polymerization that definitely exceeds the values obtained
from SEC.
Comparison of the optical absorption data for the polymers
in chloroform solution and as films cast from chloroform
solution reveals a significant red-shift in all cases (Table 1).
Typical electrochemical behavior for polythiophene thin
films is revealed by characterization of solution cast thin
films of the PolyDOPTs (Poly4a,b) and PolyDHPTs
(Poly5a,b) on conducting indium tin oxide (ITO) glass plates
(Table 1). The oxidative behavior of Poly4a,b and Poly5a,b
is very similar to poly(3-alkylthiophene)s, whereas thin films
of Poly4a,b upon electrochemical reduction show a distinct
reduction wave, indicating that PolyDOPTs could be poten-
tial carriers of negative charges.
Table 1. Summarized Characteristics of Poly4a,b and
Poly5a,b
Figure 2 illustrates a schematic packing motif, typical of
P oly4a
P oly4b
P oly5a
p oly5b
1
9,20
polythiophenes,
which agrees well with all the observed
λmax solution
λmax film
424 nm
460 nm
4.2 103
60 nm
1.39 V
-1.55 V
23.6 Å
3.45 Å
1.42
434 nm
473 nm
7.0 103
40 nm
1.56 V
-1.65 V
27.8 Å
3.54 Å
1.43
494 nm
508 nm
1.7 103
60 nm
516 nm
533 nm
2.5 103
30 nm
data. In particular, the model shows that efficient π-stacking
is only possible for the PolyDOPTs (Poly4a,b, Figure 2A),
max [M cm-1
]
-
1
ꢀ
3
while the sp -hybridized carbon atoms in the five-membered
film thicknessa
Epa †
Epc b
ring seriously interfere with π-stacking, as evident by the
absence of a diffraction peak corresponding to π-stacking
for the PolyDHPTs (Poly5a,b, Figure 2B).
1.39 V
1.67 V
dlamella
23.9 Å
4.5 Å
0.96
dstack
density [g cm-3]c
4.5 Å
a
Film thicknesses were estimated from Lambert-Beer’s law using the
absorbance at λmax, the extinction coefficient in solution, and the calculated
b
density. Electrochemical experiments were conducted under nitrogen in
dry acetonitrile using tetrabutylammonium tetrafluoroborate (0.10 M) as
the supporting electrolyte, an ITO glass plate as the working electrode, a
coiled platinum wire as the auxiliary electrode, and an Ag/AgCl electrode
c
as the reference system. Densities were calculated from the XRD data
using dstack, dlamella, and 7.6 Å as the three dimensions in a unit cell containing
a dimeric thiophene unit.
This typical behavior for polythiophenes indicates that the
degree of conjugation is increased in the dense thin films
compared to solution due to ordering effects in the solid state.
The red-shift, however, is not as significant as that seen for
regioregular poly(3-alkylthiophene)s, indicating a certain
degree of preaggregation in solution for Poly4a,b and
1
8
Poly5a,b. Corroborating the SEC data and the optical
absorption maxima, we note that the extinction coefficient
per monomer increases with increasing length of the alkyl
chain for both presented polythiophene systems, which
clearly indicates that the grafting of long dodecyl chains onto
the monomers provides a higher degree of polymerization
than what is observed with the shorter octyl substituents.
X-ray powder diffraction gives diffraction patterns typical
for polythiophenes encompassing two to three rather broad
peaks. At low angles, one typically observes a peak corre-
sponding to the lamellar repeat distance between polymers.
At higher angles, one observes one to two peaks correspond-
ing to alkyl chain packing and/or π-stacking, respectively;
these data are presented in Table 1.
Figure 2. Schematic model illustrating the two packing motifs
for PolyDOPTs (A) and PolyDHPTs (B).
(
11) Pomerantz, M.; Amarasekara, A. S. Synth. Met. 2003, 135-136,
Pomerantz has reported on ab initio calculations conclud-
ing that the most stable conformation of the DOPT-methyl
dimer is the coplanar structure due to favorable electrostatic
oxygen-sulfur interactions, while the most stable conforma-
tion of the DHPT-methyl dimer is characterized by a
2
1
57.
(12) MacDowell, D. W. H.; Wisowaty, J. C. J. Org. Chem. 1972, 37,
712.
(
13) Muenster, P.; Freund, W.; Steiner, G.; Walter, H.; Westphalen, K.
O.; Gerber, M. European Patent 407793, 1992.
(
(
(
14) Sic e´ , J. J. Org. Chem. 1954, 19, 70.
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Org. Lett., Vol. 6, No. 19, 2004
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