Effect of local molecular structure on the chain-length dependence of the electronic properties of thiophene-based π-conjugated systems

Article abstract of DOI:10.1021/jo034319o

Three series of thiophene-based π-conjugated oligomers built with different combinations of thiophene cycles and double bonds have been synthesized and characterized. The analysis of the chain length dependence of the electronic, electrochemical, and vibrational properties of the three series of oligomers has been carried out using cyclic voltammetry, UV-vis, IR, and Raman spectroscopies. These various investigations provide consistent results showing that incorporation of ethylenic linkages in an oligothiophene structure leads to a faster decrease of the HOMO-LUMO gap with chain extension due to the combined effects of enhanced planarity and lower overall aromatic character of the system. Although the incorporation of two consecutive double bonds in the system leads to a stabilization of the dicationic state, this structural modification does not produce the expected further decrease of the HOMO-LUMO gap at large chain extension. This phenomenon is discussed on the basis of an interplay between aromaticity and bond length alternation.

Full text of DOI:10.1021/jo034319o

Effect of Loca l Molecu la r Str u ctu r e on th e Ch a in -Len gth
Dep en d en ce of th e Electr on ic P r op er ties of Th iop h en e-Ba sed
π-Con ju ga ted System s
Pierre Fre`re,*,† J ean-Manuel Raimundo,† Philippe Blanchard,† J acques Delaunay,†
Pascal Richomme,‡ J ean-Louis Sauvajol,§ J esus Orduna,| J avier Garin,| and J ean Roncali*,†
Groupe Syste`mes Conjugue´s Line´aires, IMMO, UMR CNRS 6501, Universite´ d’Angers,
2 Bd Lavoisier, 49045 Angers, France, Service Commun d’Analyses Spectroscopiques,
16 Bd Daviers, 49100 Angers, France, Groupe de Dynamique des Phases Condense´es,
UMR CNRS 5581, Universite´ de Montpellier 2, 39095 Montpellier Cedex 5, France, and
Departamento de Quimica Organica, ICMA, Universidad de Zaragoza, CSIC,
E-50009 Zaragoza, Spain
pierre.frere@univ-angers.fr
Received March 11, 2003
Three series of thiophene-based π-conjugated oligomers built with different combinations of
thiophene cycles and double bonds have been synthesized and characterized. The analysis of the
chain length dependence of the electronic, electrochemical, and vibrational properties of the three
series of oligomers has been carried out using cyclic voltammetry, UV-vis, IR, and Raman
spectroscopies. These various investigations provide consistent results showing that incorporation
of ethylenic linkages in an oligothiophene structure leads to a faster decrease of the HOMO-LUMO
gap with chain extension due to the combined effects of enhanced planarity and lower overall
aromatic character of the system. Although the incorporation of two consecutive double bonds in
the system leads to a stabilization of the dicationic state, this structural modification does not
produce the expected further decrease of the HOMO-LUMO gap at large chain extension. This
phenomenon is discussed on the basis of an interplay between aromaticity and bond length
alternation.
In tr od u ction
quantities such as absorption maximum or oxidation
potential measured on oligomers of increasing chain
length have been used to predict the ultimate properties
of an ideal defect-free infinite polymer chain.3 Examina-
tion of the data obtained for various series of (hetero)-
aromatic oligomers shows that both the convergence limit
of ∆E and the rate at which the system converges toward
this limit depend on structural factors such as bond
length alternation, planarity, and aromatic resonance
energy of the (hetero)aromatic ring.2,3
In recent years, we have developed various series of
extended oligothienylenevinylenes (nTVs).4 Detailed analy-
ses of the chain-length dependence of the electronic
properties of these oligomers have shown that they
present the smallest ∆E value and longest convergence
limit among all classes of known extended π-conjugated
oligomers.4,5 Indeed, the limitation of rotational disorder
and the decrease of the overall aromaticity of the system
associated with the thiophene/double bond alternation
The unique electronic properties of linearly conjugated
oligomers and polymers originate from a system of
π-electrons extending over a large number of sp2 carbons.
Full delocalization of π-electrons would result in carbon-
carbon bonds of equal lengths, and chain extension
should lead to the progressive closure of the HOMO-
LUMO gap (∆E). However, for these unidimensional
systems, the electron-electron correlation combined with
the electron-phonon coupling leads to an uneven distri-
bution of π-electrons and hence to the development of
alternate single and double bonds.1 A major consequence
of this process is that chain extension leads to the
convergence of ∆E toward a finite limit which directly
depends on the specific chemical structure of the π-con-
jugated system.2
During the past decade, the chain-length dependence
of many series of well-defined π-conjugated oligomers
have been investigated in detail. Thus, extrapolation of
(3) (a) Electronic Materials: the Oligomer Approach; Mu¨llen, K.,
Wegner, G., Eds.; Wiley-VCH: Weinhiem, 1997. (b) Martin, R. E.;
Diederich, F. Angew. Chem., Int. Ed. 1999, 38, 1350. (c) Meier, H.;
Stalmach, U.; Kolshorn, H. Acta Polym. 1997, 48, 379.
(4) (a) Elandaloussi, E. H.; Fre`re, P.; Richomme, P.; Orduna, J .;
Gar´ın, J .; Roncali, J . J . Am. Chem. Soc. 1997, 119, 10774. (b) J estin,
I.; Fre`re, P.; Blanchard, P.; Roncali, J . Angew. Chem., Int. Ed. 1998,
37, 942. (c) J estin, I.; Fre`re, P.; Mercier, N.; Levillain, E.; Stievenard,
D.; Roncali, J . J . Am. Chem. Soc. 1998, 120, 8150.
* To whom correspondence should be addressed. Fax: 33(0)2 41 73
54 05.
† Universite´ d’Angers.
‡ Service Commun d’Analyses Spectroscopiques.
§ Universite´ de Montpellier 2.
| Universidad de Zaragoza.
(1) Bre´das, J . L.; Cornil, J .; Beljonne, D.; Dos Santos, D. A.; Shuai,
Z. Acc. Chem. Res. 1999, 32, 267.
(2) Roncali, J . Chem. Rev. 1997, 97, 173.
(5) Roncali, J . Acc. Chem. Res. 2000, 33, 147.
10.1021/jo034319o CCC: $25.00 © 2003 American Chemical Society
Published on Web 08/23/2003
7254
J . Org. Chem. 2003, 68, 7254-7265

Electronic Properties of Thiophene-Based π-Conjugated Systems
CHART 1
leads to a significant decrease of ∆E compared to, e.g.,
oligothiophenes (nTs) containing the same number of sp2
carbons. On such a basis, it might be expected that
insertion of additional double bonds between the thiophene
rings could contribute to a further decrease of ∆E at given
chain lengths.
To investigate this question in more detail, three series
of π-conjugated oligomers involving various combinations
of thiophene rings and double bonds have been synthe-
sized (Chart 1) and a comparative analysis of the chain
length dependence of their electronic properties has been
carried out using cyclic voltammetry, UV-vis, and
vibrational spectroscopies.
ppm corresponding to the sp2 carbons since, as demon-
strated by HMQC experiments, the two carbons at the
3′ and 4 positions of the thiophene rings are equivalent.
Compounds H-T4V2 and H-T5V2 were obtained by
double Wittig-Horner olefination of diformyl bi- or
terthiophene 3 and 4 with diethyl 2-thienylmethylphos-
phonate (P a ) or diethyl 2-(5-hexylthienyl)methyl]phos-
phonate (P b) in the presence of t-BuOK at room tem-
perature (Scheme 1). These phosphonates were obtained
by a one-step reaction of diethyl iodomethylphosphonate
1
on the corresponding 2-thienylcopper reagent.6 In the H
NMR spectrum of H-T4V2b and H-T5V2b recorded in
C6D6, the ethylenic protons appear as an AB doublet (3J
) 15.7 Hz) pointing to the whole trans configuration of
the molecules.
Resu lts a n d Discu ssion
Unsubstituted nTVs T3V2a , T4V3a , and T5V4a were
synthesized as already reported.7 End-substituted T3V2b,
T4V3b, and T5V4b have been obtained by 2-fold Wittig-
Horner olefination of dialdehydes 5, 6, and 7 with
phosphonate P b (Scheme 2). Recrystallization from
CHCl3-hexane gave pure E-isomers of T3V2b, T4V3b,
and T5V4b in 70%, 60%, and 65% yield, respectively. The
1H NMR spectrum of T3V2b and T4V3b shows the
symmetrical ethylenic parts as AB systems with a ∆ν/J
ratio of 3, leading to a trans coupling constant of the
olefinic protons (3J ) of 15.5 Hz. For T5V4b, the solvent
Syn th esis. The synthesis of the various series of
oligomers is depicted in Schemes 1-3. A McMurry
reaction of aldehydes 2a or 2b, obtained by Vilsmeier
formylation of bithiophenes 1a or 1b, gave compounds
H-T4V1 essentially as E-isomers. Recrystallization from
EtOH/CHCl3 gave pure E-isomer in 40% and 60% yield
for H-T4V1a and H-T4V1b, respectively. Although the
NMR spectra of hybrids H- T4V1 do not allow to
determine the configuration of the double bond, they
suggest the presence of a pure isomer. Thus, the 1H NMR
spectra only show the expected singlet for ethylenic
protons. On the other hand, the 13C NMR spectrum of
H-T4V1b only shows eight signals between 120 and 150
(6) Wang, C.; Dalton, L. R. Tetrahedron Lett. 2000, 41, 617.
(7) Nakayama, J .; Fujimori, T. Heterocycles 1991, 32, 991
J . Org. Chem, Vol. 68, No. 19, 2003 7255

Fre`re et al.
SCHEME 1
yield. Vilsmeier formylation of T2B1a gave monoalde-
hyde 11 in 74% yield, while dialdehyde 12 was prepared
in 56% yield by dilithiation followed by reaction with
DMF. Wittig-Horner olefination of aldehyde 11 with P c
gave T3B2a in 74% yield.
Double olefination of dialdehyde 12 gave an unsepa-
rable mixture of T3B2a and T4B3a . In the presence of
a strong base, dialdehyde 12 is quickly converted into
monoaldehyde 11 by mono-decarbonylation. Then, a
nucleophilic attack of the phosphonate anion on the
mixture of 11 and 12 leads to T3B2a and T4B3a .
Decarbonylation of a 2-thienylcarbaldehyde group by a
strong base has already been observed by Cava et al. who
concluded that this reaction was caused by the generation
of an acylanion, followed by a loss of carbon monoxyde.9
Because of the poor yield of diolefination with P c,
another route toward T4B3a and its soluble analogues
T3B2b and T4B3b has been developed. This alternative
approach involves first the formation of the acrolein
system followed by olefination with phosphonates P a or
P b. Thus, olefination of thienylacrolein with P a afforded
T2B1a in a better yield (85%) than the first route
whereas two-fold olefination of 2,5-diacrolein-thiophene
13 with P b gave T3B2b in 66% yield. Treatment of
T2B1a with n-BuLi and 3-dimethylaminoacroleine gave
bis-1,4-(2-thienylacrolein)butadiene 14 in 60% yield. Bis-
olefination of 14 with P a gave T4B3a while, T4B3b was
obtained in 70% yield from P b. Recrystallization from
CHCl3 or CHCl3-hexane, gave T3B2a , T3B2b, and
T4B3b as pure E-isomers, but the poor solubility of
T4B3a did not allow a correct purification.
SCHEME 2
The whole trans configuration of the various oligomers
1
has been confirmed by 500 MHz H NMR. Although for
the unsymmetrical monoaldehyde 11 the butadienic
protons present easily identified AB or AB2 patterns, for
compounds 12, 14 and T2B1a an AA′XX′ (J XX′ ) 0 Hz)
pattern is observed in C6D6 with ∆δ between the AA′ and
the XX′ parts varying from 0.25 to 0.40 ppm. The coupling
3
3J AX ≈ 15.0 Hz and J AA′ ≈ 10.0 Hz are characteristic of
a trans configuration for each butadiene unit.
shift effects observed in C6D6 allows to distinguish
ethylenic and aromatic protons. The ethylenic system
connected to the terminal thiophenes appears as a AB
doublet (∆ν/J ) 3 and 3J ) 15.4 Hz), whereas the two
magnetically equivalent central ethylenic linkages occur
a sharp singlet corresponding to an A2 system.
As shown in Figure 1 (top), the aromatic and ethylenic
protons of T3B2a are easily identified on the spectrum
recorded in C6D6. Protons of the butadiene system appear
as three symmetrical multiplets centered at 6.65, 6.41,
and 6.32 ppm, respectively, with a ratio of 2/1/1 in the
integral sets. Even with the same apparent chemical
shifts, protons H2 and H3 are magnetically inequivalent
whereas the chemical shifts difference between H1 and
H4 protons is only of 0.08 ppm. As these patterns are
very similar to the AA′ part of an AA′XX′ system, the
coupling constants given in Table 1 were evaluated with
usual equations for a second order spectrum. The close
agreement between the resulting theoretical spectrum
(Figure 1, bottom) and the experimental spectrum strongly
supports our interpretation.
Kossmehl and al. synthesized T3B2a by a Wittig
reaction followed by isomerization with iodine to obtain
the all-trans isomer.8 To skip the isomerization step, we
developed an alternative synthesis involving two succes-
sive Wittig-Horner olefinations. The first step involves
the formation of the butadiene system by olefination of
2-thiophenecarbaldehyde 10 with phosphonate P c bear-
ing an ethylenic group (Scheme 3). Alcohol 9, obtained
by reduction of 2-thienylacrolein 8 with NaBH4, was
treated with PBr3 to yield the bromide derivative. Due
to its high instability, this latter compound was im-
mediately reacted with the anion diethyl phosphite to
give phosphonate P c in 65% yield for the two steps.
Wittig-Horner olefination of 2-thienylcarbaldehyde with
P c, in the presence of t-BuOK, afforded T2B1a in 60%
For T3B2b, the overlap of the signals for H2 and H3
with those of the thiophenic protons does not allow an
unequivocal assignment. However, owing to the similar
resonance of H1 and H4 to those of T3B2a , the coupling
(9) Mohanackrishnan, A. K.; Lakshmikantham, M. V.; McDougal,
C.; Cava, M. P.; Baldwin, J . W.; Metzger, R. M. J . Org. Chem. 1998,
63, 3105.
(8) Kossmehl, G.; Bohn, B.; Manecke, G. Makromol. Chem. 1976,
177, 1857.
7256 J . Org. Chem., Vol. 68, No. 19, 2003

Electronic Properties of Thiophene-Based π-Conjugated Systems
SCHEME 3
constants were evaluated with the same method. The
the rotational disorder and hence a decrease of the
rigidity of the π-conjugated with chain extension.
3
3J H1-H2 and J H3-H4 thus obtained are close to 15.0 Hz
which confirms the all-trans configuration of T3B2a and
T3B2b. Finally, the configuration of T4B3b could not
be directly deduced from its 1H NMR spectrum. However,
only 14 resonances between 100 and 150 ppm appeared
on its proton decoupled 13C NMR spectrum, as expected
for a pure isomer.
As already reported, the electronic absorption spectrum
of nTVs exhibits a well resolved vibronic fine structure
characteristic of planar rigid systems.4 The constant
energy spacing between the maximum of the lowest
energy transition (λ0-0), main absorption band (λmax) and
high energy band (1400-1500 cm-1) is consistent with a
CdC stretching mode strongly coupled to the electronic
structure.12 The spectra of thienylenebutadiene (nTBs)
are very similar to those of nTVs with however a slight
enhancement of the resolution of the vibronic fine
structure. Table 2 lists the main UV-vis data of the three
series of oligomers. For each series, the +I inductive
effect of the alkyl chains at the R-position of the terminal
thiophenes produces a small bathochromic shift of ab-
sorption bands. However, a closer examination shows
that the magnitude of this electronic effect decreases with
the lengthening of the π-conjugated system. This satura-
tion effect already observed in highly extended nTVs end-
substituted by dithiafulvenyl groups, has been attributed
to the fact that as chain length increases, ∆E becomes
essentially determined by the topology of the π-conju-
gated system.13 As expected, for each series, chain
Single crystals of 14 were obtained from slow evapora-
tion of CHCl3 solution. The X-ray structure determination
(Figure 2) clearly reveals the all-trans configuration of
the butadiene spacer and of the acrolein system. To
analyze the torsion and the bond length alternation of
conjugated systems in the solid state, the X-ray data of
14, T2B1a 10 (TB series), and T3V2a 11 (TV series) can
be compared. In the TV series, the dithienylethylene
molecule is nearly planar and the dihedral angle between
the plane of the thiophene ring and that of the ethylene
bonds is inferior to 4° in T3V2a . For 14, the thiophene
cycles present a twist angle of 6° with the fully planar
butadiene unit and for T2B1a the angle reaches 10°.
Bond length alternation calculated as the difference
between the average length of single and double bonds
for the 16 carbons of the conjugated systems is around
0.08 Å for both T3V2a and 14.
UV-vis Sp ectr oscop y. Figure 3 shows the electronic
absorption spectra of the three series of end-substituted
oligomers in CH2Cl2. The spectrum of T4V1b and T4V2b
exhibits a discernible vibronic fine structure which
vanishes for T5V2b. This effect suggests an increase of
(10) Buschmann, J . F.; Ruban, G. Acta Crystallogr. 1978, B34, 1923
(11) Zobel, V. D.; Ruban, G. Acta Crystallogr. 1978, B34, 1652.
(12) Rughooputh, S. D. D. V.; Hotta, S.; Heeger, A. J .; Wudl, F. J .
Polym. Sci. 1987, 25, 1071.
(13) J estin, I.; Fre`re, P.; Levillain, E.; Roncali, J . Adv. Mater. 1999,
11, 134.
J . Org. Chem, Vol. 68, No. 19, 2003 7257

Fre`re et al.
F IGURE 1. NMR spectra of T3B2a : (top) experimental spectrum in C6D6; (bottom) theoretical spectrum.
extension leads to a bathochromic shift of λ0-0 and hence
to a decrease of ∆E.
extension data for nTVs and nTBs are located on a
common line and the two compounds corresponding to
the longest chain length namely T5V4b and T4B3b (Cn
) 28 sp2 carbons), show the same λmax and λ0-0 of 493
and 527 nm respectively (Table 2).
To compare the evolution of the absorption bands with
the lengthening of the conjugated chain (Cn), the plots
of λmax versus the reciprocal number of sp2 carbons has
been reported in Figure 4 for each series and also for
oligothiophene nTs.14 Whereas λmax scales linearly with
1/Cn for nTVs and nTBs, a deviation from linearity is
observed for nTs. The lowest slope is observed for nTs
series and the highest for nTBs. However, at larger chain
The absolute value of ∆E and the rate at which a
π-conjugated system converges toward this limit depends
on structural factors such as bond length alternation,
planarity and aromaticity.2,3 On this basis, the higher
slopes obtained for nTVs and nTBs show that a given
increment in chain length (Cn) is more efficient to
(14) Ba¨uerle, P. In ref 3a, pp 105-197.
7258 J . Org. Chem., Vol. 68, No. 19, 2003

Electronic Properties of Thiophene-Based π-Conjugated Systems
TABLE 1. Ch em ica l Sh ifts a n d Cou p lin g Con sta n ts for P r oton s of Bu ta d ien e System s for T3B2a a n d T3B2b
compd
H1
H2
H3
H4
T3B2a (R ) H)
δ ) 6.41 ppm
δ ) 6.65 ppm
δ ) 6.65 ppm
δ ) 6.32 ppm
3J H1-H2 ) 15.1 Hz
4J H1-H3 ) 0.9 Hz
3J H2-H1 ) 15.1 Hz
3J H2-H3 ) 10.7 Hz
4J H2-H4 ) 0.8 Hz
3J H3-H4 ) 15.2 Hz
3J H3-H2 ) 10.7 Hz
4J H3-H1 ) 0.9 Hz
3J H4-H3 ) 15.2 Hz
4J H4-H2 ) 0.8 Hz
T3B2b (R ) Hex)
δ ) 6.45 ppm
multiplet between 6.62 and 6.74 ppm
δ ) 6.35 ppm
3J H1-H2 ) 15.0 Hz
4J H1-H3 ) 0.9 Hz
3J H1-H2 ) 15.0 Hz
4J H1-H3 ) 0.9 Hz
F IGURE 2. ORTEP view of (E,E)-1,4-bis[2-(3-oxo-2-propenyl)-5-thienyl]buta-1,3-diene 14 with bond lengths in Å.
reducing ∆E than for the nTs series. This difference
reflects the combined effects of enhanced planarity and
lower overall aromaticity.
oxidation wave involves in fact two very close one-electron
oxidation steps. Comparison of the E0 - E0 values for
2
1
the three series of compounds shows that the decrease
of the overall aromaticity of the system by replacing a
thiophene cycle by one and then two double bonds leads
to a dramatic decrease of E02 - E01. Conversely, the data
for the hybrid compounds show that the presence of two
or more adjacent thiophene rings produces the reverse
effect with E0 - E0 reaching values similar to those
This major influence of ethylenic linkages is further
confirmed by the location of the data for the hybrid
compounds H-T4V1, H-T4V2, and H-T5V2 between the
plots corresponding to the nTs and nTVs/nTBs series.
Conversely, the data for H-T5V2 clearly show that
despite the partial rigidification induced by the two
ethylene linkages, the rotational disorder inherent to the
central terthiophene core is prevalent, making this
compound closer to the nTs series.
2
1
reported for nTs of comparable chain length.15
The magnitude of E0 - E0 reflects the Coulombic
2
1
repulsion between positive charges in the dicationic state.
Since T5V4 and T4B3 have the same number of sp2
carbons (28) the smaller E0 - E0 value observed for
Cyclic Volta m m etr y. Table 3 lists the cyclic volta-
mmetric (CV) data of the various oligomers. The CV of
all compounds presents two reversible one-electron oxi-
dation waves corresponding to the formation of the cation
2
1
T4B3b (40 mV vs 80 mV for T5V4b) shows that replace-
ment of a thiophene ring by two ethylene linkages allows
a better delocalization of the positive charges of the
dication, thus leading to a weaker Coulombic repulsion.
For short oligomers, the dication state is associated to a
strong quinoid deformation with the structural changes
located at the end of the molecule.16 In this configuration,
the decrease of the global aromatic character by the
insertion of ethylenic bonds favors charge delocalization
and the stabilization of the dication. The presence of the
ethylenic bonds plays an important rule on the energetic
cost to produce the structural deformation of the conju-
gated chain between the two charges. Hence, direct two-
electron transfer occurs for a chain length of 46 carbons
radical and dication at redox potential (E01) and (E0
)
2
except for T3V2a for which the second oxidation process
is irreversible.
For each type of structure, introduction of hexyl chains
at the R-positions of the terminal thiophene rings pro-
duces a small negative shift of E01 and E02 and a decrease
of the difference E0 - E0 indicating that the +I
2
1
inductive effect of the alkyl substituents raises the
HOMO level and stabilizes the cationic state.
As expected, chain extension produces a shift of E0
1
toward negative potentials reflecting an increase of the
HOMO level. Concurrently, E0 undergoes a negative
2
shift of larger magnitude which results in a decrease of
the difference E02 - E0 . Thus, the CV of T3B2b presents
1
two very close reversible oxidation peaks (∆E ) 100 mV)
while for T4B3b the CV consists of a single broad redox
system (Figure 5). Deconvolution reveals that this broad
(15) Ba¨uerle, P. Adv. Mater. 1992, 4, 102.
(16) Hapiot, P.; Kispert, L. D.; Konovalov, V. V.; Save´ant, J .-M. J .
Am. Chem. Soc. 2001, 123, 6669.
J . Org. Chem, Vol. 68, No. 19, 2003 7259

Fre`re et al.
TABLE 2. UV-vis Sp ectr ocop ic Da ta for th e Va r iou s
Oligom er s in CH2Cl2
compd
Cn
λ
max, nm (eV)
λ
0-0, nm (eV)
H-T4V1a
H-T4V2a
H-T5V2a
H-T4V1b
H-T4V2b
H-T5V2b
T3V2a
T4V3a
T5V4a
T3V2b
T4V3b
T5V4b
T2B1a
T3B2a
T4B3a
T3B2b
T4B3b
18
20
24
18
20
24
16
22
28
16
22
28
12
20
28
20
28
422 (2.94)
435 (2.85)
444 (2.79)
431 (2.88)
449 (2.76)
458 (2.71)
416 (2.98)
460 (2.70)
490 (2.53)
423 (2.93)
465 (2.67)
493 (2.52)
357 (3.47)
440 (2.82)
487 (2.55)
449 (2.76)
493 (2.52)
448 (2.77)
448 (2.77)
456 (2.72)
477 (2.60)
445 (2.79)
491 (2.53)
522 (2.38)
454 (2.73)
495 (2.51)
526 (2.36)
382 (3.25)
467 (2.66)
521 (2.38)
477 (2.60)
527 (2.35)
F IGURE 4. Plots of the absorption maximum versus the
reciprocal number of sp2 carbons for the various series of
conjugated oligomers (data for nTa series are taken from ref
14).
The analysis has been limited to the main characteristic
bands which were compared to those of nTs and oligo-
p-phenylenevinylene (nPVs) series. Table 4 lists the main
IR frequencies and their assignment. The spectrum of
nTVs presents strong bands around 930 cm-1 involving
a doublet for T3V2b and T4V3b and an intense broad
band for T5V4b. By analogy with nPVs which present a
strong absorption around 970 cm-1, these bands are
assigned to the C-H out-of-plane wag of the trans
vinylene.18 For nPVs, the downshift of the frequency of
the 970 cm-1 band with chain extension has been
attributed to a distortion of the conjugated chain.19 In
our case, the invariance of the frequency of the 930 cm-1
band with chain extension suggests that the nTV chain
is not subject to such a distortion.
F IGURE 3. UV-vis spectra of the oligomers in CH2Cl2: (top)
nTBb series; (middle) nTVb series; (bottom) H-TVb series.
in TVs4c and only for 20 carbons in oligoenes.16,17 For
T4B2b, the small value of ∆E shows that the convergence
of the two oxidation processes is close and should occur
for a situation intermediate between nTVs and oligoenes.
Vibr a tion a l Sp ectr oscop y. IR and Raman spectra
of nTVs and nTBs have been recorded in the solid state.
The spectrum of T5V4b presents a broad band at 797
cm-1 with a shoulder on the low-frequency side whereas
(18) (a) Tian, B.; Zerbi, G.; Mu¨llen, K. J . Chem. Phys. 1991, 95, 3198.
(b) Tian, B.; Zerbi, G.; Schenk, R.; Mu¨llen, K. J . Chem. Phys. 1991,
95, 3191.
(17) Heinze, J .; Tschuncky, P.; Smie, A. J . Solid State Electrochem.
1998, 2, 102.
(19) Sakamoto, A.; Y. Furakawa, Y.; M. Tasumi, M. J . Phys. Chem.
1992, 96, 1490.
7260 J . Org. Chem., Vol. 68, No. 19, 2003

Electronic Properties of Thiophene-Based π-Conjugated Systems
TABLE 3. Cyclic Volta m m etr ic Da ta for Oligom er s, 10-4
M in 0.1 M Bu 4NP F 6/CH2Cl2, Sca n Ra te 100 m V‚s-1
Refer en ce AgCl/Ag
,
compd
Cn
E01 (V)
E02 (V)
E0 - E01 (mV)
2
H-T4V1a
H-T4V2a
H-T5V2a a
H-T4V1b
H-T4V2b
H-T5V2b
T3V2a
T4V3a
T3V2b
T4V3b
T5V4b
18
20
24
18
20
24
16
22
16
22
28
20
28
20
28
0.86
0.80
0.73
0.74
0.71
0.68
0.87
0.74
0.76
0.67
0.62
0.76
0.62
0.67
0.60
1.07b
1.06
0.97
0.92
0.90
0.87
1.13b
0.92
0.97
0.80
0.70
0.92
0.70
0.77
0.64
210
260
240
180
190
170
260
180
220
130
80
T3B2a
160
70
100
40
T4B3a a
T3B2b
T4B3b
a
b
Saturated solution. Irreversible peak.
that of T4V3b and T3V2b shows an intense narrow band
at 800 cm-1 and two well-defined weak peaks at 780 and
770 cm-1. These bands can be assigned to the C-S
vibrations and C-H out-of-plane mode of the â-carbon
of the thiophene cycles.20 As shown in Scheme 4, three
different conformers corresponding to anti and syn
conformations are possible for T3V2 while the number
goes up to 10 for T5V4. Indeed, a syn-anti form of
T3V2a has been observed in the cristallographic struc-
ture11 and it is likely that for longer nTVs, a mixture of
the different conformers exists in the solid state which
could account for the observed broadening of the IR
bands. The effects of this conformational variability on
the vibrational spectrum have been analyzed in the case
of nTs.21
The IR spectrum of nTBbs presents a strong band
around 970 cm-1 corresponding to the C-H out-of-plane
wag of the trans-vinylene linkage. The frequency in-
creases from 970 cm-1 for T3B2b to 976 cm-1 for T4B3b
indicating that chain extension does not induce a severe
torsion of the π-conjugated system. As noted by Kos-
smhel, the presence of a cis double bond in the structure
of T2B1a , results in an intense band about 930 cm-1 8
.
The absence of such a band in the spectrum of nTBs
confirms, in agreement with NMR data, the all-trans
configuration of the molecules. The C-S vibrations and
the C-H out-of-plane mode occur as two bands at 823
and 800 cm-1. A strong and broad band is observed at
800 cm-1 in T4B3b, in contrast with T3B2b which
displays three resolved weak bands centered around 787
cm-1. Again, the increase of the number of possible
conformers when going from T3B2 to T4B3 can explain
the broadening of this band.
F IGURE 5. Cyclic voltammograms of n TBb oligomers 10-4
mol L-1 in 0.1 m Bu4NPF6/CH2Cl2, scan rate 100 mV s-1; (top)
T3B2b; (middle) T4B3b; (bottom) T4B3b deconvoluted.
Raman spectroscopy has been widely used to analyze
the chain length dependence of the effective conjugation
length in linear π-conjugated systems. Previous work on
oligomeric conjugated systems of homogeneous structure
such as oligoenes have established a linear dependence
of the Raman shifts of resonantly coupled mode with the
inverse conjugation length. For oligoenes involving 6 to
24 sp2 carbons, the CdC stretching mode was found to
shift by 130 cm-1 22
A more recent work on oligopyrroles
.
has shown that the so-called Я mode near 1500 cm-1
shifts by 45 cm-1 when increasing chain length from 3
to 7 pyrrole rings23 while for nTs oligomers pratically no
dispersion is observed with increasing the chain length.24
(20) (a) Hernandez, V.; Casado, J .; Ramirez, F. J .; Zotti, G.; Hotta,
S.; Lopez Navarrete, J . T. J . Chem. Phys. 1996, 104, 9271. (b)
Hernandez, V.; Kanamitsu, Y.; Lopez Navarrete, J . T. J . Raman
Spectrosc. 1998, 29, 617.
(21) Millefiori, S.; Alparone, A.; Millefiori, A. J . Heterocycl. Chem.
2000, 37, 847.
(22) Schaffer, H. E.; Chance, R. R.; Silbey, R. J .; Knoll, K.; Schrock,
R. R. J . Chem. Phys. 1991, 94, 4161.
J . Org. Chem, Vol. 68, No. 19, 2003 7261

Fre`re et al.
TABLE 4. IR F r equ en cies
for nTBs are limited to a couple of points, comparison of
the chain length dependence of the two types of CdC
stretching frequencies reveals two different behaviors.
For both nTVs and nTBs thiophenic CdC stretching data
are located on the same straight line and scale linearly
with 1/Cn. Conversely, data corresponding to the vinylic
CdC stretching frequencies for nTVs and nTBs follow
two parallel lines, for a given Cn value, the nTBs peaks
are observed at lower wavenumbers. These contrasting
behaviors suggest that the increase of the number of
double bonds between the thiophene rings results in the
creation of a short polyenic segment in which the
behavior typical to polyenes, namely the dimerization
becomes possible.
C-H out of plane wag for C-S and C-H out of plane for
compd the vinylene units (cm-1
)
the thiophene cycle (cm-1
)
T3V2b
933 (st)
930 (st)
800 (st)
780 (m)
770 (m)
800 (st)
780 (m)
770 (m)
797 (st)
broad signal
827 (m)
T4V3b
935 (st)
930 (st)
T5V4b
T3B2b
930 (st)
broad signal
970 (st)
787 (m)
broad signal
795 (m)
T4B3b
975 (st)
821 (m)
broad signal
Con clu sion
SCHEME 4
Three series of extended π-conjugated systems of well-
defined structure and geometry built up on various
combinations of thiophene rings and double bonds have
been synthesized and characterized. A comparative analy-
sis of the chain-length dependence of the optical, and
electrochemical properties of these various systems has
shown that, as expected, chain extension results in a
decrease of the HOMO-LUMO gap and in an increase
of the HOMO level. The results obtained with the various
series of oligomers provide a coherent picture showing
that the incorporation of double bonds in an oligothio-
phene structure leads to faster decrease of the HOMO-
LUMO gap with chain extension due to the synergistic
effects of enhanced planarity and lower overall aromatic
character. However, a further addition of ethylenic bonds
by insertion of butadiene units between the thiophene
rings does not produce the expected further narrowing
of the gap. These results lead to the conclusion that the
major effect of the ethylenic linkage concerns the pla-
narization of the π-conjugated system by suppression of
the rotational disorder inherent to oligothiophenes. Con-
sequently, while contributing to decrease the overall
aromatic character of the system, the incorporation of
second double bond brings some freedom to the system
which begins to behave as a “short polyene” in which
small changes in bond length alternation become pos-
sible. In other words, the small decrease of the overall
aromaticity associated with the insertion of an additional
double bond is counter-balanced by an increase of the
vibrational freedom.
The Raman spectra of nTVs and nTBs have been
recorded using 514.5 or 647.1 nm excitation line. As for
others π-conjugated oligomers, the Raman spectra are
rather simple despite the complex chemical structure. In
each oligomer a strong band assigned to the C-H cycle
bending in plane vibration, appears at 1044 cm-1
.
The CdC stretching region between 1200 and 1650
cm-1 of the Raman spectrum of nTVs and nTBs exhibits
two main bands around 1400 and 1600 cm-1 correspond-
ing to CdC stretching in thiophene ring deformation and
CdC stretching in ethylene linkage, respectively (Table
5). The thiophene CdC stretching frequency moves
downshifts by 16 cm-1 between T3V2b and T5V4b while
the vinylic (CdC) stretching downshifts by 10 cm-1. The
correlated shifts of these two CdC modes with chain
extension confirms that both “aromatic” and ethylenic
double bonds contribute to π-electron delocalization.
The Raman spectra of the nTBs are more complex and
show, in addition to the main bands observed in the TVs
spectra, intense bands around 1130 cm-1 assigned to the
C-C stretching of the butadiene units (short polyene
chain) (Table 5). Chain extension results in a downshift
of the main bands corresponding to the ν(CdC) modes of
both the thiophene cycle (from 1437 to 1417 cm-1 for
T3B2b and T4B3b respectively) and of the vinylene
linkages (from 1591 to 1587 cm-1). Again, these concomi-
tant shifts reflect the coupling of the two types of double
bonds of the π-conjugated system.
These various sets of results thus definitively confirm
that nTVs exhibit the smallest HOMO-LUMO gap
among extended π-conjugated oligomers.
Exp er im en ta l Section
1H and 13C spectra were recorded on a spectrometer operat-
ing at 500 MHz and 125.7 MHz respectively. δ are given in
ppm (relative to TMS) and J values in Hz. Cyclic voltammetry
experiments were performed under dry nitrogen atmosphere.
Working electrode (Pd disk) and counter electrode (Pt wire)
were used, all potentilas refer to AgCl/Ag as reference elec-
trode. EI mass spectra were obtained at 70 eV and accurate
mass measurements were performed at 10000 resolution (peak
width at 5% height) using PFK as internal reference. Room-
temperature micro-Raman experiments (x50 objective) were
measured using Ar+-Kr+ ion laser lines (514.5 and 647.1 nm)
in the backscattering geometry on a spectrometer equipped
with a liquid nitrogen cooled CCD detector. In all these
Figure 6 shows plots of the two types of CdC stretching
frequency versus 1/Cn for nTVs and nTBs. Although data
(23) Zerbi, G.; Veronelli, M.; Martina, S.; Schluˆtter, A.-D.; Wegner,
G. Adv. Mater. 1995, 5, 385.
(24) Hernandez, V.; Castiglioni, C.; Del Zoppo, M.; Zerbi, G. Phys.
Rev. B, 1994, 50, 9815.
7262 J . Org. Chem., Vol. 68, No. 19, 2003

Electronic Properties of Thiophene-Based π-Conjugated Systems
TABLE 5. Ra m a n F r equ en cies
C-H bending
CdC stretching
CdC stretching
C-C stretching
compd
(in plane) (cm-1
)
(thiophene) (cm-1
)
(vinylene) (cm-1
)
(butadiene) (cm-1
)
T3V2b
T4V3b
T5V4b
T3B2b
1044 (st)
1044 (st)
1045 (w)
1044 (st)
1434 (st)
1424 (st)
1418 (st)
1427 (sh)
1437 (st)
1468 (m)
1418 (st)
1437 (m)
1470 (m)
1603 (st)
1596 (st)
1593 (w)
1591 (st)
1600 (w)
1139 (st)
T4B3b
1045 (w)
1587 (st)
broad signal
1135 (m)
1162 (m)
1189 (m)
thiophene 8 and 2,5-bis(3-oxo-1propenyl)thiophene 13 were
synthesized by Wittig oxopropenylation with 1,3-dioxan-2-
ylmethyltributylphosphorane on the corresponding mono- or
dialdehyde.25
3-(2-Th ien yl)-2-p r op en ol (9). To a solution of 2-(3-oxo-1-
propenyl)thiophene 8 (3.3 g) in 40 mL of CH2Cl2/MeOH (1/1)
at 0 °C was added portionwise NaBH4 (0.9 g), and the mixture
was allowed to warm at room temperature and stirred for 30
mn. After addition of water (30 mL), the mixture was extracted
with twice 20 mL of dichloromethane. The organic phases were
dried over MgSO4 and evaporated in vacuo. The resulting oil
was distilled under reduced pressure (0.2 mbar, 101-102 °C)
to give a clear yellow oil (3.0 g, 88% yield): 1H NMR (CDCl3)
3
3
δ 7.15 (dd, 1H, J ) 3.5 Hz, J ) 2.3 Hz); 6.95 (m, 2H), 6.75
3
4
3
(dd, 1H, J ) 15.5 Hz, J ) 1.4 Hz); 6.18 (td, 1H, J ) 5.6 Hz,
3J ) 15.5 Hz); 4.26 (dd, 2H, J ) 5.6 Hz, J ) 1.4 Hz); 2.01 (s,
3
4
1H).
Dieth yl [3-(2-Th ien yl)-2-p r op en yl]p h osp h on a te (P c).
P r ep a r a tion of th e Solu tion of An ion of Dieth yl P h os-
p h ite. Under N2 atmosphere, a suspension of 2.8 g of NaH
(60% in oil, washed with twice 10 mL of dry THF) in 10 mL of
THF was cooled at -15 °C. A solution of 8 mL of diethyl
phosphite in 40 mL of dry THF was added dropwise over a
period of 1 h. After 1 h of stirring, the mixture was allowed to
stand at -5 °C.
P r ep a r a tion of P h osp h on a te. To a solution of 2.95 g of
alcohol 9 in benzene (20 mL)-toluene (10 mL) cooled at -20
°C was added 3.5 mL of PBr3, and the mixture was allowed to
warm at -10 °C. The solution of bromide derivative was
rapidly transferred in a dropping funnel and was added to the
solution of anion of diethyl phosphite at -10 °C. The mixture
was allowed to warm at room temperature and stirred for 2
h. After addition of water (100 mL), the mixture was extracted
with twice 50 mL of Et2O. The organic phase was separated,
and the aqueous phase was extracted with Et2O. The organic
phases were dried over MgSO4 and evaporated in vacuo. Flash
chromatography on silica gel (AcOEt/CH2Cl2 2/1) afforded oil
which was distilled with a Kugelrhor apparatus (0.2 mbar, 173
°C) to give a clear yellow oil (3.7 g, 74% yield): MS (EI) m/z )
1
260 [M•+]; H NMR (CDCl3) δ 7.10 (d, 3J ) 4.7 Hz, 1H), 6.9
3
4
(m, 2H), 6.65 (dd, J ) 15.5 Hz, J H-P ) 5.1 Hz, 1H), 5.9 (td,
3J ) 15.5 Hz, J H-P ) 7.7 Hz, 1H), 4.10 (m, 4H), 2.70 (dd, J
) 7.7 Hz, J H-P ) 22.3 Hz, 2H), 1.30 (m, 6H).
3
3
2
Gen er a l P r oced u r e for th e Vilsm eier Rea ction . To a
solution of 2.3 mmol of bithiophene derivatives 1a ,b or T2B1a
and 0.25 mL of DMF in 25 mL of dry dichloroethane, in a
round-bottomed flask kept under a nitrogen atmosphere, was
added 0.26 mL of POCl3, and then the mixture was refluxed
for 8 h. After cooling to room temperature, 1 M sodium acetate
was added to neutrality and the mixture was stirred vigorously
for 1 h. The solution was extracted with dichloromethane, and
the organic phase was dried over Na2SO4. After evaporation
of the solvent, the crude product was purified by chromatog-
raphy on silica gel (eluent, CH2Cl2).
F IGURE 6. Shift of the CdC stretching mode vs reciprocal
chain length for nTVbs and nTBbs: (top) thiophene CdC
stretching; (bottom) vinylic CdC stretching.
experiments, the average laser power density was kept below
50 mW/mm2 in order to minimize the local heating and to avoid
sample degradation. Room-temperature IR transmission was
carried out on samples pelletized with KBr using a fast-scan
Fourier transform spectrometer. The investigated frequency
range was 450-4000 cm-1. The resolution was fixed at 2 cm-1
.
2-Thiophenecarbaldehyde and 2-hexylthiophene were pur-
chased and used as received. Dialdehyde derivatives with
thiophene (5), bithiophene (3), terthiophene (4), and thienyl-
enevinylene (6 and 7) units as spacer were synthesized by a
n-BuLi-DMF sequence.25 Bithiophene 1a and 1b were ob-
tained from Kumada or Stille coupling. 2-(3-Oxo-1-propenyl)-
(25) (a) Feringa, B. L.; Hulst, R.; Rikers, R.; Brandsma, L Synthesis
1998, 316. (b) Nakayama, J .; Fujimori, T.; Sulfur Lett. 1990, 11, 29.
(c) Elandaloussi, E. H.; Fre`re, P.; Roncali, J .; Richomme, P.; J ubault,
M.; Gorgues, A. Adv. Mater. 1995, 7, 390.
J . Org. Chem, Vol. 68, No. 19, 2003 7263

Fre`re et al.
3
3
(E-E)-1-(2-Th ien yl)-4-(5-for m yl-2-t h ien yl)b u t a -1,3-d i-
en e (11): orange powder (74% yield); mp 116 °C; 1H NMR
(CDCl3) δ 9.83 (s, 1H), 7.63 (d, 1H,3J ) 3.8 Hz), 7.24 (d, 1H,3J
) 5.0 Hz), 7.06 (d, 2H,3J ) 3.8 Hz), 7.00 (dd, 1H,3J ) 5.0 Hz,
3J ) 3.8 Hz), 6.91 (dd, 1H,3J ) 15.1 Hz, 3J ) 10.5 Hz), 6.89 (d,
1H,3J ) 15.3 Hz), 6.74 (d, 1H,3J ) 15.1 Hz), 6.70 (dd, 1H,3J )
Hz), 6.67 (d, 2H, J ) 3.7 Hz), 2.80 (t, 4H, J ) 7.5 Hz), 1.70
(m, 4H), 1.45-1.25 (m, 12 H), 0.92 (t, 6H, 3J ) 7.3 Hz); 13C
NMR (CDCl3) δ 145.7, 140.8, 136.8, 134.8, 126.9, 124.8, 123.4
(2C), 121.1, 31.6, 31.5, 30.2, 28.7, 22.5; HRMS (EI) for C30H36S4,
calcd 524.1699, obsd 524.1694.
Gen er a l P r oced u r e for th e Wittig-Hor n er Olefin a -
tion . Under nitrogen atmosphere, potassium tert-butoxide was
added portionwise to a mixture containing the aldehyde and
the phosphonate in dry THF. The mixture was stirred at room
temperature for 1 h. After evaporation of the solvent, the
residue was taken with MeOH to give a precipitate. The solid
was filtered then was washed twice with MeOH. The crude
product was purified by recrystallization.
3
15.3 Hz, J ) 10.5 Hz); C13H10OS2, MS (EI) m/z ) 246 [M•+].
5-F or m yl-2,2′-bith iop h en e (2a ): yellow powder (80%
1
yield); mp 58-59 °C (lit.27 mp 57-59 °C); H NMR (CDCl3) δ
9.86 (s, 1H), 7.67 (d, 1H,3J ) 3.9 Hz), 7.35 (d, 2H,3J ) 4.4 Hz),
7.25 (d, 1H,3J ) 3.9 Hz), 7.10 (t, 1H,3J ) 4.4 Hz); C9H6OS2,
MS (EI) m/z ) 194 [M•+].
5-F or m yl-5′-n -h exyl-2,2′-bith iop h en e (2b): yellow oil
1
(85% yield); H NMR (CDCl3) δ 9.85 (s, 1H), 7.65 (d, 1H,3J )
(E,E)-5,5′-Bis[2-(2-th en yl)-1-eth en yl]th ien ylvin yl]-2,2′-
bith iop h en e (H-T4V2a ). orange powder recrystallized from
ethanol; yield 87%; mp 210 °C; 1H NMR (C6D6) δ 6.99 (d, 2H,
3J ) 15.7 Hz), 6.93 (d, 2H, 3J ) 15.7 Hz), 6.86 (d, 2H, 3J ) 3.7
4.0 Hz), 7.18 (d, 1H,3J ) 3.8 Hz), 7.16 (d, 1H,3J ) 3.9 Hz),
6.78 (d, 1H,3J ) 3.8 Hz); C15H18OS2, MS (EI) m/z ) 278 [M•+].
(E,E)-1,4-Bis(5-for m yl-2-th ien yl)bu ta -1,3-d ien e (12). To
a solution of T2B1a (0.634 g, 2.9 mmol) in 100 mL of Et2O +
50 mL of THF, cooled at 0 °C under nitrogen, was added
dropwise a solution of n-BuLi 1.6 M in hexane (5.5 mL, 3
equiv). The mixture was stirred at 0 °C for 2 h, and 7 mL (90
mmol) of DMF was added. After 2 h of stirring at ambient
temperature and hydrolysis with 3 N aqueous NH4Cl, the
mixture was extracted with CH2Cl2, and the organic phases
were washed with water, dried over MgSO4, and evaporated.
Column chromatography of the residue (silica gel, CH2Cl2/ethyl
acetate 10:1) gave a red solid (0.424 g, 56% yield): mp 200 °C
3
3
Hz), 6.73 (d, 2H, J ) 4.8 Hz), 6.68 (d, 2H, J ) 3.2 Hz), 6.65
3
3
3
(dd, 2H, J ) 4.8, Hz, J ) 3.2 Hz), 6.50 (d, 2H, J ) 3.7 Hz);
HRMS (EI) for C20H14S4, calcd 381.9978, obsd 381.9966.
(E ,E )-5,5′-Bis[2-(5-h e xyl-2-t h ie n yl)-1-e t h e n yl]-2,2′-
bith iop h en e (H-T4V2b): orange powder recrystallized from
ethanol; yield 65%; mp 154 °C; 1H NMR (C6D6) δ 7.07 (d, 2H,
3J ) 15.7 Hz), 7.00 (d, 2H, 3J ) 15.7 Hz), 6.93 (d, 2H, 3J ) 3.8
3
3
Hz), 6.69 (d, 2H, J ) 3.5 Hz), 6.58 (d, 2H, J ) 3.8 Hz), 6.54
3
3
(d, 2H, J ) 3.5 Hz), 2.62 (t, 4H, J ) 7.5 Hz), 1.62-1.57 (m,
4H), 1.29-1.21 (m, 12 H), 0.92 (t, 6H, J ) 7.3 Hz); 13C NMR
3
1
(lit.28 mp 200 °C); H NMR (C6D6) δ 9.47 (s, 2H), 6.82 (d, 2H,
(CDCl3) δ 145.8, 141.8, 140.0, 135.8, 126.6, 126.3, 124.7, 124.1,
3
3
3J ) 3.9 Hz), 6.45 (d, 2H, J ) 3.9 Hz), 6.40 (2H, AA′, J AX
)
122.1, 121.0, 31.6, 31.5, 30.4, 28.7, 22.5, 14.0; MALDI-TOF for
3
3
3
15.3 Hz, J AA′ ) 10.5 Hz, J AX′ ) 0.9 Hz, J XX′ ) 0 Hz), 6.01
C
32H38S4, calcd 550.18, obsd 550.16; HRMS (EI) for C32H38S4,
calcd 550.1856, obsd 550.1867. Anal. (Calcd): C, 69.95 (69.77);
H, 6.94 (6.95).
3
3
3
3
(2H, AA′, J AX ) 15.3 Hz, J AA′ ) 10.5 Hz, J AX′ ) 0.9 Hz, J XX′
) 0 Hz); C14H10O2S2, MS (EI) m/z ) 274 [M•+].
(E,E)-1,4-Bis(5-a cr olein -2-th ien yl)bu ta -1,3-d ien e (14).
To the solution of 2 mmol of dilithiated derivative of compound
T2B1a was added 0.7 mL (7 mmol) of 3-dimethylaminoacrolein
dropwise at 0 °C under nitrogen. After 4 h of stirring at
ambient temperature and hydrolysis with 3 N aqueous NH4-
Cl, the usual workup and column chromatography (silica gel,
CH2Cl2/ethyl acetate 10:1) gave a red solid (220 mg, 33%
yield): mp 191 °C; 1H NMR (C6D6) δ 9.35 (d, 2H, 3J ) 7.4 Hz),
6.67 (d, 2H, 3J ) 15.6 Hz), 6.56 (d, 2H, 3J ) 3.8 Hz), 6.49 (2H,
(E,E)-5,5′′-Bis[2-(2-th ien yl)-1-eth en yl]th ien ylvin yl]-2,2′:
5′,2′′-ter th iop h en e (H-T5V2a ): orange powder; yield 85%;
mp 260 °C dec; HRMS (EI) for C24H16S5, calcd 463.9856, obsd
463.9865. Solubility was too low to perform spectroscopic
analyses.
(E, E)-5,5′′-Bis[2-(5-h exyl-2-th ien yl)-1-eth en yl]-2,2′:5′,2′′-
ter th iop h en e (H-T5V2b): orange powder recrystallized from
1
CHCl3-hexane; yield 72%; mp 228 °C dec; H NMR (C6D6) δ
3
3
7.03 (d, 2H, J ) 15.7 Hz), 6.95 (d, 2H, J ) 15.7 Hz), 6.86 (s,
2H), 6.88 (d, 2H, 3J ) 3.7 Hz), 6.65 (d, 2H, 3J ) 3.5 Hz), 6.52-
6.49 (m, 4H), 2.57 (t, 4H, 3J ) 7.5 Hz), 1.61-1.52 (m, 4H),
3
3
3
3
AA′, J AX ) 15.1 Hz, J AA′ ) 10.3 Hz, J AX′ ) 0.7 Hz, J XX′ ) 0
3
3
3
Hz), 6.47 (d, 2H, J ) 3.8 Hz), 6.40 (dd, 2H, J ) 15.6 Hz, J
) 7.4 Hz), 6.22 (2H, AA′, 3J AX ) 15.1 Hz, 3J AA′ ) 10.3 Hz, 3J AX′
) 0.7 Hz, 3J XX′ ) 0 Hz); 13C NMR (CDCl3) δ 192.6, 147.4, 143.9,
138.5, 133.5, 130.7, 128.0, 127.1, 126.5; C18H14O2S2, MS (EI)
m/z ) 326 [M•+].
3
1.24-1.19 (m, 12 H), 0.86 (t, 6H, J ) 7.0 Hz); MALDI-TOF
for
C36H40S5, calcd 632.17, obsd 632.21; HRMS (EI) for
C36H40S5, calcd 632.1734, obsd 632.1716. Anal. (Calcd): C, 68.47
(68,30); H, 6.25 (6.37).
Gen er a l P r oced u r e for th e Mc Mu r r y Rea ction . To a
suspension of low valent Ti prepared from TiCl4 (0.66 mL, 6
mmol) and Zn (0.78 g, 12 mmol) in 30 mL of dry THF under
N2 at 0 °C was added a dry solution of aldehyde 1a or 1b (5
mmol) in 10 mL of THF. After 2 h of refluxing, the mixture
was cooled to room temperature, poured into water, and then
extracted with CH2Cl2. The organic phase was washed with
water and dried over MgSO4. After solvent removal, the crude
solid was recrystallized.
(E ,E )-2,5-B is [2-(5-h e x y l-2-t h ie n y l)-1-e t h e n y l]t h io -
p h en e (T3V2b): orange powder recrystallized from ethanol;
3
yield 70%; mp 82-83 °C; 1H NMR (CDCl3) δ 6.93 (d, 2H, J )
3
15.7 Hz), 6.86 (d, 2H, J ) 15.7 Hz), 6.85 (s, 2H), 6.83 (d, 2H,
3
3
3J ) 3.6 Hz), 6.65 (d, 2H, J ) 3.6 Hz), 2.78 (t, 4H, J ) 7.5
Hz), 1.70-1.65 (m, 4H), 1.34-1.30 (m, 12 H), 0.90 (t, 6H, 3J )
7.3 Hz); 13C NMR (CDCl3) δ 145.71, 141.30, 140.06, 126.59,
126.22, 124.71, 121.95, 120.27, 31.56, 31.47, 30.42, 28.74,
22.55, 14.06; HRMS (EI) for C28H36S3, calcd 468.1979, obsd
468.1996. Anal. (Calcd): C, 70.88 (71.74); H, 7.73 (7.74); S,
20.00 (20.52).
1,2-Bis[2-(2-th ien yl)-5-th ien yl]eth en e (H-T4V1a ): or-
ange powder recrystallized from ethanol; yield 40%; mp 190
1
3
°C dec; H NMR (CDCl3) δ 7.22 (d, 2H, J ) 5.0 Hz), 7.18 (d,
(E,E,E)-1,2-Bis[5-[2-(5-h exyl-2-th ien yl)-1-eth yn yl]-2-th ie-
n yl]eth en e (T4V3b): red powder recrystallized from CHCl3-
3
3
3
2H, J ) 3.5 Hz), 7.07 (d, 2H, J ) 3.7 Hz), 7.03 (dd, 2H, J )
3
3
5.0 Hz, J ) 3.7 Hz), 6.95 (s, 2H), 6.93 (d, 2H, J ) 3.7 Hz);
HRMS (EI) for C18H12S4, calcd 355.9821, obsd 355.9832.
1,2-Bis[2-((5-h e xyl-2-t h ie n yl)-5-t h ie n yl]e t h e n e (H -
T4V1b): orange powder recrystallized from ethanol; yield
1
hexane; yield 60%; mp 146-147 °C; H NMR (CDCl3) δ 6.95
3
3
(d, 2H, J ) 15.7 Hz), 6.92 (s, 2H), 6.89 (d, 2H, J ) 4,0 Hz),
3
3
6.87 (d, 2H, J ) 4,0 Hz), 6.86 (d, 2H, J ) 15.7 Hz), 6.84 (d,
3
3
3
2H, J ) 3.8 Hz), 6.65 (d, 2H, J ) 3.8 Hz), 2.78 (t, 4H, J )
7.3 Hz), 1.70-1.65 (m, 4H), 1.35-1.29 (m, 12 H), 0.90 (t, 6H,
3J ) 6.4 Hz); 13C NMR (CDCl3) δ 145.8, 141.8, 141.0, 140.0,
127.7, 127.2, 126.7, 126.4, 122.2, 121.4, 120.2, 31.5, 31.4, 30.4,
28.7, 22.5, 14.0; HRMS (EI) for C34H40S4, calcd 576.2013, obsd
576.2036. Anal. (Calcd): C, 69.95 (70.78); H, 6.94 (6.99); S,
21.81 (22.23).
60%; mp 137 °C; 1H NMR (CDCl3) δ 6.98 (d, 2H, J ) 3.8 Hz),
3
6.96 (d, 2H, 3J ) 3.8 Hz), 6.91 (s, 2H), 6.89 (d, 2H, 3J ) 3.7
(26) (a) Spangler, C. W.; McCoy, R. Synth. Commun. 1988, 18, 51.
(b) Koâmehl, G.; Bohn, G. Chem. Ber. 1974, 107, 2791.
(27) Wei, Y.; Yang, Y.; Yeh, J .-M. Chem. Mater. 1996, 8, 2659.
(28) Benahmed-Gasmi, A.; Fre`re, P.; Elandaloussi, E. H.; Roncali,
J .; Orduna, J .; Gar´ın, J .; J ubault, M.; Riou, A.; Gorgues, A. Chem.
Mater. 1996, 8, 2291.
(E,E,E,E)-2,5-Bis[2-[5-(5-h exyl-2-t h ien yl)-2-t h ien yl]-1-
eth en yl]th iop h en e (T5V4b): red powder recrystallized from
7264 J . Org. Chem., Vol. 68, No. 19, 2003

Electronic Properties of Thiophene-Based π-Conjugated Systems
1
CH2Cl2-hexane; yield 65%; mp 208-209 °C; H NMR (C6D6)
(m, 12 H), 0.87 (t, 6H, 3J ) 7.2 Hz); 13C NMR (CDCl3) δ 145.87,
141.99, 140.53, 128.91, 127.31, 126.87, 126.17, 126.09, 125.60,
124.76, 31.48, 30.42, 29.69, 28.74, 22.60, 14.08; HRMS (EI)
for C32H40S3, calcd 520.2292, obsd 520.2290. Anal. (Calcd): C,
74.03 (73.79); H, 7.68 (7.74); S,18.71 (18.47).
δ 7.10 (d, 2H, 3J ) 15.4 Hz), 7.00(d, 2H, 3J ) 15.4 Hz), 6.98 (s,
4H), 6.67 (d, 2H, 3J ) 3.1 Hz), 6.57-6.54 (m, 6H), 6.50 (d, 2H,
3J ) 3.3 Hz), 2.57 (t, 4H, 3J ) 7.5 Hz), 1.60-1.50 (m, 4H),
1.36-1.30 (m, 12 H), 0.87 (t, 6H, 3J ) 7.2 Hz); 13C NMR
(CDCl3) δ 145.94, 142.02, 141.61, 141.00, 140.02, 127.42,
127.35, 126.76, 126.48, 124.81, 122.30, 121.72, 121.37, 120.19,
31.61, 31.51, 30.48, 28.79, 22.61,14.31; HRMS (EI) for C40H44S5,
calcd 684.2047, obsd 684.2071. Anal. (Calcd): C, 69.85 (70.12);
H, 6.12 (6.47); S, 23.14 (23.40).
(E,E,E,E,E,E)-1,4-Bis[5-[4-(5-h exyl-2-th ien yl)-1,3-bu ta -
d ien yl]-2-th ien yl]bu ta -1,3-d ien e (T4B3b): red powder re-
1
crystallized from CHCl3; yield 70%; mp 195 °C dec; H NMR
3
3
(CDCl3) δ 6.85 (d 2H, J ) 3.8 Hz), 6.83 (d 2H, J ) 3.8 Hz),
3
3
6.80 (d 2H, J ) 3.5 Hz), 6.74-6.54 (m, 14H), 2.8 (t, 4H, J )
7.5 Hz), 1.60-1.50 (m, 4H), 1.30-1.10 (m, 12 H), 0.87 (t, 6H,
3J ) 7.2 Hz); 13C NMR (CDCl3) δ 145.94, 142.35, 141.85,
140.85, 131.12, 129.03, 128.65, 127.27, 126.93, 126.22, 125.59,
124,77, 124.70, 31.55, 30.42, 28.74, 22.56, 17.14, 14.07, 13.08;
HRMS (EI) for C40H46S4, calcd 654.2482, obsd 654.2478. Anal.
(Calcd): C, 73.18 (73.34); H, 7.10 (7.08).
(E,E)-1,4-Bis(2-th ien yl)bu ta -1,3-d ien e (T2B1a ). Rou te
A. From phosphonate P a and 2-(3-oxo-1-propenyl)thiophene
8, 85% yield. Rou te B. from phosphonate P c and 2-carbalde-
hydethiophene 10: 60% yield; yellow powder recrystallized
from ethanol; mp 174 °C (lit.29 mp 173 °C); H NMR (C6D6) δ
1
3
3
6.74 (d, 2H, J ) 5.0 Hz), 6.71 (d, 2H, J ) 3.6 Hz), 6.68 (dd,
2H, 3J ) 5.0 Hz, 3J ) 3.6 Hz), 6.60 (2H, AA′ part of the AA′XX′
system, J AX ) 15.3 Hz, J AA′ ) 10.3 Hz, J AX′ ) 0.5 Hz, J XX′ ) 0
Hz), 6.35 (2H, XX′ part of the AA′XX′ system′, J AX ) 15.3 Hz,
J AA′ ) 10.3 Hz, J AX′ ) 0.5 Hz, J XX′ ) 0 Hz).
X-r ay Cr ystallogr aph y of 14. Single crystals were mounted
on an Enraf-Nonius MACH3 diffractometer with graphite
monochromator and Mo KR (λ ) 0.717 03 Å) radiation at T )
294 K. The data collections were performed with the ω/2θ scan
technique. The crystal structures were solved by direct method
(SIR) and refined by full matrix least squares techniques using
ShelX software. The positions of hydrogen atoms were obtained
from Fourier difference.
Crystal data of 14: C18H14S2O2, triclinic space group P-1, a
) 3.929(2) Å, b ) 10.03(1) Å, c ) 11.00(1) Å, R ) 114.3(1)°, â
) 92.98(8)°, γ ) 92.90(7)°, V ) 393(1) Å3, Z ) 1, T ) 293 K,
F(000) ) 170, µ(Mo KR) ) 0.341 mm-1; 1458 reflections
collected from which 1381 (Rint ) 0.032) were independent,
1032 reflections with I > 2σ(I), 128 refined parameters, R1 )
0.045, wR2 (all data) ) 0.114.
(E ,E ,E ,E )-2,5-Bis[4-(2-t h ie n yl)-1,3-b u t a d ie n yl]t h io-
p h en e (T3B2a ): yellow powder recrystallized from CHCl3-
hexane; yield 74%; mp 212 °C (lit.7 mp 212 °C); 1H NMR (C6D6;
see text and Table 1 for the discussion) δ 6.78 (d, 2H, 3J ) 5.0
Hz), 6.72 (d, 2H, J ) 3.5 Hz), 6.68 (dd, 2H, J ) 5.0 Hz, J )
3.5 Hz), 6.65 (AA′,4H), 6.62 (s, 2H), 6.41 (XX′, 2H), 6.32 (XX′,
2H); 13C NMR (CDCl3) δ 142.89, 142.02, 128.73, 128.51, 127.72,
127.16, 126.02, 125.61, 125.46, 124.56; HRMS (EI) for C20H16S3,
calcd 352.0414, obsd 352.0420. Anal. (Calcd): C, 67.74 (68.14);
H, 4.80 (4.57).
3
3
3
(E,E,E,E,E,E)-1,4-Bis[5-[1-(4-(2-th ien yl))-1,3-bu tadien yl]-
2-th ien yl]bu ta -1,3-d ien e (T4B3a ): orange powder; yield
60%; HRMS (EI) for C28H22S4, calcd 486.0604, obsd 486.0601.
Solubility was too low to perform spectroscopic analyses.
(E,E,E,E)-2,5-Bis[1-[4-(5-h exyl-2-th ien yl)]-1,3-bu tadien yl]-
t h iop h en e (T3B2b ): orange powder recrystallized from
Ack n ow led gm en t. Financial support from MCyT-
FEDER (Project No. BQU2002-00219) and Gobierno de
Arago´n-Fondo Social Europeo (Project No. P-009-2001)
is gratefully acknowledged.
1
CHCl3-hexane; yield 66%; mp 119-120 °C; H NMR (C6D6;
see text and Table 1 for the discussion) δ 6.74-6.62 (m, 6H),
Su p p or tin g In for m a tion Ava ila ble: The X-ray data of
14 in CIF format. This material is available free of charge via
the Internet at http://pubs.acs.org.
6.60 (s, 2H), 6.53 (d, 2H, 3J ) 3.7 Hz), 6.45 (XX′, 2H), 6.35
3
(XX′, 2H), 2.75 (t, 4H, J ) 7.5 Hz), 1.70-1.60 (m, 4H), 1.33
(29) Ribereau, P.; Pastour, P. Bull. Soc. Chim. Fr. 1969, 6, 2076.
J O034319O
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