Highly Air‐Stable Thieno [3, 2‐b] thiophene‐Thiophene‐Thiazolo [5, 4‐d] thiazole‐Based...

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1890

Highly Air-Stable Thieno[3,2-b]thiophene-Thiophene-Thiazolo[5,4-d]thiazole-BasedPolymers for Light-Emitting Diodes

Sarada P. Mishra, Akshaya K. Palai, Amit Kumar, Ritu Srivastava,Modeeparampil N. Kamalasanan, Manoranjan Patri*

A series of highly air-stable, low-bandgap poly(3-alkylthiophene)s containing electron-richthieno[3,2-b]thiophene and electron-deficient thiazolo[5,4-d]thiazole rings were synthesizedby the Stille coupling reaction. The polymers exhibited good thermal stability and solubilitywith excellent film forming properties when drop- or spin-cast from solution. A strongabsorption at 564–568nm and a shoulder at614–616nm were observed. The optical bandgapof the polymers was found to be 1.82–1.85 eV. TheIP of the polymers was found to be 5.62–5.65 eV.All polymers showed strong fluorescent emissionboth in solution and in the solid state. EL deviceswere fabricated using the polymers as an emis-sive layer and red emissionwas observedwith theemission range of 649–679nm.

S

S S

SSn

SnS

NS

N

S

SBr

Br

R

R

R

R

Stille Coupling

S

S S

S S

NS

N

S

S

R

R

R

R

n

R = C12H25, C14H29, C16H33

Introduction

Organic light emitting diodes (OLEDs) are of great interest

becauseof theiruse in largeareaflatpaneldisplayandother

optoelectronic devices.[1] Since the discovery of OLEDs by

Tang et al.,[2] there has been considerable progress on the

designanddevelopment of various kinds ofnovel polymers

and small molecules,[3] in addition to modifications to the

fabrication techniques.[4] Thisnewclass of plasticmaterials

provides great advantages, as it can be processed by a

simple solution deposition technique.[5] While the modifica-

tions in the fabrication technique increase the lifetime and

S. P. Mishra, A. K. Palai, M. PatriNaval Materials Research Laboratory, Shil-Badlapur Road,Ambernath 421506, IndiaE-mail: [email protected]. Kumar, R. Srivastava, M. N. KamalasananNational Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi110012, India

Macromol. Chem. Phys. 2010, 211, 1890–1899

� 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

efficiency of an OLED device,[4a,6] the development of new

molecules and polymers helps us to obtain all possible color

emissions with increased lifetime and stability.[7] A large

number of conjugated polymers, such as poly(p-pheny-

lene)s,[8] poly(p-phenylenevinylene)s,[9] polyfluorenes,[10]

polycarbazoles,[11] polythiophenes[12] and fused-ring-

containing polymers,[13] have been synthesized in order to

achieve all possible color emissions. Amongst these, fused

ring polythiophenes are known for showing interesting and

unique chemical and physical properties.[14] The better

electronic properties of polythiophenes are attributed to

the more rigid and planar structure of the fused ring in

the backbone, which in turn leads to improvements in the

effective conjugation.[15] Furthermore, it also helps prevent

chain folding and lowers the bandgap.[16] In addition to this,

the fused ring in polymeric backbone also lowers the

reorganization energy of the polymer and facilitates inter-

molecular hopping and charge carriermobility.[17] Therefore,

when used as an active layer in light-emitting organic field-

effect transistors (LEOFET),[18] these types of polymers are

expected to give promising results.

DOI: 10.1002/macp.201000132

Highly Air-Stable Thieno[3,2-b]thiophene-Thiophene-Thiazolo[5,4-d]thiazole-Based . . .

Among the fused rings, thieno[3,2-b]thiophene is exten-

sively used for fabricating polymer based organic thin film

transistors.[13,19] However, the use of thieno[3,2-b]thio-

phene based polymers as an emissive layer in electro-

luminescent (EL) devices has not been fully explored.[20]

This is because thieno[3,2-b]thiophene-based polymers are

predominately hole-transportingmaterials possessing low

electronaffinities. Therefore,whenthesepolymersareused

in a device structure, it creates an imbalance of electron

injection (from the low work function cathode) and hole

injection (from the highwork function anode). This leads to

charge recombination near the polymer/cathode interface

and a lowering of EL efficiency due to quenching of excitons

by the metal electrode.[21] To achieve high quantum

efficiency in light-emittingdiodes (LEDs)usingthepolymer,

it is also necessary to have balanced charge injection and

transport of both holes and electrons in the emissive

materials.[22] To improve the electron transport property, a

common strategy is to incorporate ann-typemonomer into

the polymeric backbone.[23] Among various n-type mono-

mers, thiazolo[5,4-d]thiazole is an attractive heterocyclic

compound having a strong electron affinity.[24] Due to its

unique characteristics and stability, oligomers and poly-

mers of thiazolo[5,4-d]thiazole and related compounds

have recently found extensive applications in organic thin-

film transistors (OTFTs).[25] Surprisingly, the use of thia-

zolo[5,4- d]thiazole polymers in polymer light emitting

diodes is very limited.[26]

In this regard, we have undertaken the synthesis and

detailed characterization of a series of novel low bandgap

copolymers derived fromvarious alkyl thiophenes contain-

ing thieno[3,2-b]thiophene and thiazolo[5,4-d]thiazole

units alternatively in the main chain. It was observed that

the combination of thieno[3,2-b]thiophene and thia-

zolo[5,4-d]thiazole units in the polythiophene backbone

not only leads to low bandgap polymers, but also increases

theair stability of thepolymerby lowering theHOMO level.

Their thermal, electrochemical and optical properties have

beenalso studied. Theyhavebeenusedasanactive layer for

fabricating polymer LED devices. Because of their high air

stability, the devices were fabricated in air without any

additional precautions.

Experimental Part

General

Gel permeation chromatography (GPC) of the polymers was

performed on a Waters 2690 separation module apparatus and a

Waters 2487 dual l absorbance detector, with chloroform or

chlorobenzene as the eluent and polystyrene as standards.Melting

points of the monomers were recorded in an Electrothermal 9100

(S. d. Fine, India) instrument and were reported uncorrected.

Elemental analysis was performed in a Perkin-Elmer 2400 series II



instrument. 1H NMR spectra were recorded on a Bruker Avance

300MHz spectrometer. Thermogravimetric analysis (TGA) was

carried out in a Hi-Res TGA 2950 from TA instruments with a

heating rate of 10 8C �min�1 in an N2 atmosphere. UV-vis/near

infrared (NIR) spectra of all the polymerswere recordedon aVarian

Cary 5000 spectrometer in chloroform solution and polymer thin

films cast onto 2� 2 cm2 cover glass slide. PL spectrawere recorded

onaFluorolog Jobinspectrophotometer (Yvon-Horiba,Model-3-11).

Cyclic voltammetrywascarriedout onAutolabPGSTAT100usinga

three-electrode cell. EL spectra were recorded using an HR 2000

Ocean Optics Spectrometer, having a CCD array and fiber optic

probe. The current/voltage/luminescence characteristics were

recorded using the Keithley 617 electrometer and the luminance

meter (LMT L1009).

EL Device Fabrication

For the device fabrication, the ITO coated glass substrate was

patterned with a photolithography technique. Later, the surface

was cleaned using a soap solution followed by the vapor of boiling

trichloroethylene and isopropyl alcohol, and finally dried under a

vacuum. The double layer LED with a configuration of indium/tin

oxide (ITO)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfo-

nate) (PEDOT:PSS) /emitting polymer/2,9-dimethyl-4,7-diphenyl-

1,10-phenanthroline (BCP)/Alq3/LiF/Al was then fabricated, as

reported previously.[27] The concentration of the polymer solution

taken for the spin castingwas 5mg �mL�1 in chlorobenzenewhich

was filtered through a 0.5mm filter (Millipore Co.). All the

fabrications, as well as the measurements, were carried out in

the ambient atmosphere.

Synthesis of Monomers

Compounds 2,5-bis(3-alkylthiophen-2-yl)thiazolo[5,4-d]thiazole

(12a,b) and 2,5-bis(5-bromo-3-alkylthiophen-2-yl)thiazolo[5,4-

d]thiazole (13a,b) were synthesized following a literature proce-

dure.[28]

General Procedure for the Synthesis of

2,5-Bis(3-alkyl-2-yl)thieno[3,2-b]thiophenes (6)

A mixture of 2,5-dibromothieno[3,2-b]thiophene (5) (3 g,

10.06mmol), 4,4,5,5-tetramethyl-2-(3-alkylthiophen-2-yl)-1,3,2-

dioxaborolane (25.16mmol) and tetrakis(triphenylphosphine)pal-

ladium (0.40mmol) in toluene (50mL) was degassed by bubbling

nitrogen for 15min. To the mixture, a degassed aqueous solution

of K2CO3 (50mL of a 30% solution) was added. The mixture was

allowed to reflux under nitrogen for 48h. The resulting yellowish

brown solution was extracted with chloroform. The combined

organic layer was then washed with water, dried over Na2SO4

(anhydrous) and evaporated under reduced pressure. The crude

materials were then purified by silica gel column chromatography

using a hexane and chloroform mixture (95:5) as the eluent to

obtain the desired product as a yellow solid.

2,5-Bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene (6a)

Yield¼ 73%, melting point (m.p.)¼ 68 8C. 1H NMR (CDCl3): d¼0.87 (t,

J¼ 6.3Hz, 6H), 1.24–1.32 (m, 36H), 1.65 (quin, J¼ 7.5Hz, 4H), 2.78 (t,

J¼ 7.5Hz,4H),6.94(d, J¼5.4Hz,2H),7.20(d, J¼5.4Hz,2H),7.22(s,2H).

www.mcp-journal.de 1891

S. P. Mishra, A. K. Palai, A. Kumar, R. Srivastava, M. N. Kamalasanan, M. Patri

1892

13C NMR (CDCl3): d¼14.15, 22.73, 29.27, 29.40, 29.53, 29.72, 30.83,

31.97, 118.03, 124.31, 130.05, 130.78, 137.67, 139.17, 140.26.

(C38H56S4):Calcd.C71.19,H8.80,S20.01;FoundC71.02,H8.03,S19.61.

2,5-Bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene (6b)

Yield¼85%,m.p.¼64 8C. 1HNMR (CDCl3): d¼0.87 (t, J¼ 6.6Hz, 6H),

1.25–1.32 (m, 44H), 1.65 (quin, J¼7.5Hz, 4H), 2.78 (t, J¼ 7.5Hz, 4H),

6.95 (d, J¼ 5.1Hz, 2H), 7.20 (d, J¼5.1Hz, 2H), 7.22 (s, 2H). 13C NMR

(CDCl3): d¼14.17, 22.75, 29.30, 29.42, 29.55, 29.72, 30.43, 30.57,

30.85, 31.99, 118.03, 124.32, 130.06, 130.81, 137.69, 139.19, 140.25.

(C42H64S4): Calcd. C 72.35, H 9.25, S 18.40; Found C 72.13, H 9.05, S

17.89.

2,5-Bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene (6c)



6.94 (d, J¼ 5.1Hz, 2H), 7.20 (d, J¼5.1Hz, 2H), 7.22 (s, 2H). 13C NMR

(CDCl3, 75MHz): d¼14.14, 22.72, 29.26, 29.40, 29.52, 29.73, 30.83,

31.96, 118.03, 124.31, 130.04, 130.78, 137.66, 139.17, 140.26.

(C46H72S4): Calcd. C 73.34, H 9.63, S 17.03; Found C 73.11, H 9.02,

S 16.90.

General Procedure for the Synthesis of 2,5-Bis(5-

bromo-3-alkyl-2-yl)thieno[3,2-b]thiophenes (7)

A solution of 2,5-bis(3-alkyl-2-yl)thieno[3,2-b]thiophene (3.0mmol)

in tetrahydrofuran (THF, 30mL) was cooled to 0 8C. To the cold

solution,NBS (1.12 g, 6.3mmol)was added inportions. Themixture

was stirred at room temperature overnight. The solvent was

evaporated under reduced pressure. The compound was then

purified in a silica gel column using a hexane and chloroform

mixture (95:5) to obtain a dark yellow solid.

2,5-Bis(5-bromo-3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene (7a)



6.90 (s, 2H), 7.16 (s, 2H). 13C NMR (CDCl3): d¼14.14, 22.72, 29.20,

29.40, 29.44, 29.58, 29.70, 30.65, 31.96, 111.22, 118.34, 132.08,

132.69,136.60,139.32,140.98. (C38H54Br2S4):Calcd.C57.13,H6.81, S

16.05; Found C 56.91, H 6.24, S 15.71.

2,5-Bis(5-bromo-3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene (7b)



6.90 (s, 2H), 7.16 (s, 2H). 13C NMR (CDCl3): d¼14.15, 22.73, 29.21,

29.40, 29.46, 29.60, 29.69, 30.66, 31.96, 111.22, 118.33, 132.10,

132.70,136.61,139.32,140.97. (C42H62Br2S4):Calcd.C59.00,H7.31, S

15.00; Found C 58.86, H 7.06, S 14.67.

2,5-Bis(5-bromo-3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene (7c)



6.90 (s, 2H), 7.16 (s, 2H). 13C NMR (CDCl3): d¼14.13, 22.71, 29.19,

29.39, 29.43, 29.57, 29.72, 30.65, 31.95, 111.22, 118.36, 132.07,



132.69,136.59,139.32,140.99. (C46H70Br2S4):Calcd.C60.64,H7.74, S

14.08; Found C 60.35, H 7.37, S 13.76.

General Procedure for the Synthesis of

2,5-Bis[3-dodecyl-5-(trimethylstannyl)thiophen-2-

yl]thieno[3,2-b]thiophenes (8)

A solution of 2,5-bis(5-bromo-3-alkyl-2-yl)thieno[3,2-b]thiophene

(1.25mmol) in THF (80mL) was cooled to �78 8C under a nitrogen

atmosphere. To the cold solution, BuLi (1.7mL of 1.6M solution in

hexane) was added dropwise and the resulting dark solution was

stirred at the same temperature for an additional 1.5 h. Me3SnCl

(0.57 g, 2.87mmol) in THF (5mL) was added drop wise to the

reactionmixture and the cooling bathwas removed. The resulting

mixture was allowed to stir at room temperature overnight. The

solvent was evaporated and the crude materials were dissolved in

chloroform. The organic layer was washed several times with

water, dried over Na2SO4 (anhydrous) and evaporated in vacuo to

get a brown sticky liquid. Hot methanol (10mL) was added to the

compound and it was kept in a refrigerator for 1 h. The methanol

layer was decanted and the sticky compound was dried under a

high vacuum to obtain the compound as a yellow brown low-

melting solid.

2,5-Bis[3-dodecyl-5-(trimethylstannyl)thiophen-2-yl]thieno[3,2-b]thiophene (8a)

Yield¼71%. 1HNMR (CDCl3): d¼0.37 (s, 18H), 0.86 (t, J¼ 6.4Hz, 6H),


7.02 (s, 2H), 7.20 (s, 2H). 13C NMR (CDCl3): d¼ 8.18, 14.15, 22.73,

29.21, 29.40, 29.51, 29.73, 30.83, 30.98, 31.97, 117.48, 136.57, 137.17,

137.81, 138.37, 139.16, 141.30.

2,5-Bis[3-tetradecyl-5-(trimethylstannyl)thiophen-2-yl]thieno[3,2-b]thiophene (8b)



7.0 (s, 2H), 7.21 (s, 2H). 13CNMR (CDCl3): d¼ 8.18, 14.16, 22.73, 29.21,

29.40, 29.52, 29.71, 30.84, 30.98, 31.97, 117.48, 136.58, 137.18,

137.81, 138.38, 139.17, 141.30.

2,5-Bis[3-hexadecyl-5-(trimethylstannyl)thiophen-2-yl]thieno[3,2-b]thiophene (8c)



7.01 (s, 2H), 7.20 (s, 2H). 13C NMR (CDCl3): d¼ 8.18, 14.16, 22.74,

29.22, 29.41, 29.53, 29.75, 30.84, 30.99, 31.97, 117.48, 136.59, 137.17,

137.82, 138.37, 139.17, 141.29.

General Procedure for the Synthesis of 2,5-Bis(3-

alkylthiophen-2-yl)thiazolo[5,4-d]thiazoles (12)

2,5-Bis(3-alkylthiophen-2-yl)thiazolo[5,4-d]thiazoles (12) were

synthesized following a modified literature procedure and the

spectroscopic data for compound 12a and 12bwere well-matched

with the literature.[25c,28] In this procedure, a mixture of 3-

alkylthiophene-2-carbaldehyde (11) (35.7mmol) and dithiooxa-

mide (2.74 g, 21.42mol) in 1mL phenol was stirred at 200 8Covernight. The water formed during the reaction was removed

DOI: 10.1002/macp.201000132


using a Dean-Stark apparatus. The reaction mixture was cooled to

roomtemperatureandadded to coldmethanol. Theprecipitatewas

collected and purified by silica gel column chromatography using

a mixture of hexane and chloroform (3:1) to obtain the pure

compound as a yellow solid.

2,5-Bis(3-hexadecylthiophen-2-yl)thiazolo[5,4-d]thiazole(12c)



7.02 (d, J¼5.2Hz, 2H), 7.36 (d, J¼5.2Hz, 2H). 13C NMR (CDCl3):

d¼ 14.15, 22.73, 29.54, 29.63, 29.77, 30.06, 30.11, 30.42, 31.94,

127.31, 130.79, 131.78, 143.11, 150.05, 161.62. (C44H70N2S4): Calcd. C

69.97, H 9.34, N 3.71, S 16.98; Found C 69.78, H 9.22, N 3.93, S 16.45.

Synthesis of 2,5-Bis(5-bromo-3-alkylthiophen-2-

yl)thiazolo[5,4-d]thiazoles (13)

2,5-Bis(5-bromo-3-alkylthiophen-2-yl)thiazolo[5,4-d]thiazoles (13)

were synthesized following the literature procedure and the

spectroscopic data for compound 13a and 13bwere well-matched

with the literature.[25c,28]

2,5-Bis(5-bromo-3-hexadecylthiophen-2-yl)thiazolo[5,4-d]thiazole (13c)


1.24–1.31 (m, 52H), 1.68 (quin, J¼ 7.8Hz, 4H), 2.90 (t, J¼7.8Hz, 4H)

6.98 (s, 2H). 13C NMR (CDCl3): d¼14.11, 22.71, 29.33, 29.42, 29.61,

29.73, 29.84, 30.22, 31.89, 115.39, 133.44, 143.42, 150.08, 160.29.

(C44H68N2Br2S4): C 57.88, H 7.51, N 3.07, S 14.05; Found C 57.47, H

6.99, N 3.35, S 13.53.

A Representative Example of the Polymerization

An equimolar amount of 2,5-bis[3-dodecyl-5-(trimethylstan-

nyl)thiophen-2-yl]thieno[3,2-b]thiophene (8a) (200mg, 0.2mmol)

and 2,5-bis(5-bromo-3-dodecylthiophen-2-yl)thiazolo[5,4-d]thia-

S

Br

S

Br

CHO S

S COO

S

S Br

Br S

SS

R

S

SS

S

R

RSn

Sn

i (87%) ii (89%) i

v (94%) vi (85-63%)

viii (78-71% )R = C12H2

C14H2C16H3

i) L DA, T HF, -78 oC, DMF; ii) K2CO3, THF, ethyl-2 mercaptoqunoline, ref lux; v) NBS, DMF, rt; vi) 4,4,5,5-tetramethyl-2-K2CO3, toluene, water, ref lux; vii) NBS, THF, rt; viii) n-BuLi,

1 2

5 6(a-c)

8(a-c)

Scheme 1. Synthesis of thieno[3,2-b]thiophene/alkylthiophene oligom



zole (13a) (165mg, 0.2mmol) were taken with anhydrous

chlorobenzene (8mL). To the solution, tris(dibenzylideneacetone)-

dipalladium (0) (3.66mg, 2 mol-%), tri(o-tolyl)phosphine (4.8mg,

8mol-%)andCuO (15.3mg,0.2mmol)wereadded.Themixturewas

heated to reflux for 72h under a N2 atmosphere. After cooling to

room temperature, the solution was poured into a mixture of

methanol (100mL) and conc. HCl (5mL). The dark precipitateswere

filtered and were Soxhlet-extracted with methanol and hexane to

remove the impurities and the low-molecular-weight polymers.

Finally the chloroform and chlorobenzene soluble fractions of the

polymers were isolated by Soxhlet extraction with the respective

solvents followed by evaporation of the solvent.

Results and Discussion

Synthesis and Characterization

2,5-bis(3-alkyl-2-yl)thieno[3,2-b]thiophene monomers were

synthesized from dibrominated thieno[3,2-b]thiophene (5),

which in turn was synthesized from 3-bromothiophene

adopting a previously reported procedure (Scheme 1).[29] A

typical Suzuki coupling reaction between dibrominated

thieno[3,2-b]thiophene (5) and 4,4,5,5-tetramethyl-2-(3-

alkylthiophen-2-yl)-1,3,2-dioxaborolanes was performed to

afford compounds6a–cwithhighyields. Several attempts to

make thedistannyl compounds8a–c fromtheoligomers6a–

c using BuLi and Me3SnCl were unsuccessful because of the

formation of large amount of monostannyl products which

were difficult to separate from the desired products.

Therefore, an alternate route was followed in which a

dibromination reaction of 6a–c in the presence of NBS and

THFyielded 7a–c asdarkyellow solids after purification. The

distannyl compounds were then prepared from the dibro-

minated compounds 7a–c by treatment of BuLi at �78 8C,followedbytrimethyltinchloride.Distannyl compounds8a–

c were found very sensitive while purifying by column

Et

S

S COOH

S

S

S

R

S

SS

S

R

RBr

Br

)%19(vi)%18(ii

vii (94-91%)

5 (a)9 (b)3 (c)

acetate, rt; iii) NaOH, MeOH, THF, rt; iv) 2CuO.Cr2O3,(3-alkylthiophen-2-yl)-1,3,2-dioxaborolane, Pd(PPh3)4,-78 oC, Me3SnCl

43

7(a-c)

ers.



S

R

i (92-96%)

S

R

CHOiii (25-32%)

S

R

N

S

S

N S

R

S

R

N

S

S

N S

R

BrBr

iv (91-95%)

9(a-c)S

R

Brii (88-79%)

10(a-c) 11(a-c) 12(a-c)

13(a-c)

i) NBS, THF, rt; ii) n-BuLi, THF, -78 oC, DMF; iii) Dithioxamide, p -TSA, 150 oC; iv) NBS, chlorof orm, ref lux

R = C12H25 (a)C14H29 (b )C16H33 (c)

Scheme 2. Synthesis of thiazolo[5,4-d]thiazole/alkylthiophene oligomers.

1894

chromatography and thus were used without further

purification.

The thiazolo[5,4-d]thiazole oligomers 12a–c could easily

be synthesized from the 3-alkylthiophene-2-carbaldehyde

following the reported one step reaction of the correspond-

ing aldehydes with dithiooxamide.[28] Thiazolo[5,4-d]thia-

zole oligomers 12a–c were then dibrominated in the

presence ofN-bromosuccinimide (NBS) in refluxing chloro-

form to obtain 13a–cwhich were used as one precursor for

the polymerization reactions (Scheme 2).

Different thieno[3,2-b]thiophene 8a–c and thia-

zolo[5,4-d]thiazole monomers 13a–c were then copoly-

merized by Stille cross coupling in the presence of CuO

to obtain the polymers with a high molecular weight

(Scheme3).[30] The reaction conditionof the Stille coupling

was optimized to obtain highmolecularweight polymers.

It was found that coupling in the presence of Pd2(dba)3as the catalyst, (o-tol)3P as the base and chlorobenzene

as a solvent with 1 equivalent of CuO was found to

produce a polymer of high molecular weight. The

polymerization yields and the molecular weights of

the copolymers are summarized in Table 1. Initially the

polymerizations were carried out without CuO and the

majority of the polymers were found to be soluble in

chloroform. The molecular weights of the chloroform

soluble polymers were found to be (by GPC analysis)

+S

R

Br

S

R

8(a-c)

i) Pd2(dba)3, (o-tol)3P, CuO, Chlorobenzene, ref lux

S

RS

S

S

R

SnSn

Scheme 3. Polymerization of thieno[3,2-b]thiophene and thiazolo[5,4



between 8 000–11 000. However, in the presence of CuO,

only a small amount (5–10%) of the polymers were found

to be soluble in chloroform, and the rest of the polymers

were found to be soluble in chlorobenzene and dichlor-

obenzene. Themolecularweight of the chloroformsoluble

polymers obtained from both the methods were found

to be similar. However, the molecular weights of

chlorobenzene-soluble polymers were found to be in

the range 15 000–16 500.

Thermal Properties

The thermal stability of the polymers is an important

parameter for their application in optoelectronic devices.

The thermal properties of the polymers were investigated

by thermogravimetric analysis (TGA). The degradation

pattern was observed to be similar for all polymers due to

their similar polymeric structure. TGA analysis indicated

that the synthesized polymers possess an excellent

thermal stability up to 251 8C with only 2% weight loss

(Figure 1). Such high thermal stability of these polymers

could be attributed to the presence of the nitrogen rich

thiazolo[5,4-d]thiazole aromatic ring in the polymeric

backbone. The temperatures for 5% weight loss of the

polymers were observed at 340 8C, 344 8C and 336 8Crespectively,when going from the polymer containing the

N

S

S

N S

R

Br

S

S

S

R

N

S

S

NS

RS

R

13(a-c)

i (72-79%)

PTT-Tz-C12 (R = C12H25)PTT-Tz-C14 (R = C14H29)PTT-Tz-C16 (R = C16H33)

n

-d]thiazole oligomers.

DOI: 10.1002/macp.201000132


Table 1. Yield and molecular weight of the polymers.

Polymer Mna) Mw=Mn Yield

%

Chlorobenzene-soluble fraction Chloroform-soluble fraction

PTT-Tz-C12 15 900 3.1 73 5

PTT-Tz-C14 15 000 3.1 66 6

PTT-Tz-C16 16 500 2.9 65 10

a)Determined by GPC from the chlorobenzene-soluble fractions (with chlorobenzene as eluent and polystyrene as standard).

shortest side-chain PTT-Tz-C12 to the polymer containing

the longest side-chain PTT-Tz-C16. However, the char

yields were similar for all the polymers.

Electrochemistry

To investigate the charge injection and transport in the

polymers, it is very important to determine the energy

levels of the highest occupied molecular orbital (HOMO)

and the lowest unoccupiedmolecular orbital (LUMO) using

cyclic voltammetry experiments. In this experiment, a

platinum electrode was taken as the working electrode at

a scan rateof 100mV � s�1, against anAg/AgCl electrodeasa

reference electrode with a nitrogen-saturated anhydrous

solution of 0.1M tetrabutylammonium tetrafluoroborate

in acetonitrile. The solvent evaporation deposition of the

polymers onto theworking electrodewasperformed froma

dilute chlorobenzene solution.

All the polymers exhibited an irreversible oxidation

process (Figure 2). It is important to note thatwhen the side

chain length of the polymers is shorter, the oxidation peak

0 100 200 300 400 500 600 700 800

0

20

40

60

80

100

Wei

ght (

%)

Temperature (0C)

PTT-Tz-C12PTT-Tz-C14PTT-Tz-C16

Figure 1. TGA thermograms of polymers.



of the polymers becomes sharper. In this regard, the

sharpest oxidation peakwas obtained for the polymer PTT-

Tz-C12, compared to the other polymers. This could be due

to the better p-stacking of the shorter side chain containing

polymer compared to the longer side chains. Because of

better p-stacking of the polymers, the movement of charge

through thepolymeric chains is easier and results ina sharp

oxidationpeak. Theonsets of the oxidationpotential values

are 0.88, 0.75 and0.87V, respectively, for thepolymerswith

asequential increasinginthesidechainlength.Theoxidation

potentialof thepresentsetofpolymerswasfoundtobemuch

higherthanregioregularpoly(3-hexylthiophene) (rr-P3HT)[16]

and the poly(thieno[3,2-b]thiophene)s.[31] The relatively high

oxidationpotentialof thepolymers is due to thepresenceof

the highly electron deficient thiazolothiazole ring.

Because of the higher oxidation potential of the current

set of polymers, they may not require any special inert

conditionswhile fabricatingELdevices.HOMOlevel of the

polymers were calculated from the onset of the oxidation

peak from the CV, as reported in the literature. The

calculated HOMO levels [ionization potentials (IP)] of the

-0.5 0.0 0.5 1.0 1.5 2.0

PTT-Tz-C12

PTT-Tz-C14

Cur

rent

(A)

Potential (V)

PTT-Tz-C16

Figure 2. Cyclic voltammograms of polymers.



Table 2. Optoelectronic properties of polymers.

Polymer lmax lPL lELf) Egg) Eoxh) EHOMO

i) ELUMOj) Tdk)

nm nm nm eV eV eV eV -C

Filma) Filmb) Solutionc) Solutiond) Filme)

PTT-Tz-C12 564, 614 529, 564, 612 571, 618 589, 635 678 649 1.82 0.88 �5.65 �3.83 340

PTT-Tz-C14 569, 618 532, 566, 615 578, 624 589, 632 674 656 1.85 0.85 �5.62 �3.77 344

PTT-Tz-C16 568, 616 532, 566, 614 577, 625 588, 632 671 679 1.83 0.87 �5.64 �3.81 336

a)UV-Vis spectra of films deposited by slow evaporation from solution on top of glass slides; b)UV-Vis spectra of films after annealing at

150 8C for 30min; c)UV-Vis spectra of polymers in CHCl3 solution; d)Photoluminescence emission spectra of the polymers recorded on

CHCl3 solution;e)Photoluminescence emission spectra of the polymer films deposited by slow evaporation from solution on top of glass

slides; f)EL spectra of the polymers recorded from the devices; g)Optical bandgap; h)Vs. Ag/Agþ; i)All potentials are reported vs. Fc/Fcþ based

on the assumption that the redox couple of Fc/Fcþ is 4.8 eV relative to vacuum; j)LUMO levels were calculated from the HOMO levels and

the optical bandgap; k)Temperature at 5% weight loss.

1896

polymers are around 5.6 eV, which is 0.7 and 0.5 eV lower

than that of rr-P3HT and thiazolothiazole-containing

polythiophenes, respectively.[28] The higher IP of these

polymers not only helps them to become stable in the

neutral state in air, but also possibly helps in decreasing

the number of trap states due to oxygen doping.[32] It is

also important to note that, although donor/acceptor-

type polymers normally show a smaller IP[33] since they

possess a smaller bandgap, the described polymers were

found to combine a larger IP with a smaller bandgap.

LUMO levels of the polymers calculated from the

difference betweenHOMO levels and the optical bandgap

are depicted in Table 2.

350 400 450 500 550 600 650 700 7500.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Abs

orba

nce

Wavelength (nm)


Figure 3. UV-Vis spectra of polymer films spin-coated on a glassplate.

Absorption and Emission Studies

The investigations of the absorption and photolumines-

cence properties of the polymers were carried out in

solution aswell as in solid films. The solutions of polymer

sample (typically 2mg of polymer per mL of chloroben-

zene) were drop-cast and evaporated slowly to get the

thinfilm. Theabsorptionmaximaarepresented inTable 2.

The polymers showed two absorption peaks at �564 and

614 nm, respectively, with a weak shoulder at around

630 nm (Figure 3). The splitting of the absorption spectra

is probably due to the vibronic coupling, which is

well known in the literature.[28] In the absorption

spectra, the lowest energy absorption peaks for the

polymers were found to be between the thiazolothiazole/

thiophene copolymers and rr-P3HT.[28] This is attributed

to the enhanced intramolecular charge transfer in the

donor/acceptor-type of polymer backbone, due to

the presence of electron rich thieno[3,2-b]thiophene

and electron poor thiazolo[5,4-d]thiazole rings.

Furthermore, the absorption maxima of the current

set of polymers were also found to be red-shifted



when compared to poly[2,5-bis(3-alkylthiophen-2-

yl)thieno[3,2-b]thiophene].[15]

Interestingly, after annealing the thin film at 150 8C for

30min, a slight blue shift of �2–3nm was observed (see

Table 2). However, the weak shoulder at around 530nm

became slightly more prominent after annealing. The UV-

vis spectra of polymers were also recorded in dilute

chlorobenzene solution. Two absorption maxima of the

polymers in solution were observed, one at 571–577 and

the other at 618–625nm. Surprisingly, the absorption

maxima of these polymers in solution are red-shifted

compared to its filmas cast or annealed. Although the exact

cause of this is not known to us, a similar observation has

been reported for thiazolothiazole polymers.[28] The band-

gaps of the polymers calculated from the onset of the

absorption maxima are depicted in Table 2. As can be seen

DOI: 10.1002/macp.201000132


600 650 700 750 800 850

PL In

tens

ity (a

.u.)

Wavelength (nm)


Figure 4. Photoluminescence spectra of polymer films spin-coated on a glass plate.

400 500 600 700 800 900 1000

0

25

50

75

100

125

150

175

200

225

EL In

tens

ity

Wavelength (nm)


Figure 5. EL spectra of polymeric devices.

150

200

250

300

350

400

Inte

nsity

8V9V10V12V

from Table 2, the polymers exhibited a bandgap of 1.82 to

1.85 eV. This smaller bandgap compared to rr-P3HT (2.0 eV)

is due to the more coplanar structure, as well as the donor-

acceptor effect.

The emission spectra of the polymers were measured in

solution aswell as in solid films and are depicted in Table 2.

Photoluminescence data were obtained upon excitation at

the wavelength of the absorption maxima. In solution, the

polymers exhibited a very similar red emissionwith a lmax

of 588 to 589nm along with a shoulder at 635nm. The

shoulder in the emission spectra indicates a better resolved

vibronic structure, indicating a higher order polymeric

structure. In the solid state, the polymers exhibited

fluorescence emission at lmax¼ 671 to 678nm, as shown

in Figure 4. With increasing the alkyl side chain length, a

bathochromic shift was observed in the emission spectra.

The red shifting of the absorption and emission spectra of

the polymers could be due to the better packing of the

polymers when increasing the length of the side-chain.[27]

The photoluminescence quantum yield of the polymers

wasmeasured followinga reportedprocedure.[34]Moderate

quantum yields of 22, 27 and 29% were obtained,

respectively, for the polymers with increasing side chain

length.

400 500 600 700 800 900 10000

50

100

EL

Wavelength (nm)

Figure 6. EL spectra of the ITO/PEDOT:PSS/ PTT-Tz-C14/BCP/Alq3 /LiF/Al device at varying forward applied bias voltages.

Electroluminescent Properties

To investigate the luminescence properties of the

polymers and the color stability under electrical

bias, solution processed electroluminescent devices of

the polymers were fabricated using the polymers as

the emissive layer. The configuration of the LED devices

was glass:ITO/PEDOT:PSS/emitting polymer/BCP/Alq3/



LiF/Al. Figure 5 displays the EL spectra of the devices.

The observed EL maxima are noted in Table 2. The

maximum emission peaks of the polymers were

observed at 649, 656 and 679 nm, respectively, with

increasing side chain length (Figure 5). All the devices

showed a bright red emission when appropriate positive

bias was applied. As the applied voltage increased,

the EL intensity of the polymer increased. Figure 6 shows

the voltage independent EL spectrum of PTT-Tz-C14.

We believe that this new series of polymers are

promising materials for polymer LEDs as well as for

LEOFET applications.

Figure7andFigure8 showthe current/voltage (I/V) and

voltage/luminance (V/L) characteristics of the polymers,

respectively. The device showed typical diode character-

istic properties. When a forward bias was applied, an

increase of current, as well as luminescent intensity, was



0 2 4 6 8 10 120

100

200

300

400

500

600

700

Cur

rent

Den

sity

(mA

/cm

2 )

Voltage (V)


Figure 7. Current/voltage (I/V) curve of polymeric devices.

0 50 100 150 200 2500.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

Cur

rent

Eff

icie

ncy

(cd/

A)

Brightness (cd/m2)


Figure 9. Current efficiency/brightness characteristics of the stu-died devices.

1898

observed for the polymer devices. As shown in Figure 7,

the turn-on voltage for the polymers was found to be

approximately 4.0 V. The maximum luminances of the

resulting electroactive polymers were found to be 198,

224 and 182 cd �m�2 respectively with gradually

increasing the side-chain length. As shown in Figure 9,

a maximum current efficiency of 0.036, 0.036 and

0.029 cd �A�1 was realized for the polymers PTT-Tz-C12,

PTT-Tz-14 and PTT-Tz-C16, respectively. The poor

device performance could be due to the strong inter-

molecular interactions and aggregate formation of

the polymers, which quench fluorescence in the solid

state. However, we also believe that the luminescence

of these polymers can be improved by material and

device optimization.

0 1 2 3 4 5 6 7 8 9 10 11 12 130

50

100

150

200

250

Lum

ines

cenc

e (c

d/m

2 )

Voltage (V)


Figure 8. Luminescence/voltage (V/L) characteristics of the ITO/PEDOT:PSS/PTT- Tz/BCP/Alq3 /LiF/Al device.



Conclusion

A series of low-bandgap polymers containing alkylthio-

phene and two other fused rings, namely electron-rich

thieno[3,2-b]thiophene and electron-poor thiazolo[5,4-

d]thiazole rings, have been designed and synthesized by

Stille coupling. To obtain high molecular weight polymers,

CuO was used during the polymerization reactions. All

the copolymers were characterized by GPC, TGA, cyclic

voltammetry, UV-vis and fluorescence spectroscopy.

Thermal analysis of the polymers indicated that all the

polymers have good thermal stability. The HOMO levels of

the polymerswere found to be�5.62 to�5.65 eV and had a

bandgap ranging between 1.82 and 1.85 eV. The polymers

were found to be highly fluorescent in nature, both in

solution and the solid state. The polymers were used as the

active layer for fabricating EL devices and a bright red

emission was obtained from themwhen positive bias was

applied. In particular, the physical and electronic properties

of these polymers makes these thieno[3,2-b]thiophene-

alkylthiophene-thiazolo[5,4-d]thiazole copolymerspromis-

ing materials for polymer LEDs and LEOFETs.

Acknowledgements: The authors wish to thank Mr. R. S. Hastak,Director of the NMRL, for his encouragement and the Head ofPolymer Division, Dr. B. C. Chakraborty, for his continuous supportduring this study.

Received: March 15, 2010; Revised: May 28, 2010; Publishedonline: July 27, 2010; DOI: 10.1002/macp.201000132

Keywords: electrochemistry; light-emitting diodes (LED);luminescence; polymerization (general); synthesis

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