO-PPV (dashed lines).85

Figure 21, shows the photoluminescence quenching of five Nile red concentrations

with respect to increasing PCBM concentrations.

Page | 28

Figure 21. Photoluminescence quenching of Nile red at 5 concentrations against PCBM wt % in solid

polystyrene films. The quenching indicated was calculated using the Forster energy transfer equation and

electron transfer with a Forster radius of 3.1 nm.84

It was suggested that the FRET from Nile red to PC61BM is clearly evident from the

presence of high fluorescence quenching at low donor and acceptor concentrations. On the

other hand, if the quenching did increase with increasing concentration of Nile red, it would

have confirmed the transfer of excitons between the Nile red molecules to reach the

quencher.87

The outcome of this study suggests that FRET may extend the effective exciton

diffusion lengths in conjugated polymers beyond the estimated 4 - 20 nm limits.3,41-44

However, the extent to which the FRET mechanism can operate depends on important

parameters, such as the (i) difference between the LUMO and HOMO energy levels of the

donor and acceptor materials, (ii) a strong overlap between the emissions of the donor

conjugated polymer and the absorption of the acceptor, and (iii) an increased

photoluminescence quantum yield for the donor.17

In spite of the benefits of the FRET

mechanism, the inability of conjugated polymer emissions, with energy band gaps below

~1.8 eV, to overlap with the absorption of PCBM may prevent the operation of such energy

transfer mechanism.84

Consequently, the FRET photon conversion mechanism may not be

applicable to that operating in polymer/PCBM OPV cells which utilises low band gap

semiconducting polymers.

Although, the different photocurrent generating mechanisms explored above have all

or partially been observed in different BHJ solar cells, it will be imprudent to disregard the

suggestion that more than one of these may be operational at any given time. However, this is

highly dependent upon the type of donor/acceptor semiconducting materials utilised, and the

operating atmospheric conditions; that is, temperature, etc. More importantly, the variation of

Page | 29

the buckminsterfullerene (C60) acceptor content has been shown to profoundly affect both the

effective dissociation of excited electron-hole pair (excitons) relative to geminate

recombination, and the transport of free carriers in many polymer-fullerene type solar

cells.79,92,93

This could be attributed to the fact that an increase in the concentration of PCBM

may result to a surge in the effective dielectric permittivity of the composite film. As a result,

the probability of charge separation occurring prior to germinate recombination becomes

greater. Furthermore, an increase in the rate of exciton dissociation may also be induced due

to the proliferated population of the acceptor moiety.79

Despite the differences highlighted between the simple photon conversion mechanism

(see scheme 1) and the CSS, CTC and FRET models, certain commonalities do exist between

them. Crucially, the energy gap between the frontier molecular orbitals (HOMO and LUMO)

of both the donor and acceptor components of the polymer-fullerene BHJ solar cell is

fundamental, not only in terms of the resultant open circuit voltage, but also pertaining to the

efficient harvesting of photogenerated currents from the sun. Therefore, when devising

energetically enhanced organic solar cell, it is imperative to consider the various aspects of

the different mechanisms herein explored.

1.3 Organic Field Transistors: A Brief Insight

The metal-oxide-semiconductor field effect transistor (MOSFET), also known as

MISFET (metal-insulator-semiconductor FET) or IGFET (insulator gate FET) comprising of

a polymeric semiconducting layer has been proposed to be the best alternative replacement

candidate in place of those based on silicon. Since the ingenious FET concept was proposed

by Lilienfeld over 8 decades ago,94,95

the resultant development of the first silicon-based

MOSFET by Kahng and Atalla,96

in 1970 set a precedence and rejuvenized the field of

microelectronics. The interest in silicon-based MOSFETs is due to the superior quality of the

interface between the single-crystalline silicon and the silicon oxide (SiO2). Although, the

performance (i.e. field effect mobility ≥ 50 cm2 V

-1 s

-1)97

of the single-crystalline silicon

FETs far exceeded that of the organic polymer-based counterparts, they were found to be

unsuitable for applications requiring large areas. This together with their relatively high cost

of fabrication has helped reinvigorate enormous drive to research and develop high

performance organic FETs (OFETs) comparable to those of the widely used hydrogenated

amorphous silicon MOSFET technology.20

Interestingly, the aSi:H FETs or thin film

transistors (TFTs) typically displayed charge carrier mobility of 0.5 cm-2

V-1

s-1

, almost zero

Page | 30

threshold voltage, on/off current (ION/IOFF) of 108, and sub-threshold slope of less than ca. 1.0

V/decade.98

Although OFETs were known since the 1970s, it was not until 1987 that the work of

Tsumura et al.,99,100

on an OFET incorporating polythiophene, synthesized via

electrochemically polymerization, reaffirmed their usefulness as viable electronic device

components.99,100

However, the field effect mobility was still too low (~ 10-5

cm2 V

-1 s

-1)100

for the device to be commercialized. Following this publication, in 1997, Horowitz et al.

fabricated a top-contact bottom-gate OFET with sexithiophene oligomers as the active

semiconductor. However, the measured field effect was approximately 0.07 cm2 V

-1 s

-1 was

only just lower than that of the amorphous silicon FETs, with a ION/IOFF of 104 and a

threshold voltage – VT of 6.4 V.101

These promising results heralded an explosion of interest, and an important

breakthrough came in the same year (1997), by the report of a pentacene-based OFET with

field effect mobility as high as ca. 0.7 cm-2

V-1

s-1

(ION/IOFF ~ 108) which, for the first time,

exceeded that of amorphous hydrogenated silicon-based FET architectures.102

Such high field

effect mobility was ascribed to the highly ordered microstructure of the pentacene molecule,

akin to that of a single crystal.103

However, the sub-threshold slope was still very high (ca. 5

V/decade) with near zero threshold voltage, and thus required incredibly high gate voltage to

switch-off the device. In addition, a particularly limitation of the pentacene OFET relates to

the lack of reproducibility of the field effect mobilities, due to the variation in the optimum

substrate temperature with the performance of the device.104

Another issue of concern

involved the use of voltages ranging from -100 to +100 V for the recording of the ION/IOFF

(108), which exceeded that used in microelectronics.

Further improvements were the result of detailed optimisation of the transistor

geometry, and film deposition conditions. For example, when OFET device configuration

was altered to a bottom-contact bottom-gate arrangement (see figure 22, vide infra), with

treatment of the silicon dioxide with a self-organising OTS (octadecyltrichlorosilane) layer,

mobilities were doubled (1.5 cm-2

V-1

s-1

) with a VT of - 8V (ION/IOFF ~ 108), and a low sub-

threshold slope of 1.6 V/decade.98

The optimized charge carrier mobility was ascribed to the

improvement of the interface between the semiconducting layer and substrate/electrode

surface by use of a self-organizing material (OTS), and alleviated any further doubts about

the scope of improved performance that could be actualized in OFET devices. Consequently,

Page | 31

huge interest was generated from the academic and industrial microelectronics communities.

Figure 6 depicts the structure of the OFET device used in the aforementioned investigations

and the structures of the sexithiophene and pentacene oligomers.

Figure 22. OFET configuration (a) top-contact bottom gate, (b) stacked bottom-contact bottom-gate. Included

are the structures of the oligomers (sexithiophene and pentacene).

On the other hand, due to the insolubility of pentacene in organic solvents, these

oligomers are mostly deposited by high vacuum vapour evaporation techniques. In order to

fabricate truly low cost electronic devices, many researchers believe that the deposition of the

semiconductor by solution based printing processes is critical.104

In realization of this

drawback and the increased understanding of the polymer structural pre-requisites for high

performance FETs, solution-processable thiophene and thieno[3,2-b]thiophene polymer

derivatives consisting of different alkyl groups with varying chain lengths appended to their

π-conjugated mainframe were explored. Recently, poly(2,5-bis(alkylthiophen-2-

yl)thieno[3,2-b]thiophene) (pBTTT) bearing C10, C12, C14 side chains displayed charge

carrier mobilities ranging from 0.2 to 0.7 cm2 V

-1 s

-1, with high ION/IOFF ratios (~ 10

6).

105,106

The latter is important in relation to the ability of device applications, such as, active matrix

displays, and logic circuits to switch-off or short down.104

According to McCulloch et al.,105

these high values were attributed to the enhancement of the polymer crystallinity.105

Crucially, the carrier mobilities displayed by pBTTTs are higher than that of amorphous

silicon FETs. Further comprehensive reviews on other conjugated polymer-based OFETs

and the advantages of these over those based on small molecules can be found in the

literature.20,104

Page | 32

1.3.1 Organic Field Effect Transistors: Device Architecture and Applications

Polymer-based OFET or OTFT are inexpensive replacements to silicon-based

transistors,107,108

and can be best described as three terminal devices comprising of a source,

drain, and gate electrode contacts. The device works as an electrical switch, where the gate

electrode allows for the modulation of the current flow (or conductivity) into the source-drain

channel via accumulation of charges at the organic semiconductor/gate dielectric (insulator)

interface (figure 23).20,104,109

Figure 23. Top-contact bottom-gate device showing the direction of slow of holes.

For p-type semiconductors, the OFET device is switched on by applying a negative

voltage to the gate electrode, thus resulting in the creation of a inversion layer between the

insulator/semiconductor interface.104

This inversion layer functions as conductivity pathway

between the source and drain Ohmic contacts on the OFET. Importantly, the chemistry of the

interface and microstructure of the conjugated polymer are crucial to the performance of the

device.109

In addition, the operation of the device also depends strongly on the geometry of

the transistor.110

Besides those device configuration displayed in figure 22(a) and 22(b), other

types do exist, depending on the positions of the semiconducting layer and the electrodes (see

figure 24).

Over the years, semiconducting π-conjugated polymers have found widespread

application as active layers in numerous OFET architectures.111-115

Optimisation of these

device potentially enables applications such as biosensors,116,117

inexpensive large area

memories, integrated circuits,118,119

smart cards, and driving circuits for large size display

Page | 33

technologies.120

Most important, the solution processability of soluble semiconducting

polymers is advantageous in terms of reduced fabrication costs through easy solution-based

patterning techniques (i.e. ink-jet printing or screen printing), which has made OFETs

compatible with flexible plastic substrates.120,121

Other electronic devices where

semiconducting polymers play imperative roles includes; organic light emitting diodes

(OLEDs),122

organic phototransistors,123,124

and others.125,126

Figure 24. OFET device configurations (a) top-gate staggered and (b) top-gate coplanar.109

The performance of the OFET can be assessed from the output and transfer

characteristics current-voltage plots (not shown).20,104

From these plots, parameters such as

the source-drain currents (ISD), mobility (μ – cm2 V

-1 s

-1), current on-and-off ratios (ION/IOFF),

threshold voltage (VT - V), and sub-threshold swing (V/decade) can be estimated. By using

the MOSFET gradual channel approximation model, the mobility in the linear and saturation

regimes can be calculated from the equation 9 and 10.20,104,109

SDSD

TSDiLinSD VV

VVCL

ZI

2. (9)

2.2

TSGiSatSD VVCL

ZI

(10)

Where, ISD and Ci represent the source-drain currents, and capacitance per unit area of

the insulating layer. Z and L are the channel width and length. VSD, VT and VSG stands for the

source-drain, threshold, and source-gate voltages. ISD equals to zero when no voltage is

applied to the gate electrode. When VG ≠ 0 V the device switches on, ISD ≠ 0 A.20,104

Polymeric semiconductors used in OFET can be classified as p-type (hole-transporting), n-

type (electron-transporting) and ambipolar (possess both p-type and n-type

characteristics).20,105,106,127-130

However, these classifications depends on the position of the

Page | 34

HOMO and LUMO of the conjugated polymer relative to the work-function of the

electrodes.127

1.4 Semiconducting Conjugated Polymers: An Introduction and The Need for Device Enhancement

In the 20th

century semiconducting polymers possessing unique electrical conducting

characteristics suitable for electronic device application drew significant interests from both

industry and academia. Prior to this era, most synthetic and non-synthetic π-conjugated

polymers were mostly utilized in encapsulations/insulators, and plastic containers, to name a

few. It was not until the 1950s that the field of conducting polymer really began to gain

recognition, as a result of the discovery of the first conducting polymer (polyacetylene - black

powder).97

Electrical conductivities ranging from 10-9

to 10-1

S/cm were measured for the

then new polymer semiconductor, which closely correlated with that displayed by other

semiconductors, with respect to its synthetic form.131

In 1963, a report by Weiss at al.,

unveiled the discovery of the highly conductive iodine-doped black polypyrrole with

comparatively high conductivity, compared to that of polyacetylenes (1 S/cm).132-134

The

rationale for such high electrical conductivity was ascribed to the microstructure of

polypyrrole, which was shown to consist of pyrrole aromatic units linked by C-C single bond.

A decade later, a new form of polyacetylene (silvery solid film) was published by

Shirakawa and co-workers.135,136

During this particular period, this discovery represented a

major breakthrough in the field of polymer science, in that it was relatively soluble in most

organic solvents tested and, therefore, could easily be characterized by the then available

analytical techniques, unlike the powder form. The discovery of polyacetylene incepted a

revolution in the field of conducting polymers, thus attracting interests from scientific

institutions around the world. In the year 2000, the noble prize for Chemistry was shared

between three prominent scientists, A.G MacDiarmid, H. Shirakawa and A.J. Heeger, for

their collective contribution towards the discovery of semiconducting polymers.137,138

Although, aromatic π-conjugated polymer derivatives such as polythiophene (II), polypyrrole

(III), and polyaniline (IV) (see figure 25), were known prior to the discovery of the

polyacetylene, very little were known about them until the 1980s.

Page | 35

Figure 25. Structures of the conjugated polymers.

Following the discovery of semiconducting polymers and the emergence of new

synthetic methodologies, the field of polymer chemistry has continued to evolve over the

many decades with the development of an ever-growing catalogue of novel semiconducting

structures with interesting optoelectronic properties.18,20,139,140

Intrinsic properties, such as;

high absorption coefficients,55,141

tuneable optoelectronic properties, structural and

mechanical flexibility,142

low weight,143

solution processability to facilitate roll-to-roll

manufacturing,112,144

formation of high thermally stable thin films,112

inexpensive production

costs,64

and abundant precursor material,143

makes them attractive for application in organic

electronic devices (that is, OPVs,16,51,140,145

OFETs,111-115,146,147

organic light emitting diodes

(OLEDs),122,131,148

biosensors,149

electrochromic displays,150,151

organic

phototransistors.123,124

).

One of the most extensively studied semiconducting polymer is that based on poly(3-

alkylthiophene)s (P3AT)s, due to their interesting structural-property relationship. The

properties exhibited by P3ATs, such as, good environmental stability, narrow band gap,

relative solubility (or processability), and broad absorption spectra have been correlated to

the tunability of their morphology or microstructure.152,153

Another class of conjugated

polymers which has attracted insurmountable studies are the poly(thienylene vinylene) (PTV)

derivatives. These polymers have been researched as potential candidate materials for

OLEDs, OFETs and smart window application.142

This was attributed to their good field

effect mobilities (up to 0.22 cm2 V

-1 s

-1),

154-158 low HOMO/LUMO band gaps (< 1.8 eV

159,160)

compared to that of P3ATs,161

and optical transparency in the visible region.142

Despite these

useful electronic and optical properties, PTVs generally display very poor power conversion

efficiencies (PCE) in OSCs due to the non-luminescent nature.162-164

Furthermore, in a

comparative study involving poly(3-hexylthienylene vinylene) (P3HTV) and poly(3,6-

dihexylthieno[3,2-b]thiophene vinylene) (DH-PTTV), it was revealed that the incorporation

of fused thiophene ring (thieno[3,2-b]thiophene) into the polymer backbone resulted to a hole

mobility (up to 0.032 cm2

V-1

s-1

) and PCE (from 0.19 % for P3HTV and 0.28 % for DH-

PTTV, respectively).154

The introduction of vinylene linkages serves to further improve the

Page | 36

coplanarity of the polymer main chain, thus, extending the effective conjugation length (vide

infra) by alleviating intramolecular torsional interactions between neighbouring repeat

units.154

However, the reported side group modification of a PTV derivative showed an

improved PCE of 2.01 % after thermal treatments and observed flourescnece.165

Thieno[3,2-b]thiophene containing polymers have been shown to display very high

mobilities in OFET, and therefore can prove vital for enhanced OPV or OSC device

efficiencies.105,106

Thus, in order to produce OFETs and PSCs with superior performance,

strategies for the design and synthesis of new state-of-the-art conjugated polymer

architectures with high charge carrier mobilities need to be explored. Furthermore, polymer

candidates exhibiting lower energy band gaps (<1.8 eV), broad absorption with extended

band edge, and high VOC through suitably aligned electronic energy levels with the fullerene

acceptors, are crucial pre-requisites for the development of future PSCs with higher

efficiency (>10 %).139,166,167

It is very challenging to achieve a balance between the structural

modifications and the desired polymer properties, in order to attain higher performance

OFETs and OPVs there is need for a careful consideration of the structural-property

relationships involved when designing future π-conjugated semiconducting architectures.

1.4.1 Engineering the Band Gap of Conjugated Polymer for Optimized Device Performance

Recent advancement in our understanding of the structural-property relationship of

many π-conjugated polymers (CPs) used as active materials in well-explored organic

electronic devices, particularly, BHJ PSCs and OFETs, and its impact on their performance

has led to monumental enhancements in PCEs (7 % - 10 %) and charge carrier mobilities.63-

65,168,169 On the other hand, in order to further optimize the performance of the new generation

of OPV devices, strategies are needed to bring about improvements in parameters, such as the

short-circuit current (JSC) and open-circuit voltage (VOC).

In the case of OFETs, the design of conjugated polymers with low band gaps (or

suitable band gaps) may influence the ease of charge injection/extraction between the

semiconductor and the electrode contacts. Moreover, it is a well established fact that the JSC

and VOC are both influenced by the frontier orbital energy levels (HOMO and LUMO) of

conjugated polymers. On the other hand, the tuning or engineering of the HOMO-LUMO

band gap in semiconducting polymers may also lead to the extension of their absorption into

Page | 37

the near-infrared (NIR) region of the solar spectra, thus allowing for the harvesting of low

energy photons in excitonic devices.18,170

This is important, seeing that the energetic edge of

the absorption spectrum determines the optical band gap (Eg) of the conjugated polymer.

However, the absorption spectrum peaks upon the polymer attaining a specific conjugation

length (CL), which is known as the effective conjugation length (ECL).171

Therefore, the

adjustment of the HOMO (valence band) and LUMO (conduction band) energy offset by the

modification of the polymeric structure is vital to the enhancement of electronic and

optoelectronic devices.

The Eg of many conjugated polymers is directly affected by known factors, which

include: bond length alternation, side groups substituent effects, molecular weight, aromatic

resonance energy, backbone coplanarity, and intermolecular interaction.18,172

Moreover, since

these parameters are inter-related, the modification of the Eg may invariably effect some

changes in other chemical and physical properties of the conjugated polymer.18

Several preferred strategies exist in the literature for tuning and narrowing the Eg of

conjugated polymers.18

These include: the reduction of bond length alternation (BLA) by the

use of conjugated systems with enhanced quinoid character, and the state-of-the-art D-A

copolymer concept, involving strong electron-donor and electron-accepting units.

The minimization of BLA principle involves the use of fused ring aromatic units with

enhanced quinoid character. This can be demonstrated by considering the resonance

structures of the polythiophene (a), polythienothiophene (b) and (c),173-175

polyisothianaphthene (d),176

and polythienopyrazine (e)177

repeating units displayed in figure

26.18,178

Page | 38

Figure 26. Schematics of the aromatic and resonance stabilized quinoid repeat units.

Polythiophene derivatives (a) are known to possess high Eg (between 1.85 and 2.0 eV)

which limits their absorption to the short wavelength direction of the absorption spectra.178,179

This is partially due to the unfavourably high energy of the quinoid form (a*) in the ground

state of the conjugated polymer, which leads to enhanced single bond character between the

thiophene units.18,178

In other words, the quinoid form of the thiophene unit is higher in

energy relative to the aromatic structure. In addition, the aromaticity of the thiophene ring is

lost, thus resulting to the increase in BLA and the larger energy gap. In contrast, by fusing the

thiophene moiety with other high resonance energy heterocyclic units (b – e), the quinoid

structure can be stabilized, and the aromaticity is retained.18,140

This leads to a reduction in

BLA, together with a concomitant narrowing of the HOMO-LUMO energy gap. Other

detailed approaches towards minimizing the BLA in polymeric structures are present in the

literature.167,178

The „quinoid‟ principle has been employed numerously in the synthesis of

copolymers with resulting band gaps ranging from 0.8 to 1.1 eV.176,180,181

Page | 39

The donor-acceptor concept, first proposed in 1993,182,183

entailed the

copolymerization of a strong electron-donor heterocycle with a „high-lying‟ HOMO, and a

strong electron-acceptor moiety with a „low-lying‟ LUMO to form an alternating π-

conjugated polymer network (figure 27).184,185

As a result, the intramolecular charge transfer

(ICT) between the donor (D) and acceptor (A) units effectively leads to a reduction of the

HOMO-LUMO band gap.2,182,186

This is due to the fact that, the ICT or enhanced

intramolecular interaction inherent with the D-A system, enhances the advantageous double-

bond character (reduced BLA) between adjacent units, thus leading to the planarization of the

conjugated polymer backbone. Consequently, the delocalization of π-electrons along the

polymer main-chain is unimpeded, hence the narrowing of the energy gap (Eg) between the

frontier orbitals.18,140,170

In addition, the ECL of the conjugated system is expanded, as a

result of the broadening of the conduction and valence band.18

The vantage point of this

approach is that it allows for the control of the HOMO-LUMO energy gap of the polymer via

varying the strength of the electron-donating ability of the donor unit and the electron-affinity

of the acceptor chromophore.140,187

Figure 27. Hybridization of the donor and acceptor molecular orbitals leading to the formation of a strong

intermolecular interaction and a reduced band gap.

The D-A copolymerization concept has been utilized extensively in the literature for

application in both OPVs and OFETs with exceptionally high performance.20,140

The above

band gap control strategies and examples of the copolymers which are relevant to this project

will be presented in subsequent chapters (2, 3, and 4).

Page | 40

1.5 Fullerene Acceptors for BHJ OPV Cells

The realisation that donor acceptor blends or bilayer were required for higher

efficiency devices has lead to much research into both the donor and acceptor material.

Whilst many different types of high efficiency donor polymer were developed, the only high

efficiency class of electron acceptor so far developed has been fullerene derivatives.49,188,189

Two important of the derivatives of C60 utilized commonly utilized in bulk-heterojunction

OPV devices are depicted in figure 28.

PC60BM PC70BM

Figure 28. PC60BM190-192

and PC70BM70,193

fullerene acceptors.

The unique position of these fullerenes as exceptional acceptor semiconductors is thought

to rely on the following;

They possess high electron affinity(s), owning to their energetically low-lying three-

fold degenerate LUMO.194,195

The latter, together with their electron-deficient nature,

enables fullerenes to undergo reversible reduction with up to six electrons.190,196,197

As

a result, these classes of acceptor materials are able to effectively stabilise negative

charge(s).

The presence of very high ionization energy,34

thus, signifying the presence of a low-

lying HOMO energy level.

In field-effect transistors (FETs), fullerenes have been shown to induce high electron

mobility up to 6 cm2 V

-1 s

-1, which is crucial to the performance of organic solar

cells.192,198,199

Page | 41

Electrochemical studies indicate that the LUMO of the soluble fullerenes can be fine

tuned by the nature and number of substituents attached to the ring. This is evident in

the negligible variations in the first reduction potentials (+ 100 mV relative to C60)

observed in various cyclic voltammetric studies.191,200

This is particularly beneficial

since the structure of fullerenes can be altered without compromising their electronic

integrity.

The tendency of soluble fullerenes to orient in order to form packed crystalline

structures can be beneficial for efficient charge transfer processes to occur.197

Figure

29, shows the types of packing involved in the crystalline structures of solution

soluble C60 explored by Rispens el al.192

(a)

(b)

(c) (d)

Figure 29. Crystal packing structures of C60 (a) view along the [-1,0,-1]- and (b) along the [0,0,1]-direction, 2

x 2 x 2 unit cells; (c) with neighbouring moieties (d = 12.95, 13.15 and 13.76 Å) and (d) with neighbouring

moieties (9.85 ˂ d ˂ 10.13 Ǻ) (reproduced from ref 192).192

The aforementioned attributes together with the formation of efficient charge transfer

complexes between fullerenes and a variety of strong donor conjugated polymeric materials

Page | 42

has made fullerenes the favoured acceptor candidates for bulk-heterojunction OPV device

applications to date.

1.6 Conclusion

In summary, the field of organic electronics has evolved over the years, in particular,

the intensive research and development of strategies towards enhancing the performance of

bulk-heterojunction OPVs and OFETs. In this regards, the design and synthesis of novel

donor-acceptor-type conjugated polymer architectures is very important, as is the

modification of the device and processing techniques. Albeit quite challenging, an in-depth

understanding of the structural pre-requisites for achieving a balance between the desired

band gap (Eg) and other physical, mechanical and chemical properties in emerging

semiconducting polymers should be taken into consideration. Herein, a broad overview into

the motivation of the project, an insight into the different organic solar cells, organic field

effect transistors, and a concise introduction into the field of semiconducting polymers,

together with the most important band gap engineering strategies currently in vogue, are

presented.

1.7 References

(1) The United States Environmental Protection Agency. Federal Registry, Ed.

Rules and Regulations, 2009; Vol. 74, p 66497 - 66546.

(2) Bundgaard, E.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2007, 91, 954 - 985

and references therein.

(3) Rostalski, J.; Meissner, D. Sol. Energy Mater. Sol. Cells 2000, 61, 87 - 89.

(4) Davies, P.; Bates, C.; Cole, T.; Prentice, A.; Clarke, P. Eur. J. Clin. Nutr.

1999, 53, 195 - 198.

(5) IEC-Norm 904-3. Measurement principles for terrestrial photovoltaic solar

devices with reference spectral irradiance data. IEC Geneva, 1987.

(6) Heidler, K. In Photovoltaische MeBtechnik, Ii; Meissner, D., Ed.; Solarzellen

Friedich Vieweg & Sohn.

(7) Hulstrom, R.; Bird, R.; Riordan, C. Sol. Cells 1985, 15, 365.

(8) van Duren, J. K. J.; Dhanabalan, A.; van Hal, P. A.; Janssen, R. A. J. Synth.

Met. 2001, 121, 1587 - 1588.

(9) Dhanabalan, A.; van Duren, J. K. J.; van Hal, P. A.; van Dogen, J. L. J.;

Janssen, R. A. J. Adv. Funct. Mater. 2001, 11, 255 - 262.

(10) Brustad, M.; Edvardsen, K.; Wilsgaard, T.; Engelsen, O.; Aksnesc, L.; Lunda,

E. Photochem. Photobiol. Sci. 2007, 6, 903 - 908.

(11) Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger,

A. J.; Brabec, C. J. Adv. Mater. 2006, 18, 789 - 794.

(12) Koster, L. J. A.; Mihailetchi, V. D.; Blom, P. W. M. Appl. Phys. Lett. 2006,

88, 093511.

Page | 43

(13) Brabec, C. J. Sol. Energy Mater. Sol. Cells 2004, 83, 273 - 292.

(14) Jenekhe, S. A.; Yi, S. Appl. Phys. Lett. 2000, 77, 2635 - 2637.

(15) O‟Regan, B.; Gratzel, M. Nature 1991, 353, 737 - 740.

(16) Coakley, K. M.; McGehee, M. D. Chem. Mater. 2004, 16, 4533 - 4542.

(17) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183 - 185.

(18) Chochos, C. L.; Choulis, S. A. Prog. Polym. Sci. 2011, 36, 1326 - 1414.

(19) Volker, S. F.; Uemura, S.; Limpinsel, M.; Mingebach, M.; Deibel, C.;

Dyakonov, V.; Lambert, C. Macromol. Chem. Phys., 2010, 211, 1098 - 1108.

(20) Facchetti, A. Chem. Mater. 2011, 23, 733-758.

(21) Vandewal, K.; Gadisa, A.; Oosterbaan W. D; Bertho, S.; Banishoeib, F.;

Severen, I.; Lutsen, L.; Cleij, T. J.; Vanderzande, D.; Manca, J. V. Adv. Funct. Mater. 2008,

18, 2064 - 2070.

(22) Tvingstedt, K.; Vandewal, K.; Gadisa, A.; Zhang, F.; Manca, J.; Inganas, O. J.

Am. Chem. Soc. 2009, 131, 11819 - 11824.

(23) Veldman, D.; Ipek, O.; Meskers, S. C. J.; Sweelssen, J.; Koetse, M. M.;

Veenstra, S. C.; Kroon, J. M.; van Bavel, S. S.; Loos, J.; Janssen, R. A. J. J. Am. Chem. Soc.

2008, 130, 7721 - 7735.

(24) Veldman, D.; Meskers, S. C. J.; Janssen, R. A. J. Adv. Funct. Mater. 2009, 19,

1939 - 1948.

(25) Nunzi, J. M. C. R. Phys. 2002, 3, 523 - 542.

(26) Gregg, B. A. J. Phys. Chem., B. 2003, 107, 4688 - 4698.

(27) Peumans, P.; Yakimov, A.; Forrest, S. J. Appl. Phys., 2003, 93, 3693 - 3724.

(28) Arango, A. C.; Carter, S. A.; Brock, P. J. Appl. Phys. Lett. 1999, 74, 1698 -

1700

(29) Antoniadis, H.; Hsieh, B. R.; Abkowitz, M. A.; Stolka, M.; Jenekhe, S. A.

Polym. Prepr. 1993, 34, 490.

(30) Yu, G.; Zhang, C.; Heeger, A. J. Appl. Phys. Lett. 1994, 64, 1540 - 1543.

(31) Karg, S.; Riess, W.; Dyakonov, V.; Schwoerer, M. Synth. Met. 1993, 54, 427 -

433.

(32) Savenije, T. J.; Warman, J. M.; Goossens, A. Chem. Phys. Lett. 1998, 287, 148

- 153.

(33) Marks, R. N.; Halls, J. J. M.; Bradley, D. D. C.; Friend, R. H.; Homes, A. B. J.

Phys. Condens. Matter. 1994, 6, 1379 - 1394.

(34) Brabec, C. J.; Cravino, A.; Meissner, D.; Sariciftci, N. S.; Fromherz, T.;

Rispens, M. T.; Sanchez, L.; Hummelen, J. C. Adv. Funct. Mater. 2001, 11, 374 - 380.

(35) Ramsdale, C. M.; Barker, J. A.; Arias, A. C.; Mackenzie, J. D.; Friend, R. H.;

Greenham, N. C. J. Appl. Phys. 2002, 92, 4266 - 4270.

(36) Malliaras, G. G.; Salem, J. R.; Brock, P. J.; Scott, J. C. J. Appl. Phys. 1998,

84, 1583 - 1589.

(37) Maksudul, M. A.; Samson, A. J. Chem. Mater. 2004, 16, 4647 - 4656

references therein.

(38) Frechet, J. M. J.; Thompson, B. C. Angew. Chem. Int. Ed. 2008, 47, 58 - 77.

(39) Samuel, I. D. W.; Rumbles, G.; Collison, C. J. Phys. Rev. B 1995, 52, R11573.

(40) Greenham, N. C.; Samuel, I. D. W.; Hayes, G. R.; Phillips, R. T.; Kessener, Y.

A. R. R.; Moratti, S. C.; Holmes, A. B.; Friend, R. H. Chem. Phys. Lett. 1995, 241, 89 - 96.

(41) Theander, M.; Yartsev, A.; Zigmantas, D.; Sundstrom, V.; Mammo, W.;

Anderson, M. R.; Inganas, O. Phys. Rev. B 2000, 61, 12957 - 12963.

(42) Haugeneder, A.; Neges, M.; Kallinger, C.; Spirkl, W.; Lemmer, U.; Feldman,

J.; Scherf, U.; Harth, E.; Gugel, A.; Mullen, K. Phys. Rev., B 1999, 59, 15346 - 15351.

Page | 44

(43) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. Adv. Funct. Mater. 2001, 11,

15 - 26.

(44) Wohrle, D.; Meissner, D. Adv. Mater. 1991, 3, 129 - 138.

(45) Desilvestro, J.; Graetzel, M.; Kavan, L.; Moser, J. J. Am. Chem. Soc. 1985,

107, 2988 - 2990.

(46) Gerlin, T.; Graetzel, M.; Walder, L. Progr. Inorg. Chem. 1997, 44, 345 - 393.

(47) Kroto, H. W.; Heath, J. R.; O'Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature

1985, 318, 162 - 163.

(48) Kroto, H. W.; Allaf, A. W.; Balm, S. P. Chem. Rev. 1991, 91, 1213 - 1235.

(49) Allemand, P. M.; Koch, A.; Wudl, F. J. Am. Chem. Soc. 1991, 113, 1050 -

1051.

(50) Hoppe, H.; Sariciftci, N. S. Adv. Polym. Sci. 2007, 12, 121 - 205.

(51) Ma, W. L.; Yang, C. Y.; Gang, X.; Lee, K.; Heeger, A. J. Adv. Funct. Mater.,

2005, 15, 1617-1622.

(52) Halls, J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R.

H.; Moratti, S. C.; Holmes, A. B. Nature 1995, 376, 498 - 500.

(53) Mihailetchi, V. D.; Koster, L. J. A.; Hummelen, J. C.; Blom, P. W. M. Phys.

Rev. Lett. 2004, 93, 216601.

(54) Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y.

Nat. Mater. 2005, 4, 864 - 868.

(55) Gunes, S.; Neugebauer, H.; Sariciftci, N. S. Chem. Rev. 2007, 107, 1324-1338.

(56) Chen, D.; Yang, Y.; Zhong, C.; Yi, Z.; Wu, F.; Qu, L.; Li, Y.; Li, Y.; Qin, J. J

Polym Sci A: Polym Chem., 2011, 49, 3852-3862.

(57) He, Y. J.; Chen, H.-Y.; Hou, J. H.; Li, Y. F. J. Am. Chem. Soc., 2010, 132,

1377-1383.

(58) Kim, Y.; Cook, S.; Tuladhar, S. M.; Choulis, S. A.; Nelson, J.; Durrant, J. R.;

Bradley, D. D. C.; Giles, M.; MuCulloch, I.; Ha, C. S.; Ree, M. Nat. Mater., 2006, 5, 197-

203.

(59) Zhao, G.; He, Y.; Li, Y. Adv. Mater., 2010, 22, 4355-4358.

(60) Moulé, A. J.; Meerholz, K. Adv. Mater., 2008, 20, 240-245.

(61) Li, G. H. L.; Yang, L., X. J Mater. Chem., 2008, 18, 1984-1990.

(62) Wang, W. L.; Wu, H. B.; Yang, C. Y.; Luo, C.; Zhang, Y.; Chen, J. W.; Cao,

Y. Appl. Phys. Lett., 2007, 90, 183512-1-183512-3.

(63) Liu, M.; Wang, Y.; Zhang, Z.-G.; Li, J.; Liu, Y.; Tan, H.; Ni, M.; Lei, G.; Zhu,

M.; Zhu, W. J Polym Sci A: Polym Chem., 2011, 49, 3874-3881.

(64) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv.

Mater., 2010, 22, 1-4.

(65) Siddiki, M. K.; Li, J.; Galipeau, D.; Qiao, Q. Energy Environ. Sci., 2010 3,

867-883.

(66) Li, C.; Liu, M.; Pschirer, N. G.; Baumgarten, M.; Mullen, K. Chem. Rev.,

2010, 110, 6817-6855.

(67) Arkhipov, V. I.; Bassler, H. Phys. Status Solidi, A 2004, 201, 1152 - 1187.

(68) Knupfer, M. Appl. Phys. A 2003, 77, 623 - 626.

(69) Campbell, I. H.; Hagler, T. W.; Smith, D. L.; Ferraris, J. P. Phys Rev Lett.

1996, 76, 1900 - 1903.

(70) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258,

1789 - 1791.

(71) Nelson, J. Phys. Rev. B 2003, 67, 155 - 209.

(72) Nogueira, A. F.; Montari, I.; Nelson, J.; Durrant, J. R.; Winder, C.; Sariciftci,

N. S.; Brabec, C. J. Phys. Chem. B 2003, 107, 1567 - 1573.

Page | 45

(73) Montanari, I.; Nogueira, A. F.; Nelson, J.; Durrant, J. R.; Winder, C.; Loi, M.

A.; Sariciftci, N. S.; Brabec, C. J. Appl. Phys. Lett. 2001, 81, 3001 - 3004.

(74) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270,

1789-1791.

(75) Koster, L. J. A.; Smiths, E. C. P.; Mihailetchi, V. D.; Blom, P. W. M. Phys.

Rev. B 2005, 72, 085205.

(76) Hoppe, H.; Glatzel, T.; Niggemann, M.; Schwinger, W.; Schaeffler, F.;

Hinsch, A.; Lux-Steiner, M. C.; Sariciftci, N. S. Thin Solid Films 2006, 587, 511 - 512.

(77) Hoppe, H.; Sariciftci, N. S. J. Mater. Chem. 2006, 16, 45 - 61.

(78) Deibel, C.; Strobel, T.; Dyakonov, V. Adv. Mater. 2010, 22, 4097 - 4111.

(79) Benson-Smith, J. J.; Goris, L.; Vandewal, K.; Haenen, K.; Manca, J. V.;

Vanderzande, D.; Bradley, D. D. C.; J., N. Adv. Funct. Mater. 2007, 17, 451 - 457.

(80) Pensack, R. D.; Asbury, J. B. J. Phys. Chem. Lett. 2010, 1, 2255 - 2263.

(81) Clarke, T. M.; Durrant, J. R. Chem. Rev. 2010, 110, 6736 - 6767.

(82) Bredas, J. L.; Norton, J. E.; Cornil, J.; Coropceanu, V. Acc. Chem. Res. 2009,

42, 1691 - 1699.

(83) Hwang, I.-W.; Kim, J. Y.; Cho, S.; Yuen, J.; Coates, N.; Lee, K.; Heeney, M.;

McCulloch, I.; Moses, D.; Heeger, A. J. J. Phys. Chem. C 2008, 119, 7853 - 7857.

(84) Liu, Y. X.; Summers, M. A.; Scully, S. R.; McGehee, M. D. J. Appl. Phys.

2006, 99, 093521.

(85) Ohkita, H.; Cook, S.; Astuti, Y.; Duffy, W.; Heeney, M.; Tierney, S.;

McCulloch, I.; Bradley, D. D. C.; Durrant, J. R. Chem. Commun. 2006, 3939 - 3941and

references therein.

(86) Morteani, A. C.; Sreearunothai, P.; Herz, L. M.; Friend, R. H.; Silva, C. Phys.

Rev. Lett., 2004, 92, 247-402.

(87) Morteani, A. C.; Friend, R. H.; Silva, C. J. Chem. Phys. 2005, 122, 244906.

(88) Scully, S. R.; McGehee, M. D. J. Appl. Phys. 2006, 100, 034907.

(89) Hasharoni, K.; Keshavarz, K. M.; Sastre, A.; Gonzalez, R.; Bellavia-Lund, C.;

Greenwald, Y. J. Chem. Phys. 1997, 107, 2308 - 2312.

(90) Scharber, M. C.; Schultz, N. A.; Sariciftci, N. S.; Brabec, C. J. Phys. Rev. B:

Condens. Matter Mater. Phys. 2003, 67, 085202.

(91) Kim, H.; Kim, J. Y.; Park, S. H.; Lee, K.; Jin, Y.; Kim, J.; Suh, H. Appl. Phys.

Lett. 2005, 86, 183 - 502.

(92) Mihailetchi, V. D.; Koster, L. A. A.; Blom, P. W. M.; Melzer, C.; de Boer, B.;

van Duren, J. K. J.; Janssen, R. A. J. Adv. Funct. Mater. 2005, 15, 795 - 801

(93) Tudlahar, S. M.; Poplavsky, D.; Choulis, S.; Durrant, J. R.; Bradley, D. D. C.;

Nelson, J. Adv. Funct. Mater. 2005, 15, 1171 - 1181.

(94) Lilienfeld, J. E.; U.S. Patent 1 745 175: United States, 1930.

(95) Lilienfeld, J. E.; U.S. Patent 1 900 018: United States, 1933.

(96) Kahng, D.; Atalla, M. M. In IRE Solid-State Devices Research Conference

Carnegie Institute of Technology, Pittsburg, PA, 1960.

(97) Wong, W. S.; Raychaudhuri, S.; Lujan, R.; Sambandan, S.; Street, R. A. Nano

Lett. 2011, 11, 2214 - 2218.

(98) Lin, Y. Y.; Gundlach, D. J.; Nelson, S. F.; Jackson, T. N. IEEE Electron.

Device Lett. 1997, 18, 606 - 608.

(99) Tsumura, A.; Koezuka, H.; Ando, Y. Synth. Met. 1988, 25, 11 - 23.

(100) Tsumura, A.; Koezuka, K.; Ando, T. Appl. Phys. Lett. 1986, 49, 1210 - 1212.

(101) Horowitz, G.; Garnier, F.; Yassar, A.; Hajlaoui, R.; Kouki, F. Adv. Mater.

1996, 8, 52 - 54.

Page | 46

(102) Gundlach, D. J.; Lin, Y.-Y.; Jackson, T. N. IEEE Electron. Device Lett. 1997,

18, 87 - 89.

(103) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J.; Dodabalapur, A. Chem.

Mater. 1996, 8, 2542 - 2544.

(104) Horowitz, G. Adv. Mater. 1998, 10, 365 - 377.

(105) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.;

Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R.

J.; McGehee, M. D.; Toney, M. F. Nat. Mater., 2006, 5, 328-333.

(106) Heeney, M.; Bailey, C.; Genevicius, K.; Shkunov, M.; Sparrowe, D.; Tierney,

S.; McCulloch, I. J. Am. Chem. Soc., 2005, 127, 1078-1079.

(107) Flexible flat panel display; Crawford, G. P., Ed.; Wiley: New York, 2005.

(108) Organic Electronics: Materials,Manufacturing, and Applications; Klauk, H.,

Ed.; Wiley-VCH: Weinheim, Germany, 2006.

(109) Salleo, A. Materialstoday 2007, 10, 38 - 45.

(110) Sirringhaus, H. Adv. Mater. 2005, 17, 2411 - 2425.

(111) Tierney, S.; Heeney, M.; McCulloch, I. Synth. Met., 2008, 148, 195-198.

(112) Osaka, I.; Abe, T.; Shinamura, S.; Miyazaki, E.; Takimiya, K. J. Am. Chem.

Soc., 2010, 132, 5000-5001.

(113) Lee, J.-K.; Gwinner, M. C.; Berger, R.; Newby, C.; Zentel, R.; Friend, R. H.;

Sirringhaus, H.; Ober, C. K. J. Am. Chem. Soc. 2011, 133, 9949-9951.

(114) McCulloch, I.; Bailey, C.; Giles, M.; Heeney, M.; Love, I.; Shkunov, M.;

Sparrowe, D.; Tierney, S. Chem. Mater., 2005, 17, 1381-1385.

(115) Zou, Y.; Wu, W.; Sang, G.; Yang, Y.; Liu, Y.; Li, Y. Macromolecules 2008,

40, 7231-7237.

(116) Crone, B.; Dodabalapur, A.; Gelperin, A.; Torsi, L.; Katz, H. E.; Lovinger, A.

J.; Bao, Z. Appl. Phys. Lett., 2001, 78, 2229-2231.

(117) Yan, F.; Mok, S. M.; Yu, J. J.; Chan, H. L. W.; Yang, M. Biosens.

Bioelectron., 2009, 24, 1241-1245.

(118) Drury, C. J.; Mutsaers, C. M. J.; Hart, C. M.; Matters, M.; de Leeuw, D. M.

Appl. Phys. Lett., 1998, 73, 108-111.

(119) Crone, B.; Dodabalapur, A.; Lin, Y. Y.; Filas, R. W.; Bao, Z.; Sarpeshkar, R.;

Katz, H. E.; Li, W. Nature, in press.

(120) Bao, Z. Adv. Mater., 2000, 12, 227.

(121) Hong, Y.; Yan, F.; Migliorato, P.; Han, S. H.; Jang, J. Thin Solid Films 2007

515, 4032-4035.

(122) Ahn, S. H.; Czae, M.; Kim, E. R.; Lee, H.; Han, S. H.; Noh, J.; Hara, M.

Macromolecules 2001, 34, 2522-2527.

(123) Mok, S. M.; Yan, F.; Chan, H. L. W. Appl. Phys. Lett., 2008, 93, 023310-1-

023310-3.

(124) Yan, F.; Li, J.; Mok, S. M. J. Appl. Phys., 2009, 106, 074501-1-074501-7.

(125) Chen, C.-C.; Chiu, M.-Y.; Sheu, J.-T.; K.-H, W. Appl. Phys. Lett., 2008, 92,

143105-1-143105-3.

(126) Mabeck, J. T.; Malliaras, G. G. Anal. Bioanal. Chem., 2006, 384, 343-353.

(127) Meijer, E. J.; de Leeuw, D. M.; Setayesh, S.; van Veenendaal, E.; Huisman,

B.-H.; Blom, P. W. M.; Hummelen, J. C.; Scherf, U.; Klapwijk, T. M. Nat. Mater. 2003, 2,

678 - 682.

(128) McCulloch, I.; Heeney, M.; Chabinyc, M. L.; DeLongchamp, D.; Kline, R. J.;

Colle, M.; Duffy, W.; Fischer, D.; Gundlach, D.; Hamadani, B.; Hamilton, R.; Richter, L.;

Salleo, A.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Zhang, W. Adv. Mater., 2009, 21, 1091 -

1109.

Page | 47

(129) Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Dehm, C.; Schutz, M.;

Maisch, S.; Effenberger, F.; Brunnbauer, M.; Stellacci, F. Nature 2004, 431, 963 - 966.

(130) Gwinner, M. C.; Jakubka, F.; Gannott, F.; Sirringhaus, H.; Zaumseil, J. ACS

Nano 2012, 6, 539 - 548.

(131) Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay,

K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature 1990, 347, 539 - 541.

(132) McNeill, R. M.; Sindak, R.; Wardlaw, J. H.; Weiss, D. E. Aust. J. Chem. 1963,

16, 1056 - 1075.

(133) Weiss, D. E.; Bolto, B. A. Aust. J. Chem. 1963, 6, 1076 - 1089.

(134) Weiss, D. E.; McNeill, R.; Bolto, B. A. Aust. J. Chem. 1963, 6, 1090 - 1103.

(135) Shirakawa, H. Synth. Met. 2001, 125, 3 - 10, 281 - 286.

(136) Ikeda, S.; Shirakawa, H.; Ito, T. J. Polym. Sci., Polym. Chem. Edu. 1974, 12,

11.

(137) Chiang, C. K.; Finder, C. R. J.; Park, Y. W.; Heeger, A. J.; Shirakawa, H.;

Louis, E. J.; Gan, S. C.; MacDiarmid, A. J. Phys, Rev. Lett. 1977, 39, 1098 - 1101.

(138) Shirakawa, H.; Louis, E. J.; MacDiarmid, A. G.; Chiang, C. K.; Heeger, A. J.

J. Chem. Soc., Chem. Commun. 1997, 578 - 580.

(139) Chen, J.; Cao, Y. Acc. Chem. Res. 2009, 42, 1709-1718.

(140) Zhou, H.; Yang, L.; You, W. Macromolecules 2012, 45 607 - 632.

(141) Chen, J.; Hou, J.; Li, Y.; Zhou, X.; Zhang, J.; Li, X.; Xiao, X.; Lin, Y. Chin.

Sci. Bull., 2009, 54, 1669.

(142) Loewe, R. S.; McCullough, R. D. Chem. Mater., 2000, 12, 3214-3221.

(143) Lewis, N. S. Science 2007, 315, 798-801.

(144) Krebs, F. C.; Gevorgyan, S. A.; Alstrup, J. J. Mater. Chem., 2009, 19, 5442-

5451.

(145) Hujuh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425 - 2427.

(146) Ong, B. S.; Wu, Y. L.; Li, Y. N.; Liu, P.; Pan, H. L. Chem. Eur. J. 2008, 14,

4766 - 4778.

(147) Bao, Z. N.; Lovinger, A. J. Chem. Mater. 1999, 11, 2607 - 2612

(148) Perepichka, I. F.; Perepichka, D. F.; Mary, H.; Wudi, F. Adv. Mater. 2005, 17,

2281 - 2305.

(149) Le Floch, F.; Ho, H. A.; Harding-Lepage, P.; Bedard, M.; Neugu-Plesu, R.;

Leclerc, M. Adv. Mater. 2005, 17, 1249 - 1256

(150) Kumar, A.; Welsh, D. M.; Morrart, M. C.; Piroux, F.; Abboud, K. A.;

Reynolds, J. R. Chem. Mater. 1998, 10, 896 - 902.

(151) Sonmez, G.; Shen, C. K. F.; Rubin, Y.; Wudi, F. Angew. Chem., Int. Ed. 2004,

43, 1498 - 1502.

(152) Zhang, X.; Khlar, M.; Matzger, A. J. Macromolecules 2004, 37, 6306 - 6315.

(153) Chen, T. A.; Wu, X. M.; Rieke, R. D. J. Am. Chem, Soc. 1995, 117, 233 - 244.

(154) He, Y.; Wu, W.; Zhao, G.; Liu, Y.; Li, Y. Macromolecules 2008, 41, 9760-

9766.

(155) Fuchigami, H.; Tsumura, A.; Koezuka, H. Appl. Phys. Lett., 1993, 63, 1372-

1374.

(156) Gillissen, S.; Henckens, A.; Lutsen, L.; Vandersande, D.; Gelan, J. Synth. Met.

2003, 255, 135 - 136.

(157) Prins, P.; Candeias, L. P.; van Breemen, A. J. J. M.; Sweelssem, J.; Herwig, P.

T.; Schoo, H. F. M.; Siebbles, L. D. A. Adv. Mater., 2005, 17, 718 - 723

(158) Henckens, A.; Lutsen, L.; Vanderzande, D.; Knipper, M.; Manca, J.; Aernouts,

T.; Poortsmans, J. Proc. SPIE Int. Soc. Opt. Eng. 2004, 5464, 52 - 59.

Page | 48

(159) Huitema, H. E. A.; Gelinck, G. H.; Van der Putten, J. B. P. H.; Kuijk, K. E.;

Hart, C. M.; Cantatore, E.; Herwig, P. T.; Van Breemen, A. J. J. M.; De Leeuw, D. M. Nature

2001, 414, 5 - 99.

(160) Huitema, H. E. A.; Gelinck, G. H.; Van der Putten, J. B. P. H.; Kuijk, K. E.;

Hart, C. M.; Cantatore, E.; Herwig, P. T.; Van Breemen, A. J. J. M.; De Leeuw, D. M. Adv.

Mater. 2002, 14, 1201 - 1204.

(161) Roncali, J. Chem. Rev. 1997, 97, 173 - 206.

(162) Smith, A. P.; Smith, R. R.; Taylor, B. E.; Durstock, M. F. Chem. Mater., 2004,

16, 4687-4692.

(163) Hou, J. H.; Tan, Z. A.; He, Y. J.; Yang, C. H.; Li, Y. F. Macromolecules 2006,

39, 4657-4662.

(164) Banishoeib, F.; Henckens, A.; Fourier, S.; Vanhooyland, G.; Breselge, M.;

Manca, J.; Cleij, T. J.; Lutsen, L.; Vanderzande, D.; Nguyen, L. H.; Neugebauer, H.;

Sariciftci, N. S. Thin Solid Films 2008, 516, 3978 - 3988.

(165) Hou, L.; Chen, T. L.; Zhou, Y.; Hou, J.; Chen, H. Y.; Yang, Y.; Li, Y.

Macromolecules 2009, 42, 4377-4380.

(166) Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger,

A. J.; Brabec, C. J. Adv. Mater., 2006, 18, 789-795.

(167) Dennler, G.; Scharber, M. C.; Brabec, C. J. Adv. Mater., 2009, 21, 1-16.

(168) Huo, L.; Hou, J.; Zhang, S.; Chen, H.-Y.; Yang, Y. Angew. Chem. Int. Ed.,

2010, 49, 1500 -1503.

(169) Nie, W.; MacNeill, C. M.; Li, Y.; Noftle, R. E.; Carroll, D. L.; Coffin, R. C.

Macromol. Rapid Commun., 2011, 32, 1163-1168.

(170) Roncali, J. Chem. Rev., 1997, 97, 173 - 206.

(171) Salzner, U.; Lagowski, J. B.; Pickup, P. G.; Poirier, R. A. Synth. Met. 1998,

96, 177 - 189.

(172) Winder, C.; Sariciftci, N. S. J. Mater. Chem., 2004, 14, 1077-1087.

(173) Arbizzani, C.; Catellani, M.; Cerroni, M. G.; Mastragostoni, M. Synth. Met.

1997, 84, 249 - 250.

(174) Inaoka, S.; Collard, D. M. Synth. Met. 1997, 84, 193 - 194.

(175) Pomerantz, M.; Xiaormin, G. Synth. Met. 1997, 84, 243 - 244.

(176) Wudl, F.; Kobayashi, N.; Heeger, A. J. J. Org. Chem. 1984, 49, 3381 - 3384.

(177) Pomerantz, M.; Chaloner-Gill, B.; Harding, L. O.; Tseng, J. J.; Pomerantz, W.

J. Synth. Met. 1993, 55 - 57, 960 - 965.

(178) van Mullekom, H. A. M.; Vekemans, J. A. J. M.; Havinga, E. E.; Meijer, E.

W. Mater. Sci. Eng. 2001, 32, 1 - 40.

(179) Cai, W.; Gong, X.; Cao, Y. Sol. Energy Mater. Sol. Cells 2010, 94, 114 - 127.

(180) Pomerantz, M.; Chalonergill, B.; Harding, L. O.; Tseng, J. J.; Pomerantz, W.

J. J. Chem. Soc., Chem. Commun. 1992, 22, 1672 - 1673.

(181) Hong, S. Y.; Marynick, D. S. Macromolecules 1992, 25, 4652 - 4657.

(182) Ajayaghosh, A. Chem. Soc. Rev. 2003, 32, 181 - 191.

(183) Havinga, E. E.; ten Hoeve, W.; Wynberg, H. Synth. Met. 1993, 55, 299 - 306.

(184) Kitamura, C.; Tanaka, S.; Yamashita, Y. Chem. Mater. 1996, 8, 570 - 578.

(185) Jayakannan, M.; van Hal, P. A.; Janssen, R. A. J. J. Pol. Scien. A, Pol. Chem.

2002, 40, 251 - 261.

(186) Roncali, J. Acc. Chem. Res. 2009, 42, 1719 - 1730.

(187) Zhou, H.; Yang, L.; Stoneking, S.; You, W. ACS Appl. Mater. Interfaces 2010,

2, 1377 - 1383.

(188) Haddon, R. C. Acc. Chem. 1992, 25, 127 - 133.

Page | 49

(189) Haufler, R. E.; Conceicao, J.; Chibante, L.; Chai, Y.; Byrne, N.; Flanagan, S.;

Haley, M.; O‟Brien, S.; Pan, C.; Xiao, Z.; Billips, W.; Ciufolini, M.; Hauge, R.; Margrave, J.;

Wilson, L. J.; Curl, R.; Smalley, R. J. Phys. Chem. 1990, 8634 - 8636.

(190) Kooistra, F. B.; Knoll, J.; Kastenberg, F.; Popescu, L. M.; Verhees, W. J. H.;

Kroon, J. M.; Hummelen, J. C. Org. Lett. 2007, 9, 551 - 554.

(191) Riedel, I.; Parisi, E.; Martin, N.; Giacalone, F.; Dyakonov, V. Adv. Funct.

Mater. 2005, 15, 1979 - 1987.

(192) Rispens, M. T.; Meetsma, A.; Rittberger, R.; Brabec, C. J.; Sariciftci, N. S.;

Hummelen, J. C. Chem. Commun. 2003, 2116 - 2118.

(193) Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. J. Am. Chem.

Soc. 2009, 131, 7792-7799.

(194) Meier, M. S.; Corbin, P. S.; Vance, V. K.; Clayton, M.; Mollman, M.;

Poplawska, M. Tetrahedron Lett. 1994, 35, 5789 - 5792.

(195) Wei, X.; Wu, M.; Qi, L.; Xu, Z. J. Chem. Soc., Perkin Trans. 2 1997, 1389 -

1393.

(196) Allemand, P. M.; Koch, A.; Wudl, F.; Rubin, Y.; Diederich, F.; Alvarez, M.

M.; Anz, S. J.; Whetten, R. L. J. Am. Chem. Soc. 1991, 113, 1050 - 1051.

(197) Singh, T. B.; Marjanovic, N.; Matt, G. J.; Gunes, S.; Sariciftci, N. S.; Ramil,

A. M.; Andreev, A.; Sitter, H.; Schwodiauer, R.; Bauer, S. Org. Electron. 2005, 6, 105 - 110.

(198) Labram, J. G.; Kirkpatrick, J.; Bradley, D. D. C.; Anthopoulos, T. D. Adv.

Energy Mater. 2011, 1, 1176 - 1183.

(199) Anthony, J. E.; Facchetti, A.; Heeney, M.; Marder, S. R.; Zhan, X. Adv.

Mater. 2010, 22, 3876 - 3892.

(200) Backer, S.; Sivula, K.; Kavulak, D. F.; Frechet, J. M. J. Chem. Mater. 2007,

19, 2927 - 2929.

Page | 50

Chapter 2

Microwave-Assisted Synthesis of Novel Conjugated Poly(thieno[3,2-b]thiophene vinylene)s: Effects of Linear and

Branched Alkyl-substituents on the Organic Field Effect Transistor and Organic Solar Cell

Performance

Page | 51

2.0 Microwave-Assisted Synthesis of Novel Conjugated Poly(thieno[3,2-b]thiophene vinylene)s: Effects of Linear and Branched Alkyl-substituents

on the Organic Field Effect Transistor and Organic Solar Cell Performance

2.1 Introduction 2.2 Synthesis of Fused Thienothiophenes

Thieno[3,2-b]thiophene (T32bT) is a stable 10π electron-rich heterocylic compound

and one of four isomeric derivatives of the thienothiophene (TT) chromophore (see figure

1).1,2

In 1967, a study published by Wynburg and Zwanenburg revealed that T32bT exhibited

very interesting and bathochromically shifted UV-vis absorption spectra relative to the other

isomeric derivatives, and is therefore eminently attractive for the synthesis of π-conjugated

polymers for photovoltaic applications.3 In contrast, T34cT is highly unstable and thus the

least investigated. Nonetheless, since the T23bT, T34bT, and T34cT isomers do not form any

part of this project, they therefore will not be tackled further.

Figure 1. Isomers of thienothiophene, thieno[2,3-b]thiophene (T23bT), thieno[3,2-b]thiophene (T32bT),

thieno[3,4-b]thiophene (T34bT) and thieno[3,4-c]thiophene.

Over the years, various reaction methodologies have been developed to synthesise

T32bT and its derivatives.1,4-8

Scheme 1 shows one of such route for the synthesis of

unsubstituted T32bT derivatives following a palladium/copper co-catalyzed route. This

initially involves the Sonogashira cross-coupling of 2-bromo-3-iodothiophene (1) and

trimethylsilylacetylene (2) in the presence of diethylamine base to afford 2-bromo-3-[2-

(trimethylsilyl)ethynyl]thiophene (3). Reduction of the triple bond, and subsequent capture of

the aluminium derivative with an electrophilic source of bromine, afforded the key

dibromothiophene derivative (4). This was lithiated with two equivalents of n-BuLi and ring

closed by reaction with an electrophilic source of sulfur. Following desilyation with tetra-n-

butylammonium fluoride (TBAF) T32bT (5) is afforded in good yield. Different isomers can

Page | 52

be formed (either T23bT or T32bT or T34bT) depending on the positions of the bromine and

ethynylene on the thiophene aromatic system.1

Scheme 1. Synthesis route to unsubstituted thieno[3,2-b]thiophene.

However this reported synthesis route to T32bT was not amenable to scale-up, and

usually resulted in low yields, therefore alternative pathways that could be scaled-up were

investigated. Scheme 2, shows a more convenient route for large scale synthesis of T32bT.9,10

Scheme 2. Alternative synthesis of thieno[3,2-b]thiophene.

Here, the commercially available 3-bromothiophene (a)11

was selectively

deprotonated at the 2-position with lithium diisopropylamine (LDA) in tetrahydrofuran

(THF) at 0 oC, followed by quenching with N-formylpiperidine to afford 3-bromothiophene-

2-carboxyaldehyde (b). The treatment of the thiophene aldehyde with ethyl 2-sulfanylacetate

in a base gave ethyl-2-thieno[3,2-b]thiophenecarboxylate (c) in 81 % yield. Finally, the

saponification (hydrolysis) of (c) with lithium hydroxide (LiOH) in THF, and the thermal

decarboxylation of the resulting thieno[3,2-b]thiophene carboxylic acid (d) in hot quinoline

with metallic copper afforded T32bT (e) in an overall yield of 50 % (upon purification by

flash chromatography).9,12

Page | 53

Scheme 3. High yield thieno[3,2-b]thiophene synthesis under milder temperatures.

Frere et. al. published an even higher yielding and scalable route (scheme 3),6 in

which the high temperature decarboxylation step in scheme 2 is by-passed. In Frere‟s route,

the Friedel-Crafts acylation and cyclization of compound (8) were achieved in a one-pot

reaction with thionyl chloride (SOCl2, 1 equivalent) in diethyl ether (Et2O), followed by the

introduction of aluminium chloride (AlCl3). The reaction was warmed under reflux in 1,2-

dichloromethane (ClCH2CH2Cl) or carbon disulfide (CS2) to afford (9) in moderate yield (50

%). The reduction of (9) with sodium borohydride (NaBH4) followed by, the treatment of the

resultant alcohol (10) with hydrochloric acid (HCl) or on silica gel, formed T32bT (11) in 84

% yield.6

The synthesis of T32bT derivatives with substituents in the 3,6-positions is generally

more challenging. Nakayama et. al. reported a facile preparation of 3,6-dimethylsubstituted

T32bT [3,6-dimethylthieno[3,2-b]thiophene] via a one-pot method, by vigorously heating

2,5-dimethyl-3-hexyne-2,5-diol (12) with elemental sulfur in benzene at elevated

temperatures (190 oC - 200

oC) in approximately 26 % yield (scheme 4).

4 Although yields

were relatively low, the simplicity of the reaction is attractive. However this route was not

amenable to the synthesis of derivatives with alkyl side groups longer than methyl.

Page | 54

Scheme 4. Nakayama‟s one-pot synthesis of 3,6-dimethylthieno[3,2-b]thiophene.

The synthesis of T32bT with longer alkyl chains necessitated a complex multi-step

synthesis as reported by Matzger et. al. (scheme 5).13

The Friedel-Crafts acylation of 3,4-

dibromothieno[3,2-b]thiophene (14) with decanonyl chloride formed 2-decanonyl-3,4-

dibromothiophene (15) in 80 % yield. Upon treatment with ethyl thioglycolate and in a

mixture of sodium hydroxide (NaOH) in ethanol (EtOH), the mono-alkylated intermediate, 3-

nonylthieno[3,2-b]thiophene-2-carboxylate (17) was afforded. The hydrolysis of (17) and

decarboxylation of the resulting acid produced (18). Then, the attachment of a second alkyl

side chain by the Pd-catalyzed Sonogashira cross-coupling of 1-nonyne and the bromine

group, followed by the hydrogenation of the ethyne group formed the target 3,6-

dinonylthieno[3,2-b]thiophene derivative (20) (scheme 5). In another publication, the 3,6-

dipentadecylthieno[3,2-b]thiophene analogue was reportedly synthesized following this same

experiment protocol.14

Scheme 5. Matzger‟s synthesis of 3,6-dinonylthieno[3,2-b]thiophene.

Page | 55

With the drawbacks of the above synthesis pathways in mind, Heeney and co-workers

pioneered a more efficient and versatile method for the preparation of 3,6-dialkyl substituted

T32bTs.15-17

Amongst all existing methodologies, this alternative route is advantageous due

to; (I) the reduced number of reaction steps involved, (II) the elimination of unnecessary

reagents, and (III) the flexibility of attaching a variety of alkyl chains at a later stage in the

synthesis (vide infra). Scheme 6, shows the Heeney synthesis route to 3,6-disubstituted

T32bT derivatives.

Scheme 6. Heeney‟s synthesis pathway to 3,6-dialkylthieno[3,2-b]thiophene derivatives.17

The 3 step synthesis route illustrated in scheme 6, begins with the excess bromination

of the readily accessible thieno[3,2-b]thiophene-2-carboxylic acid (see 8 in scheme 3) in an

acidic medium to obtain 2,3,5,6-tetrabromothieno[3,2-b]thiophene (II). Following reduction

of the tetrabrominated T32bT with zinc powder in acetic acid, the 3,6-dibrominated analogue

(III) was afforded. A variety of alkyl side groups of varying chain lengths and configurations

could be introduced by applying the palladium-catalyzed Negishi cross-coupling

methodology. It was discovered that microwave heating was required to afford high product

yields.17

The use of microwave in such reaction allows for the use of high boiling solvents,

short reaction completion times, and less side products, as previously discussed.

2.2.1 Conjugated Polymers Containing Thienothiophene

Numerous semiconducting polymers bearing the linearly symmetrical T32bT moiety

in their backbone structure have been synthesized mainly by the Stille cross-coupling

methodology (vide infra) with comparatively high field effect mobilities (μ) ranging from

0.25 to 1.0 cm2 V

-1 s

-1.14,18-20

Recent analogues of these T32bT-containing π-conjugated

polymer derivatives include; poly(3,6-dialkylthieno[3,2-b]thiophenes) (pATTs),13,16

poly(1-

Page | 56

(thiophene-2-yl)benzothieno[3,2-b]benzothiophene) (pTBTBT),21

poly(2,5-bis(3-

alkylthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT),18,22,23

poly(3,6-bis-(5-thiophen-2-yl-

N,N-2bis(2-octyl-1-dodecyl)-1,4-dioxo-pyrrolo[3,2-c]pyrrole-co-thieno[3,2-b]thiophene)

(pDBT-co-TT),20

poly(3,6-alkyl-[2,2‟]bi-[thiophene[3,2-b]thiophene]) (pDABTT),23,24

and

poly[2,5-bis(thieno-2-yl)-3,6-dipentadecylthieno[3,2-b]thiophene-co-2,1,3-benzothiadiazole]

(pBTDPTT-co-BT).25

Figure 2. Structure of thieno[3,2-b]thiophene-bearing conjugated polymers.

In general, the rigidity and planarity of fused thieno[3,2-b]thiophene was found to

impart stronger inter-chain π-π stacking geometry and enhanced crystallinity in the solid-

state, over simple 2,2‟-bithiophene derviatives.26

However a drawback of the inclusion of the

unsubstituted T32bT is that the resulting polymers can display poor solubility in common

organic solvents at room temperature, which hinders their use in potential device

Page | 57

applications.27-30

As discussed in the opening section, a route to improve solubility is to

incorporate two alkyl side chains into the β-positions of the T32bT moiety.13,23

Homopolymerization of this monomer, by oxidative methods, gave a highly soluble polymer

(pATT, figure 2).13,23

This material demonstrated poor performance in organic field effect

transistors however, with very low charge carrier mobility. This is likely due to the close

proximity of the adjacent alkyl groups, reminiscence of H-H couplings in poly(3-

hexylthiophene) (P3HT),31,32

which resulted in a disruption of the conjugation and planarity

due to steric repulsions along the polymer main-chain.23

Such disruptions led to an

amorphous polymer with poor overlap of polymer backbones, and poor FET performance.

Therefore, in an effort to circumvent the problems posed by steric repulsions, He and co-

workers reported the synthesis of a poly(3,6-alkylthieno[3,2-b]thiophene) with flexible

vinylene-linkages in the backbone structure (DH-PTTV) and hexyl (C6) side functionalities

in the 3 and 6 positions again via Stille cross-coupling polymerization (scheme 7).27

Scheme 7. Synthesis of poly(3,6-dihexylthieno[3,2-b]thiophene-2-yl vinylene).

The vantage points of the incorporated vinylene linkage includes; (a) the fact that they

function as spacers to prevent disruptive torsional H-H interactions between neighbouring

repeat units, (b) the narrowing of the polymer energy band gaps by extending the effective

conjugation length of the polymeric chains, (c) the facilitation of rotational flexibility along

the conjugated polymer framework to allow for structural re-organization in the solution and

solid phases.33

Despite this strategy, the performance of the conjugated polymer was not very

promising for OPV and FET applications, when compared to poly(3-hexylthiophenes)

(P3HT), with PCE of just 0.28% obtained for the unoptimized OPV blend device. He

reported another route (scheme 8) for the synthesis of the dialkylated monomer, used in the

preparation of DH-PTTV. Here a 7 step reaction pathway (scheme 8) was adopted to access

the 3,6-hexylthieno[3,2-b]thiophene precursor (M8), which warranted further consideration.

This route has similar drawbacks to the route of Matzger et al discussed earlier, namely the

difficulty in accessing a variety of probable monomers with different alkyl pendant groups of

varying chain lengths and configuration. This is because they are introduced at the beginning

Page | 58

of the reaction sequence. Furthermore, the high temperature (260 oC) decarboxylation stage is

low yielding and difficult to purify from residual quinoline, making this route both

unattractive and time consuming in my opinion.

Scheme 8. Synthesis route for 2,5-bromo-3,6-dialkylthieno[3,2-b]thiophene monomer by He et al.27

Our aims in this research were to further investigate the synthetic utility of thieno[3,2-

b]thiophene vinylene (TTV) containing π-conjugated polymers (scheme 9) (this research

started before the work of He was published).

Scheme 9. Representative structures of the target poly(3,6-dialkylthieno[3,2-b]thiophene-2-yl vinylenes).

Page | 59

In particular we were interested in examining the effect of the side-chain length and

conformation on the performance of the polymer in both FET and OPV devices. We

identified that the short hexyl groups used in He‟s work could possibly result in a low

solubility of the resulting polymer, and therefore a low molecular weight, so the development

of semiconducting polymers with longer side groups and therefore higher molecular weight

was desirable. In addition, we were also interested to explore if the nature of the

polymerization route used would influence the properties of the resulting polymer. In the first

half of this chapter, I focused on the synthesis of the novel monomers and their

polymerization by Stille cross-coupling reaction (vide infra). In the second half of the

chapter, I introduce another polymerisation method, Gilch polymerisation for making TTV,

and compare the properties of the conjugated polymers synthesized by both the Stille and

Gilch route. Firstly, I briefly review the use of microwave irradiation in polymer research,

and summarise the important cross-coupling mechanisms used in my research.

Page | 60

2.2.2 Microwave Irradiation in Polymer Research and Advancement

Over the past three decades, microwave (μW) irradiation has emerged as the defacto

heating technique in the preparation of a catalogue of diverse organic compounds.34,35

Figure

3 shows a typical modern microwave synthesizer.

Figure 3. Biotage Initiator 2.5 microwave synthesizer.

In polymer chemistry the usefulness of this technique has been highlighted in an

increasing number of publications since the initial study published by Teffal and Gourdenne

in the 1980s.36,37

The application for the synthesis of conjugated polymers (CPs) was quickly

realised, and a numbers of reports and reviews have summarised this progress.38-40

So far,

μW heating has been successfully utilized in polymerizations involving Heck, Stille, Suzuki,

and Sonogashira cross-coupling methodologies, catalyzed by metal-ligand complexes.38,40-45

The advantages of μW heating in polymer synthesis are impressive and exceed those of the

traditional heating methods. μW allows for accessibility to controllable and elevated reaction

temperatures by the use of a pressurized (or closed-vessel microwave chemistry) system.35

This results in optimised reaction rates, significantly shortened reaction times, improved

safety, and often cleaner experimental procedures, which are major pros of the μW heating

option.40

In addition, microwave irradiation permits the targeted transfer of energy to the

constituting reactants and solvents, thus facilitating a highly homogeneous and enhanced

heating rate.35,41

Consequently, the probability of side-product formation is sometimes

reduced or circumvented entirely and a high product yield is afforded.41,46

Furthermore, the

Page | 61

μW heating method enables the introduction of energy remotely without any contact with the

sample and the source. This energy is then distributed evenly throughout the bulk of the

product and not isolated to the surface.35

2.2.3 Stille Cross-coupling Reaction and Polymerization

The cross-coupling of organostannanes with organic aryl halides in the presence of a

group 10 palladium catalyst complex form the basis of the Stille reaction methodology (see

fig. 4)47

.48-50

In addition, the reaction is highly regioselective and proceeds well under mild

experimental conditions with high product yields.48

Figure 4. Schematic Stille cross-coupling reaction.

The publication of the first successful Stille cross-coupling reaction of an

allyltributyltin reagent with an arylbromide catalyzed by the

tetrakis(triphenyphosphine)palladium(0) complex [Pd(PPh3)4] in 1977,51

spurred the

synthesis of a wide range of synthetic organic compounds through the formation of the

carbon-to-carbon bonds.47,52,53

Moreover, the advantage of the Stille cross-coupling reaction

lies in the versatility of the organostannane reagents. In polymer chemistry, the accessibility

of organo-stannyl compounds and organo-halides bearing a combination of different

functional groups (that is, alkyl, alkenyl, akynyl, allyl, and aryl groups) has led to the rapid

development of a catalogue of novel low band gap donor-acceptor type conjugated polymers,

whose properties are key to the operation and efficiency of organic photovoltaic

devices.23,27,54-64

Figure 5 represent the Stille type copolymer synthesis.

Page | 62

Figure 5. Copolymerization by Stille cross-coupling reaction.

Another crucial point to highlight is the ability of the Stille reaction to couple a

variety of donor and acceptor units, with either coupling partners bearing either an electron

rich (D) and/or an electron deficient (A) functional group without requiring any ardous

deprotection reactions.48

2.2.3.1 Stille Cross-coupling Reaction: A Mechanistic Perspective

Scheme 10 depicts the typical catalytic cycle of the Stille cross-coupling reaction.

Scheme 10. Palladium catalyzed Stille cross-coupling mechanism.48,65,66

Page | 63

The Stille reaction follows the generally accepted mechanism for all transition metal-

catalyzed cross-coupling reactions and constitutes four consecutive stages; (I) ligand

dissociation, (II) oxidative addition, (III) transmetallation, and (IV) reductive elimination.49,66

In the first step of the catalytic cycle, the zerovalent palladium catalyst (PdL4, where L =

ligand) undergoes ligand dissociation to form a coordinatively unsaturated complex (that is,

the creation of a vacant attachment site).66

This results to the formation of the highly reactive

Pd(0) (PdL2) catalyst, where the σ-donicity of the ligand (usually, a phosphine derivative) to

the empty d-orbital of the metal centre through the metal-ligand (M-L) bond, leads to

increased electron density on the metal centre. The increased nucleophilicity of the catalyst

favours the oxidative addition step.65,67

According to evidence in the literature, the oxidative

addition reaction proceeds via a concerted insertion of the PdL2 catalyst into the R-X ζ-bond

to afford a trans-[PdRXL2] product.68-71

However, the preservation of the trans-configuration

of the Pd complex at this stage of the coupling mechanism has been disputed based on in-

depth kinetic studies by Casado and Espinet.72

It was discovered that the reaction (scheme

11) of 3,5-dichlorotrifluorophenyl iodide (C6Cl2F3I) with the Pd(PPh3)4 catalyst produced cis-

[Pd(C6Cl2F3)I(PPh3)2] which slowly isomerizes to trans-[Pd(C6Cl2F3)I(PPh3)2].

Scheme 11. Reaction of C6Cl2F3I and Pd catalyst.

Regardless, the transmetallation step is usually considered to be the slowest and rate-

determining step, but the mechanism by which it occurs is still not fully elucidated.66

As

depicted in scheme 10, it is thought to proceed via a dissociative X-for-R2 substitution with

the preservation of the trans-configuration at the palladium centre. This claim was based on

the premise that the addition of a neutral ligand (L) leads to a retarded coupling process,

which suggests that the dissociation of L from complex (a) (scheme 10) was indeed an

important step in the transmetallation stage. Furthermore, the dissociative mechanism implies

that L first dissociates from (a) in scheme 14 prior to transmetallation to form a

coordinatively unsaturated complex (bearing a solvent molecule), which then reacts with the

organostannane reagent. This posed some striking questions relating to the viability of the

dissociative mechanism and the fact that it is the spectator ligand which actually undergoes

Page | 64

the dissociation.66,73

In order to address the inconsistencies posed by the dissociative

mechanism (see step (III) scheme 10) an associative transmetallation mechanism was

proposed. The latter involved an associative L-for-R2 substitution which directly affords a cis

R1/R2 arrangement rather than the trans derivative. Finally, the mechanism of the reductive

elimination process also not well understood, but, it is known to occur on the cis Pd

complex.66

This process is preceded by the regeneration of the palladium catalyst (PdL4).

Scheme 12 illustrates the catalytic cycle of the proposed alternative mechanism.

Scheme 12. Alternative Stille cross-coupling catalytic cycle.66

2.3 Synthesis of Soluble Low Band Gap poly(thieno[3,2-b]thiophene vinylene) [pTTV]

2.3.1 Synthesis of the 2,5-dibromo-3,6-dialkylthieno[3,2-b]thiophene Monomer Derivatives

The initial synthesis route to the 2,5-dibromo-3,6-dialkylthieno[3,2-b]thiophene

monomers was similar to that described earlier by Heeney and co-workers (Scheme 6).

However, I implemented two modifications to the synthesis of the key 3,6-

dibromothieno[3,2-b]thiophene derivative (scheme 13).15,16

Page | 65

Scheme 13. Synthesis of 3,6-dialkylthieno[3,2-b]thiophene.9,74

Heeney et. al., had originally synthesized this via the reduction of the

tetrabromothieno[3,2-b]thiophene derivative, itself prepared by bromination of the 2-

carboxylic acid derivative of thienothiophene.9,74

Fortunately, I was able to secure a ready

commercial source of thieno[3,2-b]thiophene which enabled some simplification to the

synthetic scheme. The first modification was the tetrabromination of T32bT itself, rather than

the carboxylic acid. This proceeded under less forcing conditions than reaction of the acid.

Thus reaction with excess bromine in carbon disulfide (CS2) solution at reflux for 24 h gave

almost quantitative yields. Upon purification via repeated recrystallization from hot toluene

and drying in vacuum, needle-like solid crystals were obtained in very high yield (ca. 95 %).

The use of excess bromine was to ensure that all four unsaturated sites on the T32bT aromatic

system were completely substituted, and to prevent the occurrence of unwanted side products,

due to incomplete bromination. In addition, the purity of the tetrabrominated T32bT

precursor (24) was ascertained by GC-MS and 1H-NMR (not shown). The absence of the

aromatic hydrogen (H) peaks of the starting material confirmed the successful synthesis of

compound (24).

Subsequent reduction with two equivalents of zinc dust was attempted under identical

conditions to those reported earlier (Zn powder in refluxing acetic acid (AcOH).

Disappointingly attempts to repeat this at a large scale failed, with no reduction products

identified at all by GC-MS (only starting material). Despite increasing reaction times, and

amount of zinc dust, no reduction products could be identified upon several attempts.

Previous attempts had successfully worked when the tetrabromide was prepared via the

carboxylic acid route, so it was thought that a minor impurity present in this „tetrabromide‟

may be facilitating the reaction. However at this stage, an alternative route was developed

which did not require the formation of the tetrabromide intermediate at all. This was

Page | 66

beneficial since the tetrabromide was poorly soluble, and also the synthetic methodology was

quite wasteful in terms of the introduction and subsequent removal of 2 bromine atoms.

The new preparation of 3,6-dibromothieno[3,2-b]thiophene is outlined in Scheme

1412,16,55,75

(vide infra) and is based on the work of Miguel et. al.23

Here, a 2,5-

dibromothieno[3,2-b]thiophene is treated with two equivalents of a non-nucleophilic lithium

base (LDA) to generate the 3,6-dilithio anion at low temperature. Upon warming the salt re-

arranges by the so-called halogen dance mechanism76

to give the more thermodynamically

stable 2,5-dilithium salt, with the halogens moving to the 3,6-positions. The mechanism most

likely proceeds in an inter molecular manner, rather than intra.76

Scheme 14. Synthesis of 2,5-dibromo-3,6-dialkylthieno[3,2-b]thiophene derivatives.

Thus, 2,5-dibromothieno[3,2-b]thiophene (M11) was synthesized in accordance with

the literature,12,75,77,78

by treatment with 2 equivalent of NBS in a mixture of dichloromethane

(DCM) and glacial acetic acid (AcOH).12

Since the 2,5 positions are more susceptible to

electrophilic attack, due to the donating effect of the sulfur group on the aryl group, they are

easily brominated. The solvent used in this case (DCM) was different to that suggested in one

of the literature procedure75

(dimethylformamide – DMF). This facilitates work-up. After the

reaction progressed for about an hour, the analysis of the crude by GC-MS, indicated the

complete conversion of the unsubstituted T32bT to M11 (m/z = 298 g mol-1

). This

preliminary reaction suggests a facile debromination of T32bT. Then, the reaction was scaled

up from 5 g to a total of 60 g with the yield of the target compound (M11) exceeding 95 %,

after processing and overnight drying under vacuum at 40 oC. The precursor (M11)

Page | 67

synthesized was analyzed by 1H-NMR, with a single aromatic H (2Hs) peak appearing at

chemical shift (δ) of 7.19 ppm, and high resolution mass spectroscopy (HRMS).

For the next stage of the synthesis route, the procedure of Miguel and co-workers was

followed.63

3,6-Dibromothieno[3,2-b]thiophene (M12), was initially prepared on a small

scale,76

to ascertain its viability. Lithium diisopropylamine (LDA) was generated in-situ by

the reacting an equimolar amount of the organolithium reagent (n-butyllithium – n-BuLi) and

the basic diisopropylamine (DPA) at -78 oC. After a period of 30 minutes, a solution of M11

in tetrahydrofuran (THF) was added via canulation, while maintaining the temperature below

-78 oC. The temperature of the reaction was slowly increased to ambient temperature

overnight. On completion, the analysis of the crude by GC-MS showed a complete

transformation of M11 to M12. After work-up, M12 was obtained in 70 % yield, with the

aromatic H singlet peak at δ 7.19 ppm in the starting material shifting upfield to δ 7.37 ppm

in the product (see appendix). As a result of this successful synthesis, the reaction was

performed repeatedly on a much bigger scale (~50 g) with high yields (~80 %) for the target

compound (M12).

2.3.2 Synthesis of 3,6-dialkylthieno[3,2-b]thiophene via Microwave-assisted Negishi Cross-coupling Methodology

After the synthesis of 3,6-dibromothieno[3,2-b]thiophene (M12), we proceeded to

synthesize four structurally distinct dialkylated T32bT derivatives (see step III in scheme

12), prior to polymerization. The alkylation of the aryl bromide compound (M12) was

performed by the palladium-catalyzed Negishi cross-coupling methodology using organozinc

bromide (RZnBr) reagents, under temperature controlled microwave heating. The reaction

was carried out in a 20 mL capacity sealed microwave tube (Biotage). However, due to the

limited size of the reaction vessel, the scale of each synthesis was limited to a starting mass of

1 g each time. Also in this case, several test reactions were performed in small scales, so as to

accurately establish the optimal conditions required for complete transformation and high

yields of the target compound (M13a – M13

d scheme 12). In one trial experiment using a

sealed microwave tube, two molar equivalent of the decylzinc bromide reagent in a solution

of 0.5 M THF was reacted with one equivalent of M12 in the presence a palladium (II)

catalyst [Pd(dppf)Cl2]. The reaction mixture was sufficiently deaerated with argon under

stirring for a period of 10 - 20 minutes, to prevent atmospheric oxygen from interfering with

the coupling process. Then, the reaction mixture was placed in a microwave reactor and

Page | 68

sequential heating at 100 oC for 1 minute, 120

oC for 5 minutes, and 135

oC for 15 minutes.

However, upon analyzing the crude via GC-MS, three peaks of varying intensities were

identified the resulting spectra (not shown). These were assigned to the starting material

(M12 – 419 g mol-1

), 3-bromo-6-decyl-thieno[3,2-b]thiophene derivate (BDTT, 358 g mol-1

),

and the compound of interest (M13a

– 297 g mol-1

). Clearly, this signified an incomplete

cross-coupling reaction and thus, necessitated some adjustments to the microwave conditions.

Upon numerous modifications of the microwave conditions, a complete conversion of

M12 to M13a was observed by GC-MS and thin layer chromatography analysis. The

microwave conditions applied hereafter were 100 oC for 1 minute, 120

oC for 5 minutes, and

140 oC for 25 minutes, in all successive Negishi cross-coupling reaction of M12 with various

alkylzinc bromide reagents (that is, C10H21ZnBr, C12H25ZnBr, C16H33ZnBr, and 2-

ethylhexylZnBr). Moderate to high yields were recorded for compound M13a (~65 %, solid),

M13b (~70 %, solid), M13

c (~95 %, solid) and M13

d (~75 %, viscous oil). The derivatives

bearing straight alkyl side-chains were all solid at room temperature and were easily purified

by recrystallization from boiling acetone after silica-gel column chromatography. However,

the branched derivate (M13d) was not crystalline and purification involved painstaking and

repeated silica-gel column chromatography at slower solvent flow rate, as it could not be

further purified by recrystallization.

Page | 69

Scheme 15. Negishi cross-coupling mechanism to dialkylthieno[3,2-b]thiophene derivatives.79

The mechanism of the Negishi cross-coupling reaction (scheme 15) is akin to those

involved in other metal-catalyzed coupling processes. The co-ordinatively unsaturated

palladium (0) catalyst [Pd(dppf)(0)] is generated upon the dissociation of the halide ligands

from the palladium (II) complex (Pd(II)) in-situ,80

followed by the insertion into the aryl-

bromide bond.41,46

The exchange of ligand between the Pd and Zn metal centres (3) is

followed by a non-dissociative reductive elimination (4). Reports have shown that facile

reductive elimination is facilitated by utilizing Pd catalysts bearing bulky ligands with good

ζ-donicity.41,81

2.3.3 Synthesis of 2,5-dibromo-3,6-dialkylthieno[3,2-b]thiophenes

The bromination of the intermediates (M13a – M13

d) using NBS in anhydrous THF,

afforded the target monomers (M14a – M14

d) in very high yields (80 % - 95 %). Again,

difficulties were encountered during the purification of 2,5-dibromo-3,6-bis(2-

ethylhexyl)thieno[3,2-b]thiophene (M14d) crude. Since the resulting crude was viscous oil,

the elimination of the tribrominated (which we believe may be from side chain bromination)

and monobrominated polar impurities (detected by GC-MS analysis) by normal-phase silica-

Page | 70

gel (Si-OH) chromatography (eluted with petroleum hexane) presented a major challenge due

to similar retention times for these very non-polar materials. This problem was rectified by

analyzing the viscous crude with reverse-phase Si-C18 coated aluminium thin layer

chromatography plates and then employing the reverse-phase silica-gel (Si-C18H37)

chromatography (Biotage® Flash Master Personal) (eluent: various mixtures of acetonitrile

and THF attempted, with 3:1 providing the best separation). Since, Si-C18 is non-polar, the

most polar impurities should elute first, followed by the least polar (target M14d). Although,

this did affect the yield of the target compound, the purity of the monomer is crucial in order

to obtain high yield and high molecular weight polymers during polymerization. The

monomers (M14a – M14

d) were determined to be of high purity by

1H and

13C NMR spectra

(not shown).

2.3.4 Synthesis of poly(3,6-alkylthieno[3,2-b]thiophene vinylene)s via Microwave-assisted Stille Cross-coupling Polymerization of 2,5-dibromo-3,6-dialkylthieno[3,2-b]thiophene Derivates

Scheme 16, delineates the polymerization route to the synthesized semiconducting

polymers.

Scheme 16. Poly(2,5-dialkylthieno[3,2-b]thiophene-2-yl vinylene) derivatives by Pd-catalyzed microwave-

accelerated Stille cross-coupling reaction.

Page | 71

Three target π-conjugated polymers, PDC12TTV, PDC16TTV, and PBEHTTV were

synthesized. However, we decided to investigate the C16 and EH bearing polymer derivates

in more detail, in order to further probe the impact of the variations in the nature of the side

group attachments on their performance in PSC and OFET device architectures. The use of

microwave irradiation in Stille cross-coupling is a robust strategy, which makes possible the

high throughput synthesis of a number of polymers bearing different electron-rich and

electron-deficient units, within a short period of time. In this particular case, preliminary

Stille coupling polymerizations were instigated, in order to ascertain the most suitable giving

high molecular weight and high yield polymers. This involved the polymerization of an

equimolar amount of the EH monomer (M14d, scheme 14) and the commercially available

organostannane reagent (trans-1,2-bis(tributylstannyl)ethane M15 (95 % purity) scheme 16)

in the high boiling chlorobenzene (CB) solvent (with the addition of 0.01 % triethylamine –

Et3N).

The addition of triethylamine was to neutralise any possible HCl present in the

chlorinated solvent, since this could result in undesirable protodestannylation of the tin

reagent. In addition, the reaction was catalyzed by a bulky palladium catalyst [Pd(P(o-

tolyl)3)4] generated in-situ from the reaction of tris(dibenzylideneacetone)dipalladium (0)

[Pd2(dba)3] and tri(o-tolyl)phosphine [P(o-tolyl)3].18,42,46

The palladium catalyzed Stille

coupling process was accelerated by sequential microwave heating at 140 oC for 2 minutes,

160 oC for another 2 minutes, and then at 180

oC for 15 minutes. The polymerization was

quenched by the precipitation of the highly viscous and violet-coloured crude into acidic

methanol (that is, ca. 7 % HCl in methanol), to eliminate tri-n-butylstannyl end groups.82

The

resulting polymer precipitate was subjected to purification by Soxhlet extraction in methanol,

acetone, hexane, and chloroform, for 24 hours consecutively. The purpose of utilizing

methanol and hexane is to remove unreacted monomers/residual impurities, and oligomers

(or low molecular weight polymer fraction).83

Chloroform was used to isolate the high

molecular weight polymer fraction trapped in the extraction thimble. GPC analysis of the

chloroform extract for polymer, PBEHTTV, showed a moderate number average molecular

weight (Mn) of 16.5 kDa, with a polydispersity index (PDI) of ca. 2.4. Increasing the length

of the reaction via various modifications of the microwave conditions [140 oC (2 - 4 minutes),

160 oC (2 – 6 minutes, and 180

oC (15 – 40 minutes)] did not result in an increase in

molecular weight (table 1). Similarly increasing the catalyst concentration (table 2) resulted

in a reduction in MW, not an improvement.

Page | 72

PBEHTTV - Molecular weight versus Microwave Conditions

Method Microwave Conditions

(Temperature / oC, Time / min) Mn / kDa PDI DPn Yield / %

1 140, 2 160, 2 180, 15 16.5 2.4 42 80

2 140, 3 160, 3 180, 20 8.0 2.2 21 60

3 140, 4 160, 6 180, 40 10.0 1.9 26 75

Table 1. Effects of modified microwave heating conditions. DPn represents the degree of polymerization.

PBEHTTV - Correlation of Catalyst Loading with Molecular weight

Method Pd2(dba)3 / Mol % P(o-tolyl)3 / Mol % Mn / kDa PDI Yield

1 2.0 8.0 16.5 2.4 80

3 4.0 16.0 10.0 1.9 75

Table 2. Effect of catalyst loading on the molecular weight of PBEHTTV.

Based on these findings we decided to proceed with the best conditions (that is, row 1

in table 1 and table 2) for polymerization of the other linear monomer (M5b and M5

c).

Therefore, the palladium-catalyzed polymerizations were repeatedly scaled up in yields

exceeding 80 % (see scheme 16).

In relation to the choice of catalyst, previous studies suggests that the use of the

Pd2(dba)3/P(o-toly)3 system in Stille coupling polymerizations generally generates high

molecular weight polymers.46

This is due to the fact that, the good ζ-donicity of the P(o-

toly)3 ligand and its enhanced steric environment may increase the coupling rate, and as a

result, facilitate the elimination of side reactions caused by aryl migration from the ligand to

Pd and then to the polymer main chain.46,84

Additionally, the formation of oligomers from

polymer end-capping is greatly minimised. Interestingly, the application of microwave-

irradiation in Stille coupling polymerizations is a favoured technique for synthesizing

conjugated polymers with high molecular weight polymers and in high yields.41

Another

advantage of this technique is that the duration of the polymerization is drastically reduced

compared to conventional methods.42,85

Page | 73

2.4 Characterization of the Low Band Gap PDC16TTV and PBEHTTV poly(3,6-dialkylthieno[3,2-b]thiophene) Derivatives

2.4.1 Solubility

The branched side chain PBEHTTV polymer was found to be completely soluble in

organic solvents such as CH3Cl, chlorobenzene (CB), and 1,2-dichlorobenzene (DCB) at

ambient temperature. Despite the longer alkyl side chain, PDC12TTV and PDC16TTV

possessed inferior solubility in the same solvents at room temperature, and routinely

precipitated out of solution. However, at elevated temperatures (ca. 80 oC) it dissolved

completely. This suggest that the linear functional groups on PDC16TTV backbone promotes

better structural order, stronger interchain packing, which in turn, facilitate the crystallization

of the polymer at low temperatures (vide infra). This caused problems during film formation,

as observed when drop casting a hot polymer solution of PDC16TTV onto glass substrates

which resulted in reticulated films of poor quality. Evidently, the nature of the side groups on

the polymer main chain clearly influenced the solubility of the polymer in solution and the

resulting conformation in the solid state.

2.4.2 NMR Spectroscopy and Molecular Weight Measurements

The number average molecular weight (Mn), weight average molecular weight (Mw)

and polydispersity index (PDI) of the conjugated polymers are listed in table 3, as determined

by gel permeation chromatography (GPC). The previously reported polymer DH-PTTV with

a shorter side chain (C6), had a Mn as high as 28.2 kg.mol-1

with a PDI of 1.13 as determined

by GPC in THF at room temperature.27

This is surprising in view of the poor solubility of

both C16 and C12 at room temperature in our hands, and possibly is indicative of some

aggregation in the case of C6 polymer, leading to apparently higher MW polymers than

actually made.

Page | 74

The molecular structure of each conjugated polymer was probed by 1H NMR

spectroscopy in 1,2-dichlorobenzene at 100 oC (figure 20). The peaks ascribed to the α-/β-

hydrogen atoms attached to the linear hexadecyl (C16) and branched 2-ethylhexyl (EH) side-

groups on the thieno[3,2-b]thiophene units are centred at 2.84/1.83 ppm for PDC16TTV and

2.82/2.04 ppm for PBEHTTV, respectively. Furthermore, the hydrogens attached to the

vinylene-bridging bonds peaked at δ 7.33 ppm and 7.44 ppm, as expected.86

It should be

noted that the observed broadening of the resonance peaks may be due to the aggregation of

adjacent polymer chains. Attempts to record the spectrum at lower temperature showed very

broad peaks. Moreover, the ambiguous integral values of each peak in the NMR spectrum of

polymer may be a direct consequence of the latter. The other peaks are assigned as indicated

in figure 6.

Table 3. Molecular weights and thermal properties of the polymers.

Polymer aMn / g.mol

-1

bMw / g.mol

-1 PDI (Mn/Mw)

cDPn

dTd /

oC

eTd /

oC

PDC12TTV 9 200 17 500 1.90 18 370 260

PDC16TTV 16 000 30 000 1.88 23 390 275

PBEHTTV 16 500 39 000 2.39 42 380 268

a Number-average molecular weight, and

b weight-average molecular weight.

c Degree of polymerization,

determined from the Mn/Molar mass of the polymer repeat unit. d

Temperature at 5% decomposition under

nitrogen atmosphere. e

Temperature at 5% decomposition under in air.

Page | 75

Figure 6. 1H NMR spectra of (I) PDC16TTV and (II) PBEHTTV in 1,2-dichlorobenzene-d4 (1,2-DCB-d4) at

100 oC.

Further evaluation of the expanded linear alkyl region of the 1H NMR spectra (see

inserts in figure 6) of the individual π-conjugated polymer reveal the presence of polymers

chains with distinct end terminating groups (I – III in scheme 20, vide infra). This is

evidenced by the presence of additional minor resonances between δ 2.5 ppm and 3.0 ppm for

the α-CH2 attached to the thieno[3,2-b]thiophene sub-unit, besides the broad singlet peak

centred at ca. 2.82 ppm (see insert d and f in figure 3). Together with MALDI data (vide

Page | 76

infra), we tentatively ascribe these peaks to the presence of different polymeric end groups.

The major end-groups identified by MALDI appear to be Br/Br and Br/H. No trialkyltin

groups appear to be present, from the absence of peaks around 0 - 0.5 ppm. The halogen

termined end-groups are well known in other electron-donor poly(phenylene vinylene)s

(PPV)s (vide infra).87

Interestingly, less of these extra resonance peaks are noticeable in the

1H NMR spectra of PBEHTTV. With the appearance of a peak at δ ca. 2.69 ppm (see insert

of the aliphatic region for PBEHTTV in figure 6) suggesting the presence of predominantly

H/Br terminal groups (figure 7). Again, no definitive proof is available to conclusively

substantiate these minor peak assignments at this stage. This warranted further investigation

using other analytical techniques (vide infra).

Figure 7. Representation of (I) H/Br, (II) H/HC=CH (II/V) Br/Br, and (IV) H/H, termini, and (IV) cis-

configuration defects in the vinylene alkyl T32bT conjugated polymers synthesized via the Stille route.

1H NMR spectroscopy is also useful in distinguishing between the different

orientations of the vinylene-linkages along the polymer backbone. This particularly pertains

to the occurrence of the cis and trans configurational isomers (that is, cis/trans isomerism)

commonly encountered in PPV polymer derivatives, such as MEH-PV and poly(thienylene

vinylenes).86,88-90

According to the literature, the cis-oriented vinylene bonds is typically

located between δ 6.10 ppm and 6.9 ppm, while a doublet splitting is normally observed in

Page | 77

the case of the trans-configuration from δ 7.10 ppm and 7.60 ppm.88-90

Reportedly, the

positions of these peaks, to a degree, depends on the synthesis route utilized. In our

experiment, a broad singlet is clearly observed at 7.33 ppm, which we attribute to the trans

protons, and which integrates for exactly 2 H versus 4 H for the methylene region. Some

small peaks are also observed at δ 6.79 ppm and 6.89 ppm, which may be attributable to cis-

vinylene protons, but, can also be a result of terminal vinyl groups. For the PBEHTTV

polymer, the three broad singlet peaks were observed at δ 6.85 ppm, 6.89 ppm and 6.93 ppm

for the cis-vinylene bond, while we attribute the resonance peak at δ 7.44 ppm to trans

protons. Drury et al. showed that the cis content can be estimated from the integrals of the cis

and trans peaks by using the equation below;90

cis Content (%) = [(Icis/(Icis + Itrans)]p * 100 (a)

Where, I represent the sum of the integrals of each resonance signal. Assuming these

peaks are associated with cis-vinylenes, then the percentage of the cis-vinylene isomer

present in the PDC16TTV polymer was estimated to be 8.26 %, while that of PBEHTTV was

15.25 %. These indicate that both polymers contain predominantly the trans-configurated

(91.74 % vs. 84.75 %) vinylene bonds along the backbone structure. In addition, the low

intensity peaks (δ 2.6 ppm – 2.75 ppm) observed close to the α-CH2 peak (ca. 2.80 ppm) in

both polymers may also be ascribed to the occurrence of cis/trans isomerisation, as well as

end-groups, as observed in studies involving MEH-PPV. Here it was suggested that the

resonance peak of the α-OCH2 (alkoxy group) is split into two chemical shifts during

cis/trans isomerization.88-90

In addition, Fan and Lin observed a diminution of the cis-

vinylene resonance peak at δ 6.7 ppm and that of the α-OCH2 (3.4 ppm to 3.7 ppm) during

isomerisation reactions with iodine.88

Assuming this to be true, the cis/trans ratio can be

deduced by dividing the integrals of the peak at ca. δ 2.69 ppm by that at δ 2.82 ppm in the

case of PBEHTTV, which gives a value of 0.092. While a value of 0.095 was calculated for

the PDC16TTV derivative. This calculation is also indicates a low cis-configurational defect

in both conjugated polymers.

Page | 78

2.4.3 Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectroscopic (MALDI-TOF-MS) Studies

This is a powerful analytical technique with its ability to simultaneously ascertain the

molecular weights and conclusively identify the end group structure of various

semiconducting polymers.91,92

However, it should be stated that due to the under-

representation of higher mass peaks in the MALDI-TOF spectra, the determination of the

molecular weights of high molecular weight polymers by this method is virtually

impossible.93

Consequently, the molecular weight determined by GPC is preferred for high

MW polymers. The well-resolved MALDI-TOF MS (see appendix) of the C16 side chain

polymer, PDC16TTV showed the presence of a mixture of six terminal end cappers, H/H,

H/Br, Br/Br, HC=CH/Br, H/HC=CH, respectively. Moreover, two of these terminal groups

(that is, H/Br and Br/Br) were dominant. Additionally, this polymer exhibited noticeable a

peak-to-peak displacement value of 612 Da, which corresponds to the mass of the monomer

fragment, 3,2-dihexyldecylthieno[3,2-b]thiophen-2-yl vinylene. On the contrary, in addition

to the presence of H/Br and Br/Br end-cappers in high proportions, the MALDI-TOF spectra

(not shown) of PBEHTTV also reveal the presence of the H/HC=CH terminated polymer

chains in high proportions. The prominent 388 Da peak-to-peak spacing observed is

consistent with the molar mass of the repeat unit, 3,6-bis(2-ethylhexyl)thieno[3,2-

b]thiophene-2-yl vinylene.

The Br/Br end groups may be the result of a slight imbalance in the stoichiometry of

the reaction. This is mainly due to difficulties in obtaining highly purified of the trans-

bis(tributylstannyl)acetylene due to its non-crystalline nature and high boiling point.

Commercial samples were >95% with the major impurity appearing to be tributyltin chloride.

An excess of the aromatic dibromide would result in Br/Br end groups. The H endgroup

probably results from oxidative insertion, without transmetallation of the organostannane,

which is the rate limiting step. Reduction of the palladium intermediate, either during the

reaction, or upon work-up would result in the H atom. In the case of the EH polymer, the

vinyl end group is most likely the result of protodestannylation during the acidic work-up.13

These findings correlate well with the observation of the additional signals observed in the 1H

NMR spectra of each conjugated polymers.

Page | 79

2.4.4 Attenuated Total Reflectance Infrared (ATR-IR) Spectroscopy

Further structural identification was performed by ATR-FTIR spectroscopy, as

displayed in figure 8. This is a useful analytical technique for the identification of key

functional groups in many conjugated polymers.

1727

1731

962

963

720

726

91813

7914

5815

0915

89

2856

292129

5630

22

915

139715

1415

88

1466

2850

2918

3017

3600 3000 2400 1800 1200 600

Tra

nsm

itta

nce

Wavelength (cm-1

)

Figure 8. ATR-FTIR spectra of the polymers at room temperature.

In figure 8, the weak bands observed at 3017 cm-1

and 3022 cm-1

are indicative of

unsaturated aromatic C=C-H stretching vibrations. While, the bands centred at 2956.3 cm-1

and 2856 cm-1

corresponds to the asymmetric methyl (–CH3) and methylene (-CH2-) -C-H

stretching vibrations. The bands positioned at a wavelength of 1589/1509 cm-1

and

15988/1514 cm-1

are due to the asymmetric –C=C- stretching vibrations. Moreover, it is

suggested that their relative intensities increases with the extension of conjugated length of

the polymer.94

The aromatic thieno[3,2-b]thiophene stretching modes peaked at 1466 cm-1

and 1458 cm-1

, respectively. Strikingly, the presence of a weak carbonyl (C=O) bands at 1727

cm-1

and 1731 cm-1

may signify the occurrence of some photo-oxidation (vide infra).

Crucially, the characteristic vinylene bond appeared at 962/915 cm-1

and 963/918 cm-1

for

both conjugated polymers.95

The bands at a wavelength of 962 cm-1

and 963 cm-1

indicate the

adoption of a trans configuration by the vinyl bridging bonds along the polymer skeleton,

since, the cis vinyl band that usually appear as a broad band at 700 cm-1

to 800 cm-1

is almost

negligible in each polymer.94,95

This evidently corroborates the assumptions made in the 1H

Page | 80

NMR spectra and thus, confirms the successful polymerization of the target conjugated

polymers.

2.4.5 Thermal Behaviour

The thermal behaviour of the polymers was probed by TGA (see figure 9) and DSC

(see figure 10).

50 100 150 200 250 300 350 400 450 500 5500

20

40

60

80

100

50 100 150 200 250 300 350 400 450 500 5500

20

40

60

80

100

We

igh

t (%

)

Temperature (C)

PDC16

TTV in Air

PBEHTTV in Air

b

We

igh

t (%

)

Temperature (C)

PDC16

TTV in N2

PBEHTTV in N2

a

Figure 9. TGA transitions of PDC16TTV and PBEHTTV (a) under N2 and (b) under O2 (insert) at a heating rate

of 10 oC/min.

No identifiable phase transitions where observed in the DSC thermograms of both

polymers. The TGA traces reveal a 5% weight loss at 390 oC (PDC16TTV) and 380

oC

(PBEHTTV) under N2 (see table 3). However, under O2 atmosphere the decomposition

temperatures at 5% weight loss were dramatically reduced to 275 oC and 268

oC,

respectively. These temperatures are well suited to the operation and longevity of existing

PSCs. However, studies utilizing infrared spectroscopic technique have shown polymers

bearing vinyl linkages or poly(phenylene vinylene)s (PPVs) to be highly susceptible to photo-

oxidation by oxygen molecules (O2). The mechanism depicted in scheme 21 is well

documented in the literature.96-98

It was suggested the singlet oxygen generated upon the

polymer photoexcitation undergo a 2+2 cycloaddition-type reaction, which eventually results

to chain scission. This fits well with the observed C=O band in the IR spectra of both

conjugated polymers (see figure 8), which may impair their air-stability. Moreover, the rate

of photo-oxidation is commonly influenced by the nature (that is, electron donation or

Page | 81

electron withdrawing groups) of the polymer pendant groups. This highlights the

imperativeness of PSC encapsulation technology and/or the use of oxygen protective layer so

as to extend their operational lifetime. 98

20 40 60 80 100 120 140 160 180 200

-6

-4

-2

0

2

4

H

ea

t F

low

(w

/g)

Temperature (C)

PDC16

TTV

PBEHTTV

Figure 10. DSC thermograms (second cycle) of PDC16TTV and PBEHTTV under inert N2 conditions at a

heating/cooling rate of 10 oC/min.

Scheme 17. Proposed chemical degradation mechanism of polymer fragment.

Page | 82

2.4.6 Wide Angle X-Ray Diffraction (WA-XRD) Measurements

The X-ray diffraction patterns of the conjugated polymer drop-casted films are

displayed in figure 11.

0 5 10 15 20 25 30

(b)

(010)

21.74° (4.09 Å)

(100)

6.50° (13.60 Å)

(010)

22.74° (3.91 Å)

D

iffr

actio

n In

ten

sity (

a.u

)

2 / deg (CuK)

PDC16

TTV

PBEHTTV

9.54° (9.27 Å)

(a)

Figure 11. Wide angle X-ray diffraction pattern for PDC16TTV (d1 = 9.27 Å and d2 = 3.91 Å) and PBEHTTV

(d1 = 13.60 Å and d2 = 4.09 Å) solid films drop-cast from chlorobenzene solution (5 mg mL-1

).

In this study, the effect of the different alkyl side groups on molecular organization

and crystallinity was investigated on polymer films drop-casted from chlorobenzene (5

mg/ml) on polished Si substrate (Si-Mat). For both films, only weak diffraction peaks were

observed, in agreement with the DSC measurements which suggested the degree of

crystallinity was not high. The XRD pattern of the linear hexadecyl side chain PDC16TTV

polymer displayed two diffraction peaks at 2Ɵ = 9.54o (9.27 Å) and 22.74

o (3.91 Å) weak

shoulder around 20°. The peak around 3.91 Å appears to correspond to π-π packing (010).

The weak peak at 9.54° is less obvious. It appears to be too short a distance to correspond to

the lamellar distance for the polymer backbones, considering the length of the alkyl side-

chains (C16). A second order lamellar peak (200) may also be ruled out on the basis, that

there is no peak observable for the first order peak (which would be expected around 2Ɵ =

4.76°). Thienothiophene containing polymers bearing similar C16 side-chains have been

reported with lamella stacking distance of (23.8 Å) (PBTTT-C16, see scheme 8).24

In this case

however, the alkyl side-chains interdigitated between adjacent polymer backbones to reduce

to lamellar spacing. For non-interdigitated polymers like poly(3-hexadecyl)thiophene a larger

Page | 83

lamellar spacing of ca. 25Å has been reported. A repeat of the XRD measurement with

particular emphasis on the 0 - 4 Ɵ region, where such diffraction peaks would occur, showed

the absence of the interchain displacement (100) peak for PDC16TTV. We therefore conclude

this polymer is mainly amorphous, and we are not sure of the origin of peak at 9.5Å. In the

case of the PBEHTTV derivative two weak peaks were observed at 6.50o (13.60 Å) and

21.70o (4.09 Å). The peak at 6.5° (100) may be related to the lamellar like ordering of the

polymer backbones, with an interchain d1-spacing of 13.6 Å of the polymer backbones This

is close to the lamellae distance predicted for PBTTT with hexyl side-chains (13.5 Å).99

The

broad nature of the peak suggests the degree of crystallinity is low. The inclusion of the

branched side-chains also resulted in an increase in the π-π distance in comparison to the

straight chain derivative. The observed values (3.7 – 4.0 Å) are similar to those observed in

crystalline poly(3-alkylthiophene)s (P3AT)s and poly(thieno[3,2-b]thiophene)s (PTTs) with

high field effect mobilities.55,64,100-102

This is an important prerequisite for high performance

OFETs and PSCs.

2.4.7 Optical Characteristics

The photophysical characteristics of the conjugated polymers PDC16TTV and

PBEHTTV were investigated by ultraviolet-visible (UV-vis) absorption and

photoluminescence (PL) spectroscopy in dilute chloroform solution and in the solid state

(spun thin film). A summary of these properties are illustrated in Table 4.

Solution

Absorption (nm)

Solid State Absorption (nm)

Polymer λmaxa λmax

b λedge Eg

opt (eV)

c λmax

a λmax

b λedge Eg

opt (eV)

d

PDC16TTV 562 680 720 1.72 578 650 730 1.70

PBEHTTV 577 615

(670)e

700 1.77 588 660 721 1.72

Table 4. Absorption characteristics of the conjugated polymers at room temperature. aAbsorption maxima.

bShoulder.

cExtra vibronic shoulder.

dOptical band gap in the solid state. The optical band gaps were deduced

from the equation, Egopt

= 1240/λonset. Where λonset represent the absorption edge in the long wavelength

region.

The absorption spectra of the conjugated polymers in the solution and in thin film are

shown in figure 12. The solid and solution phase absorption spectra of both donor polymers

shows two distinctive peaks, a strong absorption around 500-600 nm, and a much weaker

Page | 84

peak around 350 nm. The long wavelength peak is usually ascribed to the HOMO-LUMO

transition.103

The peak shapes of both polymers is solution is quite different. PDC16TTV

displays a broad absorption with a maximum at 562 nm and a distinct longer wavelength

shoulder. Upon film formation the absorption broadens considerably, and max red shifts by

about 16 nm. The intensity of the shoulder increases significantly and also slightly red shifts.

These changes suggest an increase in polymer aggregation in the solid state.27,104

The broad

absorption and clear shoulder apparent in solution also suggest some degree of aggregation of

the polymer, even in dilute solution. This is in accordance with the high propensity for the

polymer to precipitate upon cooling from moderate concentrated solutions.

In contrast, the solution absorption profile for the branched polymer is quite different.

Here the polymer exhibits two relatively sharp absorptions of almost equal intensity at 577

nm and 615 nm, in addition to a weak longer wavelength shoulder at 670 nm. The intense

shoulder at 615 nm, which is sometimes higher in intensity, is related to vibronic type

features, similar to that seen in PTV. Despite the branched side chains, the polymer

(PBEHTTV) exhibited longer wavelength absorption than the C16 polymer in solution,

surprisingly suggesting the branched side-chain do not result in significant backbone

disruption. The longer wavelength absorption may be related to the significantly increased

degree of polymerisation for the EH polymer over the C16, or suggest the polymer is more

aggregated in the solid state. Upon film formation, one main absorption band emerges at 588

nm, in between the two main solution absorption bands. This is slightly red shifted compared

to the straight chain polymer, although since the peaks are very broad this is probably not

significant. The optical band gaps for both polymers in solution, as determined by the onset

of absorption are similar with PDC16TTV (λedge = 720 nm, 1.72 eV) which is red shifted by

20 nm compared to that of PBEHTTV (λedge = 700 nm, 1.77 eV). Given the difficulty in

gaining accurate band edge measurements, we do not believe this is significant.

Comparing our properties to the previously reported DHPTTV by He and co-workers,

we find that this polymer was blue shifted by 24 nm with respect to that of PDC16TTV,

despite the much higher reported Mn for the hexyl polymer.27

This may be further evidence

for the fact that the MW reported by He was in fact erroneous, and the polymer he isolated

was in a much lower weight regime (oligomeric). In this regime it is likely that the

conjugation limit was not reached accounting for the apparent blue shift.

Page | 85

300 400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d A

bso

rba

nce

PDC16

TTV CHCl3

PBEHTTV CHCl3

PDC16

TTV Film

PBEHTTV Film

Wavelength (nm)

Figure 12. Normalized UV-vis absorption spectra of PDC16TTV and PBEHTTV in chloroform solution and thin

films spun from hot chlorobenzene at 2000 rpm (for 70 s).

Since, poly(thienylene vinylene)s are known to be non-luminescent,60

we decided to

test the photoluminescence (PL) of poly(thieno[3,6-b]thiophene vinylene)s (PTTV)s.

Surprisingly, we found that both PTTVs were emissive in both solution and the solid state. In

the solution PL spectra (see figure 13), these polymers displayed well-resolved emission

peaks with maximum intensity (λmax) centred at 631 nm and 643 nm, respectively. This

represents a red shift of 12 nm between PBEHTTV and PDC16TTV, in agreement with the

red shift observed in the absorption measurements. However, in the solid-state (see figure

14) both polymers emitted maximum intensity at similar wavelengths, and showed similar

Stoke shifts.

Page | 86

Solution (nm) Thin Film (nm)

Polymer λmax

(PL)a

λshoulder

(PL)b

ΔEStokes

(eV)c

λmax

(PL)

λshoulder

(PL)

ΔEStokes

(eV)

PDC16TTV 631 705 0.23 717 775 0.42

PBEHTTV 643 708 0.22 716 780 0.38

Table 5. Photoluminescence (PL) properties of PDC16TTV and PBEHTTV in solution and solid film. aMaximum PL intensity.

bWavelength of the first shoulder peak in the PL spectra.

cStokes shift estimated

from the difference between the λmax(PL) and λmax(UV).

The Stokes shift (see table 5) of PBEHTTV in the solid state was slightly lower than

that observed for PDC16TTV. This is indicative of a less disrupted interchain order along the

polymer backbone and less structural rearrangement during photoexcitation in the solid

state.105

On the contrary, the small Stokes shift observed between the PL and absorption

maximum intensities in the solid state (see table 5) is symbolic of the rigid geometry of the

polymer effected by the presence of the branched side chains.106

In solution, the scenarios are

reversed with the linear C16 bearing polymer exhibiting a better structural order.

300 400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d In

ten

sity (

a.u

)

Wavelength (nm)

PDC16TTV UV Soln

PBEHTTV UV Soln

PDC16TTV PL Soln

PBEHTTV PL Soln

Figure 13. Normalized UV-vis and PL spectra of PDC16TTV (ex. λ = 560 nm) and PBEHTTV (ex. λ = 570 nm)

in dilute chloroform solution.

Page | 87

300 400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d In

ten

sity (

a.u

)

Wavelength (nm)

PDC16

TTV UV Film

PBEHTTV UV Film

PDC16

TTV PL Film

PBEHTTV PL Film

Figure 14. UV-vis and PL spectra of PDC16TTV (ex. λ = 590 nm) and PBEHTTV (ex. λ = 600 nm) thin film

spun at 2000 rpm for 70 s at room temperature.

The high photoluminescence observed in PDC16TTV and PBEHTTV may have been

influenced by the rigid fused thiophene unit in the main chain.27

This suggests that the non-

radiative relaxation pathway is suppressed, due to the longer lifetime of the singlet excited

states generated, as opposed to the occurrence in PTVs.60,107

2.4.8 Electrochemical Characteristics

The redox properties of the conjugated polymers were investigated by cyclic

voltammetric (CV) measurements. This electrochemical technique has been used to directly

ascertain the energy level of the highest occupied molecular orbital (EHOMO) and lowest

unoccupied molecular orbital (ELUMO) of many conjugated polymers.108

In addition, PESA or

ultraviolet photoelectron spectroscopy (UPS) is a technique popularly used to ascertain the

ionization potential (IP or HOMO) of conjugated polymers in the solid state. The LUMO is

indirectly deduced from the IP and the Egopt

obtained from the λedge of the polymer absorption

spectra.109

Figure 15, shows the voltammogram PDC16TTV and PBEHTTV in 0.1 M

BU4NPF6/CH3CN solution.

Page | 88

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

No

rma

lize

d C

urr

en

t D

en

sity (

a.u

)

Potential (V vs Ag/Ag+)

Ferrocene

PDC16

TTV

PBEHTTV

Figure 15. CV traces of PDC16TTV and PBEHTTV films cast on Pt working electrode at a scan rate of 50 mV

s-1

.

These HOMO and LUMO energy levels are important parameters in organic

photovoltaic (OPV) applications, as they determine the open circuit voltage (Voc) and the rate

of charge transfer/separation.110

The onset potential of the oxidation (φonset.ox) and reduction

(φonset.red) processes were deduced from the intersection of the tangents drawn at the rising

anodic current and the baseline. The resulting φonset.ox and φonset.red values were referenced

against ferrocene (–4.75 eV below the vacuum level for NHE42,111

). The energy level of the

highest occupied molecular orbital (EHOMO) and that of the lowest unoccupied molecular

orbital (ELUMO), and the corresponding electrochemical band gaps (Egecl

) of the polymers

were derived from the equations below:108,112

EHOMO = - (φonset, ox - φFc/Fc+ + 4.75) = IP (eV) (1)

ELUMO = - (φonset, red - φFc/Fc+ + 4.75) = EA (eV) (2)

Egecl

= (φonset, ox - φonset, red) (eV) (3)

The calculated half-wave potential (E1/2

) value obtained for the Fc/Fc+ redox couple

(φFc/Fc+) equals to 0.29 V vs. Ag/Ag+. Where, φonset, ox and φonset, red represent the onset

potentials of oxidation and reduction vs. Ag/Ag+. Table 6, shows the energy levels of the

frontier orbitals (EHOMO and ELUMO) determined by UV-vis absorption, PESA and CV, and

the energy band gaps (Egecl

and Egopt

) of the conjugated polymers.

Page | 89

Absorption/PESA dCyclic Voltammetry

Polymer aEg

opt

(eV)

bEHOMO

(PESA)

(eV)

cELUMO

(eV)

φonset,

ox vs.

Ag/Ag+

(V)

φonset, red

vs.Ag/

Ag+

(V)

EHOMO

(eV)

ELUMO

(eV)

Egecl

(eV)

PDC16TTV 1.70 -5.05 -3.35 +0.70 -1.04 -5.16 -3.42 1.74

PBEHTTV 1.72 -5.05 -3.33 +0.69 -1.20 -5.16 -3.26 1.89

Table 6. Absorption, PESA, and electrochemical characteristics of the polymer films. a

Optical band gap

deduced from the equation, Egopt

= 1240/λonset, where λonset is the absorption edge in the long wavelength

region. b Derived from the ionization value via PESA measurements.

c Obtained from the difference between

the HOMO and the Egopt

value. d

Electrochemical measurement performed on as cast polymer films on a Pt

working electrode in 0.1 M [Bu4]+[PF6]

- at a 50 mV s

-1 potential sweep rate.

In figure 15, the PDC16TTV and PBEHTTV both displayed a quasi-reversible

oxidation/re-reduction (p-doping/de-doping) processes during the anodic potential scan. On

the contrary, in the cathodic region the reduction (n-doping) process was totally irreversible

for both polymers. This indicates that the above polymers are strong electron donating

material and are likely unstable in the reduced state. Strikingly, the two conjugated polymers

showed similar HOMO (IP) energy levels (see table 4) in both PESA and CV measurements,

which suggests parity in air stability. In contrast, the low-lying LUMO energy level of

PDC16TTV (-3.35 eV and -3.42 eV) relative to that of PBEHTTV (-3.33 eV and 3.26 eV),

signify a weakened electron affinity (EA) induced by the presence of the two lengthy C16

side chains on the polymer thieno[3,2-b]thiophene units. Expectedly, the Egecl

and Egopt

obtained for the PDC16TTV polymer (1.71 eV and 1.70 eV) were reasonably smaller than

those of PBEHTTV (1.89 eV and 1.72 eV), and the analogous poly(3,6-dihexylthieno[3,2-

b]thiophene) polymer (DH-PTTV – 1.77 eV and 2.07 eV27

). This is due to the longer

effective conjugation length and the planarization of the polymer backbone facilitated by the

linear alkyl side group in the solid state. Furthermore, the higher Egecl

relative to the Egopt

values may be due to the increased interface between the Pt electrode surface and the coated

polymer film, which serves as a barrier to charge injection.109

Nonetheless, the determination

of the energy band gaps and the HOMO/LUMO levels by electrochemistry (that is, CV) is

preferred, due to the similarity in device configuration and operation principles to OPVs.

Page | 90

2.5 Device Fabrication

2.5.1 Field-Effect Transistor (FET) Characteristics of the Conjugated

Polymers

Top-contact bottom-gate FET devices (see figure 16) based on the spin-casted films

of PDC16TTV and PBEHTTV polymers were fabricated on OTS-treated Si/SiO2 substrate, in

order to quantify the influence of the linear and branched alkyl side chains on their electrical

performance. All OFET measurements were performed in the groups of Dr Thomas

Anthopoulos (Physics department, Imperial College London) by Dr Kim YoungJu.

Source (Au)

Drain (Au)

A

VDS

VGS

-5.10 eV

PDC TTV16

-5.16 eV

PBEHTTV

-5.10 eV

Au Au

-3.26 eV

-5.16 eV

-3.42 eV

Figure 16. Top-contact bottom-gate organic field effect transistor configuration along with the energy levels

alignment diagram of the polymer semiconductor and gold (Au) electrodes.

The output and transfer characteristics of the FET device employing the

aforementioned polymers as active layer are shown in figure 17.

Page | 91

0 -10 -20 -30 -40 -50 -60 -70 -800

-5

-10

-15

-20

-25

-30

-35

-40

Vg : 0 to -80 V

V = -10 V

I D (A

)

VD (V)

(a)

0 -20 -40 -60 -8010

-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

VD= - 60V

VD= - 5V

I D (

A)

VG (V)

Vth = - 6.2V

0

1

2

3

4

5

6

7

IDsat 1

/2(10

3A1/2)

(b)

0 -10 -20 -30 -40 -50 -60 -70 -800

-2

-4

-6

-8

-10

Vg : 0 to -80 V

V = -10 V

VD (V)

I D (A

)

(c)

0 -20 -40 -60 -8010

-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

VD= - 60V

VD= - 5V

I D (

A)

VG (V)

Vth = -9.3V

0

1

2

3

IDsat 1

/2(10

3A1/2)

(d)

Figure 17. The top contact/bottom gate OFET (a) output and (b) transfer characteristics of PDC16TTV active

layer. (c) Output and (d) transfer properties of PBEHTTV as active layer. VG was varied from -10 to -80 V in 1

V steps. VD was set at -5 (linear) and -60 V (saturation). The channel length (L) = 20 μm, while, the channel

width (W) = 1000 μm.

Both polymers displayed clear p-type FET behaviour with well-resolved linear and

saturation regimes (see figure 17). However, the observed non-linear increase at low drain

voltages (see figure 17a and 17c) implicates some contact resistance. This may be due to the

charge injection barrier caused by the displacement (0.06 eV) between the work function of

the Au electrode and the HOMO of the two polymers (-5.16 eV).27,55,113

Here, we assume a

gold work-function of 5.0 eV, although the work function is known to vary according to

deposition conditions.

Table 7 shows a summary of the FET performance with the PDC16TTV and

PBEHTTV as active layers.

Page | 92

The FET device bearing PDC16TTV as the active layer exhibited a saturated charge

carrier mobility (μ(Sat)) of 0.02 cm2 V

-1 s

-1 after thermal annealing at 200

oC for 10 minute.

Annealing is known to result in improvement in charge carrier mobility (μ) in many cases.55

Unfortunately, the OFET characteristics of the conjugated polymers at room temperature

were not available (not measured). In contrast, a carrier mobility of 0.008 cm2 V

-1 s

-1 was

calculated for the FET device incorporating the branched side chain PBEHTTV polymer as

the active layer after annealing. This represents a 2.5 times reduction in mobility, which is

rather modest considering that bulkier side-chains have been included. The relatively small

differences in mobility reflect the similar ordering observed by optical techniques in thin

films. Although the values are modest, they are reasonable for relatively amorphous

polymers.

Polymer

Annealed at 200 oC

HOMO / eV

LUMO / eV μ(Sat) / cm2 V

-1 s

-1 μ(Lin) / cm

2 V

-1 s

-1

PDC16TTV 0.02 0.006 0.015 0.001 - 5.16 - 3.42

PBEHTTV 0.008 0.0015 0.005 0.0006 - 5.16 - 3.26

Table 7. OFET device characteristics and energy levels of PDC16TTV and PBEHTTV films. Mobility values

were averaged over 3 devices for each polymer.

Page | 93

2.5.2 Organic Photovoltaic Device

The photovoltaic properties of the conjugated polymers were explored in BHJ solar

cell devices with the conventional configuration of

ITO/PEDOT:PSS/Polymer:PC71BM/Ca/Al (figure 18). All measurements were performed in

Prof James Durrant‟s group by Huang Zhenggong (Steve).

Aluminium (Al)

Calcium (Ca)

Polymer:Fullerene

PEDOT:PSS

Indium Tin Oxide (ITO)

Transparent Glass Substrate

Transp

arent A

node

Cathode

Electron transport layer (ETL)

Donor:Acceptor BHJ active layer

Hole transport layer (HTL)

Sunlight

Figure 18. Organic bulk heterojunction solar cell device structure.

The [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) serves as the acceptor

material in the BHJ photoactive layer. The hole extraction process was facilitated by utilizing

a poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) interlayer and the

Ca/Al cathode for the collection of electrons. The open-circuit voltages (Voc), short-circuit

currents (Jsc), fill factors (FF), and power conversion efficiencies (PCE) of the optimized

devices are summarized in table 8.

Polymer Solvent Blend ratio Voc [mV] Jsc [mA cm-2

] FF [%] PCE [%]

PDC16TTV DCB 1:1 567 8.23 54 2.50

PBEHTTV DCB 1:2 588 5.81 55 1.87

Table 8. Photovoltaic parameters of the optimized photovoltaic devices fabricated from PDC16TTV and

PBEHTTV.

The corresponding current density-voltage (J-V) curves of the two devices are

presented in figure 19.

Page | 94

-0.2 -0.1 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7-10

-8

-6

-4

-2

0

2

Cu

rre

nt d

en

sity (

mA

/cm

-2)

Voltage (V)

PDC16

TTV/PC71

BM (1:1)

PBEHTTV/PC71

BM (1:2)

Figure 19. The current density-voltage characteristics of the polymer/PC71BM solar cells under AM 1.5

condition (100 mW/cm2).

Interestingly, we find that the two polymers optimise at different blend ratios. For the

C16 derivative, a 1:1 blend was optimal; giving a PCE of 2.50 % was obtained with a Voc of

0.57 V, Jsc of 9.23 mA cm-2

, and an FF of 54 %. This is significantly higher than the

branched analogue, which optimized at a 1:2 blend ratio, despite the increased percentage of

non-absorbing alkyl component in the C16 polymer. The PCE obtained also represents an ca.

9 times increase in performance compared to that of the C6 side chain DH-PTTV (0.28 %)

reported by He and co-workers.27

The lower PCE observed for the device incorporating the

branched PBEHTTV (1.87 %) is mainly attributed to the difference in Jsc, which is limited in

part by the lower hole mobility (see table 7) deduced for this conjugated polymer, albeit in an

OFET device. Figure 20 shows the external quantum efficiency (EQE) spectra of the BHJ

solar cell device.

Page | 95

300 400 500 600 700 800 9000

10

20

30

40

50

EQ

E (

%)

Wavelength (nm)

PDC16

TTV/PC71

BM (1:1)

PBEHTTV/PC71

BM (1:2)

Figure 20. External quantum efficiency of the optimized PDC16TTV and PBEHTTV BHJ photovoltaic device.

It can be inferred from figure 20 that both semiconducting polymer derivatives

displayed a broad EQE response covering a wide wavelength range (300 - 740 nm).

Nonetheless, PDC16TTV displayed the highest value (44 %) of the two polymers, which is

approximately 10 times higher than that of P3HTV.60

Compared to the branched EH-bearing

analogous polymer PBEHTTV, a difference of 6 % can be observed. This suggests that more

photons are being converted to electrical current in the cell bearing the C16 pendant groups

along the polymer backbone. This, therefore, implicates the difference in alkyl substituent

configuration (that is, linear versus branched) as having some impact on the efficiency of the

OPV device. However, in order to fully ascertain the viability of the aforementioned

suggestion, the distinction between the morphologies of the two polymer/PC71BM blend

necessitates further investigation. This is due to the fact that the more crystalline the polymer

is, the larger the domain of phase segregation observed in the polymer/PCBM blend, which in

turn hinders the migration of the excitons to the interface (less PCE) and makes charge

separation unfavourable.61

Therefore, it was quite surprising to observe a higher PCE in the

PDC16TTV/PC71BM blend, since it is expected to be more crystalline than PBEHTTV.

Further studies involving photoluminescence, transient photon absorption and transmission

electron microscopy (TEM) are currently underway in order to gain an in-depth

understanding of the observed distinctions in the OPV performance of both conjugated

polymers.

Page | 96

2.6 Conclusion

In summary, two structurally distinct donor π-conjugated polymers, PDC16TTV and

PBEHTTV were synthesised via a microwave assisted Stille cross-coupling reaction

pathway. Here we ascertained the effects of the variations in the side chain solubilising

groups (linear versus branched) on the microstructural order and resulting properties of the

polymer. The structural-property relationship of the polymers was investigated by utilizing

optical absorption, photoluminescence, electrochemical, PESA, XRD, and NMR

characterization techniques. It was discovered that the difference in the side chains has no

marked impact on the electrochemically and photospectroscopically measured HOMO levels.

The optical band gap was very similar for both polymers, with electrochemical measurements

revealing a small stabilisation of the LUMO of the linear side-chain polymer over the

branched.

XRD diffraction measurements revealed both polymers displayed low degrees of

crystallinity, however optical absorption measurements showed clear signs of ordering in

solution for the branched polymer. Both polymers demonstrated red shifts in optical

absorption in the solid state, commensurate with enhanced backbone planarization and

overlap Transistor measurements demonstrated that the linear polymer exhibited a higher

saturated mobility than the branched, consistent with the π-π distances measured by XRD.

Furthermore, the PDC16TTV displayed an unoptimized power conversion efficiency of 2.65

% in a fabricated OPV device, compared to 1.85 % for PBEHTTV. Again, the higher hole

mobility and crystallinity effected by the presence of the linear C16 side chain in PDC16TTV

played a deciding role in the both the OPV and OFET device performances.

Page | 97

Chapter 2 (Part 2.7)

2.7.1 Introduction

The donor poly(3,6-dihexadecylthieno[3,2-b]thiophene-2-yl vinylene) (PDC16TTV)

based on the T32bT unit has been synthesized by the microwave-accelerated Stille cross-

coupling method (see scheme 12 and 19) with hole mobility reaching ca. 2.0 x 10-2

cm2 V

-1 s

-

1 (annealed), which is pivotal in ensuring facile charge collection in various BHJ PSC

devices, as addressed in the preceding section.114

Notwithstanding, the improved charge

transport characteristics of the polymer (albeit, after thermal treatment), and the reduced

electronic energy gap (ca. 1.74 eV) between the HOMO and the LUMO frontier orbitals of

the conjugated polymer, the recorded efficiency (2.5 %) in PSCs is still not sufficiently high

in comparison to other class of conjugated polymers. Generally, besides tuning the energy

levels of conjugated polymers [red-shifted and broad absorption] to maximise the harvesting

of photons from the solar spectra, the molecular weight (Mn) and solubility of these materials

are two vital parameters with direct effects on the performance and commercialization of

organic photovoltaic (OPV) devices.115-117

Bearing this in mind, the exploration of analogous

synthesis pathways is imperative so as to formulate high molecular weights (Mn) π-

conjugated polymers (in this case, vinylene-linked alkyl-substituted T32bT polymers)

without compromising their solution processability, if at all possible.

Recent studies have shown that, higher molecular weight conjugated polymers with

good solubility tends to display enhanced charge carrier mobility and PCE in OPVs

compared to those comprising of lower Mn polymer chains.118-121

Furthermore, good polymer

solubility is advantageous in terms of the cost and fabrication of large area/homogenous thin

films directly from solution.118

Nonetheless, the molecular weight of the polymer and its

solubility are known to be interconnected. Reports have shown that the insolubility of the

polymer generated during polymerization may result in the premature termination of the

chain growth process due to precipitation, which in-turn limit the molecular weight of the

polymer chains.118,119

2.7 Comparative GILCH and Microwave-Assisted Stille Polymerization Pathways: Towards High Molecular Weight Poly(3,2-

dihexadecylthieno[3,2-b]thiophene-2-yl vinylene)s

Page | 98

Fortunately, the side functionalities attached to the polymer backbone can be

engineered to bring about improved polymer solubility and contribute to the optimization of

the efficiency of PSCs.122,123

However, the alteration of the side chain can also affect the Mn

of the polymer.118

Although, polymerizations involving palladium-catalyzed Stille cross-

coupling methodology in conjunction with microwave heating has found widespread use in

the preparation of a catalogue of semiconducting polymers (vide supra), the occurrence of

side reactions presents a challenge in relation to their molecular weights.41,42,46,55,119,121

On the

other hand, the Gilch124

synthetic route is an established and mechanistically simplified

alternative polymerization pathway to the Stille cross-coupling strategy, in that it eliminates

the use of transition metal catalysts (usually, palladium), and most importantly, it is well

suited for the preparation of extremely high molecular weight conjugated polymer (106 g mol

-

1)125

in moderate yields at even lower temperature.87,126-128

However, the only drawback of

the Gilch protocol lies in the number of well-known microstructural irregularities [defects]

that may be present intrinsically in the resulting vinylene polymer (vide infra).87,88,129-132

It is

an indisputable fact that the microstructure (that is, structural order and morphology) of the

conjugated polymer chain is one of the most important influencing parameters on the device

characteristics, and thus, must be given due consideration when synthesizing new

materials.133

To best of our knowledge, this is the first application of the Gilch double

dehydrohalogenation of a 3,6-bis(halomethyl)thieno[3,2-b]thiophene derivative to synthesize

poly(3,6-dialkylthieno[3,2-b]thiophene-2-yl vinylene) in the literature.124

This section of the

project focuses on a comparative study of the PDC16TTV π-conjugated polymer

characteristics synthesized by the Gilch route and the palladium-catalyzed Stille cross-

coupling pathway. It is envisaged that the polymer synthesized by the Gilch method would

display higher molecular weights, which is essential for improved structural properties and

comparably optimized OPV performance.

Page | 99

2.7.1.1 The Gilch Methodology in Perspective

The Gilch reaction has a distinct advantage over other related synthetic routes (that is,

the sulfinyl/sulfonyl,129,134-136

Wessling,137,138

Wittig and Horner routes90

), commonly

encountered in literature for the preparation of alkyl/aryl-substituted poly(phenylene

vinylene)s (PPVs). This is based the fact that, these other route routinely yield polymers with

molecular weights [both Mn and Mw] lower than that obtained from the Gilch route.125

Beside

that, the polydehydrohalogenation route124

proceeds simply by the treatment of various

halomethylated alkyl-substituted aryl derivates (for example, in our case 2,5-bis(halomethyl)-

3,6-dialkylthieno[3,2-b]thiophene – BHMDATT) with excess base [potassium tert-butoxide -

tBuOK] in either THF or dioxane, under mild conditions (scheme 18).

87,125 Furthermore, the

Gilch method is a versatile synthetic route in that it tolerates numerous lateral functionalities

(R) for processability and electronic reasons.87

Over the past decade, this strategy has been instrumental in the development of well-

explored conjugated polymers for organic display applications. They include; poly(2-

methoxy-5-(2‟-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV),88,94,139

poly(2-

methoxy-5(3‟,7‟-dimethyloctyloxy)-1,4-phenylene vinylene) (MDMO-PPV),129

poly(2-

(2‟,5‟-bis(octyloxy)benzene)-1,4-phenylenevinylene) (BOP-PPV),130,132

poly(2-(2‟,5‟-

bis(octyloxy)benzene)-5-methoxy-1,4-phenylenevinylene) (BOPM-PPV),132

poly[2-(2‟-p-

phenyl-4‟,5‟-bis(3‟-methylbutoxy))phenyl-1,4-phenylene vinylene) derivatives (PPBMB-

PPV),140

poly(2-decyloxy-1,4-phenylenevinylene) (DO-PPV), poly(2-decyloxy-5-(4‟-tert-

butylphenyl)-1,4-phenylenevinylene) (DtBP-PPV),141

Poly(2,5-bis[4‟-2-(N,N-

diethylamino)ethoxyphenyl]-1,4-phenylene vinylene) (PBDEAE-PPV),89

poly(2-(2‟-

ethylhexyloxy)-5-(2‟‟-(2‟‟‟,7‟‟‟-di-tert-butyl)-9‟‟,9‟‟‟-spirobifluorenyl)-1,4-

phenylenevinylene) (EHSBF-PPV),128

and others142,143

not listed in figure 21.

Scheme 18. General representation of the dehydrohalogenation polymerization method (Gilch route).

Page | 100

Figure 21. Poly(phenylene vinylene) derivatives synthesized by the Gilch route.

Page | 101

Albeit a very important and powerful protocol, several structural and configuration

defects are routinely observed in PPVs synthesized by the Gilch route. These include; tolane-

bis-benzyl (a),128-130,132,140,144

cis-vinylene configurated bridging (b),88,89,125

polymer repeat

units consisting of halide-bearing unconjugated ethylene linkages (c), and polymer chains

comprising of halide terminal groups or chain termini (d), respectively (see figure 22).87

The

presence of such defects within the vinylene-linked thieno[3,2-b]thiophene polymer chains

may be detrimental to the their stability and performance in organic photovoltaic devices.144

Figure 22. Gilch polymerization defects in poly(phenylene vinylene)s (PPVs). X1 represent the halides (Cl and

Br), while the coloured circles indicate the aryl groups within the monomer.

The so called “tolane-bis-benzyl (TBB)” structural irregularity is by far the most

common and major defect encountered in PPVs synthesized by the Gilch route, since its

discovery by the Covion group125,130,144

. Therefore, more studies have focused on elucidating

the probable mechanism leading to its occurrence than any of the listed defects (see figure

22). TBB are random segments within the polymer chain constituted by mostly non-

conjugated arylene-vinylene (single bond) bridge (bis-benzyl moiety), and the arylene-

acetylene (triple bond) bridge (tolane moiety) (see figure 22(a)).87,130,144

The formation of

such structural defect within the polymer chains is said to be as a result of head-to-head (H-H

Page | 102

- H2C-CH2) and tail-to-tail (T-T - ClHC-CHCl) couplings in side reactions occurring mainly

during the propagating stages of the Gilch mechanism (vide infra).125,129

Also at this stage,

the prospect of the polymer chains crosslinking is highly likely.129

The presence of this defect

in the polymer chains causes conjugation breaks which are detrimental to device

performance. which ultimately impair its performance.125

Consequently, the H-H and T-T

coupling leads to the irregular linkages along the backbone of the pre-polymer during

propagation, which in-turn generates the TBB defects in the target polymers.129,140

The

mechanism leading to the formation of the TBB defects is described in scheme 19 for our

proposed monomer.129,131,144

Scheme 19. Different coupling mechanisms leading to the formation of TBB defects using T32bT building

blocks.

Page | 103

One strategy to suppress the formation of the TBB defects during the

polydehydrohalogenation reaction [Gilch route] is by utilizing the electronic and steric effects

of asymmetric side groups appended to the monomer to guide the propagation of the polymer

chain.128,132,141,144

Becker et al125

demonstrated this effectiveness by analysing the 1H NMR

spectra of poly(2-methoxy-5(3‟decyloxyphenyl)-1,4-phenylenevinylene) (MDOP-PV)

synthesized by the dehydrohalogenation of 1,4-bis(chloromethyl)-2-methoxy-5-(3‟-

decyloxyphenyl) benzene (Monomer 1) using tBuOk in dioxane (Scheme 20).

Scheme 20. Synthesis of the MDOP-PV polymer.

The NMR study revealed the absence of the characteristic TBB peak usually situated

between δ 2.8 ppm and 3.0 ppm, respectively.130,132,140,144

The rationale behind the absence of

the TBB defect is due to the fact that the halomethyl group ortho to the electron-donating

methoxy moiety is more acidic than the other halomethyl group attached to the meta position.

This difference in acidity between the two halomethyl groups, coupled with the steric effect

provided by the bulky decyloxyphenyl substituent allows the polymerization to proceed with

exclusively regular head-to-tail (H-H) couplings.125,128,141

This strategy was employed by

Chang et al141

in the synthesis of DtBP-PPV from 1,4-bis(bromomethyl)-2-decyloxy-5-(4‟-

tert-butylphenyl) benzene (Monomer 2) (scheme 21) with absolutely no TBB defects from

the 1H NMR analysis.

Scheme 21. Synthesis of DtBP-PPV Gilch-type polymer.

Page | 104

The suppression of the TBB defects and the occurrence of regular H-T couplings in

the DtBP-PPV polymer were attributed to the influence of steric hindrance due to the

presence of the bulky 4‟-tert-butylphenyl side group. This correlate well with the

observations made by Shin et. al. upon analysing the 1H-NMR spectra of EHSBF-PPV (see

scheme 23).128

The effectiveness of the use of sterically bulky phenyl side groups in

preventing the formation of TBB defects has been utilized in numerous studies.109,140,145

Moreover, studies have shown a reduction in the occurrence of the TBB structural irregularity

in the many PPVs polymerized under low temperatures.131

Despite the development of

strategies to minimize the occurrence and effects of these defects, the Gilch reaction still has

issues regarding the lack of control over the polymer chain architecture and the chain length.

This is apparently due to the suggestion that the reaction proceeds at an extremely speedy rate

even at relatively low temperatures (ca. 0 - 25 oC).

87,146,147

2.7.1.2 The Gilch Mechanism

The Gilch mechanism has been a subject of contention for many years, and is thought

to proceed via either an anionic or radical chain growth.87

However, the majority of the

evidence from studies in the literature (including those of the most closely related Gilch-type

reaction) to date have all overwhelmingly dismissed the likelihood of anionic chain growth as

the predominant mechanism in Gilch route.87,134-138,148,149

Therefore, this section will focus

mainly on the radical chain growth mechanism, since there are convincing evidence pointing

towards its dominance.146

Nonetheless, a study by the Vanderzande group surmised that

under certain conditions both anionic and radical chain growth may occur simultaneously.150

The radical chain growth mechanism for the Gilch route proceed in three stages, which

includes; initiation, propagation, and chain termination, respectively. Figure 23 shows the

Gilch mechanism and the side reactions involved in this route. The initiation stage of the

Gilch mechanism (figure 23, step 1) involves the generation of the p-quinodimethane

intermediate (VIII) via a base-induced 2,5-elimination of HCl from the 3,6-dialkyl-2,5-

bis(bromomethyl)thieno[3,2-b]thiophene precursor (VI). However, in order to obtain the p-

quinodimethane derivative, which is considered as the „active monomer‟, VI must possess a

good leaving group (usually, a halide for the Gilch route).125,144

This followed by the

dimerization of the p-quinodimethane to form the diradical (IX).146

This dimer diradical is the

main initiating moiety in the next step (propagation) of the Gilch mechanism. However,

studies involving p-xylene precursors indicate that possible side reactions involving the

Page | 105

cyclization of the diradical leads to the formation of 30 % [2,2]paracyclophanes (scheme 19

(f)) from 1,6-dihalo[2,2]paracyclophanes (scheme 19(e)) as the major side-product (see

scheme 22).146,151

It was assumed that the latter may have resulted from the intramolecular

recombination of the dimer diradicals (scheme 19(d)), which may also induce chain

propagation.87,146

The existence of such side product still requires further evidence.

Figure 23. Proposed Gilch reaction mechanism towards poly(thieno[3,2-b]thiophene-2-yl vinylenes) (PTTVs).

Page | 106

Scheme 22. Mechanism of the side reaction in the Gilch reaction leading to the formation of

[2,2]paracylophane. The alkyl groups on the 2,5-positions are omitted for clarity.146,151

The chain propagation stage (step 2, figure 23) is a complex binary process, which

usually proceeds through a head-to-tail (or regular) 2,5-type addition (in this particular case)

of the highly reactive monomer (VIII in figure 23) to the ends of the initiating diradical (IX).

Moreover, both terminals of the radical dimer can react (propagate) independently with the p-

quinodimethane monomer (VIII). If this occurs mainly in a head-to-tail manner, then a

regioregular polymer chain should ensue (step 2, fig. 23). On the contrary, if this coupling

process occurs via either a head-to-head (H-H) and/or a tail-to-tail (T-T) regime, then the

polymer chain would consist of the tolane-bis-benzyl (TBB) (see fig. 22 (a)) defects (see

scheme 19).89,125,129,141,144

The crosslinking of the polymer chain is also possible at this stage

of the Gilch reaction mechanism.129

These defects may also be as a result of the

recombination of two dimeric radical chains during propagation, which may eventually lead

to the generation of polymers with infinite chain lengths.152

This is due to the fact that the

combination of two diradicals may inevitably generate another active diradical, which would

further prolong the chain propagation process. It is important to state that in this particular

scenario, the recombination of the radical would not necessarily result to the termination of

the polymerization, as usually observed in conventional radical polymerization reaction. This

is due to the diradical nature of the initiating moiety (see IX in fig. 23), and may corroborate

the generation of extremely high molecular weights PPVs via the Gilch route.87

The final stage of the Gilch mechanism is the termination (see step 3, fig. 23) of the

chain propagation process, which is not yet well understood.87

But, it is thought to proceed by

a macromolecular elimination (-HCl) cascade involving an additional equivalent of the

base.87,125,152

In addition, the chain termination stage may also be induced by either the

presence of impurities in the reaction mixture or by other slow rearrangement or deactivation

reactions. However, these conclusions are merely speculations, due to the lack of concrete

Page | 107

evidence in the literature which points to their occurrence in the termination stage of the

Gilch mechanism.87

The notion that the Gilch synthesis proceeds via a radical chain growth mechanism is

based on extensive investigation involving the use of additives, mainly the radical scavenger

2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO), in order to influence the reaction and the

nature of the polymers generated. Studies by Vanderzande and co-workers indicate that the

introduction of 0.5 stoichiometric equivalent of TEMPO during the Gilch polymerization of

MDMO-PPV in 1,4-dioxane, resulted to a reduction in yield of the PPV generated by roughly

25 %, due to its radical capturing effect.153

However, this effect is somewhat limited by the

amount of TEMPO used. This due to the fact that, when the amount TEMPO used has been

fully converted the starting material left was able to further polymerize to form the target

PPV. In similar study by the same group, it was observed that the addition of 0.5 equivalent

of TEMPO to Gilch polymerization of MDMO-PPV in THF led to significant inhibition of

the PPV formation, as evident from the reduction in the molecular weight from

approximately 800 kDa to 180 kDa, with a concomitant decrease in the yield of the polymer

by a factor of 4 (from 74 g (78 %) to 22 g (33 %)).153

Further investigation involving the use

of varying quantity of TEMPO (0.0, 0.1, 0.2, 0.4, 0.6 and 1.0 equivalent) during the Gilch

polymerization of the monomer, 5-Methoxy-2-(β-ethylhexoxy)-1,4-bis(chloromethyl)benzene

(scheme 24(a)), revealed that the presence of 1.0 equivalent of this radical scavenger was

enough to completely suppress both the generation of the MEH-PPV polymer and the

previously observed side-product, [2,2]paracyclophane (see 32, scheme 22), in THF (at 0 oC

and 60 oC).

146 These studies all indicate the inhibition of a radical polymerization process, as

opposed to an anionic chain growth mechanism for the Gilch synthetic methodology.87

2.7.2 Synthesis of High Molecular Weight Poly(3,6-dialkylthieno[3,2-b]thiophen-2-yl vinylene) via the Gilch Polydehydrohalogenation Route

2.7.2.1 Synthesis Protocol Towards the Formation of 3,6-dialkyl-2,5-bis(bromomethyl)thieno[3,2-b]thiophene Monomers (DABBMTT)

Here our synthesis begins from the 3,6-dialkylthieno[3,2-b]thiophene derivatives

already synthesised. For the Gilch polymerisation, bromomethylene groups were desired in

the 2,5-positions. Our proposed route (scheme 23) involved direct bromomethylation by the

treatment of paraformaldehyde with HBr in acetic acid.

Page | 108

Scheme 23. Synthesis of 3,6-dialkyl-2,5-bis(bromomethyl)thieno[3,2-b]thiophenes.

As was the case for the bromination of the 3,6-dialkylthieno[3,2-b]thiophenes (see

scheme 8), extensive test reactions were performed to ascertain the best conditions to enhance

the yield of the intermediates (X1 – X4) obtained from the bromomethylation reaction shown

in scheme 23, with guidance from previously reported protocols.154-158

In one of many test

reactions, a small amount of the 3,6-dihexadecylthieno[3,2-b]thiophene (Pr3) mixed with

excess of paraformaldehyde (10 mole equivalent) was heated with 33 % hydrogen bromine

solution in acetic acid (20 mL) at 80 oC for 48 h, under inert atmosphere. The completion of

the reaction was ascertained by GC-MS analysis, which indicated a high yield of the crude

product. Following an aqueous work-up, the resultant pale-blue crude product was purified

by column chromatography, eluting with petroleum hexane (60 – 70 oC) mixed with a polar

solvent. However, only trace amounts of the starting material were obtained, with no product

isolated. Following this observation, the reaction was replicated using the other precursors

(Pr1 to Pr4) with similar outcome. This suggests that the bromomethylated monomers may

not be stable in the presence of silica-gel, probably due to its slight acidic nature, thereby

leading to its degradation. However, repeated attempts to separate the pure monomer from

the crude in a column packed with a slurry of silica-gel which was basified by washing with a

mixture of triethylamine (Et3N) and hexane in 5:95 % ratio was wholly unsuccessful. Studies

in the literature have highlighted the instability and concomitant decomposition of the

bromomethylated dialkyl-thiophene derivative during chromatographic work-up.157,159

In

order to rectify this drawback, the crude bromomethylated reaction mixture was quenched,

Page | 109

added to diethyl ether, followed by extraction with an aqueous sodium hydrocarbonate base,

deionised water, and brine. The resulting intense-coloured crude was condensed under

reduced pressure, and purified by recrystallization in hot hexane solvent. Disappointingly, the

yield of the mud-like solid was below 20 %. Thus, the hexane fraction was recovered,

combined with the previous solid, and then recrystallized from a more polar solvent

(acetone). Surprisingly, the yield of the pure target monomer (X2) was substantially higher

(70 %). Moreover, in the case of X3 synthesized using similar procedure as the X2 derivative,

the recrystallization of the off-white solid from petroleum hexane (60 oC – 70

oC) resulted in

yield far exceeding 80 % for X3.156

Moreover, the rationale behind the disparity in the

purification conditions is still unclear.

The 1H and

13C NMR spectra of the X3 monomer (figure 24) showed good purity

with a distinctive singlet at δ ca. 4.79 ppm which is commonly attributed to the –CH2Br

terminal group appended to the aromatic system.156-158

Figure 24. 1H-NMR spectra of the BBMDC16TT monomer (X3).

The absence of other impurity peaks gives a clear indication of the successful

synthesis and purification of the bromomethylated monomer via recrystallization from a non-

polar solvent (hexane). In light of the noticeable improvement in the yield attained for the

Page | 110

bromomethylated monomers (X2 and X3), the modification of the bromomethylation

conditions were initiated in order to ascertain their impact, if any, on the yield of the target

monomers. This entailed the variation of the stoichiometric ratio between the starting

precursors and the paraformaldehyde (CH2O), the amount of hydrobromic acid (HBr), and

the time of reaction completion. Table 9 shows the outcome of this evaluation with a focus on

3,6-dihexadecylthieno[3,2-b]thiophene (Pr3) as the starting material.

Batch Monomer : CH2O 33% HBr (equiv)a Duration (hour) Yield (%)

1 1 : 10 32 48 74

2 1 : 10 39 72 73

3 1 : 6 32 72 82

4 1 : 6 34 48 80

5 1 : 6 32 72 66

6 1 : 2 3 24 76

7 1 : 2 3 24 84

Table 9. Synthesis of 2,5-bis(bromomethyl)-3,6-dithieno[3,2-b]thiophene X3 under different experimental

conditions at 70 oC.

aMolar equivalent to the monomer.

Overall, it was observed that the variation in the molar ratio between the starting

material and CH2O, and the duration of reaction, did not have any significant effect on the

yield of X3. On the other hand, when scaled-up small differences in yield were noticeable

(see row 6 and 7, table 9). Henceforth, all subsequent bromomethylation scale-ups were

performed by treating 2 mole equivalent of CH2O with 1 mole equivalent of the precursor

(Pr1 - Pr4) in slight excess (~2.3 equivalent) of hydrobromic acid, at 70 oC for 24 h – 48 h.

An average product yield of 75 % was obtained upon recrystallization from hexane. The

synthesis of the branched 2,5-bis(bromomethyl)-3,6-bis(2-ethylhexyl)thieno[3,2-b]thiophene

derivative (X4, scheme 23) using the aforementioned bromomethylation conditions resulted

to the formation of a highly-viscous blue-coloured reaction mixture. After extraction, as

previously described, intense-coloured oil was obtained in approximately 90 % yield. Despite

the high yield, the oil-like nature of the product precluded its purification via

recrystallization, and when subjected to column chromatography on silica-gel (eluent: hexane

or petroleum ether), it irreversibly decomposed. Although, the 1H-NMR spectra (see figure

25) confirmed the formation of X4, the presence of some resonance peaks attributable to the

Page | 111

presence of some unidentified residual impurities were apparent. The presence of the latter

may influence the polymerization stage (vide infra).

Figure 25. 1H-NMR spectra of the branched side-chain bromomethylated thieno[3,2-b]thiophene monomer

(X4).

2.7.2.2 Synthesis of the vinylene-bridged 3,6-dialkylthieno[3,2-b]thiophene polymers via the Gilch Methodology

The Gilch polycondensation route for the preparation of the poly(3,6-

hexadecylthieno[3.2-b]thiophen-2-yl vinylene) PDC16TTV-G (P3’) and poly(3,6-bis(2-

ethylhexyl)thieno[3,2-]thiophene-2-yl vinylene) PBEHTTV-G (P3”) is outlined in scheme

24. Moreover, the Stille cross-coupling polymerization pathway is highlighted for

comparison. Several attempts to polymerize X3 were unsuccessful, either due to presence of

residual moisture in the THF solvent or in the base. In the first successful attempt, the Gilch

polydehydrohalogenation of monomer X3 was performed at ambient temperature by

treatment with 6 equiv. tBuOK (in 1 M dry THF) base in 10 mL of anhydrous THF, under the

protection of oxygen-free argon or nitrogen. The speed of the Gilch polymerization was

evident from the instant transformation in colour of the reaction mixture from greyish-green

to intense-violet, with concomitant increase in viscosity upon the portionwise addition of the

anhydrous base, as observed in the literature. In order to ensure that the formation of gel-like

Page | 112

mixture is sufficiently circumvented, the polymerization was performed with constant

stirring. After stirring for 24 hours, the crude mixture was neutralized with acidic methanol

followed by the isolation of the precipitate formed. This was then purified by sequential

Soxhlet extraction in methanol, acetone, and hexane, respectively. The solubility of the

resultant π-conjugated polymer (P3’) displayed poor solubility in many organic solvents

including; toluene, THF, dimethyl sulfoxide (DMSO), and xylene. However, reasonable

solubility was only attainable in high boiling chlorinated solvents (that is, chlorobenzene

(CB) and trichlorobenzene (TCB)). Consequently, the GPC analysis of the resultant purple

polymer in CB with a column temperature of 80 oC only showed a Mn of 12 000 g.mol

-1, Mw

of 26 000 g.mol-1

and a PDI of 2.3. The low apparent MW was thought to be due to the

removal of the higher weight material by filtration before the GPC was run (the PTFE filter

was highly coloured). Therefore, the GPC analysis was re-run in TCB at a column

temperature of 170 oC, a Mn of 60 000 g.mol

-1, Mw of 160 000 g.mol

-1, and a PDI of 2.8 were

obtained from the monomodal mass distribution peak (not shown).

Scheme 24. Gilch vs. Stille cross-coupling of poly(3,6-dialkylthineo[3,2-b]thiophene-2-yl vinylene)s.

The molecular weights of Gilch polymer (PDC16TTV-G) far exceeds that

(PDC16TTV-S) obtained via the Stille cross-coupling route (Mn = 16 000 g.mol-1

and Mw =

30 000, PDI 1.88 – table 3). On the other hand, numerous efforts to selectively polymerize

the branched side-chain monomer X4 (see scheme 23) via the Gilch route (scheme 24)

Page | 113

proved futile, due to the formation of extremely low molecular weight oligomers via GPC

analysis. Although, the rationale for this failure is not immediately apparent, assumptions

point to the observed impurity peaks in the 1H-NMR spectra (figure 25). Moreover, the high

sensitivity of the Gilch protocol to changes in the reaction conditions and the presence of

intrinsic impurities (for example, H2O) in the monomer may suppress/inhibit the

polymerization reaction, by preventing the generation of the active monomer, as observed in

other Gilch studies on MEH-PPV and MDMO-PPV polymers.132,136,146,152

Particularly, the

presence of oxygen in the polymerization reaction is known to act as a radical scavenger,

which like TEMPO, hinders the generation of the dimer radical initiating moiety (figure

23).152

Attempts to obtain reproducible molecular weights by modulating the amount of base

added during the Gilch synthesis produced some anomalous results (see table 10).

Polymer

batch

tBuOK

Mole equiv. (mL)

THF

(mL)

Mn

(g.mol-1

)

Mw

(g.mol-1

) PDI DPn

Yield

(%)

1 9 (3.74) 20 24 000** 221 000 9.0 39 81

2 9 (3.41) 15 18 500** 150 600 8.1 30 65

3 6 (4.08) 10 43 000* 151 000 3.5 70 60

4 6 (2.50) 10 60 000* 160 000 2.7 98 72

Table 10. Gilch polymerization of X3 using different base concentrations and scale-ups at room

temperature.*TCB at 170 oC, and **DCB at 80

oC.

The deviations in molecular weights when similar mole equivalents of base was

introduced (see entry 1 and 2) may be assigned to either variations in the reaction conditions,

the dryness of the solvent or other unknown factors. Since polymer batch 1 and 2 were not

soluble in CB, the GPC measurements were performed in 1,2-dichlorobenzene (DCB) at 80

oC (column temperature), with high PDIs and multimodal mass distribution peaks, due to

aggregation of the polymer chains. The latter was rectified by running the GPC analysis of

the conjugated polymers in entry 3 and 4 in TCB at elevated temperature (170 oC). The

differences in the GPC conditions under which each polymer was analysed present some

difficulties in terms of drawing a definitive correlation between the quantity of base added

and the distinctions in the resulting molecular weights. However, there are indications to

suggest that a reduction in the mole equivalent of base may affect the molecular weights of

Page | 114

the conjugated polymer (see entry 1 and 3), as observed in similar investigation by Hontis

and co-workers.147

On the other hand, the observed differences in the yields, is most probably

due to the larger losses during purification. Unlike the conjugated PDC16TTV-S polymer (Mn

= 16 kg.mol-1

) obtained from the Stille experiment, which was soluble in hot CB, all batches

of the analogous PDC16TTV-G polymer prepared by the Gilch protocol were mostly difficult

to solubilise (vide supra) due to their high molecular weights. Furthermore, poor film forming

properties were observed in the Gilch PDC16TTV-G polymers during spin coating from hot

TCB and DCB on glass substrate.

2.7.2.3 ATR-FT Infrared Spectroscopy

The infrared spectra of each conjugated polymer was compared in order to confirm

their identity and to discern any distinctions in their microstructure (see figure 26). This

analysis is particularly interesting, since the only difference between PDC16TTV-S and

PDC16TTV-G lies in their synthesis route.

3500 3000 2500 2000 1500 1000

(II)

(I)

2916 2852

717

948

11621377

14

64

81

515141727

3018

Tra

nsm

itta

nce

Wavelength (cm-1)

PDC16

TTV-G

PDC16

TTV-S

2920 2850

1464 13

97

91

3

71

7

12

38

1104

Figure 26. Comparative infrared spectra of the (I) Gilch PDC16TTV-G and (II) Stille PDC16TTV-S conjugated

polymers.

As shown in figure 26, the IR spectra of these conjugated polymers synthesised via

the Gilch and the Stille methodologies are almost similar. However, the polymer synthesised

by the former shows a simple and neater IR spectra, which suggests that PDC16TTV-G

possess better structural order than PDC16TTV-S. Upon close inspection some marked

differences are immediately apparent. The IR spectrum of PDC16TTV-G shows the

Page | 115

characteristic trans-vinylene out-of-plane vibrational band at 948 ppm, while the cis-vinylene

signal observed at 815 ppm in that of PDC16TTV-S was clearly non-existent. Moreover, in

the IR spectra of the latter, the band at trans band shifted to 913 ppm, which may suggests

some dissimilarities in the polymer backbone configuration. On the other hand, the position

of the cis-vinylene band may vary depending on the synthesis route employed, according to

the literature. In the case of the Gilch route, this band is commonly observed at ca. 873 - 875

ppm, which is apparently absent in the IR spectra of PDC16TTV-G.89,160,161

This indicates that

the configuration of the vinylene-linkages in the polymer synthesised via the Gilch route is

predominantly trans in nature. Similar observation was noted in the IR spectra other PPVs

synthesised under Gilch conditions.89

Therefore, compared to the Stille cross-coupling

pathway, it is concluded that the Gilch route is well suited for the synthesis of PPVs with

more regular molecular configuration along the polymer main-chain.89,129

In addition, the

absence of the C=O band at 1727 ppm in the IR spectrum of PDC16TTV-G, suggests that

Gilch polymer may be less susceptible to degradation by photo-oxidation (vide infra) than

that synthesised via the Stille route.

2.7.2.4 NMR Spectral Characterisation

The purity and further identification of the differently synthesized vinylene-bridged

conjugated polymers (PDC16TTV-S and PDC16TTV-G) were established by studying their

1H-NMR spectra obtained in 2,2,6,6-tetrachloroethane-d2 – TCE-d2 at 100

oC (figure 27). The

conjugated polymer synthesized via the Stille cross-coupling pathway displayed a

characteristic resonance peaks at δ 0.96 ppm, 1.35 ppm, 1.87 ppm, 2.89 ppm, and 7.15 ppm,

as anticipated. These values differs from those obtained in 1,2-dichlorobenzene-d2 (see figure

3), but, can be assigned as previously shown. In contrast, the analogous polymer

(PDC16TTV-G) synthesized via the Gilch polymerization pathway also shows similar

resonance peaks with the absence of the monomer –CH2Br chemical shift at δ 4.79 ppm

(figure 24). This further confirms the successful polymerization of the high molecular weight

PDC16TTV-G homopolymer. Furthermore, the TBB defects (see scheme 19) commonly

observed between δ 2.7 – 3.0 ppm (vide supra) in many PPV-type polymers is clearly non-

existent in the 1H-NMR spectra of PDC16TTV-G. This signifies that the Gilch polymerization

proceeded without any H-H or T-T couplings side reactions occurring (see scheme 19).

Furthermore, since no noticeable cis-trans isomerization resonances were present at δ 6.0

ppm – 6.9 ppm, as seen in the NMR spectra of PDC16TTV-S (figure 3a), it is surmised that

Page | 116

the Gilch polymerization yielded predominantly trans-configured HC=CH linkages in the

PDC16TTV-G polymer backbone.

Figure 27. 1H-NMR spectra of PDC16TTV-S (in CB-D5) and PDC16TTV-G (TCE-d2) at 100

oC.

Finally, the resonance peaks due to the formation of residual t-BuO-T32bT formed by

the reaction of potassium tert-butoxide with anionic species was not observed, which is

indicative of the Gilch polymerization taking the form of a radical mechanism.147,162

Page | 117

2.7.2.5 Optical and Electrochemical Characteristics

The optical and electronic behaviour of the differently synthesized alkyl-substituted

thieno[3,2-b]thiophene-2-yl vinylene polymers, PDC16TTV-S and PDC16TTV-G, were

evaluated by employing UV-vis absorption spectroscopy, fluorescence spectroscopy, and

electrochemistry (CV).

2.7.2.5.1 Absorption and Fluorescence Properties

The UV-vis spectra of the polymers were collated in order to ascertain the influence,

if any, of the different polymerization conditions (Stille vs. Gilch) on their effective

conjugation length and optical properties. Figure 28 shows the solution and solid-state

absorption spectra of PDC16TTV-S and PDC16TTV-G, respectively.

300 400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d A

bso

rba

nce

(a

.u)

Wavelength (nm)

PDC16

TTV-S Soln

PDC16

TTV-G Soln

PDC16

TTV-S Film

PDC16

TTV-G Film

Figure 28. UV-vis absorption spectra of PDC16TTV-G and PDC16TTV-S in solution (chlorobenzene) and thin-

film on glass substrate (spun from chlorobenzene at 2000 rpm for 70 s) at room temperature.

Although, the conjugated polymers display similar absorption characteristics covering

the 400 nm to 730 nm region of the UV-visible spectra, based on their identical structures,

there exist subtle differences when closely inspected. The polymer synthesized via the Gilch

route, PDC16TTV-G, displayed a broad intrachain π-π* transition band at an absorption

maxima (λmax) of 564 nm coupled with a weak vibronic shoulder (680 nm). The latter is

usually attributed to the occurrence of intermolecular π-π interactions (aggregation).

Interestingly, a negligible bathochromic shift of only 1 nm was deduced between the λmax of

Page | 118

PDC16TTV-G and that of the Stille polymerized PDC16TTV-S conjugated polymer,

suggesting that the conjugation limit was already reached for the Stille polymer. Similarly, an

accompanying weak shoulder was also observed at 690 nm for the latter, albeit, red shifted by

10 nm.

Such negligible bathochromic shift observed in solution is not surprising due to the

fact that the structurally analogous conjugated polymers may have adopted similar geometry

in solution, irrespective of the differences in molecular weights. Thus, the difference in

molecular weights shows little or no effect on the absorption behaviour of the conjugated

polymers in solution. Table 11 shows the absorption properties of each semiconducting

polymer.

Polymer Absorption in Solution (nm) Solid State Absorption (nm)

λmaxa λedge

b Eg

opt (eV)

c λmax λedge Eg

opt (eV)

d

PDC16TTV-S 563 (680)b 720 1.72 578 (682) 730 1.70

PDC16TTV-G 564 (690)b

718 1.73 593 (682) 737 1.68

Table 11. Absorption characteristics of the Gilch and Stille conjugated polymers. aMaximum absorption in

the longer wavelength direction. bEnergetic edge of the absorption peak. The optical band gaps were deduced

from the equation, Egopt

= 1240/λonset. Where λonset represent the absorption edge in the long wavelength

region.

As anticipated, a more pronounced red shift of 15 nm was observed between the λmax

of PDC16TTV-G and that of PDC16TTV-S, with a concomitant reduction in the optical band

gap of 0.02 eV (table 11) in the thin film spun from chlorobenzene. This suggests the

presence of very strong interchain packing order in the Gilch polymer (PDC16TTV-G) with

an increased conjugation length in the sold state. Hence, the reduction in the energy band gap

(1.68 eV). Additionally, since the energetic edge is related to the effective conjugation length

of the polymer, the bathochromic shift (7 nm) observed in the onset of the solid film

absorption spectra (see figure 28) suggest an extension of the conjugation length in the case

of PDC16TTV-G (λedge = 737 nm) relative to that of PDC16TTV-S (λedge = 730 nm).163

This

presumably coincides with the molecular weights difference (~44 kDa) between the two

conjugated polymers.

Page | 119

Furthermore, the λmax ascertained for PDC16TTV-G was roughly 593 nm, which is

bathochromically shifted by 93 nm in comparison to the reported value (ca. 500 nm in thin

film) for the phenyl-based MEH-PPVs counterpart synthesized via similar Gilch

methodology.164

Thus implying that the fused thiophene containing PPVs possess superior

structural packing order, lower optical band gap, and extended effective conjugation length

than those based on the single phenyl aromatic system. The photoluminescence (PL) of the

conjugated polymers were investigated by fluorescence spectroscopy in dilute chlorobenzene

solution and solid films spun from the same solution (10 mg/mL), via excitation at the

absorption maxima. Figure 29 shows the fluorescence spectra of the conjugated polymers in

solution at room temperature (UV-vis absorption spectra included for comparison).

300 400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d In

ten

sity (

a.u

)

Wavelength (nm)

PDC16TTV-S (UV)

PDC16TTV-G (UV)

PDC16TTV-S (PL)

PDC16TTV-G (PL)

Solution

Figure 29. Normalized UV-vis and fluorescence (PL) spectra of PDC16TTV-S (ex. λ = 550 nm) and PDC16TTV-

G (ex. λ = 550 nm) in chlorobenzene at room temperature.

In solution, unlike the UV-vis spectra, there were noticeable distinctions in the

fluorescence spectrum of the polymers. In particular, the spectrum of PDC16TTV-G features a

well-resolved emission band peak between 550 nm and 860 nm, peaking at an emission

maxima (λ(em.max)) of ca. 624 nm, and a weak vibronic shoulder band centred at 720 nm (λ0-0).

The increased non-thermal relaxation (or recombination) of the excited state (π*) back to the

ground state (π) during emission is known to be responsible for the observed PL signals of

the thieno[3,2-b]thiophene-2yl vinylene-bearing polymers and their thienylene counterpart in

solution.60

The reduced thermalized relaxation due to the existence of PL in the polymers

may ensure the long life time of the excited state, which may be beneficial to performance of

Page | 120

OPV devices by enhancing the photoinduced charge transfer between the donor polymer and

the acceptor fullerene (PC70BM).60

Table 12 summarizes the data from the solution and spin-

casted thin film fluorescence spectrum of the fused thiophene conjugated polymers.

Polymer

Solution (nm)

ΔEStokesd

/eV (nm)

Solid State (nm)

ΔEStokes

/eV (nm)

λ(em. max)

(PL)a

λmax

(UV)b

λ0-0

(PL)c

λ(em.max)

(PL)

λmax

(UV)

λ0-0

(PL)

PDC16TTV-S 631 563 705 0.25 (68) 717 578 775 0.42 (139)

PDC16TTV-G 624 564 720 0.21 (60 ) 716 593 780 0.36 (123)

Table 12. Photoluminescence (PL) properties of PDC16TTV and PBEHTTV in solution and solid film. aMaximum emission intensity.

bWavelength of the first vibronic shoulder band peak in the PL spectrum.

cShoulder.

d Stokes shift estimated based on the difference between the PL and UV-vis absorption maxima.

In the case of PDC16TTV-S in solution, the λ(em.max) (631 nm) of the fluorescence

spectrum is red shifted by ca. 7 nm with respect to PDC16TTV-G, and accompanied by a

more pronounced but hypsochromically shifted vibronic shoulder peak positioned in the long

wavelength direction (λ0-0, ca. 705 nm). Firstly, when compared to PDC16TTV-G, the red

shifted emission maxima λ(em.max) suggests some variation in the polymer conformation and

conjugation during fluorescence in solution. This may be ascribed to differences in their

molecular weights, with the higher molecular weight of the TTV-G perhaps resulting in

enhanced chain folding compared to the lower molecular weight of the Stille polymer..

Latterly, the higher intensity shoulder peak displayed by PDC16TTV-S may signify either

presence of agglomerate formation or the vibronic structures of the polymer in solution, as

observed in poly(thienylene vinylenes).165

Again differences in chain folding in solution may

be responsible.

Strikingly, in the solid state PL spectra of both π-conjugated polymers (see figure 30

below) maximum intensities were observed at the same wavelength (717 nm and 716 nm,

table 13) and shape were observed, despite the different synthetic methods employed in the

polymerization stages. This indicates similar structural or π-π stacking order in the solid state

for the differently synthesised conjugated polymers.

Page | 121

300 400 500 600 700 800 900

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d In

ten

sity (

a.u

)

Wavelength (nm)

PDC16TTV-S (UV)

PDC16TTV-G (UV)

PDC16TTV-S (PL)

PDC16TTV-G (PL)

Film

Figure 30. Normalized UV-vis and fluorescence spectra of PDC16TTV-S (ex. = 580 nm, max = 715 nm) and

PDC16TTV-G (ex. = 590 nm, max = 715 nm) in chlorobenzene at room temperature.

The large spatial separation between the UV-vis λmax and the λ(int.max) (commonly

referred to as the Stokes shift) observed for the Stille synthesized polymer (PDC16TTV-S) in

both the solution and solid phase, suggests the occurrence of substantial reorganization of the

conjugated polymer structure and/or the influence of defects in the polymer microstructure.105

This may also be responsible for the almost double Stokes shift values observed in the solid

state in comparison to the values in solution (see table 13) for both semiconducting polymers.

Furthermore, in the PL spectrum of PDC16TTV-S a red shift of ca. 87 nm was

observed between the solution and solid state emission maxima. While in the case of

PDC16TTV-G, a slightly larger bathochromic shift (ca. 92 nm) was observed. As reported for

thieno[3,2-b]thiophene, thiophene and related conjugated copolymers, the occurrence of

energy transfer from the higher energy segment to lower energy sites along the main frame of

the conjugated polymers is very common.156,166-168

Low energy sites in this case are probably

extended all trans-polymer segments. Therefore, the larger red shift observed in the emission

maxima of PDC16TTV-G from solution to the solid state compared to that of PDC16TTV-S,

suggest a more efficient intramolecular energy transfer from the excitons generated on the

thieno[3,2-b]thiophene moiety to the lower energy sites involving the vinylene units along

the polymer chain. However, the energy transfer mechanism involved could not be ascertain

and would require more in-depth study, which currently is beyond the scope of this project.

Page | 122

2.7.2.5.2 Cyclic Voltammetric Characterization

The redox behaviour, the energy levels of the Frontier orbitals (HOMO and LUMO),

and the electronic band gaps of PDC16TTV-S and PDC16TTV-G were ascertained by CV

measurements.108

In addition, for the purpose of comparison UPS spectroscopy was also

employed for the determination of the ionization potential (IP) of conjugated polymers in the

solid state. Moreover, since the IP is related to the HOMO of the conjugated polymer, this

technique, in conjunction with CV data, should allow for a more accurate discernment of the

synthetic route leading to the formation of the most photo-oxidatively stable polymer. The

cyclic voltammograms of the structurally similar poly(thieno[3,2-b]thiophene-2-yl vinylene)s

taken in 0.1 M BU4NPF6/CH3CN solution are displayed in figure 31.

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

No

rma

lize

d C

urr

en

t D

en

sity

(a

.u)

Potential (V vs Ag/Ag+)

PDC16TTV-S

PDC16TTV-G

Figure 31. Comparative cyclic voltammograms of PDC16TTV-S and PDC16TTV-G thin-films drop-casted on a

Pt working electrode at a scan rate of 50 mV s-1

.

As discussed previously, in the absence of the CV data, the LUMO could be indirectly

deduced from the IP (HOMO) and the Egopt

of the polymer absorption spectra.109

The HOMO

and LUMO energy levels (EHOMO and ELUMO) were determined by imputing the onset

potential of oxidation (φonset.ox) and reduction (φonset.red) deduced from the intersection of the

tangents drawn at the rising anodic current and the baseline, into equations (1) – (3).108,112

The E1/2

of the Fc/Fc+ redox couple (φFc/Fc+) is 0.29 V vs. Ag/Ag

+ (see figure 11). Whereby,

φonset, ox and φonset, red represent the onset potentials of oxidation and reduction vs. Ag/Ag+. As

previously highlighted, the potentials were referenced against ferrocene (–4.75 eV below the

Page | 123

vacuum level for NHE42,111

). Table 13, shows a summary of the electrochemical data for the

Stille and Gilch conjugated polymers.

Absorption/UPS dElectrochemistry

Polymer

aEg

opt

(eV)

bEHOMO

(UPS)

(eV)

cELUMO

(eV)

φonset,ox vs.

Ag/Ag+

(V)

φonset,red

vs.Ag/Ag+ (V)

EHOMO

(eV)

ELUMO

(eV)

Egecl

(eV)

PDC16TTV-S 1.70 -5.05 -3.35 +0.70 -1.04 -5.16 -3.42 1.74

PDC16TTV-G 1.68 -5.27 -3.59 +1.05 -1.20 -5.51 -3.26 2.25

Table 13. Absorption and electrochemical characteristic data of the polymer solid films. aOptical band gap

deduced from the equation, Egopt

= 1240/λonset, where λonset is the absorption edge in the long wavelength region. bDerived from the ionization value via PESA measurements.

cObtained from the difference between the HOMO

and the Egopt

value. dElectrochemical measurement performed on as cast polymer films on a Pt working

electrode in 0.1 M [Bu4]+[PF6]

- at a 50 mV s

-1 potential sweep rate.

As shown in figure 31, the conjugated polymers synthesized by the Stille and Gilch

routes both displayed characteristic pseudo-reversible redox p-doping and n-doping regime in

the anodic (or positive) potential direction. However, during the negative potential scan the

reduction (n-doping) process was found to be totally irreversible, which may suggests that

both polymers may be strong electron donating materials and/or may be unstable in the

reduced state. However, several repetition (not shown) of the potential scan in the negative

direction (up to -1.8 V vs. Ag/Ag+) show no reversibility. Surprisingly, and despite, the

similar polymer structures, the PDC16TTV-G showed significantly different ionisation

potential to PDC16TTV-S. This is evident from the low lying HOMO energy values recorded

both from the UPS/Egopt

values and the CV (-5.27 eV/-5.51 eV, see table 13). From the

electrochemical and UPS data shown in table 13, the large -0.35 eV and -0.22 eV differences

in the HOMO energy levels may be ascribed to difference in their molecular weights and

their microstructure. This suggests that the conjugated polymer synthesized by the Gilch

route is comparatively more stable and less susceptible to ambient oxidative doping.

Moreover, this may partly explain the absence of the carbonyl (C=O) peak observed in the IR

spectra of PDC16TTV-S (see figure 26(I), 1727 cm-1

).

The onset of reduction for PDC16TTV-G is not clearly discernible from the CV, but a

small deviation occurs at -1.20 V, which we assume to be the reduction potential. Hence, the

LUMO energy levels of PDC16TTV-G (-3.59 eV, table 14) are -0.24 eV higher than that of

Page | 124

PDC16TTV-S obtained from the UPS/UV method, which indicate a reduced electron affinity.

This corresponds to an Egecl

of 1.96 eV for PDC16TTV-G, while that of PDC16TTV-S (1.74

eV) was lower by ca. 0.22 eV. The differences may be partly due to the difficulty in

observing a clean reduction peak. On the contrary, a slightly narrower Egopt

value (1.68 eV)

was deduced in the case of PDC16TTV-G in the solid-state. According to the literature, such

ambiguity between Egecl

and Egopt

may be ascribed to the greater interface between the Pt

electrode surface and the drop-coated polymer film, which represent a barrier to charge

injection.109

In order words, the potential of the Pt electrode must be lower than the HOMO

of the neutral conjugated polymer for oxidation (removal of a charge) to occur, and must be

higher than its LUMO to facilitate charge injection from the Pt-electrode into the LUMO

orbital.165

Nonetheless, the impact of the differences in molecular weights and effective

conjugation lengths of the conjugated polymers on their energy band gaps cannot be

dismissed, as these play crucial roles in the optimization of both OPV and OFET device

performance.

2.7.2.6 Thermal Characteristics

The phase transition and thermal behaviour of the conjugated polymers were

investigated by DSC and TGA. The DSC thermograms (not shown) exhibited no identifiable

phase transitions. The TGA traces of the conjugated polymers recorded in both air and inert

atmosphere is as shown in figure 32.

100 200 300 400 500 600 700

0

20

40

60

80

100

We

igh

t (%

)

Temperature (oC)

PDC16

TTV-S N2

PDC16

TTV-G N2

PDC16

TTV-S Air

PDC16

TTV-G Air

Figure 32. Comparative TGA curves of PDC16TTV-S and PDC16TTV-G under N2 and O2 at a heating rate of 10 oC/min.

Page | 125

The TGA curve of PDC16TTV-S shows an initial 1 % weight loss centred at

temperature of ca. 270 oC under nitrogen atmosphere. But similar occurrence was not

noticeable in the TGA of PDC16TTV-G. Therefore, such an early weight loss may probably

be due to the presence of structural/configurational defects within the polymer backbone.

Notwithstanding the high molecular weight displayed by the latter, both conjugated polymers

showed the same onset of decomposition temperature at 380 oC, which corresponds to a 5 %

weight loss. Surprisingly, the Stille polymer exhibited a slightly better thermal stability under

oxygen with 5 % weight degradation at ca. 285 oC. While the PDC16TTV-G polymer showed

a similar weight loss at ca. 280 oC.

2.7.2.7 Crystalline Behaviour

The impact of the molecular weight differences on the crystallinity of the conjugated

polymers was elucidated by X-ray diffraction measurements. Figure 33 shows the X-ray

diffraction patterns of the drop-casted films of PDC16TTV-S and PDC16TTV-G on silicon

substrates (Si-Mat). Due to differences in solubility, the conjugated polymers were dissolved

in different solvents (CB and o-DCB).

0 5 10 15 20 25 30

9.75° (9.07 Å)

(200)

21.17° (4.19 Å)

(010)

22.74° (3.91 Å)

(010)

(300)

19.52° (4.55 Å)

9.54° (9.27 Å)

(200)

Diffr

actio

n In

ten

sity (

a.u

)

2 / deg (CuK)

PDC16

TTV-S

PDC16

TTV-G

(a)

(b)

Figure 33. X-ray diffraction pattern of PDC16TTV-S (same as PDC16TTV) and PDC16TTV-G solid films drop-

cast from CB and 1,2-DCB solution (5 mg mL-1

).

Immediately noticeable, is the absence of the peak associated with the (100) reflection

from the diffraction pattern of each conjugated polymer (figure 33) as discussed earlier. This

is despite several repetitions of the measurement at concentration exceeding 10 mg mL-1

. In

Page | 126

comparison to PDC16TTV-G, the X-ray diffraction pattern of PDC16TTV-S show slightly

better defined and higher intensity peaks at 9.54o and 22.74

o (010), with a shoulder at 19.52

o,

respectively. This signifies a slightly higher degree of structural order along the polymer

chain, which fits well with its lower recorded molecular weight. Furthermore, a reduced π-π

stacking distance was deduced for PDC16TTV-S (3.91 Å), which may imply that the polymer

chains are more closely packed than those of PDC16TTV-G. The broadening of the (010)

diffraction peak of the latter is indicative of the presence of amorphous domains within the

polymer microstructure, and the diminishing π-overlap (4.19 Å vs. 3.91 Å), is an indication

of reducing packing. The increased amorphous character may partly indicate the increased

oxidation potential of the Gilch over the Stille polymer.169,170

2.7.3 Conclusion

In conclusion, the comparative study of two analogous PPV-type conjugated

homopolymers, PDC16TTV-S and PDC16TTV-G synthesized by the Stille cross-coupling and

Gilch polymerization methodologies were actualized. Interestingly, despite the application of

microwave-irradiation in the cross-coupling of PDC16TTV-S, the palladium-free Gilch route

yielded the target polymer PDC16TTV-G with substantially higher molecular weights (Mn =

60.0 kDa vs. Mn = 16.0 kDa), which may be beneficial for efficient OPV and OFET device

operations.79

High and comparable thermal stability (ca. 280 oC) were obtained for the

polymers synthesized from the different methods. However, the UPS and CV experiments

revealed increased ionization potential for the higher molecular weight PDC16TTV-G

conjugated polymer compared to that of PDC16TTV-S (-5.27 eV/-5.67 eV vs. -5.05 eV/-5.17

eV , see table 14).

Furthermore, both synthesis routes afforded polymers with similar optical band gaps

(ca. 1.68 eV and 1.70 eV), with only slight differences in those obtained by electrochemical

measurements (1.99 eV vs. 1.74 eV), in spite of the large disparity in molecular weights.

These observations suggest that the effective conjugation length and structural

conformation/order of the conjugated polymer plays a deciding role here. X-ray diffraction

measurements gave an indication of the existence of better crystallinity in PDC16TTV-S

compared to that of PDC16TTV-G, although both polymers appear largely amorphous. This is

not at all surprising, studies on P3HT have shown that lower molecular weight conjugated

polymers tend to possess better structural order than their higher molecular weight

counterparts.79

Nevertheless, due to the high molecular weight of the polymer obtained from

Page | 127

the Gilch route (PDC16TTV-G), its solubility was compromised. Although, good solubility

was possible at elevated temperatures in the high boiling DCB and TCB chlorinated solvents.

Despite this drawback, PDC16TTV-G displayed attractive characteristics for application in

high performance OPV and OFET devices.

2.8 Experimental Section for Chapter 2

2.8.1 General Instrumentation

1H and

13C nuclear magnetic resonance (NMR) spectra were recorded on a Bruker

AV-400 (400 MHz) spectrometer, using the chemical shifts of deuterated solvents (CDCl3

and CD5Cl) as internal reference. The signal splitting patterns were designated as follows: br

(broad), d (doublet), q (quartet), s (singlet), t (triplet), and m (multiplet), respectively. Solid-

state Infrared spectra of the polymers were taken on a Perkin Elmer Spectrum 100 series

FTIR spectrophotometer, equipped with a universal attenuated total reflection (ATR)

accessory. Matrix-assisted laser desorption ionization time-of-flight (MALDI-TOF) mass

spectrometry was performed in an Applied Biosystems 4800 series MALDI-TOF mass

spectrometer. Number-average (Mn) and weight-average (Mw) molecular weights were

determined by gel permeation chromatography (GPC) using an Agilent 1200 series

instrument (calibrated against polystyrene standards) in chlorobenzene (HPLC grade) at 80

oC. Differential scanning caloriemetry (DSC) was performed in a Mettler Toledo DSC822e

instrument under N2 at a heating/cooling rate of 20 oC min

-1. Thermogravimetric analysis

(TGA) was carried out on a TA Instruments Q500 series thermogravimetric analyzer kept

under N2 and O2 atmosphere, at a 20 oC min

-1 heating rate. X-ray diffraction (XRD)

measurements were obtained on a Panalytical X‟Pert Pro diffractometer (Cu K α radiation, λ

= 1.5406 Å) with polymer films drop-casted on silicon (Si) substrate. Polymer thin films

were spin-coated from hot chlorobenzene (HPLC grade) on pre-treated glass substrates using

a Waters SPIN150 single substrate spin processor, at a spin rate of 2000 rpm for 70 s under

ambient conditions. Ultraviolet absorption spectra were recorded on a Perkin Elmer Lambda

25 UV-Vis spectrometer. Photoluminescence spectra (PL) were performed on a Varian Cary

Eclipse Fluorescence spectrophotometer. Electrospray ionization spectrometry was

performed on Thermo Electron Corporation DSQII mass spectrometer. Cyclic voltammetric

measurements were conducted in Metrohm Autolab Potentiostat/Galvanostat with a three-

electrode setup. The three-electrode cell used comprised of a platinum (Pt) wire working

electrode (WE), a Pt-mesh counter electrode (CE), and a silver wire (Ag) quasi-reference

Page | 128

electrode (RE), respectively. A concentrated polymer solution in chlorobenzene (5 mg/mL)

was deposited on the Pt WE and dried before each measurement. All voltammogram were

obtained at a potential scan rate of 50 mV s-1

(three cycles), in a quiescent electrolyte. The

latter comprised of a 0.1 M solution of tetrabutylammonium hexafluorophosphate (Bu4NBF6)

in anhydrous acetonitrile (CH3CN). The CV measurements were preceded by the deaeration

of the electrolyte solution with argon for 10 minutes to eliminate oxygen interference. The

reference electrode was calibrated with 1 mM ferrocene (oxidation potential (E0) = 0.4 V vs.

normal hydrogen electrode (NHE)42,171,172

), as an internal standard, in the Bu4NF6/CH3CN

electrolyte, after each measurement. Photoelectron Spectroscopy in Air (PESA)

measurements were recorded using a Riken Keiki AC-2 PESA (or UPS) spectrometer with a

power setting of 5 nW and a power number of 0.5. Polymer thin films for PESA were spun

on indium tin oxide (ITO) glass substrates.

2.8.1.1 Fabrication of Organic Field-Effect Transistor Devices (OFET)

Bottom-gate-top-contact OFET devices were fabricated on highly doped p-type Si

with thermally grown 400nm-thick silicon oxide (SiO2) dielectric layer. The Si/SiO2 layer

was treated with silylating agent octyltrichlorosilane (OTS) before depositing the

semiconducting polymer layer. Subsequently, a thin film of the polymers were spin-casted

from a hot chlorobenzene solution (5mg/ml) at 1000 rpm on the doped Si/SiO2 substrate,

dried, and annealed at 200 °C under N2(g) for 10 min. For the source and drain electrodes,

60nm gold (Au) electrodes were deposited onto the polymer layer by thermal evaporation to

form the top-contacts. A Keithley 4200 semiconductor parameter analyzer was used to for the

electrical characterization of the OFET devices constructed.

2.8.1.2 Organic Photovoltaic device Fabrication and Measurements

The photovoltaic characteristics of the PDC16TTV and PBEHTTV was evaluated in a

BHJ solar cell device with a configuration of ITO/PEDOT:PSS/Polymer:PC71BM/Ca/Al. The

glass substrates coated with indium-tin oxide (ITO) were sequentially (15 minutes) cleaned

with detergent, distilled water, acetone, and isopropanol under sonication. Then, the

substrates were dried under a stream of nitrogen gas and pre-treated using ultraviolet ozone

plasma for 7 minutes. A solution of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)

(PEDOT:PSS) was spin-coated onto the pre-treated ITO substrates to form a PEDOT:PSS

layer of 30nm thickness. The substrates were baked at 150 C for 20 minutes prior to

Page | 129

coating of the active layers (15 mg/mL Polymer:PC71BM blend in chlorobenzene) and the

deposition of the electrodes. Preliminary optimization of the devices was performed by

varying the coating spin-rate (not shown here) and the composition of the active layer. All

measurements were carried out in an oxygen-excluded glovebox. The current density-voltage

(J-V) curves of the devices were measured with exposure to a 150 W Xenon lamp filtered to

simulate AM 1.5 (air mass 1.5). On the other hand, the current generated in the devices were

measured using a Keithley source meter with varying voltage bias. Furthermore, the

equilibrium quantum efficiency (EQE) measurements of the devices were performed using a

100 W tungsten halogen lamp equipped with a monochromator.

2.8.2 Synthesis Section

2.8.2.1 Materials and Chemical Reagents

The unsubstituted thieno[3,2-b]thiophene (M10) was used as purchased. The

precursors, 2,5-dibromothieno[3,2-b]thiophene (M11) was synthesized as prescribed in the

literature,12

and 3,6-dibromothieno[3thiophene (M12) was prepared according to the

modified literature procedure.63

Acetonitrile (CH3CN, anhydrous, 99.8 %), Ferrocene (Fc, 98

%) tetrahydrofuran (THF, anhydrous, inhibitor free, 99.9%), tetrabutylammonium

hexafluorophosphate (Bu4PF6, for electrochemical analysis, ≥ 99.0 %), diisopropylamine

(DPA, redistilled, 99.95 %), butyl lithium solution (n-BuLi, 1.6 M in hexane), [1,1-

bis(diphenylphosphino)ferrocene] dichloropalladium(II) (Pd(dppf)Cl2),

tris(benzylideneacetone) dipaladium(0) (Pd2(dba)3), tri(o-tolyl) phosphine (P(o-tolyl)3 ,97%),

N-bromosuccinimide (NBS, ReagentPlus, 99%), sodium hydride (NaH, dry, 95 %),

chlorobenzene (CH5Cl, anhydrous, 99.8 %), chloroform-d (CHCl3, 99.9 atom % D), carbon

disulfide (anhydrous, ≥ 99 %), bromine (reagent grade), and zinc powder (purum grade),

were procured from Sigma-Aldrich. Chlorobenzene-d5 (CD5Cl, 99 atom % D) was obtained

from Cambridge Isotope Laboratories. The organotin reagent, trans-1,2-bis(tri-n-

butylstannyl)ethylene (95%) was purchased from Acros Organics. The organozinc reagents,

n-(2-ethylhexyl)zinc bromide and n-hexadecylzinc bromide solution in THF (0.5 M) were

purchased from Reike Metals. Acetic acid (AcOH, analytical grade), acetone (analytical

grade), chloroform (CH3Cl, HPLC grade), dichloromethane (DCM, HPLC), hydrochloric

acid (HCl, analytical grade, 37%), hexane 67-70 oC (GPR), methanol (MeOH, analytical

grade), petroleum ether 40 – 60 oC (analytical grade), sodium sulfate (Na2SO4, anhydrous),

sodium thiosulfate, magnesium sulphate (MgSO4, anhydrous), sodium hydrogen carbonate

Page | 130

(NaHCO3, analytical grade), and sodium chloride (NaCl, analytical grade), were obtained

from VWR and used as received. Silica gel (SiO2) was obtained from Merck. Triethylamine

(Et3N, 99%, Alfa Aesar) was distilled and dried over NaH before use. Dichloromethane

(DCM) was freshly dried and distilled in a PureSolvTM

MD Solvent Purification System

(Innovative Technologies) prior to use. Brine was freshly prepared in the laboratory.

2.8.2.2 Experimental Protocol

2.8.2.2.1 Synthesis of Precursors and Target Monomers

2.8.2.2.2 Synthesis of 2,5-dibromothieno[3,2-b]thiophene M11.12

To a two neck round-bottom flask (500 mL) was added a mixture of thieno[3,2-

b]thiophene M10 (16.76 g, 119.7 mmol), anhydrous DCM (280 mL), and AcOH (133 mL).

Stirring was initiated, followed by the portionwise addition of NBS (42.60 g, 239.4 mmol) for

1 h under argon protection at room temperature. A colour transformation from green to

orange ensued. Upon stirring for about 3 h, the analysis of the mixture by GC-MS indicated

the formation of M11. The reaction was neutralised with 5% aqueous NaHCO3 solution,

extracted with another portion of DCM, and the combined organics was washed with distilled

water (3x), respectively. Then the combined organic extract was dried with MgSO4, filtered,

concentrated in a rotary evaporator, and dried in a vacuum oven to afford M11 as a white

powder (33.77 g, 95%). MS (EI): m/z (M+): 298 (C6H2S2

+).

1H NMR (400 MHz, CDCl3):

7.19 (s, 2H). 13

C NMR (400 MHz, CDCl3): 113.65, 121.77, 138.29. HRMS EI calcd for

C6H2Br2S2 295.7965, found 295.7964.

2.8.2.2.3 Synthesis of 3,6-dibromothieno[3,2-b]thiophene M12.63

Page | 131

To a magnetically stirred mixture of anhydrous THF (1400 mL) and diisopropylamine

(50.9 mL, 362.29 mmol) cooled to 0 oC in an ice bath, was added a solution of n-BuLi (1.6 M

in hexane, 194 mL, 309.65 mmol) under N2(g) atmosphere. After stirring for 30 mins, the

reaction mixture was cooled to -78 oC followed by the slow introduction of M11 (38.64 g,

130.54 mmol) in anhydrous THF (300 mL), and left to slowly warm to room temperature for

16 h with stirring. On completion, the reaction mixture was cooled to -78 oC followed by

dilution with hexane (200 mL), and quenched with brine (400 mL) under N2(g) protection.

Then the reaction mixture was allowed to warm to room temperature over 4 h, extracted with

hexane/THF, and condensed under reduced pressure. The resultant brown solid was purified

by flash chromatography (SiO2, eluent: toluene), washed with methanol to eliminate toluene

traces, and dried in a vacuum oven at room temperature for 24 h to afford M12 as a white

solid (25 g, 66% yield). MS (EI): m/z (M+): 298 (C6H2Br2S2

+).

1H NMR (400 MHz, CDCl3):

7.37 (s, 2H). 13

C NMR (400 MHz, CDCl3): 103.03, 125.16, 139.82; HRMS EI calcd for

C6H2Br2S2 295.7965, found 295.7958.

2.8.2.2.4 Synthesis of 3,6-dihexadecylthieno[3,2-b]thiophene M13c.

A 0.5 M solution of n-hexadecylzinc bromide in anhydrous THF (16 mL, 8 mmol)

was added to a mixture of compound M12 (1.00 g, 3.40 mmol) and Pd(dppf)Cl2 (54.5 mg,

0.07 mmol, 2 mol%) in a sealed Biotage® microwave vial (20 ml) under N2 atmosphere. The

reaction mixture was deaerated with N2 for 15 min with stirring. The vial was placed in a

microwave reactor (Initiator, Biotage®

) and sequentially heated at 100 oC (1 min), 120

oC (5

min), and 140 oC (25 min), respectively. This procedure was replicated 7 times (8 g in total)

due to the limited capacity of the microwave vial (x7). The intense brown reaction mixture

was cooled to ambient temperature, dissolved in CHCl3, and quenched with 10% aqueous

HCl solution. Subsequently, the organic layer was collected, extracted with another portion of

10% aqueous HCl solution to eliminate zinc bromide, and washed with brine. The combined

aqueous phase was extracted with CHCl3, combined with the organic layer, dried over

anhydrous MgSO4, filtered, and concentrated by rotary evaporation. Further purification was

Page | 132

carried out by recrystallization from acetone twice, followed by drying in a vacuum oven at

room temperature for 24 h to afford M13c as white crystals (11.0 g, 70 %). MS (EI): m/z: 588

(C38H68S2+).

1H NMR (400 MHz, CDCl3): 6.98 (s, 2H); 2.73 (t, 4H, J = 8.0 Hz); 1.77 (m,

4H); 1.28 (m, 52 H); 0.90 (t, 6 H, J = 8.0 Hz). 13

C NMR (400 MHz, CDCl3): 139.27,

135.46, 120.87, 31.95, 29.80, 29.71, 29.59, 29.42, 29.39, 28.73, 22.72, 14.14. HRMS EI

calcd for C38H68S2 588.4762, found 588.4741.

2.8.2.2.5 Microwave-assisted Negishi synthesis of 3,6-bis(2-ethylhexyl)thieno[3,2-

b]thiophene M13d.

This compound was synthesized according to the above procedure used for M13c. In

this experiment, a 0.5 M solution of 2-ethylhexylzinc bromide in anhydrous THF (250 mL,

125 mmol), Pd(dppf)Cl2 (878.4 mg, 1.08 mmol, 2 mol%), and M12 (16.00 g, 53.69 mmol),

were utilised. A first attempt at synthesizing the titled compound, produced a viscous and

intense coloured oil, which comprised of a mixture of differently brominated alkylthieno[3,2-

b]thiophenes (that is, monobromo, dibromo, and tribromo). This was ascertained by GC-MS

analysis and normal phase (Si-OH) thin layer chromatography (vide supra), thus their

separation was only successful via reverse phase (Si-C18H37) column chromatography (eluent:

3:1 acetonitrile/THF). However, the crude product obtained from subsequent synthesis, using

other batches of compound M12 was purified sufficiently by utilizing the normal phase

silica-gel column chromatography (eluent: petroleum spirit 40 – 60 oC). By drying under high

pressure at RT for 24 h M13d was obtained as clear-colour oil (14 g, 70 % yield). MS (EI):

m/z (M+): 364 (C22H36S2

+). 1H NMR (400 MHz, CDCl3): 6.97 (s, 2H); 2.69 (d, 4H, J = 8.0

Hz); 1.84 (m, 2H); 1.35 (m, 16 H); 0.94 (m, 12 H). 13

C NMR (400 MHz, CDCl3): 139.53,

134.51, 121.65, 38.70, 34.24, 32.74, 28.85, 25.89, 23.05, 14.35, 10.80. HRMS EI calcd for

C22H36S2 364.2258, found 364.2259.

Page | 133

2.8.2.2.6 Synthesis of 3,6-didecylthieno[3,2-b]thiophene M13a.

This precursor was synthesized as described for M13c. The titled compound was

obtained as white crystals (4.41 g, 62% yield), by the reaction of Pd(dppf)Cl2 (273.23 mg,

0.335 mmol), 0.5 M solution of decylzinc bromide in anhydrous THF (82 mL, 41 mmol) and

compound M12 (5.017 g, 16.84 mmol). MS (EI): m/z (M+): 420 (C26H44S2

+).

1H NMR (400

MHz, CDCl3): δ 0.94 (t, 6H, J = 8.0 Hz); 1.32 (br. m, 36H); 1.78 (m, 4H); 2.74 (t, 4H, J = 8.0

Hz); 6.99 (s, 2H). 13

C NMR (400 MHz, CDCl3): δ 14.17; 22.74; 28.77; 29.39; 29.46; 29.63;

29.67; 29.82; 31.96; 120.87; 135.45; 139.30. HRMS EI calcd for C26H44S2 420.2956, found

420.2952.

2.8.2.2.7 Synthesis of 3,6-didodecylthieno[3,2-b]thiophene M13b.

The above target compound was synthesized in accordance with the experimental

procedure detailed for M13c derivate. In this case, a stoichiometric amount of 3,6-

dibromothieno[3,2-b]thiophene M12 (5.00 g, 16.78 mmol) was reacted with 0.5 M solution

of dodecylzinc bromide in anhydrous THF (75 mL, 37.5 mmol), and catalyzed by

Pd(dppf)Cl2 (272.50 mg, 0.334 mmol). The purification, recrystallization and drying of the

processed crude, afforded white M13b crystals (5.62 g, 70% yield). MS (EI): m/z (M

+): 476

(C30H52S2+).

1H NMR (400 MHz, CDCl3): δ 0.88 (t, 6H, J = 8.0 Hz); 1.22-1.42 (br, 36H);

1.74 (m, 4H); 2.70 (t, J = 8.0 Hz, 4H); 6.95 (s, 2H). 13

C NMR (400 MHz, CDCl3): δ 14.20;

22.70; 28.70; 29.37; 29.42; 29.6; 29.67; 29.69; 29.8; 31.9; 120.8; 135.5; 139.3. HRMS EI

calcd for C30H52S2 476.3044, found 476.3041.

Page | 134

2.8.2.2.8 Synthesis of 2,5-dibromo-3,6-dihexadecylthieno[3,2-b]thiophene M14c.

To a stirred solution of M13c (3.67 g, 6.25 mmol) in anhydrous THF (50 mL) under

N2 protection, was added NBS (2.23 g, 12.50 mmol) portionwise for 40 min at 0 oC. Stirring

continued for 24 h at room temperature. Then the reaction was quenched with petroleum

hexane 67 – 70 oC (400 mL), filtered, and concentrated in a rotary evaporator. The resultant

orange oil was purified by SiO2 column chromatography (eluent: petroleum ether 40 – 60 oC)

and recrystallized from acetone to afford M14c as white crystals (4.43 g, 95% yield). MS

(EI): m/z (M+): 746 (C38H66Br2S2

+).

1H NMR (400 MHz, CDCl3): 2.70 (t, 4H, J = 8 Hz);

1.67 (m, 4H); 1.29 (m, 52H); 0.91 (t, 6H, J = 8 Hz). 13

C NMR (400 MHz, CDCl3): 136.09,

134.40, 109.42, 31.94, 29.70, 29.50, 29.37, 29.26, 28.93, 28.07, 22.70, 14.13. HRMS EI

calcd for C38H66Br2S2 746.2973, found 746.2971.

2.8.2.2.9 Synthesis of 2,5-dibromo-3,6-bis(2-ethylhexyl)thieno[3,2-b]thiophene M14d.

This was synthesized as delineated for M14

c. Compound M13

d (4.01 g, 11.00 mmol)

and NBS (3.92 g, 22.00 mmol) were used. M14d was obtained as colourless oil (4.13 g, 72%

yield). MS (EI): m/z (M+): 520 (C22H34Br2S2

+).

1H NMR (400 MHz, CDCl3): 2.63 (d, 4H, J =

16 Hz); 1.84 (m, 2H); 1.35 (m, 16 H); 0.94 (t, 12 H, J = 8 Hz). 13

C NMR (400 MHz, CDCl3):

136.52, 133.88, 110.10, 38.70, 33.49, 32.72, 31.62, 28.75, 25.97, 23.01, 22.69, 14.15, 10.83.

HRMS EI for calcd C22H34Br2S2 520.0469, found 520.0480.

Page | 135

2.8.2.2.10 Synthesis of 2,5-dibromo-3,6-didecylthineo[3,2-b]thiophene M14a.

The experimental conditions used in synthesizing M14c were employed here. The

precursor M13a (1.32 g, 3.14 mmol), NBS (1.12 g, 6.28 mmol) and anhydrous THF (100 mL)

constituted the reaction mixture. White M14a crystals were obtained (1.72 g, 95% yield). MS

(EI): m/z (M+): 578 (C26H42Br2S2

+).

1H NMR (400 MHz, CDCl3): δ 0.90 (t, 6H, J = 8.0 Hz);

1.28 (br, 28H); 1.74 (m, 4H); 2.73 (t, 4H, J = 8.0 Hz); 13

C NMR (400 MHz, CDCl3): δ 14.13;

22.69; 28.08; 28.93; 29.26; 29.33; 29.50; 29.59; 29.72; 32.77; 109.43; 134.40; 136.09.

HRMS EI for calcd C26H42Br2S2 578.1487, found 578.1831.

2.8.2.2.11 Synthesis of 2,5-dibromo-3,6-didodecylthineo[3,2-b]thiophene M14b.

Monomer M14b was synthesized as described for M14

c. A stoichiometric amount of

compound M14b (0.78 g, 1.64 mmol) was dissolved in anhydrous THF (40 mL), and then

reacted with 2 molar equivalent of NBS (0.58 g, 3.28 mmol). Upon purification and drying,

white crystals of the target monomer M14b was obtained (0.77 g, 74% yield). MS (EI): m/z

(M+): 634 (C30H50Br2S2

+).

1H NMR (400 MHz, CDCl3): δ 0.90 (t, 6H, J = 8.0 Hz); 1.28 (br,

28H); 1.68 (m, 4H); 2.69 (t, 4H, J = 8.0 Hz); 13

C NMR (400 MHz, CDCl3): δ 14.14; 22.67;

28.72; 29.33; 29.40; 29.57; 29.61; 29.78; 31.91; 120.85; 135.45. HRMS EI for calcd

C30H50Br2S2 634.2192, found 634.2264.

Page | 136

2.8.2.2.12 Synthesis of Poly(3,6-dihexadecylthieno[3,2-b]thiophene-2-yl vinylene)

PDC16TTV via microwave-accelerated Stille cross-coupling Polymerization.

To a Biotage microwave vial (20 mL) was added M14c (998.0 mg, 1.34 mmol),

Pd2(dba)3 (24.5 mg, 2 mol%), P(o-tolyl)3 (32.6 mg, 8 mol%), and trans-1,2-bis(tri-n-

butylstannyl)ethylene (810.9 mg, 1.34 mmol) in chlorobenzene (anhydrous, 20 mL with

0.01% triethylamine). The vial was charged with a magnetic stirring bar, sealed and purged

with a stream of deoxygenated nitrogen gas at ambient temperature for 20 min. Subsequently,

the reaction was pre-stirred in a microwave reactor at ambient temperature for 10 s, followed

by sequential heating a 140 oC for 2 min, 160

oC for 2 min and 180

oC for 15 min,

respectively. On completion, the intense-purple reaction was cooled to 50 oC and precipitated

dropwise into a vigorously stirred acidic methanol (10 mL hydrochloric acid and 150 mL

methanol) solution. After stirring for 3 h, the precipitate was filtered directly into an

extraction thimble and purified by Soxhlet extraction in methanol (24 h), acetone (24 h),

hexane (67–70 oC) (24 h), and chloroform (24 h), respectively. The chloroform fraction was

concentrated, reprecipitated into methanol and collected by filtration. An intense-purple

PDC16TTV solid (798.0 mg, 97% yield) was obtained after drying under vacuum at 40 oC for

24 h. GPC: Mn=16.0K, Mw=30.0K, PDI=1.90. 1H NMR (400 MHz, DCB): 0.89 (t, 6H);

1.20-1.45 (br, 52H); 1.83 (m, 4H); 2.84 (m, 4H); 7.33 (s, 2H vinylene). Found: C, 73.61; H,

11.32 %. Calcd. (C40H70S2)n: C, 78.36; 11.18 %.

Page | 137

2.8.2.2.13 Synthesis of Poly(3,6-bis(2-ethylhexyl)thieno[3,2-b]thiophene-2-yl vinylene)

PBEHTTV via microwave-accelerated Stille cross-coupling Polymerization.

This polymer was prepared in accordance with the protocol employed for the

synthesis of PBEHTTV, starting from M14d (413.0 mg, 0.78 mmol), Pd2(dba)3 (14.5 mg, 2

mol%), P(o-tolyl)3 (19.2 mg, 8 mol%), trans-1,2-(tri-n-butylstannyl)phosphine (478.0 mg,

0.789), and chlorobenzene (anhydrous, 15 mL with 0.01% triethylamine). PBEHTTV was

obtained as an intense-blue solid polymer (193.0 mg, 63% yield). GPC: Mn=16.5K,

Mw=39.5K, PDI=2.40. 1H NMR (400 MHz, C6D5Cl): 1.01 (t, 12H); 1.26-1.70 (br, 16H);

2.04 (m, 2H); 2.82 (m, 4H); 7.44 (s, 2H vinylene). Found: C, 71.94; H, 9.08 %. Calcd.

(C24H36S2)n: C, 74.17; 9.34 %.

2.8.2.2.14 Synthesis of Poly(3,6-didodecylthieno[3,2-b]thiophene-2-yl vinylene)

PDC12TTV via microwave-accelerated Stille cross-coupling Polymerization.

This polymer was synthesised following the same protocol described for PDC16TTV,

derivate. A catalytic reaction mixture consisted of monomer M14b (490.4 mg, 0.77 mmol),

Pd2(dba)3 (14.2 mg, 2 mol%), P(o-tolyl)3 (18.8 mg, 8 mol%), and trans-1,2-(tri-n-

butylstannyl)phosphine (469.2 mg, 0.77 mmol) in anhydrous chlorobenzene (15 mL with

0.01% triethylamine). A purple PDC12TTV solid polymer was obtained (224.9 mg, 58%

Page | 138

yield). GPC: Mn=9.2K, Mw=17.5K, PDI=1.90. 1H NMR (400 MHz, DCB): δ 0.88 (t, 6H);

1.15-1.45 (br, 18H); 1.80 (m, 4H); 2.83 (m, 4H); 7.34 (s, 2H vinylene).

2.8.2.2.15 Alternative synthesis of 3,6-dibromothieno[3,2-b]thiophene (Failed Synthesis)

2.8.2.2.15.1 Synthesis of 2,5,3,6-tetrabromothieno[3,2-b]thiophene X.

A two-neck flask was charged with thieno[3,2-b]thiophene (2.02 g, 14.4 mmol) and

anhydrous carbon disulfide (CS2) solution (50 mL). The reaction was deaerated with N2 for

20 mins, followed by the introduction (dropwise) of bromine (24 mL, 493.4 mmol) in CS2

solution (30 mL). The reaction mixture was refluxed with stirring under N2 atmosphere, at 60

oC for 24 h. On completion, the orange-coloured reaction mixture was added to a saturated

sodium thiosulfate solution (200 mL). The resultant precipitate was filtered, washed with

water and ethanol, followed by drying in a vacuum oven at 40 oC for 24 h. The organic layer

of the filtrate was washed with brine, 5 % NaHCO3(aq), and another portion of brine.

However, the mixture was not further dried with MgSO4, due to the presence of a solid and

organics. Upon condensation in a rotary evaporator, the solid was combined with that isolated

previously, and recrystallized from toluene to afford needle-like cream solid (6.23 g, 95 %

yield). 1H NMR (not shown) showed no aromatic peaks, which is indicative of the formation

of the target compound X.

2.8.2.2.15.2 Synthesis of 3,6-dibromothieno[3,2-b]thiophene Y.

A suspension of compound X (6.08 g, 13.3 mmol) in acetic acid (700 mL) was

refluxed at 100 oC under N2 protection to ensure complete solubility. Afterwards, zinc

powder (2.40 g, 36.7 mmol) was added, and allowed to reflux overnight. Then a second

Page | 139

portion of powdered zinc was introduced into the cloudy reaction mixture, under N2

atmosphere. The progress of the reaction was monitored by repeated GC-MS analysis.

Despite the addition of extra zinc dust, the starting material X remained tetrabrominated. The

reaction was repeated severally with fresh zinc powder and for longer periods (48 -72 h), with

similar outcome.

2.8.3 Experimental Section for Chapter 2 (Part 2.7)

2.8.3.1 Materials and Chemical Reagents

The precursors, 3,6-bis(2-ethyl-1-hexyl)thieno[3,2-b]thiophene (BEHTT) and 3,2-

dihexyldecylthieno[3.2-b]thiophene (DHDTT) were synthesized as described in scheme 12

(M4c and M4

d). Potassium tert-butoxide (1.0 M in THF), tetrahydrofuran (THF, anhydrous,

≥99.9 %, inhibitor-free), and para-formaldehyde (powder, 95%) were obtained from Sigma-

Aldrich and used as received. Hydrobromic acid solution (˃33% hydrogen bromide (HBr) in

acetic acid, purum grade) was purchased from Fluka, and magnesium sulphate (MgSO4,

dried, reagent grade) from Fisher Scientific. Acetone (analytical grade), chloroform (CHCl3,

GPR RECTAPUR), glacial acetic acid (99.8%), petroleum hexane 67 – 70 oC (GPR

RECTAPUR), sodium chloride (NaCl, analytical grade), and sodium hydrogen carbonate

(NaHCO3, analytical grade), were purchased from VWR. Brine and saturated aqueous

NaHCO3 were prepared in the laboratory.

2.8.3.2 Experimental Protocol

2.8.3.2.1 Synthesis of Bromomethylated T32bT Monomers

2.8.3.2.2 Synthesis of 2,5-bis(bromomethyl)-3,6-dihexadecylthieno[3,2-b]thiophene X3.

The scale-up synthesis of X3 was carried out following experimental protocols

detailed in previous reports on the preparation of 2,5-bis(bromomethyl)thiophene (BBMT)

compound.154,156,157,159,173

To a mixture of monomer M13c (4.00 g, 6.80 mmol) and para-

formaldehyde (0.47 g, 15.65 mmol) in glacial acetic acid (40 mL), was slowly added an HBr

solution (33 % in acetic acid, 3.12 mL, 15.65 mmol) at 0 oC under constant stirring and argon

Page | 140

atmosphere. The reaction was stirred at 70 oC for 24 h. On completion, the green/pale blue

solid/solution mixture was poured into diethyl ether, and extracted with distilled water,

saturated NaHCO3 solution, and brine. The aqueous layer was re-extracted, combined with

the intense-coloured organic phase, and concentrated in a rotary evaporator at 40 oC to afford

a green solid. Then the pale-brown solid was purified by recrystallization from petroleum

hexane (67 – 70 oC) to form the titled monomer (M12) as a cream-coloured solid (4.00 g, 76

%). MS (EI): m/z (M+): 774 (C40H70S2Br2

+).

1H NMR (400 MHz, CDCl3): δ 0.91 (t, 6H, J =

8.0 Hz); 1.29 (br. m, 52H); 1.73 (m, 4H); 2.74 (t, 4H, J = 8.0 Hz); 4.79 (s, 4H). 13

C NMR

(400 MHz, CDCl3): δ 14.13; 23.00; 26.49; 27.62; 27.95; 28.94; 29.14; 29.52; 29.92; 135.12;

135.24; 138.69. HRMS EI calcd for C40H70S2Br2 772.3286, found 772.3262.

2.8.3.2.3 Synthesis of 2,5-bis(bromomethyl)-3,6-bis(2-ethylhexyl)thieno[3,2-b]thiophene

X4.

This monomer derivate was prepared in accordance with the above procedure. A

stirred mixture of the precursor M13d ((1.41 g, 3.87 mmol) and para-formaldehyde (0.27 g,

8.89 mmol) in glacial acetic acid (10 mL) was treated with portionwise addition of 33 % HBr

solution in acetic acid (1.77 mL, 8.89 mmol) at 70 oC for 24 h. After the reaction was

completed, the blue-coloured crude was quenched and extracted as described above. The

concentration of the combined organic layer afforded X4 as a viscous and blackish oil (1.90

g, 90% yield). Attempt to further purify the oil via silica-gel column chromatography (eluent:

petroleum hexane 67 – 70 oC or benzene) resulted to its degradation. However,

1H-NMR and

GPC analysis revealed that the product was reasonably pure for the next step. MS (EI): m/z

(M+): 550 (C24H38S2Br2

+).

1H NMR (400 MHz, CDCl3): δ 0.92 (br m, 12H); 1.28 (br, 16H);

1.87 (m, 4H); 2.64 (d, J = 8.0 Hz, 2H); 4.79 (d, 4H). HRMS EI calcd for C24H38S2Br2

548.0782, found 548.0790.

Page | 141

2.8.3.2.4 General Gilch Polymerization Route

2.8.3.2.5 Synthesis of Poly(3,6-dihexadecylthieno[3,2-b]thiophene-2-yl vinylene) via Gilch

Polydehydrohalogenation Route PDC16TTV-G.

The polymerization was performed as prescribed in the literature.142

In a typical

experiment, a two-neck round-bottom flask was charged with X3 (0.317 g, 0.409 mmol),

equipped with a Teflon-coated magnetic stirring bar and placed under argon deaeration for 10

min. Anhydrous THF (10 mL) was introduced via a degassed syringe, followed by the

dropwise addition of potassium tert-butoxide (1.0 M in THF solution, 2.5 mmol, 2.5 mL) for

25 min. This was accompanied by a colour transformation from violet to intense-purple with

a concomitant increased viscosity. After continuous stirring for 48 h, the viscous polymer

crude was neutralized (quenched) with 1.0 M acidic methanol (10 mL HCl in 150 mL). The

resulting polymer mixture was poured into methanol (500 mL), followed by the isolation of

the precipitate formed in an extraction thimble. Further purification was performed by

Soxhlet extraction with acetone (24 h), methanol (24 h), hexane (24 h) and chloroform (24 h).

After stirring for 3 h, the precipitate was filtered directly into an extraction thimble and

purified by Soxhlet extraction in methanol (24 h), acetone (24 h), hexane (67–70 oC) (24 h),

and chloroform (24 h), respectively. The chloroform fraction was reprecipitated into

methanol, isolated by filtration, and dried at 40 oC under vacuum for 24 h to yield the target

polymer as a purple solid (0.200 g, 80 %). GPC (TCB at 170 oC): Mn = 60 kDa, Mw = 160

kDa, PDI = 2.8. 1H NMR (400 MHz, TCE-d2): 0.96 (s, 6H); 1.36 (br, 52H); 1.79 (s, 4H);

2.76 (br, 4H); 7.35, 7.40 (d, d, 2H vinylene). Found: C, 73.61; H, 11.32 %. Calcd.

(C40H70S2)n: C, 75.72; 10.89 %.

Page | 142

2.8.3.2.6 Synthesis of Poly(3,6-bis(2-ethylhexyl)thieno[3,2-b]thiophene-2-yl vinylene)

PBEHTTV via Gilch Polydehydrohalogenation Route PBEHTTV-G.

Attempts to synthesis the above polymer through the Gilch methodology yielded

oligomers due to the insufficient purity of the oily nature of the monomer (X4).

Page | 143

2.9 References

(1) Conjugated Polymers: Theory, Synthesis, Properties, and Characterization

Third Edition ed.; Skotheim, T. A.; Reynolds, J. R., Eds.; Taylor and Francis Group, LLC.:

Ney York, 2006.

(2) Clark, D. T. J. Mol. Spec., 1968, 26, 181-188.

(3) Wynberg, H.; Zwanenberg, D. J. Tet. Lett., 1967, 9, 761 - 764.

(4) Choi, K. S.; Sawada, K.; Dong, H.; Hoshino, M.; Nakayama, J. Heterocycles

1994, 39, 143-149.

(5) Archer, W. J.; Taylor, R. J. Chem. Soc. Perkin Trans. 2 1982, 295-299.

(6) Leriche, P.; Raimundo, J.-M.; Turbiez, M.; Monroche, V.; Allain, M.;

Sauvage, F.-X.; Roncali, J.; Frère, P.; Skabara, P. J. J. Mater. Chem., 2003, 13, 1324-1332.

(7) Wright Jr., W. B. Heterocycl. Chem., 1972, 9, 879 - 882.

(8) Litvinov, V. P.; Gol‟dfarb, Y. L. Adv. Heterocycl. Chem., 1976, 19, 123.

(9) Fuller, L. S.; Iddon, B.; Smith, K. A. J. Chem. Soc., Perkin Trans. 1 1997,

3465 - 3470.

(10) Mazaki, Y.; Kobayashi, K. Tetrahedron Lett., 1989, 30, 3315-3318.

(11) Gronowitz, S.; Raznikiewicz, T. Org. Synth., 1973, Coll. Vol. 5,, 149.

(12) Liu, P.; Wu, Y.; Pan, H.; Li, Y.; Gardner, S.; Ong, B. S.; Zhu, S. Chem.

Mater., 2009, 21, 2727-2732.

(13) Zhang, X.; Kohler, M.; Matzger, A. J. Macromolecules 2004, 37, 6306-6315.

(14) Li, Y.; Wu, Y.; Liu, P.; Birau, M.; Pan, H.; Ong, B. S. Adv. Mater., 2006, 18,

3029-3032.

(15) Heeney, M.; Bailey, C.; Duffy, W.; Shkunov, M.; Sparrowe, D.; Tierney, S.;

Zhang, W.; McCulloch, I. Polym. Mater.: Sci. & Eng. Prepr., 2006, 95, 101.

(16) Heeney, M.; Wagner, R.; McCulloch, I.; Tierney, S. 2005; Vol.

WO2005111045.

(17) McCulloch, I.; Heeney, M.; Chabinyc, M. L.; DeLongchamp, D.; Kline, R. J.;

Colle, M.; Duffy, W.; Fischer, D.; Gundlach, D.; Hamadani, B.; Hamilton, R.; Richter, L.;

Salleo, A.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Zhang, W. Adv. Mater., 2009, 21, 1091 -

1109.

(18) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.;

Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R.

J.; McGehee, M. D.; Toney, M. F. Nat. Mater., 2006, 5, 328-333.

(19) Lucas, L. A.; DeLongchamp, D. M.; Vogel, B. M.; Lin, E. K.; Fasolka, M. J.;

Fischer, D. A.; McCulloch, I.; Heeney, M.; Jabbour, G. E. Appl. Phys. Lett., 2007, 90,

012112-1-012112-3.

(20) Li, Y.; Singh, S. P.; Sonar, P. Adv. Mater., 2010, 22, 4862 - 4866.

(21) Lo, C.; Adenier, A.; Chane-Ching, K. I.; Maurel, F.; Aaron, J. J.; Kosata, B.;

Svoboda, J. Synth. Met., 2006, 156, 256 - 269.

(22) DeLongchamp, D. M.; Kline, R. J.; Lin, E. K.; Fischer, D. A.; Richter, L. J.;

Lucas, L. A.; Heeney, M.; McCulloch, I.; Northrup, J. E. Adv. Mater., 2007, 19, 833 - 837.

(23) Miguel, L. S.; Matzger, A. J. Macromolecules 2007, 40, 9233-9237.

(24) Northrup, J. E.; Chabinyc, M. L.; Hamilton, R.; McCulloch, I.; Heeney, M. J.

Appl. Phys., 2008, 104, 083705.

(25) Lee, J. Y.; Heo, S. W.; Choi, H.; Kwon, Y. J.; Hawa, J. R.; Moon, D. K. Sol.

Energy Mater. Sol. Cells 2009, 93, 1932 - 1938.

(26) Mishra, S. P.; Javier, A. E.; Zhang, R.; Liu, J.; Belot, J. A.; Osaka, I.;

McCullough, R. D. J. Mater. Chem. 2011, 21, 1551-1561.

Page | 144

(27) He, Y.; Wu, W.; Zhao, G.; Liu, Y.; Li, Y. Macromolecules 2008, 41, 9760-

9766.

(28) Rutherford, D. R.; Stille, J. K.; Elliott, C. M.; Reichert, V. R. Macromolecules

1992, 25, 2294-2306.

(29) Mastragostino, M.; Marinangeli, A. M.; Corradini, A.; Arbizzani, C.

Electrochim. Acta 1987, 32, 1589-1593.

(30) Danieli, R.; Taliani, C.; Zamboni, R.; Giro, G.; Biserni, M.; Mastragostino,

M.; Testoni, A. Synth. Met., 1986, 13, 325-328.

(31) Iovu, M. C.; Sheina, E. E.; Gil, R. R.; McCullough, R. D. Macromolecules

2005, 38, 8649-8656.

(32) McCullough, R. D.; Lowe, R. D.; Jayaraman, M.; Anderson, D. L. J. Org.

Chem., 1993, 58, 904-912.

(33) Mei, J.; Heston, N. C.; Vasilyeva, S. V.; Ryenolds, J. R. Macromolecules

2009, 42, 1482-1487.

(34) Kappe, C. O.; Dallinger, D. Nat. Rev. Drug Discovery 2006, 5, 51-63.

(35) Strauss, C. R.; Varma, R. S. Top. Curr. Chem., 2006, 266, 199-231.

(36) Teffal, M.; Gourdenne, A. Eur. Polym. J., 1983, 19, 543-549.

(37) Giguere, R. J.; Bray, T. L.; Duncan, S. M.; Majetich, G. Tetrahedron Lett.,

1986, 27, 4945-4948.

(38) Hoogenboom, R.; Schubert, U. S. Macromol. Rapid Comm., 2007, 28, 368-

386.

(39) Zhang, C.; Liao, L.; Gong, S. Green Chem., 2007, 9, 303-314.

(40) Sinnwell, S.; Ritter, H. Aust. J. Chem., 2007, 60, 729 - 743.

(41) Nehls, B. S.; Asawapirom, U.; Fuldner, S.; Preis, E.; Farrell, T.; Scherf, U.

Adv. Funct. Mater., 2004, 14, 352-356.

(42) Coffin, R. C.; Peet, J.; Rogers, J.; Bazan, G. C. Nat. Chem., 2009, 1, 657-661.

(43) Tierney, S.; Heeney, M.; McCulloch, I. Synth. Met., 2008, 148, 195-198.

(44) Beinhoff, M.; Bozano, L. D.; Scott, J. C.; Carter, K. R. Macromolecules 2005,

38, 4147-4156.

(45) Wu, Z.; Fan, B.; Li, A.; Xue, F.; Ouyang, J. Org. Elec., 2011, 12, 993-1002.

(46) Tierney, S.; Heeney, M.; McCulloch, I. Synth. Met., 2005, 148, 195-198.

(47) De Souza, M. V. N. Curr. Org. Synth., 2006, 3, 313-326.

(48) Stille, J. K. Angew. Chem. Int. Ed. Engl., 1986, 25, 508-524.

(49) Scott, W. J.; Stille, J. K. J. Am. Chem. Soc., 1986, 108, 3033-3040.

(50) Beletskaya, J. P. J. Organomet. Chem., 1983, 250, 551-564.

(51) M. Kosugi; K. Sasazawa; Shimizu., I.; Migita., T. Chem. Lett., 1977, 6, 301-

302.

(52) Handbook of Organopalladium Chemistry for Oragnic Synthesis Negishi, E.-

i., Ed.; Wley Interscience New York 2002.

(53) Metal -Catalyzed Cross-coupling Reactions Diederich, F.; Stang, P. J., Eds.;

Wiley-VCH: New York 1998.

(54) Huo, L.; Tan, Z.; Wang, X.; Zhou, Y.; Han, M.; Li, Y. J. Polym. Sci.: Part A:

Polym. Chem., 2008, 46, 4038-4049.

(55) Heeney, M.; Bailey, C.; Genevicius, K.; Shkunov, M.; Sparrowe, D.; Tierney,

S.; McCulloch, I. J. Am. Chem. Soc., 2005, 127, 1078-1079.

(56) Guo, X.; Watson, M. D. Org. Lett., 2008, 10, 5333-5336.

(57) De Cremer, L.; Verbiest, T.; Koeckelberghs, G. Macromolecules 2008, 41,

568-578.

(58) Dhanabalan, A.; van Dongen, J. L. J.; van Duren, J. K. J.; Janssen, H. M.; van

Hal, P. A.; Janssen, R. A. J. Macromolecules 2001, 34, 2495-2501.

Page | 145

(59) Zhang, S.; He, C.; Liu, Y.; Zhan, X.; Chen, J. Polymer 2009, 50, 3595-3599.

(60) Hou, L.; Chen, T. L.; Zhou, Y.; Hou, J.; Chen, H. Y.; Yang, Y.; Li, Y.

Macromolecules 2009, 42, 4377-4380.

(61) Liang, Y.; Xu, Z.; Xia, J.; Tsai, S.-T.; Wu, Y.; Li, G.; Ray, C.; Yu, L. Adv.

Mater., 2010, 22, 1-4.

(62) Yuan, M.-C.; Chiu, M.-Y.; Liu, S.-P.; Chen, C.-M.; Wei, K.-H.

Macromolecules 2010, 43, 6936-6938.

(63) Miguel, L. S.; Porter III, W. W.; Matzger, A. J. Org. Lett., 2007, 9, 1005-

1008.

(64) McCulloch, I.; Bailey, C.; Giles, M.; Heeney, M.; Love, I.; Shkunov, M.;

Sparrowe, D.; Tierney, S. Chem. Mater., 2005, 17, 1381-1385.

(65) Stille, J. K.; Lau, K. S. Y. Acc. Chem. Res., 1977, 10, 434-442.

(66) Casado, A. L.; Espinet, P. J. Am. Chem. Soc., 1998, 120, 8978-8985.

(67) Hills, I. D.; Netherton, M. R.; Fu, G. C. Angew. Chem. Int. Ed., 2003, 42,

5749-5752.

(68) Minniti, D. Inorg. Chem. 1994, 33, 2631-2634.

(69) Komiya, S.; Albright, T. A.; Hoffmann, R.; Kochi, J. K. J. am. Chem. Soc.,

1976, 98, 7255-7265.

(70) Ozawa, F.; Ito, T.; Nakamura, Y.; Yamamoto, A. Bull. Chem. Soc. Jpn. 1981,

54, 1868-1880.

(71) Tatsumi, K.; Hoffmann, R.; Yamamoto, A.; Stille, J. K. Bull. Chem. Soc. Jpn.,

1981, 54, 1857-1867.

(72) Casado, A. L.; Espinet, P. Organometallics 1998, 17, 954-959.

(73) Louise, J.; Hartwig, J. F. J. Am. Chem. Soc., 1991, 113, 9585-9595.

(74) Yui, K.; Ishida, H.; Aso, Y.; Otsubo, T.; Ogura, F.; Kawamoto, A.; Tanaka, J.

Bull. Chem. Soc. Jpn., 1989, 62, 1547 - 1555.

(75) Moon, I. K.; Kim, N. Dyes and Pigments 2008, 78, 207 - 212.

(76) Kano, S.; Yunsa, Y.; Yokomatsu, T.; Shibuya, S. Heterocycles 1983, 20,

2035-2037.

(77) Lim, E.; Jung, B.-J.; Shim, H.-K. Macromolecules 2003, 36, 4288-4293.

(78) Kong, H.; Lee, D. H.; Kang, I.-N.; Lim, E.; Jung, Y. K.; Park, J.-H.; Ahn, T.;

Yi, M. H.; Park, C. E.; Shim, H.-K. J. Mater. Chem., 2008, 18, 1895 - 1902.

(79) Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Frechet, J. M. J. Adv.

Mater., 2003, 15, 1519-1522.

(80) Sinnwell, S.; Ritter, H. Aust. J. Chem., 2007, 60, 729-743.

(81) Fischer, F.; Tabib, R.; Freitag, R. Eur. Polym. J., 2005, 41, 403-408.

(82) Huitema, H. E. A.; Gelinck, G. H.; Van der Putten, J. B. P. H.; Kuijk, K. E.;

Hart, C. M.; Cantatore, E.; Herwig, P. T.; Van Breemen, A. J. J. M.; de Leeuw, D. M. Nature

2001, 414, 599.

(83) Roncali, J. Chem. Rev., 1997, 97, 173 - 206.

(84) Farina, V.; Krishnan, B. J. Am. Chem. Soc., 1991, 113, 9585-9595.

(85) Carter, K. Macromolecules 2002, 35, 6757-6759.

(86) Hou, J.; Tan, Z.; He, Y.; Yang, C.; Li, Y. Macromolecules 2006, 39, 4657-

4662.

(87) Schwalm, T.; Wiesecke, J.; Immel, S.; Rehahn, M. Macromol. Rapid Comm.,

2009, 20, 1295-1322.

(88) Fan, Y.-L.; Lin, K.-F. J. Polym. Sci.: A: Polym. Chem., 2005, 43, 2520 - 2526.

(89) Fan, Q.-L.; Lu, S.; Lai, Y.-H.; Hou, X.-Y.; Huang, W. Macromolecules 2003,

36, 6976 - 6984.

Page | 146

(90) Drury, A.; Maier, S.; Ruther, M.; Blau, W. J. J. Mater. Chem., 2003, 13, 485 -

490.

(91) Mutule, I.; Sun, E. Tetrahedron 2005, 61, 11168 - 11176.

(92) Myaura, N.; Suzuki, A. Chem. Rev. 1995, 95, 2457 - 2483.

(93) Hadei, N.; Kantchev, E. A. B.; O‟Brien, C. J.; Organ, M. G. Org. Lett., 2005,

7, 3805 - 3807.

(94) Neef, C. J.; Ferraris, J. P. Macromolecules 2000, 33, 2311-2314.

(95) Coates, J. In Encyclopedia of Analytical Chemistry; Meyers, R. A., Ed.; John

Wiley & Sons Ltd: Chichester, 2000, p 10815 - 10837.

(96) Jørgensen, M.; Norrman, K.; Krebs, F. C. Sol. Energy Mater. Sol. Cells 2008,

92, 686-714.

(97) Cumpston, B. H.; Jensen, K. F. J. Appl. Polym. Sci., 1998, 69, 2451-2458.

(98) Neugebauer, H.; Brabec, C. J.; Hummelen, J. C.; Janssen, R. A. J.; Sariciftci,

N. S. Synth. Met., 1999, 102, 1002-1003.

(99) Kline, R. J.; DeLongchamp, D. M.; Fischer, D. A.; Lin, E. K.; Richter, L. J.;

Chabinyc, M. L.; Toney, M. F.; Heeney, M.; McCulloch, I. Macromolecules 2007, 40, 7960 -

7965.

(100) Osaka, I.; Abe, T.; Shinamura, S.; Miyazaki, E.; Takimiya, K. J. Am. Chem.

Soc., 2010, 132, 5000-5001.

(101) Ong, B. S.; Wu, Y.; Liu, P.; Gardner, S. j. Am. Chem. Soc., 2004, 126, 3378-

3379.

(102) McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.;

Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R.

J.; McGehee, M. D.; Toney, M. F. Nat. Mater., 2006, 5, 328-333.

(103) Hou, J. H.; Tan, Z. A.; He, Y. J.; Yang, C. H.; Li, Y. F. Macromolecules 2006,

39, 4657-4662.

(104) Huo, L.; Chen, T. L.; Zhou, Y.; Hou, J.; Chen, H. Y.; Yang, Y.; Li, Y.

Macromolecules 2009, 42, 4377-4380.

(105) Morteani, A. C.; Sreearunothai, P.; Herz, L. M.; Friend, R. H.; Silva, C. Phys.

Rev. Lett., 2004, 92, 247-402.

(106) Nehls, B. S.; Fuldner, S.; Preis, E.; Farrell, T.; Scherf, U. Macromolecules

2005, 38, 687-694.

(107) Hwang, I. W.; Xu, Q. H.; Soci, C.; Chen, B. Q.; Jen, A. K. Y.; Moses, D.;

Heeger, A. J. Adv. Funct. Mater., 2007, 17, 563-568.

(108) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv.

Mater., 2011, 23, 2367-2371.

(109) Chen, Z.-K.; W. Huang; L.-H. Wang; E.-T. Kang; B. J. Chen; C. S. Lee; Lee,

S. T. Macromolecules 2000, 33, 9015-9025.

(110) Wua, Z.; Fan, B.; Li, A.; Xue, F.; Ouyang, J. Org. Elec., 2011, 12, 893-1002.

(111) Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger,

A. J.; Brabec, C. J. Adv. Mater., 2006, 18, 789-795.

(112) Baran, D.; Balan, A.; Celebi, S.; Esteban, B. M.; Neugebauer, H.; Sariciftci,

N. S.; Toppare, L. Chem. Mater., 2010, 22, 2978-2987.

(113) Lee, J.-K.; Gwinner, M. C.; Berger, R.; Newby, C.; Zentel, R.; Friend, R. H.;

Sirringhaus, H.; Ober, C. K. J. Am. Chem. Soc. 2011, 133, 9949-9951.

(114) Li, W.; Lee, T.; Oh, S. J.; Kagan, C. R. Appl. Mater. Interfaces 2011, 3, 3874-

3883.

(115) Liang, Y.; Feng, D.; Wu, Y.; Tsai, S.-T.; Li, G.; Ray, C.; Yu, L. J. Am. Chem.

Soc. 2009, 131, 7792-7799.

Page | 147

(116) Nie, W.; MacNeill, C. M.; Li, Y.; Noftle, R. E.; Carroll, D. L.; Coffin, R. C.

Macromol. Rapid Commun., 2011, 32, 1163-1168.

(117) Zou, Y.; Najari, A.; Berrouard, P.; Beaupre, S.; Aıch, B. R.; Tao, Y.; Leclerc,

M. J. Am. Chem. Soc., 2010, 132, 5330-5331.

(118) Coffin, R. C.; MacNeill, C. M.; Peterson, E. D.; Ward, J. W.; Owen, J. W.;

McLellan, C. A.; Smith, G. M.; Noftle, R. E.; Jurchescu, O. D.; Carroll, D. L. J. Nanotechno.,

2011, 2011, 1 - 10.

(119) Nie, W.; MacNeill, C. M.; Li, Y.; Noftle, R. E.; Carroll, D. L.; Coffin, R. C.

Macromol. Rapid Commun., 2011 32, 1163 -1168.

(120) Zhang, G.; Fu, Y.; Zhang, Q.; Xie, Z. Chem. Commun., 2010, 46, 4997 - 4999.

(121) Park, J. K.; Jo, J.; Seo, J. H.; Moon, J. S.; Park, Y. D.; Lee, K.; Heeger, A. J.;

Bazan, G. C. Adv. Mater., 2011, 23, 2430 - 2435.

(122) Huo, L.; Guo, X.; Li, Y.; Hou, J. Chem. Commun., 2011 47, 8850 - 8852.

(123) Szarko, J. M.; Guo, J.; Liang, Y.; Lee, B.; Rolczynski, B. S.; Strzalka, J.; Xu,

T.; Loser, S.; Marks, T. J.; Yu, L.; Chen, L. X. Adv. Mater., 2010, , 22, 5468 - 5472.

(124) Gilch, H. G.; Wheelwright, W. L. J. Polym. Sci.: A 1966, 4, 1337 - 1349.

(125) Becker, H.; Spreitzer, H.; Kreuder, W.; Kluge, E.; Vestweber, H.; Schenk, H.;

Treacher, K. Synth. Met., 2001, 122, 105 - 110.

(126) Spreitzer, H.; Becker, H.; Kluge, E.; Kreuder, W.; Schenk, H.; Demandt, R.;

Schoo, H. Adv. Mater., 1998, 10, 1340 - 1343.

(127) Yu, J.; Lee, K. H.; Zhang, Y.; Klein, M. F. G.; Colsmann, A.; Lemmer, U.;

Burn, P. L.; Loa, S.-C.; Meredith, P. Polym. Chem., 2011, 2, 2668 - 2673.

(128) Shin, D.-C.; Kim, Y.-H.; You, H.; Kwon, S.-K. Macromolecules 2003, 36,

3222 - 3227.

(129) Roex, H.; Adriaensens, P.; Vanderzande, D.; Gelan, J. Macromolecules 2003,

36, 5613 - 5622.

(130) Becker, H.; Spreitzer, H.; Ibrom, K.; Kreuder, W. Macromolecules 1999, 32,

4925 - 4932.

(131) Johansson, D. M.; Theander, M.; Srdanov, G.; Yu, G.; Inganas, O.;

Andersson, M. R. Macromolecules 2001, 34, 3716 - 3719.

(132) Johansson, D. M.; Wang, X.; Johansson, T.; Ingana, O.; Yu, G.; Srdanov, G.;

Andersson, M. R. Macromolecules 2002, 35, 4997 - 5003.

(133) Yu, C.-Y.; Turner, M. L. In Complex Macromolecular Architectures:

Synthesis, Characterization, and Self-Assembly, First Edition; Hadjichristidis, N., Hirao, A.,

Tezuka, Y., Prez, F. D., Eds.; John Wiley & Sons: Asia, 2011.

(134) Issaris, A.; Vanderzande, D.; Gelan, J. Polymer 1997, 38, 2571 - 2574.

(135) Vanderzande, D. J.; Issaris, A. C.; Van Der Borght, M. J.; van Breemen, A. J.;

de Kok, M. M.; Gelan, J. M. Macromol. Symp., 1997, 125, 189 - 203.

(136) Issaris, A.; vanderzande, D.; Adriaensens, P.; Gelan, J. Macromolecules 1998,

31, 4426 - 4431.

(137) Wessling, R. A. J. Polym, Sci., Polym. Symp., 1985, 72, 55 - 66.

(138) Denton, F. R.; Sarker, A.; Lahti, P. M.; Garay, R. O.; Karasz, F. E. J. Polym.

Sci., Part A: Polym. Chem., 1992, 30, 2233 - 2240.

(139) Parekh, B. P.; Tangonan, A. A.; Newaz, S. S.; Sanduja, S. K.; Ashraf, A. Q.;

Krishnamoorti, R.; Lee, T. R. Macromolecules 2004, 37, 8883 - 8887.

(140) Huang, C.; Zhen, C.-G.; Su, S. P.; Vijila, C.; Balakrisnan, B.; Auch, M. D. J.;

Loh, K. P.; Chen, Z.-K. Polymer 2006, 47, 1820 - 1829.

(141) Chang, H.-T.; Lee, H.-T.; Chang, E.-C.; Yeh, M.-Y. Polym. Eng. Sci. 2007,

47, 1380 - 1387.

Page | 148

(142) Lee, J.-H.; Kee, I.-S.; Kang, I.-N.; Son, J.-M.; Kim, K.-S.; Park, M.-J.;

Hwang, D.-H. Curr. Appl. Phys., 2009, 9, 861 - 865.

(143) Liao, S.-C.; Lai, C.-S.; Yeh, D.-D.; Rahman, M. H.; Hsu, C.-S.; Chen, H.-L.; Chen, S.-A.

React. Funct. Polym., 2009, 69, 498 - 506.

(144) Becker, H.; Spreitzer, H.; Kreuder, W.; Kluge, E.; Schenk, H.; Parker, I.; Cao,

Y. Adv. Mater., 2000, 12, 42 - 48.

(145) Chen, Z.-K.; Lee, N. H. S.; Huang, W.; Xu, Y. S.; Cao, Y. 2003, 36, 1009 -

1020.

(146) Wiesecke, J.; Rehahn, M. Macromol. Rapid Comm., 2007, 28, 78 - 83.

(147) Hontis, L.; Vrindts. V; Vanderzande, D.; Lutsen, L. Macromolecules 2003, 36,

3035 - 3044.

(148) Brandt, M. W.; Mulvaney, J. E.; Hall, H. K. Macromolecules 1988, 21, 1553 -

1556.

(149) Cho, B. R.; Han, M. S.; Suh, Y. S.; Oh, K. J.; Jeon, S. J. Chem. Comm., 1993,

564 - 566.

(150) Hontis, L.; Van Der Borght, M.; Vanderzande, D.; Gelan, J. Polymer 1999, 40,

6615 - 6617.

(151) Wiesecke, J.; Rehahn, M. Angew. Chem. Int. Ed., 2003, 42, 567 - 570.

(152) Schwalm, T.; Rehahn, M. Macromol. Rapid Comm., 2008, 29, 207 - 213.

(153) Hontis, L.; V, v.; Lutsen, L.; vanderzande, D.; Gelan, J. Polymer 2001, 42,

5793 - 5796.

(154) Shahid, M.; Ashraf, R. S.; Klemm, E.; Sensfuss, S. Macromolecules 2006, 39,

7844 - 7853.

(155) Wen, S.; Pei, J.; Zhou, Y.; Li, P.; Xue, L.; Li, Y.; Xu, B.; Tian, W.

Macromolecules 2009, 42, 4977 - 4984.

(156) Yu, N.; Zhu, R.; Peng, B.; Huang, W.; Wei, W. J. Appl. Polym. Sci., 2008,

108, 2438 - 2445.

(157) Zhang, C.; Nguyen, T. H.; Sun, J.; Li, R.; Black, S.; Bonner, C. E.; Sun, S.-S.

Macromolecules 2009, 42, 663 - 670.

(158) van der Made, A. W.; van der Made, R. H. J. Org. Chem., 1993, 58, 1262 -

1263.

(159) Nakayama, J.; Kawamura, T.; Kuroda, K.; Fujita, A. Tet. Lett., 1993, 34, 5725

- 5728.

(160) Pang, Y.; Hu, B.; Li, J.; Karasz, F. E. Macromolecules 1999, 32, 3946.

(161) Huang, C.; Huang, W.; Guo, J.; Yang, C.-Z.; Kang, E.-T. Polymer 2001, 42,

3929 - 3938.

(162) Zheng, S.-J.; Kun, W.; Kobayashi, T. Mater. Sci. Eng. 2011, 21, 012027.

(163) Loewe, R. S.; McCullough, R. D. Chem. Mater., 2000, 12, 3214-3221.

(164) Xiao-Dong, L.; Su-Ling, Z.; Zheng, X.; Fu-Jun, Z.; Tian-Hui, Z.; Wei, G.;

Guang, Y.; Chao, K.; Yong-Sheng, W.; Xu-Rong, X. Chin. Phys. B 2011, 20, 068801.

(165) Zhang, C.; Matos, T.; Li, R.; Sun, S.-S.; Lewis, J. E.; Zhang, J.; Jiang, X.

Polym. Chem. 2010, 1, 663 - 669.

(166) Lim, E.; Jung, B.-J.; Shim, H.-K. J. Polym. Sci., Part A: Polym. Chem., 2006,

44, 243 - 253.

(167) Herguth, P.; Jiang, X.; Liu, M. S.; Jen, A. K.-Y. Macromolecules 2002, 35,

6094 - 6100.

(168) Yang, R.; Tian, R.; Yan, J.; Zhang, Y.; Yang, J.; Hou, Q.; Yang, W.; Zhang,

C.; Cao, Y. Macromolecules 2005, 38, 244 - 253.

(169) Ho, C.-C.; Liu, Y.-C.; Lin, S.-H.; Su, W.-F. Macromolecules 2011.

Page | 149

(170) Beaujuge, P. M.; Frechet, J. M. J. J. Am. Chem. Soc., 2011, 133, 20009 -

20029.

(171) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger,

A. J.; Brabec, C. J. Adv. Mater., 2006, 18, 789-794.

(172) Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Mühlbacher, D.; Scharber, M.;

Brabec, C. Macromolecules 2007, 40, 1981-1986.

(173) Pridgen, L. N.; K. Huang; Mills, R. J.; Shilcrat, S.; Tickner, A. Synth. Comm.,

1998, 28, 3479 - 3489.

Page | 150

Chapter 3

Novel Donor-Acceptor Acetylene-linked Thieno[3,2-b]thiophene Copolymers Bearing Electron-deficient Units for OFET and

OPV Applications

Page | 151

3.0 Novel Donor-Acceptor Acetylene-linked Thieno[3,2-b]thiophene Co-polymers Bearing Electron-deficient Units for OFET and OPV

Applications

3.1 Introduction

Conjugated polymers bearing an acetylene-bridged arylene units, the so called,

“poly(arylene acetylenes) (PAEs),1-7

are commonly tagged as the dehydrogenated congeners

of the well known poly(phenylene vinylenes) (PPVs). They continue to attract interest due to

their combination of intrinsically rigid structural alignment, formation of photo-oxidative

stable films, and interesting photophysical properties (that is, high electroluminescence and

photoluminescence).8-11

According to Beeby et. al.,10

the majority of these attractive

characteristics stem from the linear and extended π-conjugation that exists along the

molecular axis, which relies heavily on the relative conformation of the planar aromatic units

in the polymer chains.11, 12

In addition, like their PPV counterpart, these class of polymers

also exhibit good solution processability and interesting electro-optical properties6 which has

led to their investigation as the active semiconducting materials in organic light emitting

diode (OLED) technology,13-18

organic field effect transistors (FETs),19, 20

organic

photovoltaics (OPVs),21-26

liquid crystal displays,27, 28

non-linear optics,8 and chemical

sensors.29-31

However, the majority of the reported π-conjugated PAEs generally possess large

HOMO - LUMO energy band gaps (~2.30 to 3.5 eV) which may limit their use in certain

applications like OPV where a relatively narrow band gap is desired. For such OPV

applications, polymers with band gaps that match the maximum solar flux are desired. On

earth, this peaks around 700 nm.2, 24

Similarly, the large band gaps in PAEs may also impair

the OFET performance as too deep a HOMO can present a barrier to charge injection from

common electrode materials (vide infra). Generally, an important pre-requite to design

conjugated polymers with low band gaps is the existence of co-planarity along the polymer

backbone, in turn promotes strong intermolecular electronic interactions and increased

effective polymer conjugation lengths, in addition to enhancing crystallinity in the solid-

state.2, 32, 33

The insertion of the ethynylene moiety between the aryl units as in the case of

PAEs does indeed facilitate the planarization of the polymer chains by reducing steric

interactions between adjacent H atoms/alkyl chains in the ortho positions of biphenylene

rings. However co-planarization on its own does not seem sufficient to result in a low energy

Page | 152

band gap as discussed above. Therefore, strategies to fine-tune the structures of these

important classes of π-conjugated polymers were needed in order to effectively lower their

energy gap. This led to the adoption of the donor-acceptor (D-A) copolymerization concept.

The incorporation of various electron-withdrawing units into the PAE motif has

resulted, in most cases, in the reduction of their energy band gap and a correlated

enhancement of their performance in optoelectronic devices.23, 34

The reduced band gap may

likely be due to the intermolecular charge transfer (ICT) within the polymer chain is induced

by the co-polymerization of electron-rich and electron-deficient arylene units.35, 36

35, 36

In

addition, the donor – acceptor character can lead to intermolecular electrostatic interactions to

form strong π-π stacking structures, which is favourable for effective charge transfer in

OFETs and OPVs.35

Using this concept, Roy and co-workers were able to obtain charge

carrier mobility as high as 0.3 cm2 V

-1 s

-1 in the OFET of a PAE donor acceptor co-polymer.

The donor-acceptor concept has also been widely used in the design of donor

polymers for organic photovoltaic devices.19

Numerous examples of acetylene linked D-A

co-polymers have been reported. In 2005, Ashraf et. al., reported the synthesis of four D-A or

push-pull-type PAE co-polymers bearing electron-donating thiophene (T) units, and electron-

withdrawing benzothiadiazole (BT), and quinoxaline (Q) moieties (see P1 – P4 in figure 1)

with optical band gaps ranging from 1.85 eV to 2.08 eV (figure 1).34

As anticipated, these

PAE co-polymers showed π-stacked polymer structures due to self organization, and the

formation of aggregates in solution.34

Following this success, an unoptimized PCE of 2.37 %

was published for an OPV device fabricated from the thieno[3,4-b]pyrazine-based P6 (figure

1) with an open circuit voltage (VOC) of 0.67 V, a short circuit current (ISC) of 10.72 mA.cm-2

,

and a fill factor (FF) of 0.33.23

The analogous polymer derivative (P7) incorporating a

substituted phenylene unit only gave a PCE of 1.36 % with a VOC of 0.70 V, ISC of 4.45

mA.cm-2

, and a FF of 0.43.23

The observed difference in PCEs was mainly ascribed to the

reduced JSC for P7 over P6 reflecting the larger band gap for this phenylene polymer over the

thiophene. This highlights the importance of band gap engineering in the design of novel low

band gap PAEs, since the HOMO and LUMO levels strongly influence the performance of

OPV device.

In 2009, Zhang and co-workers reported a D-A PAE comprising of an 1,4-

diketopyrrolpyrrole (DPP) and triphenylpyrazoline (TPP) electron-withdrawing units (P8 and

P9).37

The co-polymers displayed an electrochemical band gap (Egecl

) of 1.73 eV (Egopt

= 2.05

Page | 153

eV) and 2.37 eV (2.45 eV). In the same period, the OPV performance (0.16 % PCE) of

another novel DPP and phenylene PAE co-polymer (P10) was reported with a VOC of 0.55 V,

JSC equals to 0.95 mA.cm-2

and a FF of 29.6 %.38

In spite, of the low-lying HOMO level (5.8

eV) the VOC was still relatively low. This together with the abysmal current density and fill

factor points towards the poor charge transport characteristics of the polymer. Palai and co-

workers, also reported a series of ethynyl-bridged co-polymers of DPP (P12 – P15) segments

with the electron-donating 9,9-bis(2-ethylhexyl)-9H-fluorene (P11), triphenylamine (P12),

1,4-dialkoxybenzene (P13 and P14) and 9-(2-ethylhexyl)-9H-carbazole (P15) units.18

The

band gap ranged from 1.8 eV to 2.3 eV, coupled with extremely deep HOMO and LUMO

energy levels (ca. 6.10 - 6.80 eV and ca. 4.1 – 4.3 eV, respectively).18

Recently, one of the

most improved OPV device based on D-A PAE π-systems was reported by Wu and co-

workers.39

The two structurally analogous copolymers (P16 and P17) consisted of alternating

electron-withdrawing 3,7,11-trimethyldodecane-substituted DPP units flanked on either side

by 3,6-thienyl donors, copolymerized with dialkylfluorene. The OPV device based on P17

(Egecl

= 1.72 eV) displayed a PCE of 2.25 % compared to 1.25 % for the P16 (Egecl

= 1.83 eV)

derivative with PC16BM acceptor, under AM 1.5 illumination.39

Page | 154

Figure 1. Structures of donor-acceptor poly(arylene ethynylenes).

Page | 155

The difference in OPV performance between P16 and P17 was tentatively attributed

to the differences in the hole mobilities for the two polymer, as evidenced by the higher

mobility recorded in the P17/PC61BM blend film (3.05 x 10-5

cm-2

V-1

s-1

) compared to that

involving the P16 (1.86 x 10-5

cm-2

V-1

s-1

) counterpart. Moreover, this is also reflected in the

short circuit current generated in the OPV device of the former (4.24 mA.cm-2

), which was

higher than that of the latter (3.41 mA.cm-2

).39

High carrier mobility is important in BHJ solar

cells to ensure the effective transfer of charge to the electrodes, and prevent the loss of

photocurrent through recombination mechanisms.40, 41

The application of diverse copolymers

incorporating the rigid acetylene linkage between aromatic units along the backbone structure

in OPV device has been extensively reviewed by Silvestri and Marrocchi.24

Most of these

conjugated PAEs copolymers have been synthesized via the palladium-catalyzed

Sonogashira-Hagihara reaction (vide infra), which is hereafter referred to as „Sonogashira

cross-coupling reaction‟, as it is widely known.42-45

The impetus for this project relies predominantly on the D-A principle,46

which is

beneficial in regards to achieving lower band gap PAEs. We hereby report the synthesis,

structural, and optoelectronic, and device (OFETs and BHJ OPV) characteristics of several

novel ethynyl-bridged alkyl-substituted thieno[3,2-b]thiophene homopolymers (figure 2, PA1

and PA2), and their corresponding alternating PAE copolymers (figure 2, PA3 – PA8)

comprising of the π-donor thieno[3,2-b]thiophene unit, and selected π-acceptor moieties,

which include 2,1,3-benzothiadiazole (BT), thieno[3,4-c]pyrrole-4,6-dione (TPD), and DPP,

respectively.

Figure 2. Structures of the target PAE homopolymers and copolymers.

Page | 156

The ability of the electron-deficient BT to significantly alter the HOMO and LUMO

energy levels has been utilized in many D-A copolymeric π-systems to obtain high

efficiencies in OPV (up to 6.0 %) and OFET performance.20, 23, 34, 47-54

Similarly, the TPD π-

acceptor moiety was chosen due to its strong electron-withdrawing behaviour, together with a

rigid, symmetric, and coplanar fused structure.51, 55-58

This makes it very suitable for

effectively modifying the energy band gap of the frontier molecular orbitals, with a

concomitant reduction of the HOMO energy level, and increased

intermolecular/intramolecular interactions when conjugated with various electron-donating

moieties.51, 58

Recently, BHJ OPVs device testing reveal PCE between 4.0 % and 6.8 % using

the blend films of TPD copolymers with PCBM as the active layer.57-60

The highly conjugated DPP, first synthesized in 1974,61, 62

is a bicyclic 10π-electron

heterocylic system which consists of two fused lactam units. This combined with its planar

structure makes it a desirable building block for conjugated polymers (in our case, PAE-type

copolymer), as this promotes strong π-π interactions to facilitate efficient charge carrier

mobility in optoelectronic devices.18, 38

Moreover, the electron-withdrawing ability of this π-

acceptor lies in the presence of the lactam groups within its core, with the existence of very

strong π-π intermolecular interactions provided by the two 1H-pyrrolo-2(3H)-one aromatic

system.38, 63-65

Furthermore, DPP possess interesting optical properties (that is, absorption,

electroluminescence, and photoluminescence)66

, and since the synthesis of the first

conjugated DPP copolymers were pioneered by Yu,67, 68

and Tieke,69, 70

it has been

copolymerized with a variety of electron-rich units (that is, thiophene, fluorene, carbazole,

and others) for BHJ OPVs and OFET application.61, 62, 65, 71-75

Unfortunately, BT, TPD and

DPP electron-deficient units have not been widely explored in PAE copolymers.18, 34, 38

This

forms part of our motivation for this investigation. Besides, the copolymer effect, the

influence of the different pendant groups (C16 vs. EH) appended to the thieno[3,2-

b]thiophene aromatic unit on the microstructure, photophysical properties, and device

performance of the conjugated polymers will also form part of this project.

3.1.1 The Sonogashira Cross-coupling Methodology

The palladium-copper-catalyzed (Pd-Cu) coupling reaction (Heck-Cassar-

Sonogashira-Hagihara) of terminal alkynes and aryl halides (chlorides, bromides, iodides, or

triflates) has been in existence since 1975.42, 43, 76, 77

This synthetic protocol is widely

Page | 157

employed in organic chemistry for the formation of carbon-carbon single bonds (C-C)

between a sp- and a sp2-hybridized carbon centre.

43-45, 78 Most important, in relation to the

synthesis of PAE conjugated polymers, the reliability and regioselectivity of the acetylenic

homo- and hetero-coupling processes has enabled access to a catalogue of new conjugated π-

systems.2, 24

As is the case for other Palladium-catalyzed cross-coupling processes, such as

Heck,79, 80

Stille,81, 82

Suzuki,83, 84

and Negishi,44

the Sonogashira mechanism comprises of

three key stages (that is, oxidative addition, transmetallation, and reductive elimination) as

illustrated in scheme 1.85, 86

Scheme 1. Sonogashira-Hagihara cross-coupling cycle and activation of the catalyst.

Page | 158

The catalyst activation stage of the Pd-Cu coupling process involves the generation of

the palladium complex III via the transmetallation reaction of the Pd(II) catalyst [PdL2Cl2]

with the cuprate acetylene compound II, followed by the facile reductive elimination (C) of

the III (unstable) to form the active catalyst (IV) plus a butadiyne by-product (V). Therefore,

in the catalytic cycle, during oxidative addition (A) of IV to an aryl halide (Br or I) a co-

ordinatively saturated intermediate Pd-complex VII is formed, which upon undergoing

transmetallation (B) and reductive elimination (C) generates the desired product X, and then

re-form the active catalyst (IV). In PAE synthesis, selected diethynylarylenes (XI) and dihalo

arenes (XII) are usually coupled under basic conditions (amine solvents), using copper iodide

as a co-catalyst (scheme 2).1 As indicated in scheme 2, the formation of the target PAE XIII

is not without defects.

Scheme 2. Pd-Cu-catalyzed cross-coupling of PAE conjugated polymer.

Mechanistic studies pioneered by Goodson et. al.87

in phenyl-based systems have

shown that the terminal defects (1 and 3, scheme 2) may arise due to competing

dehalogention reaction and the formation of phosphonium salts (with excess phosphine)

during the polymerization.87

On the other hand, diyne defects (2) may be present in the

polymer chain, as a result of the presence of oxygen or the reduction of the Pd2+

catalyst

precursor III (scheme 1).1, 88

Thus, an inert atmosphere is crucial when synthesizing PAEs

via the Sonogashira coupling route. We also did not use a Pd(II) pre-catalyst (which would be

reduced in situ by dimerization of the alkyne), but used a pre-formed Pd(0) catalyst, which

did not require in situ reduction. Furthermore, the majority of reports detailing the cross-

coupling of aryl bromides indicate the use of elevated temperatures (ca. 80 oC).

The polymerization is reported to proceed at a much faster rate and under milder

condition (room temperature), when iodide is utilised as the leaving group, rather than

Page | 159

bromide. This is claimed to drastically reduce the formation of defects and crosslinking.2, 24

The oxidative addition stage (scheme 1 (A)) is faster with aromatic iodides as opposed to the

bromides, which was narrowed down to both kinetic and thermodynamic reasons. In other

words, since the active Pd catalyst is electron rich, the oxidative addition would be strongly

influenced by the type of halide appended to the aromatic unit. Therefore, the less

electronegative the halide group, the faster the rate of oxidative addition.2, 24

On the other

hand, studies by Hertz,89

Swager,90

Wrighton,91

and Moore,92

all indicate that the choice and

amount of Pd-catalyst utilized also influences the outcome of the polymerization and the

molecular weight of the resulting PAE.93

Osakada et. al. have carried out an in-depth

investigation into the role of the CuI co-catalyst and concluded that its presence serves to

facilitate the selective and reversible ethynyl-ligand transfer between the metal centres.94

Whilst, Linstrumello et. al. suggests the formation of ζ- or π-acetylide in order to active the

alkyne towards transmetallation.95

Page | 160

3.2 Synthesis of Monomers and PAE π-Conjugated Polymers

3.2.1 Donor Acetylene Functionalized thieno[3,2-b]thiophene derivatives

The general pathway showing the two stage synthesis of the diacetylenic alkyl-

substituted thieno[3,2-b]thiophene monomers is depicted in scheme 3. In this investigation, a

linear hexadecyl (C16H33) and a branched 2-ethylhexyl (EH) were chosen as side-groups.

Scheme 3. Synthesis of 3,6-dialkyl-2,5-bis(ethyne)thieno[3,2-b]thiophene monomer.

The dibrominated alkyl-substituted thieno[3,2-b]thiophene precursors (Pr1’ and P1”)

were synthesized from the dialkylated thieno[3,2-b]thiophene starting material as described

in chapter 2. Initially, the Pd-catalyzed Sonogashira reaction (step 1, scheme 3) was

performed by reacting 2,5-dibromo-3,6-dihexadecylthieno[3,2-b]thiophene Pr1’ with 2 mole

equivalents of trimethylsilylacetylene (TMSA) Pr2 in the presence of 2 mol %

tetrakis(triphenylphosphine)Pd(0) [Pd(PPh3)4] catalyst, 10 mol % triphenylphosphine (PPh3),

and 2 mol % cuprous iodide (CuI) as activator, in the tertiary triethylamine (NEt3) base at 80

oC for 24 h. The pale-orange crude was purified by silica-gel chromatography to afford the

desired disilylacetylene thieno[3,2-b]thiophene P3’ in yields exceeding ca. 80 %. The purity

of the product was ascertained by disappearance of the aromatic hydrogen resonance peak

Page | 161

usually at δ 6.9 ppm, and the presence of the trimethylsilyl (TMS) CH3 groups at δ 0.29 ppm

in the 1H NMR spectra (fig. 3 (1)). Moreover, the ethynyl-linkage was observed at δ 97.93

ppm and δ 103.24 ppm in the 13

C NMR spectra (fig. 3 (2)).

Figure 3. Identification of the bis(trimethyl)silylacetylene based thieno[3,2-b]thiophene via (1) 1H NMR and

(2) 13

C NMR spectroscopy.

Page | 162

Expt. Pd(PPh3)4

/ Mol %

PPh3

/ Mol %

TMSA

/ equiv.d

Solvent

system

Solvent

Ratio

Yield

(Pr3’) /

%

Yield

(Pr3”) /

%

1 2.0 10.0 2 NEt3 1 98 87

2 2.0 10.0 1 NEt3 1 78e, 80

e 50

c

3a 2.0 10.0 1 NEt3 1 80 71

4 2.0 10.0 2 DIPA/THF 1:1 90 -

5b 4.0 10.0 3 DIPA/Tol

f 1:2 60, 80 60

Table 1. Synthesis of 2,5-dibromo-3,6-hexadecylthieno[3,2-b]thiophene (Pr3’) and 2,5-dibromo-2,5-

bis(2ethylhexyl)thieno[3,2-b]thiophene (Pr3”) under various experimental conditions (CuI = 2 mol % at 80 oC for 24 h).

ascale-up of entry 2.

bCuI = 4 mol %.

cFormation of predominantly monosilylated T32bT.

dMole

equivalent to that of the starting material. eMixture of products.

fToluene solvent.

However, a repeat of the aforementioned procedure in a mixture of a using 1

equivalent of TMSA, a secondary amine (diisopropylamine (DIPA)) and anhydrous THF

(1:1) under similar conditions furnished the product, which was found to consist of a mixture

(in ~80 % yield) of the monosilyacetylene (Pr3’’’), and the unreacted starting material (Pr1)

(entry 2, table 1). Consequently, it was initially surmised that the nature of the base and the

amount of TMSA may have had an effect on the yield of the silylation reaction. Therefore, in

order to distinguish between the effects (if at all valid), of the type of base, the ratio of

catalyst, and the co-solvent used, the reaction was performed using different experimental

conditions, as detailed in table 1.

The investigation of the silylation reaction under different conditions indicated that

the yield of the silylated product was largely independent of the amount of catalyst, co-

catalyst, the nature of the base, and the solvent ratio. However, the use of excess TMSA

(entry 4 and 5) was shown to be crucial in order to furnish the desired products (Pr3’ and

Pr3”).93

After establishing the optimized conditions (entry 1), the silylation reaction was

scaled up for Pr1’ and Pr1” to afford reasonable amounts of materials.

The deprotection (protodesilylation) of the trimethylsilyl (TMS) groups was

investigated in basic conditions using either potassium carbonate (K2CO3) or an aqueous

potassium hydroxide (KOH dissolved in H2O) in methanol to give the diethynyl thieno[3,2-

b]thiophene derivatives (P4’ and P4”). The purity of the latter was probed by GC-MS, 1H-

and 13

C-NMR. The 1H-NMR resonance peaks of the TMS observed between δ 0.24 ppm and

δ 0.30 ppm in the starting material were absent, coupled with the appearance of a single

Page | 163

ethynyl resonance peak centred at δ 3.61 ppm (P4’) and δ 3.81 ppm (P4”).96

The yield of the

diethynylenes could not be accurately ascertained due to their inherent instability in vacuum,

air and light. This was indicated by the intensification of colour or a transformation from

yellowish solid (P4’)/pale-orange oil (P4”) to intense-grey/orange when dried under vacuum

at 40 oC or room temperature. Additionally, the degradation also proceeded when stored or

exposed to light and air. This is not particularly surprising, as the triple carbon-carbon bonds

may be susceptible to photo-oxidation as observed in their vinyl counterpart.97-99

The electron

rich nature of the fused alkylated thienothiophene clearly enhanced the degradation process,

such that the materials could not be usefully stored in the solid state. In order to prevent the

degradation process, the diacetylenes were stored in the solvent (petroleum hexane) in the

dark, where no apparent degradation was observed. They were concentrated in vacuo and

immediately used for the polymerization stage.

3.2.2 Synthesis of Electron-Acceptor Derivatives

3.2.2.1 Synthesis of 1,4-diketopyrrolopyrrole Acceptor with Brominated Thiophene Termini

The electron-deficient 3,6-dithien-2-yl-2,5-dioctyl-pyrrolo[3,4-c]pyrrole-1,4-dione

monomer 4 (scheme 4), popularly known as DPP,62, 67, 100

was synthesized as described in the

literature (scheme 4).71, 72, 101

Thus 2-thiophenecarbonitrile was reacted with 0.5 equivalent of

di-n-butyl succinate in the presence 2 equivalents of potassium tert-petoxide in t-amyl

alcohol at 120 oC. A catalytic amount of iron(III) chloride was added following the literature

protocol. The resulting, poorly soluble, maroon-brown coloured solid (2) was purified by

repeated washings with warm distilled water, and hot methanol. The DPP was alkylated by

reaction with 2.5 equivalents of octyl bromide in the presence of potassium carbonate in

dimethylformamide (DMF) at 130 oC to afford 3 in 35% yield. The low yield may be partly

due to impurities present in the crude DPP 2. The pre-monomer (3) was purified by column

chromatography. The final monomer 4 was brominated with 2 equivalents of N-

bromosuccinimide in chloroform at room temperature. After washing the resulting solid with

warm distilled water and hot methanol, a intense-purple powder 4 was obtained in moderate

yield.71, 101

Page | 164

Scheme 4. Synthesis of 3,6-di(2-bromothien-5-yl)-2,5-dioctyl-pyrrolo[3,4-c]pyrrole-1,4-dione.

3.2.2.2 Synthesis of Dibromo N-octylthieno[3,4-c]pyrrole-1,4-dione Acceptor

This electron-deficient chromophore was synthesized according to procedures

described in the literature.55, 102

Thus commercially available thiophene-3,4-dicarboxylic acid

5103

was treated with excess bromine in glacial acetic acid for 24 hours. The reaction was

quenched with aqueous sodium bisulphate solution until the red colour was not longer

present. Recrystallization of the resulting grey solid from water afforded the 2,5-

dibromothiophene-3,4-dicarboxylic acid precursor in yields of 80 %. This was then reacted

with 4 equivalent of oxalyl chloride in a mixture of DMF and benzene (1:50) to afford the

2,5-dibromothiophene-3,4-dicarboxylic acid chloride 6 in quantitative yield. To access the

target 3,4-(N-n-octylimido)-2,5-dibromothiophene 7, the acid chloride 6 was simply reacted

with n-octylamine at 140 C for 30 minutes, with a yield of ca. 80 %, following prification.56

Scheme 5. Synthesis of 3,4-(N-n-octylimido)-2,5-dibromothiophene electron-withdrawing unit.

3.2.2.3 Synthesis of Dibrominated Benzo[2,1,3]thiadiazole Acceptor

The 4,7-dibromo-2,1,3-benzothiadiazole 9 (scheme 6) was synthesized following

established procedures in the literature.104-107

A stoichiometric amount of the precursor 8,

2,1,3-benzothiadiazole, was selectively brominated with 3 equivalent of bromine (Br2) in

Page | 165

hydrobromic acid (HBr, 48 %). The stirred reaction mixture was refluxed for 2 hours until the

bromine addition was complete. The solution was filtered hot and washed with distilled

water. The resulting solid was recrystallized from methanol to afford 9 in 80 % yield.

Scheme 6. Synthesis of 4,7-dibromo-2,1,3-benzothiadiazole.

3.2.3 Conventional and Microwave-Assisted Sonogashira Synthesis of PAE Homopolymer and Donor-Acceptor Copolymers based on alkyl-substituted thieno[3,2-b]thiophene Moieties

The PAE type homopolymers and copolymers were synthesized via Sonogashira

polycondensation reaction, as shown in scheme 7. Initially, the homopolymer P5’ was

polymerized by the reaction of an equimolar amount of the dibromo monomer M4’ (scheme

3) and with the di-acetylene monomer M5’ in the presence of 6 mol % of Pd(PPh3)4/CuI

under inert atmosphere at 80 oC for 48 h. A mixture of DIPA and toluene (1:3), with the

toluene co-solvent added to aid the solubility of the growing polymer chain. After

precipitation, the crude polymer was obtained as an orange solid. This was further purified by

sequential Soxhlet extraction and re-precipitation to afford the final material in 60 % yield.

The GPC analysis of P5’ taken in chlorobenzene (80 oC) yielded a Mn of 14.0 kDa, Mw of

24.0 kDa, and PDI of 1.71. P5’ was soluble in all chlorinated solvents, including; chloroform,

and chlorobenzene.

Page | 166

Scheme 7. Sonogashira synthesis of acetylene-linked fused thiophene co-polymer derivatives.

A repeat of the polymerization reaction under identical conditions demonstrated good

reproducibility, with a similar molecular isolated. In an attempt to improve molecular weight,

a longer reaction time was utilised (72 h) without success. Therefore, microwave-assisted

heating was investigated. Since toluene does not absorb microwave radiation very readily, we

changed to chlorobenzene as a co-solvent. This is also a better solvent for the growing

polymer chain, and may help improve molecular weight by preventing premature

precipitation of the polymer. By using a solvent mixture of chlorobenzene and DIPA (3:1),

and varying heating conditions for a total of 31 minutes (5 minutes at 100 oC, 6 minutes at

140 oC, and 20 minutes at 160

oC) a polymer was obtained with a Mn of 29 kDa, Mw of 54

kDa, and PDI of 1.86. Studies employing microwave heating in Sonogashira cross-coupling

Page | 167

and similar reactions have all reported enhanced molecular weights, accelerated

polymerization rates, and high polymer yields, in comparison to that of conventional thermal

heating.18, 108-113

In order to synthesis the D-A PAE copolymers, a stoichiometric amounts of

diacetylene monomer M4’ was co-polymerized with one equivalent of the acceptor units, 4

and 9 in a DIPA/toluene solvent mixture (1:3) heated at 80 oC in an oil bath for 48 – 72 hours

to give P7’ and P8’. The number average molecular weights of the conjugated copolymers

were quite low (ca. 7.0 kDa) (table 2). On the other hand, a replication of the Sonogashira co-

polymerizations using microwave-accelerated heating conditions in a mixture of

chlorobenzene and the secondary base (DIPA) gave PAE copolymers P7 – P8 with

comparatively higher molecular weights and yields (table 3). The data from each

polymerization are listed in table 2 and 3.

Conventional Heating Method

Polymer Pd (PPh3)4 /

Mol %

CuI /

Mol %

Duration /

Hours

Yield /

%

Mn /

kDa

Mw /

kDa PDI DP

P5’-C 6 6 48 – 72 60 14.0 24.0 1.71 23

P5”-C 6 6 72 25 7.0 18.0 2.50 18

P7’-C 8 8 72 - 120 36 7.0 16.3 2.33 6

P8’-C 8 8 48 – 72 28 7.0 15.1 2.14 6

Table 2. Sonogashira cross-coupling data under conventional heating in DIPA/toluene (1:3) at 80 oC. Note,

P-c depicts the PAEs synthesized under conventional heating conditions.

Page | 168

Microwave Heating Method

Polymer Pd(PPh3)4 /

Mol %

CuI /

Mol %

Conditions oC (min)

Yield

/ %

bMn /

kDa

cMw /

kDa

dPDI

DP

P5’-M 6 12 100 (5), 140

(6), 160 (20) 77 29.0 54.0 1.86 48

P5”-M 6 12 100 (5), 140

(6), 160 (20) 95 8.0 28.0 3.50 21

P6’-M 6 12 100 (5), 140

(6), 160 (20) 83 8.0 12.1 1.50 9

P6”-M 6 12 100 (5), 140

(6), 160 (20) 86 12.0 25.2 2.20 18

P7’-M 6 12 100 (5), 140

(6), 160 (20) 76 13.5 101 7.40

a 12

P7”-M 6 12 100 (5), 140

(6), 160 (20) 70 14.5 30.0 2.07 16

P8’-M 6 12 100 (5), 140

(6), 160 (20) 97 11.0 21.0 1.97 14

P7”-M 4 4 100 (5), 140

(6), 160 (40) 62 7.1 19.0 2.60 8

P7”-M 7 4 100 (5), 140

(6), 160 (40) 35 14.0 29.6 2.10 16

Table 3. Sonogashira cross-coupling data under sequential microwave heating conditions in 1:3

DIPA/chlorobenzene solvent mixtures under anaerobic atmosphere. a

Multimodal mass distribution curve in

GPC measurement, due to aggregation and solubility in hot CB. b, c, d

Ascertained by GPC with polystyrene as

standard and CB as solvent at 80 oC column temperature.

From the data presented in table 2, it is immediately apparent that the application of

thermal heating in the Sonogashira coupling reactions formed polymers with low yields,

except in the case of P5’-C, which showed moderate yield. This may be attributed to

inefficient coupling or the higher amount of oligomers generated and subsequently eliminated

during purification. This is also reflected in the low molecular weights obtained for both the

homopolymers (entry 1 and 2, table 1) and copolymers (entry 3 and 4, table 1), despite the

long duration times of each polymerization and the balanced Pd/Cu co-catalyst stoichiometry.

Page | 169

Therefore, several alterations were made in the microwave-assisted Sonogashira coupling

reactions. This involved doubling the molar equivalent of the CuI co-catalyst with respect to

the Pd-catalyst, since it is known to play a deciding role in transferring the alkyne ligand to

the metal centre of the arylpalladium halo complex, and the generation of the active catalyst

(see scheme 1).94

This approach in conjunction with the application of the microwave reaction

condition was clearly advantageous. As shown in table 2, a dramatic rise in the yield of both

the homopolymers (entry 1 and 2) and copolymers (entry 3- 7) were observed. This suggests

that the polymerization is more efficient and fewer oligomers were eliminated during

purification in methanol, acetone, and hexane. Moreover, the polymer crude obtained in each

experiment (entry 1 – 5) after microwave irradiation was more viscous and intensely-

coloured compared to those synthesized by conventional heating (at equivalent

concentrations). The molecular weights (29.0 kDa vs 14.0 kDa) of the C16 P5’-M (as wine-

red solid film) homopolymer was double that of P5’-C (as brick-red compact solid). This was

also the case for the EH bearing P5”-M and P5”-C polymer derivatives, albeit to a lower

degree (8.0 kDa vs 7.0 kDa). However, the Mw and PDI of the former (28.0 kDa and 3.5)

were significantly bigger than that of P5”-C (18.0 kDa and 2.50), which may be as a result of

aggregation of adjacent polymer chains in solution, even in low concentrations. Not

surprising, the copolymers comprising of the dithienyl DDP electron-withdrawing moiety

P7’-C and P7’-M also displayed similar trend. The Mn, Mw and PDI of the latter (13.5 kDa,

101.0 kDa, and 7.4) were substantially higher relative to the former (7.0 kDa, 16.3 kDa, and

2.33), due the observation of a multimodal mass distribution peaks during GPC analysis. This

is most certainly due to either the formation of polymer clusters or the limited solubility of

the polymer in CB at low temperatures (˂ 90 oC). Overall, the microwave-irradiation

conditions resulted to the higher polymer yields, with contributions from the increased

amount of the Cu-co-catalyst.

Furthermore, in order to differentiate between the effect of the Pd-Cu catalyst

stoichiometry and the microwave conditions, the amount of catalysts were kept uniform with

slight modulation of the microwave conditions. The outcomes of the polymerization indicate

a slight reduction in the polymer yield and lower molecular weight values. Interestingly,

when the amount of the Pd catalyst was doubled for the same coupling reaction, the yield of

the resulting copolymer was drastically reduced with improved molecular weights. These

Page | 170

modulated reactions highlights the importance of stoichiometric balances between the

palladium and cuprous iodide catalyst couple in the microwave-accelerated Sonogashira

cross-coupling polymerizations.94, 95

The conjugated homopolymers (P5), and copolymers (P6 – P8) were all soluble in

chlorinated solvents, such as chloroform (CF), chlorobenzene (CB), dichlorobenzene (DCB),

tetrachloroethane (TCE), and trichlorobenzene (TCB), as well as ethereal solvents

(tetrahydrofuran (THF)), aromatics (toluene (Tol)), and even dimethylsulfoxide (DMSO)

with some exception for P7’-M. Due to the high molecular weight nature of this copolymer,

its absolute solubility was only possible at elevated temperatures (˃ 100 oC) in the

aforementioned solvents, with the exclusion of CF and the aprotic solvents. The solubility of

these copolymers is a testament to the ability of the lengthy C16 alkyl and branched EH

substituents to induce high solubility when appended to the conjugated polymer skeleton,

which is crucial for device fabrication.

3.2.4 Attempted PAE Synthesis

3.2.4.1 Thiazole-Bearing Donor-Acceptor Poly(arylene ethynylene) Derivatives

We also investigated the synthesis of a series of novel donor acetylene-bridged

copolymers utilising fused thiazole114, 115

as the acceptor units as illustrated in scheme 8. The

thiazole monomers were provided by Dr Mohammed Al-Hashimi from a separate project in

the group. Synthesis was attempted under the conditions above, coupling fused thiazole unit

(A1) with one equivalent of the di-acetylene monomer (M4’) in a mixture of DIPA and CB in

the presence of Pd-Cu co-catalyst. As above, the microwave heating was ramped from 100 oC

to 160 oC for a total of 31 minutes. During attempted purification we observed that the

resultant copolymer completely dissolved in hexane, indicating that a low molecular weight

was likely. This was confirmed by GPC analysis (Mn = 3.0 kDa, PDI = 1.2). In addition,

modification of the coupling conditions, using a mixture DIPA and toluene (1:3) yielded a

similar outcome.

Page | 171

Scheme 8. Synthesis of thiazole-bearing PAE copolymers.

In another reaction involving the same conditions as explained above, the bisthiazole

monomer A2 was utilized. In this case, the resulting copolymer PTA2 was obtained in 80 %

yield (purple solid). The GPC trace (bimodal) obtained before purification showed a Mn, Mw,

and PDI of 8.5 kDa, 48.5 kDa and 5.8. However, after Soxhlet extraction in methanol,

acetone, and hexane, the crude polymer could not be redissolved in any solvent. This may be

due to the fact that the C16 side-groups may not be sufficient to ensure sufficiently solubility

in solution. Therefore, the polymerization was replicated using the highly solubilising EH

branched pendant groups (M4”). On this occasion, the copolymer PTA3 crashed out of the

solvent during microwave irradiation. However, when repeated using a higher solvent

volume, the resultant copolymer became intractable after purification. This led to their

exclusion from this investigation.

Page | 172

3.2.4.2 PAE Homopolymers Synthesis Via Microwave-assisted Stille cross-coupling Reaction

The polymerization of the PAE homopolymers via an alternative microwave-

accelerated Stille cross-coupling pathway was also investigated, as depicted in scheme 9.

Scheme 9. PAE synthesis via microwave-accelerated Stille cross-coupling methodology.

The coupling of the dibrominated thieno[3,2-b]thiophene monomers (M5’ and M6)

with the organostannane, trans-1,2-tri-n-butylstannyl)ethylene (TTBSE) in chlorobenzene

using the microwave conditions shown in scheme 9, resulted in the formation of oligomeric

PAE homopolymers of the type PDC12TTE-S (P1) and PDC16TTE-S (P2) (scheme 9). This

may be attributed to the low purity of the commercial organotin reagent (ca. 95 %). Longer

polymerization times [140 oC (4 minutes), 160

oC (4 minutes), and 180

oC (25 minutes)] did

not help, and low weight oligomers were obtained. Unfortunately, there were no

improvements in the molecular weights of either conjugated polymers (Mn = ca. 3.0 kDa, Mw

= ca. 4.0 kDa, and PDI = 1.4).

The low molecular weights found with the Stille polymerisation were surprising, since

Matzger at al.,116

reported a similar method to prepare the PAE homopolymer from 3,6-

dinonylthieno[3,2-b]thiophene. In his case lower reaction temperatures and longer reaction

time were used. The resulting polymer was found to be soluble in THF, and high molecular

weight (Mn = 58 KDa, PDI 2.1). The improved results in his case compared to ours may be

Page | 173

due to a higher purity of organotin reagent, or possibly to the lower temperatures used. We

compare the reported properties of the polymer of Matzger to that of P5 in the subsequent

sections.

3.3 Polymer Characterization

3.3.1 Nuclear Magnetic Resonance (NMR) Spectroscopy

The structures of the polymers was probed by 1H NMR spectroscopy in deuterated

1,2-dichlorobenzene-d2 (o-DCB-d2) as reference solvent at 100 oC. Figure 4 shows the

1H

NMR spectra of the ethynylene-spaced homopolymers, P5’ and P5”.

Page | 174

Figure 4. 1H-NMR spectra of poly(3,6-hexadecylthieno[3,2-b]thiophene-2-yl ethynylene) – PDC16TTE-M

(P5’) and poly(3,6-bis(2-ethylhexyl)thieno[3,2-b]thiophene-2-yl ethynylene – PBEHTTE-M (P5”) in o-DCB-

d2 at 100 oC.

The spectra are complicated by the tendency of the polymers to aggregate in solution,

which leads to line broadening. However, some assignments can still be deduced from them.

As shown in figure 4, the methylene signals of the alkyl side-chains are clearly visible as

broad signals between 3.04 - 2.82 ppm (P5’) and 3.17 - 2.87 ppm (P5”). Here it is unclear

why the peaks appear to be split into 2 signals for the straight chain polymer (P5’) and three

for the branched polymer (P5”). The signals integrate correctly for the expected number of

protons versus the rest of the alkyl chains present. We also note Matzger et al.,116

also

reported broad signals in his analogous nonyl substituted homo polymer, with broad signals

from 3-2.38 (ppm).

One possible reason for the splitting could be due to very high degrees of butadiyne

defects within the polymer backbone, since this would lead to two different environments for

the methylene protons on the alkyl spacer. If this was the case, based on the integration

values for P5’ and P5”, butadiyne would be approximately 40% of the backbone linkages.

We would expect to see such high percentages of defects very clearly in the IR spectra for

both polymers, due to the differences in absorption for alkyne versus butadiyne links (vide

infra). The fact that only one significant IR absorption band was observed for both

Page | 175

homopolymers, suggests, that very high amounts of butadiyne defects can be ruled out.

Similarly, the relatively high degrees of polymerization for both polymers (>20), would rule

out end-groups effects. We therefore suspect that the splitting of these peaks is indicative of

some restricted rotation about the alkyne bond, which leads to two different types of

environment for the alkyl chains. Thus for example the alkyl chains of adjacent thiophenes

can be either „trans‟ or „cis‟ with respect to the triple bond. For simple dimers and low

molecular weight compounds, rapid rotation in solution would average these out on the NMR

timescale. However in the case of the polymer, rotation would be slowed by aggregation and

partial chain planarization effects, and this may be responsible for the peak broadening.

The 1H NMR spectra of the D-A copolymers comprising of OTPD acceptors (P6’ and

P6”) (figure 5), also exhibited broadened resonance peaks as observed above, which again

creates some challenges in ascertaining the correct peak area ratio (integration).

Page | 176

Figure 5. 1H NMR spectra of poly(3,6-bis(2-ethylhexyl)thieno[3,2-b]thiophene-2,5-yl ethynyl-alt-2,8-(N-

octyl)thieno[3,4-c]pyrrole-4,6-dione) – PDC16TTE-OTPD-M (P6’) (in o-DCB-d2 at 100 oC) and poly(3,6-bis(2-

ethylhexyl)thieno[3,2-b]thiophene-2,5-yl ethynyl-alt-2,8-(N-octyl)thieno[3,4-c]pyrrole-4,6-dione) –

PBEHTTE-OTPD-M (P6”) (in CDCl3 at room temperature).

In the case of P6’, the resonance peaks centred at δ 1.97 ppm, and 3.18 ppm, can be

attributed to and protons of the linear C16 side chain. The characteristic chemical shifts

of the imine N-α-CH2 and N-β-CH2 groups on the N-octylthieno[3,4-c]pyrrole-4,6-dione

moiety appear at δ 3.89 ppm and δ 2.08 ppm, as observed in the literature for similar

copolymer structures.59, 117, 118

In the case of the EH-bearing analogue (P6”), the α-CH2 and

β-CH groups of the EH pendant groups appeared at δ 2.82 ppm and 2.31 ppm, as seen in the

corresponding homopolymer (P5”). Further assignments are highlighted in the figure 5.

Figure 6 shows the 1H NMR spectra of the P7’ and P7” copolymers comprising of

octyl-substituted DPP electron-withdrawing unit. The straight chain polymer (Mn = 13.5

kDa, PDI = 7.4) is better resolved than the branched derivative (Mn = 14.0 kDa, PDI = 2.1),

may be due to differences in their molecular weights. Unlike the OTPD-bearing PAEs above,

the chemical shifts of the signal due to the α-CH2 groups on the lactam nitrogen (N) of the

ODPP unit in P7’ and P7”, are shifted downfield to δ 4.37 ppm and 4.34 ppm (figure 6).18, 38

According to previous studies, this may be due to the stronger electron-withdrawing effect

exerted by DPP acceptor unit compared to that of TPD fused ring. The signals for the

Page | 177

thiophene H atoms are positioned at δ 7.56 ppm and δ 9.32 ppm for both DPP copolymers, as

anticipated.39, 119

All other peaks are assigned as shown in figure 6.

Figure 6. 1H NMR spectra of poly(3,6-hexadecylthieno[3,2-b]thiophene-2,5-yl ethynyl-alt-3,6-dithien-2-yl-

2,5-dioctylpyrrolo[3,4-c]pyrrole-1,4-dione-5‟,5”-diyl) – PDC16TTE-ODPP-M (P7’) and poly(3,6-bis(2-

ethylhexyl)thieno[3,2-b]thiophene-2,5-diyl ethynyl-alt-3,6-bis(thien-2-yl)-2,5-dioctylpyrrolo[3,4-c]pyrrole-

1,4-dione-5‟,5”-diyl) – PBEHTTE-ODPP-M (P7”) in o-DCB-d2 at 100 oC.

Page | 178

Figure 7 shows the 1H NMR spectra of the BT-containing PAE copolymer (P8’). The

resonance signal of the aromatic H atoms on the BT acceptor can be observed as a multiplet

at δ 7.86 ppm, as seen in other analogous BT PAEs and non-PAE type BT polymers.50, 120, 121

Furthermore, two extra signals are noticeable at δ 1.67 pp and 1.80 ppm, which could not be

clearly identified. However, the peaks at δ 1.08 ppm, 1.49 ppm, 2.20 ppm, and 3.34 ppm,

were assigned to the aliphatic side chain, as shown in figure 7.

Figure 7. 1H NMR spectra of poly(3,6-dihexadecylthieno[3,2-b]thiophene-2,5-diyl ethynyl-alt-

benzo[1,2,5]thiadiazole-4,7-diyl) – PDC16TTE-BT-M (P8’) in o-DCB-d2 at 100 oC.

3.3.2 ATR-FT Infrared Spectroscopy

This is a powerful analytical tool used extensively in organic chemistry and is

instrumental in the identification of key functional groups within a conjugated polymer

structure.122

Therefore, in order to further confirm the successful polymerization of the novel

PAE type homopolymers, PDC16TTE-M (P5’-M) and PBEHTTE-M (P5”-M), and their

copolymer derivates, PDC16TTE-OTPD (P6’-M), PBEHTTE-OTPD (P6”-M), PDC16TTE-

ODPP-M (P7’-M), PBEHTTE-ODPP-M (P7”-M), and PDC16TTE-BT-M (P8’-M), the

structural constituents of each acetylene-linked conjugated polymer was unravelled by

measuring their IR spectra (figure 8) in the solid state. As shown in figure 8, all the PAEs

exhibited similar aliphatic methylene (-CH2), and methyl (-CH3) C-H stretching vibration

Page | 179

bands at ca. 2858 cm-1

(symmetric), ca. 2925 cm-1

(asymmetric) and ca. 2960 cm-1

(asymmetric) of varying intensities.122

1000 1500 2000 2500 3000 3500 4000

1109 1393 14662129

833 1105 13801455

1349954

1233 1400

1346833

884

1179

1466

1086

1377

1462

1706 1753

2180

734

81

3

11

00

12

27

13

69

14

55

15

54

1664

1743 2858

2925

PBEHTTE-ODPP-M

PDC16

TTE-ODPP-M

PBEHTTE-OTPD-M

PDC16

TTE-OTPD-M

PDC16

TTE-BT-M

PBEHTTE-M

T

ran

sm

itta

nce

Wavenumber (cm-1)

PDC16

TTE-M

2960

Figure 8. Infrared spectra of the poly(thieno[3,2]thiophene ethynylene) conjugated copolymers recorded in the

solid state and at room temperature (see appendix for high intensity IR spectra).

Importantly, the characteristic ethynylene (C≡C) vibration band located between 2100

cm-1

and 2200 cm-1

, was observed in both the homopolymer and copolymer PAEs, with the

absence of the at terminal (C≡C-H) band, which is usually expected at 3290 cm-1

.8, 18, 34, 108,

122 However, in light of the absence of MALDI-TOF mass spectroscopy data, the latter

cannot be accurately corroborated. In addition, the location of the alkynyl shift moved from

2129 cm-1

in the homopolymers closer to 2180 cm-1

for the D-A polymers. In addition the

intensity of the stretch increased, may be due to the reduction in symmetry and the

enhancement in dipole characterization for the D-A polymer. We are not able to completely

rule out the presence of some butadiyne links (defects) by IR. For 1,4-diphenylbutadiyne the

alkyne stretch occurs at 2250 cm-1

, versus 2220 cm-1

for phenyl acetylene, with a slight

increase in intensity.123

Thus we believe the conjugated polymers consist of predominately

disusbstituted C≡C spacers without the monosubstituted type C≡C-H (of the starting

material), as observed in similar DPP and BT containing PAE copolymers.18, 34, 108

In

Page | 180

addition, the ethynylene C-H out-of-plane vibration normally at 680 cm-1

is also non-existent

in the IR of each conjugated polymer, as anticipated.122

The presence of the two aromatic carbonyl groups (C=O) on the TPD and DPP

acceptor units are confirmed by the presence of the bands at ca. 1706 cm-1

/1753 cm-1

(P6‟

and P6”) and 1664 cm-1

/1743 cm-1

(P7’-M and P7”-M). Located close to these signals are the

C=C-C aromatic ring stretching vibrations (1455 cm-1

– 1466 cm-1

, and ca. 1554 cm-1

) for

both the homopolymer and copolymer PAEs. The broad peak signifying the existence of the

C-N stretching of the TPD tertiary amine bearing copolymers (P6’-M and P6”-M) are

observed at ca. 1377 cm-1

. In addition, those of P7’-M and P7”-M are visible at 1400 cm-1

and 1369 cm-1

, respectively. The C-H stretching signals of the thienylene terminals on the

DPP units can be observed at 1233 cm-1

(P7’-M) and 1227 cm-1

(P7”-M). Moreover, that of

the BT unit can be seen as a weak peak centred at 1179 cm-1

. Furthermore, a weak C=N peak

is observable at ca, 1555 cm-1

for the P8’-M. In summary, the 1H NMR in conjunction with

the IR spectra clearly affirms the structures of the conjugated PAEs synthesized.

Page | 181

3.3.3 Absorption and Emission Characteristics

The absorption characteristics of the PAE homopolymers (PDC16TTE-M - P5’-M and

PBEHTTE-M - P5”-M) recorded at room temperature in chlorobenzene solution and as-spun

thin films from the same solvent are presented in figure 9. As shown in figure 9, both the

linear (P5’-M) and branched (P5”-M) PAE homopolymers displayed a two absorption peaks

in the short wavelength region of the spectra.

300 400 500 600 700 800 900 1000

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d A

bso

rba

nce

(a

.u)

Wavelength (nm)

PC16

TTE-M Soln

PBEHTTE-M Soln

PC16

TTE-M Film

PBEHTTE-M Film

Figure 9. Absorption spectra of linear and branched acetylene-linked thieno[3,2-b]thiophene homopolymers

(P5’ and P5”) in chlorobenzene solution and spin-casted thin films at a speed of 1000 rpm for 70 s .

The π-π* transition peak involving the conjugated polymer chains are observed the

wavelength region from 360 nm to 560 nm.124, 125

In contrast to the vinylene containing

polymers, a bathochromic shift of 21 nm was observed between the λmax of C16 bearing PAE

(479 nm) relative to that of the EH derivative (458 nm) in solution. This is indicative of the

formation of a better structural order partially promoted by the linear C16 pendant groups on

the polymer backbone in solution. Moreover, PDC16TTE-M (P5’-M) possess a longer

effective conjugation length compared to PBEHTTE-M (P5”-M), as shown by the

differences in DP (48 vs 21, see table 3). Compared to the solution absorption of P3HT (448

nm), poly(2,5-bis(dodecyloxy)phen-2-yl ethynylene) (PBDDPE - 453 nm), and the C12-

sustituted thiophene-based PAE (451 nm), the incorporation of the fused thienothiophene red

shifts the polymer absorption in solution slightly, suggesting enhanced delocalization along

the backbone and perhaps the ability to planarize in solution due to the acetylene spacer

reducing strain between the alkyl side groups, and adjacent thiophene (or phenyl) rings.

Page | 182

In the solid state, both polymers exhibit modest red shifts of about 20 nm with the

λmax of P5’-M (498 nm) red shifted by 19 nm with respect to that of P5”-M (478 nm). The

longer wavelength absorption of the C16 polymer may reflect the enhanced degree of

polymerization over the EH analogue, or may indicate that the straight chain polymer is

better able to pack and planarize in the solid state. Additional evidence of the ability of the

C16 to pack closely is seen in the observation of the vibronic structure in the solid state UV,

with a distinct shoulder at 537.126

In comparison to the vinylene-linked conjugated polymer

counterparts, PDC16TTV (λmax = 578 nm, λedge = 730, Egopt

= 1.70 eV) and PBETTV (λmax =

588 nm, λedge = 721 nm, Egopt

= 1.72 eV), investigated in chapter 2, the thin film absorption

bands of P5’-M and P5”-M are largely blue shifted, in common with phenylene substituted

polymers (PPE vs PPV). This has been previously ascribed mainly to electronic effects, with

the reduced ability of the acetylene group to fully delocalise although steric affects may also

play a role. Nonetheless, the effective conjugation length of P5’-M is more extended than

that of the branched EH derivative P5”-M, as evidenced by the lower band gap observed for

the former (2.03 eV vs. 2.14 eV). The optical band gap of P5’-M is substantially narrower

than that of the phenyl-based derivative PBDDPE (2.73 eV).8 Table 4 shows the absorption

and fluorescence data of the PAE homopolymers.

Homopolymer

Solution Absorption (nm) Solid State Absorption (nm)

λmaxa λedge

b Eg

opt (eV) λmax λedge Eg

opt (eV)

P5’-M 479 530 2.34 497 (537)c 610 2.03

P5”-M 458

539 2.30 478 580 2.14

Table 4. Absorption characteristics of PDC16TTE-M - P5’-M and PBEHTTE-M - P5”-M conjugated PAEs. aMaximum absorption in the long wavelength direction.

bEnergetic edge or onset of absorption of the UV-vis

peak. The optical band gaps were deduced from the equation, Egopt

= 1240/λonset. Where λonset represent the

absorption edge in the long wavelength region. cShoulder peak.

The fluorescence (or photoluminescence – PL) spectra of the solution and solid state

PAEs are exhibited in figure 10 and 11. In solution, the excitation of PDC16TTE - P5’-M at

the absorption maxima (479 nm) produced a fluorescence signal with a maximum PL

intensity (λmax(PL)) of 526 nm, accompanied by a weak vibronic shoulder at 556 nm.

Surprisingly, similar λmax(PL) (522 nm) was observed in the case of the branched side chain

Page | 183

counterpart (P5”), with a slightly shifted and very weak vibronic shoulder centred at 567 nm.

These observations indicate that the conjugated PAEs both adopted similar conformation in

solution, with slight aggregation of the polymer chain, which is more enhanced in the

branched derivative (11 nm red shift).

300 400 500 600 700 800 900 1000

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d In

ten

sity (

a.u

)

Wavelength (nm)

PDC16

TTE-M UV Soln

PBEHTTE-M UV Soln

PDC16

TTE-M PL Soln

PBEHTTE-M PL Soln

Figure 10. Absorption and fluorescence spectra of the acetylene-linked PAEs, PDC16TTE-M - P5’-M (ex. λ =

479 nm) and PBEHTTE-M - P5”-M (ex. λ = 458 nm) in chlorobenzene solution.

However, in the solid state (see figure 11) a different scenario ensued. Broader and

bathochromically shifted fluorescence peaks are observed for both polymers, in relation to

their solution traces (figure 10). The PDC16TTE-M - P5’-M derivative displayed an emission

spectra with a λmax(PL) of 720 nm with a pre-shoulder at 660 nm. On the other hand, the

λmax(PL) of the P5”-M derivate (616 nm) was hypsochromically shifted by 104 nm relative to

that of the linear C16 side chain P5’-M polymer. Such significant blue shift in emission

intensity may be ascribed to the disruption of conjugation effected by the steric hindrance of

the bulky branched EH side chain in PBEHTTE-M - P5”-M during excitation. In solution,

this out-of-plane twisting of the polymer backbone is greatly minimised, due to less

intermolecular interaction between neighbouring polymer chains, hence the small (2 nm)

bathochromic shift observed.

Studies have shown that the fluorescence spectral features and dynamics of the

excited state of PAEs are influenced by the twisting of the aromatic rings with respect to the

planar conformation.127, 128

This is because in the ground state PAEs are known to favour the

planar configuration, where it possesses a lower energy minimum.127

As a consequence of

Page | 184

this shallow ground-state potential, there is a broad distribution of planar configurations in

the ground state.127

However, this favourable planar conformation can be easily perturbed

based on the nature of the alkyl side chain along the PAE backbone, which would in-turn

result to a large blue shift (104 nm) in emission intensity, as observed in figure 10 and 11. In

the solid state, the close interaction of the polymer chains implies that the disruption of the

planarity or crystallinity of the polymer chains would be more enhanced in the branched EH-

bearing homopolymer PBEHTTE-M - P5”-M.

300 400 500 600 700 800 900 1000

0.0

0.2

0.4

0.6

0.8

1.0

1.2

No

rma

lize

d In

ten

sity (

a.u

)

Wavelength (nm)

PC16TTE-M UV Film

PBEHTTE-M UV Film

PC16TTE-M PL Film

PBEHTTE-M PL Film

Figure 11. Absorption and fluorescence spectra of the acetylene-linked thieno[3,2-b]thiophene homopolymers

PDC16TTE-M - P5’-M (ex. λ = 497 nm) and PBEHTTE-M - P5”-M (ex. λ = 478 nm) films.

Table 5 shows a summary of the data deducted from the solution and solid film

fluorescence spectra of the PAE homopolymers. PDC16TTE-M - P5’-M shows a smaller

Stokes shift in solution compared that of PBEHTTE-M - P5”-M (0.22 eV vs. 0.33 eV).

However, the reverse scenario is observed in the solid-state, where the disparity between the

Stokes shift value between P5’-M and P5”-M is much larger (0.20 eV). Again these

differences may be related to the supposed conformational changes within the polymeric

backbone, which would inadvertently affect the coplanarity of the polymer chains during

excitation.

Page | 185

Polymer

Solution / nm

ΔEStokesc

/eV (nm)

Thin Film /nm

ΔEStokes

/eV (nm)

λ(em. max)

(PL)a

λmax

(UV)

λ0-0

(PL)b

λ(em.max)

(PL)

λmax

(UV)

λ0-0

(PL)

P5’-M 520 479 556 0.21 (41) 720 497 621 0.78 (223)

P5”-M 522 458 567 0.33 (64 ) 616 478 0 0.58 (138)

Table 5. Fluorescence properties of PDC16TTE-M - P5’-M and PBEHTTE-M - P5”-M in solution and

solid film. aMaximum emission intensity.

bWavelength of the first vibronic shoulder band peak in the PL

spectrum. cStokes shift estimated from the difference between the emission and UV-vis absorption maxima.

The UV-visible absorption spectra of the D-A PAE copolymers in chlorobenzene

solution are shown in figure 12. PDC16TTE-BT-M (P8’-M) shows a distinctive peak between

400 nm and 650 nm, with a λmax of 545 nm. The latter is red shifted by 66 nm compared to

the corresponding donor homopolymer P5’-M, which is may be attributed to the expansion of

the π-conjugated system by the incorporation of the electron-deficient BT unit into the

polymer chain. This also resulted to the reduction of the optical band gap from 2.34 eV to

1.91 eV.

300 400 500 600 700 800 900 1000

0.0

0.2

0.4

0.6

0.8

1.0

1.2

No

rma

lize

d A

bso

rba

nce

(a

.u)

Wavelength (nm)

PDC16

TTE-BT-M

PDC16

TTE-OTPD-M

PBEHTTE-OTPD-M

PDC16

TTE-ODPP-M

PBEHTTE-ODPP-M

Solution

Figure 12. Absorption spectra of the PAE type donor-acceptor copolymers in chlorobenzene solution.

In addition, PDC16TTE-BT-M (P8’-M) possesses a bathochromically shifted λmax in

comparison to the analogous copolymers with 2,5-hexyloxybenzene (513 nm, Egopt

of 2.16

eV)53

and 3,4-didodecylthiophene (540 nm and Egopt

of 1.95 eV)34

. This suggests that the

fused thiophene also contributes to the extension of the conjugation length of the D-A π-

system. The OTPD-acceptor-bearing copolymers PDC16TTE-OTPD-M (P6’-M) and

Page | 186

PBEHTTE-OTPD-M (P6”-M) both exhibited similarly shaped absorption bands which cover

most of the short wavelength region of the spectra. P6’-M shows a comparatively red shifted

λmax (46 nm) relative to that of P6”-M (539 nm vs. 493 nm). This difference may be as result

of the impact of the linear C16 and branched EH alkyl side chains on the alignment of the

repeat units along the polymer backbone. Interestingly, the absorption maxima of P6’-M is

blue shifted by 82 nm with respect to that PDC16TTE-BT-M - P8’-M. Such surprisingly large

displacement signifies either a stronger intramolecular interaction promoted by the BT unit

and/or the extended conjugation in P8’. Though, the latter possess a slightly lower optical

band gap than that of P6”-M (1.88 eV vs. 1.91 eV).

Strikingly, PDC16TTE-ODPP-M (P7’-M) and PBEHTTE-ODPP-M (P7”-M) both

demonstrated far superior absorption characteristics than P6’-M, P6”-M and P8”-M, which

extends into the near-infrared (NIR) direction. The absorption band of P7’-M peaked at a

λmax of 652 nm, coupled with a vibronic shoulder centred at 758 nm. This represents a

bathochromic shift of 30 nm relative to that of P7”-M (622 nm) with no discernible shoulder

peak. The red shift observed in P7’-M together with the appearance of the pre-shoulder peak,

both indicate the presence of strong interchain packing and the aggregation of the polymer

chains in solution, which is strengthened by the C16 linear side chains and the strong polarity

of the DPP lactam groups.36, 66, 129

Compared to P6’-M and P8’-M, the absorption maxima of

P7’-M is red shifted by 113 nm and 107 nm. This indicates that the DPP unit is a stronger

electron-withdrawing unit than both BT and TPD. In addition, the presence of the extra

thienylene units in P7’-M serves to further extend the effective conjugation length of the

copolymer, thus leading to a reduction in the HOMO and LUMO energy band gap.18, 39

In the absorption spectra of the copolymers, unlike their homopolymer counterparts,

the major absorption peaks are attributed to the intramolecular charge transfer interactions

(ICT) between the donor-acceptor polymer molecules.36, 39, 53, 72, 106

Unfortunately, the optical

characteristics of the TPD-containing copolymers could not be compared with that of similar

PAE derivatives bearing this same acceptor, due to the lack of existing publications.

However, in the case of P7’- and P7”-M, the analogous copolymers bearing a fluorene donor

unit has been reported.39

But, their optical band gaps were comparatively larger (ca. 1.85 eV

in solution), which again highlights the importance of the fused thiophene donor unit. Besides

P7’-M, P8’-M also displayed aggregation in solution, due to the noticeable shoulder peak at

621 nm. This phenomenon has been well document by Ashraf and Klemm,34

in similar

Page | 187

thienylene-based PAE copolymer with the BT acceptor. Table 6 summarizes the UV-vis

absorption data obtained in solution and solid state.

Copolymer

Solution Absorption (nm) Solid State Absorption (nm)

λmaxa λedge

b Eg

opt (eV) λmax λedge Eg

opt (eV)

P6’-M 539 658 1.88 547 709 1.75

P6”-M 493

641 1.93 479 674 1.84

P7’-M 409, 652

(758)c

806 1.54 410, 689,

(750, 650)

827 1.50

P7”-M 622 (369)c 780 1.59 421, 650

(750)

810 1.53

P8’-M 362, 545,

(621)c

650 1.91 390, 613

(563)

674 1.84

Table 6. Absorption characteristics of the PAE copolymers. aMaximum absorption in the long wavelength

direction. bEnergetic edge or onset of absorption of the UV-vis peak. The optical band gaps were deduced

from the equation, Egopt

= 1240/λonset. Where λonset represent the absorption edge in the long wavelength

region. cShoulder peak. P6’-M – PDC16TTE-OTPD-M, P6”-M – PBEHTTE-OTPD-M, P7’-M – PDC16TTE-

ODPP-M, P7”-M – PBEHTTE-ODPP-M, and P8’-M – PDC16TTE-BT-M.

The UV-visible spectra of the copolymers were also obtained on films spin-casted on

ITO glass substrates, as shown in figure 13.

300 400 500 600 700 800 900 1000

0.0

0.2

0.4

0.6

0.8

1.0

1.2

No

rma

lize

d A

bso

rba

nce

(a

.u)

Wavelength (nm)

PDC16

TTE-BT-M

PDC16

TTE-OTPD-M

PBEHTTE-OTPD-M

PDC16

TTE-ODPP-M

PBEHTTE-ODPP-M

Film

Figure 13. Absorption spectra of the acetylene-bridged thieno[3,2-b]thiophene donor-acceptor copolymers in

thin film spun from chlorobenzene on ITO glass substrate at 1000 rpm for 70 s.

Page | 188

As shown in figure 13, the copolymers displayed broader and red shifted absorption

bands in the solid films, compared to those of the homopolymers (see figure 9). The λmax of

the BT copolymer PDC16TTE-BT-M - P8’-M (613 nm) was bathochromically shifted by 116

nm with respect to the donor C16 homopolymer PDC16TTE-M - P5’-M. In contrast, when

compared to its solution absorption, the red shift was still relatively significant (68 nm).

Additionally, the presence of a pre-shoulder peak at 563 nm, suggests that the occurrence of

close interchain π-π stacking interactions between the polymer chains in the solid state, which

is probably aided by the planarization of the backbone by the rigid C≡C linkages.130, 131

The stacking of PAE copolymer with alternating BT units, and other π-acceptors has

been reported previously.2, 23, 34, 130

The red shifted absorption spectra in the solid state, may

be due to the expanded conjugation length provided by the push-pull (or D-A) effect of the

copolymer. According to the theory of Kuhn,131, 132

the optical band gap is inversely related to

the conjugation length of the polymer. Therefore, it is not surprise that the extension of the

polymer effective conjugation length by the incorporation of the BT unit, resulted to further

reduction in the optical band gap in the solid state, compared to that of the homopolymer P5’-

M (1.84 eV vs. 2.03 eV).

In the solid state absorption spectra of the DPP bearing copolymer (PDC16TTE-

ODPP-M - P7’-M) which comprises of the same donor unit as that of the BT (PDC16TTE-

BT-M - P8’-M) derivative, the D-A effect is much more enhanced, with an even narrower

optical band gap (1.50 eV) attained. This a similar scenario to that observed in the solution

spectra of the BT and DPP copolymers (figure 12), which implies that the DPP unit exact

stronger ICT interactions in the polymer structure than BT. This may be due to two

symmetric lactam aromatic rings of the DPP moiety, hence, the lower optical band gap. In the

case of the TPD copolymer PDC16TTE-OTPD-M - P6’-M, its λmax (547 nm) is shifted in the

blue direction by 66 nm in the solid state compared to that bearing the BT chromophore (616

nm). On the other, a larger hypsochromic shift (147 nm) is observed relative to the DPP

bearing copolymer P7’-M. This suggests that the TPD acceptor possess a less potent ICT

effect than that of BT and DPP. Similarly, the EH bearing copolymer of TPD (PBEHTTE-

OTPD-M - P6”-M) and DPP (PBEHTTE-ODPP-M - P7”-M) both displayed a blue shifted

absorption bands with λmax at 479 nm and 650 nm. Thus suggesting that the steric

environment of the bulky branched EH side chains does have a adverse effect on the packing

order of the conjugated copolymers.39, 72

Page | 189

These results emphasises the importance of side group functionalization and the D-A

effect on the optical properties of conjugated PAE copolymers. The fluorescence spectra of

the PAE copolymers in dilute chlorobenzene solution are plotted in figure 14. The emission

maxima (λ(em. max)) of P6’-M, P6”-M, P7’-M, P7”-M, and P8’-M were at 624 nm, 620 nm,

692 nm, 696 nm, and 601 nm, respectively. Again, these values are shifted bathochromically

in relation to that of the corresponding homopolymers (figure 11). PDC16TTE-OTPD-M -

P7’-M exhibited a stronger vibronic shoulder compared to that of other copolymers, which

suggest increased aggregation/structural ordering in solution. Table 7 shows the fluorescence

emission data obtained from the solution and solid state spectra.

600 700 800 900 1000

0.0

0.2

0.4

0.6

0.8

1.0

1.2

No

rma

lize

d In

ten

sity

(a

.u)

Wavelength (nm)

PDC16TTE-BT-M

PDC16TTE-OTPD-M

PBEHTTE-OTPD-M

PDC16TTE-ODPP-M

PBEHTTE-ODPP-M

Solution

Figure 14. Normalized fluorescence spectra of the donor-acceptor poly(thieno[3,2-b]thiophene-2-yl ethynylene)

derivatives in chlorobenzene solution.

Copolymer

Solution / nm

ΔEStokesc

/eV (nm)

Thin Film /nm

ΔEStokes

/eV (nm) λ(em. max)

(PL)a

λmax

(UV)

λ0-0

(PL)b

λ(em.max)

(PL)

λmax

(UV)

λ0-0

(PL)

P6’ 624 539 701 0.31 (85) 717 547 718 0.54 (170)

P6” 620 493

701 0.52 (127 ) 616 479 0 0.58 (137)

P7’ 692 652 760 0.11 (40) 731 689 820 0.10 (42)

P7” 696 622 760 0.21 (74) 737 650 819 0.23 (87)

P8’ 601 545 641 0.22 (56) 649 613 712 0.11 (36)

Table 7. Fluorescence properties of the D-A copolymers in solution and solid film. aMaximum emission

intensity. bWavelength of the first vibronic shoulder band peak in the PL spectrum.

cStokes shift estimated from

the difference between the emission and UV-vis absorption maxima. λ0-0 represent the shoulder peaks.

Page | 190

The Stokes shift (table 7) observed in solution for the copolymers are somewhat low

and comparable to those of the simple PAEs (P5’-M and P5”-M), except for that of the

branched EH bearing TPD copolymers (P6”-M). This signifies little changes in the polymer

backbone conformation during excitation in solution. However, in the solid state fluorescence

spectra of the copolymers presented in figure 15, a clear indication of aggregation and strong

interchain π-π stacking interactions were observed. However, this is more pronounced the BT

(P8’-M) and DPP copolymers (P7’-M and P7”-M) which all possessed intense vibronic

shoulder peaks at 712 nm, 820 nm and 819 nm (see table 6). This can be attributed to the

increased self-quenching process of excitons, as evidence by the low Stokes shifts observed

for these copolymers (0.11 eV, 0.10 eV and 0.23 eV).23, 34, 133-135

On the other hand, the large

Stokes shifts shown by the TPD polymers (entry 1 and 2 in table 7) in the solid state show the

reverse effect. This may be due to the greater structural re-organization, which more

pronounced in P6’-M as indicated by the large difference between Stokes shift in solution

(0.31 eV) and the solid state (0.54 eV), but negligible in the case of the EH bearing derivative

P6”-M (0.52 eV and 0.58 eV).

600 700 800 900 1000

0.0

0.2

0.4

0.6

0.8

1.0

1.2

No

rma

lize

d In

ten

sity (

a.u

)

Wavelength (nm)

PDC16

TTE-BT-M PDC16

TTE-OTPD-M

PBEHTTE-OTPD-M PDC16

TTE-ODPP-M

PBEHTTE-ODPP-M

Film

Figure 15. Normalized fluorescence spectra of the donor-acceptor poly(thieno[3,2-b]thiophene-2-yl ethynylene)

derivatives in as-spun thin-films.

3.3.4 Determination of Energy Levels by Electrochemical Method

The HOMO and LUMO energy levels and electronic band gaps of many conjugated

polymers can be effectively ascertained by applying electrochemical cyclic voltammetric

Page | 191

technique.136, 137

The determination of these electronic parameters for the conjugated PAE

homopolymers (P5’-M and P5”-M) and copolymers (P6’-M, P6”-M, P7’-M, P7”-M and

P8’-M) under consideration is vital to their performance when utilized as active charge

generating and/or transport layers in OPV and OFET device. Furthermore, by measuring the

intersection of the tangents of the onset of oxidation and reduction, the HOMO (EHOMO) and

LUMO (ELUMO) levels can be correctly deduced, which corresponds to the ionization

potential (IP) and electron affinity (EA) of the conjugated polymers.138

Table 8 shows a

summary of the electronic HOMO and LUMO energy levels, and the orbital offset values

(Egecl

) obtained from the CV and UPS (or PESA) measurements.

Absorption/PESA dElectrochemical CV

Polymer

aEHOMO

(UPS) (eV)

bELUMO

(eV)

cEg

opt

(eV)

Eonset.ox

vs.

Ag/Ag+

(V)

Eonset.red

vs.

Ag/Ag+

(V)

EHOMO

(eV)

ELUMO

(eV)

Egecl

(eV)

P5’-M -5.35 -3.32 2.03 +1.30 -1.02 -5.76 -3.44 2.32

P5”-M -5.34 -3.20 2.14 +1.21 -1.00 -5.67 -3.46 2.21

P6’-M -5.36 -3.61 1.75 +1.20 -0.96 -5.66 -3.50 2.16

P6”-M -5.53 -3.69 1.84 +1.10 -0.89 -5.56 -3.57 1.99

P7’-C -5.27 -3.78 1.49 +1.18 -0.81 -5.64 -3.65 1.99

P7’-M -5.28 -3.78 1.50 +1.00 -0.98 -5.46 -3.48 1.98

P7”-M -5.22 -3.69 1.53 +1.02 -0.89 -5.48 -3.57 1.91

P8’-C -5.45 -3.94 1.51 +1.21 -1.02 -5.77 -3.54 2.23

P8’-M -5.41 -3.57 1.84 +1.31 -0.86 -5.77 -3.60 2.18

Table 8. Absorption and electrochemical characteristic data of PAE homopolymer and copolymer solid films. aDerived from the ionization value via PESA measurements.

bObtained from the difference between the

HOMO and the Egopt

value. cOptical band gap deduced from the equation, Eg

opt = 1240/λonset, where λonset is

the absorption edge in the long wavelength region. dElectrochemical measurement performed on as cast

polymer films on a Pt working electrode in 0.1 M [Bu4]+[PF6]

- at a 50 mV s

-1 potential sweep rate.

EHOMO, ELUMO, and electrochemical band gaps, were estimated from the onset

potential of oxidation (φonset.ox) and reduction (Eonset.red) deduced from the CV traces by using

the equations: EHOMO = - e (Eonset.ox – EFc/Fc+ + 4.75) = IP (eV), ELUMO = - e (Eonset.red - EFc/Fc+ +

4.75) (eV) and Egecl

= (Eonset.ox - Eonset.red) (eV).139-141

The E1/2

of the ferrocene/ferrocenium

(Fc/Fc+) redox couple (EFc/Fc+) is 0.29 V vs. Ag/Ag

+ (see figure 16 below). Where, Eonset.ox

and Eonset.red represent the onset potentials of oxidation and reduction vs. Ag/Ag+. The

Page | 192

potentials were referenced against ferrocene (–4.75 eV below vacuum level for NHE142, 143

).

The electrochemical measurements were performed in 0.1 M Bu4+NPF6

-/CH3CN electrolyte

using a Pt, Pt-mesh, and Ag-wire as working, counter, pseudo-reference electrodes. Polymer

films were coated from hot chlorobenzene solution onto the surface of the Pt-working

electrode, and scanned at a potential sweep rate of 50 mV s-1

.

For comparison, the IP values were obtained by PESA measurements and used in

conjunction with the energetic edges from the solid state absorption data of the polymer

films, to deduce the alternative HOMO and LUMO energy levels for each conjugated

polymer (see table 8).144

Figure 16 and 17 shows the CV of the p-doping/dedoping redox

processes of PDC16TTE-M (P5’-M) and PBEHTTE-M (P5”-M) in the cathodic (positive)

direction.

0.0 0.5 1.0 1.5 2.0 2.5

No

rma

lize

d C

urr

en

t D

en

sity (

a.u

)

Potential (V vs Ag/Ag+)

Ferrocene

PDC16

TTE-M

PBEHTTE-M

Figure 16. Comparative CV transitions of the as-cast films (on the Pt-wire working electrode) of the

poly(thieno[3,2-b]thiophene homopolymers at a scan rate of 50 mV s-1

in (a) the positive potential direction in

0.1 M Bu4+NPF6

-/CH3CN electrolyte.

It is immediately apparent that the p-doping process is irreversible for both conjugated

polymers. Similarly, in the reverse process (figure 17), the n-doping/dedoping redox couple

in both cases were also non-reversible. As anticipated, the electron affinity (LUMO) of the

C16 P5’-M and EH P5”-M polymers were very close (entry 1 and 2, table 7), with only

slight difference in their HOMO levels (0.09 eV). Moreover, a similar trend is observed in the

data obtained from the PESA measurements, and thus, could be attributed to strong electron

donicity of the alkyl side chains appended to the polymer backbone. Nonetheless, the Eonset.ox

Page | 193

of P5’-M (+1.30 eV) is higher than that of P5”-M (+1.21 eV), which shows some influence

of the linear C16 and branched EH side groups on the polymer‟s ionization potential by CV.

Surprisingly, the Egecl

of PDC16TTE-M - P5’-M (2.32 eV) was found to be higher than that of

PBEHTTE-M - P5”-M (2.21 eV). This is a reversal of those obtained from the UV-vis/UPS

method (2.03 eV and 2.14 eV). Interestingly, such a reversal was also noticed in the case of

the other copolymers, and may be rationalized based on the differences in morphology

between the spin-coated films (UV-vis), the drop-casted films (CV and UPS measurements).

-2.5 -2.0 -1.5 -1.0 -0.5 0.0

No

rma

lize

d C

urr

en

t D

en

sity (

a.u

)

Potential ( V vs Ag/Ag+)

PDC16

TTE-M

PBEHTTE-M

Figure 17. Comparative CV transitions of the as-cast films (on the Pt-wire working electrode) of the

poly(thieno[3,2-b]thiophene homopolymers at 50 mV s-1

in the negative potential direction in 0.1 M Bu4+NPF6

-

/CH3CN electrolyte.

Figure 18 and 19 shows the CV curves of the PAE copolymers derivatives in the

cathodic and anodic potential sweep directions. In this instance, the copolymers all displayed

irreversible p-doping/dedoping and n-doping/dedoping redox processes, with the exception of

the BT containing derivative (PDC16TTE-BT-M - P8’-M) and the C16-bearing PDC16TTE-

OTPD-M - P6’-M. The highlighted copolymer P8’-M showed a non-reversible oxidation/re-

oxidation couple, while the reduction/re-oxidation process was quasi-reversible (figure 19).

On the other hand, the TPD bearing PBEHTTE-OTPD-M - P6”-M conjugated copolymer

exhibited a two electron (2e-) p-doping process, with 1e

- reduction (dedoping).

Page | 194

0.0 0.5 1.0 1.5 2.0 2.5

No

rma

lize

d C

urr

en

t D

esn

ity (

a.u

)

Potential (V vs Ag/Ag+)

Ferrocene PDC16

TTE-BT-M

PDC16

TTE-OTPD-M PBEHTTE-OTPD-M

PDC16

TTE-ODPP-M PBEHTTE-ODPP-M

Figure 18. Comparative CV transitions of the acetylene-bridged thieno[3,2-b]thiophene co-polymers drop-

casted on Pt-wire working electrode at 50 mV s-1

in the positive potential direction in 0.1 M Bu4+NPF6

-/CH3CN.

-2.5 -2.0 -1.5 -1.0 -0.5 0.0

No

rma

lize

d C

urr

en

t D

en

sity (

a.u

)

Potential (V vs Ag/Ag+)

PDC16TTE-BT-M PDC16TTE-OTPD-M

PBEHTTE-OTPD-M PDC16TTE-ODPP-M

PBEHTTE-ODPP-M

Figure 19. CV transitions of the acetylene-bridged thieno[3,2-b]thiophene co-polymers drop-casted on Pt-wire

working electrode at a scan rate of 50 mV s-1

in the negative potential direction in 0.1 M Bu4+NPF6

-/CH3CN.

The HOMO levels of C16-bearing copolymers PDC16TTE-OTPD-M (P6’-M),

PBEHTTE-OTPD-M (P6”-M), PDC16TTE-ODPP-M (P7’-M), PBEHTTE-ODPP-M (P7”-

M), PDC16TTE-BT-M (P8’-M) were -5.66 eV, -5.56 eV, -5.46 eV, -5.48 eV, -5.77 eV by

CV. On the other hand, those of the copolymers (PDC16TTE-ODPP-C - P7’-C and

PDC16TTE-BT-C - P8’-C) synthesized using the conventional heating method gave HOMO

Page | 195

levels of -5.64 eV and -5.77 eV, respectively. The difference for P7’-C and P7’-M of almost

0.2 eV is probably due to the significant differences in molecular weight for the conventional

versus microwave obtained polymers, with the low weight P7’-C probably being below the

conjugation limit.

The HOMO levels obtained from the PESA method were consistently higher (NB.

HOMO are negative values) than those values measured by CV, but they gave qualitatively

similar results (i.e. all ionisation potentials for the polymers are similar within 0.2 eV). The

differences are probably due to both the method of film deposition, in addition to the

electrochemical method requiring the diffusion of the counterion into the film to stabilise the

charge generated. The values obtained from the PESA and CV measurements are relatively

lower (0.34 – 0.41 eV and 0.46 – 0.77 eV) than that of P3HT (-4.86 eV)145

, implying that

they are more stable to photo-oxidation.

In addition, these copolymers should be capable of generating higher Voc values,

which may lead to optimised OPV device efficiencies, since the Voc is linearly correlated to

the displacement between the HOMO of the donor polymer and the LUMO of the PCBM

acceptor.24, 146

The low-lying HOMO values observed in both the homopolymers and the

corresponding copolymers suggest that the electron-deficient acetylene bond strongly

influence the HOMO levels. In addition, the differences in the nature of the alkyl side groups

seem to have little effect on the HOMO levels. On the other hand, the LUMO of the

homopolymers and copolymers were influence more by the acceptor units (BT, TPD and

DPP) incorporated into the polymer chains. The homopolymers P5’-M and P5”-M showed

LUMO levels of -3.44 eV (-3.32 eV) and -3.46 eV (-3.50 eV). While those of the copolymers

consisting of TPD acceptor (P6’-M and P6”-M) were -3.50 eV (-3.61 eV) and -3.57 eV (-

3.69 eV), respectively. In the other copolymers involving the DPP units (P7’-C, P7’-M, and

P”-M), the LUMO levels were lower (-3.65 eV (-3.78 eV), -3.48 eV (-3.78 eV), and -3.57 eV

(-3.69 eV)), especially the values obtained from UV-vis/PESA measurements. A similar

scenario is observed for the BT-bearing copolymers, P8’-C and P8’-M, which displayed

LUMO levels lower than those comprising of the TPD unit (-3.54 eV (-3.94 eV) and -3.60 eV

(-3.57 eV)). When comparing like-for-like (EH vs. EH), the DPP and TPD units exerted

similar electron-withdrawing effect on the D-A conjugated polymer. However, in the case of

the C16 bearing copolymers, the DPP unit is shown to be the strongest electron-withdrawing

Page | 196

group. Overall, the insertion of the electron-deficient BT, TPD and DPP units into simple

PAE backbone resulted in significant reduction in the LUMO energy levels, as expected.147

Moreover, the HOMO and LUMO energy gaps of the copolymers were also narrowed due to

the electron-withdrawing effect of the acceptor units, especially those comprising of the DPP

moiety.147

Furthermore, the difference observed between the optical and electrochemical

band gaps (see table 8) is due to the increased interface barrier for charge injection and/or the

tendency of the copolymers to aggregate strong in the solid state.34, 148

3.3.5 Thermal Stability and Transitions

The thermal stability of the conjugated PAE homopolymers and copolymer were

measured by thermogravimetric analysis (TGA) under inert conditions and oxygen at a

heating rate of 20oC.min

-1. Figure 20 and 21 shows the TGA thermograms of PDC16TTE-M

(P5’-M), PBEHTTE-M (P5”-M), PDC16TTE-OTPD-M (P6’-M), PBEHTTE-OTPD-M

(P6”-M), PDC16TTE-ODPP-M (P7’-M), PBEHTTE-ODPP-M (P7”-M), and PDC16TTE-

BT-M (P8’-M).

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750-10

0

10

20

30

40

50

60

70

80

90

100

110

120

We

igh

t (%

)

Temperature (oC)

PDC16

TTE-M Air

PDC16

TTE-BT-M Air

PDC16

TTE-OTPD-M Air

PDC16

TTE-ODPP-M Air

(a)

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750

-10

0

10

20

30

40

50

60

70

80

90

100

110

120

We

igh

t (%

)

Temperature (oC)

PBEHTTE-M Air

PBEHTTE-OTPD-M Air

PBEHTTE-ODPP-M Air

(b)

Figure 20. TGA thermograms of the C16- and EH-bearing PAE homopolymers and copolymers at a heating

rate of 10oC/min under oxygen.

Page | 197

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750-10

0

10

20

30

40

50

60

70

80

90

100

110

120

We

igh

t (%

)

Temperature (oC)

PDC16

TTE-M N2

PDC16

TTE-BT-M N2

PDC16

TTE-OTPD-M N2

PDC16

TTE-ODPP-M N2

(c)

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750-10

0

10

20

30

40

50

60

70

80

90

100

110

120

We

igh

t (%

)

Temperature (oC)

PBEHTTE-M N2

PBEHTTE-OTPD-M N2

PBEHTTE-ODPP-M N2

(d)

Figure 21. TGA thermograms of the C16- and EH-bearing homopolymers and copolymers recorded at a heating

rate of 10oC/min under inert (N2) atmosphere.

The data extracted from the TGA thermograms (figure 20 and 21) are summarized in

table 9.

Polymer aTd /

oC (N2)

bTd /

oC (Air)

P5’-M 3005%

, 33050%,

, 480100%

2905%

, 37050%

, 540100%

P5”-M 3655%

, 472100%

3305%

, 47050%

, 630100%

P6’-M 3905%

, 45060%

, 590100%

3855%

, 49050%

, 575100%

P6”-M 3455%

, 600100%

3305%

, 45060%

, 590100%

P7’-M 3955%

, 58060%

, 660100%

3905%

, 49050%

, 650100%

P7”-M 3605%

, 55050%

, 610100%

3255%

, 45050%

, 620100%

P8’-M 3605%

, 488100%

3155%

, 48070%

, 680100%

Table 9. Thermogravimetric data of the C16 and EH PAE homopolymers (PDC16TTE-M – P5’-M, and

PBEHTTE-M – P5”-M) and copolymers (PDC16TTE-OTPD-M – P6’-M, PBEHTTE-OTPD-M – P6”-M,

PDC16TTE-ODPP-M – P7’-M, PBEHTTE-ODPP-M – P7”-M, and PDC16TTE-BT-M – P8’-M) recorded at

20 oC/min.

aOnset of decomposition temperature under inert atmosphere.

bDecomposition temperature under

oxygen.

Considering the C16-bearing homopolymer, PDC16TTE-M (P5’-M), an onset of

degradation can be observed at 290 oC in air (figure 20a), which corresponds to a weight loss

of 5 %. While under inert atmosphere, this increased slightly to 300 oC. In the case of that

comprising of EH side chains (PBEHTTE-M – P5”-M), a 5 % weight loss is noticeable at Td

of 330 oC, but, in N2 this value is 35

oC higher (see table 9). For the copolymer consisting of

the TPD acceptor (PDC16TTE-OTPD-M – P6’-M) similar Td values were observed for the 5

Page | 198

% lose in weight in both air (385 oC) and inert (390

oC) atmosphere. On the contrary, the

branched derivative PBEHTTE-OTPD-M (P6”-M) displayed a comparatively lower Td of

330 oC in air for the same weight loss (5 %). Moreover, under N2, this value was 345

oC.

Similarly, the Td of PDC16TTE-ODPP-M (P7’-M) showed negligible differences between the

onset of decomposition temperature in air (390 oC) and under N2 atmosphere (395

oC) for the

5 % weight reduction. On the other hand, the 5 % weight loss under N2 for the EH-bearing

analogue PBEHTTE-ODPPM (P7”-M) occurred at a higher Td (360 oC) relative to that

observed in air (325 oC). Generally, the above results suggest that the thermal stability of

both the homopolymers and copolymers was indeed enhanced by the absence of oxygen,

which is known to induce degradation in many π-conjugated polymers (see chapter 2).

As anticipated, the decomposition temperatures measured in both air and nitrogen

indicate that the conjugated polymers can be used to fabricate high thermally stable OPV and

OFET devices. Figure 22, shows the differential scanning caloriemetry (DSC) transitions of

the PAE derivatives. Unfortunately, after repeated attempts, no discernible phase transitions

were observed in the DSC plots taken below the TGA decomposition temperatures of the

both the PAE homopolymers and copolymers.

50 100 150 200 250 300 350

-15

-10

-5

0

5

10

15

He

at F

low

(W

/g)

Temperature (oC)

PDC16

TTE-M

PBEHTTE-M

PDC16

TTE-BT-M

PDC16

TTE-OTPD-M

PBEHTTE-OTPD-M

PDC16

TTE-ODPP-M

PBEHTTE-ODPP-M

Figure 22. DSC thermograms of PAE homopolymers (PDC16TTE-M – P5’-M, and PBEHTTE-M – P5”-M)

and copolymers (PDC16TTE-OTPD-M – P6’-M, PBEHTTE-OTPD-M – P6”-M, PDC16TTE-ODPP-M – P7’-

M, PBEHTTE-ODPP-M – P7”-M, and PDC16TTE-BT-M – P8’-M) at a heating/cooling rate of 10oC/min (2

nd

cycle), under inert conditions (N2).

Page | 199

3.3.6 Thin Film X-Ray Diffraction

The impact of the different alkyl substituents and the D-A

intermolecular/intramolecular interactions on the crystallinity of the PAE homopolymer and

copolymer π-systems was investigated by performing X-ray diffraction (XRD) measurements

on films drop-casted on silicon substrate (S-Mart). The XRD pattern of the homopolymers

(P5’-M and P5”-M) and the BT-bearing copolymers are shown in figure 23.

0 5 10 15 20 25 30

(d)

(c)

(b)

(12.29)

7.19 Å (8.41)

10.49 Å

(4.67)

18.87 Å

100 (6.23)

14.20 Å

300 (11.27)

7.85 Å

200 (7.60)

11.62 Å

100 (3.89)

22.63 Å

Diffr

actio

n In

ten

sity (

a.u

)

2 / degree (CuK

Silicon Substrate PDC16

TTE-M

PBEHTTE-M PDC16

TTE-BT-M

(a)

Figure 23. X-ray diffraction pattern of (a) PDC16TTE-M (P5’-M), (b) PBEHTTE-M (P5”-M), and (c)

PDC16TTE-BT-M (P8’-M) drop-casted films from chlorobenzene at room temperature, and (c) neat silicon

substrate.

The distinctive peak corresponding to the (100) reflection is clearly noticeable for all

three conjugated polymers. This peak is attributed to the lamellar spacing between polymer

backbones, or more correctly the intermolecular d1-spacing between the polymer backbones

separated by the alkyl side groups.59, 149

Since this lamellar spacing depends on the length of

the alkyl side chains, its values are expected to vary with the different linear and branched

anchoring groups on the polymer backbone.150

The homopolymer, PDC16TTE-M (P5’-M)

displayed a small intensity peak at 2Ɵ = 3.89o (100) which corresponds to a interchain d1-

spacing of 22.63 Å, due to the long C16 alkyl side group appended to the polymer main

chain. This value is in close agreement with that reported for poly(3,6-dihexadecylthieno[3,2-

b]thiophene-co-thieno[3,2-b]thiophene) - PATT-C16 (23.8 Å).150

Moreover, additional

diffraction peaks can be seen at 7.60o (11.62 Å), and 11.27

o (7.85 Å) corresponding to the

(200) and (300) reflections. The slight errors in the expected d-spacings compared to the 100

Page | 200

peak, are due to the difficulties in obtaining accurate values for the maximum peak height due

to the broad nature of the diffractions. No clear peak is ascribable to the π-π distance (or 010),

which we would expect around 4Å.59

The lamellar spacing of 22.6 Å, which is smaller than

that of PATT-C16 suggest that the acetylene copolymer packs via interdigitation of the alkyl

chains, in a similar manner to PATT-C16.150

The branched EH polymer also displays a clear diffraction peak at 2Ɵ = 6.23°, which

corresponds to a reduced d spacing of 14.20 Å. The reduction in d-spacing over the C16

polymer is expected due to the reduction in side-chain length. There is no evidence for

second or third order diffraction peaks for the branched polymer, and similar to the straight

chain polymer the π-π stacking peak is not discernible. The lamellar distance is slightly lower

than those reported for P3HT (16.11 - 16.7 Å149, 151

) consisting of non-interdigitated hexyl

(C6) side chains, which may suggest a degree of interdigitation even for the 2-ethylhexyl

groups. The diffraction peaks of the TPD and DPP copolymers are shown in figures 24 - 26.

0 5 10 15 20 25 30

(c)

(b)

Diffr

actio

n In

ten

sity (

a.u

)

2 / degree (CuK)

Silicon substrate

PDC16

TTE-OTPD-M

PBEHTTE-OTPD-M

100 (5.49)

16.08 Å

(a)

Figure 24. X-ray diffraction pattern of the (a) PDC16TTE-OTPD-M (P6’-M) and (b) PBEHTTE-OTPD-M

(P6”-M) drop-casted films at room temperature, and (c) neat silicon substrate.

Firstly, considering all the linear C16 substituted acetylene copolymers, significant

differences were observed in the diffraction patterns for the three copolymers (PDC16TTE-

OTPD-M – P6’-M, PDC16TTE-ODPP-M – P7’-M, and PDC16TTE-BT-M – P8’-M). The BT

copolymer (P8’-M) exhibited clear diffraction peaks (figure 23), with a series of three peaks

(2 Ɵ = 4.67, 8.41 and 12.29°) that at first glance might be attributable to first, second and

third order lamellar diffractions, similar to the C16 homopolymer. However the d-spacing for

Page | 201

the three peaks are 18.87, 10.49 and 7.19 Å, respectively, and it is therefore clear that the

latter two peaks are not second and third order diffractions of the first peak. These two peaks

do appear to be second and third order reflections of an unseen peak, expected around 21 Å,

which is close to the lamellar spacing of the homopolymer. The reason this is not observed is

unclear, as is the origin of the peak around 18.87 Å. This peak is close to the length of the

monomer unit (ca. 16.8 Å) and may reflect some diffraction related to the monomer spacing.

In addition this peak is relatively broad and intense compared to the 200 and 300 peaks, so

may obscure the expected 100 peak. In contrast the TPD copolymer displayed little sign of

crystallinity (figure 25) except for a small diffraction peak at 2 Ɵ = 5.49° (16.08 Å). We

believe this is too small to be assigned to the lamellar spacing, but again is close to the

calculated monomer length (ca. 15.6 Å) and may be related to diffraction from this. The EH

substituted polymer also shows no signs of crystallinity, appearing completely amorphous

(figure 24). The reduced crystallinity for both polymers compared to the BT copolymer, or

the homopolymers may be a reflection of the different side chain lengths on each monomer

(C16 and C8), which may disrupt packing by interdigitation of adjacent polymer chains.

Figure 25 shows the XRD of the DPP copolymers.

0 5 10 15 20 25 30

(b)

(c)

010 (23.87)

3.72Å

100 (6.13)

14.41Å

400(15.55)

5.69Å300(12.58)

7.03Å

200 (8.33)

10.60Å

010 (22.14) 4.01Å

Diffr

actio

n In

ten

sity (

a.u

)

2 / degree (CuK

Silicon Substrate PDC16

ETTT-ODPP-M

PBEHETTT-ODPP-M100 (4.52)

19.52 Å (a)

Figure 25. X-ray diffraction pattern of the (a) PDC16TTE-ODPP-M (P7’-M) and (b) PBEHTTE-ODPP-M

(P7”-M) drop-casted films at room temperature, and (c) neat silicon substrate.

As shown in figure 25, the DPP acetylene copolymers both show evidence of thin

film crystallinity. Again the side group on the DPP (C8) is smaller than that on the

thieno[3,2-b]thiophene unit, but in any case, the disruption to side chain interdigitation may

be overcome by the strong tendency of the DPP cores to pack in the solid state.38, 72, 74

This

Page | 202

strong tendency may be driven by electrostatic interactions.38

The C16 copolymer

(PDC16TTE-ODPP-M – P7’-M) shows a progression of 4 peaks, the later three of which

appear to correspond to second, third and fourth order reflections of the lamellar distance.

Surprisingly, and similar to the BT copolymer, the first diffraction peak does not appear at a

d-spacing commensurate with the later spacings, and the 19.5 Å may again reflect the

backbone monomer spacing (ca. 21 Å). This copolymer also demonstrated a broad peak

around 2 Ɵ = 21°, which we ascribe to the π-π packing distance. As observed for the other

co-polymers, the branched EH polymer (PBEHTTE-ODPP-M – P7”-M) appeared less

crystalline, with a single diffraction peak at 2 Ɵ = 6.13° observable. We believe this is likely

the lamellar distance, which is similar to the homopolymer.

3.4 Evaluation of Device Characteristics

3.4.1 Fabrication of Organic Field Effect Transistors (OFETs)

The charge mobility of the PAE conjugated copolymers were tested using a top-gate

bottom-contact device configuration. Test patterns comprising of highly doped n-type Si

(100) wafer (resistivity = 1.5-3.0 Ω*cm), as substrate and common gate, a gate dielectric

layer (capacitance = 17.25 nF cm-2

) made up of thermally grown SiO2 (200 nm thickness)

modified by treatment with hexylmethyldisilazane (HMDS), and 100 nm thick titanium and

gold (Ti-Au) electrodes with an interdigitated geometry channel length (L) of 20 µm and a

channel width (W) of 10 mm, providing an aspect ratio of 500. Prior to active layer

deposition, the test patterns were initially rinsed with acetone to remove any protective layer

(photoresist), followed by sonication in acetone for 5 min, rinsed in propanol, and dried under

a stream of N2 gas. Each conjugated polymer was dissolved in chlorobenzene at a

concentration of 5mg/ml in a N2-filled glove box. To ensure a complete dissolution of the

conjugated polymers, the solutions were kept under constant stirring at 900 rpm on a hot plate

(60 ºC) for 1 hour. The OFET devices were constructed in the glove box by spin coating the

solutions at 1000 rpm for 30 s on the substrate, followed by thermal annealing at 80 ºC for 2

minutes, so as to eliminate any residual solvent before proceeding with the measurements.

The transfer characteristics in the linear (drain-source voltage VDS = -10V) and saturation

(VDS = -50V) regimes were measured by varying the gate-to-source voltage (VGS) between

+20 V and – 50V. While, the output characteristics were measured by fixing VGS in the range

from 0V to -50V and varying VDS between 0V and -50V, under a controlled atmosphere (N2 –

filled glove box), on as prepared devices and after 2 hours annealing at 120 oC. The output

Page | 203

characteristics were recorded by varying VDS from 0V to -50 V at several fixed VGS values

(0V to -50V, voltage step ΔVGS = 10V). These measurements were carried out on a Keithley

4200-SCS Semiconductor Parameter Analyzer. All measurements were performed by Dr

Pasquale D‟Angelo of Dr Thomas Anthropolous group (Imperial College London). The

transfer curves were modelled by using the conventional metal-oxide-semiconductor FET

(MOSFET) equations describing the device channel current IDS in the linear and saturation

regimes;152, 153

IDS(lin) = (W/L)μFETCi(VGS – Vth)VDS [VGS – Vth] ˃ VDS (1)

IDS(lin) = (W/2L)μFETCi(VGS – Vth)2 [VGS – Vth] ≤ VDS (2)

Here, W and L represent the channel width and length, VDS is the Drain-Source

voltage, VGS is the Gate-Source voltage, Ci the gate dielectric capacitance per unit area of the

dielectric layer (SiO2) (Ci = 17.25 nF cm-2

), and Vth is the threshold voltage (that is, the gate

voltage at which the charge carriers transport in the device channel is dominated by field

effect doping). The charge mobility (µ) was evaluated by applying the following relationships

in the linear and saturation regimes:152

(3)

(4)

Furthermore, Vth was calculated by evaluating the intersection of the linear fit of the

plots IDS vs VG (linear regime) and (IDS)1/2

vs VG (saturation regime) with the gate voltage

axis. On the other hand, the current on-and-off ratios (ION/IOFF) in the linear and saturation

regimes were ascertained from the transfer characteristics by evaluating the ratio between the

maximum IDS value (ON state) and the minimum IDS (OFF state).

Disappointingly we were unable to obtain any measurable transfer characteristics on

any of the EH polymers, including the homopolymer. Similarly the C16 homopolymer (P5’-

M – PDC16TTE-M) did not show any measurable characteristics. The reasons for this are not

immediately clear, but for all these polymers we had problems forming smooth and coherent

films by spin coating, with extensive dewetting a problem. This was probably due to a

combination of low molecular weight, in combination with the highly hydrophilic side

Page | 204

chains. In particular the high side chain density for the homopolymers may have caused

problems with good wetting. The high ionisation potentials for the polymers (table 8) may

also have resulted in difficulties with charge injection from the gold source drain electrodes.

However, we were able to measure the FET characteristics of the C16 DPP and BT

copolymers (P7’-M – PDC16TTE-ODPP-M, P7’-C - PDC16TTE-ODPP-C, and P8’-M –

PDC16TTE-BT-M). Figure 26 and 27 shows the transfer and output characteristics of the

conjugated copolymers recorded in the linear and saturation regimes, without (as-deposited)

and with (post-deposition) thermal annealing. The transfer and output curves show large

hysteresis despite annealing. Since the OFET devices were characterized under a controlled

environment (glove box filled with N2(g)), trapping phenomenon due to extrinsic agents such

as atmospheric oxygen and relative humidity (H2O) can be excluded here.154, 155

Studies by

Chabinyc et. al., on the effect of humidity of the FET performance of poly[5,5‟-bis(3-

dodecyl-2-thienyl)-2,2‟-bithiophene] (PQT-12), shows an increase in the rate of charge

carrier trapping with increased relative humidity.156

This was also accompanied by

concomitant reduction in the charge carrier mobility of the OFET device. Therefore, a

probable explanation for this hysteresis may be due to the low degree of purity of the

conjugated polymers (perhaps catalyst residue), the presence of structural irregularities or

simply the processing conditions.157

It is noticeable that P7’-M (PDC16TTE-DPP-M), which

exhibited a much higher molecular weight than P7’-C (PDC16TTE-DPP-C) shows markedly

less hysteresis despite the same purification conditions. This suggest that the primary cause of

the poor performance may be structural, with the lower MW polymer giving less

homogeneous films and perhaps more pronounced grain boundaries. Table 10, shows a

summary of the OFET properties of the copolymers under investigation.

Page | 205

-60 -50 -40 -30 -20 -10 0 10 20 3010

-15

10-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

PDC16

TTE-BT-M linear regime

PDC16

TTE-BT-M linear annealed

-ID

S(A

)

VGS

(V)

a)

-60 -50 -40 -30 -20 -10 0 10 20 3010

-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

PDC16

TTE-BT-M saturation regime

PDC16

TTE-BT-M saturation annealed

-ID

S(A

)

VGS

(V)

b)

-60 -50 -40 -30 -20 -10 0 10 20 3010

-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

PDC16

TTE-ODPP-M linear regime

PDC16

TTE-ODPP-M linear annealed

-ID

S(A

)

VGS

(V)

c)

-60 -50 -40 -30 -20 -10 0 10 20 3010

-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

PDC16

TTE-ODPP-M saturation regime

PDC16

TTE-ODPP-M saturation annealed

-ID

S(A

)

VGS

(V)

d)

-60 -50 -40 -30 -20 -10 0 10 20 3010

-15

10-14

10-13

10-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

PDC16

TTE-ODPP-C linear regime

PDC16

TTE-ODPP-C linear annealed

-ID

S(A

)

VGS

(V)

e)

-60 -50 -40 -30 -20 -10 0 10 20 3010

-11

10-10

10-9

10-8

10-7

10-6

PDC16

TTE-ODPP-C saturation

PDC16

TTE-ODPP-C saturation annealed

- I D

S (

A)

VGS

(V)

f)

Figure 26. Transfer characteristics of PDC16TTE-BT-M (P8’-M) based OTFT recorded in the linear (a) and

saturation (b) regimes; Transfer curves of PDC16TTE-ODPP-C (P7’-C) in the linear (c) and saturation (d)

regimes; Transfer curves of PDC16TTE-ODPP-M in the linear (e) and saturation (f) regimes. The black line

represent the as-prepared devices, while the red line refer to the post-annealed (that is, at 120 °C for 2 hours)

devices.

Page | 206

-50 -40 -30 -20 -10 0-1x10

-7

0

1x10-7

2x10-7

3x10-7

4x10-7

5x10-7

6x10-7

PDC16

TTE-BT-M

VGS

=0V

VGS

=-10V

VGS

=-20V

VGS

=-30V

VGS

=-40V

VGS

=-50V

-ID

S(A

)

VDS

(V)

a)

-50 -40 -30 -20 -10 0

0.0

2.0x10-7

4.0x10-7

6.0x10-7

8.0x10-7

1.0x10-6

1.2x10-6

PDC16

TTE-BT-M annealed

VGS

=0V

VGS

=-10V

VGS

=-20V

VGS

=-30V

VGS

=-40V

VGS

=-50V

-ID

S(A

)

VDS

(V)

b)

-50 -40 -30 -20 -10 0-2.0x10

-9

0.0

2.0x10-9

4.0x10-9

6.0x10-9

8.0x10-9

1.0x10-8

1.2x10-8

1.4x10-8

1.6x10-8

1.8x10-8

PDC16

TTE-ODPP-C

VGS

=0V

VGS

=-10V

VGS

=-20V

VGS

=-30V

VGS

=-40V

VGS

=-50V

-ID

S(A

)

VDS

(V)

c)

-50 -40 -30 -20 -10 0-2.0x10

-8

0.0

2.0x10-8

4.0x10-8

6.0x10-8

8.0x10-8

1.0x10-7

1.2x10-7

1.4x10-7

PDC16

TTE-ODPP-C annealed

VGS

=0V

VGS

=-10V

VGS

=-20V

VGS

=-30V

VGS

=-40V

VGS

=-50V

-ID

S(A

)

VDS

(V)

d)

-50 -40 -30 -20 -10 0

0

2x10-7

4x10-7

6x10-7

8x10-7 V

GS=0V

VGS

=-10V

VGS

=-20V

VGS

=-30V

VGS

=-40V

VGS

=-50V

PDC16

TTE-ODPP-M

-ID

S(A

)

VDS(V)

e)

-50 -40 -30 -20 -10 0

0

2x10-7

4x10-7

6x10-7

8x10-7

-ID

S(A

)

VDS

(V)

VGS

=0V

VGS

=-10V

VGS

=-20V

VGS

=-30V

VGS

=-40V

VGS

=-50V

PDC16

TTE-ODPP-M annealed

f)

Figure 27. Output characteristics of the PDC16TTE-BT-M (P8’-M) based OTFT recorded for the as-casted (a)

and the post-annealed (b) devices; Output characteristics of the PDC16TTE- ODPP-C (P7’-C) based OTFT

recorded for the as-casted (c) and the post-annealed (d) devices; Output characteristics of the PDC16TTE-

ODPP-M (P7’-M) based OTFT recorded for the as-prepared (e) and post-annealed (f) devices.

Figure 26a and 26b shows the transfer characteristics of the BT-bearing copolymer,

PDC16TTE-BT-M (P8’-M), which displayed very low drain-source currents or conductivity

(10-10

– 10-11

A) at zero voltage. This reinforces its high stability, as a result of the low lying

HOMO level (-5.77 eV) it possess. However, as can be seen from the output characteristics at

low voltage, there are clear injection issues. Nevertheless, a charge carrier mobility of 8.6 x

10-4

cm2 V

-1s

-1, a threshold voltage (Vth) of -12.5 V and a high current on/off ratio (Ion/Ioff) of

106, were obtained without thermal treatment in the saturation regime. However, when

Page | 207

annealed at 120 oC for 30 minutes, the field effect mobility improved by 1 order of magnitude

to 1.0 x 10-3

cm2 V

-1s

-1, accompanied by a Vth of -16 V. Moreover, the Ion/Ioff was maintained

at 106, which is very high due to the increased drain currents and low current at zero voltage

(ca. 10-11

A). In comparison, the current on/off ratio recorded for the P8’-M is higher than

that of regioregular P3HT (103 – 10

5).

158, 159 McCullough at. al., attributed a similarly high

Ion/Ioff value (106) to the low-lying HOMO level (-5.22 eV) of the bithiazole-containing D-A

conjugated polymer, poly{2,6-bis(4-dodecyl-1,3-thiazole-5-yl)-4-hexyldecan-4H-

bisthienopyrroles} (PBTzDTP-C12).160

Polymer As-casted Annealed at 120 oC

P7’-C μ / cm2 V

-1s

-1 Ion/Ioff Vth / V μ / cm

2 V

-1s

-1 Ion/Ioff Vth / V

Linear 8*10-6

104 -9.5 2*10

-5 10

5 -10.8

Saturation 1*10-5

103 +0.5 2.5*10

-5 10

4 +0.5

P7’-M μ / cm2 V

-1s

-1 Ion/Ioff Vth / V μ / cm

2 V

-1s

-1 Ion/Ioff Vth / V

Linear 1*10-4

105 -8.6 2*10

-4 10

5 -17

Saturation 1.5*10-4

104 +3.8 2.25*10

-4 10

4 +0.5

P8’-M μ / cm2 V

-1s

-1 Ion/Ioff Vth / V μ / cm

2 V

-1s

-1 Ion/Ioff Vth / V

Linear 4.8*10-4

106 -17.4 8*10

-4 10

6 -25.3

Saturation 8.6*10-4

106 -12.5 1*10

-3 10

6 -16

Table 10. The charge carrier mobility (µ), threshold voltage (Vth) and current ON/OFF ratio calculated from

the transfer curves. PDC16TTE-ODPP-C (P7’-C), PDC16TTE-ODPP-M (P7’-M), and PDC16TTE-BT-M

(P8’-M).

As shown in table 10, the DPP PAE copolymer synthesized under microwave

conditions, P7’-M, exhibited a surprisingly low hole mobility of 1.5 x 10-4

cm2 V

-1s

-1 in the

saturation regime as-casted, with a higher Vth (+ 3.8 V) and a correspondingly low Ion/Ioff

(104) relative to that of the P8’-M. However, when annealed, a slightly improved mobility

and better threshold voltage were obtained (2.25 x 10-4

cm-2

V-1

s-1

and +0.5 V). Seeing that

the XRD measurements reveal a highly crystalline microstructure in the case of P7’-M

(figure 25) compared to that of P8’-M (figure 23), the distinction between their field-effect

Page | 208

mobilities can be probably be ascribed to the differences in their number average molecular

weights and PDI (13.5 kDa [7.40] vs. 11.0 kDa [1.97], see table 3). Previous studies by

Menom and co-workers have concluded that conjugated polymers with high PDIs possess

lower mobilities, which was attributed to the notion that long chains within the polymer

structure are likely to contain segments with longer conjugations, this in-turn could act as

trapping sites and therefore affect the mobility of the charge carriers.158, 161

In addition, the

ease of dissolution of the polymers in CB at room temperature may have a marked effect on

the OFET device performance. On the contrary, the compromised current on/off ratio of P7’-

M suggests a relationship with its lower IP value (- 5.46 V) compared to that of P8’-M (-

5.77 eV), which could be further improved with extensive purification of the copolymer.155,

162 In the case of the DPP copolymer synthesized via conventional heating, the observed low

mobility (2.5 x 10-5

cm2

V-1

s-1

after thermal treatment at 120 oC for 1 hour may be ascribed

to the lack of structural order, as revealed by XRD measurements (not shown) and may be

related to the lower molecular weight. Furthermore, the presence of structural disorder may

be another possibility, which is evident in the 1 order of magnitude improvement in the

current on/off ratio (103 to 10

4) upon annealing the as-cast film (see entry 3, table 10).

-4.80

ITO-5.00

PE

DO

T:P

SS

-2.70

-4.80

P3

HT

-3.44

-5.76

PD

C1

6T

TE

-M

PB

EH

TT

E-M

-5.67

-3.46 -3.50

-5.66

PD

C1

6T

TE

-OT

PD

-M

PB

EH

TT

E-O

TP

D-M

-5.56

-3.57-3.65

-5.64

-3.48

-5.46

-3.57

-5.48

PD

C1

6T

TE

-OD

PP

-C

PD

C1

6T

TE

-OD

PP

-M

PB

EH

TT

E-O

DP

P-M

-3.54

PD

C1

6T

TE

-BT

-C

PD

C1

6T

TE

-BT

-M

-5.77 -5.77

-3.60

-6.50

-4.00

PC

71B

M -5.00

-4.30

Al

Au

Figure 28. Illustration of the relative HOMO/LUMO Energy levels (eV) of the homopolymers, copolymers,

P3HT and PC71BM.

Additionally, the output characteristics of P8’-M (figure 29a and 29b) and P7’-C

(figure 29c and 29d) based OFET device responses are apparently affected by contact

Page | 209

resistance effect (that is, bending of the curves at higher VDS and a sigmoid shape around VDS

= 0V). This may be due to the fact the energy offset between the HOMO level of P8’-M (-

0.77 eV) and P7’-C (-0.64 eV), and that of the gold (Au) work-function (-5.0 eV), are more

pronounced than that of P7’-M (-0.46 eV) (see figure 28 for the band gap diagram of the

PAE homopolymer and copolymers). Generally, the OFET device based on P8’-M gave the

best performance of all the copolymers investigated. On the other hand, the copolymer

synthesized by the microwave-assisted Sonogashira route (P7’-M) showed a better OFET

hole mobility than that prepared via conventional heating (P7’-C), which may be due to the

differences in molecular weights (Mn = 13.5 kDa vs 7.0 kDa). Nonetheless, this is a

preliminary study, thus, more in-depth investigation is needed to fully elucidate the impact of

the different synthesis conditions on the OFET performance.

3.4.2 Organic Photovoltaics

The performance of the homopolymers, PD16TTE-M (P5’-M) and PBEHTTE-M

(P5”-M) were ascertained in an OPV device made up of ITO-PEDOT:PSS-

Polymer:PC71BM-Ca-Al configuration. As described in chapter 2, the glass substrate was

coated with ITO, sonicated consecutively with detergent, distilled water, acetone, and

isopropanol for 15 minutes, and dried under N2(g). This was followed by pre-treatment using

ultraviolet ozone plasma for a additional 7 minutes. Then, using spin coating technique, a

solution of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) was

coated onto the pre-treated ITO substrates to form a PEDOT:PSS layer (30nm thick), and

baked at 150 C for 20 minutes prior to the coating of the active layer blends (15 mg/mL

Polymer:PC71BM blend in chlorobenzene) and the deposition of the electrode contacts. All

measurements were carried out in an N2-filled glovebox. The current density-voltage (J-V)

curves of the devices were measured with exposure to a 150 W Xenon lamp filtered to

simulate AM 1.5. The current measurements were obtained in a Keithley source meter with

varying voltage bias. Furthermore, the equilibrium quantum efficiency (EQE) of the devices

was carried out using a 100 W tungsten halogen lamp equipped with a monochromator. The

OPV device measurements were carried out by Huang Zhenggang (Steve) of Prof James

Durrant‟s group (Imperial College London). Figure 29 and 30 shows the current density-

voltage (J-V) curves of the OPV devices based PDC16TTE-M and PBEHTTE-M under the

illumination of AM 1.5 and 100 mWcm-2

. Table 11 shows a summary of the data obtained

from the J-V plots.

Page | 210

Figure 29. The current density-voltage characteristics of the PDC16TTE-M (P5’-M)/PC71BM solar cells under

AM 1.5 condition (100 mW/cm2).

Figure 30. The current density-voltage characteristics of the PBEHTTE-M (P5”-M)/PC71BM solar cells under

AM 1.5 condition (100 mW/cm2).

As anticipated, both PAE homopolymer derivatives displayed high Voc values (0.75 V

and 0.88 V) due to their high lying HOMO values (-5.76 eV and -5.67 eV). The C16-bearing

derivate P5‟-M showed the lowest OPV efficiency (0.26 %) with a short circuit current (Jsc)

of 0.97 mA cm-2

and fill factor (FF) of 35 %. On the other hand, the power conversion

Polymer Solvent Blend ratio Voc / mV Jsc / mA cm-2

FF / % PCE / %

PDC16TTE-M DCB 1:2 746 0.97 35 0.26

PBEHTT-M DCB 1:3 880 1.69 29 0.43

Table 11. Photovoltaic parameters of the preliminary OPV devices fabricated from the PDC16TTE-M (P5’-

M) and PBEHTTE-M (P5”-M) copolymers.

PDC16TTE-M:PC71BM (1:2)

PBEHTTE-M:PC71BM (1:3)

Page | 211

efficiency (PCE) of the branched EH analogous polymer (P5”-M) was almost doubled (0.43

%) that of the C16-containing counterpart. This increase in PCE may be attributed to the

higher Jsc recorded in the P5”-M device. In order to gain an insight into the reason for such

large distinctions in the OPV performance, the equilibrium quantum efficiency (EQE) spectra

of the conjugated PAEs were obtained (figure 31 and 32).

Figure 31. The EQE spectra of the PDC16TTE-M (P5’-M)/PC71BM solar cell.

Figure 32. The EQE spectra of the PBEHTTE-M (P5”-M)/PC71BM solar cell.

It can be surmised from figure 31 that the P5’-M/PC71BM shows a relatively low

EQE of 8 % compared to 16 % for P5”-M (figure 32). This explains the differences between

the short circuit currents observed in figure 29 and 30, in that the charge transfer efficiency is

greater in the P5”-M donor/PC71BM acceptor blend. Nevertheless, these PAE homopolymers

show very poor PCEs compared to their vinylene-bridged counterparts (PDC16TTV - 2.5 %

PDC16TTE-M:PC71BM (1:2)

PBEHTTE-M:PC71BM (1:3)

Page | 212

and PBEHTTV – 1.87 %). This may be due to several factors (photophysical, and

morphological) which could have grave influence on the PCE of the OPV device.

Importantly, the vinylene-bridge fused thiophene conjugated polymers displayed broader

absorption spectra covering from the 300 nm to 750 nm region of the solar spectra, which

means that more photons would be harvested from the sun. In addition, the OFET devices

based on these polymers showed considerably higher hole mobilities (PDC16TTV - 0.02 cm-2

V-1

s-1

, and PBEHTTV - 0.008 cm2 V

-1 s

-1) than even the PAE copolymers. Unfortunately, the

OPV device incorporating the PAE copolymers were not recorded.

3.5 Conclusion

In this project novel conjugated poly(arylene ethynylene) homopolymer and push-pull

type copolymers bearing hexadecyl (C16) and 2-ethylhexyl (EH) functionalities were

successfully synthesized by the Sonogashira cross-coupling reaction assisted by microwave

irradiation and/or conventional heating methods. The properties of these conjugated polymers

were characterized via various analytical techniques to ascertain their suitability as active

materials for organic electronic (OPV and OFET) applications.

The homopolymers, poly(3,6-dialkylthieno[3,2-b]thiophene-2,5-diyl ethynylene)s

(PDC16TTE-M and PBEHTTE-M) were made up of predominantly fused alkylthiophenes

(alkylthieno[3,2-b]thiophene) bridged by the rigid triple bonded carbon-carbon covalent bond

(C≡C). On the other hand, the D-A copolymers consisted of alternating electron-donating

dialkyl-thieno[3,2-b]thiophene unit and a host of electron-withdrawing moieties, OTPD

(PDC16TTE-OTPD-M and PBEHTTE-OTPD), ODPP (PDC16TTE-ODPP-M and PBEHTTE-

ODPP-M), and BT (PDC16TTE-BT-M). The impact of the alkyl side groups (C16 vs EH), the

electron-acceptors, and the C≡C linkages, present in the conjugated homopolymers and

copolymers, on the performance of the OFET and OPV devices were of particular interest to

us. As expected, the homopolymers, PDC16TTE-M and PBEHTTE-M, exhibited good

solution processability, with absorption mostly limited to the visible region (300 nm – 600

nm) of the solar spectra. Consequently, they exhibited large optical band gaps even in the

solid state (2.03 eV - C16, and 2.14 eV - EH). Moreover, electrochemical measurements

revealed higher HOMO and LUMO energy offsets (2.31 eV – C16 and 2.21 eV – EH). These

indicated that the planarizing C≡C spacer did not sufficiently extend the effective conjugation

length of the homopolymers, unlike the vinylene-bridged congener (PDC16TTV and

PBEHTTV).

Page | 213

In contrast, the push-pull type copolymers showed broad absorption characteristics

covering both the visible and NIR regions of the solar spectra, especially in the case of the

DPP-bearing copolymers (PDC16TTE-ODPP-M and PBEHTTE-ODPP-M). The latter suggest

that the presence of the extra quinoid character provided by the thienylene units may have

played an important role to further extend the conjugation length of the DPP containing

copolymers (see figure 13 and 14). Nonetheless, the different alkyl side chains did have a

marked effect on the opto-electronic properties of the homopolymers and copolymers alike.

High thermal (~330 oC on average) and photo-oxidative stabilities (low lying HOMO levels)

were observed for both the homopolymers and copolymers. The presence of the electron-

deficient C≡C bonds serves to planarize the polymer backbone and, thus precluded any steric

hindrance between adjacent aromatic units, hence the high IPs observed in the homopolymer

(- 5.77 eV). In the copolymers, this effect combined with the strong D-A interactions

provided by the aromatic acceptors, also contributed to the adjustment of the HOMO and

LUMO energy levels alike.

XRD measurements showed ordered and crystalline microstructure for the high Mn

PDC16TTE-ODPP-M, PDC16TTE-BT-M, and PDC16TTE-M. In contrast, the branched EH

analogues, PBEHTTE-M, PBEHTTE-OTPD-M, and PBEHTTE-ODPP-M, all displayed less

ordered structures. This suggests that the alkyl side chains do have a degenerating effect on

the crystallinity of the conjugated polymer, besides enhancing their solubility. Preliminary

OPV measurements involving the homopolymers in blends with PC71BM acceptor yielded

very low efficiencies (0.26 % - C16 and 0.43 % - EH), in spite of the high Voc (~ 0.80 V on

average) obtained. The efficiency of the device was mostly limited by the low Jsc of the

device. The performance of the copolymers in organic solar cells is currently underway.

Furthermore, copolymers, PDC16TTE-BT-M showed the highest field effect mobility (1.0 x

10-3

cm2 V

-1s

-1) after thermal annealing. On the other hand, under similar conditions the

mobility of the microwave-synthesized PDC16TTE-ODPP-M copolymer (2.25 x 10-4

cm2 V

-

1s

-1) was higher than that of prepared by under conventional heating, PD16TTE-ODPP-C (2.5

x 10-5

cm2 V

-1s

-1). Unfortunately, the charge carrier mobility of PDC16TTE-M, PBEHTTE-M,

PBEHTTE-ODPP-M, PDC16TTE-OTPD and PBEHTTE-OTPD were not measureable.

Further work is needed to ascertain the cause and to optimize the processing conditions, so as

to enhance the performance of these PAE conjugated polymers in OPV and OFET devices.

Page | 214

3.6 Experimental Section

3.6.1 Materials and Chemical Reagents

The catalyst, tetrakis(triphenylphosphine) Palladium(0) (Pd(PPh3)4, 99 %),

trimethylsilylacetylene (TMSA, assay, 98 %), diisopropylamine (DIPA, ≥ 99.5 %) and

Cuprous Iodide (CuI, ≈ 98 %) were purchased from Aldrich Chemical Company.

Triethylamine (Et3N, 99 %) and triphenylphosphine (PPh3, ˃ 99 %) were obtained from Alfa

Aesar. The triethylamine base was distilled over potassium hydride (VWR) to ensure

complete dryness, before use. Potassium carbonate (NaHCO3), acetonitrile, methanol

(CH3OH), tetrahydrofuran (THF), petroleum hexane (60 – 70 oC), and calcium hydroxide

(Ca(OH)2) were procured from VWR PROLABO. The dibrominated acceptor monomers,

3,4-(N-n-octylimido)-2,5-dibromothiophene (M6),56

3,6-di(2-bromothien-5-yl)-2,5-dioctyl-

pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (M7),71, 72, 163

and 4,7-dibromo-2,1,3-

benzothiadiazole (M8),106, 164

were synthesized as described in the literature. On the other

hand, the synthesis of the dibrominated donors, 3,6-dihexadecyl-2,5-dibromothieno[3,2-

b]thiophene (M5’) and 3,6-bis(2thylhexyl)-2,5-dibromothieno[3,2-b]thiophene (M5”), were

performed as delineated in chapter 2.

3.6.2 Experimental Protocol

3.6.2.1 Synthesis of 3,6-dihexadecyl-2,5-bis(trimethylsilylethynyl)thieno[3,2-b]thiophene

Pr3’.

Compound Pr3’ was synthesized by a modification of a literature protocol.165

Procedure 1: In a typical reaction, 2,5-dibromo-3,6-dihexadecylthieno[3,2-b]thiophene Pr1

(1.045 g, 1.40 mmol), Pd(PPh3)4 (65.0 mg, 0.056 mmol, 4 mol%), and CuI (11.0 mg, 0.056

mmol, 4 mol%) were dissolved in a mixture of anhydrous DIPA (10 mL) and dry toluene (20

mL) under N2 atmosphere. After 10 mins of constant deaeration and stirring at 50 oC, TMSA

(0.60 mL, 4.20 mmol) was introduced via a degassed syringe. The temperature of the reaction

Page | 215

mixture was raised to 75 oC with constant stirring overnight. After cooling to RT, the white

ammonium bromide precipitate formed was removed by filtration. The resulting

orange/yellow viscous oil obtained after concentration in vacuo, was purified by silica-gel

column chromatography (eluent: petroleum hexane 67 – 70 oC) to afford a pale-yellow

semisolid. After drying in a vacuum oven for 24 h at 40 oC Pr3’ was furnished (0.88 g, 81 %

yield). Procedure 2: To a dry triethylamine solution (30 mL) was added Pr1 (2.31 g, 3.10

mmol), Pd(PPh3)4 (71.66 mg, 2 mol%), CuI (11.81 mg, 2 mol%), and PPh3 (81.31 mg, 10

mol%), under a stream of N2. The reaction temperature was raised to 50 oC, followed by the

introduction of TMSA (0.43 mL, 3.10 mmol). Stirring was continued for 24 h at 80 °C under

N2 protection. On completion, the yellow-coloured reaction mixture was left to cool, diluted

with diethylether (150 mL), filtered, and condensed in a rotary evaporator. The resultant oil

was purified as described in procedure 1 to afford a pale-yellow solid Pr3’ (2.28 g, 94%

yield). MS (EI): m/z (M+): 780.5 (C48H84Si2S2

+).

1H NMR (400 MHz, CDCl3): (ppm) =

0.29 (s, 18H), 0.91 (t, 6H, J = 8.0 Hz); 1.29 (br, 52H); 1.74 (m, 4H); 2.80 (t, 4H, J = 8.0 Hz).

13C NMR (400 MHz, CDCl3): (ppm) = 0.07 (6C); 14.13 (2C); 22.71; 28.38; 28.82, 29.38

(3C); 29.57; 29.71 (24 C); 30.93; 31.95; 97.81; 103.24; 121.00; 137.70; 140.77. HRMS EI

calcd for C44H84Si2S2 780.5553, found 780.5538.

3.6.2.2 Synthesis of 3,6-bis(2-ethylhexyl)-2,5-bis(trimethylsilylethynyl)thieno[3,2-

b]thiophene Pr3”.

The procedures described for the synthesis of Pr3’ were also employed in the

preparation of the Pr3” derivative. Using procedure 1, Pr3” was obtained as a yellow oil

(1.26 g, 73 % yield) by reacting Pr1 (1.624 g, 3.11 mmol), Pd(PPh3)4 (72.0 mg, 0.062 mmol,

2 mol %), CuI (11.9 mg, 0.062 mmol, 2 mol %), and PPh3 (81.6 mg, 0.311 mmol, 10 mol %)

in anhydrous triethylamine base (40 mL). By applying procedure 2, a higher yield was

attained (0.95 g, 89 %) from the reaction of Pr1 (0.889 g, 1.70 mmol), Pd(PPh3)4 (72.0 mg,

0.068 mmol, 4 mol %), and CuI (13.0 mg, 0.068 mmol, 4 mol %) in a dry solvent mixture of

Page | 216

DIPA (10 mL) and toluene (20 mL). MS (EI): m/z (M+): 556.3 (C32H52Si2S2

+).

1H NMR (400

MHz, CDCl3): (ppm) = 0.29 (s, 18H), 0.93 (t, 12H, J = 8.0 Hz); 1.32 (m.br, 22H); 1.89 (m,

2H); 2.71 (d, 4H, J = 8.0 Hz). 13

C NMR (400 MHz, CDCl3): (ppm) = 0.10; 10.63; 14.10;

23.02; 25.87; 28.76, 32.79; 33.47; 39.11; 98.05; 103.27; 121.38; 137.98; 140.22. HRMS EI

calcd for C32H52Si2S2 556.3049, found 556.3043.

3.6.2.3 Synthesis of 3,6-dihexadecyl-2,5-bis(ethynyl)thieno[3,2-b]thiophene M4’.

Pr3’ (0.88 g, 1.13 mmol) was reacted with aqueous potassium hydroxide (0.15 g, 2.60

mmol in 1 mL H2O) in a mixture of THF and methanol (30 mL, 2:1 v/v), under N2. The

yellow-coloured reaction mixture was stirred at room temperature for 3 – 4 h. On completion,

the reaction mixture was dried by the addition of anhydrous MgSO4, and the solvent removed

under reduced pressure. The resultant crude product was purified by column chromatography

on silica-gel (eluent: petroleum hexane 67 – 70 oC), and concentrated in a rotary evaporator

to afford the protodesilylated monomer M4’ (814 mg, 99 % yield). The yield is not absolute

due to the facile decomposition of the titled product when removed from solution and /or

dried under vacuum at room temperature. Therefore, in order to prevent this decomposition,

M5’ was stored in hexane and used immediately for the next step after concentration. MS

(EI) (M+): m/z: 636.5 (C42H68S2

+).

1H NMR (400 MHz, CDCl3): (ppm) = 0.91 (t, 6H, J = 8

Hz); 1.28 (br, 52H); 1.74 (m, 4H); 2.83 (t, 4H, J = 8 Hz); 3.62 (s, 2H). 13

C NMR (400 MHz,

CDCl3): (ppm) = 14.13 (2C); 22.70 (4C); 28.49; 28.92, 29.28; 29.37; 29.51; 29.68 (8C);

29.71(2C); 32.12 (4C); 85.30 (2C); 97.86; 103.26; 121.42; 137.62; 140.77. HRMS EI calcd

for C42H68S2 636.4762, found 636.4769.

Page | 217

3.6.2.4 Synthesis of 3,6-bis(2-ethylhexyl)-2,5-bis(ethynyl)thieno[3,2-b]thiophene M4”.

The synthesis of M4” was performed in accordance with the protocol used for M4’.

MS (EI) (M+): m/z: 412.2 (C26H36S2

+).

1H NMR (400 MHz, CDCl3): (ppm) = 0.93 (t, 12H,

J = 8 Hz); 1.33 (br, 22H); 1.89 (m, 2H); 2.75 (d, 4H, J = 8 Hz); 3.61 (s, 2H). 13

C NMR (400

MHz, CDCl3): (ppm) = 10.27; 14.10; 22.98; 26.00; 28.72, 32.76; 33.32; 39.01; 85.28 (2C);

120.50; 124.23; 129.60; 137.98; 140.66. HRMS EI calcd for C26H36S2 412.2258, found

412.2256.

3.6.2.5 Synthesis of Poly(3,6-dihexadecylthieno[3,2-b]thiophene-2-yl ethynylene) –

PDC16TTE-M P5’-M via Microwave-accelerated Sonogashira-Hagihara cross-coupling

Polycondensation.

This polymerization was performed following similar procedure to the literature.23

Microwave Heating: A microwave vial was charged with the freshly prepared diethynyl

monomer M4’ (0.175 mg, 0.275 mmol), the dibromo monomer M5’ (0.205 g, 0.275 mmol),

Pd(PPh3)4 (6 mol %), CuI (12 mol %), a stirring bar and sealed. The reaction mixture was

immediately placed under N2(g) or argon and degassed for 10 mins, followed by the addition

of anhydrous chlorobenzene (15 mL) and distilled diisopropylamine (5 mL). Then the

reaction mixture was deaerated for a further 20 mins with stirring. Afterwards, the mixture

heated sequentially in a microwave reactor at 100 oC (5 mins), 140

oC (6 mins), and 160

oC

(20 mins). On completion, the viscous orange-coloured crude was cool to room temperature

and transferred by pipette into methanol (200 mL). The red precipitate formed was collected

Page | 218

by filtration through an extraction thimble, washed with methanol, and subjected to further

purification by Soxhlet extraction in methanol (24 h), acetone (24 h), and hexane (24 h). The

resulting solid was re-dissolved in chloroform, precipitated (2x) in methanol and isolated.

The polymer was dried under vacuum at 40 oC for 24 h to afford P5’-M (0.141 g, 88 %).

Conventional Method: A mixture of DIPA (10 mL) and toluene (30 mL) was introduced

using a degassed syringe into sealed three-neck round bottom flask, charged with monomer

M5’ (0.275 mmol), monomer 3 (0.275 mmol), Pd(PPh3)4 (6 mol %), and CuI (6 mol %)

under N2(g) protection. The mixture was stirred for 48 h at 80 oC. Effervescence was

observed, accompanied by an intense orange colour transformation. On completion, the

reaction was cooled to room temperature and poured into methanol (200 mL). The red

precipitate was filtered, and processed as described above. 1H NMR (400 MHz, 1,2-DCB-d2):

(ppm) 1.08 (br, 6H); 1.50 (br, 52H); 1.99 (br, 4H); 3.04 (br, 4H). Found: C, 78.62; H, 10.89

%. Calcd. (C40H66S2)n: C, 76.51; H, 9.83 %.

3.6.2.6 Synthesis of Poly(3,6-bis(2-ethyhexyl)thieno[3,2-b]thiophen-2-yl ethynylene) –

PBEHTTE-M P5”-M via Microwave-accelerated Sonogashira-Hagihara cross-coupling

Polycondensation.

This polymer was synthesized from M4” (0.322 g, 0.774 mmol), M5” (0.392 g, 0.774

mmol), Pd(PPh3)4 (6 mol %), and CuI (12 mol %) using microwave-heating. 1H NMR (400

MHz, 1,2-DCB-d2): (ppm) 1.16 (br, 12H); 1.62 (br, 22H); 2.35 (br, 2H); 3.17 (br, 4H).

Found: C, 74.55; H, 8.89 %. Calcd. (C24H24S2)n: C, 71.15; H, 9.24 %.

Page | 219

3.6.2.7 Synthesis of poly(3,6-hexadecylthieno[3,2-b]thiophene-2,5-yl ethynyl-alt-2,8-(N-

octyl)thieno[3,4-c]pyrrole-4,6-dione) – PDC16TTE-OTPD-M P6’-M via Microwave-

accelerated Sonogashira-Hagihara cross-coupling Polycondensation.

This polymer was synthesized from the diacetylene M4’ (0.201 g, 0.314 mmol),

dibromo thieno[3,4-c]pyrrole-4,6-dione M6’ (0.133 g, 0.314 mmol), Pd(PPh3)4 (6 mol %),

and CuI (12 mol %) using microwave-heating. 1H NMR (400 MHz, 1,2-DCB-d2): (ppm)

1.08 (br, 9H); 1.49 (br, 62H); 1.97 (br, 4H); 2.08 (br, 2H); 3.18 (br, 4H); 3.89 (br, 2H).

Found: C, 74.86; H, 9.31; N, 1.56 %. Calcd. (C56H83S3NO2)n: C, 74.58; H, 9.73, N, 1.82 %.

3.6.2.8 Synthesis of poly(3,6-bis(2-ethylhexyl)lthieno[3,2-b]thiophene-2,5-yl ethynyl-alt-

2,8-(N-octyl)thieno[3,4-c]pyrrole-4,6-dione) – PBEHTTE-OTPD-M P6”-M via Microwave-

accelerated Sonogashira-Hagihara cross-coupling Polycondensation.

P6”-M was synthesized from the diacetylene M4” (0.115 g, 0.279 mmol), the

dibromo thieno[3,4-c]pyrrole-4,6-dione M6” (0.118 g, 0.279 mmol), Pd(PPh3)4 (6 mol %),

and CuI (12 mol %) using microwave-heating method. 1H NMR (400 MHz, 1,2-DCB-d2):

(ppm) 0.88 (br, 15H); 1.28 (br, 21H); 1.58 (s, 2H); 2.31 (br, 2H); 2.82 (br, 4H); 3.65 (br, 2H).

Found: C, 71.28; H, 7.63; N, 2.08 %. Calcd. (C40H51S3NO2)n: C, 74.32; H, 10.29, N, 1.28 %.

3.6.2.9 Synthesis of poly(3,6-hexadecylthieno[3,2-b]thiophene-2,5-yl ethynyl-alt-3,6-

dithien-2-yl-2,5-dioctylpyrrolo[3,4-c]pyrrole-1,4-dione-5’,5”-diyl) – PDC16TTE-ODPP-M

Page | 220

P7’-M via Microwave-accelerated Sonogashira-Hagihara cross-coupling

Polycondensation.

This polymer was synthesized from the diethynylene M4’ (0.218 g, 0.343 mmol),

dibromo 2,5-dioctylpyrrolo[3,4-c]pyrrole-1,4(2H, 5H)-dione M7’ (0.234 g, 0.343 mmol),

Pd(PPh3)4 (6 mol %), and CuI (12 mol %) by employing the microwave-heating method. 1H

NMR (400 MHz, 1,2-DCB-d2): (ppm) 1.16 (br, 12H); 1.59 (br, 72H); 2.08 (br, 4H); 2.15

(br, 4H); 3.20 (br, 4H); 4.27 (br, 4H). Found: C, 74.69; H, 9.05; N, 2.42 %. Calcd.

(C72H104S4N2O2)n: C, 72.86; H, 9.42, N, 2.41 %.

3.6.2.10 Synthesis of poly(3,6-bis(2-ethylhexyl)thieno[3,2-b]thiophene-2,5-diyl ethynyl-alt-

3,6-bis(thien-2-yl)-2,5-dioctylpyrrolo[3,4-c]pyrrole-1,4-dione-5’,5”-diyl) – PBEHTTE-

ODPP-M P7”-M via Microwave-accelerated Sonogashira-Hagihara cross-coupling

Polycondensation.

This polymer was synthesized from the diethynylene M4” (0.148 g, 0.356 mmol),

dibromo 2,5-dioctylpyrrolo[3,4-c]pyrrole-1,4(2H, 5H)-dione M7’ (0.243 g, 0.356 mmol),

Pd(PPh3)4 (6 mol %), and CuI (12 mol %) by employing the microwave-heating method. 1H

NMR (400 MHz, 1,2-DCB-d2): (ppm) 1.14 (br, 18H); 1.55 (br, 26H); 2.05 (br, 4H); 2.32

(br, 2H); 3.12 (br, 4H); 4.34 (br, 4H); 7.57 (s, 2H); 9.32 (br, 2H). Found: C, 74.67; H, 7.77;

N, 3.11 %. Calcd. (C56H70S4N2O2)n: C, 71.43; H, 7.79, N, 2.94 %.

Page | 221

3.6.2.11 Synthesis of poly(3,6-dihexadecylthieno[3,2-b]thiophene-2,5-diyl ethynyl-alt-

benzo[1,2,5]thiadiazole-4,7-diyl) – PDC16TTE-BT-M P8’-M via Microwave-accelerated

Sonogashira-Hagihara cross-coupling Polycondensation.

This polymer was synthesized from the diacetylene M4’ (0.180 g, 0.284 mmol), the

dibromo benzo[2,1,3]thiazole M8’ (0.092 g, 0.284 mmol), Pd(PPh3)4 (6 mol %), and CuI (12

mol %) using microwave-heating. 1H NMR (400 MHz, 1,2-DCB-d2): (ppm) 1.08 (br, 6H);

1.49 (br, 52H); 2.08 (br, 4H); 3.34 (br, 4H). Found: C, 74.95; H, 8.91; N, 3.62 %. Calcd.

(C48H68N2S3)n: C, 77.92; H, 10.97, N, 1.40 %.

Page | 222

3.7 References

1. Bunz, U. H. F., Acc. Chem. Res., 2001, 34, 998 - 1010.

2. Bunz, U. H. F., Chem. Rev. 2000, 100, 1605 - 1644.

3. Moore, J. S., Acc. Chem. Res., 1997, 30, 402.

4. Giesa, R.; Schulz, R. C., Macromol. Chem. Phys., 1990, 191, 857 - 867.

5. Yamamoto, T.; Honda, K.; Ooba, N.; Tomaru, S., Macromolecules 1998, 31, 7 - 14.

6. Yamamoto, T.; Yamada, W.; Takagi, M.; Kizu, K.; Maruyama, T.; Ooba, N.; Tomaru,

S.; Kurihara, T.; Kaino, T.; Kubota, K., Macromolecules 1994, 27, 6620 - 6626.

7. Halik, M.; Klauk, H.; Zschieschang, U.; Schmid, G.; Dehm, C.; Schutz, M.; Maisch,

S.; Effenberger, F.; Brunnbauer, M.; Stellacci, F., Nature 2004, 431, 963 - 966.

8. Moroni, M.; Le Moigne, J.; Luzzati, S., Macromolecules 1994, 27, 562 - 571.

9. Weder, C.; Wrighton, M. S., Macromolecules 1996, 29, 5157 - 5165.

10. Beeby, A.; Findlay, K.; Low, P. J.; Marder, T. B., J. Am. Chem. Soc., 2002, 124, 8280

-8284.

11. Levitus, M.; Schmieder, K.; Ricks, H.; Shimizu, K. D.; Bunz, U. H. F.; Garcia-

Garibay, M. A., J. Am. Chem. Soc., 2001, 123, 4259 - 4265.

12. Donhauser, Z. J.; Mantooth, B. A.; Kelly, K. F.; Bumm, L. A.; Monnell, J. D.;

Stapleton, J. J.; Price Jr., D. W.; Rawlett, A. M.; Allara, D. L.; Tour, J. M.; Weiss, P. S.,

Science 2001, 292, 2303 - 2307.

13. Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R. N.; Mackay, K.;

Friend, R. H.; Burns, P. L.; Holmes, A. B., Nature 1990, 347, 539 - 541.

14. Gustafsson, G.; Cao, Y.; Treacy, M.; Klavetter, F.; Colaneri, N.; Heeger, A. J., Nature

1992, 357, 477 - 479.

15. Anderson, S., Chem. Eur. J. 2001, 7, 4706 - 4714.

16. Schmitz, C.; Posch, P.; Thelakkat, M.; Schmidt, H. W.; Montak, A.; Feldman, K.;

Smith, P.; Weder, C., Adv. Funct. Mater., 2001, 11, 41 - 46.

17. Grimsdale, A. C.; Chan, K. L.; Martin, R. E.; Jokisz, P. G.; Holmes, A. B., Chem.

Rev., 2009, 109, 897 - 1091.

18. Palai, A. K.; Mishra, S. P.; Kumar, A.; Srivastava, R.; Kamalasanan, M. N.; Patri, M.,

Eur. Polym. J., 2010, 46, 1940-1951.

19. Roy, V. A. L.; Zhi, Y.-G.; Xu, Z.-X.; Yu, S.-C.; Chan, P. W. H.; Che, C.-M., Adv.

Mater. 2005, 17, 1258 - 1261.

20. Baek, N. S.; Hau, S. K.; Yip, H.-L.; Acton, O.; Chen, K.-S.; Jen, A. K.-Y., Chem.

Mater. 2008, 20, 5734 - 5736.

21. Mwaura, J. K.; Pinto, M. R.; Witker, D.; Ananthakrishnan, N.; Schanze, K. S.;

Reynolds, J. R., Langmuir 2005, 21, 10119 - 10126.

22. Mwaura, J. K.; Zhao, X.; Jiang, H.; Schanze, K. S.; Reynolds, J. R., Chem. Mater.,

2006, 18, 6109 - 6111.

23. Ashraf, R. S.; Shahid, M.; Klemm, E.; Al-Ibrahim, M.; Sensfuss, S., Macromol. Rapid

Commun., 2006, 27, 1454 - 1459.

24. Silvestri, F.; Marrocchi, A., Int. J. Mol. Sci. 2010, 11, 1471 - 1508.

25. Kim, Y. S.; Park, J. H.; Lee, S.-H.; Lee, Y., Sol. Energy Mater. Sol. Cells 2009, 33,

1398 - 1403.

26. Lu, S.; Yang, M.; Luo, J.; Cao, Y., Synth. Met. 2004, 140, 199 - 202.

27. Weder, C. l.; Sarwa, C.; Montali, A.; Bastiaanseen, C.; Smith, P., Science 1998, 279,

835 - 837.

28. Montali, A.; Bastiaansen, C.; Smith, P.; Weder, C., Nature 1998, 329, 261 - 264.

29. Thomas III, S. W.; Joly, G. D.; Swager, T. M., Chem. Rev. 2007, 107, 1339 - 1386.

30. Kim, I.-B.; Bunz, U. H. F., J. Am. Chem. Soc., 2006, 128, 2818 - 2819.

Page | 223

31. Rose, A.; Lugmair, C. G.; Swager, T. M., J. Am. Chem. Soc., 2001, 123, 11298 -

11299.

32. Wautelet, P.; Moroni, M.; Oswald, L.; Le Moigne, J.; Pham, A.; Bigot, J.-Y.; Luzzati,

S., Macromolecules 1996, 29, 446 - 455.

33. Ohkita, H.; Cook, S.; Astuti, Y.; Duffy, W.; Tierney, S.; Zhang, W.; Heeney, M.;

McCulloch, I.; Nelson, J.; Bradley, D. D. C.; Durrant, J. R., J. Am. Chem. Soc. 2008, 130,

3030 - 3042.

34. Ashraf, R. S.; Klemm, E., J. Polym. Sci., Part A: Polym. Chem., 2005, 43, 6445 -

6454.

35. Morikita, T.; Yamaguchi, I.; Yamamoto, T., Adv. Mater., 2001, 13, 1862 - 1864.

36. Yamamoto, T.; Zhou, Z.-H.; Kanbara, T.; Shimura, M.; K. Kizu; Maruyama, T.;

Nakamura, Y.; Fukuda, T.; Lee, B.-L.; Ooba, N.; Tomaru, S.; Kurihara, T.; Kaino, T.;

Kubota, K.; Sasaki, S., J. Am. Chem. Soc., 1996, 118, 10389 - 10399.

37. Zhang, G.; Liu, K.; Li, Y.; Yang, M., Polym. Int., 2009, 58, 665 - 673.

38. Zhang, G.; Liu, K.; Fan, H.; Li, Y.; Zhan, X.; Li, Y.; Yang, M., Synth. Met., 2009,

159, 1991 - 1995.

39. Wu, Z.; Fan, B.; Li, A.; Xue, F.; Ouyang, J., Org. Elec., 2011, 12, 993-1002.

40. Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y., Nat.

Mater. 2005, 4, 864 - 868.

41. Brabec, C. J., Sol. Energy Mater. Sol. Cells 2004, 83, 273.

42. Sonogashira, K.; Tohda, Y.; Hagihara, N., Tet. Lett. 1975, 16, 4467 - 4470.

43. Sonogashira, K., J. Organomet. Chem. 2002, 653, 46 - 49.

44. Negishi, E. J.; Anastasia, L., Chem. Rev. 2003, 103, 1979 - 2018.

45. Chinchilla, R.; Carmen Nájera, C., Chem. Rev. 2007, 107, 874 - 922.

46. van Mullekom, H. A. M.; Vekemans, J. A. J. M.; Havinga, E. E.; Meijer, E. W.,

Mater. Sci. Eng. 2001, 32, 1 - 40.

47. Hou, J.; Chen, H.-Y.; Zhang, S.; Li, G.; Yang, Y., J. Am. Chem. Soc. 2008, 130,

16144 - 16145.

48. Park, S. H.; Roy, A.; S., B.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.;

Lee, K.; Heeger, A. J., Nature Photonics 2009, 3, 297 - 303.

49. Blouin, N.; Michaud, A.; Leclerc, M., Adv. Mater. 2007, 19, 2295 - 2300.

50. Price, S. C.; Stuart, A. C.; You, W., Macromolecules 2010, 43, 4609 - 4612.

51. Guo, X.; Kim, F. S.; Jenekhe, S. A.; Watson, M. D., J. Am. Chem. Soc. 2009, 131,

7206 - 7207.

52. Huo, L.; Hou, J.; Zhang, S.; Chen, H.-Y.; Yang, Y., Angew. Chem. Int. Ed. 2010, 49,

1500 - 1503.

53. Morikita, T.; Yamaguchi, I.; Yamamoto, T., Adv. Mater. 2001, 13, 1862 - 1864.

54. Zhang, X.; Steckler, T. T.; Dasari, R. R.; Ohira, S.; Potscavage, J., W. J.; Tiwari, S.

P.; Coppee, S.; Ellinger, S.; Barlow, S.; Bredas, J.-L.; Kippelen, B.; Reynolds, J. R.; Mardera,

S. R., J. Mater. Chem. 2010, 20, 123 - 134.

55. Nielsen, C. B.; Bjørnholm, T., Org. Lett., 2004, 6, 3381 - 3384.

56. Zhang, Q. T.; Tour, J. M., J. Am. Chem. Soc. 1998, 120, 5355 - 5362

57. Yuan, M.-C.; Chiu, M.-Y.; Liu, S.-P.; Chen, C.-M.; Wei, K.-H., Macromolecules

2011, 44, 269 - 277.

58. Yuan, M.-C.; Chiu, M.-Y.; Liu, S.-P.; Chen, C.-M.; Wei, K.-H., Macromolecules

2010, 43, 6936 - 6938.

59. Piliego, C.; Holcombe, T. W.; Douglas, J. D.; Woo, C. H.; Beaujuge, P. M.; Frechet,

J. M. J., J. Am. Chem. Soc. 2010, 132, 7595 - 7597.

60. Zou, Y.; Najari, A.; Berrouard, P.; Beaupre, S.; Aıch, B. R.; Tao, Y.; Leclerc, M., J.

Am. Chem. Soc., 2010, 132, 5330-5331.

Page | 224

61. Farnum, D. G.; Metha, G.; Moore, G. G. I.; Siegal, F. P., Tetrahedron Lett., 1974, 29,

2549 - 2552.

62. Tieke, B.; Rabindranath, A. R.; Zhang, K.; Zhu, Y., Beilstein J. Org. Chem. 2010, 6,

830 - 845.

63. Thetford, D.; Cherryman, J.; Chorlton, A. P.; Docherty, R., Dyes and Pigments 2004,

63, 259 - 276.

64. Li, Y.; Singh, S. P.; Sonar, P., Adv. Mater., 2010, 22 (43), 4862 - 4866.

65. Li, W.; Lee, T.; Oh, S. J.; Kagan, C. R., Appl. Mater. Interfaces 2011, 3, 3874 - 3883.

66. Burckstummer, H.; Weissenstein, A.; Bialas, D.; Wurthner, F., J. Org. Chem., 2011,

76, 2426 - 2432.

67. Chan, W. K.; Chen, Y.; Peng, Z.; Yu, L., J. Am. Chem. Soc. 1993, 115, 11735 -

11743.

68. Bao, Z.; Chan, W. K.; Yu, L., J. Am. Chem. Soc. 1995, 117, 12426 - 12435.

69. Zhu, Y.; Zhang, K.; Tieke, B., Macromol. Chem. Phys., 2009, 210, 431 - 439.

70. Behnke, M.; Tieke, B., Langmuir 2002, 18, 3815 - 3821.

71. Stas, S.; Sergeyev, S.; Geerts, Y., Tetrahedron 2010, 66, 1837 - 1845.

72. Zhou, E.; Yamakawa, S.; Tajima, K.; Yang, C.; Hashimoto, K., Chem. Mater. 2009,

21, 4055 - 4061.

73. Burgi, L.; Trubiez, M.; Pfeiffer, R.; Bienewald, F.; Kirner, H. J.; Winnewisser, C.,

Adv. Mater. 2008, 20, 2217 - 2224.

74. Wienk, M. M.; Turbiez, M.; Gilot, J.; Janssen, R. A. J., Adv. Mater. 2008, 20, 2556 -

2560.

75. Zou, Y. P.; Gendron, D.; Badrou-Aich, R.; Najari, A.; Tao, Y.; Leclerc, M.,

Macromolecules 2009, 42, 2891 - 2894.

76. Dieck, H. A.; Heck, R. F., 93, 259., J. Organomet. Chem. 1975, 93, 259 - 263.

77. Cassar, I., J. Organomet. Chem. 1975, 93, 253 - 257.

78. Tykwinski, R. R., Angew. Chem. Int. Ed. 2003, 42, 1566 - 1568.

79. Heck, R. F.; Nolley, J. P., Jr. , J. Org. Chem., 1972, 37, 2320 - 2322.

80. Mizoroki, T.; Mori, K.; Ozaki, A., Bull. Chem. Soc. Jpn. 1971, 44, 581.

81. Stille, J. K., Angew. Chem. Int. Ed. Engl., 1986, 25, 508-524.

82. Stille, J. K.; Lau, K. S. Y., Acc. Chem. Res., 1977, 10, 434-442.

83. Suzuki, A., J. Org. Chem. 1999, 576, 147 - 168.

84. Miyaura, N.; Suzuki, A., Chem. Rev. 1995, 95, 2457 - 2483.

85. Beletskaya, J. P., J. Organomet. Chem., 1983, 250, 551-564.

86. Scott, W. J.; Stille, J. K., J. Am. Chem. Soc., 1986, 108, 3033-3040.

87. Goodson, F. E.; Wallow, T. I.; Novak, B. M., J. Am. Chem. Soc. 1997, 119, 12441 -

12453.

88. Khan, A.; Hecht, S., Chem. Comm. 2004, 300 - 301.

89. Hager, H.; Heitz, W., Macromol. Chem. Phys., 1998, 199, 1821 = 1826.

90. Zhou, Q.; Swager, T. M., J. Am. Chem. Soc. 1995, 117, 12593 - 12602.

91. Ofer, D.; Swager, T. M.; Wrighton, M. S., Chem. Mater. 1995, 7, 418 - 425.

92. Moore, J. S.; S., Z. J., Angew. Chem. Int. Ed. Engl. 1992, 31, 922 - 924.

93. Nishihara, Y.; Inoue, E.; Ogawa, D.; Okada, Y.; Noyori, S.; Takagi, K., Tet. Lett.

2009, 50, 4643 - 4646.

94. Osakada, K.; Sakata, R.; Yamamoto, T., Organometallics 1997, 16, 5354 - 5364.

95. Alami, M.; Ferri, F.; Linstrumelle, G., Tet. Lett. 1993, 34, 6403 - 6406.

96. Yamamoto, T.; Honda, K.; Ooba, N.; Tomarus, S., Macromolecules 1998, 31, 7 - 14.

97. Jørgensen, M.; Norrman, K.; Krebs, F. C., Sol. Energy Mater. Sol. Cells 2008, 92,

686-714.

Page | 225

98. Neugebauer, H.; Brabec, C. J.; Hummelen, J. C.; Janssen, R. A. J.; Sariciftci, N. S.,

Synth. Met., 1999, 102, 1002-1003.

99. Cumpston, B. H.; Jensen, K. F., J. Appl. Polym. Sci., 1998, 69, 2451-2458.

100. Yuning, L. Diketopyrrolopyrrole-based derivatives for thin film transistors. 2009,

2009.

101. Tamayo, A. B.; Tantiwiwat, M.; Walker, B.; Nguyen, T.-Q., J. Phys. Chem. C 2008,

112, 15543 - 15552.

102. Zhang, Q. T.; Tour, J. M., J. Am. Chem. Soc. 1997, 119, 9624 - 9631.

103. Pomerantz, M.; Yang, H.; Cheng, Y., Macromolecules 1995, 28, 5706 - 5708

104. Pilgram, K.; Zupan, M.; Skiles, R., J. Heterocycl. Chem. 1970, 7, 629 - 633.

105. Kanbara, T.; Yamamoto, T., Chem. Lett., 1993, 22, 419.

106. Huo, L.; Hou, J.; Chen, H.-Y.; Zhang, S.; Jiang, Y.; Chen, T. L.; Yang, Y.,

Macromolecules 2009, 42 (6564 - 6571), 6564.

107. Edelmann, M. J.; Raimundo, J.-M.; Utesch, N. F.; Diederich, F., Helv. Chem. Acta

2002, 85, 2195 - 2213.

108. Palai, A. K.; Mishra, S. P.; Kumar, A.; Srivasrtava, R.; Kamalasanan, M. N.; Patri,

M., Macromol. Chem. Phys., 2009, 211, 1043 - 1053.

109. Hoogenboom, R.; Schubert, U. S., Macromol. Rapid Comm., 2007, 28, 368-386.

110. Sinnwell, S.; Ritter, H., Aust. J. Chem., 2007, 60, 729 - 743.

111. Nehls, B. S.; Fuldner, S.; Preis, E.; Farrell, T.; Scherf, U., Macromolecules 2005, 38,

687-694.

112. Nehls, B. S.; Asawapirom, U.; Fuldner, S.; Preis, E.; Farrell, T.; Scherf, U., Adv.

Funct. Mater., 2004, 14, 352-356.

113. Wan, J.; Zhu, R.; Xia, Y.; Qu, F.; Wu, Q.; Yang, G.; Neyts, J.; Peng, L., Tet. Lett.

2006, 47, 6727 - 6731.

114. Rössler, A.; Boldt, P., Synthesis 1998, 980 - 982.

115. Sarkis, G. Y.; Al-Azawe, S., J. Chem. Eng. Data 1972, 17, 516 - 518.

116. Miguel, L. S.; Matzger, A. J., Macromolecules 2007, 40, 9233-9237.

117. Witzel, S.; Ott, C.; Klemm, E., Macromol. Rapid Commun., 2005, 26, 889 - 894.

118. Yuan, M.-C.; Chiu, M.-Y.; Liu, S.-P.; Chen, C.-M.; Wei, K.-H., Macromolecules

2010, 43, 6936-6938.

119. Zhou, E.; Yamakawa, S.; Tajima, K.; Yang, C.; K., H., Chem. Mater. 2009, 21, 4055 -

4061.

120. Ashraf, R. S.; Klemm, E., J. Polym. Sci.: Part A: Polym. Chem., 2005, 43, 6445 -

6454.

121. Huo, L.; Hou, J.; Zhang, S.; Chen, H.-Y.; Yang, Y., Angew. Chem. Int. Ed., 2010, 49,

1500 -1503.

122. Coates, J., Interpretation of Infrared Spectra, A Practical Approach. In Encyclopedia

of Analytical Chemistry, Meyers, R. A., Ed. John Wiley & Sons Ltd: Chichester, 2000; pp

10815 - 10837.

123. AIST, Spectra Database for Organic Compounds

124. Zhou, E. J.; He, C.; Tan, Z. A.; Yang, C. H.; Li, Y. F., J. Polym. Sci., Part A: Polym.

Chem., 2006, 44, 4916 - 4922.

125. Zhang, X.; Wang, C.; Lai, G.; Zhang, L.; Shen, Y., New J. Chem. 2010, 34, 318 - 324.

126. Cremer, J.; Bäuerle, P.; Wienk, M. M.; Janssen, R. A. J., Chem. Mater. 2006, 18,

5832 - 5834.

127. Sluch, M. I.; Godt, A.; Bunz, U. H. F.; Berg, M. A., J. Am. Chem. Soc. 2001, 123,

6447 - 6448.

128. Miteva, T.; Palmer, L.; Kloppenburg, L.; Neher, D.; Bunz, U. H. F., Macromolecules

2000, 33, 652 - 654.

Page | 226

129. Wallquist, O.; Lenz, R., Macromol. Symp. 2002, 187, 617 - 629

130. Apperloo, J. J.; Janssen, R. A. J.; Malenfant, P. R. L.; Frechet, J. M. J.,

Macromolecules 2000, 33, 7038 - 7043.

131. Wenz, G.; Muller, M. A.; Schmidt, M.; Wegner, G., Macromolecules 1984, 17, 837 -

850.

132. Kuhn, H., Fortschr. Chem. Org. Naturst. 1958, 16, 169.

133. McBranch, D. W.; Sinclai, M. B., In The Nature of the Photoexcitations in

Conjugated Polymers. Sariciftci, N. S., Ed. World Scientific: Singapore, 1997; p 608.

134. Rothberg, L. J.; Yan, M.; Papadimitrakopoulos, F.; Galvin, M. E.; Kwock, E. W.;

Miller, T. M., Synth. Met. 1996, 80, 41 - 58.

135. Peng, Z., Polym. News 2000, 25, 185 -191.

136. Sun, Q. J.; Wang, H. Q.; Yang, C. H.; Li, Y. F., J. Mater. Chem. 2003, 13, 800 - 806.

137. Li, Y. F.; Cao, Y.; Gao, J.; Wang, D. L.; Yu, G.; Heeger, A. J., Synth. Met. 1999, 99,

243 - 248.

138. Wang, X. C.; Wang, H. Q.; Yang, Y.; He, Y. J.; Zhang, L.; Li, Y. F.; Li, X. Y.,

Macromolecules 2010, 43, 709 - 715.

139. Baran, D.; Balan, A.; Celebi, S.; Esteban, B. M.; Neugebauer, H.; Sariciftci, N. S.;

Toppare, L., Chem. Mater., 2010, 22, 2978-2987.

140. Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C., Adv. Mater., 2011,

23, 2367-2371.

141. Hou, J. H.; Tan, Z. A.; Yan, Y.; He, Y. J.; Yang, C. H.; Li, Y. F., J. Am. Chem. Soc.

2006, 128, 4911 - 4916.

142. Scharber, M. C.; Muhlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.;

Brabec, C. J., Adv. Mater., 2006, 18, 789-795.

143. Coffin, R. C.; Peet, J.; Rogers, J.; Bazan, G. C., Nat. Chem., 2009, 1, 657-661.

144. Chen, Z.-K.; W. Huang; L.-H. Wang; E.-T. Kang; B. J. Chen; C. S. Lee; Lee, S. T.,

Macromolecules 2000, 33 (24), 9015-9025.

145. Yue, W.; Zhao, Y.; Shao, S.; Tian, H.; Xie, Z.; Geng, Y.; Wang, F., J. Mater. Chem.,

2009, 19, 2199 - 2206.

146. Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.;

Brabec, C. J., Adv. Mater., 2006, 18, 789-794.

147. He, Y.; Wang, X.; Zhang, J.; Li, Y., Macromol. Rapid Comm., 2009, 30, 45 - 51.

148. Wang, X.; Luo, H.; Sun, Y.; Zhang, M.; Li, X.; Yu, G.; Liu, Y.; Li, Y.; Wang, H., J.

Polym. Sci., Part A: Polym. Chem., 2012, 50, 371 - 377.

149. Kline, R. J.; DeLongchamp, D. M.; Fischer, D. A.; Lin, E. K.; Richter, L. J.;

Chabinyc, M. L.; Toney, M. F.; Heeney, M.; McCulloch, I., Macromolecules 2007, 40, 7960

- 7965.

150. Northrup, J. E.; Chabinyc, M. L.; Hamilton, R.; McCulloch, I.; Heeney, M., J. Appl.

Phys., 2008, 104, 083705.

151. Ho, C.-C.; Liu, Y.-C.; Lin, S.-H.; Su, W.-F., Macromolecules 2012, 45, 813 - 820.

152. Sze, S. M.; Ng, K. K., Physics of Semiconductor Devices. Wiley-Interscience: New

York, 1981.

153. Horowitz, G., Adv. Mater. 1998, 10, 366 - 377.

154. D‟Angelo, P.; Stoliar, P.; Cramer, T.; Cassinese, A.; Zerbetto, F.; Biscarini, F., Appl.

Phys. A-Mater., 2008, 95, 55 - 60.

155. Huisman, B.; Valeton, J. J. P.; Nijssen, W.; Lub, J.; Hoeve, W., Adv. Mater. 2003, 15,

2002 - 2005.

156. Chabinyc, M. L.; Endicott, F.; Vogt, B. D.; DeLongchamp, D. M.; Lin, E. K.; Wu, Y.;

Liu, P.; Ong, B. S., Appl. Phys. Lett., 2006, 88, 113514/1.

Page | 227

157. Brown, A. R.; Jarrett, C. P.; de Leeuw, D. M.; Matters, M., Synth. Met. 1997, 88, 37 -

55.

158. Kline, R. J.; McGehee, M. D.; Kadnikova, E. N.; Liu, J.; Frechet, J. M. J., Adv.

Mater., 2003, 15, 1519-1522.

159. Liu, J.; Zhang, R.; Sauve, G.; Kowalewski, T.; McCullough, R. D., J. Am. Chem. Soc.

2008, 130, 13167 - 13176.

160. Liu, J.; Zhang, R.; Osaka, I.; Mishra, S.; Javier, A. E.; Smilgies, D.-M.; Kowalewski,

T.; McCullough, R. D., Adv. Funct. Mater., 2009, 19, 3427 - 3434.

161. Menom, A.; Dong, H. P.; Niazimbetova, Z. I.; Rothberg, L. J.; Galvin, M. E., Chem.

Mater. 2002, 14, 3668 - 3675.

162. Facchetti, A., Chem. Mater. 2011, 23, 733-758.

163. Tamayo, A. B.; Tantiwiwat, M.; Walker, B.; Nguyen, T.-Q., J. Phys. Chem. C 2008,

112, 15543 - 15552.

164. Kanbara, T.; Yamamoto, T., Chem. Lett., 1993, 419 - 422

165. Khan, A.; Hecht, S., Chem. Comm. 2004, 300 - 3001.

Page | 228

Chapter 4

Synthesis of Novel Narrow Band-Gap and Near-IR Absorbing Squaraine-based Conjugated Polymers for Organic Bulk-

Heterojunction Solar Cell Application

Page | 229

4.0 Synthesis of Novel Narrow Band-Gap and Near-IR Absorbing Squaraine-based Conjugated Polymers for Organic BHJ Solar Cell

Application

4.1 Introduction

The poly(3-hexylthiophene) (P3HT) and poly(2-methoxy-5-(2‟-ethylhexyloxy)-1,4-

phenylenevinylene) (MEH-PPV) conjugated polymers are by far the most extensively

investigated electron-donor candidate materials in blends with the [6,6]-phenyl C61-butyric

acid methyl ester (PC61BM) acceptor in BHJ solar cells. Furthermore, highly optimized

organic PV devices incorporating the P3HT and PC61BM has achieved substantially

improved power conversion efficiencies (η) ranging from 5.0 % to 6.0 %.1-4

However, the

scope of further improvements in η for future OPVs is hampered by the limited absorptions

of these conjugated π-systems to the visible region (short wavelength) of the solar spectrum

(300 – 700 nm)5, 6

, which in-turn limits the amount of excitons generated. In addition, their

HOMO/LUMO energy band-gaps are often large (2.0 eV and 3.0 eV).7, 8

Consequently, there

is great impetus for the development and design of new narrow band-gap conjugated polymer

with advanced π-extended structures capable of harnessing the low energy photons extending

well into the near-infrared (NIR) direction (ca. 900 nm) and for optimal device performance.

4.1.1 Squaraine Molecular Dyes

Squaraines are symmetric organic dyes characterized by sharp/intense absorptions

(molar extinction coefficient ε ˃ 3.0 x 105

M-1

cm-1

) and high quantum yield fluorescence

emissions (Øf up to 0.99) in solution, with absorption/emission maxima at long wavelengths

(λmax.λem ˃ 600 nm9). Nonetheless, in the solid state their intense absorption and emission

spectra are broadened/panchromatic (400 – 1000 nm), as a result of their strong donor-

acceptor-donor-type charge-transfer interaction.10-14

In addition, squaraines are known to

display unique and strong „exciton interaction‟ associated with the formation of H-dimer

(vide infra). This so called „exciton interaction‟ is akin to a pseudo-excimeric interaction

often used in the literature to describe the impact of electronic interactions between adjacent

squaraine pairs.15

These highly attractive optoelectronic properties of the squaraine molecular

dyes has continued to fuel their exploration in a myriad of commercial device applications,

some of which includes; organic solar cells (OSC),16-23

light emitting and ambipolar

OFETs,24, 25

non-linear optics (NLO),15, 26

photoconductors for xerography,16

active materials

Page | 230

for fluorescence patterning/makers,27, 28

and media for diode laser optical recording.29

Additionally, extended D-A-D-type squaraines were recently discovered to be highly

efficient 2 photon absorbers (2PA),30, 31

for utilization in photodynamic therapy, two-photon

fluorescence microscopy, and 3D optical data storage, amongst others.32

The name “squaraines” was first coined in the 1980s by Schmidt for these class of

molecular dyes.33

However, the origin of squaraines can be tracked as early as 1960s to the

pioneering work by Treibs and Jacob,34, 35

which involved the condensation of a quadratic

acid or squaric acid (3,4-dihydroxycyclobut-3-ene-1,2-dione - I)36

and highly reactive

pyrroles (II) in refluxing ethanol to form a series of bis(pyrrol-2-yl)squaraines (III) as red

dyes, which are very similar to cyclotrimethine dyes (scheme1).

Scheme 1. Synthesis of bis(pyrrole-2-yl)squaraine dyes.

The squaraine dye (III) formed (scheme 1) is clearly symmetrical with equivalent

pyrroles in the 1 and 3 positions. Although, their inherent insolubility precluded any further

probing of their structure via NMR measurements, IR analysis revealed a carbonyl stretching

at 1620 cm-1

,34

which can be attributed to considerable single bond character of the

carbonyl.37

In addition, these squaraines possess highly delocalized π-systems with 2π

aromaticity,37

as shown by the three possible canonical or resonance-stabilized zwitterionic

structures depicted in scheme 2.37-39

Scheme 2. Zwitterionic resonance structures of the squaraine dye.

Page | 231

The zwitterionic resonance structures depicted in scheme 2 highlight their symmetric

electronic distribution. Following the first squaraine synthesis, a plethora of novel squaraine

zwitterionic dyes have been synthesized by the generally condensation mechanism of squaric

acid with two equivalents of a variety of electron-rich heterocyclic derivatives, such as

thiazoles (SI),40

indolizines (SII and SIII),41, 42

azulenes (SIV),43

N,N-dialkylanilines (tertiary

amines - SV),44

pyrroles (SVI),36

and phenols (SVII),34

as depicted in scheme figure 1.

Figure 1. Selected squaraine organic dye molecules.

Despite these advances, the absorption spectra of the above squaraines were mostly

sharp and not sufficiently broad to cover large portions of the solar spectrum. In addition,

their solubility was a major stumbling-block in relation to their processability. Studies by

Buschel et. al.,45

showed that the expansion of the conjugation length in squaraine dyes could

be achieved by inserting vinylarylenes at the terminal positions of the squarylium moiety,

thus resulting in enhanced optoelectronic properties. Therefore the squaraine dye Sq1, Sq2,

Sq3, Sq4, Sq5, Sq6, Sq7, and Sq8 were prepared, as represented in figure 2.18, 19, 38, 45

These new classes of π-extended squaraine dyes displayed broad absorptions

extending into the near-IR region of the solar spectra (600 – 900 nm). As a result, the

application of Sq4/PC71BM (1:3) and Sq6/PC71BM (1:3) in unoptimized bulk-heterojunction

solar cell devices, produced a maximum PCE of 1.47 % (Jsc = 7.16 mA cm-2

, Voc = 0.55, and

Page | 232

FF = 37 %) and 2.05 % (Jsc = 9.32 mA cm-2

, Voc = 0.57, and FF = 37 %), respectively.18

On

the other hand, the PCEs of the Sq7/PC61BM (1:3) and Sq8 /PC61BM (1:3) devices were

much lower, 0.89 % (Jsc = 4.72 mA cm-2

, Voc = 0.59, and FF = 32 %) and 1.24 % (Jsc = 5.70

mA cm-2

, Voc = 0.62, and FF = 35 %).19

In addition, Wobkenberg et. al., have discovered

ambipolar transistor properties in an azulene-type (figure 1, SIV) dye with a squarylium core,

with balanced hole and electron mobilities of approximately 10-4

cm-2

V-1

s-1

in a bottom-gate

bottom-contact OFET device configuration.25

Figure 2. Structures of π-extended squaraine dyes.

Synthetically, the one-step condensation of these 1,3-disustituted squaraine dye

molecules usually occurs in acidic medium (acetic acid) or alcohols of high boiling nature

(butanol). However, in order to facilitate the azeotropic isolation of the water molecules

generated during the condensation reaction, aromatic hydrocarbons such as benzene or

toluene are utilized as co-solvents. In reactions involving highly reactive electron-rich

reagents like 2,4-dimethylpyrrole, refluxing ethanol was discovered to be sufficient.34

Page | 233

It is generally accepted that the mechanism of the condensation reaction proceeds by

the condensation of one equivalent of the electron-rich aromatic precursor with the squaric

acid, thus yielding a semisquaraine intermediate. The final product is afforded by the reaction

of the second equivalent of the electron-rich aromatic. At this stage of the condensation

reaction the 1,2-disubstituted squaraine derivative can also be generated, due to the non-

regioselective nature of the reaction (scheme 3). This derivative is said to be more soluble

than the 1,3-substituted product (SVIII and SIX, scheme 3), and they can usually be

separated by a simple filtration or recrystallization.10

Scheme 3. Condensation reaction yielding both 1,2- and 1,3-disubstituted squaraine dyes.

4.1.2 Polysquaraines: From Squaraine dye molecules

The later realization of low HOMO-LUMO band gaps and the broad NIR absorption

in small π-extended squarylium dyes has paved the way for their implementation as acceptor

building blocks in the synthesis of a myriad of novel UV-vis-NIR conjugated polymer

architectures with attractive properties for device applications. This idea is corroborated by

the comprehensive theoretical experiments pioneered by Brocks and Tol in 1996 on the

polymerization of squaraine dye molecules.46

This study predicted energy band gaps as low

as 0.2 eV for polysquaraines.

The first attempts to synthesis these polysquaraines were unsuccessful due to the

intractable nature and insolubility in organic solvents of the isolated polymer.47, 48

Further

work by Havinga et al.,49-51

employed the successful condensation of benzothiazoles and

benzobispyrrolines with squaric acid to afford soluble polysquaraines (Psq1 and Psq2, figure

3) with an optical band-gap of about 1.15 eV obtained for Psq1, while the water soluble

derivative Psq2 was 0.7 eV. Moreover, the former showed conductivities ranging from 10-5

–

10-6

S/cm at room temperature.49

The observed low band-gaps were attributed to the regular

alternation of strong π-donor and π-acceptor units along the polymer mainframe.39

It was

discovered that the incorporation of the strong electron-withdrawing squaric acid into the

Page | 234

polymer backbone resulted in a strong delocalization of the negative charge on the oxygen

atom along the main-chain structure.49, 50

More recently, the polycondensation of pyrrole derivatives, 1-dodecyl- and 3-

dodecylpyrrole and squaric acid resulted in the formation of the corresponding

polysquaraines (Psq3 and Psq4).52

Interestingly, these conjugated polysquaraines were found

to exhibit solvatochromic behaviours in various organic solvents possessing different

polarities and dielectric constant (benzene, tetrachloroethane, chloroform, isopropanol, 1-

butanol, 2-propanol, ethanol and methanol). Furthermore, the reaction conditions had a

deciding effect on the nature of the polysquaraine obtained. For instance, when the

polymerization was performed under azeotropic distillation using a solvent mixture of 1-

butanol and benzene (2:1), the polysquaraine generated comprised predominantly of the 1,3-

disubstituted zwitterionic squarylium repeat units (Psq3 and Psq4). On the contrary, in the

presence of DMSO-acetic acid, the conjugated polysquaraine chains consisted of a mixture of

the 1,3- and 1,2-disubstituted zwitterionic segments (Psq5 and Psq6). Interestingly, the 1,3-

disubstituted polysquaraines were shown to display negative solvatochromic properties in the

above named solvents, which gave a simple method to determine the nature of the

polymerization.53-55

Although, NIR absorption characteristics were envisaged in these

polysquaraines and similar derivatives,56, 57

disappointingly, Psq3 and Psq4 showed a

negligible bathochromically shifted absorption maxima (6 nm) from that of the corresponding

squaraine monomeric dye (585 nm, SVII in figure 1) in chloroform.52

Therefore, it became

apparent that the extension of the conjugation length of the polymer did not adequately

narrow the HOMO and LUMO energy gaps of the polysquaraines.

Figure 3. Structures of polysquaraines.

Page | 235

This was rationalized by Ajayaghosh39

based on the fact that the D-A-D squaraine

repeat unit is bridged by an electron-withdrawing squarylium moiety (A) (scheme 4, PQ1).

As a result, the D-A-D interaction is severely weakened by the electron-pull of the squaraine

bridge (A), which in turn adversely affects the absorption characteristics of the resulting

conjugated polymer.39

Based on this rationale, it was therefore logical to adopt the proposed

insertion of a electron-rich linkage into the polymer backbone, which would hopefully

reinforce the monomer D-A-D interaction, thus generating a quinoid-type structure (PQ2,

scheme 4).58, 59

This should result to an extension of the polysquaraine conjugation length and

thereby, narrow the HOMO and LUMO energy band-gap. Therefore, the polycondensation of

a bifunctional monomer (P-D-P) consisting of two reactive terminal groups linked to an π-

excess aromatic centre (D) with squaric acid may result to the generation of the squaraine dye

in situ at specific positions of the polymer backbone (PQ2, scheme 4).39

Scheme 4. Structures highlighting the modification of the polysquaraine structures towards low band-gap.

This strategy of structural engineering was employed by Ajayaghosh and Eldo in the

synthesis of the first set of soluble π-extended polysquaraines.60, 61

In a typical

polycondensation (scheme 5), the electron-rich 2,5-dialkoxybenzenevinylene-bridged bis(N-

alkyl)pyrroles 1 and squaric acid 2 in a mixture of 1-butanol and benzene under azeotropic

removal of water, resulted to the formation of the intensely coloured polysquaraines (Psq7) in

high yields (67 – 80 %).60

The reaction conditions could be optimized by varying the ratio of

the reactants and/or that of the solvent mixture.60

Due to the extensively expanded

conjugation of these new polysquaraine derivatives, very broad absorptions spanning the

visible and NIR region (300 – 1300 nm) of the solar spectrum ensued, with optical band gaps

as low as 0.79 eV.60

Furthermore, the polysquaraines displayed in scheme 8 showed strong

Page | 236

aggregation in solution and solid state, as evident by their high PDIs (7 – 10) and multimodal

absorption traces. In addition, these polymers were found to exhibit fluorescence emissions

with a relatively high quantum yields (ΦF up to 0.4161

) together with room temperature

electrical conductivities reaching 10-4

S/cm (Psq7(a) and Psq7(b)) without doping.60, 61

Generally, the nature of the alkyl side chains greatly influenced the optoelectronic properties

and solubility of the different conjugated polysquaraines.

Scheme 5. Synthesis of polysquaraines with extended π-conjugated structures.

In 2007, Wu et al.,62

synthesised two soluble fluorene-based polysquaraine (Psq8 and

Psq9) by the condensation of (E,E)-2,7-bis[2-(1-hexylpyrrol-2-yl)vinyl]-9,9‟-di-n-

octylfluorene (3) and (E,E)-2,7-bis[2-(1-hexadecylpyrrol-2-yl)vinyl]-9,9‟-di-n-octylfluorene

(4) with squaric acid in a solvent mixture of n-butanol and toluene (1:2). Like the above

polysquaraines (Psq7), their absorption ranged from 300 – 1100 nm, with high IPs (-5.54 eV

and -5.63 eV) and good thermal stability (407 – 421 oC).

62

Scheme 6. Synthesis of fluorene-based polysquaraines.

Recently, Volker et al.,63

pursued the synthesis of an indolenine-based polysquaraine

(Psq10) with the intention of investigating its performance in a BHJ solar cell device.

However, as opposed to synthesizing the conjugated copolymer via the normal

polycondensation pathway (scheme 6), a nickel-catalyzed Yamamoto cross-coupling of the

Page | 237

monomeric dibromoindolenine squaraine dye 6, accessed via the condensation of the

brominated indole (5-bromo-1-hexadecyl-3,3-dimethyl-2-methyl-2-methylene-2,3-

dihydroindole) 5 and 3,4-dihydroxyl-3-cyclobutene-1,2-dione Sq in a mixture of n-butanol

and toluene (1:1), was employed as shown in scheme 7.

Scheme 7. Yamamoto cross-coupling synthesis of indolenine-based conjugated polysquaraine (Psq10).

Interestingly, the polysquaraine obtained via this method (Psq10) exhibited a

narrowed PDI (1.72) compared to those obtained from the standard condensation method

(scheme 6). Moreover, the solution/solid state absorption spectrum was less broad (550 – 770

nm/810 nm63

), which was attributed to exciton coupling or excitonic interchain interactions

(solid film).30, 39

This was accompanied by a higher lying HOMO level (-5.09 eV63

), relative

to that (~ -5.60 eV62

) of the derivatives synthesized by the alternative condensation pathway.

This may be due to a reduced occurrence of the 1,2 substituted squaraines, which act to

interrupt the backbone conjugation thereby increasing the ionisation potential. The OPV

device incorporating a Psq10/PC61BM (1:1) active blend film, yielded a low PCE of 0.45 %

(Voc = 0.48 V, JSC = 2.82 mA cm-2

, FF = 34 %), compared to that based on P3HT/PC61BM

(2.83 %) obtained using similar conditions.63

The disparity in performance was due to the

attributed to the low EQE of the polysquaraine (20 %), with that of the latter reaching 60 %.

Related polysquaraine derivatives based on pyridopyrazine,64

carbazole,65

and

dipyridine65

(not shown) have also recently been developed with electronic band gaps raging

from 1.0 to 1.38 eV.

Page | 238

As shown by the above investigations, unlike the squaraine dye molecules, the

research into high performance polysquaraines is relatively limited, hence the motivation for

embarking on this project. We hereby present the synthesis and characterization of

dialkylthieno[3,2-b]thiophene-based polysquaraines (Psq13 - Psq15) with an extended

conjugation. In addition, their non-extended counterparts (Psq11 and Psq12) were also

included in this investigation for comparative reasons. It is envisaged that the different alkyl

side groups (linear hexadecyl and branched 2-ethylhexyl) would impact the optoelectronic,

morphological and OPV/OFET device properties of these new set of low band-gap

polysquaraines. Figure 4 depicts the structures of the target polysquaraines considered in this

study.

Figure 4. Structures of polysquaraines synthesized.

4.2 Synthesis of Monomeric Derivatives

The nature of the targeted polysquaraines (figure 4) for the purpose of this

investigation necessitated the need for the design of a variety of electron-rich thieno[3,2-

b]thiophene-based monomer derivatives with the presence or exclusion of the conjugation

extending vinylene-linkages between aromatic moieties. Therefore, in order to achieve these

structurally distinct monomers, the synthesis of two distinct π-donor units namely, 3,6-

dialkyl-2,5-bis(N-alkylpyrrol-2-yl)thieno[3,2-b]thiophene and (E,E)-2,5-bis[2-(N-

alkylpyrrol-2-yl)vinyl]-3,6-dialkylthieno[3,2-b]thiophene, were pursued.

Page | 239

4.2.1 Synthesis of the 3,6-dialkyl-2,5-bis(N-alkyl-1H-pyrrolyl)thieno[3,2-b]thiophene monomeric derivatives

The synthesis pathway employed in the formation of the above named monomers is as

illustrated in scheme 8.

Scheme 8. Synthesis of Alkyl-functionalized thieno[3,2-b]thiophene-2,5-bispyrroles.

In step 1, the palladium-catalyzed Stille cross-coupling reaction of two equivalents of

the commercially available N-methyl-2-(tributylstannyl)pyrrole precursor 9 with the

previously synthesized 2,5-dibromo-3,6-dihexadecylthieno[3,2-b]thiophene 7 was attempted,

by microwave heating in anhydrous chlorobenzene solvent at 140 oC (2 min), 160

oC (2 min),

and 180 oC (15 min). However, the yield of the resulting pale-green product 10 obtained after

purification in a silica-gel packed column (eluent: 9:1 mixture of hexane and ethyl acetate),

Page | 240

and recrystallization from acetone, was only 30 %, which suggests an incomplete coupling

process.

A repeat of the reaction protocol under modified microwave conditions (140 oC (4

min), 160 oC (4 min), 180

oC (25 min)), resulted in a yield of over 70 %. The

1H NMR

spectra (see appendix) shows the characteristic -NCH3 signal at 3.61 ppm with the pyrrole

aromatic H clearly observed between 6.24 – 6.79 ppm. In addition, signals belonging to the

α-/β-CH2 groups on the thieno[3,2-b]thiophene unit were observed at 2.85 ppm and 1.70

ppm. On the other hand, the application of the aforementioned microwave conditions to

access the EH -bearing 3,6-bis(2-ethyl-1-hexyl)-2,5-bis(N-methyl-1H-pyrrolyl)thieno[3,2-

b]thiophene derivative 11, amounted to a reduction in yield (40 %). The lower yield is

probably due to difficulties encountered while purifying compound 8 since it is an oil which

precluded purification by recrystallization, and necessitated repeated chromatography until a

pure fraction was obtained, as shown in the 1H and

13C NMR spectra obtained (not shown).

We were aware that the polymers resulting from monomers (scheme 8, 7 – 9) might

have some issues with solubility because of the short methyl chain used on the pyrrole, so

therefore we also prepared Stille reagents of longer chain pyrroles. We utilised 1-octylpyrrole

as starting material because of its commercial availability. The stannylation of the

alkylpyrrole was attempted with variable success (step 2, scheme 8).66

For example, efforts to

stannylate N-octylpyrrole by treatment with tri-n-butyltin chloride in anhydrous THF or in

conjunction with a base (2,2,6,6-tetramethylpiperidine) (step 2, route a and b), as described in

the literature,67

did not afford any product. Surprisingly, upon analysis via TLC and 1H NMR,

the crude mixture showed the presence of the starting material 12 and the unreacted trialkyl

tin chloride without any trace of the target compound 13. This may be due to unnecessary

complexities presented by the literature procedure, pertaining to the interplay between

reaction temperatures (0 oC, -65

oC and -75

oC) and solvent.

67 Despite repeated alteration of

the reaction conditions (that is, variable reaction temperatures), the outcome remained

unchanged with THF as solvent. Gratifyingly, as shown in step 2c (scheme 8), when THF

was replaced with anhydrous diethyl ether (Et2O) we found that the reaction worked well.

In addition, we found that cooling to -65 oC to the dissolved N-octylpyrrole in dry

Et2O, was not necessary, and the addition could be performed at 0 oC before the addition of

the n-BuLi, followed by stirring overnight under N2(g) protection at room temperature. Then

the temperature of the now turbid reaction mixture was cooled to -78 oC, at which point

Page | 241

tributyltin chloride was added with further stirring for an additional 18 hours. After

quenching and extraction using Et2O, and drying, the stannylated alkylpyrrole 13 was

obtained in 60 % yield.

This optimized procedure was employed in further scale-up reactions with calculated

conversion rates, based on 1H NMR analysis, ranging from 20 – 50% (see figure 5). The

1H

NMR is somewhat surprising, since the aromatic protons in the product appear as two signals,

at 6.96 ppm (integrating for 1 H) and a broader signal at 6.30 ppm (integrating for 2 H). We

believe the signal at 6.96 is the aromatic proton in the 5 position, in agreement with many

other pyrrole structures.68

This was further corroborated by 1H NMR prediction (ACD NMR

predictor) for a simple 2-trimethyl tin derivative. The predicted spectrum is shown in the

inset of figure 5. Interestingly, the attempted purification of 13 by column chromatography

resulted in its degradation and/or adsorption on the silica-gel, therefore the stannyl

compounds were used crude. The final stage of the reaction sequence involved the cross-

coupling of 13 with the alkyl-substituted dibromothieno[3,2-b]thiophene (7 or 8, scheme 8)

as delineated in step 1. The purity of the monomeric derivatives 10, 11, 14, and 15 were

ascertained by GC-MS and 1H- and

13C NMR (not shown).

Figure 5. 1H NMR spectra of N-octylpyrrole-2-tributylstannane.

Page | 242

4.2.2 Synthesis of the 3,6-dialkyl-2,5-bis[2-(N-alkyl-1H-pyrrol-2-yl)vinyl]thieno[3,2-b]thiophene monomeric derivatives

The synthesis pathway employed in the formation of the alkylthieno[3,2-b]thiophene

and the N-octylpyrrole chromophore bridged by vinyl C=C bonds as shown in scheme 9.

Scheme 9. Synthesis of vinylene-linked alkyl-substituted thieno[3,2-b]thiophene pyrrole monomers.

In this case, commercially available pyrrole-2-carboxaldehyde 16 (scheme 9) was

alkylated by the reaction of sodium hydride (NaH) in DMF. To the resulting white

suspension at 0 oC, was added an equimolar amount of hexadecyl- or 2-ethylhexylbromide.

Longer alkyl chains and/or branched EH were used to ensure the solubility of the resulting

polymers. After reaction for 72 hours at room temperature, the reaction mixture was poured

into ice-cold deionized water, and extracted. Further purification (although, not

recommended by the literature procedure31, 69

) was performed by silica-gel column

chromatography (eluent: hexane). For both substituted pyrrole derivatives (17 and 18, scheme

9) highly viscous pale-orange coloured oils were obtained in almost 90 % yield. Their 1H-

and 13

C NMR spectra (not shown) showed characteristic resonance signals which clearly

confirms the success of the substitution reaction.

The coupling partner for the Wittig reaction was prepared by the so called „Michaelis-

Arbuzov70

‟ reaction of the bromomethylated thieno[3,2-b]thiophene 19 and triethyl phosphite

Page | 243

(P(OEt)3). Reaction at elevated temperature afforded the phosphonates 20 in 65 % yield. The

latter was purified by silica-gel chromatography (eluent: acetone:hexane (1:3)). However, the

concentration of the intense-coloured crude mixture in a rotary evaporator under high vacuum

(60 oC) gave an end-product (20) with comparable level of purity, as ascertained by

1H,

13C

and 31

P NMR spectroscopy (not shown). This reaction proved very viable and therefore was

subsequently scaled up for the next step.

The electron-rich bispyrrole monomer derivatives 21, 22, and 23 (scheme 9) were

prepared by the Wittig-Horner-Emmons reaction of specific alkylpyrrole-2-carboxaldehydes

(17 and 18) and the phosphonates (20) in the presence of potassium tert-butoxide, using

anhydrous THF as solvent at reflux overnight. The highly fluorescent intense-green reaction

mixture obtained after work-up was purified by column chromatography (eluent: hexane:

ethyl acetate (9:1)), with yields around 65 %, with the exception of highly branched 23 (31 %

intense-orange oil). Subsequently, an alternative purification was developed which involved

repeated precipitation in methanol (3x) from chloroform, followed by recrystallization from

methanol, the bispyrrole derivatives 21 and 22 were furnished in the form of intense green

solids in high yields (~80 %). The analysis of the methanol fraction by GC-MS and NMR

techniques (spectra not shown) shows the presence of predominantly unreacted alkylpyrrole-

2-carcoxaldehyde derivatives (17 and 18). Despite the different purification methods

attempted, the NMR spectra and EI-MS data (not shown) of the products were no different.

4.2.3 Novel Polysquaraine Synthesis 4.2.3.1 Synthesis of π-Conjugated Poly(3,6-dialkyl-2,5-bis(N-alkyl-1H-pyrrol-2-yl)thieno[3,2-b]thiophene-co-squaric acid) derivatives

Firstly, the A-B type condensation of the 2,5-bis(N-alkylpyrrol-2-yl)-3,6-

dialkylthieno[3,2-b]thiophene monomeric derivatives 10, 11, 14, and 15 (scheme 8) with 2,4-

dihydroxy-3-cyclobetadiene-3,4-dione (squaric acid - Sq) was performed in a solvent mixture

of anhydrous n-butanol and toluene with surprising outcomes (Scheme 10). In a trial

polycondensation experiment, an equimolar amount of monomer 14 (or 15) was reacted with

that of the squaric acid (Sq) in dry n-butanol/toluene solvent using a mixing ratio of 1 to 3

(80 mL) under azeotropic distillation conditions. The polymerization reaction was continued

at 120 oC for 72 hours under argon atmosphere, with the observation of an intense-blue

coloured solution over time. At completion the reaction was cooled to room temperature and

Page | 244

precipitated into petroleum ether. Subsequently, the precipitate was washed repeatedly with

copious amounts of diethyl ether in order to eliminate any remnants of the unreacted

bispyrrole monomer that may be present, followed by methanol.

Further purification by Soxhlet extraction in methanol (24 h) and acetone (24 h) to

remove oligomeric and low weight material, afforded an intense-blue solid (95 % yield).

Disappointingly, after drying in a vacuum oven at 40 oC for 24 h, the resultant polysquaraine

Psq16 (scheme 10) was completely insoluble during repeated attempts to dissolved it in

selected organic solvents (chloroform, chlorobenzene, dichloromethane, dichlorobenzene,

toluene, and n-butanol) both a room temperature and elevated temperatures (up to140 oC).

The replication of this protocol gave similar results. As observed in the literature,61

the

solubility of polysquaraines is highly dependent on the nature of the alkyl side groups and the

substitution pattern along the backbone structure. Therefore, in realisation of this previous

observation, the monomer bearing the hexadecyl (-C16H33) substituent on the thieno[3,2-

b]thiophene moiety 10 was replaced with the more solubilising 2-ethylhexyl (EH)

functionality 11. However, the polycondensation of the latter with squaric acid under

analogous reaction conditions produced a polymer derivative Psq17 with the same solubility

issues. Since, this approach proved futile, another strategy was adopted in order to further

modify the structure of the zwitterionic conjugated polymer.

In this case, the pyrrole substituted with octyl groups (14) was utilized. This was

condensed with squaric acid in a mixture of n-butanol and lower boiling point benzene (2:1,

75 mL)71

, as opposed to toluene, under reflux for 24 h. On completion, the intense-blue crude

was filtered and concentrated, followed by re-precipitation in petroleum ether and

cyclohexane solvent mixture. The precipitate was isolated and dried in vacuum to afford

Psq11 (also see figure 4) in 70 % yield. Although, this derivative was soluble in the

previously listed organic solvents, GPC analysis indicated the presence of a low molecular

weight polymer (Mn = 8.0 kDa).

Page | 245

Scheme 10. Polycondensation of 2,5-bis(N-alkylpyrrol-2-yl)-3,6-dialkylthieno[3,2-b]thiophene monomers and

squaric acid.

In order to improve molecular weight, a re-run of the same polycondensation for

between 48 and 72 hours resulted to the formation of a dark-blue conjugated polysquaraine,

which could only be precipitated in methanol (soluble in petroleum ether/cyclohexane

mixture). However, after sequential Soxhlet extractions in diethyl ether, methanol and

acetone, the resultant polymer Psq11 was obtained in 30 % yield with very low solubility in

trichlorobenzene (heated to 220 oC for 2 hours). The polymer was insoluble in all other

solvents tested. Here we suspected we had isolated a high molecular weight fraction that was

insoluble. Further repetition of the above polycondensation under high solvent dilution (n-

butanol/toluene, 1/3, 133 nmL) at 140 oC for 72 hours resulted to the formation of a new

batch of Psq11 (57 % yield) which was initially 100 % soluble in trichlorobenzene when

heated to 170 oC. GPC analysis revealed a promising Mn of 21.0 kDa (PDI = 1.8) of the crude

before precipitation and drying (no Soxhlet). Afterwards, a Mn of 95.0 kDa (PDI = 1.3) was

obtained. However, when left in storage for 5 days as a fibre-like solid, the polymer became

intractable and could not be re-dissolved.

Furthermore, the polycondensation of the bispyrrole derivative 9 bearing the 2-

ethylhexyl (EH) pendant group at the 3 and 6 positions of the thieno[3,2-b]thiophene core,

with squaric acid under high dilution, afforded a green polymer Psq12 possessing very low

molecular weight (Mn = 4.0 kDa). However, after storing as a powder under vacuum for a

prolonged period of time, the polysquaraine also became intractable, as seen in the other

Page | 246

derivatives. Although initially surprising, the lack of solubility of polysquaraines of these

types have been a long standing issue, which has so far limited their applications.37, 72

This

intractable nature of the polysquaraines has been mostly attributed to strong intermolecular

electrostatic interactions between the zwitterionic moieties.72, 73

On the other hand, the

differences in solubility during the various polymerizations, even before solid state material

was obtained may suggest some structural abnormalities or isomerism of the squaraine (1,2-

and 1,3-disubstitution) within the polymer backbone (figure 3, Psq5/6), as suggested in

earlier reports.34, 74

However, a more recent experimental and FTIR study by

Chenthamarakshan et al.,52

has concluded the opposite, and shown the formation of

predominantly 1,2-disubtituted squarylium units within the conjugated polymer structure. In

addition, it was also highlighted that the constitution of the polymer backbone may depend on

the reaction conditions.52

Thus, the presence of the 1,3-disubstituted squarylium moiety

cannot be ruled out entirely, and my be distinguished from the 1,2-derivative by FTIR

analysis of the resulting polysquaraine.52, 71, 72, 75

Disappointingly, the polysquaraines

synthesized here could not be characterized via GPC and 1H NMR spectroscopy due to their

observed insolubility. The solubility drawbacks observed led to the pursuance of alternative

polysquaraine structures with greater flexibility and expanded conjugation.

Page | 247

4.2.3.2 Synthesis of π-Extended Poly(3,6-dialkyl-2,5-bis[2-(N-alkyl-1H-pyrrol-2-yl)vinyl]thieno[3,2-b]thiophene-co-squaric acid) derivatives.

The synthesis of the π-extended polysquaraines bearing vinyl bridging bonds were

achieved by employing the analogous, but modified experimental protocol described for the

preparation of the non-extended candidates Psq11, Psq12, Psq16 and Psq17 (see scheme

10). Scheme 11 shows a depiction of the synthesis route leading to formation of the target

vinylene-linked polysquaraine derivatives (PSQ6 – PSQ8).

Scheme 11. Synthesis of π-extended polysquaraine derivatives.

In an initial attempt, the polycondensation of 1 equivalent of the 3,6-dialkyl-2,5-bis[2-

(N-alkyl-1H-pyrrol-2-yl)vinyl]thieno[3,2-b]thiophene monomer 21 and that of the quadratic

acid (Sq) was performed in an n-butanol and toluene mixture (1:3), with the azeotropic

removal of water at 140 oC for 120 hours (or 5 days).

39, 62 Unlike that of Psq11/12/16/17, the

reaction mixture (mixture of solid and solution), upon cooling, was notably intense-green.

The solid was subsequently isolated, accompanied by the concentration of the collected

filtrate in vacuo to afford the highly viscous polymer crude. The GPC analysis of the crude

yielded a Mn of 11.0 kDa and PDI of 1.5. Moreover, after repeated precipitation in methanol,

followed by washing with hexane, diethyl ether, and methanol, the polymer was dried under

vacuum at 40 oC for 48 hours. The Mn of the resulting dark-green polysquaraine PSQ6 (46 %

yield) increased by approximately 2000 Da (13.5 kDa, and PDI = 1.8). Crucially, the

insertion of the vinylene-linkage into the conjugated backbone of the polysquaraine led to its

complete solubility in trichlorobenzene and dichlorobenzene (at 150 oC). However, after

several days of storage, the polysquaraine again became sparingly soluble in toluene,

chloroform and chlorobenzene. However, in trichlorobenzene and dichlorobenzene, the

Page | 248

polymer was still soluble. The table 1 shows the data for the optimized polycondensation

conditions and their impact on the polymer yield and molecular weights.

PSQ Batch

No.

Butanol/

Toluene

(mL, v/v)

SMa

(mmol)

Time

(hour)

Yield

(%)

Mn

(kDa)

Mw

(kDa) PDI DPd

PSQ6

1 40:120c 0.17 144 57 13.5 25.0 1.8 10

2 23:70b 0.21 120 67 66.0 85.0 1.3 51

3 50:150c 0.26 168 70 75.0 91.0 1.2 58

PSQ7

1 23:70b 0.25 144 75 12.0 27.5 2.4 11

2 40:120c 0.32 168 83 12.0 25.0 2.1 11

PSQ8 1 40:120c 0.46 212 75 15.0 40.0 2.7 18

Table 1. Polycondensation data for the π-extended polysquaraines. All polymerizations were performed at 140 oC.

aStarting material (SM).

b100 mL round-bottom flask.

c500 mL flask.

dDP is the degree of polymerization.

It can be inferred from table 1, that under variable solvent dilutions (entry 1 – 3) the

resulting percentage yields of the different batches of PSQ6 were not significantly affected.

However, under high concentration (2.25 mM, entry 2), the polymer yield and molecular

weight were substantially higher (Mn = 66.0 kDa vs 13.5 kDa). A similar trend was observed

between entry 1 and 3 (13.5 kDa vs 75.0 kDa). This may be due to the fact that, the less of the

1,2-disubstituted squarylium-bearing polymer defects were generated under high

concentration and dilution, as visually observed during the polycondensation reaction. On the

contrary, in the case of PSQ7 consisting of pyrrole-EH units in the polymer backbone, the

effect of concentration seems negligible. GPC analysis revealed a Mn of 12.0 kDa with

slightly varied PDIs in both attempts. Last but not least, the all branched EH-bearing

conjugated polymer PSQ8 was obtained in 75 % yield as intense-green solid, with a Mn of

15.0 kDa and PDI of 2.7, following the optimized condensation protocol. Furthermore, PSQ7

and PSQ8 were soluble in chloroform, chlorobenzene, dichlorobenzene, and dichloromethane

even after prolonged storage. This indicates that the elongation of the polymer chain by the

vinylene-spacer may have resulted in enhanced rotational freedom between adjacent repeat

units along the polymer backbone, thereby, enhancing their solution solubility over the

directly coupled polymers (Psq11, 12, 16 and 17, scheme 10).76

Page | 249

4.2.4 Proton NMR Spectroscopy

The structure of the π-conjugated polysquaraines was probed by 1H NMR

spectroscopy. Figure 6, 7, and 8 shows the 1H NMR of the π-extended zwitterionic

polysquaraine derivatives (PDC16BNC16PVSQTT – PSQ6, PDC16BNEHPVSQTT – PSQ7,

and PBEHBNEHPVSQTT – PSQ8).

Figure 6. 1H NMR spectra of polysquaraine derivative PSQ6 in 1,2-dichlorobenzene-d2 at 100

oC.

Due to the slightly poor solubility of PSQ6 in chlorobenzene, this squaraine polymer

was solubilised in 1,2-dichlorobenzene-d2 at 100 oC. On the other hand, PSQ7 and PSQ8

were both soluble in deuterated 2,2,6,6-tetraochloroethane-d4 (2,2,6,6-TCE-d4) either at room

temperature or when heated to 100 oC. Since, these conjugated polymers aggregate quite

strongly; heating was applied during 1H NMR measurements. It should be noted that when

spectra were obtained in chlorobenzene (100 oC), the spectra were so broadened that no clear

peaks were discernible, presumable due to aggregation. Even in DCB or TCE all the observed

signals were very broad in nature, with the aromatic peaks appearing very small in relation to

the peaks associated with the alkyl side-chains. The peaks remained broad and poorly

resolved even at elevated temperatures, which we attribute to the segmental aggregation of

the polymer in solution and the presence of a variety of chain conformations which cannot

freely rotate, even at elevated temperatures.76

Because of the poor resolution peak resolution

Page | 250

is difficult, both for all three polymers we can make an attempt. All polymers showed

resonance peaks centred at δ 0.90 – 0.97 ppm due to the presence of the terminal –CH3 group

on the alkyl side functionalities. In the same linear region, the broad chemical signals at δ

1.37 – 1.40 ppm and 1.80 ppm can be assigned to the presence of the -CH2- protons and that

of the β-CH2 group on the alkyl pendant group attached to the thieno[3,2-b]thiophene

aromatic system. The protons are integrated much higher than the backbone aromatic protons

due to the enhanced structural freedom associated with the freely rotating side-chains.

Figure 7.

1H NMR spectra of polysquaraine derivative PSQ7 in 2,2,6,6-tetrachloroethane-d4 at 100

oC.

The resonance peak of the β-CH protons of the 2-ethylhexyl (EH) group appended to

the pyrrole imine N atom (N-CH2-CH-) appeared as a weak and broad multiplet signal at δ

2.34 ppm (see figure 7 and 8). Moreover, the protons of β-CH group of the EH side chain on

the thieno[3,2-b]thiophene ring in PSQ8 (see figure 8) is also seen in the broad resonance

peak at δ 2.34 ppm. Furthermore, the poorly resolved and broad multiplet signal at δ 2.65 –

2.75 ppm can be attributed to the α-CH2 protons directly attached to the thieno[3,2-

b]thiophene moiety. On the other hand, those appended to the pyrrole N atom (-N-CH2 and -

N+-CH2) are noticeable at δ 5.30 ppm PSQ6.

62 While those of the PSQ7 and PSQ8 are

centred at δ 4.84 – 4.86 ppm.65

That said, the presence of diethyl ether solvent peak at δ 3.69

ppm (see figure 7 and 8), may be due to insufficient dryness. The pyrrole and vinylene

Page | 251

protons are partly obscured by solvent residues for PSQ6. For the other polymers, the

aromatic signals are very strongly broadened and difficult to resolve.

Figure 8. 1H NMR spectra of polysquaraine derivative PSQ8 in 2,2,6,6-tetrachloroenthane-d4 at 100

oC.

Page | 252

4.2.5 Solid-State FT-IR Spectral Characterization

The FT-IR spectra of the π-extended conjugated polysquaraines (PSQ6, PSQ7 and

PSQ8) are displayed in figure 9 between 500 cm-1

and 4000 cm-1

. In addition, for

comparative reasons that of the sparingly soluble and non-extended polymer Psq11 polymer

counterpart was also recorded as shown in figure 10.

4000 3500 3000 2500 2000 1500 1000 500

92

49

24

64

97

19

92

8

1093

12

90

13

52

13

81

14

55

15

00

15

87

16

2017

39

28

50

29

20

Tra

nsm

itta

nce

(%

)

Wavenumber (cm-1)

PDC16

BNC16

PVSQTT

PDC16

BNEHPVSQTT

PBEHNEHPVSQTT

29

57

Figure 9. Attenuated total reflectance FT-IR spectra of the π-extended polysquaraine derivatives

(PDC16BNC16PVSQTT - PSQ6, PDC16BNEHPVSQTT - PSQ7 and PBEHBNEHPVSQTT - PSQ8) at room

temperature.

3500 3000 2500 2000 1500 1000 500

29

62

70

47

94

86

41

01

5

14

60

13

60

12

58

14

95

16

28

17

35

28

50

29

23

Tra

nsm

itta

nce

(%

)

Wavenumber (cm-1)

PDC16

BNC8PSQTT

29

62

Figure 10. Attenuated total reflection FT-IR spectra of the polysquaraine, PDC16BNC8PSQTT (Psq11), at room

temperature

Page | 253

This FTIR technique serves to ascertain the structural compositions and the presence

of structural irregularity(s) within the polymer backbone structure (see figure 3, vide supra).

As expected, the presence of intense bands at approximately 2957 cm-1

, 2920 cm-1

and 2850

cm-1

, is indicative of the presence of C-H stretching vibrations of the alkyl side chains

presence in all four polysquaraine backbones. Importantly, the nature of the carbonyl groups

(-CO) of either the 1,2-disubstituted or 1,3-disubstituted squarate unit in the polymer chain

can be distinguished by the studying the bands between the 1750 and 1600 cm-1

regions of

the IR spectra (figure 9 and 10). This is due to the fact polysquaraine zwitterionic CO groups

routinely display strong characteristic absorptions in the aforementioned IR spectral

regions.71, 77, 78

The bands at ca. 1739 cm-1

observed in the IR spectra of the extended

polysquaraine (PSQ6, 7 and 8, figure 9) was assigned to the covalent C=O group of the

squarate moiety, as observed in the literature.52, 71, 75

Similar values (1735 cm-1

) was observed

in the IR spectra of the analogous polysquaraine Psq11 without the vinyl-bridges (figure 5).

These values are lower relative to that recorded for the squaric acid (1810 cm-1

) and higher

than that of the squarate anion (1706 cm-1

), which signify the consistency of electron

delocalization within the polymer structures.71, 75

Crucially, the polysquaraines synthesized

also show strong absorptions at a wavenumber of ca. 1620 cm-1

(PSQ6, 7 and 8) and 1628

cm-1

(Psq11), which corresponds to the zwitterionic carbonyl (C-O) of the squarate moiety

along the polymer chain.52, 61

This indicates the presence of the 1,3-disubstituted

cyclobutenediylium-1,3-diolate unit along the polymer mainframe, and a resonance-stabilized

zwitterionic structure.52

Taken together, this is strong evidence for the formation of mainly

1,3 substituted squaraines in the polymer backbone.

The peaks at 1455 – 1460 cm-1

can be attributed to the presence of CH2 bending

vibrations. The C-N stretching bands of the pyrrole units are positioned between 1360 cm-1

and 1381 cm-1

. The C=C bond within the squarylium core can be attributed to the presence of

a weak band at 1500 cm-1

for the π-extended conjugated polysquaraines (PSQ6 – PSQ8) and

1495 cm-1

for the non-extended derivative (Psq11).30

While the characteristic C-H out-of-

plane vibrations of the trans-vinyl (HC=CH)79

are situated between 924 cm-1

and 928 cm-1

for the PSQ6 – PSQ8, but absent in the IR spectra of the non-extended Psq11 analogous

polysquaraine. Thus, the absence of the cis-vinyl band at ca. 875 cm-1

, indicate that the π-

conjugated polysquaraines consists of trans-vinylene configuration along the polymer

backbone. The evidence based on the FT-IR further elucidates the structural composition of

the novel conjugated polysquaraines synthesized.

Page | 254

4.2.6 Optical Characteristics

The polysquaraines are π-conjugated structures which are known to produce intense

absorption in the visible region and near infrared (NIR) region of the solar spectra. Therefore,

this technique is well-suited to analysing these characteristics. For this purpose, the UV-vis of

the simple D-A polysquaraine derivative (PDC16BNC8PSQTT or Psq11) is first analyzed,

taking note of its limited solubility in chlorobenzene. As, shown in figure 6, Psq11 displayed

an intense and narrow absorption band covering a small portion (500 – 740 nm) of the solar

spectra in chlorobenzene solution, which is typical of squaraine dyes and simple

polysquaraines (Psq4, figure 3).11, 39, 71

An absorption maxima (λmax) of 636 nm was deduced

together with an energetic edge of 710 nm, which corresponds to a optical band gap (Egopt

) of

ca. 1.75 eV. The λmax of Psq11 red shifted by 56 nm compared to that of the previously

reported polymer Psq4 (580 nm39

). This relatively large band gap can be attributed to the

charge transfer (CT) interaction involving primarily the cyclobutadiylium-1,3-diolate unit and

the pyrrole moiety (scheme 4, PQ1).11, 71, 76

Nonetheless, the bathochromic shift (56 nm)

observed may be ascribed to the strong electron-donating effect provided by the thieno[3,2-

b]thiophene units, as seen in other squaraine dye bearing strong π-donor chromophores.11, 76

300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d In

ten

sity (

a.u

)

Wavelength (nm)

PDC16BNC8PSQTT Soln UV

PDC16BNC8PSQTT Soln PL

PDC16BNC8PSQTT Film UV

PDC16BNC8PSQTT Film PL

Figure 11. UV-vis absorption and fluorescence spectra of PDC16BNC8PSQTT (Psq11) in chlorobenzene

solution (ex. λ = 613 nm) and as-spun film (ex. λ = 635 nm) at room temperature.

However, the lack of the anticipated spectral broadening and UV-vis-NIR absorption

indicate the introduction of the electron-donating 3,6-dihexadecylthieno[3,2-b]thiophene unit

into the polysquaraine backbone did not have a significant affect compared to the simple

Page | 255

pyrrole-squaraine copolymer. Strikingly, in the solid state absorption (see figure 11), the

absorption maxima was blue-shifted by 23 nm. This may be attributed to the weakening of

the intramolecular D-A-D CT interaction within the solid state microstructure caused by the

disruption of the polymer backbone planarity and steric effects. 11, 80

In particular, there may

be steric strain between the alkyl chains of the thieno[3,2-b]thiophene and the adjacent

pyrrole groups which prevents effective backbone planarization. What is surprising is this

steric strain is not sufficient to impart good solubility on the polymer, and overcome the

strong electrostatic interactions in the solid state. The introduction of the vinylene spacer

between the thieno[3,2-b]thiophene and pyrrole is expected to reduce this undesirable steric

interaction and allow the polymer to fully planarize in the solid state, and also in solution.

Polymer

Solution / nm Thin Film / nm

λmaxa λedge

b Eg

opt (eV) λmax λedge Eg

opt (eV)

Psq11 636 710 1.75 613 710 1.75

PSQ6 803 (920)c

1140 1.09 828 1181 1.05

PSQ7 784 (940)c 1060 1.17 825 1140 1.09

PSQ8 800 (940)c 1050 1.18 808 1120 1.11

Table 2. UV-vis and NIR absorption data of the polysquaraine derivatives in chlorobenzene and as-spun

thin films. a

Absorption maxima. b

Energetic edge/onset of absorption. The optical band gaps were deduced

from the equation, Egopt

= 1240/λonset. Where λonset represent the absorption edge in the long wavelength

region. c Vibronic shoulder peak.

Figure 12 displays the UV-vis-NIR absorption spectra of the conjugated vinylene-

containing polysquaraines and those of the corresponding bispyrrole monomers in dilute

chlorobenzene solutions. The data for all the polymers is also summarised in Table 2. The

bispyrrole monomers all exhibited absorptions in the short wavelength region with λmax

centred at approximately 440 nm, and λedge of 500 nm (Egopt

= 2.48 eV). In all cases, the

polymerization of the monomers with squaric acid, resulted in a considerable red-shift and

broadening of the absorption spectra, with broad absorptions encompassing both the visible

and NIR directions of the UV spectra (300 – 1200 nm). This is similar to observations by

Ajayaghosh et al.,39, 61

in poly(phenylene-alt-squaraines), Zhang et al.,65

in poly(carbazole-

alt-squaraines)/poly(bipyridyl-alt-squaraine), and Wu et al.,62

in poly(fluorene-alt-squaraine),

Page | 256

all bearing vinyl linkages within the polymer backbone. This suggests that the addition of the

vinyl link facilitates planarization of the backbone in solution, therefore allowing full

delocalisation along the backbone. In chlorobenzene solution, the structurally analogous

polysquaraines PSQ6 - 8 all displayed broadly similar absorption profiles, with a maximum

absorption around 800 nm, and a smaller absorption around 400 nm. They all exhibited a

pronounced red-shift in comparison to polymer Psq11, which did not contain the vinylene

link. Subtle differences in the spectra were observed depending on the side group substitution

pattern of the conjugated polymers. The polymer with linear hexadecyl side-chains on both

the thieno[3,2-b]thiophene and pyrrole displayed the longest wavelength maximum

absorption at 803 nm, and a pronounced shoulder in the longer wavelength region around 920

nm. This shoulder is typically associated with signs of aggregation. The branched EH

polymers displayed less prominent shoulders, which might imply they are less aggregated in

solution due to the sterically bulky branched pendant groups.

300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d A

bso

rba

nce

(a

.u)

Wavelength (nm)

DC16

BNC16

PVTT Soln

DC16

BNEHPVTT Soln

BEHBNEHPVTT Soln

PDC16

BNC16

PVSQTT Soln

PDC16

BNEHPVSQTT Soln

PBEHBNEHPVSQTT Soln

Figure 12. UV-vis-NIR spectra of the vinylene-bridged bispyrrole monomers 6, 7 and 8 (see scheme 9) and the

corresponding π-extended squaraine copolymers (PDC16BNC16PVSQTT – PSQ6, PDC16BNEHPVSQTT –

PSQ7, and PBEHBNEHPVSQTT – PSQ8) in dilute chlorobenzene solution at room temperature.

In the solid state (figure 13), PSQ6, PSQ7 and PSQ8 each displayed a bathochromic

shift of 25 nm, 31 nm and 8 nm, relative to their λmax values in solution (figure 12). In

addition, the onset of absorption is further extended by 50 nm to 1200 nm. This evidently

shows that the polysquaraines in the solid state assume enhanced intramolecular and/or

intermolecular interactions, due to their closer interchain packing order and expanded

effective conjugation lengths. Moreover, these effects seems to be more pronounced in PSQ6

and PSQ7 based on the large red-shifts (25 and 31 nm), which indicate that the alkyl pendant

Page | 257

groups thus influence the optical properties and microstructural order of the polysquaraines.61

The small shift for PSQ8 which contains the branched EH groups on both the thieno[3,2-

b]thiophene and pyrrole is perhaps not surprising, and indicates that the degree of structural

order is not significantly enhanced in the solid state, most likely due to the large number of

bulky side groups present.

Overall, the incorporation of the vinyl spacer into the polymer backbone resulted in

significant broadening of the absorption spectra and the expansion of the effective

conjugation length of the polysquaraines, compared to that of the simple Psq11 derivative.

We believe this is most likely due to the reduction of steric strain between the alkyl

substituted thieno[3,2-b]thiophene and pyrrole groups. As a result, the solid state optical band

gaps of the π-extended polysquaraines ranged from 1.04 eV to 1.11 eV, which is a reduction

of approximately 0.7 eV with respect to that of Psq11 (1.75 eV) (see table 2). Moreover,

these values are lower than those reported for vinylene-linked poly(carbazole-alt-squaraine)

and poly(bipyridyl-alt-squaraine) (1.38 eV65

) and poly(fluorene-alt-squaraine) (1.20 eV62

),

deduced from their solid state absorption spectra.

300 400 500 600 700 800 900 1000 1100 1200 1300 1400 1500

0.0

0.2

0.4

0.6

0.8

1.0

No

rma

lize

d A

bso

rba

nce

(a

.u)

Wavelength (nm)

PDC16

BNC16

PVSQTT Film

PDC16

BNEHPVSQTT Film

PBEHBNEHPVSQTT Film

Figure 13. UV-vis-NIR spectra of the π-extended zwitterionic squaraine copolymer thin films spun from hot

chlorobenzene (4mg/ml) on ITO glass substrate at 1000 rpm for 70 seconds.

Surprisingly, the fluorescence emission of the non-extended conjugated polysquaraine

Psq11 in the solution and solid state are similar (figure 11), which indicates some similarities

in the conformation of the polysquaraine in both states. This is further evidence that there is

considerable steric strain between the fused thiophene and pyrrole units, which prevents

backbone planarization in both the solid state and solution. On the other hand, the conjugated

Page | 258

vinylene polysquaraines possessing the extended conjugated structures displayed absolutely

no fluorescence. This according to Liang et al.,81

may be due to the fact the extended

squaraines are known to adopt folded or H-dimer conformations. Consequently, the absence

of fluorescence in the dimer or folded chains probably arises from the reduced oscillator

strength for the low-energy component of the split exciton band.76, 81

In addition, the low

energy of this state is likely to facilitate fast non-radiative decay, as anticipated.81

4.2.7 Electrochemical Determination of Energy Levels

The electrochemical processes exhibited by the novel π-conjugated polysquaraine

derivatives (PSQ6, PSQ7 and PSQ8) were elucidated by cyclic voltammetric (CV)

experiments, and utilized in the estimation of their HOMO and LUMO energy levels/band

gaps. The CV measurements were performed in a three-electrode cell set-up, as described in

chapter 2 and 3. However, in this instance, the Ag-wire pseudo-reference electrode

previously utilized was replaced with a Pt-wire due to difficulties in obtaining clear and

discernible redox peaks. PSQ7 and 8 were deposited from hot CB onto the Pt-wire working

electrode, while PSQ6 was deposited from TCB. The Pt pseudo-reference electrode was

calibrated after each CV measurements using 0.001 M ferrocene (Fc) (E = 0.4 V vs the

normal hydrogen electrode (NHE)82-84

in CH3CN/Bu4NPF6, as detailed in chapter 2. The

potential onsets deduced from the voltammograms were corrected to the NHE and calculated

with respect to vacuum level using 4.75 eV vs vacuum.85-87

Figure 14 shows the

voltammogram of ferrocene and those of the vinylene-linked polysquaraines (PSQ6, 7 and

8). The half-wave potential - E1/2 of the ferrocenium/ferrocene (Fc+/Fc) redox couple was

estimated to be around 0.182 V vs Pt, whereas the cathodic (Epc) and anodic (Epa) peak

potentials of the Fc/Fc+ redox couple were 0.131 V and 0.232 V vs Pt.

Page | 259

0.0 0.5 1.0 1.5 2.0

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

No

rma

lize

d C

urr

en

t D

en

sity (

a.u

)

Potential (V vs Pt)

Ferrocene

PDC16

BNC16

PVSQTT

PDC16

BNEHPVSQTT

PBEHBNEHPVSQTT

Figure 14. Cyclic voltammogram of the π-extended polysquaraine derivative PDC16BNC16PVSQTT - PSQ6,

PDC16BNEHPVQTT - PSQ7, and PBEHBNEHPVSQTT - PSQ8 drop-casted films on Pt-working electrode

and neat ferrocene, scanned at a rate of 50 mV s-1

in 0.1 M Bu4NPF6/CH3CN in the positive direction.

Thus, the HOMO and LUMO energy levels of the polysquaraine can be calculated

using the following empirical equations;86, 88

EHOMO = - (φonset, ox + 4.57) = IP (eV) (1)

ELUMO = - (φonset, red + 4.57) = EA (eV) (2)

Egecl

= (φonset, ox - φonset, red) (eV) (3)

Here, υox and υred represent the onset of oxidation and reduction, respectively. The

acronym EA stands for the electron affinity of the conjugated polymer, and IP is the

ionization potential. Additionally, Egec

represent the electrochemical band-gap estimated from

the onset potentials of oxidation and reduction of cyclic voltammogram. The cyclic

voltammogram of PDC16BNC16PVSQTT – PSQ6 (figure 14) displayed an electrochemically

quasi-reversible 1 electron oxidation process in the cathodic direction at peak potential Epa =

1.40 V and Epc = 0.71 V. In the case of the PDC16BNEHPVSQTT – PSQ7 the redox process

is similar to that occurring in the analogous polysquaraine PSQ6. However, PSQ7 showed

reduced oxidation/reduction peak potentials (Epa = 0.90 V and Epc = 0.31 V) relative to that of

PSQ7, which may imply some differences in the kinetics of the charge transfer process

during oxidation. On the other hand, the cyclic voltammogram of PBEHBNEHPVSQTTV –

PSQ8 is noticeably broadened in comparison to that of PSQ6 and PSQ7, with no apparent

Page | 260

reversibility of the oxidation process. In addition, PSQ6 and PSQ7 both showed similar onset

potential of oxidation (φonset.ox = 0.5 eV), while that of PSQ8 was reduced by almost 0.2 eV

(φonset.ox = 0.35 eV). These observations indicate that the oxidation process is heavily

influenced by the nature of the alkyl groups attached to the thieno[3,2-b]thiophene moiety,

which as discussed above influence the ability of the polymer to aggregate and pack closely.

-2.5 -2.0 -1.5 -1.0 -0.5 0.0-25

-20

-15

-10

-5

0

5

Nor

mal

ized

Cur

rent

Den

sity

(a.

u)

Potential (V vs Pt)

PDC16BNC16PVSQTT PDC16BNEHPVSQTT

PBEHBNEHPVSQTT

Figure 15. Cyclic voltammogram of the π-extended polysquaraine derivative PDC16BNC16PVSQTT - PSQ6,

PDC16BNEHPVQTT - PSQ7, and PBEHBNEHPVSQTT - PSQ8 drop-casted films on Pt-working electrode

and scanned at a rate of 50 mV s-1

in 0.1 M Bu4NPF6/CH3CN in the negative directions.

In the anodic scan (negative) shown in figure 15, the reduction/re-oxidation

(dedoping/doping) processes involving the C16 bearing PSQ6 squaraine polymer again were

quasi-reversible. In contrast, the other derivatives both displayed irreversible de-

doping/doping redox processes (figure 15). Table 3 shows a summary of the electrochemical

data (EHOMO, ELUMO, Eonset.ox, Eonset.red, and Egecl

) extracted from the cyclic voltammograms of

the PSQ6, PSQ7 and PSQ8.

Page | 261

Polymer

dCyclic Voltammetry

Eonset.ox vs.

Pt (V)

Eonset.red vs.

Pt (V)

EHOMO

(eV)

ELUMO

(eV)

Egecl

(eV)

PSQ6 0.51 -0.95 -5.08 -3.62 1.46

PSQ7 0.50 -0.76 -5.07 -3.81 1.26

PSQ8 0.35 -0.90 -4.92 -3.67 1.25

Table 3. Electrochemical data of as-cast polysquaraine films. Electrochemical measurement performed on

as-cast polymer films on Pt working electrode in 0.1 M [Bu4]+[PF6]

- at a 50 mV s

-1 potential sweep rate.

As presented in table 3, PSQ6 comprising exclusively of C16 side groups on the

thieno[3,2-b]thiophene moiety and pyrrole unit, produced an onset potential of 0.51 V vs Pt,

which corresponds to a HOMO energy level of -5.08 eV. Similarly, the C16 and EH bearing

polymer derivative PSQ7 also displayed a similar value (-5.07 eV). In the case of the all EH

polysquaraine PSQ8, a high-lying HOMO level (-4.92 eV), was observed. This is somewhat

surprising since any disruption to backbone delocalization caused by steric twisting would be

expected to increase the ionisation potential and afford a lower-lying HOMO (more

negative). We speculate that the reason may be related to the fact that the polymer chains can

pack less closely in the all EH polymer (see UV data), which facilitates interdiffusion of the

counterion from the electrolyte during the CV scan, influencing the onset potential. The film

morphology is known to have a big influence during thin film CV measurements. Generally,

based on their onset of oxidative values (0.51 V, 0.50 V and 0.35 V), all these polysquaraines

are not very stable towards electrochemical oxidation. Previous electrochemical studies on

poly(fluorene-alt-squaraine) and poly(carbazole-alt-squaraine) bearing vinylene bridges also

showed similar Eonset.ox values (~0.50 V).62, 65

The LUMO energy levels of PSQ6 and PSQ8 show negligible differences (-3.62 eV

and -3.67 eV), although we note that the onset of reduction is not very clearly defined for

PSQ8. Interestingly, the C16 and EH bearing polymer derivative (PSQ7) showed a higher

electron affinity based on its low-lying LUMO energy level (-3.87 eV). Although, the LUMO

energy is dependent of the electron-withdrawing strength of the π-acceptor, which in this

instance is the squarylium unit, changes in the electron-donating ability of the π-donor

bispyrrole moiety may impact this, hence, the variation in the LUMO energy levels.

Furthermore, PSQ6 displayed an electrochemical band gap of 1.46 eV, which slightly differs

Page | 262

from those of PSQ7 and PSQ8 (~1.25 eV). Furthermore, the disparity between the solid state

optical band gaps and those deduced from the electrochemical measurements may be due to

the aggregation of the polymer chains in the solid state, albeit, within the error margin (0.2 –

0.5 eV).89-91

4.2.8 Thermal Stability Studies

4.2.8.1 Thermogravimetric Analysis

The thermal decomposition and stability of the novel polysquaraines were

investigated both in air and under inert atmosphere at a heating rate of 10 oC min

-1 from 25 -

750 oC. Figure 16 shows the TGA thermograms of the non-extended polysquaraine Psq11 in

air and nitrogen.

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750-10

0

10

20

30

40

50

60

70

80

90

100

110

We

igh

t (%

)

Temperature (oC)

PDC16

BNC8PSQTT Air

PDC16

BNC8PSQTT N

2

Figure 16. Thermogram of PDC16BNC8PSQTT – Psq8 in air and under inert conditions at heating rate of 10 oC/min.

This conjugated polymer (Psq11) was stable up to 300 oC in air and 380

oC under N2.

This was followed by a 5 % weight loss at a thermal decomposition (Td) temperature of 380

oC (air) and 430

oC (N2). The sharp drop in weight beyond these temperatures may be

ascribed to the degradation of the polymer backbone structure. In the case of the vinylene π-

conjugated polymer derivatives (PSQ6 – PSQ8), no decomposition was observed up to ca.

300 oC in air (figure 17, vide infra). However, a 5 % reduction in weight was observed at 320

oC for PSQ6, 310

oC for PSQ7 and 320

oC for PSQ8. The three stage weight loss observed in

figure 17 corresponds to the degradation of the various structural components of the

conjugated polymer.

Page | 263

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750-10

0

10

20

30

40

50

60

70

80

90

100

110

W

eig

ht (%

)

Temperature (oC)

PDC16

BNC16

PVSQTT Air

PDC16

BNEHPVSQTT Air

PBEHBNEHPVSQTT Air

50 100 150 200 250 300 350 400 450 500 550 600 650 700 750

-10

0

10

20

30

40

50

60

70

80

90

100

110

We

igh

t (%

)

Temperature (oC)

PDC16

BNC16

PVSQTT N2

PDC16

BNEHPVSQTT N2

PBEHBNEHPVSQTT N2

Figure 17. TGA thermograms of the π-extended polysquaraine derivatives PDC16BNC16PCSQTT – PSQ6,

PDC16BNEHPVSQTT – PSQ7 and PBEHBNEHPVSQTT – PSQ8 obtained in air and N2 at heating rate of 10 oC/min.

Under N2 atmosphere (figure 17), PSQ6 was observed to be thermally stable up to

340 oC, while that of both PSQ7 and PSQ8 were lower (ca. 310

oC). Most notably, a single

large drop was observed in this case, whereby, the decomposition temperature at 5 % weight

loss for PSQ6 (400 oC) was substantially higher than that of PSQ7 (370

oC) and PSQ8 (330

oC), but lower (by about 30

oC) than that of observed for Psq11 (400

oC). These observations

highlight the fact that the extension of the π-conjugated structures of the polysquaraines by

incorporating flexible vinylene-linkages in between aromatic donor and acceptor units may

weaken their structural rigidity. In addition, the nature of the alkyl chain attachments has only

a small influence on thermal stability, with the incorporation of branched groups resulting in

a slight reduction in stability.

4.2.8.2 Differential Scanning Caloriemetry

The thermal phase-transition behaviour of the conjugated polysquaraines were

investigated by differential scanning caloriemetry (DSC) under N2 atmosphere from room

temperature to 200 oC. Figure 18 displays the DSC transitions of PDC16BNC16PVSQTT -

PSQ6, PDC16BNEHPVSQTT - PSQ7 and PBEHNEHPVSQTT - PSQ8, respectively.

Page | 264

0 20 40 60 80 100 120 140 160 180 200-3

-2

-1

0

1

2

3

4

5

6

7

He

at F

low

(m

W)

Temperature (oC)

PDC16

BNC16

PVSQTT PDC16

BNEHPVSQTT

PBEHBNEHPVSQTT

94.4 oC

77.5 oC

63.4 oC

87.8 oC

62.2 oC

95.1 oC

Figure 18. DSC transitions of the π-extended polysquaraine derivatives at a heating rate of 10

oC min

-1.

As shown in figure 18, the transition of the all C16-bearing PSQ6 derivate clearly

revealed an endothermic peak at a temperature of approximately 94.4 oC during heating. This

may be assigned to the glass transition temperature (Tg) of the polysquaraine. At this

temperature, the polymer chains have increased segmental motions due to the transformation

to a rubbery/flexible state during heating. Interestingly, this Tg value is in close agreement

with that of PSQ7 (95.1 oC), which suggests that probably the C16 side chain appended to the

thieno[3,2-b]thiophene unit does play a significant role in relation to how strongly the

polymer chains are packed together, and some similarity of microstructural order. In contrast,

the EH-bearing polysquaraine (PSQ8) possessed a significantly low Tg (87.8 C), which again

corroborated the aforementioned claim. In this case, it is surmised that the bulky EH side

groups on the polymer backbone disrupts the packing order of conjugated system, thereby

reducing the Tg observed. Upon cooling, two exothermic transitions (or recrystallization

peaks) were observed for PSQ6 (77.5 oC and 63.4

oC). However, PSQ7 showed no such

transition, while, PSQ8 showed an exotherm transition (62.2 oC) close to that seen in PSQ6

(63.4 o

C). Surprisingly, no transitions were observed in the DSC curves of the simple

polysquaraine derivative – Psq8, as shown in figure 19.

Page | 265

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 300-5

-4

-3

-2

-1

0

1

He

at F

low

(m

W)

Temperature (C)

PDC16

BNC8PSQTT

Figure 19. DSC transitions of PDC16BNC8PSQTT – Psq11 at a heating/cooling rate of 10 oC min

-1 under N2

atmosphere.

4.2.9 Crystallinity Study by Wide Angle X-ray Diffraction

The impact of the strategically appended linear C16 and branched EH alkyl

functionalities, and the vinylene-spacer on the crystallinity and microstructural order of the

conjugated polysquaraines were elucidated by XRD measurements on thick films drop-casted

from hot concentrated CB (PSQ7 and 8) and TCB (PSQ6) solvent unto polished silicon

substrate. The resulting diffraction patterns are displayed in figure 20. The non-elongated

polysquaraine derivative showed a low intensity and broadened diffraction peak at 4.5o,

which corresponds to an interchain d1-spacing of 19.8 Å. When compared with the value

(23.5o) deduced from the XRD pattern of intercalated poly(2,5-bis(3-hexadecylthiophen-2-

yl)thieno[3,2-b]thiophene (pBTTT-C16) bearing C16 pendant groups, which is known to

form lamellar packing, the calculated d1-spacing showed a 2.7 Å difference.92, 93

This suggest

that this zwitterionic conjugated polymer also possesses an interdigitated interchain packing,

with closer lamellar spacing than that of p-BTTT-C16. However, we note that the diffraction

peaks are much broader than for p-BTTT and without higher order reflections, suggesting the

degree of crystallinity is much lower. Previous studies have shown that polysquaraines

bearing elongated vinylene-spacers and linear alkyl side chains are capable of interdigitation

with the adoption of a slipped stacking orientation for maximum electrostatic interaction.61

For all other polymers, there was a notable absence of any strong diffraction peaks in the low

angle regions. For PSQ6 a very weak peak was just discernible at 2 Ɵ = 7.1º, which

Page | 266

corresponds to a spacing of 12.6 Å. It is possible that this is a second order reflection from a

lamellar spacing, and the first order peak (expected around 2 Ɵ = 3.6º) is not observed. In the

case of PSQ7 which comprised of both C16 and EH alkyl side chains, and the all EH-bearing

polysquaraine (PSQ8) no clear peaks were apparent. This indicates that PSQ7 and PSQ8 are

largely amorphous.

0 5 10 15 20 25 30

100 (4.5)

19.8 Å

010 (21.6) 4.2 Å

010 (20.2)

4.6 Å

010 (21.8) 4.1 Å

010 (19.8)

4.5 Å

010 (19.0)

4.7 Å

010 (19.0) 4.7 Å

Diffr

actio

n In

ten

sity (

a.u

)

2 / degree (CuK)

PDC16

BNC8PSQTT

PDC16

BNC16

PVSQTT

PDC16

BNEHPVSQTT

PBEHBNEHPVSQTT

Silicon substrate

(a)

(b)

(c)

(d)

(e)

Figure 20. Wide angle X-ray diffraction pattern of films of (a) PDC16BNC8PSQTT (Psq11), (b)

PDC16BNC16PVSQTT (PSQ6), (c) PDC16BNEHPVSQTT (PSQ7), (d) PBEHBNEHPVSQTT (PSQ8) and (e)

Neat polished silicon substrate.

Most of the polymers display a broad diffraction around 2Ɵ = 20º, which likely

corresponds to the π-π distance. Interestingly, both the vinylene and non-vinylene

polysquaraines displayed little changes in their π-π stacking distances (4.5 – 4.7 Å), which

indicate that the crystallinity of the polysquaraines are influenced mainly by the alkyl side

chains along the polymer backbone structure. PSQ8 with all branched side chains, shows the

weakest peak indicating the bulky alkyl side-chains act to suppress close contacts between

polymer backbones.

4.3 Conclusion

The synthesis of three novel π-extended zwitterionic poly(thieno[3,2-b]thiophene-

vinyl-pyrrole-alt-squaraine) derivatives, designated as PSQ6, PSQ7 and PSQ8, has been

successfully accomplished. The impact of the strategically appended alkyl functionalities

along the polymer backbone and the elongation of the conjugated system on the morphology,

Page | 267

and physical, optical, and electrochemical characteristics of the resulting polysquaraines was

investigated by modulating the structure of the electron-rich bispyrrole monomers prior to

condensation with squaric acid. This primarily entailed the alternation of the linear hexadecyl

(C16) and 2-ethylhexyl (EH) alkyl groups between the thieno[3,2-b]thiophene and pyrrole

aromatic systems (see structures 10, 11, 14 and 15, scheme 10) and the insertion of the

vinylene C=C linkage in-between aromatic units so as to expand the effective conjugation

length of the afforded polysquaraine (scheme 11, structures 21 – 23). Unfortunately, it was

discovered that the polysquaraines without the vinylene linkages were intrinsically insolubile

in most organic solvents, including; CB, o-DCB, TCB, THF, DCM, ethanol/CB, and toluene,

despite the presence of the solubilising alkyl groups (C16, C8, and EH) along the polymer

mainframe. This was assigned to the rigidity and strong intermolecular/intramolecular

electrostatic interactions in these new set of polysquaraines. Therefore, this precluded the

characterization and processability of Psq16, Psq17, Psq11, and Psq12 (scheme 10). On the

contrary, dramatic improvements were observed in the solubility of PSQ6 – PSQ8 (scheme

11) by the incorporation of the vinylene linkages, which presumably allowed some freedom

of rotation in solution and less electrostatic interactions.

In addition to the notable improvements in solubility, the implementation of the

vinylene-linkers resulted in the considerable broadening (300 – 1200 nm, figure 11 and 12)

and extension of the absorption spectra of the π-extended polysquaraines (PSQ6 – PSQ8)

into the long-wavelength (near-IR) region of the UV spectrum. In contrast, Psq11 showed a

narrowed UV absorption mostly restricted to the visible region (500 – 700 nm, figure 10).

This suggests the vinylene-linkage reduced the steric twisting strain between the adjacent

alkylated thieno[3,2-b]thiophene and the pyrrole units, thus facilitating the polymer backbone

planarization and conjugation. The Mn of PSQ6 – PSQ8 ranged from 66.0 kDa to 12.0 kDa

(PDI = 1.8 – 2.7) with thermal stabilities up to 400 oC under inert conditions. Furthermore,

these π-extended polysquaraines showed small electrochemical band gaps (1.25 to 1.46 eV).

The optical band gaps (1.06 ev to 1.14 eV) were found to be lower than the electrochemical

band gaps. XRD studies indicate that the morphology of the conjugated polymers is

dependent on the alkyl side chains attached to the polymer backbone and the vinylene

linkages. According to the aforementioned evidence, these novel conjugated polymers

displayed all the pre-requisites for OFET and OPV device applications, which are currently

being ascertained.

Page | 268

4.4 Experimental Section

4.4.1 Materials and Chemical Reagents

The dibrominated 3,2-dialkylthieno[3,2-b]thiophene precursors (1 and 2,

scheme 11), and the bromomethylated analogous derivatives (4, scheme 12) were synthesized

as described in chapter 2. A solution of N-methyl-2-(tributylstannyl) pyrrole – 3 (scheme 11)

(95 %) was purchased from Frontier Scientific. 1-Bromo-2-ethyl-1-hexane (95 %), N-

octylpyrrole, tributyltin chloride (95 %), tris(dibenzylidene acetone) dipalladium (0) -

(Pd2(dba)3), tri(o-tolyl)phosphine - P((o-tolyl)3) (97 %), triethylphosphite (98 %),

butyllithium (2.5 M solution in hexanes), 1-bromohexadecane (97 %), para-formaldehyde

powder (95 %), pyrrole-2-carboxyaldehyde (98 %), potassium tert-butoxide (1.0 M solution

in THF), hydrobromic acid solution (˃ 33 % HBr in acetic acid, purum grade), sodium

hydride (60 % dispersion in mineral oil) were obtained from Sigma-Aldrich. In addition

solvent such as, anhydrous n-butanol (≥ 99.5 %), anhydrous toluene, anhydrous

chlorobenzene, anhydrous tetrahydrofuran (≥ 99.9 %, inhibitor free), and anhydrous dimethyl

formarmide (DMF) (99.8 %) were procured from latter. Toluene (AnalaR NORMAPUR),

glacial acetic acid (analytical grade), chloroform, hydrochloric acid, tetrahydrofuran (HPLC

grade), sodium hydrocarbonate (NaHCO3), and magnesium sulfate (MgSO4) were supplied

by VWR BHD PROLABO. All solvents purchased were distilled over calcium hydride

before usage, except otherwise stated.

4.4.2 Experimental Protocol

4.4.2.1 Synthesis of 3,6-dihexadecyl-2,5-bis(N-methyl-1H-pyrrol-2yl)thiophene[3,2-

b]thiophene (DC16BNMPTT) 10.

A Biotage microwave process vial (20 mL) equipped with a teflon-coated magnetic

stirring bar, was charged with 2,5-dibromo-3,6-dihexadecylthieno[3,2-b]thiophene

(DBDC16TT - 7) (509.31 mg, 0.683 mmol), Pd2(dba)3 (12.51 mg, 2 mol %), and P(o-tolyl)3

Page | 269

(16.63 mg, 8 mol %) and C6H5Cl (15 mL with 0.01 % Et3N). The reaction mixture was

degassed with N2 for 10 minutes, followed by the introduction of N-methyl-tributylstannyl

pyrrole – N-MTBSP (0.45 mL, 13.7 mmol) using a syringe (evacuated and filled with N2

several times due to the labile nature of N-MTBSP in air). The reaction was degassed for an

additional 10 minutes with constant stirring. Subsequently, the mixture was subjected to

sequential microwave irradiation at 140 oC for 4 minutes, 160

oC at 4 minutes, and 180

oC for

25 minutes, respectively. On completion, the dark-grey mixture was cooled to ambient

temperature and diluted with diethylether (150 mL). The reaction mixture was washed with

distilled water (400 mL), NaHCO3 (200 mL), and brine (300 mL). The black palladium

catalyst residue formed in the process of extraction was discarded. Then the combined

orange/yellow organic phase was concentrated in vacuo to afford an intensely-coloured crude

product. Purification was performed by silica-gel column chromatography (eluent:

hexane/ethyl acetate 9/1 v/v) and the resulting crude product recrystallized from acetone to

afford 10 as fine pale-green crystals (0.376 g, 74 % yield). MS (EI) (M+): m/z: 747

(C48H78N2S2+).

1H NMR (400 MHz, CDCl3): (ppm) = 0.90 (t, 6H, J = 8 Hz); 1.27 (m, 52H);

1.67 (q, 4H); 2.65 (t, 4H, J = 8Hz); 3.61 (s, 6H); 6.25 (m, 2H), 6.29 (m, 2H); 6.79 (m, 2H).

13C NMR (400 MHz, CDCl3): (ppm) = 14.12 (2C); 22.69; 28.55; 28.74; 29.31; 29.37; 29.48

(4C); 29.53; 29.70; 31.93; 34.57 (2C); 107.66; 111.89; 123.29; 125.66; 134.88; 138.36.

HRMS EI calcd for C48H78N2S2 746.5606, found 746.5594.

4.4.2.2 Synthesis of 3,6-Bis(2-ethyl-1-hexyl)-2,5-bis(N-methyl-1H-pyrrol-2yl)thieno[3,2-

b]thiophene (BEHBNMPTT) 11.

The aforementioned protocol was employed in the synthesis of 5. The product

obtained from the reaction of the oil-like precursor - 2,5-dibromo-3,6-bis(2-ethyl-1-

hexyl)thieno[3,2-b]thiophene (DBBEHTT) (361 mg, 0.692 mmol), Pd2(dba)3 (12.60 mg, 2

mol%), and P(o-tolyl)3 (16.85 mg, 8 mol%), N-MTBSP (0.46 mL, 1.384 mmol) and C6H5Cl

Page | 270

(15 mL with 0.01 % Et3N) was a pale-green oil. The latter was purified by column

chromatography twice as described above. The resulting pale-green oil was dried in a vacuum

oven at 40 oC for 24 h to afford the target monomer 5 (140 mg, 40% yield). MS (EI) (M

+):

m/z: 522 (C32H46N2S2+).

1H NMR (400 MHz, CDCl3): (ppm) = 0.92 (t, 12H, J = 8Hz); 1.30

(m, 16H); 1.87 (m, 4H); 2.59 (d, 4H, J = 8Hz); 3.60 (s, 6H); 6.24 (m, 2H), 6.29 (m, 2H); 6.79

(d, 2H, J = 8Hz). 13

C NMR (400 MHz, CDCl3): (ppm) = 10.63; 14.09; 22.97; 25.92; 28.66;

32.78 (2C); 34.51; 38.44; 107.66; 111.96; 123.14; 125.69; 129.47134.39; 138.54. HRMS EI

calcd for C32H46N2S2 522.3102, found 522.3092.

4.4.2.3 Synthesis of N-octyl-2-(tributylstannyl) pyrrole – 13.

A dry three-neck round bottom flask was equipped with a magnetic stirrer bar and

thermometer and charged with N-octylpyrrole (0.563 g, 3.14 mmol), followed by deaeration

for 10 mins. Anhydrous diethylether (20 mL) was introduced via a syringe. The reaction

mixture was cooled to 0 oC, followed by the dropwise addition of 2.5 M solution of n-BuLi

(1.31 mL, 3.27 mmol) in hexanes, while maintaining the temperature at 0 oC. At this stage, a

colour transformation from light-brown to intense brown ensued. Stirring continued for a

further 2 h. Afterwards the reaction temperature was raised to room temperature and stirred

overnight. The resulting turbid reaction mixture was cooled to -78 C with the addition of

tributyltin chloride in a dropwise manner. The temperature was allowed to rise to room

temperature, accompanied by a colour change to a cream-like mixture. After stirring for 18 h,

the reaction mixture was diluted with diethylether, and washed with distilled water. The

combine organic phase was dried with MgSO4, and condensed in a rotary evaporator to

afford the titled compound 7 (1.3 g, 60 % yield). This product decomposed when purified by

silica-gel column chromatography. However, 1H NMR analysis showed that it was 50 %.

1H

NMR (400 MHz, CDCl3): (ppm) = 0.96 (m, 12H); 1.33 (m, 16H); 1.57 (m, 6H); 1.78 (m, 2

H), 3.88 (t, 2 H, J = 8 Hz); 6.30 (m, 2 H); 6.97 (m, 1H).

Page | 271

4.4.2.4 Synthesis of 3,6-dihexadecyl-2,5-bis(N-octyl-1H-pyrrol-2yl)thiophene[3,2-

b]thiophene (DC16BNC8PTT) 14.

This monomer was synthesized following the protocol employed for compound 10.

The reaction of DC16BNC8PSQTT - 14 (1.25 g, 2.67 mmol), DBDC16TT – 7 (1.00 g, 1.33

mmol), Pd2(dba)3 (0.024 g, 0.027 mmol), P(o-tolyl)3 (0.033 g, 0.11 mmol) in chlorobenzene

(20 mL with 0.01 % Et3N) under microwave conditions, 140 oC/4 mins, 160

oC/4 mins, and

180 oC/35 mins afforded 8 as a pale-green semi-solid (0.903 g, 72 % yield). MS (EI) (M

+):

m/z: 943 (C62H106N2S2+).

1H NMR (400 MHz, CDCl3): (ppm) = 0.92 (m, 12H, J = 8 Hz);

1.30 (m, 72H); 1.70 (m, 8H); 2.65 (t, 4H, J = 8Hz); 6.28 (br, 4H), 6.86 (m, 2H). 13

C NMR

(400 MHz, CDCl3): (ppm) = 8.78; 13.74; 17.17; 22.62; 22.76; 26.74; 26.89; 27.45; 27.71;

28.87; 29.18; 29.33; 29.76 (4C); 29.98; 31.98; 47.21; 107.71; 111.92; 121.83; 125.06;

129.01; 134.96; 138.37. HRMS (not available due to the high molecular weight).

4.4.2.5 Synthesis of 3,6-Bis(2-ethyl-1-hexyl)-2,5-bis(N-octyl-1H-pyrrol-2yl)thieno[3,2-

b]thiophene (BEHBNC8PTT) 15.

Monomer 15 was synthesized as described for the analogous derivative 10. Here the

Stille cross-coupling reaction of BEHDBTT – 8 (0.67 g, 1.28 mmol) and NC8TPSP - 13 (1.17

g, 2.55 mmol), catalyzed by Pd2(dba)3 (0.024 g, 0.026 mmol) and P(o-tolyl)3 (0.031 g, 0.10

mmol) in chlorobenzene (15 mL with 0.01 % Et3N) yielded 9 as a green viscous oil (0.370 g,

Page | 272

40 %). MS (EI) (M+): m/z: 719 (C42H74N2S2

+).

1H NMR (400 MHz, CDCl3): (ppm) = 0.91

(m, 18H); 1.25 (m, 32H); 1.69 (m, 4H); 1.82 (m, 2H); 1.90 (m, 2H), 2.68 (d, 4H, J = 4Hz);

6.24 (m, 4H), 6.83 (m, 2H). 13

C NMR (400 MHz, CDCl3): (ppm) = 14.12 (2C); 22.69;

28.55; 28.74; 28.84; 29.31; 29.37; 29.48; 29.53 (4C); 29.70; 31.93; 34.57; 107.66; 111.89;

123.29; 125.66; 129.03; 134.88; 138.36. HRMS EI calcd for C42H74N2S2 718.5293, found

718.5302.

4.4.2.6 Synthesis of poly(3,6-dihexyldecyl-2-N-methylpyrrol-2-yl-5-(5-cyclobutenediylium-

1,3-diolate-N-methylpyrrol-2-yl)thieno[3.2-b]thiophene (PDC16NMPSQTT) Psq16.

The reaction was performed following the literature protocol.52

To a two neck round-

bottom flask (250 mL) was added the monomer 10 (114.91 mg, 0.154 mmol) and 3,4-

dihdroxy-3-cyclobutene-1,2-dione (squaric acid) (17.56 mg, 0.154 mmol). A Dean-stark

apparatus and a reflux condenser were attached, followed by deaeration with a constant

stream of dry argon for 10 mins. Then, a 1:3 mixture of anhydrous n-butanol (20 mL) and

toluene (60 mL) was introduced via a degassed syringe (20 mL). The reaction mixture was

refluxed at 140 oC for 72 h under argon with the removal of water azeotropically. On

completion, the intense green reaction mixture was cooled to room temperature and filtered.

The intense green filtrate was concentrated in vacuo, dissolved in dichloromethane and

precipitated in n-hexane. The precipitate was collected by filtration, washed with diethyl

ether (150 mL) to eliminate any unreacted bispyrrole monomer, and methanol (150 mL). The

precipitate was Soxhlet extracted with methanol and acetone for 24 h, which produced

polymer 16 (120 mg, 95 % yield). The latter was insoluble is chloroform, dichloromethane

(DCM), chlorobenzene (CB), dichlorobenzene (DCB), trichlorobenzene (TCB), dimethyl

sulfoxide (DMSO), methanol, hexane and xylene, respectively.

Page | 273

4.4.2.7 Synthesis of poly(3,6-bis(2-ethylhexyl)-2-N-methylpyrrol-2-yl-5-(5-

cyclobutenediylium-1,3-diolate-N-methylpyrrol-2-yl)thieno[3.2-b]thiophene)

(PBEHBNMPSQTT) Psq17.

This polysquaraine derivative was synthesized according to condensation procedure

employed for Psq16. The polycondensation of 15 (0.100 g, 0.192 mmol) and squaric acid

(0.022 g, 0.192 mmol) afforded Psq17. This conjugated polymer was predominantly

insoluble in most organic solvents.

4.4.2.8 Synthesis of poly(3,6-dihexadecyl-2-N-octylpyrrol-2-yl-5-(5-cyclobutenediylium-

1,3-diolate-N-octylpyrrol-2-yl)thieno[3.2-b]thiophene) (PDC16BNC8PSQTT) Psq11.

This polysquaraine derivative was synthesized as described for Psq16. The

polycondensation of 8 (0.903 g, 0.958 mmol) and squaric acid (0.109 g, 0.958 mmol) formed

Psq11 in a solvent mixture of n-butanol and benzene (2:1) under azeotropic distillation at 120

oC for 48 h under N2 atmosphere, in the form of a blue solid (after purification, re-

precipitation, and Soxhlet extraction). Nonetheless, this zwitterionic polymer derivate was

profoundly insoluble in most organic solvents. Some solubility was observed in TCB when

heated to 200 oC for 24 h.

Page | 274

4.4.2.9 Synthesis of poly(3,6-bis(2-ethylhexyl)-2-N-octylpyrrol-2-yl-5-(5-

cyclobutenediylium-1,3-diolate-N-octylpyrrol-2-yl)thieno[3.2-b]thiophene)

(PBEHBNC8PSQTT) Psq12.

This polysquaraine derivative was synthesized as described for Psq16. The

polycondensation of 9 (0.066 g, 0.0922 mmol) and squaric acid (0.0105 g, 0.0922 mmol) in a

solvent mixture of n-butanol and toluene (1:3) under azeotropic distillation at 140 oC for 72 h

under N2 atmosphere, produced Psq12 as a green solid after purification and precipitation in

methanol. This conjugated polymer was predominantly insoluble in most organic solvents.

This polymer was initially soluble in CB, DCM and chloroform. But, after 24 h storage, it

came intractable. In addition, the product obtained consisted of predominantly oligomers (Mn

= 3.6 kDa, Mw = 4.2 kDa, and PDI = 1.2).

4.4.2.10 Synthesis of 3,6-Dihexadecyl-2,5-bis(methylenediethylphosphate)thieno[3,2-

b]thiophene (DC16BMDEPTT).

This compound was synthesized as described in the literature.61, 62

To a round bottom

flask (100 mL) was added a mixture of 2,5-dibromomethyl-3,6-dihexadecylthieno[3,2-

b]thiophene (1.01 g, 1.30 mmol) and triethylphosphite (1.4 mL, 7.80 mmol) and refluxed

under argon at 140 oC for 24 h with stirring. On completion, the dark coloured solution was

cooled to room temperature and the unreacted triethylphosphite removed under in a rotary

Page | 275

evaporator. Further drying in a high vacuum oven at 40 oC for 24 h to afforded

DC16BMDEPTT as a pale-brown viscous oil/solid (1.31 g, 96% yield). MS (EI) (M+): m/z:

889 (C48H90O6P2S2+).

1H NMR (400 MHz, CDCl3) 0.90 (t, 18H, J = 8 Hz); 1.28 (m, 52H);

1.69 (m, 4H), 2.68 (t, 4H, J = 8 Hz), 3.40 (d, 4H, J = 20 Hz), 4.11 (m, 8H), 13

C NMR (400

MHz, CDCl3) 14.12; 16.42; 16.70; 16.88; 22.69; 26.47; 27.79; 28.72; 29.52; 29.70 (4C);

31.83; 58.05 (2C); 62.66; 126.36; 133.08;137.05. 32

P NMR (400 MHz, CDCl3) δ 24.22.

HRMS calcd for C48H90O6P2S2 (M+): 888.5654, found 888.5630.

4.4.2.11 Synthesis of 3,6-bis(2-ethylhexyl)-2,5-bis(methylenediethylphosphate)thieno[3,2-

b]thiophene (BEHBMDEPTT).

The procedure employed in the synthesis of DC16BMDEPTT was adopted for the

preparation of the EH-bearing analogue. BEHBMDEPTT was obtained as brown viscous oil

(1.12 g, 66% yield). MS (EI) (M+): m/z: 664 (C32H58O6P2S2

+).

1H NMR (400 MHz, CDCl3)

0.85 (m, 24H); 1.34 (m, 16H); 1.80 (m, 4H); 2.59 (d, 4H, J = 8 Hz); 3.36 (d, 4H, J = 20 Hz);

4.09 (m, 8H). 13

C NMR (400 MHz, CDCl3) 10.92; 14.04; 16.07; 16.47; 22.97; 25.99; 28.90;

32.50; 38.90; 63.62; 127.05; 133.57; 137.24. 32

P NMR (400 MHz, CDCl3) δ 24.33. HRMS

calcd for C32H58O6P2S2 (M+): 664.3150, found 664.3156.

4.4.2.12 Synthesis of N-alkylpyrrole-2-carboxaldehyde derivatives (NAPC).

Page | 276

N-hexadecylpyrrole-2-carboxaldehyde 17. A suspension of NaH (1.60 g, of 60 %

suspension in mineral oil, 40.0 mmol) in anhydrous DMF (15 mL) was placed in a three-neck

round bottom flask (250 mL) and sealed. The suspension was deaerated with N2 via a syringe,

cooled to 0 oC and stirred continuously for 10 min. Then, a degassed solution of pyrrole-2-

carbaldehyde (3.40 g, 30.75 mmol) in DMF (30 mL) was introduced dropwise via a syringe.

The resultant white coloured suspension was stirred at 0 oC for a further 30 min. Afterwards a

degassed solution of 1-bromohexadecane (11.45 mL, 37.5 mmol) in anhydrous DMF (30 mL)

was added dropwise via a syringe and effervescence was observed. The grey coloured

mixture was stirred at ambient temperature under N2 protection for 72 h. On completion, the

reaction mixture was poured into ice-cold distilled water (300 mL) and extracted with diethyl

ether. The combined organic phase was washed with distilled water, dried over anhydrous

MgSO4, and concentrated under reduced pressure. Upon drying in a vacuum oven at 40 oC

for 48 h a viscous orange-oil 17 was obtained (7.1 g, 63 % yield). MS (EI) (M+): m/z: 319

(C32H58NO). The product was sufficiently pure for usage in the next step. 1

H NMR (400

MHz, CDCl3) 0.90 (t, 3H, J = 8 Hz); 1.28 (m, 24H); 1.76 (q, 2H); 4.32 (t, 2H, J = 8 Hz);

6.24 (d d, 1H, J = 4.4 Hz, J = 2.8); 6.94 (m, 2H); 9.55 (s, 1H). 13

C NMR (400 MHz, CDCl3)

14.13 (2C); 22.70; 26.52; 29.22; 29.38; 29.53; 29.57; 29.68 (4C); 31.37; 31.94; 49.13;

109.39; 124.70; 131.17; 131.46; 179.18. HRMS calcd for C21H27NO (M+): 319.2875, found

319.2867.

N-(2-ethylhexyl)-2-carboxyaldehyde 18. This compound was prepared as described

for 2 above. In this case, a deaerated solution of 2-ethylhexybromine (6.67 mL, 37.5 mmol)

in anhydrous DMF (30 mL) was utilized. After purification and prolonged drying under

vacuum compound 3 was obtained as orange-coloured oil (6.3 g, 86 % yield). MS (EI) (M+):

m/z: 207 (C13H21NO). 1

H NMR (400 MHz, CDCl3) 0.88 (t, 6H, J = 8 Hz); 1.27 (m, 8H);

1.81 (m, 1H); 4.21 (m, 2H); 6.21 (m, 1H); 7.29 (m, 2H); 9.54 (s, 1H). 13

C NMR (400 MHz,

CDCl3) 10.37; 14.11; 22.99; 23.44; 28.33; 29.71; 40.41; 52.74; 109.29; 124.79; 131.63;

131.86 179.21. HRMS calcd for C13H21NO: 207.1623, found 207.1622.

Page | 277

4.4.2.13 Synthesis of (E,E)-2,5-bis[2-(1-hexadecylpyrrol-2-yl)vinyl]-3,6-

dihexadecylthieno[3,2-b]thiophene (BC16PVDC16TT) 21.

To a three-neck round bottom flask (100 mL) was added a mixture of

DC16BMDEPTT (1.20 g, 1.35 mmol) and N-hexadecylpyrrole-2-carboxaldehyde 3 (0.86g,

2.7 mmol). The flask was equipped with a reflux condenser and a magnetic stirring bar, and

placed under argon atmosphere. A solution of anhydrous THF (20 mL) was introduced via a

degassed syringe. After deaeration for 1 h at room temperature, a solution of potassium tert-

butoxide in anhydrous THF (1.0 M, 2.7 mL, 2.7 mmol) was added dropwise for 30 min,

followed by reflux for 15 h. On completion, the intensely-green and highly fluorescent

reaction mixture was cooled to room temperature, condensed under reduced pressure and

neutralised with 5 % hydrochloric acid solution. The organic phase was extracted with

CHCl3, and then washed with distilled water, saturated aqueous NaHCO3, and brine. After

drying over anhydrous MgSO4, and concentration under reduced pressure, the resultant crude

product was purified by precipitation (x3) from chloroform into methanol. Alternatively, the

extracted crude mixture can be purified by silica-gel column chromatography (eluent:

hexane/ethyl acetate 9:1 v/v). Then after drying in under vacuum for 48 h a green solid 21

was afforded (0.934, 70 % yield). MS (EI) (M+): 1219 (C82H142N2S2

+).

1H NMR (400 MHz,

CDCl3) 0.90 (t, 12H, J = 6 Hz); 1.28 (m, 108H); 1.79 (m, 4H); 2.70 (t, 4H, J = 8Hz); 3.97

(t, 4H, J = 6H Hz); 6.19 (t, 2H, J = 4 Hz); 6.51 (d, 2H, J = 4 Hz); 6.71-6.75 (m, 4H); 7.04-

7.07 (d, 2H, J = 12 Hz). 13

C NMR (400 MHz, CDCl3) 14.12 (4C); 22.70 (4C); 26.80; 28.05;

29.22; 29.37; 29.44 (4C); 29.51; 29.54 (12 C); 29.61; 29.67; 29.72; 31.51; 31.93; 47.03 (2C);

106.49; 108.34; 115.82; 118.17; 122.68; 131.25; 132.23; 137.13; 138.34.

Page | 278

4.4.2.14 Synthesis of (E,E)-2,5-bis[2-(N-(2-ethylhexyl)pyrrol-2-yl)vinyl]-3,6-

dihexadecylthieno[3,2-b]thiophene (BNEHPVDC16TT) 22.

Following the aforementioned procedure, the titled monomer 22 (0.66 g, 70 %) was

synthesized from a mixture of DC16BMDEPTT (0.85 g, 0.96 mmol) and N-(2-

ethylhexyl)pyrrole-2-carboxaldehyde 18 (0.40 g, 1.91 mmol). MS (EI) (M+): 994

(C66H110N2S2+).

1H NMR (400 MHz, CDCl3) 0.90 (t, 18H, J = 6 Hz); 1.28 (m, 108H); 1.72

(m, 4H); 2.76 (t, 4H, J = 8 Hz); 3.83 (m, 4H); 6.18 (t, 2H, J = 4 Hz); 6.51 (d d, 2H, J = 1.2

Hz, J = 1.2 Hz); 6.67 (m, 4H); 6.76 (d, 2H, J = 16 Hz); 7.03 (d, 2H, J = 16 Hz). 13

C NMR

(400 MHz, CDCl3) 10.67; 14.12 (2C); 22.67; 26.80; 28.05; 29.22; 29.37; 29.44; 29.51;

29.54; 29.61; 29.68 (6C); 29.72; 30.51; 31.94; 47.03; 105.50; 106.50; 108.34; 115.82;

118.17; 122.68; 131.26; 132.28; 137.14; 138.39.

4.4.2.15 Synthesis of (E,E)-2,5-bis[2-(1-(2-ethylhexyl)pyrrol-2-yl)vinyl]-3,6-

dihexadecylthieno[3,2-b]thiophene (BC16PVDC16TT) 23.

The procedure employed in this section was adopted from that delineated for the

preparation of the titled monomer 21. BEHPVBEHTT – 23 was obtained as an intense-green

Page | 279

solid (0.425 g, 31 %). Due to oil-like nature of this compound, satisfactory purity was

attained by silica-gel chromatography (eluent: DCM/hexane 1:2), followed by thorough

drying under vacuum at 40 oC for 48 h. MS (EI) (M

+): 771 (C59H78N2S2

+).

1H NMR (400

MHz, CDCl3) 0.94 (m, 24H); 1.34 (m, 24H); 1.77 (m, 2H); 1.81 (m, 2H), 2.67 (m, 4H);

3.84 (m, 4H); 6.17 (d d, 2H, J = 4 Hz, J = 4 Hz); 6.49 (d d, 2H, J = 1.2 Hz, J = 1.2 Hz); 6.66

(m, 2H); 6.75 (d, 2H, J = 16 Hz), 7.03 (d, 2H, J = 16 Hz). 13

C NMR (400 MHz, CDCl3)

10.21; 10.67; 10.79; 10.90; 13.97; 14.09; 22.63; 22.97; 23.40; 28.31; 28.80; 29.67; 30.12;

31.56; 40.39; 52.74; 105.50; 106.08; 108.08; 109.25; 115.84; 118.27; 124.78; 125.13; 131.60;

131.84; 137.24. HRMS calcd for C50H78N2S2: 770.5606, found 770.5606.

4.4.2.16 Synthesis of Poly(3,6-dihexadecyl-2-(N-hexadecylpyrrol-2-yl)-5-(5-

cyclobutadienediylium-1,3-diolate-N-hexadecylpyrrol-2-yl-5-vinyl)thieno[3,2-b]thiophene)

(PDC16NC16PCBDDC16PVTT or PDC16BNC16PVSQTT – PSQ6.

This polysquaraine was synthesized as described in the literature.39, 61, 62

A two-neck

round bottom flask (500 mL) was charged with DC16BNC16PVSQTT 6 (0.314 g, 0.257

mmol) and the squaric acid (0.0293 g, 0.257 mmol). A Dean-Stark reflux condenser was

attached, followed by, the introduction of anhydrous toluene (150 mL) and anhydrous n-

butanol (50 mL) via degassed syringes at room temperature. The reaction mixture was then

refluxed at 140 oC for 7 days with the removal of water azeotropically. On completion, the

intense green crude product was cooled to room temperature, filtered and concentrated under

reduced pressure. The resultant product was dissolved in chloroform, precipitated in

methanol (3x), and dried under vacuum at 40 oC for 48 h to afford the target polymer PSQ6

as an intense-green solid. (0.428 g, 60 % yield). GPC: Mn = 75.2 kDa, Mw = 90.5 kDa, and

PDI = 1.2. 1H NMR (400 MHz, 1,2-DCB-D4) 0.97 (m, 12H); 1.40 (br, m, 112H); 1.82 (m,

4H); 2.84 (m, 4H); 4.49 (m, 2H); 5.30 (m, 2H); 7.13 – 7.18 (m, 4H); 7.27 (d, 2H). Found: C,

74.54; H, 11.75; N, 0.93 %. Calcd. (C82H142N2S2): C, 79.57; H, 10.87, N, 2.16 %.

Page | 280

4.4.2.17 Synthesis of Poly(3,6-dihexadecyl-2-N-2-ethylhexylpyrrol-2-yl)-5-(5-

cyclobutadienediylium-1,3-diolate-N-2-ethylhexylpyrrol-2-yl-5-vinyl)thieno[3,2-b]thiophene)

(PDC16NEHPCBDDNEHPVTT or PDC16BNEHPVSQTT – PSQ7.

The above procedure was applied in the synthesis of PSQ7 (0.278 g, 83 % yield).

GPC: Mn = 12.0 kDa, Mw = 27.5 kDa, and PDI = 2.4. 1H NMR (400 MHz, CDCl3) 0.96 (m,

18H); 1.37 (br, m, 68H); 1.80 (m, 4H); 2.34 (m, 2H); 2.76 (m, 4H); 4.23 (m, 2H); 4.86 (m,

2H); 6.94 (br, 4H); 7.91 (s,s, 2H). Found: C, 78.91; H, 11.76; N, 1.14 %. Calcd.

(C70H108N2O2S2)n: C, 78.30; H, 10.14, N, 2.61 %.

4.4.2.18 Synthesis of Poly(3,6-bis(N-2-ethylhexyl-2-N-2-ethylhexylpyrrol-2-yl)-5-(5-

cyclobutadienediylium-1,3-diolate-N-2-ethylhexylpyrrol-2-yl-5-vinyl)thieno[3,2-b]thiophene)

(PBEHNEHPCBDDNEHPVTT or PBEHBNEHPVSQTT – PSQ8.

The protocol used in the synthesis of PSQ8 (0.285 g, 75 % yield) was as described for

the preparation of PSQ6. GPC: Mn = 15.0 kDa, Mw = 40.0 kDa, and PDI = 2.7. 1H NMR

(400 MHz, CDCl3) 0.96 (m, 24H); 1.37 (br, m, 24H); 1.88 (m, 2H); 2.34 (m, 2H); 2.65 (br,

Page | 281

m, 8H); 4.16 (m, 2H); 4.84 (m, 2H); 6.92 (br, 4H); 7.91 (d, 2H). Found: C, 78.47; H, 11.23;

N, 1.39 %. Calcd. (C70H108N2O2S2)n: C, 76.73; H, 8.59, N, 3.31 %.

Page | 282

4.5 Reference

1. Ma, W. L.; Yang, C. Y.; Gang, X.; Lee, K.; Heeger, A. J., Adv. Funct. Mater., 2005,

15, 1617-1622.

2. Li, G.; Shrotriya, V.; Huang, J. S.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y., Nat.

Mater. 2005, 4, 864 - 868.

3. Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A.

J., Science 2007, 317, 222 - 225.

4. Park, S. H.; Roy, A.; S., B.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.;

Lee, K.; Heeger, A. J., Nature Photonics 2009, 3, 297 - 303.

5. Oku, T.; Nagaoka, S.; Suzuki, A.; Kikuchi, K.; Hayashi, Y.; Sakuragi, H.; Soga, T.,

J.Ceram. Process. Res. 2008, 9 (6), 549 - 552.

6. Ma, C.-Q.; Fonrodona, M.; Schikora, M. C.; Wienk, M. M.; Janssen, R. A. J.;

Bauerle, P., Adv. Funct. Mater. 2008, 18, 3323 - 3331.

7. McCullough, R. D.; Lowe, R. D.; Jayaraman, M.; Anderson, D. L., J. Org. Chem.,

1993, 58, 904-912.

8. Parekh, B. P.; Tangonan, A. A.; Newaz, S. S.; Sanduja, S. K.; Ashraf, A. Q.;

Krishnamoorti, R.; Lee, T. R., Macromolecules 2004, 37, 8883 - 8887.

9. Das, S.; Thomas, K. G.; George, M. V., Mol. Supramol. Photochem. 1997, 1, 467 -

517.

10. Beverina, L.; Salice, P., Eur. J. Org. Chem. 2010, 1207 - 1225.

11. Law, K.-Y., J. Phys. Chem. 1987, 91, 5184 - 5193.

12. Detty, M. R.; Henne, B., Heterocycles 1993, 35, 1149

13. Bello, K. A.; Corns, S. N.; Griffiths, J., J. Chem. Soc., Chem. Commun. 1993, 5, 452 -

454.

14. Keil, D.; Hartmann, H.; Moschny, T., Dyes Pigm. 1991, 17, 19.

15. Chen, C.-T.; Marder, S. R.; Cheng, L. T., J. Am. Chem. Soc. 1994, 116, 3117 - 3118.

16. Law, K.-Y., Chem. Rev. 1993, 93, 449 - 486.

17. Loutfy, R. O.; K., H. C.; Kazmaier, P. M., Photogr. Sci. Eng. 1983, 27, 5 - 9.

18. Bagnis, D.; Beverina, L.; Huang, H.; Silvestri, F.; Yao, Y.; Yan, H.; Pagani, G. A.;

Marks, T. J.; Facchetti, A., J. Am. Chem. Soc. 2010, 132, 4074 - 4075.

19. Silvestri, F.; Irwin, M. D.; Beverina, L.; Facchetti, A.; Pagani, G. A.; Marks, T. J., J.

Am. Chem. Soc. 2010, 130, 17640 - 17641.

20. Piechowski, A.; Bird, G.; Morel, D.; Stogryn, E., J. Phys. Chem. 1984, 88, 934 - 950.

21. Liang, K.; Law, K.-Y.; Whitten, D. G., J. Phys. Chem. 2002, 99, 16704 - 16708.

22. Yum, J.-H.; Walter, P.; Huber, S.; Rentsch, D.; Geiger, T.; Nuesch, F.; De Angelis,

F.; Gratzel, M.; Nazeeruddin, M. K., J. Am. Chem. Soc. 2007, 129, 10320 - 10321.

23. Law, K.-Y., J. Phys. Chem. 1988, 92, 4226 - 4231.

24. Smits, E. C. P.; Setayesh, S.; Anthopoulos, T. D.; Buechel, M.; Nijssen, W.;

Coehoorn, R.; Blom, P. W. M.; de Boer, B.; de Leeuw, D. M., Adv. Mater. 2007, 19, 734 -

738.

25. Wobkenberg, P. H.; Labram, J. G.; Swiecicki, J.-M.; Parkhomenko, K.; Sredojevic,

D.; Gisselbrecht, J.-P.; de Leeuw, D. M.; Bradley, D. D. C.; Djukic, J.-P.; Anthopoulos, T.

D., J. Mater. Chem. 2010, 20, 3676 - 3680.

26. Ashwell, G. J.; Jefferies, G.; Hamilton, D. G.; Lynch, D. E.; Roberts, M. P.; Bahra, G.

S.; Brown, C. R., Nature 1995, 375, 385 - 388.

27. Liu, L.-H.; Nakatani, K.; Pansu, R.; Vachon, J.-J.; Tauc, P.; Ishow, E., Adv. Mater.

2007, 19, 433 - 436.

28. Ghazarossian, V.; Pease, J. S.; Ru, M. W. L.; Laney, M.; Tarnowski, T. L., Chem.

Abstr. 1990, 113, 61319j.

Page | 283

29. Schmidt, A. H., In Oxocarbons, West, R., Ed. Academic Press: New York, 1980.

30. Scherer, D.; Dörfler, R.; Feldner, A.; Vogtmann, T.; Schwoerer, M.; Lawrentz, U.;

Grahn, W.; Lambert, C., Chem. Phys. 2002, 279, 179 -207.

31. Chung, S.-J.; Zheng, S.; Odani, T.; Beverina, L.; Fu, J.; Padilha, L. A.; Biesso, A.;

Hales, J. M.; Zhan, X.; Schmidt, K.; Ye, A.; Zojer, E.; Barlow, S.; Hagan, D. J.; Van

Stryland, E. W.; Yuanping, Y.; Shuai, Z.; Pagani, G. A.; Bredas, J.-L.; Perry, J. W.; Marder,

S. R., J. Am. Chem. Soc. 2006, 128, 14444 - 14445.

32. Lin, T.-Z.; Chung, S.-J.; Kim, K.-S.; Wang, X.; He, G. S.; Swiatkiewicz, J.; Pudavar,

H. E.; Prasad, P. N., Adv. Polym. Sci. 2003, 161, 157 - 193.

33. Schmidt, H. A., Synthesis 1980, 961 - 994.

34. Treibs, A.; Jacob, K., Angew. Chem. Int. Ed. Engl. 1965, 4, 694.

35. Treibs, A.; Jacob, K., Liebig's Ann. Chem. 1966, 699, 153 - 167.

36. Maahs, G.; Hegenberg, P., Angew. Chem. Int. Ed. 1966, 5, 888 - 893.

37. Bonnett, R.; Motevalli, M.; Siu, J., Tetrahedron 2004, 60, 8913 - 8918.

38. Ajayaghosh, A., Acc. Chem. Res. 2005, 38, 449 - 459.

39. Ajayaghosh, A., Chem. Soc. Rev. 2003, 32, 181 - 191.

40. Sprenger, H.-E.; Ziegenbein, W., Angew. Chem. Int. Ed. Engl. 1967, 6, 553 - 554.

41. Beverina, L.; Abbotto, A.; Landenna, M.; Cerminara, M.; Tubino, R.; Meinardi, F.;

Bradamante, S.; Pagani, G. A., Org. Lett., 2005, 7, 4257 - 4260.

42. Beverina, L.; Crippa, M.; Salice, P.; Ruffo, R.; Ferrante, C.; Fortunati, I.; Signorini,

R.; Mari, C. M.; Bozio, R.; Facchetti, A.; Pagani, G. A., Chem. Mater. 2008, 20, 3242 - 3244.

43. Ziegenbein, W.; Sprenger, H.-E., Angew. Chem. Int. Ed. Engl. 1966, 5, 893 - 894.

44. Sprenger, H.-E.; Ziegenbein, W., Angew. Chem. Int. Ed. Engl. 1966, 5, 894.

45. Buschel, M.; Ajayaghosh, A.; Arunkumar, E.; Daub, J., Org. Lett. 2003, 5, 2957 -

2978

46. Brocks, G.; Tol, A., J. Phys. Chem. 1996, 100, 1838 - 1846.

47. Chen, Y.-Y.; Hall, H. K., Jr. , Polym. Bull. 1986, 16, 419 - 425.

48. Treibs, A.; Jacob, K., Angew. Chem., Int. Ed. Engl. 1965, 4, 694 - 695.

49. Havinga, E. E.; ten Hoeve, W.; Wynberg, H., Polym. Bull. 1992, 29, 119 - 126.

50. Havinga, E. E.; ten Hoeve, W.; Wynberg, H., Synth. Met. 1993, 55, 299 - 306.

51. Havinga, E. E.; Pomp, A.; ten Hoeve, W.; Wynberg, H., Synth. Met. 1995, 69, 581 -

582.

52. Chenthamarakshan, C. R.; Eldo, J.; Ajayaghosh, A., Macromolecules 1999, 32, 251 -

257.

53. Reichardt, C., Solvents and Solvent Effects in Organic Chemistry. 2nd ed.; VHC:

Verlagsgesellschaft, Weinheim, 1988.

54. Kamlet, M. J.; Abboud, J.-L. M.; Abraham, M. H.; Taft, R. W., J. Org. Chem. 1983,

48, 2877.

55. Zollinger, H., Colour Chemistry. VCH: New York, 1991.

56. Ajayaghosh, A.; Chenthamarakshan, C. R.; Das, S.; George, M. V., Chem. Mater.

1997, 9, 644 - 646.

57. Chenthamarakshan, C. R.; Ajayaghosh, A., Chem. Mater. 1998, 10, 1657 - 1663.

58. Bredas, J.-L.; Heeger, A. J.; Wudl, F., J. Chem. Phys. 1986, 85, 4673 - 4678.

59. Jenekhe, S. A., Nature 1986, 322, 345 - 347.

60. Ajayaghosh, A.; Eldo, J., Org. Lett. 2001, 3, 2595 - 2598.

61. Eldo, J.; Ajayaghosh, A., Chem. Mater. 2002, 14, 410 - 418.

62. Wu, J.; Huo, E.; Wu, Z.; Lu, Z.; Xie, M.; Jiang, Q., e-polymers 2007, 77, 1 - 10.

63. Volker, S. F.; Uemura, S.; Limpinsel, M.; Mingebach, M.; Deibel, C.; Dyakonov, V.;

Lambert, C., Macromol. Chem. Phys., 2010, 211, 1098 - 1108.

Page | 284

64. Shi, Q.; Chen, W.-Q.; Xiang, J.; Duan, X.-M.; Zhan, X., Macromolecules 2011, 44,

3759 - 3765.

65. Zhang, W.; Wang, Z.; Tang, Y. S.; Xu, Z. G.; Li, Y.; Jiang, Q., Chin. Chem. Lett.

2010, 21, 245 - 248.

66. Dhanabalan, A.; van Dongen, J. L. J.; van Duren, J. K. J.; Janssen, H. M.; van Hal, P.

A.; Janssen, R. A. J., Macromolecules 2001, 34, 2495-2501.

67. Salman, H.; Abraham, Y.; Tal, S.; Meltzman, S.; Kapon, M.; Tessler, N.; Speiser, S.;

Eichen, Y., Eur. J. Org. Chem. 2005, 2207 - 2212.

68. AIST, Spectra Database for Organic Compounds

69. Balakrishnan, K.; Datar, A.; Naddo, T.; Huang, J.; Oitker, R.; Yen, M.; Zhao, J.;

Zang, L., J. Am. Chem. Soc. 2006, 128, 7390.

70. Arbuzov, B. A., Pure Appl. Chem. 1964, 9, 307 - 335.

71. Lu, H.-C.; Whang, W.-T.; Cheng, B.-M., Synth. Met. 2010, 160, 1002 - 1007.

72. Lynch, D. E.; Geissler, U.; Kwiatkowski, J.; Whittaker, A. K., Polym. Bull. 1997, 38,

439 - 499.

73. Bonnett, R.; Davies, J. E.; Hursthouse, M. B.; Sheldrick, G. M., Proc. R. Soc. London

1978, B202, 249 - 268.

74. Neuse, E. W.; Green, B. R., Polymer 1974, 15, 339

75. Santana, A. C.; de Siqueira, L. J. A.; Santos, P. S.; Temperini, M. L. A., J. Raman

Spectrosc. 2006, 37, 1346 - 1353.

76. Block, M. A. B.; Hecht, S., Macromolecules 2004, 37, 4761 - 4769.

77. Stuart, B.; Ando, D., Modern Infrared Spectroscopy. 1st ed.; John Wiley & Sons:

Chichester, 1996.

78. Smith, B. C., Infrared Spectral Interpretation a Systematic Approach. 1st ed.; CRC

Press LLC: 1998.

79. Drury, A.; Maier, S.; Ruther, M.; Blau, W. J., J. Mater. Chem., 2003, 13, 485 - 490.

80. Meier, H.; Stalmach, U.; Kolshorn, H., Acta Polym. 1997, 48, 379.

81. Liang, K.; Farahat, M. S.; Perlstein, J.; Law, K.-Y.; Whitten, D. G., J. Am. Chem. Soc.

1997, 119, 830 - 831.

82. Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.;

Brabec, C. J., Adv. Mater., 2006, 18, 789-794.

83. Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Mühlbacher, D.; Scharber, M.;

Brabec, C., Macromolecules 2007, 40, 1981-1986.

84. Coffin, R. C.; Peet, J.; Rogers, J.; Bazan, G. C., Nat. Chem., 2009, 1, 657-661.

85. Gomer, R. J.; Tryson, G., J. Chem. Phys. 1977, 66, 4413.

86. Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C., Adv. Mater., 2011,

23, 2367-2371.

87. Kötz, R.; Neff, H.; Müller, K. A., J. Electroanal. Chem. Interfacial Electrochem.

1986, 215, 331.

88. Baran, D.; Balan, A.; Celebi, S.; Esteban, B. M.; Neugebauer, H.; Sariciftci, N. S.;

Toppare, L., Chem. Mater., 2010, 22, 2978-2987.

89. Ashraf, R. S.; Klemm, E., J. Polym. Sci., Part A: Polym. Chem., 2005, 43, 6445 -

6454.

90. Wang, X.; Luo, H.; Sun, Y.; Zhang, M.; Li, X.; Yu, G.; Liu, Y.; Li, Y.; Wang, H., J.

Polym. Sci., Part A: Polym. Chem., 2012, 50, 371 - 377.

91. Zhang, G.; Liu, K.; Fan, H.; Li, Y.; Zhan, X.; Li, Y.; Yang, M., Synth. Met., 2009,

159, 1991 - 1995.

92. Cates, N. C.; Gysel, R.; Beiley, Z.; Miller, C. E.; Toney, M. F.; Heeney, M.;

McCulloch, I.; McGehee, M. D., Nano Lett. 2009, 9, 4153 - 4157.

Page | 285

93. McCulloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; MacDonald, I.; Shkunov, M.;

Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.; Chabinyc, M. L.; Kline, R. J.; McGehee,

M. D.; Toney, M. F., Nat. Mater., 2006, 5, 328-333.

Page | 286

Chapter 5

General Thesis Conclusions

Page | 287

5.0 General Thesis Conclusions

Organic electronics continues to attract much interest in a variety of applications, with

a particular current focus on optimizing the performance of organic photovoltaics (OPVs) and

organic field effect transistors (OFETs). At the forefront of this device optimization strategy

is the design and synthesis of a plethora of novel intrinsic π-conjugated semiconducting

polymer structures, with the aim of narrowing the HOMO-LUMO band gap (via extension of

their effective conjugation length) and enhancing their morphology. As highlighted in chapter

1, by lowering the band gap of conjugated polymers (CPs), their photon harvesting capacity

can be substantially increased by the broadening of the absorption spectra to cover both the

visible and near-infrared (NIR) regions (300 – 1200 nm) of the solar spectrum. In addition, it

was envisaged that this would translate into higher efficiencies in the widely-explored BHJ

OPV device configuration. Although, there are many approaches in the literature for lowering

the band gap and enhancing the morphology of conjugated polymers, the concept of

copolymerizing an electron-donor (D) moiety and an electron-acceptor unit (A), both

involving fused heterocyclic systems, is generally considered the start-of-the-art synthesis

technique, and has so far proven to be the best strategy with its BHJ OPVs generating PCEs

currently at 8 %.

In this project, we present the synthesis and characterization of 13 novel π-conjugated

homopolymers and copolymers based on the electron-rich fused thiophene heterocycle, for

OPV and OFET device applications. In relation to the homopolymers, vinylene (C=C) and

ethynylene (C≡C) linkages were incorporated between adjacent thieno[3,2-b]thiophene donor

units, to increase their effective conjugation length (ECL) and thus effect a reduction in their

HOMO-LUMO band gaps. These conjugated polymers were mainly synthesized via Stille

and Gilch polymerization methodologies to study their effect on the molecular weight, and

polydispersity index (PDI) of the semiconducting materials.

The copolymers were comprised of thieno[3,2-b]thiophene as the main donor, along

with a variety of acceptors, such as benzo[2,1,3]thiadiazole (BT), squaric acid (SQ), N-

octylthieno[3,4-c]pyrrole-4,6-dione (OTPD), 2,5-dioctylpyrrolo[3,4-c]pyrrole-1,4-dione

(ODPP). These were synthesized by a variety of Sonogashira and condensation

polymerization routes. In addition, the alkyl substituents on the fused thiophene unit were

varied between the straight chain hexadecyl (C16), and branched chain 2-ethylhexyl (EH).

For the polysquaraines, additional branched and straight chain pyrroles were included as co-

Page | 288

monomers. The aim was to investigate the impact of the placements of linear and branched

alkyl functionalities along the polymer backbone on their solubility (or processability), film

forming ability, crystallinity, device performance, and to some extent, their HOMO and

LUMO energy levels.

The energy levels of the Frontier orbitals (HOMO and LUMO) were ascertained by

electrochemical (CV) characterization and, a combination of UV and UPS (or PESA)

measurements. We found significant differences in both the ionization potential and optical

band gap for the homopolymers of thieno[3,2-b]thiophene linked by either vinylene (TTV) or

ethynyl groups (TTE), with the PPV exhibiting typical band gaps around 1.7 - 1.8 eV, whilst

the TTE‟s were around 2.2 - 2.3 eV. The exact values depended on the nature of the side-

chain, which we rationalize through the ability of the polymers to fully planarize in the solid

state. In addition we found that the nature of the synthetic route to TTV, namely Stille versus

Gilch had a significant effect on the polymer ionization potential, possibly by the occurrence

of backbone defects. Co-polymerization of the ethynyl-linked conjugated polymers with a

range of acceptor monomers was demonstrated to be an effective way to reduce the polymer

band gap. Very low band gap polymers were prepared by the inclusion of the zwitterionic

squarine acceptor in the polymer backbone structure.

Morphological studies using the X-ray diffraction (XRD) technique on drop-casted

films of the homopolymers and copolymers, reveal that the π-conjugated systems with the

linear C16 side-chain possessed better crystalline order and stronger interchain packing,

compared to those bearing the branched EH alkyl groups along the polymer backbone, as

anticipated. However, the strength of the acceptor was also found to strongly influence the

morphology of the copolymers, most notably in the case of OTPD and SQ.

Preliminary electrical studies of the vinylene-linked homopolymers in a bottom-gate

top-contact OFET, demonstrated a hole mobilities of 2.0 x 10-2

cm2 V

-1 s

-1 (C16) and 0.8 x

10-2

cm2 V

-1 s

-1 (EH). We were unable to measure any field effect mobility for the acetylene

linked homopolymers (TTA) possibly because their ionization potentials were too large.

Furthermore, amongst the acetylene-linked copolymers, the C16 copolymer with

benzothiadiazole (BT) displayed the highest field effect mobility of 1.0 x 10-3

cm2 V

-1s

-1 after

thermal treatment at 150 oC. Somewhat surprising, the C16 DPP copolymer displayed a

comparatively lower mobility (2.25 x 10-4

cm2 V

-1s

-1) despite the high thin-film crystallinity.

Disappointingly, no field effect was found in the other copolymers.

Page | 289

In OPV devices, the C16-bearing vinylene-bridged homopolymer (PDC16TTV)

displayed an unoptimized PCE of 2.65 %, compared to 1.85 % for the branched counterpart

(PBEHTTV). In the case of the acetylene-linked homopolymers, the PCEs were substantially

lower (C16 - 0.26 % and EH - 0.43 %). Therefore, it became apparent that the morphology

and the nature of the solubilising alkyl substituents both impacted the performance of the

OFET and OPV device. The device performance of the copolymers, are yet to be ascertained.

Nonetheless, the investigation has shed new light on the importance of devising new

conjugated polymer structures for the furtherance of the development of higher efficiency

optoelectronic devices in the future.