Optoelectronic Properties of A-π-D-π-A Thiophene-Based Materials with a Dithienosilole Core: An Experimental and Theoretical Study

Article abstract of DOI:10.1002/cplu.201900092

Two A-π-D-π-A thiophene-based small molecules with a central dithienosilole core and dicyanovinyl (DCV) end groups were synthesized. These compounds differ only by the presence of alkyl and alkylsulfanyl chains, respectively, on the thiophene beta positions. Computational data together with the spectroscopic and electrochemical findings (obtained by means of absorption, steady-state/time-resolved emission techniques, and cyclic voltammetry) revealed that both molecules possess low electronic and optical band gaps, broad absorption spectra, and good stability both in p and n-doping states, which make them suitable for optoelectronic applications. In both compounds, the HOMO–LUMO transition involves an intramolecular charge transfer from the electron-donor dithienosilole unit to the two terminal electron-acceptor DCV groups. A marked positive emission solvatochromism was observed for both molecules and was interpreted on the basis of the symmetry breaking in the S₁ excited state. The two synthesized compounds were also compared to their shorter precursors and to similar oligothiophenes to understand how the nature of the building block influences the characteristics of the final materials. Furthermore, it was possible to better understand the contribution of the sulfur atom in modulating the optical properties of the small molecules studied.

Full text of DOI:10.1002/cplu.201900092

Accepted Article
Title: Optoelectronic Properties of A-π-D-π-A Thiophene-Based
Materials with a Dithienosilole Core: An Experimental and
Theoretical Study
Authors: Francesca Parenti, Monica Caselli, Davide Vanossi, Mirko
Buffagni, Manuel Imperato, Laura Pigani, and Adele Mucci
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Optoelectronic Properties of A-π-D-π-A Thiophene-Based
Materials with a Dithienosilole Core: An Experimental and
Theoretical Study
Monica Caselli[a], Davide Vanossi[a], Mirko Buffagni[a], Manuel Imperato[a], Laura Pigani[a], Adele
Mucci[a], Francesca Parenti *[a]
[a] Dr. M. Caselli, Dr. D. Vanossi, Dr. M. Buffagni, Dr. M. Imperato, Dr. L. Pigani, Prof. A. Mucci, Dr. F. Parenti,
Dipartimento di Scienze Chimiche e Geologiche, Università di Modena e Reggio Emilia, Via G. Campi 103,
41125 Modena, Italy
E-mail: francesca.parenti@unimore.it
Supporting Information is available
Abstract: Two A-π-D-π-A thiophene based small molecules, with a central dithienosilole core and dicyanovinyl
end groups were synthesized. These compounds differ only for the presence of alkyl and alkylsulfanyl chains,
respectively, on thiophene beta positions. The computational data together with the spectroscopic and
electrochemical findings (obtained by means of absorption, steady-state/time-resolved emission techniques
and cyclic voltammetry) revealed that both molecules possess low electronic and optical band gaps, broad
absorption spectra and a good stability both in p and n-doping states, properties that make them suitable for
optoelectronic applications. In either compounds the HOMO-LUMO transition involves an intramolecular
charge transfer from the electron-donor dithienosilole unit to the two terminal electron-acceptor DCV groups.
A marked positive emission solvatochromism was observed for both molecules and was interpreted on the
basis of the symmetry breaking in the S1 excited state. The two synthesized compounds were also compared
to their shorter precursors and to similar oligothiophenes to understand how the nature of the building block
influences the characteristics of the final materials. Furthermore, it was possible to better understand the
contribution of the sulfur atom in modulating the optical properties of the small molecules studied.
Introduction
1
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Organic electronics is mainly based on conducting polymers, widely studied functional materials which find
applications in organic solar cells (OSC), organic light emitting diodes (OLED), organic field-effect transistors
(OFET), fluorescence sensors and biosensors.[1–4] The versatility of these polymers is due to their stability and
to the uniqueness of their electronic and optical features. Conductive polymers are obtained by relatively
simple synthetic protocols, which make a fine-tuning of their properties possible, in view of modulating and
improving the performance of the final devices. The principal drawback is that, like all polymers, they suffer
from lack of synthetic reproducibility. Conjugated oligomers possess similar or even better electronic
properties, e.g. often exhibit higher charge mobility and polarizability and, in addition, are structurally well-
defined, monodisperse, free of structural defects. Moreover, oligomers are prone to form ordered
supramolecular structures and this has recently shown to have significant repercussions on the performance
of the final device.[5,6] In particular, small conjugated molecules such as oligothiophenes (OTs), beyond
retaining some of the typical molecular characteristics of their homologous with higher molecular weight, have
proved to be good materials for OSCs,[7] OLEDs[8] and OFETs.[9–11] The optoelectronic properties of
oligothiophenes and, generally, of aromatic π-conjugated systems, are largely a result of the degree of
conjugation. The conjugation length and, hence, the optical band gap in solution are influenced by four
variables: aromatic or quinoid character, bond length alternation, torsions along the polymer backbone and
electronic effects from side groups.[12] Among small molecules, the push-pull ones are attracting increasing
attention. They are characterized by a widely delocalized π-electron system, resulting in low-energy absorption
bands that make them excellent light harvesting systems in optoelectronic devices. Different building blocks
of electron donors (D) and acceptors (A), linked through a π-bridge, may be variously combined (i.e. A-π-A,
D-π-D, A-π-D, A- π-D-π-A, D-π-A-π-D) and the choice of the donor and acceptor groups makes possible a
fine tuning of their electronic band gap. Furthermore, A-π-D-π-A and D-π-A-π-D quadrupolar molecular
systems have been studied in relation to their nonlinear optical properties, which make them usable in bio-
imaging applications and photodynamic therapy.[13] Dipolar and quadrupolar molecules undergo intramolecular
charge transfer transitions, so their photophysics is dominated by a strong dependence on the nature of the
solvent. [14] Since quadrupolar molecular systems do not possess a permanent electric dipole moment, due to
their symmetry, such common observations have been interpreted on the basis of a breaking of the symmetry
upon excitation assisted by the solvent.[15]
Recently A-π-D-π-A thiophene-based small molecules, where the A moiety is a dicyanovinyl group (DCV),
have been studied.[16–20] These molecules, where the backbone has a hole-transporting function whereas the
DCV acceptor moiety expands the absorption spectrum to higher wavelengths, should lead to efficient
intramolecular charge-transfer and make possible a fine-tuning of the band gap. Moreover, DCV is planar and
2
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contributes to π-stacks formation and strong intermolecular interactions. Among the D units, dithienosilole is
intriguing since it possesses rigidity and planarity, reinforced by the interaction between the σ* orbital of silicon
and π* orbital of bithiophene, stability under ambient conditions and a lower band gap if compared to the
carbon analogues. [21] Several examples that combine the properties of DCV donor and dithienosilole acceptor
units are present in the literature.[22–24] In particular, methyl-DCV derivatives, where the vinyl hydrogen of the
DCV is substituted by a methyl group, seem to be very photostable and to possess good photovoltaic
performances.[25,26]
We have been involved in the synthesis and characterization of poly(alkylsulfanylthiophenes) for several years
[27-30]. For this type of polymers, it has been demonstrated that the electron-donating sulfur atom of the
alkylsulfanyl chain causes a red-shift in the absorption spectra and affects both LUMO and HOMO energy
levels. [31,32] In this respect, we are still interested in studying the influence of alkylsulfanyl substituents on the
electronic properties of thiophene-based small molecules.
In this paper, we present the synthesis and a combined theoretical and experimental study of the optical,
photophysical and electrochemical behavior of two A-π-D-π-A thiophene-based small molecules with a central
dithienosilole unit (Scheme 1). The latter is symmetrically bonded to an octyl (E1) and an octylsulfanyl (E2)
bithienyl unit, which in turn is connected to a methyl-DCV group. Their relevant synthetic precursors, DTS, T1
and T2 (Scheme 1) were also investigated, focusing on the progressive change of the electrochemical, optical
and photophysical properties, by means of cyclic voltammetry (CV), absorption and steady-state/time-resolved
emission techniques. A marked positive fluorescence solvatochromism was observed only for the quadrupolar-
like compounds (E1 and E2), interpreted on the basis of a symmetry-breaking in the S1 excited state. These
observations allow the influence of the nature of building blocks and of substituents on the optical and
photophysical properties to be rationalized, and further insights into the design of materials for applications in
optoelectronic devices to be gained.
Scheme 1 The A-π-D-π-A thiophene-based small molecules E1 and E2 and their building blocks (T1, T2 and DTS)
3
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Results and discussion
Synthesis
One of the key motifs of E1 and E2 here studied is the dithienosilole unit, to which thienyl and thienyl-DCV
units are bound in subsequent steps (Scheme 2).
Scheme 2 Synthesis of E1, E2 and the precursors DTS, T1 and T2: i) NBS; ii) Pd(PPh3)4, Na2CO3,; iii) LDA,
trimethyltin chloride; iv) HMDS, acetic acid, malononitrile.
The synthesis of DTS can be accomplished starting from 3,3’-dibromo-2,2’-bithiophene (Scheme 3, route a)[33–
35]
or from 3,3’-dibromo-5,5’-bis(trimethylsilyl)-2,2’-bithiophene (Scheme 3, route b).[36] Both routes were
explored by us and, eventually, we preferred route b with some synthetic modifications (Scheme 3, route c):
starting from 2-bromothiophene, the bithiophene 5 was generated by a one-pot reaction[37], instead of the more
cumbersome method usually employed.[38, 39] The one-pot procedure allows the insertion of the protecting
trimethylsilyl groups, the halogen dance of bromine from α to β thiophene positions and the copper-catalyzed
coupling of 4-bromo-2-(trimethylsilyl)thiophene without the need of isolating the intermediates. The silole ring
closure was achieved in good yield with minor modifications to the procedure reported.[36] The 1H NMR spectra
of the raw material obtained showed both the presence of α,α’-bis(trimethylsilyl) protected DTS 6 and of its α-
desilylated derivatives. Therefore, we decided to treat the crude with TFA in order to remove all the
trimethylsilyl groups, obtaining DTS (24 %, overall yield after chromatography). DTS was then brominated [32]
to afford 1 (Scheme 2).
4
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Scheme 3 Synthetic pathways for DTS: i) n-butyllithium, dioctyldichlorosilane; ii) trifluoroacetic acid; iii) LDA,
trimethylsilylchloride, then LDA, anhydrous CuCl2.
Compound 1 was then reacted with dioxaborolanes 2a,b to generate dithienyl DTSs T1 and T2 (Scheme 2).
Dioxaborolanes were, in turn, obtained by lithiation and subsequent treatment with 2-isopropoxy-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane of 2-bromo-3-octylthiophene or 2-bromo-3-(octylsulfanyl)thiophene.
Compounds T1 and T2 were reacted with LDA and trimethyltin chloride to generate trimethyltin derivatives
3a,b which were, eventually, coupled with thienyl-substituted malononitrile 4 to generate E1 and E2. For the
synthesis of compound 4 we followed the methods described in the literature for the corresponding aldehyde
using piperidine[41], beta-alanine[42] or triethylamine[43] as the base in the Knoevenagel reaction with
malononitrile, but unfortunately no product was obtained. 1D and 2D-NMR spectra of the crudes showed
signals attributable to the formation of cyclization side-products (Scheme S1). We suppose that these products
are formed after deprotonation of the acidic methyl protons of the methyl-DCV group and subsequent
nucleophilic attack of the carbanion both on the vinylic carbon of 4 and on the carbonyl group of 2-acetyl-5-
bromothiophene. The formation of these byproducts was avoided by using hexamethyldisilazane in acetic acid
as the base, following the procedure reported for different substrates[44] to selectively deprotonate the
malononitrile in the Knoevenagel step. Compound 4, here synthesized for the first time, is an interesting
building block for the synthesis of push-pull small molecules bearing methyl-DCV functionalised thiophene as
acceptor units. In fact, by taking advantage of the bromine atom of 4, it is possible to insert this unit directly on
different substrates (e.g. using a Stille coupling with a tin derivative) avoiding the use of protecting/deprotecting
steps for the carbonyl moiety, which is usually converted into the methyl-DCV group, through a Knoevenagel
reaction, at the end of the synthetic route.
5
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Optical and photophysical properties
The absorption and fluorescence spectra of DTS, T1, T2, E1, E2 in n-hexane are reported in Figure 1. Similarly
to unsubstituted oligothiophenes, DTS, T1 and T2 absorption and emission features are independent of the
solvent polarity. The maximum of the lowest-energy absorption band of DTS is at 336 nm in n-hexane and
acetonitrile, red-shifted with respect to those of 2,2’-bithiophene (maximum at 304 nm). This is consistent with
the suggestion that the silicon bridge between the two thiophene rings extends the π-conjugation thanks to
the planar structure assumed by the central, three-ring core. Also, silicon gives an appreciable contribution to
the LUMO, as shown in Figure S1 in SI, suggesting a hyperconjugative effect able to reduce the optical band
gap: such a finding is in agreement with the observed red-shifted absorption spectrum of a dithienosilole
derivative with respect to the correspondent compound where Si is substituted by C (323 nm).[45, 46] The lowest-
energy absorption band of T1, which bears two b-octylthiophenes in the 2,6 positions of the DTS central core,
is characterized by a maximum at 403 nm: it is red-shifted with respect to DTS, consistently with the fact that
the substitution by the thiophene units extends the π-conjugation system (see calculated Kohn-Sham
molecular orbitals, KS MO, Figure S1). Furthermore, it is also red-shifted with respect to the unsubstituted
quaterthiophene, which shows a maximum at 392 nm [47], and markedly red-shifted with respect to 3,3’’’-
dimethyl and 3,3’’’-dioctylquaterthiophenes (maxima at 383 and 372 nm respectively).[48, 49] It has been reported
that the planarity of OTs, and hence the degree of conjugation, increases as a function of the number of
thiophene units; however, insertion of alkyl substituents induces out-of-plane distortions which results in a
reduced degree of conjugation and in blue-shifted absorption spectra. Combined quantum-chemical and
spectroscopic studies have shown that at room temperature methyl-substituted OTs are characterized by a
distribution of torsional conformations in the electronic ground state, resulting in broad absorption spectra with
maxima depending on the number and position of the substituents.[48] The octyl substituents, because of their
sterical hindrance, induce an increased departure from planarity of the conjugated backbone, if compared with
the methyl ones. [49] Nonetheless, T1 and T2, which bear octyl or octylsulfanyl substituents on a b-position of
the outer thienyl rings, are characterized in apolar solvents by absorption maxima at 403 and 434 nm,
respectively: these results can be explained by the role of the planar DTS central core which confers a higher
degree of conjugation, if compared to dioctyl-substituted quaterthiophenes, thanks to the lack of torsional
freedom for the central thiophene units in the ground state. T2 shows a further red-shifted absorption spectrum
with respect to T1, also attributable to the sulfur contribution to π-delocalization. Another contribution to the
bathochromic shift could derive by the lower distortion in T2 with respect to T1 suggested by the stronger
deshielding of H-3 NMR signal on the dithienosilole unit going from DTS to T2. This lower distortion causes H-
3 to move closer to the plane of the adjacent thienyl ring in T2 with respect to T1 and is in agreement with
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previous observations on polythiophenes.[50] The molar absorption coefficients enhance going from DTS to
T1/T2 and to E1/E2. This trend is expected on the basis of the increased conjugation length.
In Figure S1 the relevant frontier KS MOs are reported for all the molecules studied. Because the photophysics
of these compounds is fully determined by the lowest-lying excited states, we will not dwell on a detailed
analysis of the experimental higher-energy absorption bands and computed transition energies. A comparison
of the HOMO and LUMO for T1, T2, E1 and E2 indicates the typical increased bond order of the interring
bonds upon photoexcitation in OTs. The TDDFT calculated gas-phase one-photon absorption spectra reported
in Table S2 show that for all compounds the lowest-energy absorption band is mainly attributable to the HOMO-
LUMO transition. The maxima calculated by means of the functional CAM-B3LYP (Table 1) are in rather good
agreement with the experimental ones (Table 2). An increase of the S1 oscillator strengths on passing from
DTS to T1/T2 and then to E1/E2 is also shown in Table S2 and is in agreement with the increase of the
experimental molar absorption coefficients. The low charge-transfer character exhibited by the HOMO-LUMO
transition is consistent with the lack of absorption solvatochromism for all the studied compounds.
Table 1: KS gap ( KS ) and optical band gap (ΔEgap) obtained at DFT/LR-TDDFT level.
Eg
ΔEgap/eV[a]
ΔEgap/nm
EgKS
Molecule
/eV
B3LYP
4.045
2.944
2.876
2.036
1.853
CAM-B3LYP
B3LYP
CAM-B3LYP
3.873
B3LYP
337
CAM-B3LYP
DTS
T1
6.538
5.165
5.089
4.050
3.846
3.681
2.709
2.622
1.827
1.661
320
414
425
534
562
2.998
458
T2
2.917
473
E1
2.323
679
E2
2.208
746
[a]These ΔEgap corresponds to the S0→S1 transition energies reported in Table S2.
While DTS is characterized by structureless spectra both in absorption and emission, the fluorescence spectra
of T1, T2, E1 and E2 in n-hexane (Figure 1b) show the typical features of OTs, namely they are structured and
narrow, if compared with the correspondent absorption spectra. Such behavior of OTs has been interpreted
assuming that a broad potential energy torsional surface in S0 makes accessible a rather large number of
conformations whereas, after photoexcitation in S1, OTs relax to a quinoid-type planar conformation
characterized by an increased interring bond order. Thus, the torsional potential energy curve is narrower in
comparison with the ground-state potential, resulting in a distribution of conformations very close to a planar
[51, 52]
geometry and in narrower fluorescence spectra.
The vibronic structure in emission spectra of OTs is
attributed to vibrational modes efficiently coupled to the electronic transition.[53]
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A marked difference between DTS, on one hand, and T1 and T2, on the other, can be observed also in the
photophysical behavior (Table 2).
DTS shows a very high fluorescence quantum yield in all solvents and a large Stokes shift, properties which
make it suitable as a fluorescent label and a good excitation energy donor for blue-emitting acceptors. The
photophysical behavior of DTS is markedly different from that of 2,2’-bithiophene: the high fluorescence
quantum yield (0.86 in n-hexane) and long fluorescence lifetime (7.75 ns) indicate a main radiative deactivation
path in DTS (kF=1.1x108 s-1), whereas in 2,2’-bithiophene the excited state deactivation is dominated by
intersystem crossing (triplet quantum yield in benzene is 0.99).[47] T1, characterized by a lower fluorescence
quantum yield and lifetime if compared to DTS, exhibits a behavior similar to that of quaterthiophene: whereas
kF increases by a factor around 2 (kF=1.1x108 s-1 for DTS and 2.6x108 s-1 for T1), the sum of non-radiative
constant increases by a factor around 100 (knr=1.8x107 s-1 for DTS 2.8x109 s-1 for T1), giving to T1
photophysical properties similar to those of the corresponding quaterthiophene (kF=4.1x108 s-1 and knr=1.9x109
s-1 in benzene). T2 resembles T1 in the photophysical behavior. It was shown that the fluorescence quantum
yields increase and the triplet quantum yields decrease as a function of the chain length in OTs.[47] The
calculated spin–orbit coupling strength together with the energy gaps between S1 and the triplet state involved
in intersystem crossing process account for such results: in particular, a fundamental role was attributed to the
energy gap between S1 and the T2 or T3 triplet states.[54] It has been suggested that in bithiophene and higher
OTs the non-planarity of the thiophene backbone increases the intersystem crossing efficiency. Therefore, the
high fluorescence quantum yield shown by DTS, and hence the very low efficiency of the non-radiative paths,
if compared to bithiophene, could be explained by the inhibition of the out-of-plane torsions in such a rigid
structure. Furthermore, the longer and more flexible molecules (T1, T2, E1 and E2) exhibit radiative and non-
radiative constants similar to the correspondent OTs.
E1 and E2 show further red-shifted absorption and emission spectra, due to the extended π-conjugation which
involves also the DCV end caps (see the LUMOs of E1 and E2 in Figure S1). The role of electron-withdrawing
DCV end groups in lowering the optical band gap of OTs has been studied: the absorption maximum of the
terminally DCV-substituted sexithiophene without side chains is at 532 nm in dichloromethane [55], not different
from the value we found for E1. The effect of the substitution by the octyl chains on the absorption spectrum
of E1 in low polarity solvents (maximum at 532 nm) can be understood by a comparison with the correspondent
compound lacking side alkyl chains (maximum at 542 nm).[56] Such a finding could be explained by the twisting
around interring single bonds induced by the steric hindrance of the alkyl substituents, which leads to
conformations with a slightly reduced conjugation on the whole. On the other hand, the octylsulfanyl chain, if
compared with the octyl one, induces a reduction of the band gap in E2 (maximum at 557 in dichloromethane),
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thanks also to the involvement of sulfur in π-conjugated system. These results confirm that the introduction of
alkyl side chains acts by increasing the optical band gap; on the contrary, the DTS central core in E1 and E2
is still able to increase the π-conjugation length. In E1 the positive effect of the DTS core is counterbalanced
by the presence of the octyl chain, whereas in E2 a reduction of the optical band gap, if compared to E1 and
also to the different DCV-substituted sexithiophenes reported in literature, [55, 56] can be observed, attributable
to the stronger bathochromic effect induced by the alkylsulfanyl chain.
Table 2. Absorption and emission maxima, molar absorption coefficients, Stokes shifts, quantum yields (FF)
and fluorescence lifetimes (tF), radiative and overall non-radiative decay rate constants of DTS, T1, T2, E1
and E2
labsmax (nm)
lfluomax (nm)
FF
tF (ns)
Compound Solvent
Stokes shift
(cm-1)
kF (s-1)
knr (s-1)
1.1∙108 1.8∙107
n-hexane
DTS
241,336
403
4950 0.86
7.75
(emax=11100 M-1∙cm-1)
FWMH=3900 cm-1
FWMH=4530 cm-1
dichloromethane
243 ,338
240,336
405
404
4890
acetonitrile
5010 0.89
8.42
1.1∙108 1.3∙107
2.6∙108 2.8∙109
n-hexane
T1
T2
265,308,403
FWMH=4770 cm-1
484,505
FWMH=3390 cm-1
5010 0.087 0.33
dichloromethane
266,311,407
488,510
4960
(emax=24900 M-1∙cm-1)
265,309,402
acetonitrile
486,507
495,520
FWMH=3250 cm-1
5150
270, ~325,434
FWMH=4360 cm-1
3810 0.12
3890
0.41
2.9∙108 2.1∙109
n-hexane
dichloromethane
~328, 438
(emax=34100 M-1∙cm-1)
268, ~328,433
503,528
500,524
acetonitrile
4010
4.1∙108 8.7∙108
2.1∙108 7.9∙108
1.6∙108 1.1∙109
n-hexane
E1
390, 512 620, 665 (shoulder)
3400 0.32
0.78
1.0
FWMH=5880 cm-1
FWMH=2390 cm-1
ethyl acetate
390, 513
390, 520
705
720
765
5310 0.21
5340
tetrahydrofuran
dichloromethane
395, 533
5690 0.13
0.81
(emax=55100 M-1∙cm-1)
390, 515
acetone
780
6600
3850
n-hexane
E2
365, 520
FWMH=6080 cm-1
650
FWMH=2560 cm-1
790
dichloromethane
375, 555
5300
e
max=55100 M-1∙cm-1)
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The relevant features of E1 and E2, which differentiate them from the other compounds studied in the present
work, can be understood by the comparison between the E1 OMs and those of the corresponding compound
lacking the DCV (Figure S1) and highlights the role of DCV groups in changing the electronic structures of E1
and E2: indeed, in such molecules the HOMO-LUMO transition involves a intramolecular charge transfer from
the DTS electron-donor unit to the DCV electron-acceptor groups.
The influence of the solvent polarity on absorption and emission spectra was studied in detail for E1. While
DTS, T1 and T2 show optical and photophysical properties independent of the solvent, E1 shows a weak
absorption solvatochromism (Figure 2a) and fluorescence strongly affected by the solvent polarity (Figure 2b):
a large positive solvatochromism characterizes the emission spectra, where a red shift of about 3300 cm-1
from a nonpolar solvent (n-hexane) to a polar solvent (acetone) is observed. The absorption and emission
spectra could not be collected in more polar solvents like acetonitrile or DMSO due to the low solubility of the
compound.
The examined compounds are non-centrosymmetric, so the electric dipole moments calculated (at B3LYP
level for all the molecules) for the relaxed ground state are non-null by symmetry: the values obtained for DTS,
T1 and T2 are 1.48 D, 0.34 D, 1.35 D, respectively. For E1 a value of 10.94 D was obtained, whereas for the
analog compound lacking the DCV substituents a value of 0.37 D was calculated. The dipole moment is normal
to the long molecular axis, along the C2 symmetry axis. The dramatic influence of the solvent polarity on the
emission properties of E1 (Figure 2b) can be interpreted in terms of a large change in the electric dipole
moment upon photoexcitation; however, the calculated value of the dipole moment of the Franck-Condon S1
state is 12.93 D, indicating an increase of only 1.99 D. This computational finding is in agreement with the
weak absorption solvatochromism but it does not permit to rationalize the relevant fluorescence
solvatochromism observed. For the purpose of elucidating such behavior, the emission solvatochromism was
experimentally studied for E1 in five solvents of increasing polarity, from n-hexane to acetone. Using the
Lippert-Mataga equation, a linear fit with a slope of 10770 cm-1 was obtained (Figure S2), considering a cavity
radius a=15 Å, so that the E1 molecule is fully embedded in the model spherical cavity. This slope value
corresponds to an increase of the dipole moment upon excitation of about 60 D. Though this value is probably
overestimated, due to the non-spherical shape of the investigated molecule, the marked positive emission
solvatochromic effect, along with the small difference of the dipole moment calculated between S0 and the
unrelaxed S1 state, suggests an important increase of the S1 polarity after relaxation.
Such a behavior, a weak positive absorption solvatochromism and a marked positive emission
solvatochromism, is characteristic of several quadrupolar molecules and it has been widely reported in
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literature.[15, 57] Though these chromophores, because of their symmetric structure, are characterized by a low
dipole moment in the ground state (a null dipole in the case of centrosymmetric molecules), which is maintained
after the vertical transition, the strong emission solvatochromism suggests, on the contrary, the occurrence of
a polar S1 state. It has been interpreted on the basis of the excited-state symmetry breaking model induced by
the solvent polarity: the first electronic excited state is characterized by the charge localization on one arm of
the molecule, so that the emission originates from a polar state stabilized in polar solvents. We propose that
this model, already used in order to explain the positive emission solvatochromism in V-shaped molecules, [58]
can effectively apply to these pseudo-quadrupolar dithienosilole-based molecules.
Figure 1. a) Normalized absorption spectra and b) corrected normalized fluorescence spectra of DTS (red),
T1 (black), T2 (cyan), E1 (blue), E2 (green) in n-hexane
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Figure 2. a) Normalized absorption spectra and b) corrected normalized fluorescence spectra of E1 in n-
hexane (black), ethyl acetate (cyan), tetrahydrofuran (green), dichloromethane (red), acetone (blue)
Electrochemical characterization
CV is a convenient research tool for the study of the electrochemical properties of molecules and other
compounds, among which conducting oligomers and polymers, which can undergo oxidation and reduction
processes.
The electrochemical parameters of DTS, T1, T2, E1 and E2 are reported in Table 3. All the compounds
investigated in this study undergo a p-doping process exhibiting, with the exception of DTS, two reversible
oxidation waves (I and II) attributable to the formation of cationic and dicationic sites. Figure 3 reports the
voltammetric curves of oligomers E1 and E2, while those of DTS, T1 and T2 can be found in Figure S3.
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Figure 3. CV scan of a) E1 and b) E2 in a 0.1 M solution of TBAPF6 in dichloromethane, at 100 mV/s scan
rate.
The potential peak values relative to the first oxidation process (EpoxI) of E1 and E2, as well as those of T1 and
T2, are very closed to each other, as reported in Table 3. Comparing the data relative to the second oxidation
process (EpoxII), a more marked difference is notable especially between T1 and T2. In the experimental
conditions used for this study, it was possible to observe the occurrence of n-doping only with E1 and E2;
Figure 3 shows the relevant voltammetric curves. In this case, the peak potential of E1 is located at less
cathodic values with respect to E2 (see Table 3).
From the CV curves recorded during oxidation and reduction processes is possible to determine the onset
potentials, indicated as Eonset and Eonsetred, respectively. On the basis of these values, the electrochemical
ox
band gap (Egelectr) energies can be derived (Table 3).
The obtained Egelectr values for E1 and E2 are consistent with those reported in the first column of Table 1. In
particular, and in accordance to photochemical evidences and to theoretical calculations, Egelectr of E2 is slightly
narrower than that of E1.
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Table 3: Electrochemical parameters of the studied oligomers in dichloromethane
Eonsetox (V)
+0.58
EpoxI(V)
+0.72
+0.34
+0.32
+0.31
+0.33
EpoxII(V)
+0.84
+0.72
+0.62
+0.59
+0.55
Eonsetred (V)
Epred (V)
Egelectr (eV)
DTS
T1
/
/
/
+0.22
/
/
/
T2
+0.22
/
/
/
E1
+0.20
-1.53
-1.41
-1.63
-1.48
1.73
1.63
E2
+0.22
Conclusions
Two A-π-D-π-A thiophene-based small molecules, having a central electron-donating dithienosilole unit and
terminal electron-withdrawing methyl-DCV units, differing only for the substitution at the thiophene ring (E1
bearing a n-octyl chain vs E2 bearing a n-octylsulfanyl chain), were synthesized. The spectroscopic,
photophysical and electrochemical properties of E1, E2 and of their precursors (DTS, T1 and T2) were studied.
In particular, the influence of the nature of the building blocks and substituents on the optical and photophysical
properties was rationalized in order to gain further insights into the design of materials for applications in
optoelectronic devices. A first conclusion is that the central rigid dithienosilole unit allows for an increased π-
conjugation length and hence a lower optical band-gap if compared to the correspondent oligothiophenes
lacking the dithienosilole central unit.
A progressive bathochromic shift in absorption and emission spectra is observed by passing from DTS to T1
and T2; E1 and E2 exhibit a further red-shifted absorption. The TDDFT calculations show that the lowest-
energy absorption band is mainly attributable to the HOMO-LUMO transition for all compounds; they are
characterized by an extended π-conjugation, involving in E1 and E2 also the terminal DCV groups. DTS
exhibits a peculiar photophysical behavior, if compared with that of 2,2’-bithiophene: it is characterized by a
very high fluorescence quantum yield and not efficient non-radiative deactivation paths. On the contrary, longer
and more flexible molecules T1, T2, E1 and E2 show a photophysical behavior similar to that of the
correspondent OTs, with similar radiative and non-radiative constants. The relevant features of E1 and E2,
which exhibit different optical and photophysical properties if compared with their precursors, were highlighted:
in such molecules the HOMO-LUMO transition involves a intramolecular charge transfer from the electron-
donor dithienosilole unit to the two terminal electron-acceptor DCV groups. The optical and photophysical
properties are independent of the solvent for DTS, T1 and T2, whereas a marked positive emission
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solvatochromism is shown by E1 and E2: the increased electric dipole moment upon photoexcitation in such
two quadrupolar-like molecules can be interpreted on the basis of the symmetry breaking in the S1 excited
state.
The role of the octylsulfanyl lateral chains in reducing the optical band–gap in T2 and E2 with respect to that
shown in compounds bearing octyl chains (T1 and E1) is due both to electronic and conformational effects
related to the presence of the sulfur atom. Electrochemical data confirm the decrease of the electronic band-
1
gap going from E1 to E2. Finally, the significant deshielding of the H NMR proton signal of the central
dithienosilole unit in T2 with respect to DTS and T1, is in favor of a more planar conformation of T2 with respect
to T1 in solution.
Experimental section
Synthesis: 3-octylthiophene was purchased from Sigma Aldrich, 2-bromo-3-octylthiophene was obtained as
already reported. [59] 2-bromo-3-octylsulfanylthiophene was obtained as reported in ref. [60] Medium Pressure
Liquid Chromatography (MPLC) was performed with a Grace Reveleris Flash Chromatography system with
1
evaporative light scattering (Alltech 3300 ELSD) and UV detection. H and 13C NMR spectra were recorded
with Bruker Avance 400 and Avance III HD 600 spectrometers, operating at 400.13 and 600.13 for proton and
at 100.61 and 150.90 MHz for carbon, respectively, Assignments were made applying standard HSQC and
HMBC pulse sequences.
Computational Details: All the calculations on molecules DTS, T1, T2, E, E1 and E2 were carried out (without
the inclusion of solvent effects) using both Firefly Quantum Chemistry package, [61] which is partially based on
the GAMESS (US) source code [62], as well as NWChempackage [63]. Assuming for all the molecules a model
C2 starting structure, the ground state geometry was fully optimized at DFT B3LYP/Ahalrichs-pVDZ level. We
also checked for the positive semidefinite character of the nuclear hessian in order to ensure that the located
stationary point was a true local minimum. Starting from these gas-phase optimized structures, the Kohn-Sham
(KS) HOMO and LUMO orbital energies were employed for evaluating the KS gap (
) which is an
approximation of the fundamental electronic gap Eg. The basis set Ahalrichs-pVDZ was also used for the
calculation of the optical one-photon absorption (OPA) spectra in the framework of adiabatic LR-TDDFT
exploiting both B3LYP as well as CAM-B3LYP functionals.
Optical and photophysical properties: UV-vis spectra were collected using a Varian Cary 100 Scan UV-vis
spectrophotometer. Fluorescence measurements were performed on a Horiba Jobin Yvon Fluoromax-3
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spectrofluorometer. Emission spectra extending into the NIR region were obtained with an Edinburgh
Instruments FLS920 spectrometer equipped with a Hamamatsu R5509-72 supercooled photomultiplier tube.
Spectra were corrected for the instrumental spectral sensitivity. Fluorescence quantum yields (ΦF) were
determined at T= 20–25 °C using different standards: 9,10-diphenyl-anthracene in cyclohexane (Φ F=0.90 [64]
for DTS, quinine sulfate in H2SO4 1N (Φ F=0.545 [64]) for T1, fluoresceine in NaOH 0.1 M forT2 (ΦF=0.925 [65]
)
and cresyl violet in methanol (ΦF=0.64 [66]) for E1. The correction for the refractive index of the solvents was
introduced. Fluorescence lifetimes were measured with a Horiba FluoroMax4 time correlated single-photon
counting equipment using as excitation sources different nano-LEDs, emitting at 340 nm for DTS, 450 nm for
T1 and T2, 570 nm for E1 and E2. Emission experiment were performed on solutions deoxygenated by
bubbling nitrogen.
Electrochemical measurements: the electrochemical properties were investigated by cyclic voltammetry
(CV) in a dichloromethane solution containing tetraethylammonium hexafluorophosphate (TEAPF6) (puriss.,
Fluka) as the supporting electrolyte. The measurements were carried out in a three electrodes cell in a dry
argon atmosphere using an Autolab PGSTAT 12 galvanostat/potentiostat (Eco Chemie). A 3-mm-diameter Pt
disk (Metrohm) was used as the working electrode. A platinum wire and a silver wire covered with AgCl were
the auxiliary and the pseudo-reference electrodes, respectively. The potential of the pseudo-reference
electrode with respect to the ferrocene/ferrocenium (Fc/Fc+) redox couple was always measured after each
experiment.
Synthesis of 3,3’-dibromo-5,5’-bis(trimethylsilyl)-2,2’-bithiophene: this compound was obtained as
already reported [37]
Synthesis of DTS: in a round bottom Schlenk tube a solution of 3,3’-dibromo-5,5’-bis(trimethylsilyl)-2,2’-
bithiophene (1.5 g, 3.20 mmol) in 15 mL of dry THF was cooled to −78 °C under nitrogen atmosphere. n-
Butyllithium (1.6 M in hexane, 4.6 mL, 7.36 mmol) was added dropwise to the cooled solution and stirring was
prolonged for 1 h at −78 °C. Then, 1.33 mL (3.84 mmol) of dichlorodioctylsilane were added dropwise to the
solution and stirring prolonged for 3 h at room temperature. The mixture was diluted with diethyl ether (15 mL),
washed with a saturated solution of NaCl (2×15 mL) and water (2×15 mL), and dried over MgSO4. After
removal of the solvent under reduced pressure, the NMR spectra of the brown oil thus obtained revealed the
presence of DTS as the major compound with some impurities attributable to its fully and partially deprotected
analogues. The crude was therefore dissolved in 80 mL of chloroform and treated with trifluoroacetic acid (0.6
mL, slow addition) for 3 h at room temperature. The mixture was then washed with water (50 mL), dried over
MgSO4 and the solvent removed under reduced pressure giving a brown oil. The crude was purified via MPLC
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using a prepacked column of 24 g of silica and n-hexane as the eluent, obtaining a yellow oil (0.83 g, 53%
yield).1H NMR (600 MHz, CDCl3): δ= 7.20 (d, J = 4.7 Hz, 1H), 7.05 (d, J = 4.7 Hz, 1H), 1. 38 (m, 2H), 1.32-
1.17 (m, 10H), 0.89 (m, 2H), 0.87 ppm (t, J = 7.0 Hz, 3H); 13C NMR (600 MHz, CDCl3): δ= 149.2, 141.6, 129.6,
124.9, 33.1, 31.8, 29.2, 29.1, 24.2, 22.6, 14.1, 11.9 ppm.
Synthesis of 1: in a round bottom flask protected from light, N-bromosuccinimide (0.37 g, 2.09 mmol) was
added portionwise to a solution of 1 (0.4 g, 0.95 mmol) in 25 mL of chloroform. The solution was stirred for 5
h at room temperature then washed with water (2×25 mL), dried over MgSO4, filtered and the solvent removed
under reduced pressure. The crude was purified by MPLC on silica gel, using petroleum ether 40-60 as eluent,
affording 0.387 g of a green viscous oil (70% yield) of 1. 1H NMR (400 MHz, CDCl3): δ= 6.99 (s, 1H), 1.39-1.17
(m, 12H), 0.86 (m, 2H), 0.88 ppm (t, J = 7.1 Hz, 3H); 13C NMR (400 MHz, CDCl3): δ=148.8, 140.9, 132.2,
111.1, 33.1, 31.8, 29.2, 29.1, 24.0, 22.6, 14.1, 11.6 ppm.
Synthesis of 2a: in a round bottom Schlenk tube and under argon atmosphere, a solution of 2-bromo-3-
octylthiophene (2.7 g, 9.8 mmol) in 50 mL of anhydrous THF was cooled to −78 °C. A solution of n-butyllithium
(6.44 mL, 1.6 M in hexane) was dropwise added and the mixture stirred for 2 h, then 2-isopropoxy-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane (4 mL, 19.6 mmol) was slowly added. The chiller was switched off and the
mixture stirred overnight. The solution was washed with 50 mL of a saturated solution of NaHCO3, extracted
with ethyl acetate (2×30mL), washed with water (2×50 mL), dried over MgSO4 and concentrated under reduced
pressure. The crude was purified by MPLC on silica gel column (neutralized with trimethylamine, and gradient
elution from100% n-hexane to n-hexane ethyl acetate 90:10) to yield an oil (1.7 g, 54%).1H NMR (400 MHz,
CDCl3): δ= 7.48 (d, J = 4.7 Hz, 1H), 7.01 (d, J = 4.7 Hz, 1H), 2.88 (t, J = 7.6, 2H), 1.56 (m, 2H), 1.33 (s, 12H),
1.32-1.21 (m, 10H), 0.88 (t, J = 6.7 Hz).
Synthesis of 2b: was prepared in the same way as 2a, starting from 2-bromo-3-octylsulfanylthiophene (1.05
g, 3.81 mmol) obtaining a crude (24% yield) which decomposes on neutralized silica and alumina ad therefore
was used as obtained for the next step. 1H NMR (400 MHz, CDCl3): δ= 7.55 (d, J = 4.9 Hz, 1H), 7.09 (d, J =
4.9 Hz, 1H), 2.94 (t, J = 7.5 Hz, 2H), 1.63 (qn, J=7.5 Hz, 2H), 1.41 (m, 2H), 1.35 (s, 12H), 1.33-1.21 (m, 8H),
0.89 (t, J = 6.9 Hz, 3H); 13C NMR (400 MHz, CDCl3): δ= 143.5, 131.7, 129.7, 126.3, 34.9, 31.9, 29.5, 29.2,
28.9, 24.8, 22.6, 14.1 ppm.
Synthesis of T1: in a round bottom Schlenk tube, a solution of 1 (0.37 g, 0.64 mmol) and 2a (0.522 g, 1.62
mmol) in toluene (10 mL) and ethanol (1 mL) was degassed. In another Schlenk tube a solution of Na2CO3 (2
M in water, 2.4 mL) was degassed with repeated vacuum/argon cycles for 30 min. To the first Schlenk tube
the catalyst tetrakis(triphenylphosphine)palladium(0) (0.073 g, 0.064 mmol) was added and the solution of
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carbonate was slowly cannulated. The mixture was refluxed overnight, cooled to room temperature, poured in
water (10 mL), extracted with n-hexane (2×10mL) and washed with water. The organic phases were dried over
MgSO4, filtered and concentrated. The sticky solid obtained was washed with methanol and purified by MPLC
on silica gel and petroleum ether:ethyl acetate 90:10 as the eluent, obtaining 0.42 g (81%) of a dark yellow
viscous liquid.1H NMR (600 MHz, CDCl3): δ = 7.16 (d, J = 5.1 Hz, 1H), 7.05 (s, 1H), 6.93 (d, J = 5.1 Hz, 1H),
2.78 (t, J = 7.8, 2H), 1.65 (qn,J=7.6, 2H), 1.42 (m, 2H), 1.37 (m, 2H), 1.35-1.18 (m, 18H), 0.94 (m, 2H), 0.87
(t, J = 7.0 Hz, 3H), 0.86 (t, J = 7.1 Hz, 3H); 13C NMR (400 MHz, CDCl3): δ = 148.8, 142.0, 139.3, 136.7, 131.0,
130.0, 128.6, 123.4, 33.2, 31.9, 30.6, 29.5, 29.4, 29.3, 29.2, 24.2, 22.7, 14.1, 11.8 ppm.
Synthesis of T2: The title compound was prepared in the same way as T1 obtaining 180 mg (71% yield) of
an orange viscous liquid starting from 0.17 g of 1. 1H NMR (600 MHz, CDCl3): δ= 7.32 (s, 1H), 7.14 (d, J = 5.3
Hz, 1H), 7.03 (d, J = 5.3 Hz, 1H), 2.85 (t, J = 7.4, 2H), 1.61 (qn,J=7.5, 2H), 1.40 (m, 4H), 1.36-1.18 (m, 18H),
0.94 (m, 2H), 0.85 (2 t, J = 6.8 Hz, 6H); 13C NMR (600 MHz, CDCl3): δ = 149.9, 141.7, 136.8, 136.2, 132.5,
128.9, 127.0, 122.4, 36.1, 33.2, 31.7, 29.6, 29.1, 28.7, 24.1, 22.6, 14.0, 11.7 ppm.
Synthesis of 3a: in a round bottom Schlenk tube a solution of T1 (0.12 g, 0.15 mmol) in THF (5 mL) was
cooled to −78°C and three cycles, consisting of lithium diisopropylamide (1 M in THF/n-hexane, 0.18 mL, 0.18
mmol) dropwise addition, followed by a 40 min stirring period and by a dropwise addition of a solution of
trimethyltin chloride in n-hexane (1 M, 0.18 mL, 0.18 mmol), were repeated. After the last addition of trimethyltin
chloride, stirring was prolonged for 30 min at -78 °C. The solution was left under stirring for 1 h and a half at
−30 °C then for 30 min at room temperature. Water (5 mL) was added then the mixture was extracted with n-
hexane (2×5 mL) and the organic phases washed with water (2×5 mL) and dried over Na2SO4. After removal
of the solvent, a yellow-green viscous oil (0.14 mg, 82% yield) was obtained and used as it was for the next
step. 1H NMR (600 MHz, acetone-d6): δ= 7.22 (s, 1H), 7.12 (s, 1H), 2.83 (t, J = 7.6, 2H), 1.68 (qn,J=7.6, 2H),
1.48 (m, 2H), 1.41 (m, 2H), 1.37-1.20 (m, 18H), 1.04 (m, 2H), 0.87 (t, J = 7.0 Hz, 3H), 0.85 (t, J = 7.1 Hz, 3H),
0.38 (s, 9H, JH,Sn = 56.6, 58.8 Hz); 13C NMR (600 MHz, CDCl3): δ = 149.0, 143.5, 141.4, 139.6, 138.1, 137.6,
137.0, 129.2, 33.6, 32.6, 32.7, 31.4, 30.0, 29.9, 29.8, 29.6, 24.9, 23.4, 14.4, 12.3, -8.3 ppm.
Synthesis of 3b: The title compound was prepared in the same way as 3a obtaining 160 mg (90% yield) of
an orange viscous liquid starting from 0.91 g of T2. 1H NMR (400 MHz, acetone-d6): δ= 7.52 (s, 1H), 7.26 (s,
1H), 2.98 (t, J = 7.1, 2H), 1.65 (qn,J=7.6, 2H), 1.48 (m, 2H), 1.41-1.22 (m, 20H), 1.04 (m, 2H), 0.88 (m, 6H),
0.45 (s, 9H, JH,Sn = 56.9, 58.3 Hz).
Synthesis of 4: in a round bottom flask hexamethyldisilazane (1.42 g, 8.7 mmol) was slowly added to 4.8 mL
of acetic acid, keeping the temperature below 75° C. The hexamethyldisilazane/acetic acid mixture was then
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added dropwise to a flask containing a solution of 2-acetyl-5-bromothiophene (1.5 g, 7.31 mmol) and
malononitrile (0.97 g, 14.63 mmol) in 2.4 mL of acetic acid. The mixture was stirred at 65 °C for 22 h, then
cooled to room temperature with chilled toluene (12 mL) and diluted with water (9 mL). The aqueous layer was
separated and extracted with toluene (5 mL). The combined organic extracts were washed with water (4×5mL)
and dried over MgSO4. After removal of the solvent and recrystallization from methanol a yellow solid was
obtained (0.68 g, 37% yield). 1H NMR (600 MHz, CDCl3): δ= 7.76 (d, J = 4.3 Hz, 1H), 7.22 (d, J = 4.3 Hz, 1H),
2.64 (s, 3H); 13C NMR (600 MHz, CDCl3): δ = 161.0, 139.3, 134.1, 132.0, 123.7, 113.8, 113.3, 79.0, 23.0.
Synthesis of E1: in a round bottom Schlenk tube a solution of 3a (0.11 g, 0.097 mmol) and 4 (0.054 g, 0.213
mmol) in dry toluene (5.5 mL) was degassed with argon and vacuum followed by the addition of Pd(PPh3)4 (4
mg, 0.034 mmol). After stirring at 110 °C for 24 h under inert atmosphere, the mixture was poured into water
(30 mL), and extracted with chloroform. The organic layer was washed with water (2×20 mL), dried over
Na2SO4, reduced at small volume and reprecipitated with methanol. The solid obtained was filtered in a
cellulose thimble in a Soxhlet apparatus, washed with methanol and extracted with chloroform. After removal
of the solvent, 105 mg (76% yield) of a black solid were obtained. Elemental analysis calcd (%) for
C66H82N4S6 (1150.5): C 68.82, H 7.18, N 4.86, S 16.70; found: C 68.69, H 7.20, N 4.88, S 16.64. 1H NMR
(600 MHz, CDCl3): δ= 7.95 (d, J = 4.2 Hz, 1H), 7.24 (d, J = 4.2 Hz, 1H), 7.23 (s, 1H), 7.16 (s, 1H), 2.80 (t, J =
7.8, 2H), 2.68 (s, 3H), 1.70 (qn,J=7.6, 2H), 1.42 (m, 4H), 1.39-1.20 (m, 18H), 0.97 (m, 2H), 0.88 (t, J = 7.0 Hz,
3H), 0.86 (t, J = 7.0 Hz, 3H); 13C NMR (400 MHz, CDCl3): δ = 160.9, 149.4, 146.8, 143.1, 140.7, 136.1, 135.7,
135.4, 134.0, 132.2, 129.7, 129.2, 124.6, 114.6, 114.0, 76.8, 33.1, 31.9, 30.4, 29.6, 29.5, 29.4, 29.2, 24.2,
23.0, 22.7, 14.1, 11.7 ppm.
Synthesis of E2 The title compound was prepared in the same way as E1 obtaining 0.08 g (48% yield) of a
black solid starting from 0.16 g of 3b. Elemental analysis calcd (%) for C66H82N4S8 (1215.9): C 65.19, H
6.80, N 4.61, S 21.10; found: C 65.03, H 6.83, N 4.59, S 21.02. 1H NMR (600 MHz, CDCl3): δ= 7.96 (d, J = 4.3
Hz, 1H), 7.40 (s, 1H), 7.30 (s, 1H), 7.25 (d, J = 4.3 Hz, 1H), 2.92 (t, J = 7.4, 2H), 2.68 (s, 3H), 1.67 (qn,J=7.4,
2H), 1.43 (m, 4H), 1.38-1.20 (m, 18H), 0.96 (m, 2H), 0.85 (t, J = 6.8 Hz, 6H); 13C NMR (400 MHz, CDCl3): δ =
160.9, 150.9, 145.8, 142.8, 138.8, 136.1, 136.0, 135.2, 131.5, 131.3, 129.7, 128.9, 124.8, 114.4, 113.6, 77.2,
36.5, 33.2, 31.9, 31.8, 29.6, 29.3, 29.1, 28.8, 24.2, 23.1, 22.7, 22.6, 14.1, 11.8 ppm.
Acknowledgements
The authors wish to thank Dr. Filippo Monti (Institute of Organic Synthesis and Photoreactivity - CNR Bologna) for his assistance
in collecting the fluorescence spectra in the NIR range.
Keywords: electrochemistry • fluorescence • oligothiophenes • photophysics • solvatochromism
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TOC
Two A-π-D-π-A thiophene based small molecules with a central dithienosilole core and dicyanovinyl end
groups were synthesized. Theoretical calculations, together with spectroscopic and electrochemical findings
revealed that both molecules possess low band gaps, broad absorption spectra and a good stability both in p
and n-doping states. The observed increasing of the electric dipole moment upon photoexcitation in such two
quadrupolar-like molecules has been interpreted on the basis of the symmetry breaking in the S1 excited state.
25
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