Redox states of well-defined π-conjugated oligothiophenes functionalized with poly(benzyl ether) dendrons

Article abstract of DOI:10.1021/ja994259x

The redox states of a series of well-defined hybrid dendrimers based on oligothiophene cores and poly(benzyl ether) dendrons have been studied using cyclic voltammetry and variable-temperature UV/visible/ near-IR spectroscopy. The oxidation potentials and

Full text of DOI:10.1021/ja994259x

7042
J. Am. Chem. Soc. 2000, 122, 7042-7051
Redox States of Well-Defined π-Conjugated Oligothiophenes
Functionalized with Poly(benzyl ether) Dendrons
Joke J. Apperloo,† Rene´ A. J. Janssen,*,† Patrick R. L. Malenfant,‡
Lambertus Groenendaal,‡ and Jean M. J. Fre´chet*,‡
Contribution from the Laboratory for Macromolecular and Organic Chemistry, EindhoVen UniVersity of
Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, and Department of Chemistry,
UniVersity of California, Berkeley, California 94720-1460
ReceiVed December 3, 1999
Abstract: The redox states of a series of well-defined hybrid dendrimers based on oligothiophene cores and
poly(benzyl ether) dendrons have been studied using cyclic voltammetry and variable-temperature UV/visible/
near-IR spectroscopy. The oxidation potentials and the electronic transitions of the neutral, singly oxidized,
and doubly oxidized states of these novel hybrid materials have been determined as a function of oligothiophene
conjugation length varying between 4 and 17 repeat units. The attachment of poly(benzyl ether) dendritic
wedges at the termini of these lengthy oligothiophenes considerably enhances their solubility, thus enabling
the first detailed investigation of the electronic structure of oligothiophenes having 11 and 17 repeat units with
minimal â-substitution. In the case of the undecamer and heptadecamer, we find that the dicationic state consists
of two individual polarons, rather than a single bipolaron. The effect of the dendritic poly(benzyl ether)
solubilizers on the properties of the redox states varies with the oligothiophene length and dendron size. More
specifically, we observe a kinetic limit to the electrochemical oxidation of the oligothiophene core when the
dendron is large compared to the electrophore. Finally, we have observed the first example of self-complexation
of cation radicals via π-dimerization leading to the formation of dendritic supramolecular assemblies.
Introduction
solubilize5a and segregate the conjugated polymer chain5 or as
end-groups at the termini of well-defined conjugated oligomers
to create block copolymer-like architectures.6-9
The functional properties of semiconducting conjugated
polymers result from the interplay between the intrinsic features
of the polymer chains and interchain interactions. The extent
of this interplay will ultimately dictate the role that a semicon-
ducting polymer can play as the active element in new
generations of light-emitting diodes, field-effect transistors, and
photovoltaic devices.1 In this regard, dendrimer chemistry2 may
be used as a tool to modify and control interchain interactions
and achieve supramolecular ordering of conjugated polymer
chains in solid films. Different avenues to combine dendritic
architectures and conjugated materials have been proposed in
recent years. In one approach, dendrimers were used as a core
onto which conjugated oligomers or polymer chains were
attached as electroactive and photoactive end groups.3 Alter-
natively, dendritic wedges4 have been used as side chains to
The use of block copolymers with conjugated segments is
an emerging area of research in which the repulsive and
attractive interactions of the different blocks lead to self-
organization of functional polymeric materials on the nanometer
scale.10-12 Dendrimer size and functionality as well as the
controllable length of the conjugated segment13 are well-defined
parameters that may be used to manipulate the morphology of
block copolymers in a rational way. Furthermore, by controlling
the nature of the end-groups of the dendrimers, it is possible to
(4) Hawker, C. J.; Fre´chet, J. M. J. J. Am. Chem. Soc. 1990, 112, 7638.
(5) (a) Malenfant, P. R. L.; Fre´chet, J. M. J. Macromolecules 2000, 33,
3634. (b) Karakaya, B.; Claussen, W.; Gessler, K.; Saenger, W.; Schlu¨ter,
A.-D. J. Am. Chem. Soc. 1997, 119, 3296. (c) Bao, Z.; Amundson, K. R.;
Lovinger, A. J. Macromolecules 1998, 31, 8647. (d) Schenning, A. P. J.
H.; Martin, R. E.; Ito, M.; Diederich, F.; Boudon, C.; Gisselbrecht, J.-P.;
Gross, M. Chem. Commun. 1998, 1013.
† Eindhoven University of Technology. E-mail: R.A.J.Janssen@tue.nl.
‡ University of California, Berkeley. E-mail: Frechet@cchem.berkeley.edu.
(1) (a) Handbook of Organic ConductiVe Molecules and Polymers;
Nalwa, H. S., Ed.; Wiley: New York, 1997; Vols. 1-4. (b) Handbook of
Conducting Polymers, 2nd ed.; Skotheim, T. A., Elsenbaumer, R. L.,
Reynolds, J. R., Eds.; Marcel Dekker: New York, 1998. (c) Dimitrako-
poulos, C. D.; Purushothaman, S.; Kymissis, J.; Callegari, A.; Shaw, J. M.
Science 1999, 283, 822.
(2) (a) Dendritic Molecules: Concepts, Synthesis, PerspectiVes; New-
kome, G. R., Moorefield, C. N., Vo¨gtle, F., Eds.; VCH: Weinhein, New
York, 1996. (b) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A. Angew.
Chem., Int. Ed. Engl. 1990, 29, 138. (c) Fre´chet, J. M. J. Science 1994,
263, 1710. (d) Emrick, T. E.; Fre´chet, J. M. J. Curr. Opin. Colloid Interface
Sci. 1999, 4, 15.
(6) Jestin, I.; Levillain, E.; Roncali, J. Chem. Commun. 1998, 2655.
(7) Klaerner, G.; Miller, R. D.; Hawker, C. J. Polym. Prepr. 1998, 39,
1006.
(8) (a) Malenfant, P. R. L.; Groenendaal, L.; Fre´chet, J. M. J. J. Am.
Chem. Soc. 1998, 120, 10990. (b) Malenfant, P. R. L.; Jayaraman, M.;
Fre´chet, J. M. J. Chem. Mater. 1999, 11, 3420.
(9) Miller, L. L.; Zinger, B.; Schlechte, J. S. Chem. Mater. 1999, 11,
2313.
(10) Ruokolainen, J.; Ma¨kinen, R.; Torkkeli, M.; Ma¨kela¨, T.; Serimaa,
R.; ten Brinke, G.; Ikkala, O. Science 1998, 280, 557.
(11) Li, W.; Maddux, T.; Yu, L. Macromolecules 1996, 29, 7329.
(12) Hempenius, M. A.; Langeveld-Voss, B. M. W.; van Haare, J. A. E.
H.; Janssen, R. A. J.; Sheiko, S. S.; Spatz, J. P.; Mo¨ller, M.; Meijer, E. W.
J. Am. Chem. Soc. 1998, 120, 2798.
(3) (a) Wang, F.; Rauh, R. D.; Rose, T. L. J. Am. Chem. Soc. 1997, 119,
11106. (b) Miller, L. L.; Kunugi, Y.; Canavesi, A.; Rigaut, S.; Moorefield,
C. N.; Newkome, G. R. Chem. Mater. 1998, 10, 1751. (c) Schenning, A.
P. J. H.; Peeters, E.; Meijer, E. W. J. Am. Chem. Soc. 2000. 122, 4489.
(13) Well-defined oligothiophenes have been prepared up to 27 repeat
units, see: (a) Nakanishi, H.; Sumi, N.; Aso, Y.; Otsubo, T. J. Org. Chem.
1998, 63, 8632. (b) Martin, R. E.; Diederich, F. Angew. Chem., Int. Ed.
Engl. 1999, 38, 1350.
10.1021/ja994259x CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/07/2000

Redox States of Well-Defined π-Conjugated Oligothiophenes
J. Am. Chem. Soc., Vol. 122, No. 29, 2000 7043
Chart 1. Molecular Structure of the Oligothiophenes and Abbreviations Used in the Text
Chart 2. Structure of the First Three Generations of
endow these block copolymer architectures with functions such
as enhanced adhesion1414 and energy harvesting,15 as well as
optoelectronic characteristics.16
Poly(benzyl ether) Dendrons ([G-1]-OH, [G-2]-OH,
[G-3]-OH) Used in the Functionalization of the
Oligothiophenes Depicted in Chart 1
While the incorporation of dendrimers is clearly advantageous
with respect to solubility and processability, and they may help
provide control over the morphology of thin films, little is known
about their influence on the optical and electronic properties of
such hybrid materials. The study of these properties has received
limited attention so far, and the effect of incorporating dendritic
structures on the nature and characteristics of charge carriers
has not yet been addressed. Here we present the first detailed
investigation of the redox states of well-defined diblock and
triblock hybrid dendrimers based on oligothiophene cores and
poly(benzyl ether) dendrons (Chart 1), utilizing cyclic voltam-
metry and variable-temperature UV/visible/near-IR spectros-
copy. By incorporating poly(benzyl ether)4,17 dendrons of
different generations (Chart 2) it was possible to investigate
the redox states of these systems, i.e. to determine the oxidation
potentials as a function of conjugation length, and the evolution
of the electronic transitions in the neutral, singly oxidized, and
doubly oxidized state. Furthermore, the use of dendrons as
solubilizers provides a unique opportunity to study oligothio-
phenes having minimal â-substitution; such materials would
normally be insoluble in the absence of the dendrons. Herein,
we show that, in general, poly(benzyl ether) dendritic wedges
are compatible with the charges generated in the conjugated
oligomers, yet specific “dendrimer related” effects do occur.
For instance, depending on their size, the dendritic end groups
can affect the kinetics of electrochemical oxidation reactions
by sterically preventing the electroactive group from interacting
with the electrode. We also present evidence that the presence
of bulky G3 poly(benzyl ether) dendrons in diblock materials
does not inhibit the well-established π-dimer formation of
oligothiophene cation radicals in solution, yet the enthalpy for
dimerization is slightly reduced in comparison to the nonden-
dritic analogue. This constitutes the first example in which the
π-dimerization of oligothiophene cation radicals is used to
construct a supramolecular dendritic assembly. Finally, by
(14) Fre´chet, J. M. J.; Gitsov, I.; Monteil, T.; Rochat, S.; Sassi, J. F.;
Vergelati, C.; Yu, D. Chem. Mater. 1999, 11, 1267.
(15) (a) Gilat, S. L.; Adronov, A.; Fre´chet J. M. J. Angew. Chem., Int.
Ed. 1999, 38, 1422. (b) Gilat, S. L.; Adronov, A.; Fre´chet J. M. J. J. Org.
Chem. 1999, 64, 7474. (c) Adronov, A.; Gilat, S. L.; Fre´chet, J. M. J.;
Ohta, K.; Neuwahl, F. V. R.; Fleming, G. R. J. Am. Chem. Soc. 2000, 122,
1175. (d) Adronov, A.; Malenfant, P. R. L.; Fre´chet J. M. J. Chem. Mater.
2000, 12, accepted.
(16) (a) Freeman, A. W.; Fre´chet, J. M. J.; Koene, S. C.; Thompson, M.
E. Polym. Prepr. 1999, 40, 1246. (b) Koene, S. C.; Freeman, A. W.; Killeen,
K. A.; Fre´chet, J. M. J.; Thompson, M. E. Polym. Mater. Sci. Eng. 1999,
80, 238.
(17) Hawker, C. J.; Fre´chet, J. M. J. J. Chem Soc., Chem. Commun.
1990, 1010.

7044 J. Am. Chem. Soc., Vol. 122, No. 29, 2000
Apperloo et al.
Scheme 1. Synthesis of G3-17T-G3 Using the Stille Reaction in the Final Stepa
a Conditions: (a) KOH (aq)/THF, H3O+ (99%); (b) oxalyl chloride, DMF (cat.), CH2Cl2 (quant.); (c) [G-3]-OH, Py, CH2Cl2 (76%); (d) Pd(PPh3)2Cl2,
DMF, argon (65%).
additional advantage, the thianthrene formed in the reduction reaction
exhibits an absorption band at E ) 4.83 eV, well outside the region of
interest for the redox states of the present oligothiophenes.
investigating the oxidized states of the generation three un-
decamer dumbbell (G3-11T-G3) and heptadecamer dumbbell
(G3-17T-G3), we have been able to characterize the electronic
transitions and obtain evidence that, in the doubly oxidized state
of long oligothiophenes, two singly charged deformations (also
referred to as polarons) are present on the chain, rather than a
single, doubly charged, charge carrier (bipolaron). In contrast,
for oligomers up to nine units (9T(B) and 9T(G*2)), the
dications exhibit typical characteristics of a bipolaron. Hence,
these experiments give evidence that a remarkable change in
the electronic structure of oligothiophene dications occurs for
oligomers with 10 thiophene units. At this point a single (doubly
charged) deformation (bipolaron) can dissociate on the chain
into two (singly charged) deformations (polarons).
Results and Discussion
Synthesis. The synthesis and characterization of G3-11T-
G3 and B-17T-B have been described previously.8a The poly-
(benzyl ether) dendrons were prepared as described in the
literature.4,17 Triblock compound G3-17T-G3 was prepared in
a multistep process involving the saponification of the R-bro-
minated Br-8T-B,8a to afford carboxylic acid Br-8T-COOH,
followed by quantitative conversion to the acid chloride Br-
8T-COCl using oxalyl chloride in CH2Cl2 in the presence of a
catalytic amounts of DMF. Reaction of Br-8T-COCl with the
generation three poly(benzyl ether) dendron having a benzyl
alcohol group at the focal point4,17 in CH2Cl2/pyridine afforded
Br-8T-G3. Finally, a Stille coupling between Br-8T-G3 and
2,5-bis(trimethylstannyl)thiophene provided G3-17T-G3 in 65%
yield after chromatography (Scheme 1). In contrast to B-17T-
B, which is slightly soluble in hot CS2, G3-17T-G3 is quite
soluble in common organic solvents such as THF, CHCl3, and
CH2Cl2. The Gn-4T-Gn series (n ) 1, 2, and 3) was prepared
through reaction of the dendritic alcohols with the tetramer
diacid chloride ClCO-4T-COCl. The latter was obtained via a
Stille coupling reaction between methyl 2-bromothiophene-5-
carboxylate and 5,5′-(bistrimethylstannyl)-2,2′-bithiophene to
provide the symmetric tetramer Me-4T-Me. Subsequent sa-
ponification with aqueous KOH/THF followed by chlorination
with oxalyl chloride provided the diacid chloride ClCO-4T-
COCl (Scheme 2). While Me-4T-Me was rather insoluble, it
could be sufficiently purified by trituration in boiling THF. All
of the dendritic derivatives where found to be quite soluble in
common organic solvents, thus facilitating their handling and
purification. The phenyl end-capped pentamer Ph-5T-B was
prepared via Stille coupling between Br-5T-B8a and trimeth-
ylstannylbenzene. Similarly, Ph-5T-G3 was prepared by a Stille
coupling between Br-5T-G38a and trimethylstannylbenzene.
Once again, a significant enhancement in solubility is observed
upon functionalization of the phenyl end-capped pentamer with
a generation three dendron instead of a simple benzyl moiety.
Preparation of 9T(B)15d and 9T(G*2) has been reported
elsewhere. 8b
Experimental Section
Commercial grade solvents were purified, dried, and deoxygenated
following standard methods. Cyclic voltammograms were recorded in
CH2Cl2 or CH3CN with 0.1 M tetrabutylammonium hexafluorophos-
phate (TBAPF6) as the supporting electrolyte using a Potentioscan
Wenking POS73 potentiostat. The working electrode was a platinum
disk (0.2 cm2), the counter electrode was a platinum plate (0.5 cm2),
and a saturated calomel electrode was used as the reference electrode,
calibrated against a Fc/Fc+ couple (+0.470 V vs SCE). The reported
oxidation potentials have been obtained with a scan speed of 100 mV/s
unless indicated otherwise. UV/visible/near-IR spectra were recorded
using a Perkin-Elmer Lambda 900 spectrophotometer equipped with
an Oxford Optistat cryostat for variable-temperature experiments. The
temperature was kept constant within (0.3 K and spectra were corrected
for volume changes. The concentrations in the optical experiments
varied between 10-5 and 10-3 M, using both 10 mm and 1 mm near-
IR grade suprasil quartz cells. The oxidation experiments were
performed by adding small aliquots of a solution of thianthrenium
perchlorate (THIClO4)18 of known concentration in dichloromethane
to the solutions of the oligomers from a gas-tight syringe. THIClO4
was chosen as the oxidizing agent because it combines a relatively
high oxidation potential with characteristic signals in the UV/visible
spectrum (E ) 2.23 and 4.25 eV).18,19 The appearance of these signals
in the absorption spectra indicates that further addition of THIClO4
does not result in (complete) oxidation of oligothiophene. As an
(18) Shine, H. J.; Dais, C. F.; Small, R. J. Org. Chem. 1964, 29, 21.
(19) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley
and Sons: New York, 1980; p 702.

Redox States of Well-Defined π-Conjugated Oligothiophenes
Scheme 2. Synthesis of Gn-4T-Gn Triblock Systemsa
J. Am. Chem. Soc., Vol. 122, No. 29, 2000 7045
a Conditions: (a) Pd(PPh3)2Cl2, DMF, argon (81%); (b) KOH/Me4NOH‚THF/H2O (92%); (c) oxalyl chloride, DMF (cat.), CH2Cl2 (quant.); (d)
[G-n]-OH, n ) 1 (55%), 2 (70%), and 3 (60%).
Table 1. Peak Potentials (EPa), Standard Potentials (E°), and
Anodic-Cathodic Peak Separations (∆Ep ) |EPa1 - EPc1|) for
Oxidations of Oligothiophenes Recorded in CH2Cl2
compd
Epa1 (V)
Epa2 (V)
E° (V)
∆Ep (V)
G1-4T-G1
G2-4T-G2
G3-4T-G3
Ph-5T-B
Ph-5T-G3
9T(B)
9T(G*2)
G3-11T-G3
B-17T-B
1.223
1.221
1.225
0.915
0.905
0.792
0.849
0.72
0.658
0.762a
0.812a
1.57
1.58
1.58
1.58
1.22
1.20
0.98
1.01
1.190
1.187
1.168
0.884
0.873
0.772
0.807
0.067
0.068
0.115
0.062
0.065
0.040
0.083
0.210
0.225a
0.296a
G3-17T-G3
[G-n]-OH
a Measured in CH3CN.
Figure 1. Cyclic voltammograms of Gn-4T-Gn oligothiophenes,
recorded at 295 K in CH2Cl2 containing TBAPF6 (0.1 M), scan rate
100 mV/s. Potential vs SCE calibrated against Fc/Fc+. Curves are offset
vertically for clarity.
potential up to 1.5 V. When the electrochemical potential is
further increased a second oxidation wave occurs at Epa ) 1.58
V. The anodic current associated with the second oxidation wave
is considerably larger than that of the first wave, suggesting
the transfer of more than one electron. At this point a film is
formed on the electrode and subsequent cycles show an increase
of current. In a separate experiment we found that the oxidation
of the dendritic wedge occurs at Epa1 ) 1.57 V (Table 1) and
is completely irreversible. Hence, the Epa ) 1.58 V wave of
G1-4T-G1 is attributed to the oxidation of the G1 wedge. It
must be noted, however, that the second oxidation wave of the
quaterthiophene moiety is expected around the same potential
and may therefore contribute to the current observed at 1.58 V.
Audebert, P.; Hapiot, P.; Pernaut, J.-M.; Garcia P. J. Electroanal.
Chem.1993, 361, 283. (f) Van Haare, J. A. E. H.; Groenendaal, L.; Havinga,
E. E.; Janssen, R. A. J.; Meijer, E. W. Angew. Chem., Int. Ed. Engl. 1996,
35, 638. (g) Sakamoto, A.; Furukawa, Y.; Tasumi, M. J. Phys. Chem. B
1997, 101, 1726. (h) Apperloo, J. J.; Janssen, R. A. J. Synth. Met. 1999,
101, 373.
(21) (a) Fichou, D.; Xu, B.; Horowitz, G.; Garnier, F. Synth. Met. 1991,
41-43, 463. (b) Fichou, D.; Horowitz, G.; Garnier, F. Synth. Met. 1990,
39, 125. (c) Fichou, D.; Horowitz, G.; Xu, B.; Garnier, F. Synth. Met. 1990,
39, 243.
Singly Oxidized States and π-Dimerization of Short
Oligothiophenes (n e 5) with Poly(benzyl ether) Dendrimer
End Groups. First we will address the question whether the
electrochemical properties and the nature of the oxidized states
of the shorter oligothiophenes (n e 5) are influenced by the
poly(benzyl ether) dendrons.
Figure 1 shows the one-electron oxidation wave at E° ) 1.19
V vs SCE for G1-4T-G1. The oxidation is chemically reversible,
which indicates that oxidative polymerization of the 4T moiety
does not occur. Clearly, the G1 end groups are effective at
inhibiting the coupling of cation radicals via the R-positions,
which is a well-known reaction in the one-electron oxidation
of short oligothiophenes (nTs with n < 6) lacking R-sub-
stituents.20-22 Chemical reversibility is preserved in cycling the
(20) (a) Hill, M. G.; Mann, K. R.; Miller, L. L.; Penneau, J.-F. J. Am.
Chem. Soc. 1992, 114, 2728. (b) Yu, Y.; Gunic, E.; Zinger, B.; Miller, L.
L. J. Am. Chem. Soc. 1996, 118, 1013. (c) Ba¨uerle, P.; Segelbacher, U.;
Maier, A.; Mehring, M. J. Am. Chem. Soc. 1993, 115, 10217. (d) Zotti, G.;
Schiavon, G.; Berlin, A.; Pagani, G. Chem. Mater. 1993, 5, 620. (e)
(22) Ba¨uerle, P.; Segelbacher, U.; Gaudl, K.-U.; Huttenlocher, D.;
Mehring, M. Angew. Chem., Int. Ed. Engl. 1993, 32, 76.

7046 J. Am. Chem. Soc., Vol. 122, No. 29, 2000
Apperloo et al.
solid state, it is well established that the same phenomenon
affects the absorption spectra of dye molecules in solution when
these form dimers or aggregates.24 The low-temperature spectra
do not show a charge-transfer transition between the two cation
radicals in the π-dimer which is sometimes found at energies
below the RC1 band for π-dimers of conjugated oligomer cation
radicals.25,26 In accordance with literature reports on studies of
similar systems in solution20 and X-ray crystallography in the
solid state,27 we propose an interpretation of these effects in
terms of π-dimerization, yet it must be noted that σ-dimerization
has recently been proposed as an alternative explanation.28,29
Although there is no unambiguous structural proof whether
σ-dimers or π-dimers are formed in solution, we prefer the
π-dimer hypothesis. Evidence in favor of π-dimerization is as
follows: (1) The reVersible dimerization of various oligothio-
phene radical cations reported in the literature so far20 does not
occur preferentially for oligomers with free R-positions, as can
be expected for σ-dimerization.30 (2) The two absorption bands
(RC1 and RC2), which are highly characteristic for an open-
shell conjugated radical ion, are preserved to a large extent in
the D1 and D2 bands of the dimer. This strong similarity in
electronic structure is not expected for a closed-shell σ-dimer.
(3) The Davydov model of a face-to-face π-dimer describes
qualitatively, and to a large extent quantitatively, the experi-
mentally observed hypsochromic shift of D1 and D2 as
compared to RC1 and RC2.23,24 (4) The dimerization enthalpy
generally increases with longer systems,20 while for σ-dimer-
ization one would expect the opposite behavior.
Figure 2. (a) UV/visible/near-IR spectra of G1-4T-G1 as a function
of the doping level by the stepwise addition of 1 equiv of THIClO4
recorded in a 10 mm cell. The graph shows the formation of cation
radicals (RC1 and RC2) at the cost of neutral molecules (N). (b) UV/
visible/near-IR spectra as a function of temperature (increasing from
180 to 295 K) recorded in a 1 mm cell showing the interconversion
from π-dimers (D1 and D2) at low temperatures into cation radicals
(RC1 and RC2) at room temperature. The inset shows the variation of
the dimerization constant (K′, see ref 31) with reciprocal temperature
as determined from the increase of the RC2 band in the spectra.
As a consequence of the negligible spectral overlap and high
intensity, the changes in the intensity of the RC2 transition with
temperature can be used to estimate the enthalpy of dimerization
(∆H0dim) by determining the temperature dependence of the
equilibrium constant for dimerization (K ) [dimer]/[cation
radical]2, see inset of Figure 2b).31 This analysis gives ∆H0
dim
) -34 ((5) kJ/mol for G1-4T-G1.
The higher generation dendrimer substituted quaterthiophenes,
G2-4T-G2 and G3-4T-G3, show an analogous behavior in
cyclic voltammetry and the first one-electron oxidations occur
at almost the same potentials as for G1-4T-G1 (Table 1).
However, for G3-4T-G3 the increase of ∆Ep from 67 to 115
mV reveals that a quasireversible oxidation occurs, indicative
of a kinetic limitation to the electron-transfer reaction with
increasing size of the dendritic end group. The kinetic limitation
for G3-4T-G3 is supported by the observation that the anodic
The evolution of the UV/visible/near-IR spectra of G1-4T-
G1 in CH2Cl2 solution with progressive oxidation is shown in
Figure 2a. Stepwise addition of 1 equiv of THIClO4 results in
the gradual formation of G1-4T-G1+•, which exhibits two
characteristic transitions at 1.16 and 1.89 eV (labeled RC1 and
RC2, respectively) associated with cation radicals of π-conju-
gated oligomers.20-22 The RC1 and RC2 bands are assigned to
a dipole-allowed transition from the lowest doubly occupied
level to the singly occupied electronic (polaron) level and from
the singly occupied level to the lowest unoccupied electronic
level, respectively. Bands or shoulders at higher energy, which
have a vibronic origin, accompany the two principal bands. The
high second oxidation potential of G1-4T-G1 (Epa2 ) 1.58 V)
and the irreversible nature of the second wave preclude the
formation of the corresponding dication using THIClO4. Figure
2b shows the reversible changes in the absorption spectra of
G1-4T-G1+• as a function of temperature. At low temperatures
the RC1 and RC2 bands are replaced by new bands at 1.37
(D1) and 2.31 eV (D2). The D1 and D2 bands are hypsochro-
mically shifted with respect to the RC1 and RC2 transitions.
This shift is usually observed for π-dimers and is ascribed to a
Davydov interaction between the transition dipole moments of
the RC1 and RC2 transitions on adjacent radicals in the π-dimer
complex.23 Although the Davydov shift is often observed in the
(24) Torrance, J. B.; Scott, B. A.; Welber, B.; Kaufman, F. B.; Seiden,
P. E. Phys. ReV. B 1979, 19, 730,and references therein.
(25) Yu, Y.; Gunic, E.; Zinger, B.; Miller, L. L. J. Am. Chem. Soc. 1996,
118, 1013.
(26) Van Haare, J. A. E. H.; Van Boxtel, M.; Janssen, R. A. J. Chem.
Mater. 1998, 10, 1166.
(27) Graf, D. D.; Duan, R. G.; Campbell, J. P.; Miller, L. L.; Mann, K.
R. J. Am. Chem. Soc. 1997, 119, 5888.
(28) (a) Smie, A.; Heinze, J. Angew. Chem., Int. Ed. Engl. 1997, 36,
363. (b) Tschuncky, P.; Heinze, J.; Smie, A.; Engelmann, G.; Kossmehl,
G. J. Electroanal. Chem. 1997, 433, 223. (c) Merz, A.; Kronberger, J.;
Dunsch, L.; Neudeck, A.; Petr, A.; Parkanyi, L. Angew. Chem., Int. Ed.
Engl. 1999, 38, 1442.
(29) (a) Effenberger, F.; Mack, K.-E.; Niess, R.; Reisinger, F.; Steinbach,
A.; Stohrer, W.-D.; Stezowski, J. J.; Rommel, I.; Maier, A. J. Org. Chem.
1998, 53, 4379-4386. (b) Effenberger, F.; Stohrer, W.-D.; Mack, K.-E.;
Reisinger, F.; Steufert, W.; Kramer, H. E. A.; Fo¨ll, R.; Vogelmann, E. J.
Am. Chem. Soc. 1990, 112, 4849.
(30) In fact, a recent study on the dimerization of oligothienylenevinylene
radical cations reveals a much stronger preference for dimerization of
oligomers with R-alkyl substituents than â-substituents; see ref 37.
(31) The normalized absorbance A of the RC2 transition can be used to
determine K via the relation K ) (1 - A)/2C0A2), where C0 is the total
concentration of cation radicals (present as free monomers or incorporated
in π-dimers). ∆H0dim can then be obtained from the slope of a plot of ln K
(23) π-Dimers and π-stacks in solution have last been reviewed in 1996,
see: Miller, L. L.; Mann, K. R. Acc. Chem. Res. 1996, 29, 417.
vs 1/T. Alternatively, K′ ) KC0 can be used to determine ∆H0
.
dim

Redox States of Well-Defined π-Conjugated Oligothiophenes
J. Am. Chem. Soc., Vol. 122, No. 29, 2000 7047
Table 2. Energies of the Electronic Transitions of the Neutral (N), Cation Radical (RC), Dication (DC), and π-Dimers (D) as Well as Redox
States of Oligothiophenes and the Enthalpy of Dimerization of Cation Radicals
compd
N (eV)
RC1 (eV)
RC2 (eV)
DC1 (eV)
DC2 (eV)
D1 (eV)
1.37
D2 (eV)
2.32
∆H0dim (kJ/mol)
G1-4T-G1
G2-4T-G2
G3-4T-G3
Ph-5T-B
Ph-5T-G3
9T(B)
9T(G*2)
G3-11T-G3
B-17T-B
2.93
2.93
2.93
2.86
2.85
2.83
2.76
2.60
2.56
2.59
1.16
1.16
1.16
0.93
0.94
0.63
0.63
0.69
0.77
0.66
1.89
1.89
1.89
1.66
1.65
1.44
1.47
1.42
1.82
1.74
a
a
a
1.27
1.31
0.80
0.83
0.89
a
a
a
-34 ( 5
b
b
-51 ( 3
-38 ( 2
1.09
1.11
1.99
2.00
1.62
1.62
1.68
G3-17T-G3
0.75
1.62
a Dication cannot be generated using THIClO4. b Low stability inhibited variable-temperature experiments to assess π-dimerization.
peak current (ipa) scales with the scan rate (V) according to ipa
∼ V0.5 for G1 and G2 in the range V ) 25-200 mV/s, but as ipa
∼ V0.25 for G3. Similar observations have been made with
porphyrin core dendrimers.32,33 It is interesting to note that in a
recent study done on dendritic oligothienylenevinylenes both
the oxidation potential and electron-transfer kinetics were found
to be independent of dendron generation for a system containing
five vinylene and four thienylene units (5V4T).6 The lack of
dependence of the electroactivity on dendron generation was
attributed to the length of the electrophore that prevents its
complete encapsulation by the dendritic structure. The reduced
length of 4T in Gn-4T-Gn (ca. 65%) in comparison to 5V4T
might explain the kinetic limitation of electron transfer to the
electrode observed in our system.
G2-4T-G2 and G3-4T-G3 can also be oxidized to the
corresponding cation radicals and exhibit similar UV/visible/
near-IR spectra as G1-4T-G1 (Table 2). It would have been
interesting to investigate π-dimerization of these cation radicals,
to assess a possible generation dependence. Unfortunately, the
stability of the G2-4T-G2+• and G3-4T-G3+• cation radicals
is less than that of G1-4T-G1+• and samples deteriorated
Figure 3. Selected cyclic voltammograms of oligothiophenes shown
in Chart 1, recorded at 295 K in CH2Cl2 containing TBAPF6 (0.1 M),
scan rate 100 mV/s. Potential vs SCE calibrated against Fc/Fc+. Curves
are offset vertically for clarity.
significantly in less than 10 min, thus preventing variable-
temperature experiments that typically require 2-3 h. The loss
of the RC1 and RC2 bands for G2-4T-G2+• and G3-4T-G3+•
in time is not accompanied by the full recovery of the π-π*
absorption of the neutral oligomer. This is a clear indication
that the 4T moiety is participating in an irreversible chemical
reaction. Since G1 dendritic end groups are effective in
stabilizing the otherwise very reactive 4T+• cation radical in
G1-4T-G1+•, we rationalize the decreased lifetime of G2-4T-
G2+• and G3-4T-G3+• by an intramolecular reaction of the 4T+•
cation radical with back-folded dendritic wedges. Since G1-
4T-G1+• is expected to exhibit a similar reactivity in intermo-
lecular reactions as the generation two and three isomers, but
lacks the possibility of back-folding of the dendritic wedge, we
prefer an intramolecular degradation mechanism to explain the
reduced stability of G2-4T-G2+• and G3-4T-G3+•, in compari-
son with G1-4T-G1+•. We speculate that oxidation of the
dendritic wedge (Epa ) 1.58 V) may provide a radical
intermediate that is reactive toward the 4T conjugated system,
but we were not able to confirm this experimentally.
electronic transitions (RC1 and RC2 at 0.93 and 1.66 eV,
respectively) (Figure 4a). Subsequent addition of a second
equivalent of THIClO4 produces the dication, which shows a
single strong transition at 1.27 eV (Table 2). The cation radical
of Ph-5T-B is stable on the time scale of several hours, thus
allowing the use of variable-temperature UV/visible/near-IR
spectroscopy to investigate π-dimerization at low temperatures.
Figure 4a shows the changes that occur in the spectrum as the
temperature decreases. The RC1 and RC2 bands are reversibly
replaced by two new bands attributed to the π-dimer: D1 and
D2 at 1.09 and 1.99 eV, respectively. Using the RC2 band as
a gauge to determine the equilibrium constant,31 the enthalpy
for π-dimerization was found to be ∆H0 ) -51 ((3) kJ/
dim
mol, which is in good agreement with related systems.20
The results of the oxidation experiments on Ph-5T-G3 are
analogous to those of the benzyl ester derivative Ph-5T-B. The
oxidation is fully reversible to both the cation radical (RC1 )
0.94, RC2 ) 1.65 eV) and the dication (DC ) 1.31 eV). Ph-
5T-G3+• also forms π-dimers when the temperature is de-
creased. The more extended conjugation of Ph-5T-G3 in
comparison with G3-4T-G3 enhances the intrinsic stability of
the corresponding cation radical significantly, thus allowing the
heat of dimerization to be measured. Comparison of Figure 4a
and Figure 4b shows that the π-dimerization of the dendritic
oligothiophene derivative is less pronounced. The reduced
The two quinquethiophenes Ph-5T-B and Ph-5T-G3 exhibit
two consecutive one-electron chemically reversible oxidation
waves at about Epa1 ) 0.90 and Epa2 ) 1.20 V (Table 1, Figure
3). Ph-5T-B can be oxidized to the cation radical by adding 1
equiv of THIClO4. The singly oxidized state exhibits two
(32) Dandliker, P. J.; Diederich, F.; Zingg, A.; Gisselbrecht, J. P.; Gross,
M.; Louati, A.; Sanford, E. HelV. Chim. Acta 1997, 80, 1773 and references
therein.
(33) Pollak, K. W.; Leon, J. W.; Fre´chet J. M. J.; Maskus, M.; Abruna,
H. D. Chem. Mater. 1998, 10, 30.

7048 J. Am. Chem. Soc., Vol. 122, No. 29, 2000
Apperloo et al.
the electrode as inferred from an increasing current in each
subsequent cycle.
The UV/visible/near-IR spectra of 9T(B) and 9T(G*2) as a
function of doping level are shown in Figure 6, spectra a and
b, respectively, and show that both compounds exhibit a very
similar behavior. By stepwise addition of 2 equiv of THIClO4
these nonamers can be oxidized to the cation radical and the
dication. While the cation radicals of 9T(B) and 9T(G*2) exhibit
two strong transitions below the optical band gap of the neutral
oligomer, the corresponding dications show only one strong
subgap transition. Such a single transition for a doubly charged
dication is highly characteristic of a bipolaronic charge carrier,
i.e. the two charges belong to a single geometric deformation
of the chain.35 Comparing the optical transitions of the redox
states of the two nonamers (Table 2) reveals that the π-π*
transition of neutral 9T(G*2) is at lower energy than that of
9T(B). This is rationalized by the minimal substitution of 9T-
(G*2) which induces a more planar structure and, hence, a
longer effective conjugation length. This result is consistent with
previous work comparing â-substituted and nonsubstituted
oligothiophenes.36 However, the oxidation potential of 9T(B)
is relatively lower as a consequence of the electron-releasing
alkyl groups. Apparently, the electron-releasing effect outweighs
the reduction of the conjugation length that results from the
inter-ring torsion due to steric hindrance of the alkyl groups,
thus lowering the oxidation potential.
The two polaron bands are of almost the same intensity for
these 9T derivatives. Figure 6 shows that the DC transition of
the dication has developed before the N band of the neutral
oligomer has fully disappeared. This is due to the fact that the
first two oxidation waves are close in energy, thus giving rise
to a disproportionation reaction in which the cation radicals are
in equilibrium with a neutral and doubly charged oligomer:
Figure 4. UV/visible/near-IR spectra of (a) Ph-5T-B+• as a function
of temperature (decreasing from 270 to 210 K) showing the intercon-
version from cation radicals (RC1 and RC2) at room temperature into
π-dimers (D1 and D2) at low temperatures. The inset shows the
variation of the dimerization constant (K′, see ref 31) with reciprocal
temperature as determined from the decrease of the RC2 band in the
spectra. (b) Ph-5T-G3+• in the temperature range from 280 to 220 K.
2 nT+• a nT + nT2+
tendency to form π-dimers is also reflected in the lower
dimerization enthalpy of Ph-5T-G3+• which was found to be
∆H0dim ) -38 ((2) kJ/mol as compared to ∆H0dim) -51 ((3)
kJ/mol for Ph-5T-B+•. The decreased π-dimerization may be
rationalized by an increase in steric interactions between the
two cation radicals, but it may also result from a decrease in
the solvation of the dendritic wedges when incorporated into a
π-dimer. The formation of (Ph-5T-G3)22+ π-dimers is the first
example in which the self-complexation of conjugated cation
radicals provides a supramolecular dendritic assembly. Although
the exact geometry of these assemblies is still unknown, we
speculate that the dumbbell shaped dimer depicted in Figure 5
represents a plausible arrangement.
Singly and Doubly Oxidized States of Intermediate Length
Oligothiophenes (n ) 9) with Varying Degrees of Substitu-
tion. In this section we address the redox properties of two
related nonithiophenes, 9T(B) and 9T(G*2). By introducing the
aliphatic ether dendron (G*2) it is possible to study an oligomer
of unprecedented length, having only a single solubilizing group.
The electrochemical oxidation of 9T(B) and 9T(G*2) is
chemically reversible when the voltage scans are reversed before
E ) 0.9 and 1.0 V, respectively (Figure 3). Distances between
anodic and cathodic peaks are 40 and 83 mV, respectively.
Extension of the sweep to higher potentials gives ill-resolved
quasireversible broad second oxidation waves at 0.98 and 1.01
V. This complex behavior is analogous to previous results on
an alkyl-substituted nonithiophene.34 At E )1.2 V the oxidation
of 9T(G*2) becomes irreversible and film formation occurs at
The spectral changes of 9T(B)+• and 9T(G*2)+• with temper-
ature revealed a rather complex behavior, which could not be
analyzed in detail. Although some indications of π-dimerization
were observed, the changes with temperature were relatively
small and could also be interpreted in terms of a shift in the
disproportionation equilibrium to the neutral and dicationic state
as observed recently for thienylenevinylene oligomers.37
Singly and Doubly Oxidized States of Long Oligothio-
phenes (n g 11) with Poly(benzyl ether) Dendrimer End
Groups. The results on 9T(B) and 9T(G*2) support the view
that open-shell cation radicals (polarons) and closed-shell
dications (bipolarons) are the primary redox species in oligo-
thiophenes of limited and intermediate length. The nature of
the doubly oxidized state for long oligothiophenes, however, is
less clear.34,38 With the high solubility of the present dendrimer
functionalized oligithiophenes, the oxidized states up to the
heptadecamer (G3-17T-G3) can be analyzed in detail for the
first time. The possibility that long oligothiophene dications
possess an open-shell structure with two individual polarons
has been proposed for an duodecithiophene derivative (12T).34
Such a transition with increasing chain length can be explained
(35) (a) Fesser, K.; Bishop, A. R.; Campbell, D. K. Phys. ReV. B. 1983,
27, 4804. (b) Cornil, J.; Beljonne, D.; Bre´das, J. L. J. Chem. Phys. 1995,
103, 842.
(36) Horowitz, G.; Yassar, A.; Von Bardeleben, H. J. Synth. Met. 1994,
62, 245.
(37) Apperloo, J. J.; Raimundo, J.-M.; Fre`re, P.; Roncali J.; Janssen, R.
A. J. Chem. Eur. J. 2000, 6, 1698.
(34) Van Haare, J. A. E. H.; Havinga, E. E.; Van Dongen, J. L. J.;
Janssen, R. A. J.; Cornil, J.; Bre´das, J.-L. Chem. Eur. J. 1998, 4, 1509.
(38) Nessakh, B.; Horowitz, G.; Garnier, F.; Deloffre, F.; Srivastava,
P.; Yassar, A. J. Electroanal. Chem. 1995, 399, 97.

Redox States of Well-Defined π-Conjugated Oligothiophenes
J. Am. Chem. Soc., Vol. 122, No. 29, 2000 7049
Figure 5. Proposed schematic structure of the dumbbell π-dimer of Ph-5T-G3+•
.
by the fact that the decrease of Coulomb energy obtained by
moving the two like charges apart may (at a certain length)
outweigh the concurrent energy cost that follows from the fact
that the two charges no longer share the same geometrical
deformation.
As can be seen in Figure 3 there is a loss of electrochemical
reversibility for G3-11T-G3 in comparison with shorter oligo-
thiophenes. However, repetitive cycling of the voltammogram
produces the same curve without the appearance of new peaks.
This may be due to a slow electron-transfer reaction with the
oligomer.39
G3-11T-G3 can be reversibly oxidized to the cation radical
and dication via addition of THIClO4. The evolution of the UV/
visible/near-IR spectra with increasing doping level is shown
in Figure 7. After addition of the first aliquot of THIClO4, the
RC1 and RC2 transitions of G3-11T-G3+• are found at 0.69
and 1.42 eV, respectively. However, at higher oxidation levels,
the two transitions shift more or less continuously to 0.89 and
1.68 eV, when the dication is present. The last spectrum shown
in Figure 7 corresponds to the highest degree of oxidation that
may be reached with THIClO4. A very similar complex behavior
has previously been observed for two other lengthy oligothio-
phenes (e.g. a polystyrene-undecithiophene-polystyrene tri-
block copolymer12 and a tetradodecyl duodecithiophene34,38).
In both cases the initial spectrum of the cation radical shifts
hypsochromically with increasing extent of oxidation. The inset
of Figure 7 shows that the spectrum recorded halfway through
the oxidation process is temperature independent and demon-
strates that the formed redox state has no tendency to dimerize
at low temperature.40 The reason for the complex changes in
Figure 6. UV/visible/near-IR spectra of (a) 9T(B) and (b) 9T(G*2)
as a function of the doping level by the stepwise addition of 2 equiv of
THIClO4 recorded in a 10 mm cell. The graph shows the formation of
cation radicals (RC1 and RC2) at the expense of neutral molecules
(N).
the UV/visible/near-IR spectra with increasing oxidation is a
matter still under debate.34,38 Although it is well-established that
(39) Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New
York, 1986; Vol. 14.

7050 J. Am. Chem. Soc., Vol. 122, No. 29, 2000
Apperloo et al.
Figure 7. UV/visible/near-IR spectra of G3-11T-G3 as a function of
the doping level by the stepwise addition of 2 equiv of THIClO4
recorded in a 10 mm cell. The graph shows the formation of cation
radicals (RC1 and RC2) at the expense of neutral molecules (N) and
the subsequent conversion to dications. The inset shows the variation
of the spectrum with temperature in the range from 280 to 200 K.
a disproportionation equilibrium of two monocation radicals into
a neutral and doubly charged oligomer is operative for these
long oligothiophenes, the hypsochromic shift of the spectrum
can be rationalized in two ways. First, a quadruply charged
spinless π-dimer could be formed in which each molecule carries
two noninteracting charges.38 The second possibility is that the
dicationic state of these lengthy oligothiophenes is a singlet state
(i.e. spinless) carrying two individual polarons.34 Such behavior
has been predicted based on theoretical calculations41 and can
be qualitatively rationalized by the decrease in Coulomb
repulsion that is obtained upon moving the positive polarons
further apart, which, at certain length, outweighs the energy cost
required for creating two polaronic deformations vs a single
bipolaronic structure. In accordance with this proposition, the
bands of the G3-11T-G32+ dication (0.89 and 1.68 eV, n )
11) are found at almost the same position as those of the Ph-
5T-B+• cation radical (0.93 and 1.66 eV, n ≈ 5.5) which has
half the effective conjugation length.
Cyclic voltammetry of the two 17T derivatives in solution
only produced very low currents that could not be analyzed.
Therefore, B-17T-B and G3-17T-G3 were investigated as drop-
cast films deposited on the Pt electrode and cyclic voltammo-
grams were recorded in CH3CN to prevent their dissolution (a
similar procedure is often employed for poly(alkylthiophenes)).42
The cyclic voltammogram of a thin film of B-17T-B shows a
large difference of 210-225 mV between anodic and cathodic
peak currents (Figure 3, Table 1). For G3-17T-G3 the splitting
is even larger amounting to 300 mV. The anodic peak currents
of B-17T-B and G3-17T-G3 scale linearly with the scan rate
in the range of 25 to 200 mV/s, as expected for a thin solid
film on the electrode. In both films, the reversal peak of the
discharging is shifted to lower energy compared to the original
oxidation peak. The same behavior has been observed for short-
chain oligothiophenes and oligophenylenes in the solid state but
Figure 8. UV/visible/near-IR spectra of (a) B-17T-B and (b) G3-17T-
G3 as a function of the doping level by the stepwise addition of 2
equiv of THIClO4 recorded in a 10 mm cell. Graph b shows the
formation of cation radicals (RC1 and RC2) at the expense of neutral
molecules (N) and the subsequent conversion to dications. The inset
of graph b shows the variation of the spectrum with temperature in the
range from 290 to 190 K.
not in solution.43,44 The explanation for this behavior postulated
by Meerholz and Heinze involves a two-step process during
the charging of the oligomer films:43,44 first the molecules
stabilize themselves from an originally twisted conformation
into a more planar, quinoid-like structure with enhanced
π-conjugation, thus resulting in discharge at potentials of lower
energy. The second step involves an intermolecular stabilization
through interactions between neighboring charged segments,
thus leading to increased delocalization and further stabilization.
The UV/visible/near-IR spectra of B-17T-B and G3-17T-
G3 as a function of doping level are shown in Figure 8. The
solubility of B-17T-B in CH2Cl2 is limited and the spectrum of
the neutral compound in Figure 8a reveals that the oligomer is
in an aggregated state even under these conditions. The broad
onset of the π-π* absorption and the appearance of resolved
vibronic fine structure in the absorption band are characteristic
of a system that is not molecularly dissolved. During the
oxidation two transitions are observed that shift to lower and
higher energy with increased doping level. The G3-17T-G3
oligomer dissolves molecularly and thus can be analyzed in
much more detail as a result of the solubilizing effect of the
G3 dendrons. The UV/visible/near-IR spectra of the oxidation
process are shown in Figure 8b. The spectrum recorded after
addition of the first aliquot of THIClO4 shows two transitions
(40) In fact the temperature independence only shows that the π-dimer-
ization equilibrium does not shift in the temperature range. In principle
this does not imply that π-dimers are absent, since a similar result could
have been obtained when all cation radicals are present as π-dimers, even
at the highest temperature.
(41) (a) Tol, A. J. W. Chem. Phys. 1996, 208, 73. (b) Tol, A. J. W.
Synth. Met. 1995, 74, 95.
(42) Zagorska, M.; Pron, A.; Lefrant, S. In Handbook of Organic
ConductiVe Molecules and Polymers; Nalwa, H. S., Ed.; Wiley: New York,
1997; Vol. 3, Chapter 4, p 183.
(43) Meerholz, K.; Heinze, J. Electrochim. Acta 1996, 41, 1839.
(44) Meerholz, K.; Heinze, J. Angew. Chem., Int. Ed. Engl. 1990, 29,
692.
(45) Apperloo, J. J.; Malenfant P. R. L. Unpublished results.

Redox States of Well-Defined π-Conjugated Oligothiophenes
J. Am. Chem. Soc., Vol. 122, No. 29, 2000 7051
with peaks at 0.66 and 1.74 eV (Figure 8b). As anticipated, the
first transition occurs at a position slightly lower in energy than
the RC1 band of G3-11T-G3+• (0.69 eV, Table 2), but the
second transition is unexpectedly at a much higher energy (1.74
vs 1.42 eV, Table 2). A more critical look at the spectra shown
in Figure 7 and Figure 8b reveals that the low-energy onset of
the RC2 band for both G3-11T-G3+• and G3-17T-G3+• is at
∼1.25 eV. The relatively higher energy of the RC2 peak of
G3-17T-G3+• is probably the result of an increased contribution
of the 0-1 or 0-2 vibronic transitions to the absorption band.
In this respect it should also be mentioned that the maximum
of the RC2 band of G3-11T-G3+• is not at a constant value but
shifts from 1.42, to 1.61, and finally to 1.70 eV during the first
three oxidation steps (Figure 7). Such changes do not occur for
G3-17T-G3 for which at least the first five to six spectra have
the same RC2 peak energies. However, beyond this point, a
change in the spectra occurs and the low-energy RC1 band
exhibits a hypsochromic shift whereas the high-energy RC2
band exhibits a bathochromic shift. Variable-temperature UV/
visible/near-IR spectra recorded at the oxidation level where
this transition occurs reveal that the spectrum is temperature
independent in the range from 290 to 190 K. As was the case
for G3-11T-G3+•, this demonstrates that no change in the
π-dimerization equilibrium occurs for G3-17T-G3+• in this
temperature range.40 Furthermore, the bands of the G3-17T-
G32+ dication (0.75 and 1.62 eV, n ) 17) are found at almost
the same position as those of a cation radical possessing half
the conjugation length (i.e. 8T-B+• bands are found at 0.71 and
1.49 eV, n ) 843). This behavior is analogous to that of the
G3-11T-G32+ dication and corroborates the view that long
oligothiophenes accommodate two polarons on a single chain
in their doubly oxidized state.34
proximate linear relation with the reciprocal conjugation length,
in agreement with observations for other oligothiophenes.20,21
These results demonstrate that, apart form the shortest cation
radicals (n ) 4), the principal electroactive properties of the
oligothiophenes are not affected by the poly(benzyl ether)
dendritic wedges, thus presenting an interesting option for
combining optoelectronic properties, processability, and dimen-
sional order in thin films and aggregates. Some clear “den-
drimer-related” effects could be established such as the elec-
trochemical oxidation being kinetically limited when the
dendrons are large in comparison to the electrophore. Further-
more, variable-temperature experiments on the Ph-5T-G3+•
cation radical revealed that π-dimerization is not inhibited by
the dendritic wedges, yet the dimerization enthalpy is reduced.
Furthermore, the low-temperature studies of Ph-5T-G3+• cation
radicals constitute the first example in which a supramolecular
dendritic assembly results from the π-dimerization of oxidized
oligomers. Last, with the chemical oxidation of G3-17T-G3+•
and G3-17T-G32+, we have characterized the longest well-
defined oligothiophene cation radical and dication to date. We
find that the electronic transitions of the doubly oxidized state
of the lengthy oligothiophenes G3-11T-G3 and G3-17T-G3 are
consistent with the view that they carry two independent
polarons rather than one bipolaronic deformation, which is
formed for oligomeric dications with up to nine thiophene units
such as 9T-(B) and 9T-(G*2).34 The rationale for this transition,
which occurs at about 10 thiophene units in oligothiophene
dications, is that the decrease in Coulomb repulsion which is
obtained by moving the two positive charges further apart along
the chain outweighs the energy cost required for creating two
individual geometry deformations on the chain, instead of a
single deformation associated with a bipolaron. Further studies
on the supramolecular aggregation of neutral chains in solution
and thin films are currently underway.
Conclusions
We have shown that dendrimers consisting of oligothiophene
cores and poly(benzyl ether) dendrons can be converted into
stable cation radicals and dications by chemical oxidation,
depending on the oligomer length and the dendron generation.
The first generation (G1) dendritic end groups are effective in
stabilizing the reactive 4T+• cation radical against R-coupling.
For the higher generation end groups (G2 and G3) the
stabilization of 4T+• is less effective. The decreased stabilization
for higher generations is tentatively attributed to an intramo-
lecular redox reaction of the back-folded dendritic wedges with
the rather localized charge of 4T+•. The cation radicals of the
longer oligothiophenes (n ) 5, 9, 11, and 17) functionalized
with dendritic wedges (including those with G3 substitution)
have a lower oxidation potential (Table 1) and are intrinsically
more stable as a result of the increased delocalization of charge
and electron spin. The electronic transitions of the cation radical
and dication of these novel hybrid systems follow an ap-
Acknowledgment. We would like to thank Prof. E. W.
Meijer (TUE) for stimulating discussions and advice. This
research has been supported by The Netherlands Organization
for Chemical Research (CW), The Netherlands Organization
for Scientific Research (NWO) through a grant in the PIONIER
program (J.J.A., R.A.J.J.), the Air Force Office of Scientific
Research through their MURI program, and the National Science
Foundation NSF-DMR No. 9816166 (P.R.L.M., J.M.J.F.). The
preparation of the G3 dendron by Stefan Hecht (Berkeley) is
also acknowledged.
Supporting Information Available: Experimental details
pertaining to the synthesis and characterization of novel
compounds (PDF). This material is available free of charge via
the Internet at http://pubs.acs.org.
JA994259X