Bridged Dithienylethylenes as Precursors of Small Bandgap Electrogenerated Conjugated Polymers

Article abstract of DOI:10.1021/jo9623447

Bridged dithienylethylenes (DTEs) bearing solubilizing alkyl chains at various positions (2-5) have been synthesized by McMurry dimerization of cyclopenta[b]thiophen-6-ones. In order to introduce alkyl substituents at different positions of the DTE molecule, the precursor ketones have been prepared by different strategies based on a combination of Mannich or Wittig - Horner reaction and Friedel - Craft intramolecular cyclization. The position and the length of the alkyl substituents exert a strong effect on the ability of the precursor to undergo electrochemical polymerization. Thus, whereas substitution at the α-position of the ethylene linkage (3) results in a rapid inhibition of electropolymerization, introduction of alkyl chains at the β-position (4, 5) greatly improves the efficiency of the polymerization process. The analysis of the electrochemical and optical properties of the polymers shows that rigidification of the DTE molecule leads to a significant decrease of the oxidation potential and bandgap. A comparative analysis of DTE and its bridged analogues by means of X-ray diffraction reveals, in agreement with experimental and theoretical results, that the observed reduction of both the HOMO - LUMO gap of the precursor and the bandgap of the corresponding polymers are related to a relaxation of bond length alternation in the DTE moiety.

Full text of DOI:10.1021/jo9623447

J . Org. Chem. 1997, 62, 2401-2408
2401
Br id ged Dith ien yleth ylen es a s P r ecu r sor s of Sm a ll Ba n d ga p
Electr ogen er a ted Con ju ga ted P olym er s
Philippe Blanchard, Hugues Brisset, Bertrand Illien, Ame´de´e Riou, and J ean Roncali*,†
Inge´nierie Mole´culaire et Mate´riaux Organiques, CNRS, UMR 6501, Universite´ d’Angers, 2 Bd Lavoisier,
F-49045 Angers Cedex, France
Received December 13, 1996X
Bridged dithienylethylenes (DTEs) bearing solubilizing alkyl chains at various positions (2-5) have
been synthesized by McMurry dimerization of cyclopenta[b]thiophen-6-ones. In order to introduce
alkyl substituents at different positions of the DTE molecule, the precursor ketones have been
prepared by different strategies based on a combination of Mannich or Wittig-Horner reaction
and Friedel-Craft intramolecular cyclization. The position and the length of the alkyl substituents
exert a strong effect on the ability of the precursor to undergo electrochemical polymerization. Thus,
whereas substitution at the R-position of the ethylene linkage (3) results in a rapid inhibition of
electropolymerization, introduction of alkyl chains at the â-position (4, 5) greatly improves the
efficiency of the polymerization process. The analysis of the electrochemical and optical properties
of the polymers shows that rigidification of the DTE molecule leads to a significant decrease of the
oxidation potential and bandgap. A comparative analysis of DTE and its bridged analogues by
means of X-ray diffraction reveals, in agreement with experimental and theoretical results, that
the observed reduction of both the HOMO-LUMO gap of the precursor and the bandgap of the
corresponding polymers are related to a relaxation of bond length alternation in the DTE moiety.
In tr od u ction
conjugated polymers.1a-e While this research area re-
mains very active,5 the past few years have witnessed
the emergence of alternative approaches such as the
introduction of electron-withdrawing or electron-releasing
groups at selected positions of the conjugated chain1c,g,3,6
or, more recently, the rigidification of the π-conjugated
system.1f,7
The synthesis of narrow bandgap linear π-conjugated
systems (LCSs) has become one of the main challenges
for synthetic chemists involved in the field of organic
conductors.1 Since Eg governs the intrinsic electronic
properties of the π-conjugated system, progress in this
area constitutes the key of many fundamental and
technological problems. Thus, in addition to the long
term possibility of achieving intrinsic metallic conductiv-
ity, the search for small bandgap LCSs is motivated by
the optimization of other properties such as the increase
of the number of charge carriers in organic field-effect
transistors,2 the possibility to use stable metal electrodes
for the fabrication of organic light emitting diodes,3 or
the improvement of electron transmission efficiency when
LCSs are used as π-conjugating spacers in molecules for
nonlinear optics.4
Previous works have shown that the bandgap of poly-
(thienylenevinylene) (PTV) (1.70-1.80 eV)8 is signifi-
cantly smaller than that of poly(thiophene) (PT) (2.00-
2.20 eV).9 This lower bandgap is related to the presence
of ethylene linkages of defined configuration that (i) limit
the statistical rotational disorder associated with the
interannular rotations around single bonds in PT10 and
(ii) decrease the overall aromatic character of the π-con-
jugated backbone and thus contribute to minimizing
π-electrons confinement within the aromatic thiophene
rings.11
Since the first synthesis of poly(isothianaphthene)1a the
increase of the quinonoid character of the conjugated
backbone to the detriment of its aromaticity has repre-
sented the main strategy for synthesizing small bandgap
In this context, trans-dithienylethylenes (DTEs) have
recently attracted particular attention as precursors of
π-conjugated polymers that exhibit some structural
features of PTV while retaining the advantages of the
†
To whom correspondence should be addressed. E-mail:
J ean.Roncali@univ-angers.fr.
(5) (a) Lorcy, D.; Cava, M. P. Adv. Mater. 1992, 4, 562. (b) Ba¨uerle,
P. Go¨tz, G.; Emerle, P.; Port, H. Adv. Mater. 1992, 4, 564.
(6) (a) Ferraris, J . P.; Lambert, T. M. J . Chem. Soc., Chem. Commun.
1991, 1268. (b) Havinga, E. E.; Ten Hoeve, W.; Wynberg, H. Synth.
Met. 1993, 55-57, 299. (c) Ho, A. H.; Brisset, H.; Fre`re, P.; Roncali, J .
J . Chem. Soc., Chem. Commun. 1995, 2309. (d) Kozaki, M.; Tanaka,
S.; Yamashita, Y. J . Org. Chem. 1994, 59, 442. (e) Kitamura, C.;
Tanaka, S.; Yamashita, Y. Chem. Mater. 1996, 8, 570. (f) Ho, A. H.;
Brisset, H.; Elandaloussi, E.; Fre`re, P.; Roncali, J . Adv. Mater. 1996,
8, 990.
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Roncali, J . J . Chem. Soc., Chem. Commun. 1994, 1765. (b) Roncali, J .;
Thobie-Gautier, C. Adv. Mater. 1994, 6, 841.
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X Abstract published in Advance ACS Abstracts, March 15, 1997.
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S0022-3263(96)02344-4 CCC: $14.00 © 1997 American Chemical Society

2402 J . Org. Chem., Vol. 62, No. 8, 1997
Blanchard et al.
Ch a r t 1
Ch a r t 2
Sch em e 1a
electrochemical polymerization.12 Thus, the chemical or
electrochemical polymerization of DTEs bearing solubi-
lizing alkyl chains at the 3,3′-positions of the thiophene
rings leads to polymers with bandgaps around 1.70 eV
a
Reagents: (a) Na/Hg, (b) SOCl2, (c) AlCl3, (d) TiCl4/Zn.
and conductivities of ∼10-2 S cm-1 12
.
On the other hand,
Sch em e 2a
it has been shown that electropolymerization of DTEs or
of related oligomers bearing a cyano group at the ethyl-
ene linkage leads to conjugated polymers exhibiting both
high electron affinity and reduced bandgaps.6c,f
We have shown recently that the covalent bridging of
the thiophene rings with the central ethylene linkage of
DTE leads to a reduction from 1.80 to 1.40 eV of the
bandgap of the corresponding polymer.13 However, this
structural modification also produces a considerable
decrease of the solubility of the precursor, which has
deleterious consequences for the efficiency of the elec-
tropolymerization process.
a
Reagents: (d) TiCl4/Zn, (e) Me2NH,HCl/HCHO, (f) ∆, (g)
In an attempt to solve this problem, we report here
the synthesis of bridged DTEs 2-5 (Chart 1) in which
solubilizing alkyl groups have been attached at various
positions of the molecule.
The characterization of the electrochemical and optical
properties of the resulting polymers using DTEs 1 and 2
as reference compounds shows that the length and the
position of the alkyl chain exert a strong effect on the
polymerization reaction and on the structure and proper-
ties of the polymer. These results are discussed in the
light of the crystal structures of the precursors and of
the results of MO calculations.
H2SO4.
gation. We therefore focused on the two other possible
sites R2 and R3 (3-5) (Chart 1).
While the various bridged DTEs have been synthesized
by McMurry dimerization14 of the corresponding cyclo-
penta[b]thiophen-6-one (Schemes 1-3), each type of
substitution implies the definition of the approriate
synthetic strategy for the preparation of the precursor
ketone. 1,2-Bis(2-thienyl)ethylene (1) was prepared ac-
cording to the method described in the literature.15
Cyclopenta[b]thiophen-6-one (15) was synthesized ac-
cording to the procedure described by McDowell et al.16
3-(3-Thienyl)propionic acid (17) obtained by reduction of
3-(3-thienyl)acrylic acid (18) by sodium amalgam was
converted into the corresponding acid chloride (16), which
was then subjected to intramolecular Friedel-Crafts
acylation to afford the target compound (Scheme 1).
Attempts to synthesize ketone 11 with a methyl group
at the R3 position by direct methylation of 15 with
iodomethane in the presence of potassium tert-butylate
gave mixtures of mono- and dimethylated products that
were very difficult to separate. In order to solve this
Resu lts a n d Discu ssion
Syn th esis of th e DTE P r ecu r sor s. Substitution of
the bridged DTE molecule 2 can be envisioned at three
different positions R1-R3 (Chart 2).
Since subsequent polymerization implies the formation
of thiophene-thiophene linkages, substitution at R1 will
result in a strong steric interaction between the substit-
uents grafted on adjacent thiophene rings with a depar-
ture from planarity and hence a loss of effective conju-
(12) (a) Galarini, R.; Musco, A.; Pontellini, R.; Bolognesi, A.; Catel-
lani, M.; Destri, S.; Mascherpa, M.; Zhou, G. J . Chem. Soc., Chem.
Commun. 1991, 394. (b) Catellani, M.; S. Luzzati, S.; Musco, A.;
Speroni, F. Synth. Met. 1994, 62, 223. (c) Onoda, M.; Iwasa, T.; Kawai,
T.; Yoshino, K. Synth. Met. 1993, 55-57, 1614.
(14) McMurry, J . E. Chem. Rev. 1989, 89, 1513.
(15) Miller, R. E.; Nord, F. F. J . Org. Chem. 1951, 16, 1380.
(16) MacDowell, D. W. H.; Patrick, T. B.; Frame, B. K.; Ellison, D.
L. J . Org. Chem. 1967, 32, 1227.
(17) (a) Blicke, F. F.; Burckhalter, J . H. J . Am. Chem. Soc. 1942,
64, 451. (b) Burckhalter, J . H.; Sam, J . J . Am. Chem. Soc. 1951, 73,
4460.
(13) Roncali, J .; Thobie-Gautier, C.; Elandaloussi, E.; Fre`re, P. J .
Chem. Soc., Chem. Commun. 1994, 2249.

Small Bandgap Electrogenerated Conjugated Polymers
J . Org. Chem., Vol. 62, No. 8, 1997 2403
Sch em e 3a
Ta ble 1. Electr och em ica l a n d UV-vis Sp ectr a l Da ta for
Com p ou n d s 1-5
compd
λmax (nm)
Epa1 (V/SCE)
1
2
3
4
5
330, 341, 360
331, 348, 367
333, 350, 369
331, 348, 368
332, 349, 369
1.10
0.72
0.78
0.73
0.72
observed for 2,13 a strong intensification of the resolution
of the vibronic fine structure typical of rigid conjugated
systems.19 The band maxima undergo a 7-8 nm batho-
chromic shift indicative of an extension of π-electron
delocalization, while as already observed for other rigidi-
fied π-conjugated systems,7,13 these changes are ac-
companied by the emergence of a new absorption band
of weak intensity extending up to the 460-470 nm
region.
P r ep a r a tion a n d P r op er ties of th e P olym er s. The
electrochemical polymerization of aromatic-like precur-
sors involves their electrooxidation into the corresponding
cation radical followed by a succession of chemical (C)
and electrochemical (E) steps according to a (ECCE)n
scheme.9,20 In addition to the chemical reactivity of the
cation radical, the efficiency of electropolymerization
depends also on the solubility of the precursor molecule
and on that of the various oxidized intermediates in-
volved in the polymerization process. However, since
oxidation to the cation radical state generally results in
a decrease in solubility, it is clear that a poorly soluble
precursor will lead to a limited number of couplings, thus
preventing the formation of extensively conjugated poly-
mer chains. In the extreme case, highly conjugated and
poorly soluble molecules lead to the electrodeposition of
the cation radical salt of the precursor itself, as already
observed for sexithienyls.21 In this context, it is clear
that, due to their low oxidation potential and hence to
the low reactivity of their cation radicals, electropoly-
merization of DTEs, and especially the bridged ones, can
be expected to be more difficult than that of thiophene
monomers.
a
Reagents: (a) Na/Hg, (b) SOCl2, (c) AlCl3, (d) TiCl4/Zn, (h)
n-BuMgBr, (i) CrO3, (j) (EtO)2P(O)CH2CO2Et.
problem, we adopted another synthetic strategy based
on the method of Burckhalter et al.17 A Mannich reaction
between 2-propionylthiophene, formaldehyde, and dim-
ethylamine chlorohydrate gave the salt 13, which, upon
distillation under reduced pressure, afforded the unsat-
urated ketone 12. Due to its strong propensity to
polymerize, this latter compound is immediately engaged
in the last step, which involves intramolecular cyclization
in the presence of concentrated sulfuric acid (Scheme 2).
Ketones 6a and 6b, precursors of bridged DTEs 4 and
5 substituted at R2 with methyl and butyl chains, have
been prepared using 3-acetylthiophene (9a ) and 3-pen-
tanoylthiophene18 (9b) as starting material. This latter
compound has been prepared by reaction of n-butylmag-
nesium bromide on 3-thiophenecarboxaldehyde (10) fol-
lowed by oxidation of the resulting secondary alcohol with
chromium trioxide (Scheme 3).
Figure 1 shows the cyclic voltammograms (CV) corre-
sponding to the potentiodynamic polymerization of 2-5
by application of recurrent potential scans. In each case,
the first CV trace shows an irreversible anodic wave
corresponding to the oxidation of the monomer into its
cation radical. Subsequent cycling leads to the emer-
gence, at less positive potentials, of a new redox system
associated with the oxidation and reduction of the
polymer grafted onto the electrode surface. For the
unsubstituted DTE 2, the CV shows narrow anodic and
cathodic waves indicative of the formation of a single
compound of defined structure. The polymerization CV
of 3 and 4 show that introduction of a methyl group in
the DTE structure leads to a broadening of the wave and
to a slight negative shift of their maximum. However,
for 3, the increase of the wave intensity stops after 12-
15 cycles, revealing an inhibition of the polymerization
process. Comparison of the polymerization CV of 5 with
the previous ones shows that the grafting of the n-butyl
chain induces a much larger negative shift of the polymer
redox system, suggesting the formation of a more con-
jugated polymer.
Wittig-Horner olefination of 9a and 9b with triethyl
phosphonoacetate in the presence of n-butyllithium af-
forded the corresponding R-unsaturated esters 8a and 8b
as a mixture of Z and E isomers. Saponification of the
ester group followed by reduction of the double bond with
sodium amalgam and acidification gave the correspond-
ing carboxylic acids 7a and 7b. Subsequent conversion
into the corresponding acid chlorides and intramolecular
Friedel-Crafts acylation yielded the cyclic ketones 6a
and 6b as racemic mixtures. McMurry reaction of
ketones 11, 6a , and 6b gave the substituted target
molecules 3-5 as a mixture of diastereomers.
Table 1 lists the electrochemical and electronic absorp-
tion data of the various DTEs. The values of the anodic
peak potentials (Epa1) determined by single scan cyclic
voltammetry clearly show that the bridging of the
thiophene ring to the ethylene linkage leads to a consid-
erable negative shift of Epa1 from 1.10 to ca. 0.70 V,
indicative of a parallel decrease in oxidation potential.
Comparison of the electronic spectra of the various
bridged DTEs with that of 1 reveals, as previously
(18) Campaigne, E.; Thomas, H. L. J . Am. Chem. Soc. 1955, 77,
5365.
(19) J affe´, H. H.; Orchin, M. Theory and Applications of Ultraviolet
Spectroscopy Wiley: New York 1962.
(20) Heinze, J . Topics in Current Chemistry; Springer-Verlag:
Berlin Heidelberg, 1990; Vol. 152.
(21) Roncali, J .; Giffard, M.; J ubault, M.; Gorgues, A. J . Electroanal.
Chem. 1993, 361, 185.

2404 J . Org. Chem., Vol. 62, No. 8, 1997
Blanchard et al.
F igu r e 2. Cyclic voltammograms of the polymers electrode-
posited on Pt under the conditions of Figure 1. Electrolytic
medium 10-1 M TBHP-MeCN, scan rate 100 mV s-1: (a) poly-
(1), (b) poly(2), (c) poly(4), and (d) poly(5).
Ta ble 2. Electr och em ica l a n d UV-vis Sp ectr a l Da ta for
P oly(1-5)
precursor
Epa (V/SCE)
λmax (nm)
Eg (eV)
1
2
3
4
5
0.98
0.60
0.70
0.56
0.34
500
500
430
470
486
1.80
1.30-1.40
F igu r e 1. Potentiodynamic electrodeposition curves for the
polymers (a) 2, (b) 3, (c) 4, and (d) 5. 10-2 M substrate in 0.1
1.30-1.40
1.30-1.40
M TBHP-MeCN, scan rate 100 mV s-1
.
1000 nm region. Similar spectral features have already
been observed for other small bandgap polymers.1c,f,6a,c,d
Although the shape of these spectra makes it difficult, a
precise determination of the bandgap (Eg) values around
1.30-1.40 eV can be estimated from the long-wavelength
absorption onset.
Figure 2 shows the CVs of the polymers derived from
1, 2, 4, and 5 recorded in a monomer-free electrolytic
medium. Comparison of the CVs of poly(1) and poly(2)
shows that the rigidification of the DTE molecule con-
siderably improves the symmetry of the anodic and
cathodic waves and hence the electrochemical reversibil-
ity. Concurrently, Epa1 shifts negatively from 1.00 to
0.60 V while a new weak anodic shoulder can be
discerned around 0.30-0.40 V. The introduction of a
methyl group in the structure leads to an intensification
of this shoulder and to a decrease of Epa to 0.56 V.
Finally, the CV of poly(5) shows that the first wave
becomes predominant while Epa undergoes a further
negative shift to 0.34 V, which is, to our knowledge, the
lowest value ever reported for a poly(DTE) (Table 2).
This steady decrease of Epa from poly(1) to poly(5)
suggests that the grafting of alkyl chains on the rigid
DTE backbone considerably improves the efficiency of the
electropolymerization process, thus allowing the forma-
tion of more extended π-conjugated structure.
Figures 3 and 4 show the electronic absorption of the
undoped polymers on ITO. The spectrum of poly(1)
shows a λmax at 500 nm and an absorption edge at 685
nm corresponding to a bandgap (Eg) of ca. 1.80 eV, in
agreement with previous works.12,13 In addition to a main
band with a λmax around 500 nm, the spectra of the
polymers derived from the rigid DTEs exhibit an ad-
ditional broad absorption band extending to the 900-
A further comparison of the spectra of Figures 3 and 4
shows that the intensity of the near-IR absorption is
significantly lower for poly(4) and poly(5) than for poly-
(2). This result appears in contradiction with the exten-
sion of conjugation length indicated by the low Epa values
for poly(4) and poly(5). Consequently, it seems rather
unlikely that these different spectral features exactly
reflect the intrinsic electronic properties of the polymers.
In fact, electrodeposition of homogeneous films of poly-
(4) and poly(5) on ITO was found considerably more
problematic than for poly(1) and poly(2). Although the
origin of this difficulty is not clearly understood, it seems
plausible that the presence of alkyl substituents in a
plane orthogonal to that of the π-conjugated system can
perturb the grafting of the polymer onto the ITO surface.
Such a behavior has already been reported for other
nonplanar monomers.22 Another possible relevant factor
could involve the increase of the lipophilicity of the
(22) Roncali, J .; Garnier, F.; Garreau, R.; Lemaire, M. J . Chem. Soc.,
Chem. Commun. 1987, 1500.
(23) Roncali, J .; Garreau, R.; Yassar, A.; Marque, P.; Garnier, F.;
Lemaire, M. J . Phys. Chem. 1987, 91, 6706.

Small Bandgap Electrogenerated Conjugated Polymers
J . Org. Chem., Vol. 62, No. 8, 1997 2405
F igu r e 5. ORTEP view of 3.
F igu r e 3. Electronic absorption spectra of the undoped
polymers on ITO: (a) poly(1) and (b) poly(2).
deposits was measured on compacted pellets. While poly-
(2) shows a four-probe conductivity of 1-2 × 10-2 S cm-1
,
comparable to that of poly(1),12,13 poly(5) exhibits a
significantly higher value of 0.15 S cm-1 consistent with
the more extended conjugation length indicated by
electrochemical data.
These various results show that, whereas the rigidifi-
cation of the DTE molecule leads to a significant decrease
of the oxidation potential and bandgap of the resulting
polymer, the position and the size of the alkyl group exert
a strong influence on the electropolymerization process
and on the structure and electronic properties of the
resulting polymer. Thus, while substitution at R2 im-
proves the efficiency of polymerization, substitution at
R3 inhibits the process. These different behaviors suggest
that substitution affects the geometry of the conjugated
structure and hence the electronic distribution within the
DTE molecule. In order to gain more detailed informa-
tion on this point, single crystals of 3 and 5 have been
analyzed by X-ray diffraction.
X-r a y Str u ctu r e a n d MO Ca lcu la tion s. Figures 5
and 6 show the ORTEP views of the meso forms of 3 and
5 which crystallize preferentially (see the Experimental
Section). Molecules 1,24 3, and 5 belong to the Ci
molecular point group. For 1 and 5 the thiophene ring
is almost planar and there is a dihedral angle (θ) of 3°
between the thiophene ring average plane and the C2-
C
2′ double bond. For 3, the steric interaction induced by
the methyl groups at R3 leads to a distortion of the
conjugated path with an increase of θ to 8° while the C5
atom is pushed out of plane. Although the inhibition of
polymerization observed for 3 is probably related to these
various distortions of the DTE molecule, the relationships
between the observed phenomenon and the geometry of
the precursor are not clearly understood yet.
F igu r e 4. Electronic absorption spectra of the undoped
polymers on ITO: (a) poly(4), and (b) poly(5).
precursor, which makes more difficult the grafting onto
the hydrophilic ITO surface as observed for poly-
(thiophenes) substituted by long alkyl chains.23
Larger scale electropolymerizations were performed,
and the four-probe conductivity of the resulting powdery
(24) Ruban, G.; Zobel, D. Acta Crystallogr. B 1975, 31, 2632.

2406 J . Org. Chem., Vol. 62, No. 8, 1997
Blanchard et al.
Ta ble 4. Com p u ted Va lu es of HOMO LUMO En er gies
a n d Electr on ic Absor p tion Da ta for 1, 3, a n d 5
EHOMO
(eV)
ELUMO
(eV)
λmax
(nm)
HOMO-
LUMO %
compd
f
1
3
5
-8.58
-8.19
-8.16
-0.63
-0.59
-0.59
344
361
361
78
78
82
0.54
0.58
0.67
to a 0.42 eV increase of the energy level of the HOMO,
which nicely agrees with the 0.40 V decrease of Epa
(Table 1).
Theoretical electronic absorption data show that DTE
molecules are characterized by a strong singlet-singlet
π,π* transition (where the HOMO-LUMO transition is
preponderant) with high oscillator strength (f). As
expected, the bridging of the DTE molecule leads to a
red shift of λmax and to an increase of f. These results
and the invariance of ELUMO confirm, in agreement with
experiment, that the relaxation of BLA induced by the
rigidification of the DTE molecule leads to an increase
of EHOMO and to a parallel decrease of the HOMO-LUMO
energy gap.
Con clu sion
F igu r e 6. ORTEP view of 5.
Bridged DTEs substituted by solubilizing alkyl groups
have been synthesized. The analysis of the electropoly-
merization of these new precursors and of the structure
and properties of the resulting π-conjugated polymers has
shown that rigidification of DTE allows a significant
decrease of the oxidation potential and bandgap of the
polymer. The interpretation of these results in the light
of structural and theoretical analyses has shown that the
observed narrowing of both the HOMO-LUMO gap of
the precursor and bandgap of the polymer are consecutive
to a relaxation of bond length alternation, thus confirm-
ing that the rigidification of the π-conjugated system
constitutes an efficient strategy for bandgap control.
Although introduction of solubilizing alkyl chains on
the precursor molecule can greatly enhance the efficiency
of the polymerization process and the conjugation length
of the polymer, the position and the size of the substitu-
ent appear to be as crucial and lead in some cases to
undesired effects such as inhibition of polymerization or
perturbation of the electrodeposition process. A possible
answer to this latter problem can involve the chemical
polymerization of these rigid precursors or the synthesis
of longer oligomers based on these structures. Work in
this direction is now in progress in our laboratory.
Ta ble 3. Bon d Dista n ces a lon g th e Con ju ga ted P a th for
Molecu les 1,24 3, a n d 5
compd C6-C10 C6-C4 C4-C7 C7-C2 C2-C2′ δr (Å)
1
3
5
1.350(8) 1.44(1) 1.40(1) 1.457(7) 1.309(8) 0.095
1.365(7) 1.416(6) 1.377(6) 1.455(5) 1.325(6) 0.080
1.350(1) 1.409(8) 1.356(8) 1.440(7) 1.335(6) 0.078
Comparison of the bond distances along the conjugated
path for molecules 1, 3, and 5 (Table 3) shows that the
bridging of the DTE molecule induces a deformation of
the thiophene ring with shortening of the C6-C4 and
C4-C7 bonds.
These modifications are larger for 5 than for 3, sug-
gesting that steric interaction between the methyl group
and the thiophene ring contributes to limit the deform-
ability of the latter. On the other hand, the length of
the C7-C2 single bond in the linker group decreases by
0.017 Å, while that of the C2-C2′ central double bond
increases by 0.026 Å. These various bond length modi-
fications lead to a decrease of the difference between the
average length of single and double bonds (δr) from 0.095
Å for 1 to 0.078 Å for 5.
As discussed in numerous theoretical works, the degree
of bond length alternation (BLA) related to δr represents
the main contribution to the existence of a finite bandgap
in linearly π-conjugated polymers.25 In this regard, the
relaxation of BLA induced by the rigidification of the DTE
molecule can be expected to play a major role in the
observed reduction of the polymer bandgap.
Exp er im en ta l Section
Electrochemical experiments were carried out with a PAR
273 Potentiostat-Galvanostat in a three-electrode single com-
partment cell equipped with platinum microelectrodes of 7.85
× 10-3 cm2 area, a platinum wire counter electrode, and a
saturated calomel reference electrode (SCE). Electropolymer-
izations were performed in acetonitrile solutions (HPLC grade)
containing 0.01 M of precursor and 0.1 M of tetrabutylammo-
nium hexafluorophosphate (TBAHP) (Fluka puriss). Solutions
were deaerated by nitrogen bubbling prior to each experiment,
which was run under a nitrogen atmosphere. Films for optical
and electrical characterization were grown from the same
electrolytic solutions using indium-tin oxide (ITO)-coated
glass electrodes. UV-vis absorption spectra were recorded on
a Lambda 2 Perkin-Elmer spectrometer.
In order to confirm this conclusion, AM1 calcula-
tions26,27 have been performed using the X-ray structural
data of molecules 1, 3, and 5. The computed values in
Table 4 show that the bridging of the DTE molecule leads
(25) (a) Bre´das, J . L. J . Chem. Phys. 1985, 82, 3808. (b) Bre´das, J .
L.; Street, G. B.; The´mans, B.; Andre´, J . M. J . Chem. Phys. 1985, 83,
1323. (c) Wennerstro¨m, O. Macromolecules 1985, 18, 1977. (d) Kertesz,
M.; Lee, Y.-S. J . Phys. Chem. 1987, 91, 2690.
(26) Dewar, M. J . S.; Zoerbisch, E. G.; Healy, E. F.; Stewart, J . J .
P. J . Am. Chem. Soc. 1985, 107, 3902.
X-r ay Str u ctu r al An alyses.31 Cr ystal Data for 3: C16H16S2,
MW ) 272.43, monoclinic, P21/n, Z ) 2, a ) 6.453(4) Å, b )
14.350(5) Å, c ) 7.510(3) Å, â ) 98.50(8)°, V ) 688(1) Å3, λ )
0.71069 Å.
(27) VAMPS 5. 51: Clark, T. et al. Erlangen (FRG), distributed by
Oxford Molecular Ltd., Oxford, England.

Small Bandgap Electrogenerated Conjugated Polymers
J . Org. Chem., Vol. 62, No. 8, 1997 2407
Da ta Collection . The data were collected by the zig-zag
ω scan technique on an Enraf-Nonius Mach III diffractome-
ter: 2.5° e Θ e 30°, tmax ) 40s, range h, k, l ((h 0,9; k 0,20; l
-10,10), intensity controls without appreciable decay (0.2%)
gives 2230 reflections from which 1199 were independent with
I > 3σ(I). Str u ctu r e Refin em en t. After Lorentz and polar-
ization corrections the structure was solved with direct
methods (SIR), which reveal all the non-hydrogen atoms. After
anisotropic refinement of all the C and S atoms, the coordi-
nates of H atoms were determined from the Hydro program.
The whole structure was refined by full-matrix least-squares
techniques (use of F magnitude; Uij for S and C atoms, x, y, z
and B fixed for H; 82 variables and 1199 observations,
weighting ω ) 1/σ(F0)2 ) [σ2(I) + (0.04F02)2]-1/2) with the
resulting R ) 0.062, Rw ) 0.080.
Cr ysta l d a ta for 5: C22H28S2, MW ) 356.58, monoclinic,
P21/n, Z ) 2, a ) 11.891(7) Å, b ) 5.284(15) Å, c ) 15.89(2) Å,
â ) 100,98(7)°, V ) 980(3) Å3, λ ) 0.710 69 Å. Da t a
Collection . Data collection was carried out by the zig-zag ω
scan technique 2.5° e θ e 30°, on an Enraf-Nonius Mach III
diffractometer. Conditions of measurements were tmax ) 40
s, range h, k, l (h 0,16 ; k 0,7 ; l -22,22). Intensity control
reflections were measured every 2 h without appreciable decay
(0.15%). A total of 3182 independent reflections were collected
from which 1071 correspond to I > 3σ(I). Str u ctu r e Refin e-
m en t. After Lorentz and polarization corrections the structure
was solved by direct methods (SIR), which reveal all the non-
hydrogen atoms. After anisotropic refinement of all the C and
S atoms, the coordinates of H atoms were determined from
the Hydro program. The whole structure was refined by full-
matrix least-squares techniques (refinement on F, x, y, z, Uij
for S and C atoms, x, y, z, and U fixed for H atoms; 109
variables and 1071 observations, weighting scheme: non-
Poisson contribution with w ) 1/σ(F0)2 ) 4F02/[σ(I)2 + (0.04F02)2]
with the resulting R ) 0.078, Rw ) 0.090. All calculations
were performed using the MolEN package.
Th eor etica l Ca lcu la tion s. AM1 calculations26,27 have
been based on X-ray structures; however, since some hydrogen
atoms were not experimentally defined, hydrogen atoms of all
molecules were optimized at the AM1/RHF level. Ionization
potential values (IP) are obtained with the AM1/RHF method
through the Koopmans theorem.28 Theoretical electronic
spectra have been computed at the AM1/PECI level following
the Franck-Condon approximation. Single plus pair double
excitations29,30 are included in the CI procedure. Microstates
are constructed from a 2 s molecular orbital window spanning,
the frontier orbitals (13 occupied and 12 vacant), 469 mi-
crostates (corresponding to 313 singlet configurations), are
built into the configuration procedure. The window is large
enough to include most of the π,π* molecular orbitals in the
CI procedure.
N,N-Dim eth yl-N-[2-m eth yl-3-oxo-3-(2-th ien yl)p r op yl]-
a m m on iu m Ch lor id e (13). A mixture of 2-propionylthio-
phene (1.5 g, 10.70 mmol), dimethylamine hydrochloride (1.13
g, 13.91 mmol), formaldehyde (0.43 g, 14.12 mmol), ethanol
(1.7 mL), and concd HCl (0.02 mL) was refluxed for 4 h. After
the mixture was cooled to rt, a white precipitate was filtered
and dried to give 1.21 g (49%) of the title compound, mp 154-
156 °C (lit.17a mp 154-156 °C), which was immediately used
for the next step.
4,5-Dih yd r o-5-m e t h yl-6H -cyclop e n t a [b]t h iop h e n -6-
on e (11). Compound 12 (0.57 g, 3.76 mmol) was added
dropwise to 3.2 mL of concd H2SO4 at rt. The mixture was
stirred for 2 h and cooled to 0 °C, and cold water was added.
After extraction with ether, the organic phase was dried over
CaCl2, the solvent evaporated, and the residue chromatogra-
phied on silica gel (CH2Cl2) to provide 0.46 g (81%) of a pink
oil: bp 95 °C/2 mmHg (lit.17 bp 95.5 °C/2 mmHg); 1H NMR
(CDCl3) δ 1.36 (d, 3J ) 7.5 Hz, 3H); 2.62 (dd, 3J ) 2.8 Hz, 2J )
3
3
3
17.15 Hz, 1H), 3.05 (dq, J ) 2.8 Hz, J ) 7.05 Hz, J ) 7.5
2
3
Hz, 1H); 3.29 (dd, J ) 17.15 Hz, J ) 7.05 Hz, 1H), 7.04 (dd,
3J ) 4.7 Hz, 1H), 7.91 (dd, 3J ) 4.7 Hz, 1H); 13C NMR (CDCl3)
δ 16.5, 32.4, 47.0, 123.7, 139.2, 140.3, 166.8, 199.8; UV-vis
(CH2Cl2) λmax (log ꢀ) 263 (4.14); IR (Nujol) 1702 cm-1; MS (EI)
152 (M•+).
3-P en ta n oylth iop h en e (9b).18 In a three-necked flask
fitted with a condenser and a dropping funnel were introduced
Mg shavings (2.62 g, 108 mmol) and 5 mL of dry Et2O under
N2 atmosphere. A solution of 1-bromobutane (13.56 g, 99
mmol) in 25 mL of Et2O was added dropwise in 0.5 h. After
0.5 h of stirring, a solution of 3-thiophenecarboxaldehyde (10)
(10.09 g, 90 mmol) in 30 mL of Et2O was added in 1 h. After
1 h of reflux and 20 h of stirring at rt, the mixture was poured
into a mixture of ice-water (200 mL) and concd HCl (30 mL).
After dilution with Et2O (100 mL), the organic phase was
separated and the aqueous phase was extracted with Et2O.
The organic phases were gathered, dried (MgSO4), and evapo-
rated in vacuo to afford an oil corresponding to the intermedi-
ate alcohol. The latter was dissolved in acetic acid (110 mL),
and a solution of chromium(VI) oxide (6.47 g, 65 mmol) in 135
mL of acetic acid/water (2:1) was added dropwise. After 1 h
of stirring and further addition of water (100 mL), the mixture
was extracted with Et2O and the organic phase was slowly
poured into a saturated aqueous solution of Na2CO3 until
neutralization. The organic phase was washed with water,
dried (MgSO4), and evaporated in vacuo. The resulting oil was
purified by a flash chromatography on silica gel (CH2Cl2) to
give 11.57 g (77%) of a yellow oil: 1H NMR (CDCl3) δ 0.95 (t,
3H, J ) 7.3 Hz), 1.31-1.49 (m, 2H), 1.65-1.78 (m, 2H), 2.88
(t, 2H, J ) 7.4 Hz) 7.31 (dd, 1H, J ) 5.0, 3.0 Hz), 7.55 (dd, 1H,
J ) 5.0, 1.3 Hz), 8.04 (dd, 1H, J ) 1.3, 3.0 Hz); 13C NMR
(CDCl3) δ 13.9, 22.4, 26.4, 39.6, 126.2, 126.96, 131.7, 142.4,
195.0; MS (EI) 168 (M•+); IR (KBr) 1674 cm-1 (CdO). Anal.
Calcd for C9H12OS: C, 64.25; H, 7.19; S, 19.05. Found: C,
63.77; H, 7.19; S, 19.30.
Eth yl (E a n d Z)-3-(3-Th ien yl)-2-bu ten oa te (8a ). To a
solution of triethyl phosphonoacetate (33.60 g, 150 mmol) in
anhydrous THF (120 mL) was added a solution of n-BuLi 2.5
M in hexane (63 mL, 158 mmol) dropwise at -50 °C under N2
atmosphere over a period of 2 h. After 30 min of stirring, a
solution of 3-acetylthiophene (9a ) (6.31 g, 50 mmol) in anhy-
drous THF (120 mL) was added in 1 h, and the mixture was
allowed to warm to rt and stirred for 45 h. After successive
addition of 3 M aqueous NH4Cl (80 mL) and aqueous 1 M HCl
(80 mL), the mixture was diluted with Et2O (100 mL), the
organic phase was separated, and the aqueous phase was
extracted with Et2O. The combined organic phases were then
washed with water, dried (MgSO4), and evaporated in vacuo.
The product was chromatographed on silica gel (CH2Cl2/
petroleum ether (1:1)) to give 9.39 g (96%) of a yellow oil
corresponding to a mixture of the Z and E isomers. First
eluted isomer: 1H NMR (CDCl3) δ 1.32 (t, 3H, J ) 7.1 Hz),
2.58 (d, 3H, J ) 1.2 Hz), 4.21 (q, 2H, J ) 7.1 Hz), 6.23 (q, 1H,
J ) 1.2 Hz), 7.31 (d, 2H, J ) 2.1 Hz), 7.48 (t, 1H, J ) 2.1 Hz);
13C NMR (CDCl3) δ 14.26, 17.13, 59.73, 115.23, 124.17, 125.13,
126.12, 143.30, 148.87, 167.10. Second eluted isomer: 1H
NMR (CDCl3) δ 1.20 (t, 3H, J ) 7.1 Hz), 2.20 (d, 3H, J ) 1.4
Hz), 4.10 (q, 2H, J ) 7.1 Hz), 5.88 (q, 1H, J ) 1.4 Hz), 7.16
(dd, 1H, J ) 5.0, 1.3 Hz), 7.26 (dd, 1H, J ) 5.0, 2.9 Hz), 7.41
(dd, J ) 2.9, 1.3 Hz); 13C NMR (CDCl3) δ 14.0, 26.8, 59.9, 117.2,
124.2, 124.3, 127.9, 139.8, 148.0, 166.1; MS (EI) 196 (M•+) IR
(KBr) 1708 cm-1 (CdO). Anal. Calcd for C10H12O2S: C, 61.20;
H, 6.16; S, 16.34. Found: C, 61.25; H, 6.25; S, 16.46.
Eth yl (E a n d Z)-3-(3-Th ien yl)-2-h ep ten oa te (8b). This
compound was prepared using the above-described procedure
from triethyl phosphonoacetate (24 g, 107 mmol), n-BuLi 1.6
2-Meth yl-1-oxo-1-(2-th ien yl)-2-p r op en e (12). Distilla-
tion of 13 under reduced pressure yielded 0.57 g (59%) of a
clear yellow oil that must be rapidly used for the next step
due to a strong propensity to polymerize: bp 118-119 °C/19
mmHg, (lit.17 bp 118-120 °C/19 mmHg). 1H NMR (CDCl3) δ
2.07 (s, 3H), 5.80 (s, 2H), 6.93-7.80 (m, 3H).
(28) Koopmans, T. Physica 1933, 1, 104.
(29) Clark, T.; Chandrasekhar, J . Israel J . Chem. 1993, 33, 435.
J ain, M.; Chandrasekhar, J . J . Phys. Chem. 1993, 97, 4044.
(30) A modified version of VAMP 4.3 has been used for the PECI
calculations because VAMPS. S1 computes erroneous values of the
transition dipole moment and oscillator strength.
(31) The author has deposited atomic coordinates for 3 and 5 with
the Cambridge Crystallographic Data Centre. The coordinates can be
obtained, on request, from the Director, Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge, CB2 1EZ, UK.

2408 J . Org. Chem., Vol. 62, No. 8, 1997
Blanchard et al.
M in hexane (70.3 mL, 113 mmol), and 3-pentanoylthiophene
(6 g, 36 mmol) except that completion of the reaction was
achieved by an additional 24 h of reflux, yield 8.23 g (97%) of
a yellow oil corresponding to a mixture of Z and E isomers.
First eluted isomer: 1H NMR (CDCl3) δ 0.93 (t, 3H, J ) 7.2
Hz), 1.32 (t, 3H, J ) 7.1 Hz), 1.37-1.57 (m, 4H), 3.05 (t, 2H,
J ) 7.8 Hz), 4.21 (q, 2H, J ) 7.1 Hz), 6.16 (s, 1H), 7.28 (dd,
1H, J ) 5.2, 1.4 Hz), 7.32 (dd, 1H, J ) 5.2, 2.8 Hz), 7.48 (dd,
1H, J ) 2.8, 1.4 Hz); 13C NMR (CDCl3) δ 13.88, 14.27, 22.97,
30.72, 31.76, 59.70, 115.00, 123.95, 125.63, 126.09, 142.42,
154.20, 166.70. Second eluted isomer: 1H NMR (CDCl3) δ 0.89
(t, 3H, J ) 7.1 Hz), 1.17 (t, 3H, J ) 7.2 Hz), 1.25-1.47 (m,
4H), 2.46 (t, 2H, J ) 6.9 Hz), 4.07 (q, 2H, J ) 7.1 Hz), 5.86 (s,
122.9, 140.2, 140.6, 174.0, 196.6; MS (EI) 152 (M•+); IR (NaCl)
1691 cm-1 (CdO); UV-vis (CH2Cl2) λmax nm (log ꢀ) 265 (4.21).
Anal. Calcd for C8H8OS: C, 63.13; H, 5.30; S, 21.06. Found:
C, 63.41; H, 5.55; S, 20.68.
4-Bu t yl-4,5-d ih yd r o-6H -cyclop en t a [b]t h iop h en -6-on e
(6b). This compound was obtained using the procedure
already described for the preparation of 6a from 8b (8.56 g,
1
0.036 mol): yellow oil; 2.63 g (38%); H NMR (CDCl3) δ 0.91
(t, 3H, 3J ) 7.0 Hz), 1.28-1.43 (m, 4H), 1.43-1.62 (m, 1H),
2
3
1.72-1.85 (m, 1H), 2.61 (dd, 1H, J gem ) 18.3 Hz and J ) 2.4
Hz), 3.12 (dd, 1H, J gem ) 18.3 Hz and 3J ) 6.7 Hz), 3.23-
2
3
3
3.33 (m, 1H), 7.06 (d, 1H, J ) 4.9 Hz), 7.87 (d, 1H, J ) 4.9
Hz); 13C NMR (CDCl3) δ 13.9, 22.6, 29.7, 35.2, 36.9, 47.9, 123.4,
140.3, 140.5, 172.9, 196.7; MS (EI) 194 (M•+); IR (KBr) 1697
cm-1 (CdO); UV-vis (CH2Cl2) λmax nm (log ꢀ) 266 (4.15). Anal.
Calcd for C11H14OS: C, 68.00; H, 7.26; S, 16.50. Found: C
68.42; H, 7.35; S, 16.09.
1H), 7.05 (dd, 1H, J ) 4.5 and 1.6 Hz), 7.25-7.30 (m, 2H); 13
C
NMR (CDCl3) δ 13.8, 14.0, 22.1, 30.0, 40.0, 59.8, 117.1, 123.3,
124.4, 128.0, 139.5, 153.4, 166.2; MS (EI) 238 (M•+); IR (KBr)
1709 cm-1 (CdO). Anal. Calcd for C13H18O2S: C, 65.51; H,
7.61; O, 13.43; S, 13.45. Found: C, 65.35; H, 7.71; O, 13.41;
S, 13.17.
Gen er a l P r oced u r e for th e McMu r r y Rea ction . After
dropwise addition of TiCl4 (4-7 mmol) to 20-30 mL of
anhydrous THF at 0 °C under N2 atmosphere, zinc dust (16-
28 mmol) was added portionwise. The mixture was then
refluxed for 0.5 h and cooled to 0 °C. A solution of ketone (4-7
mmol) in THF was added dropwise, and the mixture was
refluxed again for 4 h. After being cooled to rt and addition
of water, the mixture was extracted with CH2Cl2. The
combined organic phases were washed with water, dried
(MgSO4), and evaporated in vacuo.
4,5-Dih ydr o-6H-cyclopen ta[b]th ioph en -6-on e (15). 3-(3-
Thienyl)acrylic acid (Lancaster) (10 g, 65 mmol) was dissolved
in 140 mL of 0.6 M aqueous NaOH, and 2% sodium amalgam
(200 g) was added portionwise in 1 h. After 0.5 h of stirring,
the mixture was acidified with 50% aqueous HCl and extracted
with ether. The organic phase was washed with water, dried
over CaCl2, and evaporated, giving 10.7 g of a pale yellow oil
that crystallized. A solution of thionyl chloride (15 mL, 210
mmol) in 200 mL of anhydrous ether was added, and the
mixture was refluxed for 3.5 h. Removal of the solvent and
thionyl chloride in excess by evaporation under reduced
pressure gave a brown oil that was dissolved in 150 mL of
CS2. After transfer in a dropping funnel, this solution was
added dropwise to a suspension of AlCl3 (8.7 g, 65 mmol) in
100 mL of CS2. The mixture was stirred for 20 h at rt, refluxed
for 2 h, and then poured into a mixture of concd HCl (100 mL)
and ice-water (200 mL). The aqueous phase was extracted
with CH2Cl2, and the organic phase was washed with water,
dried (MgSO4), and evaporated in vacuo. The residue was
chromatographed on silica gel (petroleum ether/Et2O 4:1) to
afford 5 g of a white solid, yield 56% based on 18: mp 89-90
°C (lit.16 mp 90-91 °C); 1H NMR (CDCl3) δ 2.90-3.10 (m, 4H),
7.05 (d, 1H, 3J ) 4.85 Hz), 7.89 (d, 1H, 3J ) 4.85 Hz); 13C NMR
(CDCl3) δ 23.9, 41.2, 123.9, 140.5, 141.1, 170.0, 197.3; MS (EI)
138 (M•+); IR (KBr) 1685 cm-1 (CdO); UV-vis (CH2Cl2) λmax
nm (log ꢀ) 262 (4.21).
6,6′-Bis(4,5-d ih yd r o-6H -cyclop e n t a [b]t h ie n ylid e n e )
1
(2): yield 86%; yellow solid: mp 205 °C; H NMR (CDCl3) δ
3
3.00 (m, 4H), 3.27 (m, 2H), 6.92 (d, 2H, J ) 4.9 Hz), 7.02 d,
2H, 3J ) 4.9 Hz); 13C NMR (CDCl3) δ 27.23, 35.2, 122.5, 123.0,
128.3, 129.3, 150.1; UV-vis (CH2Cl2) λmax nm (log ꢀ) 331 (4.23),
348 (4.42), 367 (4.37); MS (EI) 244 (M•+); HRMS calcd
244.3080, found 244.3082. Anal. Calcd for C14H12S2: C, 68.84;
H, 4.96; S, 26.20. Found: C, 68.75; H, 4.95; S, 25.80.
6,6′-Bis(4,5-d ih yd r o-5-m eth yl-6H-cyclop en ta [b]th ien -
ylid en e) (3): chromatographed on silica gel (petroleum ether/
CH2Cl2 9:1): yield 0.20 g (37%) of yellow powder containing a
mixture of diastereomers; mp 180 °C; 1H NMR (CDCl3) δ 1.30
(d, 6H, 3J ) 6.8 Hz), 2.49 (dd, 2H, 2J ) 16 Hz), 3.18 (dq, 2H, 3J
2
3
) 6.8, 7.05 Hz), 3.62 (dd, 2H, J ) 16 Hz, J ) 7.05 Hz), 6.89
(d, 2H, 3J ) 4.9 Hz), 7.30 (d,2H, 3J ) 4.9 Hz); 13C NMR (CDCl3)
δ 21.5, 22.9, 36.28, 36.4, 42.9, 43.2, 123.0, 123.1, 128.5, 129.3,
133.1, 141.1, 141.2, 148.2, 148.9; UV-vis (CH2Cl2) λmax nm (log
ꢀ) 333, (4.41) 349 (4.50), 368 (4.33). MS (EI) 272 (M•+). HRMS
calcd for C16H16S2 272.0693, found 272.0703.
4-Me t h yl-4,5-d ih yd r o-6H -cyclop e n t a [b]t h iop h e n -6-
on e (6a ). An aqueous solution of NaOH 10% (130 mL) was
added to a solution of ester 8a (9.25 g, 47 mmol) in 150 mL of
EtOH, and the resulting mixture was refluxed for 2 h. After
dilution with water (100 mL), 3% sodium amalgam (128.5 g)
was added portionwise at rt in 3 h and the mixture was stirred
for 20 h. Hg was separated by decantation, and the reaction
mixture was slowly acidified at 0 °C with concd HCl (65 mL).
After extraction of the aqueous phase with CH2Cl2, the organic
phase was washed with water, dried (MgSO4), and evaporated
in vacuo to afford 7.83 g (46.06 mmol) of the crude carboxylic
acid as a yellow oil. After addition of thionyl chloride (13.80
mL) in Et2O (185 mL), the mixture was refluxed for 3 h.
Solvent evaporation gave the crude acid chloride (8.46 g, 44.9
mmol). The latter was dissolved in CS2 (100 mL), and this
solution was added dropwise to a mixture of AlCl3 (7.77 g, 58.3
mmol) in CS2 (150 mL) under N2. After 4 h of reflux, the
mixture was cooled to 20 °C and poured into a mixture of concd
HCl (100 mL) and ice-water (200 mL). The aqueous phase
was extracted with CH2Cl2, and the organic phase was washed
with water, dried (MgSO4), and evaporated in vacuo. The
residue was chromatographed on silica gel (petroleum ether/
Et2O 4:1) to afford 2.94 g of a yellow oil, yield 41% based on
8a : 1H NMR (CDCl3) δ 1.35 (d, 3H, J ) 7.3 Hz), 2.53 (dd, 1H,
6,6′-Bis(4-m et h yl-4,5-d ih yd r o-6H-cyclop en t a [b]t h ien -
ylid en e) (4): chromatographed on silica gel (petroleum ether/
CH2Cl2 9:1). Yield 0.52 g (83%) of a mixture of two diastere-
omers; beige solid; mp 68-90 °C. 1H NMR (CDCl3) δ 1.32-
1.37 (m, 6H), 2.73-2.84 (m, 2H), 3.30-3.56 (m, 4H), 6.92 (d,
3
3
2H, J ) 4.9 Hz), 7.32 (d, 2H, J ) 4.9 Hz); 13C NMR (CDCl3)
δ 22.0, 34.9, 44.3, 122.2, 125.4, 129.4, 142.8, 155.4; MS (EI)
272 (M•+); HRMS calcd for C16H16S2 272.069 344, found
272.069 690; UV-vis (CH2Cl2) λmax nm (log ꢀ) 367 (4.43), 348
(4.47), 331 (4.27). Anal. Calcd for C16H16S2: C, 70.54; H, 5.92.
Found: C, 70.02; H, 5.41.
6,6′-Bis(4-b u t yl-4,5-d ih yd r o-6H -cyclop en t a [b ]t h ien -
ylid en e) (5): chromatographed on silica gel (petroleum ether/
CH2Cl2 9:1). Yield 0.29 g of the first eluted diastereomer as
pale yellow solid (mp 57-61°C) and 0.07 g of the second one
1
as yellow oil, final yield 50%; H NMR (CDCl3) δ 0.94 (t, 6H,
J ) 6.8 Hz), 1.30-1.80 (m, 12 H), 2.80-2.90 (m, 2H), 3.25-
3.35 (m, 2H), 3.35-3.50 (m, 2H), 6.93 (d, 2H, 3J ) 4.9 Hz),
3
7.31 (d, 2H, J ) 4.9 Hz); 13C NMR (CDCl3) δ 14.1, 22.9, 29.7,
36.5, 40.4, 42.4, 122.7, 125.4, 129.1, 143.2, 154.2; MS (EI) 356
(M•+); HRMS calcd 356.163 245, found: 356.163 784. UV-
vis (CH2Cl2) λmax nm (log ꢀ) 369 (4.54), 349 (4.58), 332 (4.36).
Anal. Calcd for C22H28S2: C, 74.10; H, 7.91; S, 17.98. Found:
C, 73.79; H, 7.85; S, 18.11.
2
2J gem ) 18.4 Hz and 3J ) 2.5 Hz), 3.18 (dd, 1H, J gem ) 18.4
3
Hz, J ) 6.7 Hz), 3.31-3.44 (m, 1H), 7.04 (d, 1H 3J ) 4.8 Hz),
3
7.87 (d, 1H, J ) 4.8 Hz); 13C NMR (CDCl3) δ 20.7, 31.4, 49.8,
J O9623447