998
Published on the web September 5, 2011
n-Type Field-effect Transistors Based on Thieno[3,2-b]thiophene-2,5-dione
and the Bis(dicyanomethylene) Derivatives
Shiyan Chen, Altan Bolag, Jun-ichi Nishida, and Yoshiro Yamashita*
Department of Electronic Chemistry, Tokyo Institute of Technology,
G1-8, 4259 Nagatsuta-cho, Midori-ku, Yokohama, Kanagawa 226-8502
(Received June 21, 2011; CL-110518; E-mail: Yoshiro@echem.titech.ac.jp)
CF
3
Two new compounds, 3,6-bis(4-trifluoromethylphenyl)thieno-
S
Br
(HO)2
B
CF3
S
[3,2-b]thiophene-2,5-dione (1) and 2,5-bis(dicyanomethylene)-
3,6-bis(4-trifluoromethylphenyl)thieno[3,2-b]thiophene (2), were
synthesized and characterized. The crystal structure of 1 was
determined by single-crystal X-ray structure analysis. Organic
field-effect transistors (OFETs) showed a relatively good
NBS
CHCl3/CH3COOH
[Pd(PPh3
)
4], THF/ 1M K2CO3
S
S
Br
F C
CF
3
3
O
S
n-BuLi
1
2
3
B(OBu)3
H2O2
S
O
CF
3
¹1
mobility up to 0.039 cm2 V¹1 s in 1 and a better air stability
F
C
1
3
Br
S
CN
CF
3
in 2.
S
NC
TCNEO
1,3-dibromopropane
reflux, 2h
S
Br
F C
3
CN
CN
S
F C
3
2
Organic field-effect transistors (OFETs) have been attracting
increasing attention due to their potentials as low-cost, large-
area, and flexible organic electronic devices.1 During the past
few years, remarkable progress has been made in developing p-
type semiconducting materials such as pentacene,2 rubrene,3 and
poly(3-hexylthiophene) (P3HT).4 Their device performances
have been comparable to those of amorphous silicon transistors.
However, compared to the high performances of p-channel
OFETs, the device performance of n-channel has been unsatis-
factory1a and development of novel n-type semiconductors
remains a great challenge.
n-Type semiconductors have generally been prepared from
electron-accepting ³ systems. Introduction of fluorine-contain-
ing substituents such as fluoroalkyl, pentafluorophenyl, trifluoro-
acetyl, and trifluoromethylphenyl groups to ³-conjugated
systems has been proven to be a good way to obtain n-channel
OFETs with good performance.5 Cyano groups have also been
widely used in n-type materials.6 We have used thiazolothiazole
as electron-accepting ³ core to give high-performance n-type
semiconductors.7 As an extension of this work, we have now
focused on thieno[3,2-b]thiophene-2,5-dione for the following
reasons. First, thieno[3,2-b]thiophene is a rigid heterocyclic
system which has often been used in high-performance p-type
semiconductors. Second, the dione unit is electron-accepting
and the electron affinity can be further enhanced by substitution
with dicyanomethylene groups. Third, by introduction of aryl
substituents it is relatively easy to tune the physical properties.
These considerations show that novel n-type organic semi-
conductors would be produced by introducing electron-accept-
ing groups to the thieno[3,2-b]thiophene-2,5-dione core. We
report herein the synthesis and characterization of dione 1 and
the dicyanomethylene compound 2 with trifluoromethylphenyl
groups (Scheme 1) and the OFETs based on them.
Scheme 1. Synthetic route to compounds 1 and 2.
chromatography in a good yield of over 90%. Bromination
with NBS afforded the corresponding dibromide as a white solid
in a high yield of 95%. According to the method reported by
Tiecco et al.,9 compound 1 was obtained in a yield of 75% as
an orange solid. Compound 2 could be obtained by reaction of
2,5-dibromo-3,6-bis(4-trifluoromethylphenyl)thieno[3,2-b]thio-
phene with TCNEO (tetracyanoethylene oxide) in 1,3-dibromo-
propane in a yield of 62%.10 The two target compounds were
characterized by mass spectrometry (MS), 1H NMR, and
elemental analysis. Although the number of hydrogens of 1 is
the same as those of 2, the chemical shifts in 2 show an obvious
change relative to those of 1. The significant shifting is
attributed to the shielding caused by the close proximity of the
cyano groups in 2. The thermal properties of 1 and 2 were
determined by DSC measurements (Figure S114). They both
exhibit excellent thermal stability with relatively high melting
points at 258 and 378 °C, respectively. In addition, facile
sublimation of the molecules allowed the deposition to give
uniform thin films by vacuum evaporation for the fabrication of
OFETs.
The optical properties of the molecules were examined by
UV-vis spectroscopy in CH2Cl2 solution and in solid films
shown in Figure 1. The data are summarized in Table 1. As
shown in Figure 1, the spectrum of 1 in the film is significantly
changed from that in solution, and the longest absorption band
undergoes an obvious bathochromic shift of about 60 nm.
However, the absorption intensity obviously decreased. This
may be because the longest absorption band in solution can be
attributed to intramolecular charge transfer (ICT), and in the
solid film, the ICT may be decreased leading to the lower
intensity of the band and instead, the intermolecular charge
transfer takes place at lower energies. Further insight into the
electronic properties of these compounds was given by cyclic
voltammetry at room temperature. Figure 2 shows the reduction
waves of the compounds 1 and 2, and the data are listed in
Table 1. They both exhibit good reversible cathodic reduction
The synthetic approach of compounds 1 and 2 is outlined in
Scheme 1. First, 3,6-dibromothieno[3,2-b]thiophene8 reacted
with 4-trifluoromethylphenylboronic acid through palladium-
catalyzed Suzuki coupling reaction under reflux for 24 h in
deaerated aqueous K2CO3 solution and tetrahydrofuran (THF).
The white solid 3,6-bis(4-trifluoromethylphenyl)thieno[3,2-
b]thiophene was obtained and further purified by column
Chem. Lett. 2011, 40, 998-1000
© 2011 The Chemical Society of Japan
Chen, Shiyan
