Electronic properties of thienylene vinylene oligomers: Synthesis and theoretical study

Article abstract of DOI:10.1007/s11224-012-9979-0

(E)-1,2-bis(2-thienyl)vinylene (TV), (E)-1,2-bis (3-octyl-2-thienyl) vinylene (TOV), (E)-1,2-bis(3-(2-ethylhexyl)-2-thienyl)vinylene (T2EHV), and (E)-1,2-bis-[2,2'-bithienyl] vinylene (BTV) have been synthesized by a combination of formylation reaction and Mc Murry dimerization. UV-Vis spectra of BTV showed the longest wavelength absorption, TOV and T2EHV showed a bathochromic shift to the red compared with TV, due to an increment of delocalization of the conjugated p-system as the result of the weakening of the carbon-carbon double bonds of the thienyl rings due to the substitution of one hydrogen by the alkyl group. Based on optical data, the effect of linear and branched alkyl chain and extension of conjugation length on the electronic properties is discussed. ¹H, ¹³C-NMR, UV-Visible, Fluorescence data are discussed and theoretical DFT and TD-DFT calculations of ground state and excited states have been also considered in the analysis and explanation of results. Springer Science+Business Media, LLC 2012.

Full text of DOI:10.1007/s11224-012-9979-0

Struct Chem (2012) 23:1751–1760
DOI 10.1007/s11224-012-9979-0
ORIGINAL RESEARCH
Electronic properties of thienylene vinylene oligomers:
synthesis and theoretical study
•
G. Neculqueo V. Rojas Fuentes A. Lopez
•
•
´
R. Matute S. O. Vasquez F. Martınez
•
•
´
´
Received: 27 October 2011 / Accepted: 14 February 2012 / Published online: 9 March 2012
Ó Springer Science+Business Media, LLC 2012
Abstract (E)-1,2-bis(2-thienyl)vinylene (TV), (E)-1,2-bis
(3-octyl-2-thienyl)vinylene (TOV), (E)-1,2-bis(3-(2-ethyl-
hexyl)-2-thienyl)vinylene (T2EHV), and (E)-1,2-bis-[2,2’-
bithienyl] vinylene (BTV) have been synthesized by a com-
bination of formylation reaction and Mc Murry dimerization.
UV–Vis spectra of BTV showed the longest wavelength
absorption, TOV and T2EHV showed a bathochromic shift to
the red compared with TV, due to an increment of delocal-
ization of the conjugated p-system as the result of the
weakening of the carbon–carbon double bonds of the thienyl
rings due to the substitution of one hydrogen by the alkyl
group. Based on optical data, the effect of linear and branched
alkyl chain and extension of conjugation length on the elec-
Introduction
In recent years the thienylene vinylene derivatives, formed
from a combination of thiophene and the vinyl group, are
known to have a delocalized p system, which is an
important component for building electronic devices [1]. In
addition, the incorporation of vinylene bonds between
aromatic rings leads to an increment of the degree of co-
planarity of the system and extend the conjugation length,
reducing the steric hindrance in successive aromatic rings
which is desirable for polymers to be used as electro-
optical devices [2–5]. For instance, poly(thienylene vinyl-
ene) has a lower band gap than polythiophene [6–12] and
can be regarded as a suitable material for semiconductor
applications because of high flatness, high conductivity,
and great potential mobility. [8, 13]
1
tronic properties is discussed. H, 13C-NMR, UV–Visible,
Fluorescence data are discussed and theoretical DFT and TD-
DFT calculations of ground state and excited states have been
also considered in the analysis and explanation of results.
In this study, we compare the extension of conjugation
length between thienylvinylene and bithienylvinylene and
the effect of linear and branched alkyl chain on the elec-
tronic properties of thienylvinylene. Roncali et al. have
reported the effect of increasing number of thiophene rings
bridged by vinylene units and also compare the a and b
alkyl substituted thienylvinylene [5, 14]. They have con-
cluded that alkyl substitution at a position at the end of
thiophene rings has little or none influence on the solubility
and electronic properties, whereas mono or di-substitution
at the b position of the thiophene rings enhances strongly
the solubility. At the same time, spectroscopic and oxida-
tion potential of thienyl-vinylene oligomers show expected
bathochromic shift of kmax and decrease of the HOMO–
LUMO gap with extension of the conjugation length.
Theoretical approaches to calculate ground state and
excited states properties have shown to be useful in
this kind of systems and can yield interesting results
on geometry aspects, localization-delocalization of the
Keywords Thienylene vinylene ꢀ Oligomers ꢀ
Optoelectronic ꢀ TD-DFT
´
G. Neculqueo ꢀ V. Rojas Fuentes ꢀ A. Lopez ꢀ
´
´
S. O. Vasquez (&) ꢀ F. Martınez (&)
Departamento de Ciencia de los Materiales, Facultad de Ciencias
´
´
Fısicas y Matematicas, Universidad de Chile, Casilla,
2777 Santiago Chile, Chile
e-mail: svasqueza@ing.uchile.cl
´
F. Martınez
e-mail: polimart@ing.uchile.cl
R. Matute
Department of Chemistry and Biochemistry,
University of California, Los Angeles, USA
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Struct Chem (2012) 23:1751–1760
p-conjugated system and solvation effects [15], which are
included in experimental electronic transitions. Recently,
we have applied this approach to the study of the
observed spectral features of 4,40-bis(3-octylthienyl)bi-
phenyl (OTBP) and 4,40-bis(thienyl)biphenyl [16].
rigidity of the structure as a result of the introduction of the
vinylene unit in the structure, which reduce the steric
interactions between the aromatic rings and extend the
conjugation [2, 5, 14, 20], so it would produce a greater
planarity and electron delocalization of p electrons. This
effect would be even more pronounced when comparing
the kmax of terthienyl vinylene, which absorbs at 462 nm
(data not reported) with sexithiophene which absorbs to
434 nm, identical to our BTV with only four thiophene
rings. Now, taking into account the effect of linear (TOV)
and branched alkyl chain (T2EHV) on the electronic
spectra (Fig. 1), we observe again a bathochromic shift,
kmax 353 and 354 nm respect to the 344 nm of TV.
Theoretical calculations were performed using the true
structures for the unsubstituted TV and BTV oligomers
but simplified structures for the alkyl OTV and T2EHV
substituted oligomers. In fact, in the last cases, the eight
member alkyl chain was shortened to just a CH2–CH2–
CH2–CH3 chain for OTV and a CH2–CH–(CH3)2 branched
chain for T2EHV. As the alkyl chains are somewhat flex-
ible and have rotational freedom along the chain axis, the
longest alkyl chain, the slower the convergence of the
calculation. To maintain cpu times within reasonable fig-
ures we used this simplified structures for OTV and
T2EHV, keeping in mind that this kind of treatment has
already been tested in previous work [16].
Computational aspects
Calculations performed with Gaussian 03 and Gaussian 09
codes [17] consider optimization of the ground state
geometry of TV, BTV, TOV, and T2EHV using the density
functional theory (DFT) level of theory with the Becke, Lee,
Yang and Parr B3LYP hybrid functional for exchange and
correlation effects and the 6-31G?(d,p) basis set. Geometry
optimizations were carried out in a two-step procedure: first
as isolated gas phase molecules and subsequently we use the
Tomasi’s Polarized Continuum Model (PCM) to account for
changes under the effect of the chloroform solvent.
The vertical S0 ? S1 transition corresponding to
absorption spectra of each of the four oligomers were
resolved by time-dependent DFT (TD-DFT/B3LYP/
6-31G?(d,p)) calculations [18]. Subsequent study of the
geometry for the first excited singlet state is made by
optimization using the TD-DFT/B3LYP/6-31G?(d,p)
methodology implemented in G09 and finally data for the
energies of the S1 ? S0 emission were also obtained for
TV, BTV, TOV, and T2EHV including solvent effects.
Figure 2 show the optimal geometry obtained for the
four oligomers in ground state and the atomic labels con-
sidered for the following analysis.
Conformational aspects of the four oligomers in elec-
tronic ground state show that they are similar in nature. The
TV is the only structure that is completely planar in gas
phase and remains unchanged under the solvent effect of
the chloroform (see Table 1 for details). Bond lengths of
the nine C–C bonds of the dithienyl-vinylene chain show
Results and discussion
Absorption spectra
Figure 1 is the summary of the electronic absorption
spectra of the four oligomers diluted in chloroform. The
kmax absorption of (E)-1,2-di-(thiophene)vinylene (TV) at
344 nm is associated with the arrangement and delocal-
ization of existing five conjugated double bonds in the
molecule and its value is between the absorption wave-
lengths of 2,2’-bithiophene (300 nm and four conjugated
bonds) and 2,20:50:200-terthiophene (354 nm and six con-
jugated bonds), respectively, and consistent with the fact
that a longer conjugation increases the value of wavelength
[19]. The (E)-1,2-bis-[2,2’-bithienyl] vinylene (BTV),
kmax = 434 nm, which has nine conjugated double bonds,
however absorbs 90 nm higher than TV (kmax = 344 nm.)
but compared with the quinquethiophene [2] having 10
conjugated double bonds, which absorbs at 416 nm,
exhibits a higher bathochromic shift. This apparent con-
tradiction, lower number of conjugated bonds and higher
kmax than an analogous compound with higher number of
p-delocalized bonds can be explained due to greater
Fig. 1 Electronic absorption spectra of thienylvinylene oligomers
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Struct Chem (2012) 23:1751–1760
1753
Fig. 3 Bond lengths of the carbon–carbon conjugated p-system in:
a the ground state and b the S1 excited state for TV (black dots),
T2EHV (white triangles) and TOV (black squares)
bonding character between the C1–C2 and C5–C6 is clearly
shown and we note that C3–C4 is distorted with the
C3-S pair having a great contribution in the description of
the electronic density of this part of the conjugated system.
Comparison between the bond lengths and the HOMO
molecular orbitals of TV and the models of the alkyl
substituted oligomers T2EHV and TOV exhibit some dif-
ferences in the electronic densities and geometry: the thi-
enyl rings suffer from a small distortion due to the
stretching of the C3–C4 and C4–C5 bond lengths as con-
sequence of the substitution of hydrogen by the alkyl
groups in the C4 carbon. Then, alkyl substitution induce a
change in the electronic densities of the HOMO states by
weakening the C3–C4 and C4–C5 bonds and delocalizing in
some extent the electronic densities of T2EHV and TOV,
as suggested by contour isosurfaces that are markedly
greater in volume compared to TV. However, as the bond
lengths of the central dithienyl-vinylene chain in TOV and
T2EHV are almost identical (see Fig. 3a), we conclude that
the only nature of the linear (in TOV) or branched (in
T2EHV) chain structures makes no difference on the
behavior of the electronic p-system of these alkyl substi-
tuted oligomers.
Fig. 2 Geometries of the oligomers in the ground state: a TV,
b BTV, c T2EHV and d TOV, e labels for carbon atoms in oligomers
Table 1 Dihedral angles of the S–C–C–C central thienyl-vinyl sub-
structure in the ground state and excited state geometry
TV
T2EHV
TOV
BTV
S0 ground state (gas phase)
S0 ground state (solvent)
S1 excited state (solvent)
0°
0°
0°
0.8°
1.7°
0.1°
2.5°
0.1°
0.0°
1.2°
0.9°
0.0°
A dielectric constant e = 4.9 is considered for the chloroform solvent
an alternating shorter-longer pattern which is indicative
that the conjugated p electronic system is not completely
delocalized. This behavior is very similar both in gas phase
and in solvent.
The central C1–C2 vinylene bond is the shortest of the
series for every oligomer and, as a result of this, more
double bond character (see Fig. 3a). Other carbon–carbon
bonds show the expected zig-zag pattern of the single or
double character of each specific pair bonded. Figure 4
includes the frontier molecular orbitals diagrams. In ref-
erence to the electronic densities of the HOMO of TV, the
Dihedral angles of the S–C–C–C central thienyl-vyni-
lene sub-structure are close to 0°. In gas phase, small
differences between the four oligomers are observed
and the decreasing order is dihedral (TOV) [ dihedral
(BTV) [ dihedral (T2EHV) [ dihedral (TV) = 0° (see
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Struct Chem (2012) 23:1751–1760
Fig. 4 Frontier molecular
orbitals of models of a TV,
b T2EHV and c TOV oligomers
(contour surface with
isovalue = 0.015 for each
figure)
Table 1 for details). The effect of the substituents is
observed in the case of TOV and T2EHV, causing a small
distortion from planarity and that is somewhat bigger for
TOV but difficult to interpret as the oligomer is almost
planar as a whole. On the contrary T2EHV exhibit bran-
ched substituents located over and below the plane of the
central thienyl-vinylene sub-structure but with a lower
dihedral (see Fig. 2 for details). We postulate that the
details and relative orientation of the ethylhexyl or octyl
substituent chain with respect to the rings produce this
unpredictable effect in the geometry of the lowest energy.
For BTV the explanation is simple as the third and fourth
thiophene rings added in both sizes of the TV induces
twisting of the entire structure, a well known behavior
already observed in oligothiophenes [21].
Tomasi’s Polarized Continuum Model (PCM) and chloro-
form as the solvent. We do not expect an exact agreement
between calculated and experimental wavelengths but the
general trend is explained fairly well (see Table 2) and, in
particular, figures for the vertical absorption of TV are in
agreement to those reported by Meng et al. [22]. Differences
on the S0 ? S1 transition between TV and T2EHV and
TOV can be attributed specifically to the alkyl substituents
leading to an increment on delocalization of the p-cloud.
There is also some electronic density that extends to the first
member of the alkyl chain, en effect that is more important
in TOV (Fig. 5) and that could explain its longer wavelength
compared to T2EHV. In relation to TV, the red shifts in
absorption are: 12.25 (10.0) nm for T2EHV, 25.59 (9.0) nm
for TOV, and 119.79 (90) nm for BTV, where experimental
values are in parenthesis. Table 3 also include very similar
calculated oscillator strengths for the TV, T2EHV, and TOV
trio, which means that the absorption spectra should have
peaks of similar intensity. However, this could not be
compared directly to experimental spectra in Fig. 1, as they
were recorded at different concentration of the oligomers in
chloroform.
When we add the chloroform solvent effect into the calcu-
lations, TOV and BTV become more planar as expected con-
sidering a previous study [16] which interprets the effect of the
solvent as a compression on the oligomer, action which results
in an easier distortion along single bonds rather than more rigid
structures such as double or triple bonds or carbon rings and
making more planar structures. However, T2EHV does not
follows this trend, because of the bulky 2-ethylhexyl groups
which project over and under the almost planar central thienyl-
vinylene and causing a non expected small increment on S–C–
C–C dihedral of about 1 degree. A plausible explanation for this
difference in the geometry optimizations might come from the
fact that the compression exerted by the solvent could include a
sort of lever action on the branched substituents, particularly
due to steric hindrance between the CH3 and the thienyl ring.
Vertical absorption transitions are then calculated using
time-dependent DFT for the optimal geometries including
The overestimated red shift for TOV seems to be the
result of the optimum geometry found in the ground state
which is more planar than the T2EHV oligomer. Although
we have used shortened substituents, unreported pre-
liminary results show that calculations with longer and
more representative alkyl substituents do not change in a
significant extent this picture for the ground state optimum
geometry of both oligomers. Keeping in mind that the octyl
groups have some extent of rotational freedom along the
C–C bonds we postulate that less planar conformations of
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Struct Chem (2012) 23:1751–1760
1755
TOV, due to slightly distorted linear substituents, could
play an important role in lowering the calculated red shift
of TOV at experimental conditions.
Having nine conjugated double bonds, BTV is in a dif-
ferent category and have the most pronounced red shift and
longest wavelength of the family. Despite of the fact that it is
in qualitative agreement to the sequence of UV–Vis experi-
mental absorption maxima, a larger difference between cal-
culated and experimental energies are readily observed:
(Eexp/Ecalc) = 0.85. This kind of disparity has already been
observed for DFT calculations of an even longer oligomer:
(tetra-2,5-thienylene-1,2-vinylene having eleven conjugated
double bonds) [24] and is a proof of the limits of the meth-
odology, at least under the basis set and the exchange–cor-
relation functional choices considered in this study.
Fluorescence analysis
For an excitation wavelength of 340 nm, the fluorescence
spectrum of (E)-1,2-di-(2-thienyl) vinylene, have an emis-
sion signal at 402 nm, being greater than in the absorption
transition and generating a shift of kmax to higher values.
This phenomenon is known as Stokes shift and is charac-
teristic for such compounds and is commonly reported as a
consequence of a more planar in the excited states. From the
fluorescence spectra (Fig. 6) it can be observed that the
T2EHV and TOV monomers with thiophene-vinylene sub-
stituent groups have the same emission wavelength at
417 nm suggesting a more planar S–C–C–C central thienyl-
vinyl sub-structure and, therefore, less energy.
Fig. 5 Detail of the frontier molecular orbitals of the a T2EHV and
b TOV oligomers. The first CH2 of the alkyl substituents has been
highlighted (contour surface with isovalue = 0.012 for each figure)
Only the terminal C8–C9 and C9–C10 bonds remain almost
unchanged in lengths and nature. In Fig. 4, the LUMO
show that the C2–C3 become the more bonding character of
all the pairs, the C4–C5 pair increase its electronic density
and that the C5–C6 bond (although shorter) have an elec-
tronic density clearly distorted to the C6 position. All this
details are consistent to a change from aromatic form (in
the S0 ground state) to a quinoid form (in the S1 excited
state) of TV and the alkyl substituted oligomers. From
inspection of the changes in dihedral angles (Table 1) it is
observed that the general criteria of more planar confor-
mations for the excited state are fulfilled: TV, TOV, and
BTV have central S–C–C–C dihedrals completely flat and
T2EHV become almost planar. In the case of the T2EHV
oligomer the tiny non zero dihedral angle is the result of
the two combined movements which are not related to an
increment on conjugation : (i) The sulfur atoms, which
weaken their two S–C bonds, tend to move out of the
Optimization of geometry of the first excited electronic
state reveals interesting changes for the family of com-
pounds. Based on bond lengths, the dithienyl-vinylene
chain in TV, TOV, and T2EHV is much more delocalized:
˚
C–C distances become very similar (around 1.4 A) except
for the C3–C4 bond of T2EHV and TOV which are
somewhat larger (see Fig. 3). For BTV delocalization is
also evident in the central dithienyl-vinylene substructure
(see Fig. 7a) and resulting in a pronounced delocalization
of the p-electron cloud along the larger BTV molecule.
Table 2 Absorption S0 ? S1 vertical transition (ABS) and fluorescence S1 ? S0 vertical transition (FL) energies, transition wavelengths,
oscillator strengths, and Stokes shifts
TV
T2EHV
ABS
TOV
ABS
BTV
ABS
ABS
FL
FL
FL
FL
Calculated DE (eV) (kmax in nm)
Experimental DE (eV) (kmax in nm)
fosc (a.u)
3.445
(360.1)
3.607
(344)
0.817
952
3.332
(372.4)
3.505
(354)
0.762
2.824
(439)
2.975
(417)
0.724
3.216
(385.7)
3.515
(353)
0.777
2.75
2.585
(479.9)
2.859
(434)
1.505
2.237
(554.2)
2.64
(420)
3.086
(402)
0.783
(450.9)
2.975
(417)
0.747
(470)
1.585
Calculated data include the effect of chloroform as solvent
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Struct Chem (2012) 23:1751–1760
˚
Table 3 Sulfur–carbon bond lengths (A) in the ground state and excited state geometry, including chloroform solvent effect
State
TV
T2EHV
S–C3
TOV
S–C3
BTV
S–C3
S–C6
S–C6
S–C6
S1–C3
S1–C6
S2–C7
S2–C10
S0
S1
1.76
1.79
1.74
1.73
1.76
1.79
1.74
1.73
1.76
1.79
1.73
1.73
1.76
1.77
1.74
1.76
1.76
1.77
1.74
1.74
Fig. 6 Fluorescence spectra of thienylvinylene oligomers
thienyl ring and in the opposite direction of the bulky
branched substituent which fill a volume outside the plane
of the dithienyl-vinylene substructure and (ii) these alkyl
groups tend to separate even more off the plane. This result
warns over the need of a careful examination of all the
geometric parameters when comparing ground versus
excited electronic states.
Fig. 7 Bond lengths in BTV: a the carbon–carbon conjugated
p-system and b the sulfur–carbon bond. In both cases, white dots or
rhombs are for the S0 ground state and black dots or rhombs for the S1
excited state
Another interesting aspect is the change in sulfur–car-
bon bond length. Each thienyl ring have two different S–C
lengths: S–C3, which is longer and facing the central
HC=CH group and the S–C6 a shorter bond located to the
outside of the oligomer and less directly related to the
conjugation. Not unexpectedly, these bond lengths are very
Theoretical calculations on emission transitions consider
a re-optimization of the geometry using TD-DFT/B3LYP//
6-31G?(d,p) to the first excited singlet state and a sub-
sequent calculation of the transition. TV energies of the
S1 ? S0 emission are in very good agreement to literature:
2.95 versus 2.98 eV [22].
˚
similar for TV, TOV, T2EHV: about 1.76 and 1.74 A,
respectively, in ground state. When excited, the more de-
localized p-system induces a weakness of the S–C3 bond:
they become longer (S–C3) and remains almost unchanged
(S–C6) and with nodal planes in the S–C internuclear dis-
tance, keeping sulfur as electronic densities quasi isolated
(see Fig. 4). Similar behavior is observed in BTV:
stretching of the internal sulfur–carbon bonds and the
external S–C10 keeping its length (see Table 3).
Following a similar behavior to vertical absorption transi-
tion calculations, the S1 ? S0 emissions also show the bath-
ochromic effect due to the substitution of hydrogen with alkyl
chains: taking the transition of TV as a reference, the red shifts
in emission are: 19.0 (15.0) nm for T2EHV, 30.9 (15.0) nm for
TOV, and 134.2 (68.0) nm for BTV, where experimental
values are in parenthesis (see Table 2 for details).
Following a similar trend to the HOMO, electronic
densities of T2EHV and TOV LUMOs are less spatially
constrained and then more delocalized than TV. In these
cases it is easy to observe also the existence of a small
contribution on the alkyl substituents to the electronic
densities, causing even more delocalization.
Finally focusing on the TV, T2EHV, and TOV family,
Stokes shifts between absorption–emission maxima can be
obtained from Table 2. The shifts are in good agreement with
experimental data (see Fig. 8) with differences lesser than 6%
and is a complementary proof of the usefulness of this study.
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Struct Chem (2012) 23:1751–1760
1757
experimentalspectralfeatures,thattheresultsareinfairlywell
global agreement and that the apparent discrepancies could be
searched improving the methodology. One possibility to be
investigated is to make corrections correlating experimental
data with calculated spectroscopic features of slightly dis-
torted model structures of the stable conformers, in a similar
¨
way to the method used by Dilien et al. [23] to predict NMR
chemical shifts assignments of poly(3-octylthienilene vini-
lene) polymerization defects. However, preliminary results on
the change of the absorption/fluorescence peak maxima by
distorting the geometry of the alkyl substituents does not
produce significant enough shifts of the absorption/fluores-
cence features. The fact that Stokes shifts of the TV, T2EHV,
TOV oligomers is reasonably well explained, although with a
systematic overestimation between 2 and 6% points to the
necessity of a further investigation with different correlation-
exchange functionals or at a higher level of methodology
(MP2, for instance) butisbeyondthescopeofthisstudy.Inthe
S1 excited electronic state, we observed that the delocalization
of the electronic densities is even more marked than the S0
ground state, including a small contribution to the alkyl sub-
stituents to the electronic densities and the calculated fluo-
rescent vertical transitions follow similar trends to the
absorption transitions previously mentioned.
Fig. 8 Stokes shift for the TV, T2EHV, and TOV family. Black dots
represents experimental shifts, black squares are calculated shifts
Conclusions
We have successfully prepared alkylthienyl oligomers. The
UV–Vis absorption spectra of these compounds showed clear
difference resulted from different band gap. Such differences
were assigned to the different incorporating vinylene bonds
between thiophene rings which lead to an increasing degree
of coplanarity of the oligomer, as the vinylene bond reduces
steric hindrance on successive aromatic rings.
From the theoretical viewpoint, structural changes and
electronic densities of the ground and first excited electronic
statesareusefultoexplaintheobservedspectralchangesinthe
TV, TOV, T2EHV, and BTV family of oligomeric entities. A
first basic result is the geometry differences between both
states, which show clearly the weakening of the double bonds
of the central dithienyl-vinylene, based on the analysis of
carbon–carbon bond lengths and planarization of the dihedral
angles in the S1 excited state. These details are consistent to a
change from aromatic form (in the S0 ground state) to a
quinoid form (in the S1 excited state) of the oligomers. Con-
sidering the S0 ground state and related to TV, the alkyl sub-
stitution in the TOV and T2EHV duet induce an increment in
the delocalization of the conjugated electronic p-system and
results in a red shift of the absorption spectral features. As
expected, the longer non-substituted BTV has an even bigger
shift due to its longer conjugation length. However, the fact
that the experimental very similar maxima (both in absorption
and fluorescent vertical transitions) for TOV and T2EHV is
not well explained by our calculations could be the conse-
quence of the linear (TOV) or branched (T2EHV) alkyl sub-
stitution:the S0 groundstate ofTOV (in chloroform solvent) is
more delocalized and almost perfectly planar as the lineal
alkyl group are linear and coplanar to the central dithienyl-
vinylene structure and leads to a calculated red shift bigger
than the T2EHV oligomer, in which the branched alkyl sub-
stituents acts as levers and distorts in a small extent the central
dithienyl-vinylene and under the solvent effect. We stress that
wenever expectedanexactagreementbetweencalculatedand
Experimental section
General information
All solvents were purchased from commercial sources and
were distilled prior use, catalyst and reagents were used as
received without further purification. Thin-layer chroma-
tography (TLC) was performed using silica gel 60 F254
1
precoated plates. H and 13C NMR spectra were recorded
on NMR spectrometer operating at 300 and 75.5 MHz,
1
respectively. The H chemical shifts are reported in parts
per million relative to tetramethylsilane. UV–Vis and
fluorescence spectra were recorded on chloroform solution.
Experimental procedure
3-Octylthiophene (1a)
According to reported method of Kumada coupling [24]. A
regioselective Grignard cross coupling reaction of octylbr-
omide with 3-bromothiophene, catalyzed by Ni(dppp)Cl2
(Nickel diphenylphosphino-propane) (24) give 22.6 g (73%
yield) of 1a; refractive index 1.4921; UV–Vis kmax (CHCl3):
241 nm.
1H-NMR (CDCl3, 400 MHz) d(ppm) 7.31 (dd, 1H,
J = 3.0 Hz, J = 4.9 Hz); 7.01 (m, 2H); 2.72 (t, 2H,
123

1758
Struct Chem (2012) 23:1751–1760
J = 7.7 Hz); 1.72 (q, 2H); 1.37 (m, 10H); 0.99 (t, 3H,
J = 6.8 Hz); 13C-NMR (CDCl3, 400 MHz): d(ppm) 143.6;
128.6; 125.3; 120.1; 32.1; 30.9; 30.7; 29.1; 29.7; 29.64;
23.0; 14.5.
134.18; 131.3; 41.6; 34.2; 32.8; 28.8; 25.8; 23.0; 24.1;
10.8.
(E)-1,2-Di-(3-octylthiophene)vinylene (3a)
3-(2-Ethylhexyl)thiophene (1b)
To a solution of 4.48 g of 3-octyl-2-thiophenecarbox-
aldehyde in 75 mL of anhydrous THF is added 6.6 mL of
titanium tetrachloride at -18 °C. The mixture is stirred for
half an hour at -18 °C. To this solution were added 8.0 g
of Zn powder in small portions for 10–15 min. The mixture
was stirred for 30 min at -18°C and allowed to reach room
temperature gradually, and then heated for 4 h at reflux.
The mixture was cooled in an ice-water bath and 50 mL of
aqueous solution of sodium carbonate 10% were added.
Then, 200 mL of dichloromethane were added in portions
of 50 mL and two layers mixture is stirred after being fil-
tered. The dichloromethane layer is separated from the
filtrate and washed with water until neutral pH. Later, dried
with magnesium sulfate for 3 h and the solvent was filtered
and evaporated, leaving an orange liquid, which is purified
by column chromatography, eluting with hexane giving
2.00 g product (48% yield).
The procedure is same as above (1a) but in this case,
2-ethylhexyl bromide was used instead of octyl bromide.
The product is a colorless liquid, was obtained with 65%
yield, refractive index: 1.4877. Product weight: 20.1 g.
Percentage yield: 65%, UV–Vis kmax (CHCl3): 241 nm.
1H-NMR (CDCl3, 400 MHz) d(ppm) 7.29 (m, 1H); 6.99
(m, 2H); 2.63 (d, 2H, J = 6.9 Hz); 1.62 (tt, 1H, J =
6.3 Hz, J = 12.4 Hz); 1.36 (m, 8H); 0.94 (m, 6H). 13C-
NMR d(ppm) (CDCl3, 400 MHz): 142.0; 128.8; 124.8;
120.7; 40.5; 34.4; 33.0; 29.0; 25.7; 23.1; 14.2; 10.9.
3-Octyl-2-thiophenecarboxaldehyde (2a)
3 mL of phosphorus oxychloride (32 mmol) were added
dropwise to a solution containing 5.6 g (0.028 mmol) of
3-octylthiophene and 2.5 mL DMF (0.032 mmol) in
10 mL of 1,2-dichloroethane at 0 °C. The solution was
gradually brought to room temperature and refluxed over-
night. The solution was cooled to room temperature and
hydrolyzed with 100 mL of aqueous solution of sodium
acetate. The solution was extracted with dichloromethane.
The organic layer was washed with water, dried over
anhydrous magnesium sulfate and concentrated under
reduced pressure. The product was purified by column
chromatography (silica gel, hexane/dichloromethane) to
give a 5.73 g (89%) of light yellow oil. Refractive index:
1.5214 UV–Vis kmax (CHCl3): 275 nm.
1H-NMR (CDCl3, 400 MHz) d(ppm) 7.07 (d, 2H,
J = 5.1 Hz); 7.02 (s, 2H); 6.85 (d, 2H, J = 4.9 Hz); 2.67
(t, 2H, J = 7.6 Hz); 2.56 (t, 2H, J = 7.6 Hz); 1.60 (m,
4H); 1.27 (m, 20H); 0.87 (t, 6H); 13C-RMN (CDCl3,
400 MHz); d(ppm) 140.9; 136.5; 130.1; 122.7; 119.7; 32.1;
31.2; 29.7; 29.61; 29.5; 28.7; 22.9; 14.4.
(E)-1,2-Di-(3-(2-ethylhexyl)thienyl)vinylene (3b)
The procedure is same as 3a, but in this case 3-(2-etylh-
exil)-2-thiophenecarboxaldehyde was used instead of
3-octyl-2-thiophene carboxaldehyde. Product weight:
1.67 g (40% yield).
1H-RMN (400 MHz, CDCl3, ppm); d(ppm) 7.30 (s, 2H);
7.10 (d, 2H); 6.86 (d, 2H); 2.62 (d, 4H); 1.57 (q, 2H); 1.35
(m, 16H); 0.95 (t, 12H).
13C-RMN (400 MHz, CDCl3); d(ppm) aromatics: 139.0;
137.0; 130.5; 122.4; 120.2; aliphatics: 40.8; 34.6; 32.3;
29.0; 25.8; 22.9; 14.2; 10.9.
1H-NMR (CDCl3, 400 MHz) d(ppm) 10,4 (s, 1H); 7.65
(d, 1H, J = 4.7 Hz); 7.01 (d, 1H, J = 4.9 Hz); 2.96 (t, 1H,
J = 7.7 Hz) 2.64 (t, 1H, 7.7 Hz); 1.67 (m, 2H); 1.26 (m,
10H); 0.88 (t, 3H, J = 6.6 Hz);13C-NMR (CDCl3, 400
MHz) d(ppm); aldehyde 182.50; aromatics 153.13; 137.85;
134.6; 130.9; aliphatics 32.0; 31.7; 29.5; 29.5; 29.4; 28.7;
22.9; 14.3.
3-(2-Ethylhexyl)-2-thiophenecarboxaldehyde (2b)
(E)-1,2-Di-(2-thienyl)vinylene (3c)
The procedure is same as above (2a) but in this case, use
3-(2-ethylhexyl)thiophene instead of 3-octylthiophene.
Yield 78%, yellow oil, refractive index: 1.5211. UV–Vis
kmax (CHCl3): 302 nm.
The procedure is same as 3a, but in this case thiophene-2-
carboxaldeyde was used instead of 3-octyl-2-thiophene-
carboxaldehyde (83.9% yield) [25]
1H-NMR (400 MHz, CDCl3): d(ppm) 6.96–6.99, (dd,
2H), 7.02–7.04 (d, 2H), 7.05 (s, 2H, –CH = CH–),
7.15–7.18 (d, 2H).
1H-NMR (CDCl3 400 MHz); d(ppm) 10.07 (s, 1H); 7.65
(d, 1H, J = 4.7 Hz); 7.01 (d, 1H, J = 4.9 Hz); 2.92 (d, 1H,
J = 7.2 Hz); 2.61 (d, 1H, J = 6.9 Hz); 1.65 (dd, 1H,
J = 6.0 Hz); 1.31 (m, 8H); 0.92 (t, 6H, J = 7.3 Hz); 13C-
NMR (CDCl3, 400 MHz): d(ppm) 182.4; 152.1; 137.6;
13C-RMN (400 MHz, CDCl3); d(ppm) aromatics: 144.4;
128.1; 127.0; 125.88; 124.3 (Scheme 1).
123

Struct Chem (2012) 23:1751–1760
1759
Scheme 1 Synthetic route
of (E)-1,2-di-(3-
alkylthiophene)vinylene
Br
R
R
R
R-MgBr
POCl3
DMF
TiCl4
S
CHO
Ni(dppp)Cl2
ethyl ether
S
S
S
S
1a, 1b
2a, 2b
3a, 3b, 3c
3a; R = octyl
3b; R = 2-ethyl-hexyl
3c; R hydrogen
R
2a; R = octyl
2b; R = 2-ethyl-hexyl
1a; R = Octyl
1b; R = 2-ethyl-hexyl
S
POCl3 / DMF
CH2Cl2
TiCl4/ Zn
THF
S
S
S
CHO
S
S
S
S
4b
4a
Scheme 2 Synthetic route of (E)-1,2-di-(3-bithienyl)vinylene
5-formyl-2,2-bithiophene (4a)
washed with distilled water, dried over magnesium sulfate
during 3 h. The compound was purified by continuous
extraction in Soxhlet with dichloromethane and recrystal-
lized in the same solvent.
To a two-neck round bottom flask were added 6 g
(36 mmol) 2,20-bithiophene, 3 mL DMF (40 mmol), and
15 mL of 1,2-dichloroethane under argon atmosphere.
Subsequently, 3.8 mL (40 mmol) of phosphorus oxychlo-
ride were added, at 0 °C. The solution was gradually
brought to room temperature and allowed to reflux over-
night. Afterward, the solution was cooled to room tem-
perature and hydrolyzed with 100 mL of aqueous 1 M
sodium acetate solution. Then, the mixture was extracted
with dichloromethane and washed with water until neutral
pH. Finally, the organic layer was dried with anhydrous
magnesium sulfate for 3 h, filtered and concentrated by
rotary evaporator. The title compound was purified by a
silica column chromatography using hexane as eluant.
Yield 5.32 g., 76%, mp: 57–58 °C.
Yield 2.3 g, 82%, mp: [220 °C (d).
1H-NMR (400 MHz, CDCl3) d(ppm) 7.26 (s, 2H), 7.22
(dd, J = 5.1, 1.3 Hz, 2H), 7.18 (dd, J = 3.5, 1.2 Hz, 2H),
7.07 (d, J = 3.7 Hz, 2H), 7.02 (dd, J = 5.1, 3.6 Hz, 2H),
6.97–6.91 (m, 2H).
13C-NMR: (CDCl3,400 MHz); d(ppm) 141.5; 137.7;
136.5; 128.1; 127.4; 124.8; 124.4; 124.0; 121.5
(Scheme 2).
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13C-NMR: (CDCl3,400 MHz); d(ppm) 182.7; 147.34;
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