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Chemistry Letters Vol.38, No.5 (2009)
New n-Type Field-effect Transistors Based on Pyrimidine-containing Compounds
with (Trifluoromethyl)phenyl Groups
Takahiro Kojima,1 Jun-ichi Nishida,1 Shizuo Tokito,2 and Yoshiro Yamashitaꢀ1
1Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering,
Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 226-8502
2NHK Science and Technical Research Laboratories, Kinuta, Setagaya-ku, Tokyo 157-8510
(Received January 23, 2009; CL-090082; E-mail: yoshiro@echem.titech.ac.jp)
New co-oligomers containing pyrimidine units were pre-
respectively. These compounds were purified by sublimation.
Their structures were determined by the spectral data along with
elemental analysis.15
A single crystal of 2 suitable for X-ray structure analysis
was obtained by sublimation. The molecule 2 was found to be
almost planar, where the dihedral angles between the phenylene
and pyrimidine rings and between the pyrimidine and thienylene
rings are 0.6 and 1.5ꢁ, respectively. This molecule forms ꢀ-
stacking as shown in Figure 1.16
The optical properties and reduction potentials are shown in
Table 1. The reduction potentials of 1 and 2 were measured by
differential pulse voltammetry. These values were similar to
each other, indicating that the position of the pyrimidine units
is not related to the reduction potentials. The absorption maxi-
mum of 2 in solution is red-shifted by 23 nm compared to that
of 1, indicating that the intramolecular charge transfer from
the electron-donating bithiophene unit to the electron-accepting
pyrimidine takes place more effectively in 2 than in 1. Although
the absorption and emissions maxima of 1 and 2 are blue-shifted
in films compared to those in solution, the end absorptions of
them are red-shifted. This fact suggests the presence of the inter-
molecular interactions in the solid state (Figure S1).15
pared. The planar molecular and ꢀ-stacking structures were
revealed by X-ray structure analysis. The FET devices based
on them showed good n-type performance.
Organic thin-film transistors (OTFTs) based on ꢀ-conjugat-
ed molecules are of great interest for applications to plastic elec-
tronics such as organic light-emitting diodes (OLEDs), displays,
e-paper and radio-frequency identified tags (RFID tags). They
have advantages of low cost, lightness, flexibility, and disposa-
bility.1,2 Many p- and n-type organic OFETs have been reported,
and some of these materials show high hole or electron mobility
comparable to amorphous Si.3–9 However, the number of n-type
OFETs is still limited and the performance is not satisfactory yet.
To achieve high electron mobility, lower LUMO levels are nec-
essary. For this purpose, electron-withdrawing groups have been
introduced into well-known p-type semiconductor cores such
as acenes or oligo-thiophenes.10,11 We have also used electron-
accepting nitrogen-heterocycles such as thiazole and pyrazine
to lower the LUMO levels.12,13
We have now designed new oligomers 1 and 2 containing
pyrimidine rings (Scheme 1). The heterocycle is a stronger elec-
tron-acceptor than thiazole. The pyrimidine ring also has an ad-
vantage of reducing steric repulsion between the neighboring
rings. Planar geometry of the molecule is important for strong in-
termolecular interactions leading to high mobility. On the other
hand, nitrogen heterocycles have a disadvantage in that the nitro-
gen atoms interact with the SiO2 substrate leading to a molecular
arrangement lying on the substrate. Some oligomers containing
nitrogen heterocycles are known to lie on the SiO2 substrate,14
and such an arrangement is unfavorable for high mobility. The
control of the molecular arrangement is crucial for high-per-
formance FETs. We have found here that the FET performances
of 1 and 2 were greatly improved by the substrate treatment.
Pyrimidine-containing co-oligomers 1 and 2 were prepared
by the Stille or Suzuki–Miyaura coupling reaction in the pres-
ence of Pd catalyst in DMF or toluene in 15 and 12% yields,
FET devices based on 1 and 2 were fabricated by a vapor-
deposition method with bottom contact geometry on SiO2/Si
substrates which are bare or treated with HMDS. Cr (10 nm)/
Au (20 nm) electrodes forming channels of 25-mm length (L)
and 294-mm width (W) were photolithographycally defined.
The semiconductor layer was evaporated on the electrode/
dielectric surface, where the SiO2 layer was 300-nm thick. The
FET performances are summarized in Table 2. In the case of
bare substrates, the electron mobilities of both 1 and 2 decreased
drastically at the substrate temperature of 80 ꢁC. The on/off ratio
also decreased at the high temperature. On the other hand, in the
0.6o
1.5o
N
S
F3C
N
N
CF3
S
N
1
2
N
N
CF3
N
N
S
S
F3
C
Scheme 1.
Figure 1. ORTEP and packing structure of 2.
Copyright ꢀ 2009 The Chemical Society of Japan
Kojima, Takahiro
