Electrochemical studies of porphyrin-appended dendrimers

Electrochemical studies of porphyrin-appended dendrimers

Conor F. Hogan,aAlexander R. Harris,

bAlan M. Bond,*

bJoseph Sly

cand

Maxwell J. Crossley*c

Received 16th November 2005, Accepted 23rd February 2006

First published as an Advance Article on the web 24th March 2006

DOI: 10.1039/b516281e

The electrochemical properties of porphyrin-appended dendrimers containing 2-, 4-, 8-, 16-, 32-

and 64-porphyrin macrocycles in their free-base and zinc(II) forms have been investigated. Both

series gave diffusional based voltammetric responses in dichloromethane. There was minimal

effect of dendrimer generation on the redox potentials. Multiple p-cation and anion radicals as

well as dications and dianions were formed on the surface of the dendrimers on oxidation or

reduction as appropriate, with each cyclic voltammetric wave representing electron transfer to or

from multiple non-interacting porphyrin sites. Electrostatic interactions in the higher generation

dendrimers result in kinetic effects being observed for the highly charged species generated when

each porphyrin unit is doubly or triply oxidised. The number of electrons transferred on

reduction or oxidation of the dendrimers was evaluated using steady-state microelectrode

voltammetry. For the lower generations of species a good correlation was observed between

numbers of electrons transferred and number of porphyrin entities per molecule; for the

dendrimers containing 32 and 64 units, however, slight negative deviations were observed,

possibly due to electrostatic interactions as the porphyrins become closer packed.

Introduction

There has been considerable interest in construction of synthetic

analogues of the light-harvesting and redox-active pigment

arrays involved in photosynthesis. Structural studies of the

natural systems have shown that porphinoid pigments are used

as the photo- and electro-active chromophores.1 Arrays of these

pigments are arranged in well-defined geometry usually on or

near the surface of a 3-D shape. The light-harvesting pigments

are relatively close packed but their number is organism

dependent. In the case of photosystems I and II, the pigments

lie in the outer regions of ellipsoid molecular spaces that

encapsulate the photosynthetic reaction centre.2,3 The photo-

system I complex of the cyanobacterium Synechococcus elonga-

tus is trimeric and has 96 chlorophyll pigments so arranged in

each sub-unit.2 The light-harvesting system of green sulfur

bacteria has only 7 (bacteriochlorophyll) pigments per subunit

but has similar molecular architecture.4,5 Purple photosynthetic

bacteria have light-harvesting units with 27- and 32-porphinoid

pigments arranged on the outer periphery of a toroid.6,7

The redox properties and substantially large extinction

coefficients of systems containing multiple porphyrin units

make them particularly useful models of natural photosyn-

thetic antennas and may also underpin developments in

molecular photonic devices such as sensors, nanoscale optical

sources and solar collectors. The stability of the p-cation

radicals and p-anion radicals that are formed upon oxidation

or reduction of multiple porphyrin arrays provide the basis for

nanoscale hole/electron storage reservoirs.

The unique hyperbranched polymeric structure and well de-

fined 3-D architecture of dendrimers affords precise control of

molecule size,8 as well as the positioning of desired functionalities

at the core,9 within the dendric branches10 or on the exterior

surfaces.11 In addition, each branch in the dendrimer structure

allows the attachment of a greater number of external substitu-

ents. In an attempt to mimic the properties of light-harvesting

arrays, we constructed oligoporphyrin systems based on dendri-

mers, the porphyrins being appended to the outer surface of the

dendrimers thereby mimicking an important feature of the

natural systems. We have prepared poly(propylene) imine den-

drimers with 4-, 8- 16-, 32-, and 64-porphyrins attached to the

perimeter, thereby approximating the number of pigments from

the smaller to the larger natural light-harvesting systems.

In previous work we have thoroughly investigated the

dynamics of energy transfer using time-resolved fluorescence

anisotropy, in the first, third and fifth generation of the

dendrimers functionalised with free-base porphyrins,12 and

the first and second generation compounds have been used

in supramolecular photovoltaic cells.13–15 We have also studied

energy transfer and conformational dynamics,16 and singlet–

singlet annihilation kinetics17 in the corresponding zinc(II)

porphyrin dendrimers. In this study we have systematically

evaluated the electrochemistry of free-base (2-H2–7-H2) and

metalated (2-Zn–7-Zn) porphyrin functionalized dendrimers

containing 2-, 4-, 8-, 16-, 32- and 64-porphyrin functionalities

and compared these properties with those of the corresponding

monoporphyrin model species 1-H2 and 1-Zn, respectively,

which both have a single branch of the dendrimer as a

substituent. Electrochemical and photophysical properties of

aDepartment of Chemistry, La Trobe University, Bundoora, Victoria,3086, Australia

b School of Chemistry, Monash University, Clayton, Victoria, 3800,Australia. E-mail: [email protected]; Fax: +613 99054597; Tel: +613 9905 1338

c School of Chemistry, The University of Sydney, NSW 2006,Australia. E-mail: [email protected]

2058 | Phys. Chem. Chem. Phys., 2006, 8, 2058–2065 This journal is �c the Owner Societies 2006

PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics

these porphyrin-appended dendrimers are of particular inter-

est because of their relevance to natural systems.

Experimental

Materials

The synthesis of the free-base (2-H2–7-H2) and metalated

(2-Zn–7-Zn) porphyrin-appended dendrimers as well as the

monoporphyrin model compounds, 1-H2 and 1-Zn, will be

described elsewhere.18 Free-base and zinc 5,10,15,20-tetra-

kis(3,5-di-tert-butylphenyl)porphyrin, TDBPP-H2 and

TDBPP-Zn, and the corresponding 5,10,15,20-tetraphenyl-

porphyrins, TPP-H2 and TPP-Zn, were prepared by literature

methods.19

For UV-Vis absorption measurements, spectroscopic grade

dichloromethane (Aldrich) was used. Electrochemical mea-

surements were carried out in AR grade dichloromethane

This journal is �c the Owner Societies 2006 Phys. Chem. Chem. Phys., 2006, 8, 2058–2065 | 2059

(Aldrich). The solvent was stored over molecular sieves prior

to use and basic alumina was used to remove any residual

hydrochloric acid. The electrolyte, [Bu4N][PF6], (GFS) was

recrystallised twice from ethanol/water before use and all

solutions were deoxygenated using solvent saturated nitrogen

prior to the electrochemical experiments. All measurements

were carried out at ambient temperature (20 � 2 1C).

Absorption measurements, to evaluate the concentration of

the dendrimers in the electrochemical solutions, were made

using a Varian Carey 5 spectrophotometer. Large molar

absorptivity values of the dendrimer species necessitated using

a small path length (1 mm) cell and in some cases dilution was

necessary.

Apparatus

Voltammetric measurements were carried out with an Autolab

PGSTAT100 (ECO-Chemie, Utrecht, Netherlands), or a Mac

lab/4e (ADInstruments, NSW, Australia). A Faraday cage

was used which, in addition to isolating the cell from electrical

noise, maintained the solutions in darkened conditions. A

three-electrode assembly was used with a small volume (0.5

cm3) electrochemical cell (Cypress Systems). The working

electrode consisted of a 1 or 1.5 mm diameter glassy carbon

disk shrouded in polyether ethylketone (Cypress Systems) or a

10 mm diameter Au microelectrode shrouded in glass (Cypress

Systems), while a Pt wire served as the auxiliary electrode. The

reference electrode was an Ag wire that was separated from

the test solution by a Vycor frit. All potentials are quoted

relative to the ferrocene/ferrocenium couple (Fc0/+) measured

in situ. The supporting electrolyte was 0.1 M [Bu4N][PF6] for

all measurements. IUPAC conventions are used for presenta-

tion of all voltammograms.

Results and discussion

Free-base porphyrin dendrimers

Fig. 1 shows the cyclic voltammetric responses at a 1 mm

diameter glassy carbon electrode for solutions of free-base

mono-porphyrin 1-H2 and the second generation dendritic

octakis-porphyrin 4-H2, dissolved in dichloromethane con-

taining 0.1 M [Bu4N][PF6] at a scan rate of 1 V s�1. The

concentrations are approximately 1 and 0.1 mM for the two

compounds, respectively. In general, the electrochemical be-

haviour exhibited in Fig. 1 is mirrored in each successive

generation of dendrimer; that is, two reversible reductions

and one reversible oxidation process are observed in each case.

For each compound the voltammetry exhibited in this solvent

is characteristically diffusional, with no significant evidence of

adsorption occurring, even for the higher molecular weight

species.

The first oxidation process in each case shows an Ioxp /Iredp

ratio (Ioxp = oxidation peak current, Iredp = reduction peak

current), greater than unity when scan rates below 1 V s�1 are

used, but approaches unity and hence becomes reversible at

higher scan rates. This chemical irreversibility is due to a

secondary homogeneous reaction of the electrogenerated por-

phyrin cation-radical and gives rise to an unidentified product

that is reduced in the region of �0.3 V vs. Fc0/+. Also, over

time (>4 h), with exposure to ambient light, photolysis of the

dendrimers occurs, which is signalled by a colour change from


red to green, giving rise to a new quasi-reversible redox process

centred at�0.98 V vs. Fc0/+. For some of the lower generation

species, additional irreversible oxidation peaks could also be

discerned close to the edge of the solvent limit.

The formal potential (E0f ) data for the mono- and bis-

porphyrin species and five generations of porphyrin-appended

dendrimers are compared with the corresponding data for

model compounds, TPP-H2 and TDBPP-H2 in Table 1 and

displayed graphically in Fig. 2. E0f values have been calculated

as the average of the oxidation (Eoxp ) and reduction (Ered

p ) peak

potentials obtained at a scan rate of 1 V s�1. As expected, the

TDBPP-H2 oxidation is more facile (and reduction more

difficult) than the un-substituted TPP-H2 (by 20–40 mV),

due to the electron-donating effect of the tert-butyl groups.

As seen in Table 1 and Fig. 2, compounds 1-H2 to 7-H2 show

no significant variation in oxidation and reduction potentials

as the number of porphyrin units is increased. Moreover, the

potentials are the same within experimental error as those of

the model species TDBPP-H2, the amide linker group see-

mingly having little effect on the electron density at the

porphyrin macrocycles.

The uniformity of the redox potentials with dendrimer gen-

eration is in contrast with results reported for dendrimers with

electroactive groups located within their cores, where a strong

correlation between redox potential and degree of dendric

branching is found.20 This implies that the porphyrin entities in

the compounds investigated in this study experience similar

micro-environments regardless of dendrimer generation, as ex-

pected, given their peripheral location. The constancy in the

redox potentials is mirrored in the spectroscopic data presented

in Table 1, i.e., the position of the intense Soret band at 425 nm

does not change with generation, though the intensity of the

bands increases proportionally with increasing numbers of por-

phyrins. The positions of the Q-bands at 488, 521, 555, 593 and

649 nm are similarly invariant with increasing number of por-

phyrin units and show the same trend with respect to intensity.

An interesting feature of the data presented in Fig. 2 is the

constancy, for all dendrimers and model porphyrin com-

pounds, of the difference in potential between the first oxida-

tion and first reduction processes (2.20 � 0.03 V). The so-

called electrochemical HOMO–LUMO gap (DH�L) has often

Fig. 1 Cyclic volammograms of the monoporphyrin, 1-H2 and the

dendrimer containing eight free-base porphyrins, 4-H2 dissolved in

CH2Cl2 (0.1 M [Bu4N][PF6]) at a 1 mm diameter glassy carbon

electrode. The scan rate is 1 V s�1 in both cases.

Table 1 Electrochemical and spectroscopic data for free-base dendrimers and other compounds. Values in italics represent peak potential (Eoxp )

data for irreversible processes

p E0f /V (vs. Fc0/+) DH�L/V lSoret/nm Log (eSoret)/M

�1 cm�1

TPP-H2 1 �1.98 �1.67 0.52 0.82 — 2.19 419 5.69TDBPP-H2 1 �2.02 �1.69 0.48 0.78 — 2.17 422 5.661-H2 1 �2.03 �1.70 0.48 0.76 1.10 2.21 425 5.552-H2 2 �2.05 �1.74 0.45 0.86 1.10 2.19 425 5.803-H2 4 �2.10 �1.78 0.41 0.72 1.10 2.19 425 6.114-H2 8 �2.13 �1.77 0.44 0.76 — 2.21 425 6.385-H2 16 �2.19 �1.79 0.44 0.76 — 2.23 424.5 6.756-H2 32 �2.13 �1.80 0.41 — — 2.21 424.5 6.957-H2 64 �2.11 �1.78 0.43 — — 2.20 424 7.3

Fig. 2 Redox potential data for the free-base dendrimers and model

compounds. Open circles represent peak potential (Ep,ox) data for

irreversible processes. The dashed lines represent the limits for solvent

decomposition.


been used as a diagnostic criterion for assigning the site of

electron transfer to the metal or the macrocycle. Its value has

been given as 2.25 � 0.15 V for reactions at the conjugated

p-ring system of metalloporphyrins containing tetraphenyl-

porphyrin (TPP-M) macrocycles.21 It is also noteworthy that

the difference between E1

f’s for the first and second reductions

is also constant (Dox1,ox2 = 0.34 � 0.03 V). This constancy is

also frequently taken as evidence for ring-centred electron

transfer reactions.22

As is known to be the case for 5,10,15,20-tetraphenylpor-

phyrin23 (TPP-H2), stepwise electrooxidation of the mono-

porphyrins TDBPP-H2, and 1-H2 is presumed to yield p-cation radicals and p-dications while the stepwise reduction

yields p-anion radicals and p-dianions according to eqn (1).

P2þÐe�

P�þÐe�

PÐe�

P��Ðe�

P2� ð1Þ

The third irreversible oxidation process observed for free-

base compounds 1-H2, 2-H2 and 3-H2 in the vicinity of the

anodic solvent limit is assigned to the amide linker groups as it

is absent in the voltammetric responses of TPP-H2 and

TDBPP-H2.

The almost identical redox behaviour observed for all free-

base multi-porphyrin species and mono-porphyrin model spe-

cies as well as the insensitivity of the absorption maxima to the

number of porphyrins per molecule, indicates a lack of

electronic communication between individual sites.24 There-

fore, the waves observed for compounds 2-H2 to 7-H2 must

represent the simultaneous oxidation and reduction of multi-

ple non-interacting units. If it is assumed that each wave

results from the oxidation/reduction of all the porphyrin units

in each species, i.e., that n= p, then their redox behaviour can

be represented as follows in eqn (2).

P2nþn Ð

ne�

Pnð�þÞn Ð

ne�

PnÐne�

Pnð��Þn Ð

ne�

P2n�n ð2Þ

In order to test this, the number of electrons transferred (n) in

the first reduction wave for several of the compounds was

determined by measuring the steady state voltammetric re-

sponse at a 10 mm diameter gold microelectrode and using the

formula:

Ilim = 4nFrDc (3)

where Ilim is the limiting current, r the electrode radius, D the

diffusion coefficient and c the concentration of electroactive

species. The concentration c was determined from absorption

measurements at the wavelength of the Soret band or one of

the Q-bands, while D was evaluated using the Stokes–Einstein

relationship:

D ¼ kT

6prZð4Þ

where k is the Boltzmann constant, T is the temperature, r is

the radius of the molecule and Z is the viscosity of the solvent.

Because of the bulky porphyrin end-groups, back-folding in

these molecules is not possible, therefore, the dendrimers are

assumed to adopt either a disc or spherical shape depending on

generation12 and r has been estimated by taking the sum of the

radius of the porphyrin molecule and the radius of gyration of

the dendrimers determined from a molecular dynamics

study.25 The values of D calculated in this way were found

to vary from 5.6 � 10�6 to 2.3 � 10�6 cm2 s�1 on going from

compound 1-H2 to 7-H2.

The values of n determined for each of the compounds,

1-H2, 2-H2, 3-H2, 4-H2, and 6-H2 using eqn (3) above were 1 �0.3, 2 � 0.4, 5 � 2.1, 8 � 4.4 and 27 � 6.6, respectively. The

values for the other two compounds in the series were not

determined due to the restricted amounts of these samples

available. The results for the four compounds investigated are

plotted in Fig. 3 versus the number of porphyrin units (p) on

the dendrimer in each case. The straight line in Fig. 3

represents the situation where n = p, i.e., each porphyrin unit

receives one electron from the electrode. The correlation is

good for compounds 1-H2, 2-H2, 3-H2 and 4-H2 but there is

some deviation for the species containing 32 porphyrin units,

6-H2 in which the porphyrin units are not exhaustively elec-

trolysed for a given species. It is noteworthy that absorption

spectra suggest that the porphyrin sites of these dendrimers are

fully populated because while the absorptions are broadened

as expected for the two higher generation compounds, the

relative area per porphyrin of the absorptions remains con-

stant. The molar extinction coefficients for these compounds

are presented in Table 1. A time-resolved anisotropy study

recently conducted on these dendrimers revealed the existence

of structural defects in the higher generation species,12 with

several units confined to the internal cavity of the dendrimer.

This could result in a smaller number of electrons being

exchanged with the electrode due to strong steric hindrance

to electron transfer as a result of some porphyrins being

encapsulated and effectively isolated from the electrode surface.

Zinc porphyrin dendrimers

The electrochemical behaviour of the metalated porphyrin

dendrimers is illustrated in Fig. 4 which shows the

Fig. 3 Numbers of electrons transferred (n) in the first reduction

process of the free-base mono- and multiporphyrin species (1-H2,

2-H2, 3-H2, 4-H2 and 6-H2) versus the number of porphyrin units

per molecule (p). The theoretical situation where n = p is represented

by a solid line.


voltammetric responses at a 1 mm diameter glassy carbon

electrode, solutions of the Zn2+ monoporphyrins TDBPP-Zn

and 1-Zn, and the corresponding fourth generation dendrimer

containing 32 porphyrin entities (6-Zn), dissolved in dichlor-

omethane containing 0.1 M [Bu4N][PF6]. The formal potential

(E0f ) data for the mono- and bis-porphyrin species and the five

generations of porphyrin-appended dendrimers are compared

with the corresponding data for two model compounds TPP-

Zn and TDBPP-Zn in Table 2 and Fig. 5.

As observed for the corresponding free-base compounds,

the potentials for TDBPP-Zn are shifted negative relative to

un-substituted TPP-Zn and essentially the same redox pattern

is observed for each compound in the series. The third oxida-

tion process, ascribed to the amide linkers, is once again

absent for the two model species. The redox processes for

the metalated species are observed at more negative potentials

than the free-base compounds, due to the increased electron

density on the porphyrin macrocycles provided by the zinc.

Both the similarity in oxidation potential between successive

generations and the negative shift on co-ordination with zinc

are mirrored in the absorption spectra. The spectroscopic data

presented in Table 2 show that the position of the Soret band

does not vary between the metalated compounds and has a

value that is shifted to longer wavelengths by 2 to 5 nm relative

to the free-base compounds.

In general, for the metalated compounds, only a single

reduction wave is detected, while all three oxidation processes

are now observed in each case. As a result of the negative shift

in potentials on metalation with zinc(II), the second and third

oxidation processes for these compounds are now more clearly

defined, not being complicated by proximity to the oxidative

solvent limit, while the second reduction for most of the Zn

compounds is now completely obscured by the negative limit

where the solvent is reduced.

As can be seen from Table 2, in common with the free-base

compounds, the HOMO–LUMO gap,(DH�L) for the metal-

lated species does not vary appreciably with dendrimer gen-

eration (2.12 � 0.04 V) although it does decrease slightly on

metalation. The constancy of the gap between reduction

processes observed for the free-base compounds in Table 1 is

Fig. 4 Cyclic voltammograms at a 1 mm diameter glassy carbon

electrode, for the Zn2+ mono porphyrins TDBPP-Zn (4 mM) and 1-

Zn (1.5 mM) and the fourth generation dendrimer containing 32

metalated porphyrin entities, 6-Zn (50 mM) dissolved in dichloro-

methane (0.1 M [Bu4N][PF6]). The scan rate is 0.1 V s�1 in each case.

Table 2 Electrochemical and spectroscopic data for zinc dendrimers and other compounds. Values in italics represent peak potential data forirreversible processes

p E0f /V (vs. Fc0/+) DH�L/V lSoret/nm Log (eSoret)/M

�1 cm�1

TPP-Zn 1 �2.17 �1.79 0.42 0.71 — 2.21 420 5.45TDBPP-Zn 1 — �1.89 0.30 0.62 — 2.19 423 5.711-Zn 1 �2.16 �1.86 0.30 0.56 0.95 2.16 426.5 5.772-Zn 2 — �1.85 0.30 0.59 0.94 2.15 426.5 6.073-Zn 4 — �1.84 0.30 0.70 1.00 2.14 427 6.344-Zn 8 — �1.80 0.30 0.63 0.90 2.10 427 6.625-Zn 16 �2.18 �1.80 0.29 0.63 0.90 2.09 427 6.896-Zn 32 — �1.79 0.31 0.64 0.91 2.10 427 7.227-Zn 64 — �1.80 0.30 0.76 1.09 2.10 426 7.49

Fig. 5 Redox potential data for the zinc dendrimers and model

compounds. Open circles represent peak potential (Ep,ox) data for

irreversible processes. The dashed lines represent the limits for solvent

decomposition.


mirrored in Table 2 for the gap between adjacent oxidation

processes (Dox1, ox2 = 0.33 � 0.06 V).

The Zn(II) porphyrins displayed more stability toward

ambient light than the free-base compounds, with no evidence

of new voltammetric processes developing over time as ob-

served for the unmetalated species. A homogeneous reaction

was observed, following oxidation to the dication, which gave

rise to a reduction peak at ca. �0.5 V vs. Fc0/+, which can be

seen in the uppermost curve of Fig. 4. Doubly oxidised zinc

porphyrin compounds are known to be susceptible to nucleo-

philic attack at both the meso and b-positions resulting in the

formation of substituted products;23,26 the coupled homo-

geneous reaction observed here, therefore, may be the result

of trace levels of nucleophiles in the electrochemical medium.

As evidenced by Fig. 4, a trend toward chemical and

electrochemical irreversibility is observed as the generation

number increases. This trend is also observed for the free-base

compounds but is most apparent for the second and third

oxidations of the metalated dendrimers. With increasing num-

ber of porphyrins per molecule, the peak current per unit

concentration times the number of porphyrins decreases and

additionally a significant increase in DEp accompanied by a

broadening of the voltammetric waves is observed. For the

third oxidative process, at ca. 1.0 V vs. Fc0/+, which is

assigned to the amide functional groups, these changes could

be regarded as being consistent with a strong steric inhibition

to electron transfer as these groups are buried ever deeper

within the porphyrin core for each successive generation.

Moreover, with increasing dendrimer generation, access to

the electrode surface will be increasingly restricted as the

porphyrin surface becomes more crowded. For the first two

oxidations, however, which are assigned to the formation of

p-cation radicals and p-dications on the peripheral porphyrin

rings, significant steric crowding is unlikely. Another possible

explanation for the observed kinetic effects, therefore, may be

electrostatic interactions contributing to an increasingly large

barrier to electron transfer as ever-larger numbers of electrons

are transferred with increasing dendrimer generation. Finally

it is noted that a higher probability of kinetic and/or thermo-

dynamic dispersion is likely for higher generation dendrimers

and that these factors will lead to broadening of voltammo-

grams.

Fig. 6 shows the results for the determination of n, the

number of electrons transferred, for the series of zinc porphyr-

in compounds 1-Zn to 7-Zn. The n values determined for the

seven compounds were 1 � 0.3, 2 � 0.5, 4 � 0.4, 7 � 1.5 17 �2.3, 26 � 8.1 and 58 � 6.6. The trend observed for the

unmetalated species is mirrored here, with n increasing almost

proportionately with p, the negative deviations once again

being observed for the two highest generations. The absorp-

tivity data in Table 2 shows that, as with the unmetalated

species, the maximum intensities of the Soret bands (e) showthe same trend as for the electrons transferred (n), with a

negative divergence from simple proportionality of e to p for

the largest dendrimers. Broadening of the electronic spectra

occurs as the dendrimer generation increases, however, and

use of area rather than maximum values of absorption

significantly improves the level of linearity.

Conclusions

Cyclic voltammetry of both free-base and zinc metalated

dendrimers show diffusional responses when dissolved in

dichloromethane, with no significant evidence of adsorption

phenomena. For the free-base dendrimers and free-base

monoporphyrin models, generally two porphyrin-ring based

reversible reductions are observed, corresponding to the for-

mation of single or multiple p-anion radicals and p-dianionsand one reversible oxidation process resulting in single or

multiple p-cation radicals. For the zinc porphyrin compounds,

being more difficult to reduce, the second reduction is ob-

scured by the solvent limit, but one or two additional oxida-

tion processes are observed. The second oxidation in this case

is due to the formation of single or multiple porphyrin

p-dications, while the third oxidation process is assigned to

the amide linker groups, as it is absent in the voltammetric

response of a model compound which is not substituted at the

b-pyrrole position.

For the higher generation dendrimers, kinetic or thermo-

dynamic dispersion effects are evident, with increased DEp

values and broadening of the voltammetric waves. The effects

are most pronounced for the doubly- and triply-oxidised zinc

porphyrin arrays, which suggests they may be a consequence

of the heterogeneous kinetics being perturbed by electrostatic

interactions, though strong steric inhibition of electron trans-

fer may also be a contributing factor in the case of the

oxidation of the amide linkers, which are located at the

dendrimer core.

The positions of the redox waves are very similar for each

successive generation free-base dendrimer and also similar to

the corresponding model species. The same applies to the

series of metalated compounds but the potentials are shifted

negative relative to the free-base species, due to the increased

electron density at the porphyrin rings provided by the metal.

The electrochemical behaviour is mirrored in the spectroscopic

Fig. 6 Numbers of electrons transferred (n) in the first reduction

process of the zinc mono- and multiporphyrin species (1-Zn, to 7-Zn)

versus the number of porphyrin units per molecule (p). The theoretical

situation where n = p is represented by a solid line.


response with the maximum positions of the Soret bands in the

visible absorption spectra remaining constant with increasing

generation and shifting to longer wavelengths on metalation

with zinc(II). The absence of a dendrimer effect implies that

most or all of the porphyrins experience essentially the same

microenvironment regardless of generation. Other features of

the voltammetric response such as the HOMO–LUMO gap

and the gap between adjacent redox processes were also

invariant with generation but the magnitude of the

HOMO–LUMO gap decreased by 50–100 mV when the

porphyrin macrocycles were metalated.

The uniformity in redox potentials within each series is

consistent with each electron transfer process representing

the almost simultaneous oxidation of multiple non-interacting

centres. In order to verify this, the number of electrons

transferred (n) for each dendrimer was evaluated from limiting

current data using steady state microelectrode voltammetry,

with the diffusion coefficients in each case being estimated

from the Stokes–Einstein equation. For both free-base and

zinc metalated porphyrins, values of n determined were rea-

sonably consistent with the number of porphyrin macrocycles

(p) contained in each dendrimer. Slight deviations were ob-

served for the higher generation species in each case, which

may be a consequence of structural defects, with one dendron

of the molecule confined to the dendrimer core due to re-

stricted growth. More likely, the slight deviation might be due

to electrostatic interactions of the porphyrins as these redox-

active chromophores become closer packed in each successive

higher generation. Calculations indicate that the porphyrins

are on average about 1.8 nm apart in the tetrakis-porphyrins

3-H2 and 3-Zn but are only about 0.5 nm apart in the 64-mers

7-H2 and 7-Zn. It is interesting that in the absorptivity values

there is also a slight divergence from simple proportionality of

e with p in the fourth and fifth generation species and the

absorptions are broadened as expected while the relative area

of the absorptions remains constant.

The light harvesting potential of these molecules is clearly

demonstrated by the increased absorption properties with

array size. From an electrochemical viewpoint, dendrimers

are very interesting species because the electron transfer

properties of potentially electroactive units may be modulated

by placing them in the branches or the core, or a large number

of topologically equivalent units may be linked to the surface,

giving rise to simultaneous exchange of a large and pre-

determined number of electrons.27 Electrochemistry is also a

powerful technique for the evaluation of the degree of electro-

nic interaction among the electroactive units.28

Moreover, oxidation or reduction of the Zn porphyrin

dendrimer species results in ‘‘supercharged’’ molecules that

are stable on the cyclic voltammetric time-scale and fully

soluble in dichloromethane. This suggests that these com-

pounds may form the basis for efficient hole/electron storage

devices.

Acknowledgements

We thank the Australian Research Council for a Discovery

Research Grant (DP0208776) to M.J.C.

References

1 R. E. Blankenship, Molecular Mechanisms of Photosynthesis,Blackwell Science, Oxford, 2002.

2 P. Jordan, P. Fromme, H. T. Witt, O. Klukas, W. Seanger and N.Krauss, Nature, 2001, 411, 909–917.

3 K. N. Ferreira, T. M. Iverson, K. Maghlaoui, J. Barber and S.Iwata, Science, 2004, 303, 1831–1838.

4 A. Camara-Artigas, R. E. Blankenship and J. P. Allen, Photosynth.Res., 2003, 75, 49–55.

5 D. E. Tronrud and B. W. Matthews, Photosynthetic ReactionCentre, 1993, 1, 13–21.

6 M. Z. Papiz, S. M. Prince, T. Howard, R. J. Cogdell and N. W.Isaacs, J. Mol. Biol., 2003, 326, 1523–1538.

7 A. W. Roszak, T. D. Howard, J. Southall, A. T. Gardiner, C. J.Law, N. W. Isaacs and R. J. Cogdell, Science, 2003, 302,1969–1972.

8 G. R. Newkome, C. N. Moorefield and F. Vogtle, Dendrimers andDendrons, Wiley-VCH, Weinheim, 2001.

9 N. Tomioka, D. Takasu, T. Takahashi and T. Aida, Angew.Chem., Int. Ed., 1998, 37, 1531–1534.

10 G. R. Newkome, E. He and L. A. Godinez,Macromolecules, 1998,31, 4382–4386.

11 C.-F. Shu and H. M. Shen, J. Mater. Chem., 1997, 7, 47–51.12 E. K. L. Yeow, K. P. Ghiggino, J. N. H. Reek, M. J. Crossley, A.

W. Bosman, A. P. H. J. Schenning and E. W. Meijer, J. Phys.Chem. B, 2000, 104, 2596.

13 T. Hasobe, Y. Kashiwagi, M. A. Absalom, J. Sly, K. Hosomizu,M. J. Crossley, H. Imahori, P. V. Kamat and S. Fukuzumi, Adv.Mater., 2004, 16, 975–979.

14 S. Fukuzumi, T. Hasobe, K. Ohkubo, M. J. Crossley, P. V. Kamatand H. Imahori, J. Porphyrins Phthalocyanines, 2004, 8, 191–200.

15 T. Hasobe, P. V. Kamat, M. A. Absalom, Y. Kashiwagi, J. Sly, M.J. Crossley, K. Hosomizu, H. Imahori and S. Fukuzumi, J. Phys.Chem. B, 2004, 12865–12872.

16 J. Larsen, J. Andersson, T. Polıvka, J. Sly, M. J. Crossley, V.Sundstrom and E. Akesson, Chem. Phys. Lett., 2005, 403, 205–210.

17 J. Larsen, B. Bruggemann, T. Polıvka, V. Sundstrom, E. Akesson,J. Sly and M. J. Crossley, J. Phys. Chem. A, 2005, 109,10654–10662.

18 J. N. H. Reek, J. Sly, A. W. Bosman, M. A. Absalom, T. Khouryand M. J. Crossley, in preparation.

19 G. H. Barnett, M. F. Hudson and K. M. Smith, J. Chem. Soc.,Perkin Trans. 1, 1975, 1401–1403.

20 P. Weyermann, J.-P. Gisselbrecht, C. Boudon, F. Diedrich and M.Gross, Angew. Chem., Int. Ed., 1999, 38, 3215–3219.

21 J.-H. Fuhrhop, K. M. Kadish and K. M. Davis, J. Am. Chem. Soc.,1973, 95, 5140–5147.

22 D. W. Clack and N. S. Hush, J. Am. Chem. Soc., 1965, 87,4238–4242.

23 K. M. Kadish, L. R. Shiue, R. K. Rhodes and L. A. Bottomley,Inorg. Chem., 1981, 20, 1274–1277.

24 A. Tsuda and A. Osuka, Science, 2001, 293, 79–82.25 L. Cavallo and F. Fraternalli, Chem.-Eur. J., 1998, 4, 927–934.26 G. H. Barnett and K. M. Smith, J. Chem. Soc., Chem. Commun.,

1974, 772–773.27 C. M. Casado, B. Gonzalez, I. Cuadrado, B. Alonso, M. Moran

and J. Losada, Angew. Chem., Int. Ed., 2000, 39, 2135–2138.28 B. Garcia, C. M. Casado, I. Cuadrado, B. Alonso, M. Moran and

J. Losada, Organometallics, 1999, 18, 2349–2356.


Electrochemical studies of porphyrin-appended dendrimers

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