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Published on the web May 15, 2013
Organic Semiconductor Molecules with Nonaromatic Core of 1,4-Dithiin
Hiroki Ito,1 Daichi Watanabe,1 Tatsuya Yamamoto,2 Noboru Tsushima,2 Hiroki Muraoka,1 and Satoshi Ogawa*1
1Department of Chemistry and Bioengineering, Faculty of Engineering, Iwate University,
4-3-5 Ueda, Morioka, Iwate 020-8551
2Research Center for Composite Device Technology, Iwate University,
5-6-3 Nimaibashi, Hanamaki, Iwate 025-0312
(Received February 19, 2013; CL-130129; E-mail: ogawa@iwate-u.ac.jp)
Novel thiophene-condensed 1,4-dithiin derivatives having
S
S
styryl or phenyl groups were synthesized, and their spectro-
scopic and electrochemical properties were investigated. Organ-
ic field-effect transistor (OFET) devices using these derivatives
exhibited typical p-type FET characteristics, though the core
units of both molecules were not aromatic compounds.
S
S
S
S
BBTD-syn
DS2T
Figure 1. Structures of BBTD-syn and DS2T.
S
S
³-Conjugated thiophene derivatives offer great potential for
applications in organic electronics.1-4 The incorporation of
thiophene rings into molecules has been utilized to provide good
stability against air oxidation and strong intermolecular inter-
actions.5,6
S
S
S
S
S
S
DPDTD
DSDTD
Figure 2. Structures of DSDTD and DPDTD.
In this context, we have introduced the nonaromatic
1,4-dithiin framework into organic semiconductor molecules,
because this framework will contribute to a lower HOMO energy
level, which is favorable for the fabrication of air-stable
materials. The most well-known 1,4-dithiin derivative, thian-
threne (dibenzo-1,4-dithiin), is known to have a bent structure in
the neutral state but a planar structure in the radical cation
state.7-9 In our previous study, the 1,4-dithiin analog fused to two
benzo[b]thiophenes on both sides, bisbenzo[b]thieno[2,3-b:3¤,2¤-
e]-1,4-dithiin (BBTD-syn) (Figure 1), was reported to be a
candidate as an organic semiconductor, having a bent non-
aromatic 1,4-dithiin structure like a butterfly.10 However, it was
difficult to fabricate OFET devices through vacuum deposition
onto SiO2 substrates, since the BBTD-syn evaporates easily
owing to the radiant heat. Therefore, we needed to design more
suitable 1,4-dithiin derivatives with higher evaporation temper-
atures. Recently, Videlot-Ackermann et al. reported thiophene
derivatives such as 5,5¤-distyryl-2,2¤-bithiophene (DS2T), which
(1) and (Bu3Sn)2S with [Pd(PPh3)4] in an autoclave.14 The
formation of the 1,4-dithiin ring was achieved by the reaction of
compound 2 with n-BuLi following SCl2 to produce 2,6-
bis(trimethylsilyl)dithieno[2,3-b;3¤,2¤-e]-1,4-dithiin (3). The re-
sulting compound was converted into dithieno[2,3-b;3¤,2¤-e]-
1,4-dithiin (4) by a deprotection reaction with tetrabutylammo-
nium fluoride (TBAF).15-17 2,6-Diformyldithieno[2,3-b;3¤,2¤-e]-
1,4-dithiin (5) was then prepared from compound 4 with n-BuLi
following MeN(Ph)CHO, and DSDTD was synthesized through
the Wittig reaction in 97% yield as an orange solid. Meanwhile,
2,6-dibromodithieno[2,3-b;3¤,2¤-e]-1,4-dithiin (6) was also pre-
pared from compound 4 using n-BuLi following (CF2Br)2, and
DPDTD was synthesized through the Suzuki-Miyaura coupling
in 87% yield as a yellow solid.
Both compounds are soluble and stable in organic solvents;
therefore, the HOMO energy levels and transition energies were
estimated from their optical and electrochemical measurements.
Furthermore, DFT calculations at the B3LYP/6-311+G(d,p)
level were also performed to explore the HOMO-LUMO energy
levels, and these results are summarized in Table 1.18 The
calculated HOMO showed that the orbital distribution exists
mainly on the 1,4-dithiin ring.
The UV-vis absorption spectra of target molecules were
measured, as shown in Figure 3a. Focusing on the maximum
absorption wavelength, -max, in CH2Cl2, DSDTD (377 nm) and
DPDTD (341 nm) showed a more red-shifted absorption
compared to BBTD-syn (309 nm), depending on the terminal
substitutions. The transition energies, Eg, obtained from -max
were estimated to be 3.29 eV for DSDTD and 3.64 eV for
DPDTD, which indicated indirectly that the styryl groups
elongate the ³-conjugation length more than the phenyl groups,
as expected. Furthermore, focusing on the -max values of both
spectra from the deposited films on quartz glass, they showed a
27-nm red shift compared to the solution spectra owing to the
intermolecular interactions between molecules in the films.
¹2
showed a relatively high hole mobility of ® = 2.0 © 10
¹1 11-13
cm2 V¹1 s
.
The hole mobility was maintained for more
than 100 days under ambient conditions. Thus, they focused on
the styryl groups in terms of endcap substituents that offer a
planar structure and moderate solubility. Consequently, we
designed two novel types of organic semiconductor molecules,
i.e., 2,6-distyryldithieno[2,3-b;3¤,2¤-e]-1,4-dithiin (DSDTD),
which has the 1,4-dithiin framework and styryl groups as wing
moieties, and 2,6-diphenyldithieno[2,3-b;3¤,2¤-e]-1,4-dithiin
(DPDTD), in order to increase the molecular weight by altering
the chemical structure from condensed phenyl to linked phenyl
groups (Figure 2). In this paper, we report the first synthesis,
spectroscopic and electrochemical properties, and FET character-
istics of the novel 1,4-dithiin derivatives DSDTD and DPDTD.
The novel 1,4-dithiins DSDTD and DPDTD were synthe-
sized through the procedure shown in Scheme 1. Bis(2-
trimethylsilylthien-4-yl)sulfide (2) was synthesized by the Stille
coupling reaction between 4-bromo-2-trimethylsilylthiophene
Chem. Lett. 2013, 42, 646-648
© 2013 The Chemical Society of Japan
Ito, Hiroki
