Synthesis and chromic behaviors of dithienosiloles with push-pull substituents toward VOC detection

Article abstract of DOI:10.1080/15421406.2010.495642

2-Methylthio- and 2-n-hexylthienyl-6-(tricyanoethenyl)-4,4- diphenyldithienosilole were prepared as novel push-pull type compounds. These compounds exhibit solvatochromic behaviors with respect to their UV-vis absorption and emission spectra. Clear vapor

Full text of DOI:10.1080/15421406.2010.495642

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Synthesis and Chromic Behaviors
of Dithienosiloles with Push-Pull
Substituents Toward VOC Detection
Joji Ohshita a , Daiki Tanaka a , Kwang-Hoi Lee a , Yasunori
Kurushima a , Shotaro Kajiwara a , Yousuke Ooyama a , Yutaka
Harima a , Atsutaka Kunai a & Yoshihito Kunugi b
a Department of Applied Chemistry, Graduate School of Engineering,
Hiroshima University, Higashi-Hiroshima, Japan
b Department of Applied Chemistry, Faculty of Engineering, Tokai
University, Hiratsuka, Japan
Version of record first published: 11 Nov 2010.
To cite this article: Joji Ohshita , Daiki Tanaka , Kwang-Hoi Lee , Yasunori Kurushima , Shotaro
Kajiwara , Yousuke Ooyama , Yutaka Harima , Atsutaka Kunai & Yoshihito Kunugi (2010): Synthesis and
Chromic Behaviors of Dithienosiloles with Push-Pull Substituents Toward VOC Detection, Molecular
Crystals and Liquid Crystals, 529:1, 1-9
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Mol. Cryst. Liq. Cryst., Vol. 529: pp. 1–9, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421406.2010.495642
Synthesis and Chromic Behaviors of
Dithienosiloles with Push-Pull Substituents
Toward VOC Detection
JOJI OHSHITA,1 DAIKI TANAKA,1
KWANG-HOI LEE,1 YASUNORI KURUSHIMA,1
SHOTARO KAJIWARA,1 YOUSUKE OOYAMA,1
YUTAKA HARIMA,1 ATSUTAKA KUNAI,1
AND YOSHIHITO KUNUGI2
1Department of Applied Chemistry, Graduate School of Engineering,
Hiroshima University, Higashi-Hiroshima, Japan
2Department of Applied Chemistry, Faculty of Engineering, Tokai
University, Hiratsuka, Japan
2-Methylthio- and 2-n-hexylthienyl-6-(tricyanoethenyl)-4,4-diphenyldithienosilole
were prepared as novel push-pull type compounds. These compounds exhibit solva-
tochromic behaviors with respect to their UV-vis absorption and emission spectra.
Clear vapor chromism is also observed for their thin solid films and the color
changed clearly on exposure of the films to organic solvent vapors, indicating the
potential applications to VOC sensors.
Keywords Chromic material; push-pull compound; sensor material; silole
Introduction
Push-pull type compounds bearing electron-accepting and donating units linked by a
p-conjugated system have been extensively studied and their utilities as functional
materials, such as chromic and nonlinear optical materials have been demonstrated
[1]. In those studies, oligothiophenes are often employed as the p-conjugated linkers,
because of their efficient conjugation along the chain and the stability. Recently, we
demonstrated that the introduction of an intramolecular bridge at the b,b0-position
of bithiophene derivatives, forming a dithienosilole (DTS) ring, enhances the conju-
gation in these systems [2–4]. This is due not only to the coplanarity of two thiophene
rings linked by the bridge, but also to bonding interaction between the silicon
rꢀ-orbital and the bithiophene pꢀ-orbital, which lowers the LUMO energy level.
In this article, we report the synthesis of DTSs featuring tricyanoethenyl, and
methylthio or hexylthienyl groups as the electron-accepting and donating units,
Address correspondence to Joji Ohshita, Department of Applied Chemistry, Graduate
School of Engineering, Hiroshima University, Higashi-Hiroshima 739-8527, Japan. E-mail:
jo@hiroshima-u.ac.jp
1

2
J. Ohshita et al.
Chart 1. Tricyanoethenyl-substituted DTSs.
respectively (S-DTS-TC and T-DTS-TC in Chart 1). The tricyanoethenyl group that
can be readily introduced to a thiophene ring, has been proven useful as an acceptor
unit of push-pull type compounds [5]. We previously prepared a tricyanoethenyl-
substituted DTS (DTS-TC in Chart 1) and found that DTS-TC showed spectral
and color changes in response to organic solvent vapors in the films [6]. Sensing
materials that detect volatile organic compounds (VOC) are of growing interest,
because of their environmental concern [7]. The present S-DTS-TC and T-DTS-
TC showed solvato- and vapor chromic behaviors in the solution phase as well as
in the solid state, to larger extents than DTS-TC, indicating their potential applica-
tions as VOC-sensor materials.
Results and Discussion
Push-pull type compounds, S-DTS-TC and T-DTS-TC were obtained by the reac-
tions of S-DTS and T-DTS with tetracyanoethylene (TCNE) in DMF in 44% and
78% yields, respectively (Scheme 1). They are metallic golden colored solids, suggest-
ing strong intermolecular interaction in the solid state, as previously reported for
similar tricyanoethenyl-substituted p-conjugated compounds [5f]. We prepared also
silole-free methylthio(tricyanoethenyl)bithiophene (S-BT-TC) for comparison (see
Chart 2). Table 1 summarizes optical properties of the presently prepared com-
pounds, together with those of the related compounds, reported previously
(Chart 2) [3,6]. The UV-vis absorption and emission maxima of S-DTS-TC are lar-
gely red-shifted from that of DTS-TC [6]. Since introduction of a methylthio group
on the DTS unit moves the absorption maxima by only 7 and 18 nm in a series of
DTS1 (Chart 2, kmax ¼ 356 nm) [2b], S-DTS (363 nm), and S-DTS-S (Chart 2,
381 nm) [3], respectively, the large shift of 34 nm from DTS-TC to S-DTS-TC indi-
cates a considerable ‘‘push’’ effect of the methylthio group in this system. A similar
red shift is observed for S-BT-TC relative to BT-TC [6]. Effects of tricyanoethenyl-
substitution on the optical properties are clear, which leads to large red shifts for
S-DTS-TC and T-DTS-TC from S-DTS and T-DTS (kmax ¼ 395 nm). The silicon
bridge in S-DTS-TC also affects the optical properties, moving the kmax to longer
wavelength by about 30 nm from that of S-BT-TC. A similar red shift was already
reported for DTS-TC from BT-TC [6]. However, the kedge values are not evidently
affected by the existence of the silicon bridge. Presumably, in these compounds, it
is mainly responsible for the red shifts of the absorption maxima that the silicon

Push-Pull Type Dithienosiloles
3
Scheme 1. Synthesis of push-pull type DTSs.
bridge retains the high planarity of the bithiophene unit, while no significant elec-
tronic effects of the silicon bridge such as rꢀ-pꢀ interaction is involved, which should
affect the kedge if present.
To know the electronic states of the present systems, we carried out DFT
calculations at the B3LYP=6-31G level (Table 1). For the calculations of the DTS
derivatives, simplified model compounds that have hydrogen atoms in place of
phenyls on the silicon atoms were employed. As shown in Table 1, the HOMOs
and LUMOs lie at almost the same energies independent of the existence=absence
of the silicon bridge, in agreement with the experimental observations described above.
Presently prepared compounds, S-DTS-TC, S-BT-TC, and T-DTS-TC, exhibit
solvatochromic behaviors as shown in Table 2 and Figure 1. Although similar
Chart 2. DTS and bithiophene derivatives.

4
J. Ohshita et al.
Table 1. Optical properties of DTS- and bithiophene-based compounds
DFT calculationsa
UV in THF
max=nm edge=eV
Compd
k
k
HOMO=eV
LUMO=eV
S-DTS-TC
T-DTS-TC
S-BT-TC
DTS-TC
BT-TC
546
591
518
512
484
1.8
1.6
1.8
2.1
2.2
ꢁ6.34
ꢁ5.96
ꢁ6.31
ꢁ6.40
ꢁ6.45
ꢁ3.77
ꢁ3.60
ꢁ3.76
ꢁ3.72
ꢁ3.75
aAt the level of B3LYP=6-31G. For dithienosiloles, simplified model compounds were
employed, in which phenyl groups on the silicon atom were replaced by hydrogen atoms.
Table 2. Absorption and emission maxima [nm] of S-DTS-TC, S-BT-TC, and
T-DTS-TC in different solvents
S-DTS-TC
Emission
S-BT-TC
Emission
T-DTS-TC
Emission
Solvt
abs
abs
abs
CCl4
532
533
539
546
564
619
633
669
660
666
508
510
513
518
540
602
643
670
659
660
603
590
–
591
621
643
670
–
713
711
Ether
AcOEt
THF
CHCl3
solvatochromic behaviors were already reported for compounds DTS-TC and
BT-TC [6], the chromic shifts for DTS-TC and BT-TC were only 22 nm and
20 nm at most, respectively, on changing the solvent from ether to chloroform, which
were smaller than those of S-DTS-TC, S-BT-TC, and T-DTS-TC. Emission spectra
also change in response to the solvents as shown in Table 2. Figure 2 represents the
HOMO and LUMO profiles of the model of compound S-DTS-TC, derived from
the DFT calculations. The HOMO is rather localized on the methylthio-DTS unit,
while the LUMO mainly lies on the tricyanoethenyl unit, indicating a polarized
Figure 1. Solutions of S-DTS-TC.

Push-Pull Type Dithienosiloles
5
Figure 2. HOMO (left) and LUMO (right) profiles for a model of S-DTS-TC, derived from
DFT calculations at the level of B3LYP=6-31G.
structure in the excited state. This seems to be responsible for the larger solvatochro-
mic shifts in the emission spectra than those in the absorption spectra. No solvato-
chromic shifts larger than 2 nm are observed for S-DTS and T-DTS having no
tricyanoethenyl acceptor unit.
Compounds S-DTS-TC and T-DTS-TC are vapor chromic in the solid state.
Figure 3(a) depicts the spectral changes for S-DTS-TC with respect to the UV-vis
absorption, on exposure of the vapor deposited films to organic solvent vapors for
a few seconds. The largest vapor chromic shift of 45 nm is observed for S-DTS-
TC, accompanied with a color change from ultramarine to blue (Fig. 4), when the
film is exposed to an acetonitrile vapor. Spin coated films of T-DTS-TC also show
vapor chromism (Fig. 3(b)), but spectral changes of T-DTS-TC occur relatively
slowly with the time scale of minute, as compared with that of S-DTS-TC. No
clear vapor chromism is observed for S-BT-TC, in contrast to its sensitive
solvatochromism.
In conclusion, we prepared novel push-pull type compounds bearing a DTS sys-
tem as the linker, which exhibit remarkable solvato and vapor chromic behaviors,
potentially useful for VOC sensing. Both the experimental and theoretical studies
suggest that electronic effect of the silicon bridge is not significantly operative in
the present system. However, its steric effect making two thiophene rings coplanar
Figure 3. Spectral changes of vacuum deposited films of S-DTS-TC (A) and spin coated films
of T-DTS-TC (B) on exposure of the films to various solvent vapors.

6
J. Ohshita et al.
Figure 4. Films of S-DTS-TC as prepared by vacuum deposition (left) and exposed to an acet-
onitrile vapor (right).
significantly enhances the conjugation. Studies on applications of DTS based push-
pull compounds as organic device materials are underway.
Experimental
General Procedure
3,30-Dibromo-5-methylthio-50-trimethylsilyl-2,20-bithiophene was prepared as
reported in the literature [4]. THF and ether were dried over sodium-benzophenone
and distilled immediately before use. Toluene and DMF were distilled from sodium
and calcium hydride, respectively, and were stored over activated molecular sieves
until use. All reactions were carried out in dry nitrogen. Usual workup described
below involves hydrolysis of the reaction mixture with water, separation of the
organic layer, extraction of the aqueous layer with chloroform, drying the combined
organic layer and extracts over anhydrous magnesium sulfate, and evaporation of
the solvent, in that order.
Preparation of 3,30-Dibromo-5-methylthio-2,20-bithiophene
A mixture of 2.14 g (4.85 mmol) of 3,30-dibromo-5-methylthio-50-trimethylsilyl-
2,20-bithiophene, 40 mL of THF, and 15 mL of 35% HCl (aq) was heated to reflux
for 3 days. To the resulting mixture was added 50 mL of ether. The organic layer
was separated and washed with water, then dried over anhydrous magnesium sulfate.
After evaporation of the solvent, the residue was chromatographed on a silica gel
column eluting with hexane to give 1.71 g (95% yield) of the title compound: viscous
pale yellow oil; MS m=z 370 [Mþ];1H NMR (d in CDCl3) 2.54 (s, 3H, SMe), 6.99 (s,
1H, ring H), 7.06 (d, 1H, J ¼ 5.4 Hz, ring H), 7.38 (d, 1H, J ¼ 5.4 Hz, ring H);
13C NMR (d in CDCl3) 20.94, 111.68, 112.44, 127.46, 128.61, 129.76, 130.78,
132.18, 140.18.
Preparation of S-DTS
To a solution of 1.69 g (4.56 mmol) of 3,30-dibromo-5-methylthio-2,20-bithiophene in
50 mL of ether and 25 mL of THF was added drop wise 5.77 mL (9.12 mmol) of a
1.58 M n-butyllithium=hexane solution at ꢁ80ꢂC and the mixture was stirred at
room temperature for 1 h. The mixture was again cooled at ꢁ80ꢂC and 1.30 g
(5.90 mmol) of difluorodiphenylsilane was added slowly to the mixture, then the mix-
ture was heated to reflux for 15 h. After the usual workup, the residue was chroma-
tographed on a silica gel column eluting with hexane=ethyl acetate ¼ 40=1 to give a

Push-Pull Type Dithienosiloles
7
crude solid. Recrystallization of the crude solid from hexane=ethyl acetate afforded
0.638 g (36% yield) of compound S-DTS as a yellow needle-like solid: mp 113.8–
1
141.2ꢂC; MS m=z 392 [Mþ]; H NMR (d in CDCl3) 2.49 (s, 3H, SMe), 7.20 (s, 1H,
thiophene), 7.21 (d, 1H, J ¼ 4.8 Hz, thiophene), 7.27 (d, 1H, J ¼ 4.8 Hz, thiophene),
7.33–7.44 (m, 6H, m- and p-Ph), 7.61 (dd, 4H, J ¼ 8.0 and 1.5 Hz, o-Ph); 13C NMR (d
in CDCl3) 22.87, 126.31, 128.19, 129.62, 130.39, 131.47, 134.66, 135.35, 137.86,
139.30, 140.13, 150.31, 152.50; 29Si NMR (d in CDCl3) ꢁ20.23. Anal. Calcd for
C21H16S3Si: C, 64.24; H, 4.11. Found: C, 64.26; H, 4.20.
Preparation of 5-Methylthio-2,20-bithiophene
To a solution of 3.00 g (18.0 mmol) of bithiophene was added 11.4 mL (18.0 mmol) of
a 1.58 M n-butyllithium=hexane solution at ꢁ80ꢂC and the mixture was allowed to
warm to room temperature. A small portion of the reaction mixture was sampled
and quenched with chlorotrimethylsilane, which was then analyzed by GC indicating
the formation of a mixture consisting of bithiophene, monolithiobithiophene, and
dilithiobithiophene in an approximate ratio of 1:7:2. The mixture was again cooled
to ꢁ80ꢂC and 1.55 mL (18.0 mmol) of dimethyl disulfide was added slowly to the
mixture. The mixture was then stirred at room temperature for 3 h. After the usual
workup, the residue was chromatographed on a silica gel column eluting with hexane
to give 1.94 g (51% yield) of the title compound as yellow oil: MS m=z, 212 [Mþ];
1H NMR (d in CDCl3) 2.51 (s, 3H, SMe), 6.97–7.02 (m, 3H, thiophene H), 7.13
(d, 1H, J ¼ 3.6 Hz, thiophene H), 7.20 (d, 1H, J ¼ 5.3 Hz, thiophene H); 13C NMR
(d in CDCl3) 20.95, 111.23, 113.24, 130.26, 132.41, 133.29, 136.94, 139.85, 143.20.
Preparation of T-DTS
A mixture of 6.41g (14.0 mmol) of 5-hexyl-2-tributylstannylthiophene, 6.99g
(14.0 mmol) of 2-bromo-6-trimethylsilyl-4,4-diphenyldithienosilole, 0.81g (0.70 mmol)
of Pd(PPh3)4, 0.013 g (0.70mmol) of CuI, and 30mL of toluene was heated to reflux
for 72h. After the usual workup, the residue was chromatographed on a silica gel col-
umn eluting with hexane=toluene ¼ 3=1 to give a crude solid. Recrystallization of the
crude product from hexane gave 4.46g (62% yield) of T-DTS as a yellow solid: mp
134.7–136.9ꢂC; MS m=z 512 [Mþ]; 1H NMR (d in CDCl3) 0.88 (t, 3H, Hex),
1.40–1.24 (m, 6H, Hex), 1.69–1.62 (m, 2H, Hex), 2.77 (t, 2H, J ¼ 7.2 Hz, Hex), 6.66
(d, 1H, J ¼ 2.9 Hz, thiophene), 6.96 (d, 1H, J ¼ 3.9 Hz, thiophene), 7.20 (br d, 2H,
J ¼ 5.8 Hz, thiophene), 7.24 (d, 1H, J ¼ 3.9 Hz, thiophene), 7.33–7.44 (m, 6H,
m- and p-Ph), 7.64 (dd, 4H, J ¼ 7.8 and 1.7 Hz, o-Ph); 13C NMR (d in CDCl3)
14.08, 22.56, 28.71, 28.72, 30.16, 31.55, 123.13, 124.77, 125.43, 126.00, 128.19,
129.67, 130.35, 131.56, 134.70, 135.40, 139.27, 139.34, 140.98, 145.25, 148.20,
150.36. Exact MS Calcd for C30H28S3Si [Mþ]: 512.1122. Found: 512.1125.
Preparation of S-DTS-TC, S-BT-TC, and T-DTS-TC
A solution of 0.300 g (0.764 mmol) of S-DTS and 0.196 g (1.53 mmol) of TCNE in
2 mL of DMF was stirred at room temperature for 12 h. To this were added 5 mL
of water and 3 mL of ethyl acetate. After the usual workup, the residue was chroma-
tographed on a silica gel column eluting with hexane to give 0.167 g (44% yield) of
S-DTS-TC as a metallic green needle-like solid: mp 268.5–269.2ꢂC; MS m=z, 493

8
J. Ohshita et al.
1
[Mþ]; H NMR (d in CDCl3) 2.64 (s, 3H, SMe), 7.20 (s, 1H, thiophene), 7.41–7.51
(m, 6H, m- and p-Ph), 7.58 (d, 4H, J ¼ 6.8 Hz, o-Ph), 8.09 (s, 1H, J ¼ 4.8 Hz, thio-
phene); 13C NMR (d in CDCl3) 20.67, 112.66, 113.10, 113.16, 128.41, 128.71,
128.85, 135.32, 135.86, 142.42, 143.13, 146.88, 149.89, 150.87, 164.67 (three carbon
signals may be overlapped); 29Si NMR (d in CDCl3) ꢁ19.96; IR 2216 cmꢁ1
(CꢃN). Anal. Calcd for C26H15N3S3Si: C, 63.25; H, 3.06; N, 8.51. Found: C,
63.08; H, 3.11; N, 8.42.
Compound S-BT-TC was obtained in 47% yield by the reaction of
5-(methylthio)-2,20-bithiophene and TCNE in DMF, in a fashion similar to that
above, and was purified by silica gel column chromotography with hexane as eluent:
dark green solid; mp 178.8–180.0ꢂC; MS m=z, 313 [Mþ]; 1H NMR (d in CDCl3) 2.60
(s, 3H, SMe), 6.98 (d, 1H, J ¼ 3.9 Hz, thiophene), 7.26 (d, 1H, J ¼ 4.5 Hz, thiophene),
7.38 (d, 1H, J ¼ 3.9 Hz, thiophene), 7.96 (d, 1H, J ¼ 4.5 Hz, thiophene); 13C NMR
(d in CDCl3) 20.28, 112.16, 112.50, 112.67, 124.82, 129.26, 131.66, 131.71, 131.73,
134.08, 141.55, 146.28, 151.74 (one carbon signal may be overlapped); IR
2220 cmꢁ1 (CꢃN). Anal. Calcd for C14H7N3S3: C, 53.65; H, 2.25; N, 13.41. Found:
C, 53.67; H, 2.27; N, 13.30. Compound T-DTS-TC was obtained in 78% yield by the
reaction of 5-(hexylthienyl)-4,4-diphenyldithienosilole and TCNE in DMF, in a
fashion similar to that above, and purified by preparative GPC elutingþwith chloro-
1
form: metallic dark green solid; mp 252.58–254.9ꢂC; MS m=z 613 [M ]; H NMR
(d in CDCl3) 0.89 (t, 3H, J ¼ 6.3 Hz, Hex), 1.40–1.25 (m, 6H, Hex), 1.73–1.65 (m,
2H, Hex), 2.83 (t, 2H, J ¼ 7.2 Hz), 6.76 (d, 1H, J ¼ 3.9 Hz, thiophene), 7.13 (d,
1H, J ¼ 3.9 Hz, thiophene), 7.35 (s, 1H, thiophene), 7.54–7.42 (m, 7H, thiophene,
and m- and p-Ph), 7.62 (dd, 4H, J ¼ 7.7 and 1.9 Hz, o-Ph), 8.08 (s, 1H, thiophene);
13C NMR (d in CDCl3) 14.05, 22.53, 28.67, 30.28, 31.43, 31.49, 112.82, 113.15,
113.30, 125.67, 125.88, 126.31, 126.32, 128.44, 128.70, 129.01, 130.87, 131.41,
133.27, 135.36, 136.07, 142.90, 144.96, 148.40, 148.76, 150.92, 165.21; IR 2216cmꢁ1
(CꢃN). Exact MS Calcd for C35H27N3S3Si [Mþ]: 613.1136. Found: 613.1129.
DFT Calculations
All calculations were performed by the density functional theory (DFT) calculations.
The Becke-three-parameter-Lee-Yang-Parr hybrid functional [8] and the 6-31G basis
set was employed, which is implemented in Gaussian03 program package [9].
Acknowledgments
This work was supported in part by Electric Technology Research Foundation of
Chugoku, to which the authors’ thanks are due.
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