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Published on the web April 24, 2010
Organic Transistors Based on Octamethylenetetrathiafulvalenes
Jun-ichi Inoue,1 Masato Kanno,1 Minoru Ashizawa,2 Chayeon Seo,2 Akihiko Tanioka,2 and Takehiko Mori*2
1Department of Chemistry and Materials Science, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552
2Department of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552
(Received March 4, 2010; CL-100205; E-mail: mori.t.ae@m.titech.ac.jp)
Organic field-effect transistors (OFET) based on octamethyl-
coupling of a 1,3-dithiolium cation,10 OMTTF and 1 were
prepared on a basis of a phosphite-mediated coupling reaction
similarly to our previous synthesis of HMTTF derivatives
(Scheme 1).9,13,14 (OMTTF)(TCNQ) was prepared by mixing
hot acetonitrile solutions of OMTTF and TCNQ.15
enetetrathiafulvalene (OMTTF) and the t-butyl derivative are
investigated. The parent OMTTF shows poor transistor perform-
ance owing to the flat molecular arrangement, but the t-butyl
substitution realizes standing molecular arrangement and high
OFET performance. The tetracyanoquinodimethane (TCNQ)
complex of OMTTF exhibits n-channel properties despite the
strong donor ability of OMTTF.
The redox potentials were investigated by cyclic voltam-
metry.14 The first redox potentials of OMTTF (0.24 V vs. SCE)
and 1 (0.23 V) are approximately the same, and lower than that
of TTF (0.34 V), indicating even stronger donor ability than
TTF. These results are comparable to 0.25 V of HMTTF.9
The molecular and crystal structures of OMTTF and 1
were investigated by X-ray single-crystal structure analysis.16
OMTTF crystallizes in the space group C2/m, and the molecule
is located on a 2/m position, where a quarter of the molecule
is crystallographically independent, and the mirror plane is
perpendicular to the molecular plane (Figure 1). The outer
carbon atom of the cyclohexane ring is disordered above and
below the TTF plane so that the cyclohexane part forms chair
conformation. The molecules form a C centered lattice,
constructing a conducting network parallel to the bc plane. In
the stacking along the c axis, the molecules are slipped by 6.4 ¡
along the molecular long axis in order to avoid the steric
hindrance of the cyclohexane part. Consequently, the molecules
are largely tilted (63°) from normal to the conducting (bc) sheet.
The molecule of 1 is located on an inversion center
(Figure 2), and there are two kinds of molecules, A and B, in
each of which half of the molecule is crystallographically
independent. The final coupling reaction potentially produces cis
and trans isomers with respect to the molecular long axis. In
addition, the t-butyl groups generate potential R and S optical
isomers. Since the molecule is located on an inversion center, the
Organic field-effect transistors (OFET) have attracted con-
siderable attention for electronic applications such as large-area
low-cost integrated circuits and flexible displays.1 A variety of
organic semiconductors have been developed to be used in
¹1
OFETs,2 in which field-effect mobility exceeding 1 cm2 V¹1 s
has been attained in evaporated pentacene films and several
¹1
solution films,3 and even more than 10 cm2 V¹1 s has been
achieved in single crystal devices based on rubrene.4 Tetrathia-
fulvalene (TTF) derivatives are well known as excellent donors
in charge-transfer complexes,5 but have been also used as good
OFET materials.6,7 In particular, mobility of hexamethylene-
tetrathiafulvalene (HMTTF) has been reported to be more than
¹1
10 cm2 V¹1 s in a single crystal transistor.8 We have reported
that the HMTTF derivative substituted with t-butyl groups show
stable high performance (® = 0.98 cm¹2 V¹1 s¹1) and small
threshold voltage Vth = ¹2 V in a thin film transistor.9 In order
to explore steric effects of nonplanar fused rings, in the present
paper OFET properties of octamethylenetetrathiafulvalene
(OMTTF, 4,4¤5,5¤,6,6¤,7,7¤-octahydrodibenzotetrathiafulvalene)
with cyclohexane rings (Scheme 1) are investigated together
with the t-butyl-substituted OMTTF 1.10,11
A mixed-stack dibenzotetrathiafulvalene (DBTTF) complex
of TCNQ shows n-channel properties in single-crystal and
thin-film OFETs with (TTF)(TCNQ) source and drain (S/D)
electrodes.12 This has been attributed to the lower HOMO level
of DBTTF than that of TTF. Since (HMTTF)(TCNQ) has highly
conducting segregate stacks, this compound has been utilized
as a S/D material rather than an active layer.9 By contrast,
(OMTTF)(TCNQ) is a mixed-stack insulator,11a so we have also
investigated the thin-film OFET by using this complex as an
active layer.
Although OMTTF was previously prepared from the
Figure 1. Crystal structure of OMTTF, (a) viewed along the b
axis, (b) viewed perpendicular to the molecular plane, (c) overlap
mode of OMTTF, and (d) projection along the molecular long axis
with the intermolecular overlaps. Shortest S-S contacts are p:
3.7 ¡, a: 6.7 ¡, and c: 5.4 ¡. Transfer integrals are p: 34, a: 2.0,
and c: 14 meV.
Scheme 1. Synthesis of OMTTF and 1.
Chem. Lett. 2010, 39, 538-540
© 2010 The Chemical Society of Japan