Thallium(III) Trifluoroacetate-Trifluoroacetic Acid in the Chemistry of Polythiophenes. 2. Treatment of 3-Alkylthiophenes and Electron Paramagnetic Resonance Results

Article abstract of DOI:10.1021/jo961065g

The treatment of thiophene and 3-alkylthiophenes with thallium(III) trifluoroacetate (TTFA) in trifluoroacetic acid (TFA) gives insoluble and dark powdery solids with oxygen content and electrical conductivities ranging from 10^-4 to 10^-6 Ω-1 cm^-1. Polar and short fractions are negligible. All of them show semiconductivity (10^-3Ω-1 cm^-1) when doped in iodine atmosphere. Electron paramagnetic resonance (EPR) spectra of either as-synthesized or I₂-treated solids display characteristic single and broad lines (ΔH_PP, 1.84-7.4 G) with Lorentzian shapes and g-values in the range 2.0028-2.0038. Infrared spectra show characteristic C-H out-of-plane deformations (780 cm^-1 for polythiophene and 820-825 cm^-1 for poly(3-alkylthiophenes)) in addition to a strong peak at 1650-1690 cm^-1 which has not been conclusively assigned. EPR spectra of some disubstituted and tetrasubstituted 2,2′-bithiophene radical cations have been observed and their g-values and coupling constants assigned when the corresponding parent compounds are photolyzed with ultraviolet light in TFA. Photolysis of 3-alkylthiophenes in TFA in the EPR instrument gave the radical cations of 4,4′-dialkyl-2,2′-bithiophenes. In no case, were EPR signals of the isomeric 3,3′-dialkyl- or 3,4′-dialkyl-2,2′-bithiophene radical cations observed, indicating that dimerization of 3-alkylthiophenes occurs through the less sterically hindered 5-position. The presence of two doublet species corresponding to both conformers, syn and anti, in the radical cations is associated with a large barrier to rotation about the C(2)-C(2′) bond.

Full text of DOI:10.1021/jo961065g

878
J . Org. Chem. 1997, 62, 878-884
Th a lliu m (III) Tr iflu or oa ceta te-Tr iflu or oa cetic Acid in th e
Ch em istr y of P olyth iop h en es. 2. Tr ea tm en t of 3-Alk ylth iop h en es
a n d Electr on P a r a m a gn etic Reson a n ce Resu lts
J ordi Tormo, F. J esu´s Moreno, J ordi Ruiz, Llu´ıs Fajar´ı, and Luis J ulia´*
Departament de Quı´mica Orga`nica Biolo`gica, Centre d’Investigacio´ i Desenvolupament (CSIC),
J ordi Girona 18-26, 08034 Barcelona, Spain
Received J une 5, 1996X
The treatment of thiophene and 3-alkylthiophenes with thallium(III) trifluoroacetate (TTFA) in
trifluoroacetic acid (TFA) gives insoluble and dark powdery solids with oxygen content and electrical
conductivities ranging from 10-4 to 10-6 Ω-1 cm-1. Polar and short fractions are negligible. All of
them show semiconductivity (10-3-10-6 Ω-1 cm-1) when doped in iodine atmosphere. Electron
paramagnetic resonance (EPR) spectra of either as-synthesized or I2-treated solids display
characteristic single and broad lines (∆Hpp, 1.84-7.4 G) with Lorentzian shapes and g-values in
the range 2.0028-2.0038. Infrared spectra show characteristic C-H out-of-plane deformations
(780 cm-1 for polythiophene and 820-825 cm-1 for poly(3-alkylthiophenes)) in addition to a strong
peak at 1650-1690 cm-1 which has not been conclusively assigned. EPR spectra of some
disubstituted and tetrasubstituted 2,2′-bithiophene radical cations have been observed and their
g-values and coupling constants assigned when the corresponding parent compounds are photolyzed
with ultraviolet light in TFA. Photolysis of 3-alkylthiophenes in TFA in the EPR instrument gave
the radical cations of 4,4′-dialkyl-2,2′-bithiophenes. In no case, were EPR signals of the isomeric
3,3′-dialkyl- or 3,4′-dialkyl-2,2′-bithiophene radical cations observed, indicating that dimerization
of 3-alkylthiophenes occurs through the less sterically hindered 5-position. The presence of two
doublet species corresponding to both conformers, syn and anti, in the radical cations is associated
with a large barrier to rotation about the C(2)-C(2′) bond.
In tr od u ction
by alkyl groups. Strong signals with good hyperfine
splittings appear in the EPR spectra of these heteroare-
nes. This technique has proved to be a powerful method
of studying the spin distribution in the SOMO.2a,b
Thallium(III) trifluoroacetate (TTFA)-trifluoroacetic
acid (TFA) has been proved to be a good reagent system
to generate radical cations of aromatic substrates1 and
particularly, of five-member heterocyclic compounds,
when these are conveniently blocked in their most
reactive sites, in order to prevent polymerization reac-
tions by oxidative condensation.2
These excellent properties of the TTFA-TFA system
are due not only to the stabilization effect of radical
cations by TFA but to the low nucleophilicity of the
system which prevents any further reaction on the
primary radical cations.3 Thus, if mercury(II) trifluoro-
acetate, also a good oxidant, is used instead of thallium-
(III) trifluoroacetate, some substrates undergo, after
being oxidized to their radical cations, further mercuria-
tion on the aromatic ring.4 However, there is no report
of the observation of the analogous thallation reaction
in experiments with TTFA. Sometimes, when aromatic
substrates have low oxidation potentials, photolysis of
solutions of them in TFA may be sufficient to generate
the radical cations.
More recently, the application of the system TTFA-
TFA in 5,5′-dialkyl-2,2′-bithioph7enes has generated the
radical cations of these simple oligomers. These radical
cations are very stable and do not undergo further
secondary reactions when solutions of these substrates
were conveniently degassed at low temperatures (0 °C).5
But, if the parent compound, 2,2′-bithiophene (1), or
2,2′,5′,2′′-terthiophene (2) were treated under the same
conditions, doped polymeric fractions with good proper-
ties were obtained. This is a good test to confirm that if
polymerization in these substrates occurs, it goes by
means of oxidative condensation of the thiophene oligo-
mers through the reaction at the sterically unhindered
positions of their radical cations.6
In this context, we have been able to obtain and detect
by means of electron paramagnetic resonance (EPR)
many radical cations of furans, thiophenes, and pyrroles
when they are substituted on their C(2) and C(5) carbons
Therefore, TTFA-TFA is also an excellent chemical
method of obtaining semiconducting polymers from
bithiophenes and terthiophenes. These polymers were
characterized by their poor solubility in different organic
solvents because of the rigidity of their carbon-backbone
chains. We will now report on the results when 3-alkyl-
thiophenes and thiophene itself are treated with this
X Abstract published in Advance ACS Abstracts, February 1, 1997.
(1) Elson, I. H.; Kochi, J . K. J . Am. Chem. Soc. 1973, 95, 5060. Lan,
W.; Kochi, J . K. Ibid. 1984, 106, 7100. Lau, W.; Kochi, J . K. Ibid. 1986,
108, 6720.
(2) (a) Davies A. G.; J ulia´, L.; Yazdi, S. N. J . Chem. Soc., Chem.
Commun. 1987, 929. (b) Davies, A. G., J ulia´, L.; Yazdi, S. N. J . Chem.
Soc., Perkin Trans. 2 1989, 239. (c) Avila, D. N.; Davies, A. G. J . Chem.
Soc., Faraday Trans. 1990, 86, 3243.
(5) Alema´n, C.; Brillas, E.; Davies, A. G.; Fajar´ı, Ll.; Giro´, D.; J ulia´,
L.; Perez, J . J .; Rius, J . J . Org. Chem. 1993, 58, 3091.
(6) Gime´nez, E.; Carilla, J .; Fajar´ı, Ll.; Alema´n, C.; J ulia´, L. J . Org.
Chem. 1994, 59, 4630.
(3) Eberson, L.; Radner, F. Acta Chem. Scand. 1992, 46, 630. Davies,
A. G. Chem. Soc. Rev. 1993, 299.
(4) Davies, A. G.; McGuchan, D. C. Organometallics 1991, 10, 329.
S0022-3263(96)01065-1 CCC: $14.00 © 1997 American Chemical Society

Chemistry of Polythiophenes
J . Org. Chem., Vol. 62, No. 4, 1997 879
operating in optimized conditions.8 One of the problems
in the samples reported now is the great difficulty in
undoping them which might suggest that oxygen is
incorporated in the backbone structure as carbonyl
groups.
This behavior is completely different from that ob-
served in polythiophenes when longer oligomers, as 2,2′-
bithiophenes and 2,2′,5′,2′′-terthiophenes, were used as
starting materials and under the same reaction condi-
tions.6 Thus, washing these polymers with hot ethanol
resulted in a practically complete undoping of the samples,
the insoluble products changed their colors from dark
brown to reddish brown, and their elemental composition
after purification with CHCl3, THF, and chlorobenzene
nearly agreed with those of the neutral polymer, the
oxygen content being in negligible proportions.
Ta ble 1. Ch em ica l Com p osition of As-Syn th esized
Sa m p les
anal., % found
monomer
C
H
S
O
Total empirical formulaa
3
4
5
6
53.3 2.0 34.7 8.6
61.3 5.0 27.3 4.7
65.2 6.9 22.6 2.8
71.3 8.6 17.3 2.6
98.6
98.3
97.5
99.8
C4.1H1.8S1.0O0.5
C6.0H5.8S1.0O0.3
C7.7H9.7S1.0O0.2
C11.0H15.8S1.0O0.3
a
Empirical formulas are estimated by assuming one sulfur atom
for repeated unit in the chain.
oxidant system, and we will try to study and characterize
the paramagnetic species when these thiophenes, with
free 2- and 5-positions, are analyzed by means of EPR
spectroscopy.
In the treatment of the thiophene (3) with TTFA-TFA,
if special care is taken to avoid traces of oxygen in the
medium, by argon bubbling the TFA solutions of thiophene
and TTFA before mixing them and then keeping the inert
atmosphere during the process, the reaction practically
does not proceed, and only a very small fraction of
insoluble polymer was obtained. If the starting monomer
is 3-ethylthiophene (4) and the same precautions are
taken to avoid oxygen, the reaction goes on, but in the
purification process of the obtained sample with organic
solvents, the soluble fractions in ethanol, CHCl3, and
chlorobenzene increase in weight. The insoluble final
part is approximately 55% of the total. In this case, the
composition of the soluble fraction in CHCl3 (C, 63.3; H,
5.6; S, 26.1; O, 4.0%) and of the insoluble final part (C,
63.2; H, 5.2; S, 26.1; O, 4.1%) are similar to that displayed
in Table 2. The weight-average molecular weight (Mw)
and the number-average molecular weight (Mn) of the
CHCl3 soluble fraction, determined by gel permeation
chromatography, are 12249 and 4483, respectively, which
implies a polydispersity of 2.7. This Mn corresponds to
an average of 40 thiophenes per chain. Its UV-vis
spectrum in CHCl3 solution has a maximum absortion
Tr ea t m en t of 3-Alk ylt h iop h en es w it h TTF A-
TF A. Syn th esis. Thiophene (3) and its substituted
derivatives, 3-ethyl- (4), 3-butyl- (5), and 3-heptylthio-
phene (6), were treated with thallium(III) trifluoroacetate
in trifluoroacetic acid (1:1, molar proportion) at room
temperature. The reactions were left for 24 h and no
special precautions were taken to avoid oxygen and
moisture during the polymerization. Therefore, the
solutions were not degassed by argon bubbling prior to
treatment, and only an argon atmosphere was main-
tained during the processes.
The initial solution turned dark blue immediately after
adding the TTFA, and the precipitation of a very fine and
dark powder was observed within a few minutes. The
composition and the empirical formulae of these powders
are displayed in Table 1. All of them show the presence
of oxygen in their composition, the highest content being
in the sample prepared from thiophene 3. These results
and the analytical characterization of them, mainly by
electron paramagnetic resonance and conductivity mea-
surements, suggest that a light oxidation, by the forma-
tion of localized polarons or radical cations into the
chains, is induced simultaneously during the chemical
process.
1
at about 385 nm, and its H NMR spectrum (Figure 1)
displays two separated and broad peaks at 2.59 and 2.83
ppm, corresponding to the position of the methylene
groups in the two possible regioisomers in the polymer
backbone structure, the head-to-head and the head-to-
tail placements.
The purified samples from 3, 4, 5, and 6 were then
exposed to iodine vapors at atmospheric pressure to be
further oxidized. Analytical data of the resulting solids
are given in Table 3. All of them show the presence of
variable percentages of iodine and the lowering in the
percentages on carbon, hydrogen, and sulfur. In the
three substituted thiophenes, the longer the alkyl chain
the less the proportion of incorporated iodine. The
empirical formulas are those from the purified samples
adding the corresponding iodine, and the calculated
composition in brackets are estimated from the empirical
formulas which are in good agreement with the experi-
mental values.
The oxidized samples were treated with hot ethanol
in a Soxhlet extractor to remove polar impurities. Only
small quantities of soluble fractions were separated. The
composition of the insoluble fractions (Table 2) are
slightly different from those of the as-synthesized materi-
als. A change of color is not observed during this process
which denotes that the degree of oxidation of the samples
remains nearly unchanged. Even if purification with
ethanol is followed by extraction with CHCl3 and chlo-
robenzene, to remove low and medium molecular weight
fractions, respectively, as it was performed with the
sample obtained from 3-ethylthiophene (4), the separated
soluble fractions are very small and the elemental
composition of carbon, hydrogen, sulfur, and oxygen of
the insoluble part does not differ appreciably from the
as-synthesized material.
(7) Roncali, J . Chem. Rev. 1992, 92, 711, and references cited
therein. Rrische, B.; Zagorska, M.; Hellberg, J . Synth. Met. 1993, 58,
295. Lonarn, G.; Kruszka, J .; Lefrant, S.; Zagorska, M.; Kulszewicz-
Bayer, Y. Synth. Met. 1993, 61, 233. Barbarella, G.; Bongini, A.;
Zambianchi, M. Macromolecules 1994, 27, 3039. Miyazaki, Y.; Yama-
moto, T. Synth. Met. 1994, 64, 69. Chen, T.-A.; Wu, X.; Rieke, R. D. J .
Am. Chem. Soc. 1995, 117, 233. Arbizzani, C.; Bongini, A.; Mestra-
gostino, M.; Zanelli, A.; Barbarella, G.; Zambianchi, M. Adv. Mater.
1995, 7, 571.
The low solubilities of the above purified samples show
an important difference in the properties of the reported
poly(3-alkylthiophenes),7 although some authors suggest
that the solubility of these polymers is very limited when
(8) Roncali, J ., Garreau, R.; Yassar, A.; Marque, P.; Garnier, F.;
Lemaire, M. J . Phys. Chem. 1987, 91, 6706.

880 J . Org. Chem., Vol. 62, No. 4, 1997
Ta ble 2. Ch em ica l Com p osition of P u r ified a Sa m p les
Tormo et al.
anal. % found (calcd)b
S
monomer
C
H
O
total
empirical formula
3
4
5
6
53.5 (58.5)
62.8 (65.4)
67.5 (69.5)
71.3 (73.3)
2.0 (2.5)
5.1 (5.5)
6.8 (7.3)
8.5 (9.0)
35.6 (39.0)
28.1 (29.1)
23.0 (23.2)
17.4 (17.7)
8.4
3.0
2.3
2.0
99.6
99.0
99.6
99.2
C4.1H1.8S1.0O0.5
C6.0H5.8S1.0O0.2
C7.8H9.4S1.0O0.2
C10.9H15.5S1.0O0.2
a
b
Purification by extensive Soxhlet extraction with ethanol (6 h). Calculated as follows: 3, (C4H2S)n; 4, (C6H6S)n; 5, (C8H10S)n; 6,
(C11H16S)n.
Ta ble 3. Ch em ica l Com p osition of Iod in e-Dop ed Sa m p les
anal. % found (calcd)a
monomer
C
H
S
I
empirical formulab
3
4
5
6
46.9 (47.0)
46.1 (46.3)
60.6 (60.3)
71.0 (71.0)
1.7 (1.8)
3.8 (3.8)
6.2 (6.1)
8.5 (8.5)
31.8 (31.6)
20.6 (20.7)
20.7 (20.7)
17.0 (17.0)
12.2 (12.4)
26.9 (26.9)
11.4 (11.2)
1.5 (1.4)
C4.0H1.8S1.0O0.5I0.09
C6.0H5.8S1.0O0.2I0.33
C7.8H9.4S1.0O0.2I0.13
C10.9H15.5S1.0O0.2I0.05
a
b
Calculated compositions are estimated from the empirical formulas. Empirical formulas are those of the purified samples in Table
2 by adding the corresponding iodine.
The principal IR absortion bands observed in the
polythiophene and poly(3-alkylthiophenes) and their
assignments are presented in Table 4. The bands at 780
cm-1, characteristic of C-H out-of-plane deformation of
the 2,5-disubstituted thiophenes, and at 820-825 cm-1
,
of the 2,4,5-trisubstituted thiophenes, indicate a linear
polymer chain structure. The peak assigned to Câ-H
stretching in the range 3060-3080 cm-1 is very weak in
all of them. Besides the normal peaks corresponding to
this kind of polymers, all of them display a strong-
medium peak at 1650 cm-1 in the polythiophene and at
1690 cm-1 in the poly(3-alkylthiophenes). This absortion
is sometimes assigned to an overtone of the thiophene
ring C-H out-of-plane bend.10 When it is strong, it has
been attributed sometimes to the presence of carbonyl
groups,11 although some authors have preferred to assign
it to the aromatic ring stretching.12 Concerning the
soluble fraction in CHCl3, when 3-ethylthiophene is
treated in TFA with TTFA in a rigorously inert atmo-
sphere as mentioned above, its 13C NMR spectrum does
not display any peak corresponding to CdO, despite the
fact that its IR spectrum presents a strong peak at 1690
cm-1. On the other hand, some preliminary results from
the polymerization of 3,4-diethylthiophene with TTFA in
TFA under the same conditions show that although the
IR spectrum of the polymeric insoluble fraction presents
a strong and sharp peak at 1680 cm-1, its elemental
analysis does not indicate any traces of oxygen.
F igu r e 1. 1H NMR spectrum of CHCl3-soluble fraction of poly-
(3-ethylthiophene).
Electr on P a r a m a gn etic Reson a n ce. EPR spectral
data, including g-factors and peak-to-peak line widths are
summarized in Table 5. The spectra of all the polymeric
samples consist of a single and symmetrical line with a
Lorentzian shape corresponding to weak-conducting
samples. The g-values are included in the range 2.0028-
2.0038, comparable to that of a free electron (2.0023).
This indicates a weak spin-orbit coupling with the sulfur
atom, the free electron being mainly delocalized into the
carbon backbone structure, as is typical in this type of
polymer.
F igu r e 2. Infrared spectra in KBr pellets of (a) as-synthesized
polymer and (b) I2-doped polymer from thiophene (3).
In fr a r ed (IR) Sp ectr oscop y. As an example, typical
IR spectra of purified polythiophene before and after
iodine oxidation are displayed in Figure 2. Characteristic
broad bands at approximately 1340, 1200, 1110, and 1030
cm-1, corresponding to oxidized polythiophenes, are
observed both in the purified and in the iodine-doped
polymer. Those bands which have been interpreted on
the basis of bipolarons (dications),9 are broader and
stronger in intensity in the iodine-doped sample than in
the purified one.
Electr ica l Mea su r em en ts. The electrical conductivi-
ties of the polymers were measured either in as-
synthesized or in I2-doped samples using compressed
(10) Bradley, D. D. C.; Friend, R. H.; Lindenberger, H.; Roth, S.
Polymer 1986, 27, 1709.
(11) Tourillon, G.; Garnier, F. J . Electroanal. Chem. 1982, 135, 173.
(12) Wei, Y.; Tian, J . Macromolecules 1993, 26, 457.
(9) Moraes, F.; Schaffer, H.; Kobayashi, M.; Heeger, A. J .; Wudl, F.
Phys. Rev. B 1984, 30, 2948. Cao, Y.; Guo, D.; Pang, M.; Qian, R. Synth.
Met. 1987, 18, 189.

Chemistry of Polythiophenes
J . Org. Chem., Vol. 62, No. 4, 1997 881
Ta ble 4. In fr a r ed Ba n d P osition s (cm -1) a n d Th eir Assign m en ts for P u r ified P oly(3-a lk ylth iop h en es)
monomer
arom C-H str
aliph C-H str
ring str
methyl def
arom C-H out-of-plane
3
4
5
6
3080
3060
3070
3060
1650
1690
1690
1690
1525, 1490
1515, 1455
1510, 1450
1515, 1455
780
825
820
820
2970, 2940, 2880
2965, 2930, 2865
2960, 2920, 2850
1370
1375
1375
a
IR spectra were recorded in KBr pressed pellets.
Ta ble 5. EP R Sp ectr a l P a r a m eter s a n d Electr ica l
Con d u ctivities of Sa m p les Mea su r ed a t Room
Tem p er a tu r es
monomer
sample
as-synthesized 2.0035
I2-doped 2.0036
as-synthesized 2.0032
I2-doped 2.0030
as-synthesized 2.0038
I2-doped 2.0028
as-synthesized 2.0037
I2-doped 2.0031
g
∆Ha (G) σb (Ω-1 cm-1
)
pp
3
1.8
2.8
3.6
6.3
5.1
6.2
7.4
-
3.1 × 10-4
2.6 × 10-3
2.3 × 10-5
2.6 × 10-3
4.6 × 10-6
3.0 × 10-4
2.1 × 10-6
4.7 × 10-6
4
5
6
a
∆Hpp: peak to peak line width of the first derivative of the
b
absortion curve. σ: conductivity measurements at 20 °C on
pressed pellets by the four-probe method.13
pellets at 20 °C by the four-probe method.13 As it is
displayed in Table 5, the longer the alkyl chain of the
substituent the lower the conductivity value.
E lect r on P a r a m a gn et ic R eson a n ce on 3-Alk yl-
th iop h en es a n d 4,4′-Dia lk yl-2,2′-bith iop h en es. Al-
though a variety of methods is available to generate
radical cations, we have found a general and excellent
procedure to generate heteroaromatic radical cations in
fluid solution by photolyzing the substrates in trifluoro-
acetic acid (TFA) with or without mercury(II) or thallium-
(III) trifluoroacetate (TTFA) at low temperatures (-10
°C) or even at room temperature.
To shed further light on the behavior of 3-alkylthio-
phenes in the TTFA-TFA system, we examined very
dilute solutions of these monomers in TFA by EPR
spectroscopy, irradiating them in the same cavity of the
instrument. Therefore, when a solution of 3-ethylthio-
phene in TFA at -10 °C was irradiated, the spectrum in
Figure 3a was obtained. The assymmetry of this spec-
trum indicates the formation of two doublet species.
Photolysis of 4,4′-diethyl-2,2′-bithiophene (7) under the
same conditions displayed the same spectrum but at
higher peak intensity (Figure 3c), although slightly
different as for the composition of the two doublet species.
All of this suggests that excited 3-ethylthiophene under-
goes an oxidative dimerization to the corresponding
bithiophene radical cation. The radical cation of 3-eth-
ylthiophene is not stable enough to be detected by EPR.
This process is carried out on the least sterically hindered
site on the thiophene which corresponds to the 5-position.
The presence of two doublets suggests the formation of
the radical cations of both conformations, syn and anti.
A computer simulation of the spectrum from 3-ethylthio-
phene is shown in Figure 3b, using the hyperfine splitting
(hs) constants and other EPR parameters reported in
Table 6. A small change in the relative amounts of both
conformers simulates the spectrum starting from 7. It
is worthwhile to indicate the stability of these species
(there is not appreciable variation in the intensity of the
signals) and the invariability of the composition of the
F igu r e 3. (a) EPR spectrum of an irradiated solution of
3-ethylthiophene (4) in trifluoroacetic acid (TFA) at -10 °C.
(b) Computer simulation using the parameters given in Table
7 for 4,4′-diethyl-2,2′-bithiophene (7) radical cation. (c) EPR
spectrum of an irradiated TFA solution of 7 at -10 °C.
Ta ble 6. Exp er im en ta l Hyp er fin e Cou p lin g Con sta n ts
(G) for Ra d ica l Ca tion s of 2,2′-Bith iop h en esa
a3,3′
20.75 (2 H) 3.32 (4 H) 20.75 (2 H) 2.0065
20.70 4.10 20.70 2.0062
20.90 (2 H) 3.24 (6 H) 20.90 (2 H) 2.0074
20.85 4.06 20.85 2.0072
20.80 (2 H) 3.10 (2 H) 20.80 (2 H) 2.0064
a4,4′
a5,5′
g
∆Hpp (G)
7
8
9
0.4
0.4
0.4
0.3
20.80
3.60
20.80
2.0063
2.0025
10b 4.95 (6 H)
0.25 (2 H) 7.40 (2 H)
a
Photolysis of 3-alkylthiophenes and 4,4′-dialkyl-2,2′-bithiophenes
b
in TFA. Photolysis of 3,3′-dimethyl-2,2′-bithiophenes in TFA.
radical cations of both conformers in the range of -10
°C to 25 °C.
The same spectra were obtained when dilute solutions
of 3-butyl- and 3-heptylthiophene were irradiated under
the same conditions in TFA giving the radical cations of
the 4,4′-dialkyl-2,2′-bithiophenes in both conformations
(13) Wieder, H. H. Laboratory Notes in Electrical and Galvanomag-
netic Measurements, Materials Science Monographs; Elsevier: Am-
sterdam, 1979; Vol. 2.

882 J . Org. Chem., Vol. 62, No. 4, 1997
Tormo et al.
Ta ble 7. UV-Visible Absor p tion Ma xim a of
2,2′-Bith iop h en es in Cycloh exa n e a n d Tr iflu or oa cetic
Acid
cyclohexane λmax (ꢀa), nm
TFA, λmax, nm
7
8
9
306 (10500)
307 (12300)
309 (10500)
244 (10700)
331, 422
330, 422
339, 426
331, 397
10
a
ꢀ: absorbance (dm3 mol-1 cm-1).
which produced a quintet of quintets for every conformer.
If the starting monomer was 3-methylthiophene or 4,4′-
dimethyl-2,2′-bithiophene (8), an assymmetric spectrum,
indicating the presence of two doublets, was observed
which consisted of a quintet of septets for every conformer
with the hs constants reported in Table 6. When irradia-
tion in TFA solutions was performed on 3-cyclopentylthio-
phene or 4,4′-dicyclopentyl-2,2′-bithiophene (9), the same
assymmetric spectrum was observed; a computer simula-
tion also indicated that a mixture of two conformers
corresponding to the radical cations of the dimer was
obtained (Table 6).
When small quantities of thallium(III) trifluoroacetate
were added to the above solutions, the hyperfine splitting
of the spectra disappeared while a single and broad line
corresponding to polymeric fractions was developed.
Thus, in all cases the radical cations of 3-alkylthio-
phenes, generated in the first stage of the oxidation in
TFA, were too short-lived for EPR detection, and they
rapidly dimerize. Radical cations of the dimers were
relatively persistent, despite the fact that their most
reactive positions for polymerization, C(5) and C(5′), are
not blocked by alkyl groups. This persistence is mainly
due to the very low concentrations of these radical species
in TFA solution. As it was demonstrated for 2,2′-
bithiophene and 2,2′,5′,2′′-terthiophene, neutral dimers
of these substituted bithiophenes are almost complete
protonated in TFA medium, giving their conjugated acids.
The UV-vis absorption maxima of them in cyclohexane
and in TFA are given in Table 7. In all cases, a
considerable bathochromic shift is observed going from
the aprotic solvent to the acidic medium. Therefore, the
concentration of the neutral species which are the active
species to be oxidized by TFA is so low in this medium
that their radical cations persist long enough to be
detected before an oxidative coupling occurs. It is also
noteworthy to point out, in relation to this persistence,
the good properties of TFA in stabilizing radical cations
due to its low nucleophilicity.
The oxidative dimerization of the radical cations of
3-alkylthiophenes mainly occurs at its less sterically
hindered position. The radical cations of the isomeric
dimers, 3,3′-dialkyl- and 3,4′-dialkyl-2,2′-bithiophenes,
have not been observed by EPR, in spite of the fact that
the doublet species of the first isomer has been indepen-
dently generated from 3,3′-dimethyl-2,2′-bithiophene (10)
and has proved to be stable enough in TFA (Figure 4).
Concerning to this radical cation, the EPR spectrum of
it shows a symmetric distribution of lines, with the
coupling constants reported in Table 6, corresponding to
one doublet species, which suggests the presence of only
the anti-conformer, the syn being too unstable to be
present due to the frontal repulsion between the methyl
groups (Scheme 1).
F igu r e 4. (a) EPR spectrum of an irradiated solution of 3,3′-
dimethyl-2,2′-bithiophene (10) in trifluoroacetic acid at -10
°C. (b) Computer simulation using the parameters given in
Table 7.
indicate stronger spin-orbit couplings with the sulfur
atom in the molecule than the normal values for other
isomers, and (2) the relative high spin-coupling constants
with the four ring protons when disubstitution takes
place in the 4,4′-positions. Concerning this, some pre-
liminary results in the observation and detection by EPR
of radical cations in tetrasubstituted 2,2′-bithiophenes
indicate that when substitution takes place in 3,4,3′,4′-
positions, as in 3,4,3′,4′-tetraethyl-2,2′-bithiophene (11)
(Figure 5), the spectra are similar to those from 4,4′-
dialkyl-2,2′-bithiophenes, with a high coupling constant
for 5 and 5′ ring-positions of ∼20-23 G and a high
g-value (2.0071). However when substitution is in 4,5,4′,5′-
positions, as in 4,5,4′,5′-tetramethyl-2,2′-bithiophene (12)
(Figure 6), the highest coupling constant for 5 and 5′ ring-
positions is ∼9 G, and the g-value (2.0031) is closer to
that of the free electron.
The rigidity observed by EPR spectroscopy in these
2,2′-bithiophenes, associated with a large barrier to rotate
about C(2)-C(2′) bond which results in the presence of
both isomers, syn and anti, on the EPR timescale, is in
agreement with MINDO/3 calculations carried out on the
isomeric 5,5′-dialkyl-2,2′-bithiophenes.6 These estima-
tions indicated the existence of large barriers to intramo-
lecular rotation in radical cations and practically negli-
gible barriers in the neutral compounds, as it was noted
From data in Table 6, two remarks deserve special
comments: (1) the relative high-values of g-factors for
the radical cations of 4,4′-dialkyl-2,2′-bithiophenes which

Chemistry of Polythiophenes
J . Org. Chem., Vol. 62, No. 4, 1997 883
Sch em e 1
2,2′-bithiophene (8)16 was synthesized from 3-methylthio-
phene, by lithiation with butyllithium followed by oxidative
coupling with copper chloride.17 3,3′-Dimethyl-2,2′-bithiophene
(10)16 was prepared by iodination of 3-methylthiophene using
the method of Suzuki et al.18 and reductive dimerization of
the 2-iodo-3-methylthiophene according to the method of Iyoda
et al.19 4,5,4′,5′-Tetramethyl-2,2′-bithiophene (12) was pre-
pared by bromination of 3-methylthiophene, followed by meth-
ylation to 2,3-dimethylthiophene and oxidative dimerization
according to the method of Gronowitz et al.20 All these 2,2′-
1
bithiophenes were identified by H NMR spectra.
4,4′-Dieth yl-2,2′-bith iop h en e (7). 7 was prepared accord-
ing to a described procedure.17 A solution of N,N,N′,N′-
tetramethylethylenediamine (12.05 mL; 40.3 mmol) and n-bu-
tyllithium (50.15 mL; 40.3 mmol) in ether (150 mL) was added
dropwise to a stirred solution of 3-ethylthiophene (8.0 g; 36.0
mmol) in ether (250 mL) in inert (Ar) atmosphere and then
refluxed (1 h). After cooling down to -78 °C, CuCl2 (11.8 g,
49.0 mmol) was added, and the mixture was poured into an
excess of diluted aqueous HCl and extracted with more ether.
The organic layer, washed with water and dried over Na2SO4,
was evaporated to give a residue which was chromatographed
on column (silica gel, CCl4) and then recrystallized from
n-pentane at low temperature (-78 °C) to afford 4,4′-diethyl-
2,2′-bithiophene (3.22 g; 40.5%): 1H NMR (CDCl3) δ 7.00 (d,
2H), 6.77 (d, 2H), 2.61 (q, 4H) 1.25 (t, 6H); 13C NMR (CDCl3)
(300 MHz) 145.3, 137.4, 124.5, 118.1, 23.6, 14.5. Anal. Calcd
for C12 H14S2: C, 64.8; H, 6.3; S, 23.0. Found: C, 64.8; H, 6.2;
S, 28.8.
F igu r e 5. EPR spectrum of an irradiated solution of 3,4,3′,4′-
tetraethyl-2,2′-bithiophene in trifluoroacetic acid.
2-Iod o-3,4-d ieth ylth iop h en e. This thiophene was pre-
pared according to a described procedure.18 A mixture of 3,4-
diethylthiophene (1.0 g; 7.13 mmol), periodic acid (0.34 g; 1.5
mmol), and iodine (0.76 g; 3.0 mmol) in CH3COOH-H2O-
H2SO4 (100:20:3) (40 mL) was magnetically stirred and
warmed at 60-65 °C. After 4 h, the reaction mixture was
poured into an excess of ice-water and extracted with ether.
The organic layer was washed with an aqueous solution of
NaHSO3 and then with aqueous Na2CO3 and with water and
evaporated at reduced pressure. Distillation of the residue
(0.5 mmHg; 100 °C) gave 2-iodo-3,4-diethylthiophene (1.24 g;
65.2%): 1H NMR (CDCl3) δ 7.06 (s, 1H), 2.65-2.50 (m, 4H),
1.24 (t, 3H); 13C NMR (CDCl3) (200 MHz) 146.8, 143.0, 124.7,
74.2, 23.9, 22.6, 13.9, 13.7, and the residue was 2,5-diiodo-
3,4-diethylthiophene (0.37 g; 13.3%): mp 68-70 °C; 1H NMR
(CDCl3) δ 2.62 (q, 8H, J ) 7.6 Hz), 1.08 (t, 6H, J ) 7.6 Hz));
13C NMR (CDCl3) (200 MHz) 147.1, 77.3, 24.9, 14.4.
F igu r e 6. EPR spectrum of an irradiated solution of 4,5,4′,5′-
tetramethyl-2,2′-bithiophene in trifluoroacetic acid.
earlier by Alberti et al. applying the McLachlan proce-
dure on short oligomeric thiophenes.14
Exp er im en ta l Section
Melting points were obtained by using a Ko¨fler microscope
“Reichert” and are uncorrected. The IR spectra were recorded
with a Perkin Elmer Model 683 spectrometer, and the UV-vis
spectra with a Perkin Elmer Lambda Array 3840 spectrometer
coupled with a Perkin Elmer 7300 computer. 1H and 13C NMR
spectra were determined at 200 MHz and 300 MHz with
Varian Gemini 200 HC and XL-300 spectrometers, respec-
tively. The electrical conductivities were determined on
compressed pellets by van der Pauw’s four-probe method.
The starting monomers, 3-ethylthiophene, 3-n-butylthio-
phene, and 3-n-heptylthiophene, were synthesized via the
coupling of alkylmagnesium bromides with 3-bromothiophene
in the presence of Ni(dppp)Cl2 according to the method of
Zimmer et al.15 and were distilled before use. 4,4′-Dimethyl-
3,4,3′,4′-Tet r a et h yl-2,2′-b it h iop h en e (11). This bithio-
phene 11 was prepared according to a described procedure.21
A mixture of 2-iodo-3,4-diethylthiophene (1.0 g) and copper
powder (2.56 g) was heated at 200 °C for 20 min. After cooling,
the contents of the flask were extracted with hot CHCl3. The
organic solution was evaporated off and the residue in n-
(16) Krische, B.; Hellberg, J .; Lilja, C. J . Chem. Soc., Chem.
Commun. 1987, 1476.
(17) Zago´rska, M.; Krische, B. Polymer 1990, 31, 1379.
(18) Suzuki, H.; Nakamura, K.; Goto, R. Bull. Chem. Soc. J pn. 1966,
39, 128.
(14) Alberti, A.; Favaretto, L.; Seconi, G. J . Chem. Soc., Perkin
Trans. 2 1990, 931.
(15) van Pharm, Ch.; Mark, H. B. J r., Zimmer, H. Synth. Commun.
1986, 16, 689.
(19) Iyoda M.; Sato, K.; Oda, M. Tetrahedron Lett. 1985, 26, 3829.
(20) Wiersema, A.; Gronowitz S. Acta Chem. Scand. 1970, 24, 2593.
(21) Uhlenbroek, J . H.; Bijloo, J . D. Rec. Trav. Chim. Pays-Bas 1960,
79, 1181.

884 J . Org. Chem., Vol. 62, No. 4, 1997
Tormo et al.
hexane was passed through a column (silica gel). The evapo-
and with an excess of water, and dried in a desiccator (18 h)
and at reduced pressure (0.1 mmHg; 60 °C; 2 h).
F r om th iop h en e (3): 3 (1.03 g; 12 mmol) and TTFA (6.67
g; 12 mmol). A dark precipitate (0.65 g) was obtained.
Analytical data are given in Table 1.
F r om 3-eth ylth iop h en e (4): 4 (0.68 g; 6 mmol) and TTFA
(3.3 g; 6 mmol). A dark precipitate (0.47 g) was obtained.
Analytical data are given in Table 1.
F r om 3-n -bu th ylth iop h en e (5): 5 (0.57 g; 4 mmol) and
TTFA (2.19 g; 4 mmol). A dark precipitate (0.31 g) was
obtained. Analytical data are given in Table 1.
ration to dryness gave 3,4,3′,4′-tetraethyl-2,2′-bithiophene
1
(0.19 g; 36.5%): mp 30-2 °C (from methanol); H NMR (300
Mhz) (CDCl3) δ 6.97 (d, 2H, J ’)1.2 Hz), 2.60 (qd, 4H, J )7.5
Hz, J ’)1.2 Hz, 4,4′ CH2), 2.49 (q, 4H, J )7.5 Hz, 3,3′ CH2), 1.31
(t, 6H, J )7.5 Hz), 1.04 (t, 6H, J )7.5 Hz); 13C NMR (CDCl3)
143.5, 142.6, 129.6, 119.6, 22.3, 20.6, 15.0, 13.6. Anal. Calcd
for C16H22S2: C, 69.0; H, 8.0; S, 23.0. Found: C, 69.1; H, 7.9;
S, 23.0.
4,4′-Dicyclopen tyl-2,2′-bith ioph en e (9). This bithiophene
9 was prepared according to a described procedure.17
A
solution of N,N,N′,N′-tetramethylethylendiamine (0.62 mL; 4.2
mmol) and n-buthyllithiun (1.6 mL; 2.5 M in hexane) in ether
(7.5 mL) was added dropwise to a stirred solution of 3-cyclo-
pentylthiophene (0.5 g; 3.28 mmol) in ether (12.5 mL) in an
inert (Ar) atmosphere and then refluxed (1 h). After cooling
down to -78 °C, CuCl2 (0.6 g; 4.46 mmol) was added, and the
mixture was stirred overnight at room temperature. The
mixture was poured into an excess of diluted aqueous HCl and
extracted with more ether. The organic layer, washed with
water and dried over Na2SO4, was evaporated to give a residue
which was chromatographed on column (silica gel, n-hexane)
to afford 4,4′-dicyclopentyl-2,2′-bithiophene (0.117 g; 24%): mp
47-50 °C (from ethanol) 1H NMR (CDCl3) δ 7.01 (s, 2H), 6.79
(s, 2H), 3.00 (q, 2H), 2.06-2.00 (m, 4H), 1.79-1.56 (m, 12H);
13C NMR (CDCl3) 148.1, 137.4, 124.0, 117.5, 41.4, 33.8, 25.1.
Anal. Calcd for C18H22S2: C, 71.5; H, 7.3; S, 21.2. Found: C,
71.5; H, 7.3; S, 20.8.
F r om 3-n -h ep tylth iop h en e (6): 6 (1.64 g; 9 mmol) and
TTFA (4.96 g; 9 mmol). A dark precipitate (1.33 g) was
obtained. Analytical data are given in Table 1.
Tr ea tm en t of P olym er ic Sa m p les w ith Eth a n ol. Gen -
er a l P r oced u r e. The samples were purified by the use of a
Soxhlet extractor with ethanol at reflux during 8 h. The
soluble fractions gave, by evaporation to dryness, small
residues: 3% from 3, 4, and 6, and 4% from 5. Analytical data
of the residues from the four samples are given in Table 2.
F r om th iop h en e (3): IR (KBr) 3080 (vw), 1650 (s), 1525
(w), 1490 (m), 1415 (s), 1350 (m), 1280 (w), 1215 (m), 1170
(w), 1120 (m), 1035 (m), 825 (w), 780 (vs), 730 (w), 690 (m)
cm-1
.
F r om 3-eth ylth iop h en e (4): IR (KBr) 3060 (w), 2970 (s),
2940 (s), 2880 (m), 1780 (w), 1690 (s), 1515 (w), 1455 (s), 1370
(m), 1310 (w), 1250 (w), 1180 (m), 1045 (m), 1015 (w), 825 (vs),
770 (w) cm-1
.
EP R Exp er im en ts. EPR spectra were recorded using a
Varian E 109 spectrometer working in the X-band and using
a Varian E-257 temperature-controller. The EPR simulations
were carried out with a Hewlett-Packard 9835-B computer
using a modified version of the software package of a Varian
E-935 Data Acquisition System.
F r om 3-n -bu th ylth iop h en e (5): IR (KBr) 3070 (w), 2965
(s), 2930 (s), 2865 (s), 1775 (w), 1690 (s), 1555 (w), 1510 (w),
1450 (s), 1375 (m), 1295 (w), 1245 (w), 1210 (w), 1170 (m), 1095
(m), 1070 (w), 925 (w), 890 (w), 820 (s), 785 (m), 735 (w), 715
(w) cm-1
.
F r om 3-n -h ep tylth iop h en e (6): IR (KBr) 3060 (vw), 2960
Spectra of polymeric samples were recorded at room tem-
perature. To detect the radical cations of 2,2′-bithiophenes,
solutions of the substrates, substituted thiophenes and 2,2′-
bithiophenes, in TFA were degassed by passing a stream of
dry argon through the solutions to remove oxygen, and then
were photolyzed in the same cavity of the spectrometer at
variable temperature using the light from an Oriel 500-W high
pressure mercury arc.
Tr ea tm en t of 3-Alk ylth iop h en es w ith Th a lliu m (III)
Tr iflu or oa ceta te (TTF A) in Tr iflu or oa cetic Acid (TF A).
Gen er a l P r oced u r e. A (1:1 molar proportion) solution of
TTFA in TFA (30 mL) was added dropwise at room temper-
ature to a stirred solution of monomer (0.5-1.5 g) in TFA (20
mL) in an inert (Ar) atmosphere. The mixture was stirred
for another 1 h and then at reflux for additional time (24 h).
The precipitate was separated by filtration, washed with TFA
(s), 2920 (s), 2850 (s), 1690 (s), 1515 (w), 1455 (5), 1375 (m),
1340 (vw), 1190 (w), 1110 (w), 880 (vw), 820 (s), 715 (m) cm-1
.
Dop in g of Sa m p les w ith I2. Gen er a l P r oced u r e. The
purified samples were exposed to I2 vapors at atmospheric
pressure and at room temperature (22 h) and then dried in
vacuum (0.1 mmHg; 60 °C; 2 h). Analytical data of the iodided-
samples are given in Table 3.
Ack n ow led gm en t. Support of this research by
DGICYT of MEC (Spain) through project PB92-0031.
The authors express their gratitude to the EPR service
of Centre d’Investigacio´ i Desenvolupament (CSIC) in
Barcelona for all the facilities offered in obtaining the
EPR spectra presented here.
J O961065G