1122
Chemistry Letters Vol.37, No.11 (2008)
Development of Organic Electronic Devices
Using Boronate Esters and Related Heterocycles
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 August 13, 2008; CL-080785; E-mail: yoshiro@echem.titech.ac.jp)
New oligomers containing tricoordinated boron atoms were
prepared, and OFET devices based on these compounds were
fabricated. These are first examples of OFET devices based on
compounds containing tricoordinated boron atoms.
O
O
O
O
B
B
1
O
B
O
Organic semiconductors have attracted much attention for
electronic and optical applications such as organic field-effect
transistors (OFETs) and light-emitting diodes (OLEDs), which
have advantages of low cost, lightness, mechanical flexibility
and disposability. Many OFETs based on acenes such as penta-
cene or oligomers such as oligothiophenes have been reported,
and some of these materials show high hole or electron mobili-
ties comparable to amorphous Si (ꢁ1:0 cm2/Vs).1–8 However,
development of new materials is still important to enhance the
FET performance as well as to investigate the mechanism of car-
rier transportation. In this context, boron-containing ꢀ-conjugat-
ed compounds are attractive since the boron atom has a planar
geometry with a vacant 2p orbital. This leads to Lewis acid char-
acteristics and electron-accepting properties. Boron-containing
materials have been developed for chemical sensors, electrolu-
minescence, nonlinear optics, and hole or electron carrier
transport.9–12 However, compounds with tricoordinated borons
have not been used for OFETs, to our best knowledge, although
tetracoordinated boron-containing compounds showed FET
properties.13 This is considered to be due to the following
reasons. Tricoordinated boron-containing ꢀ-conjugated mole-
cules are generally unstable and need bulky substituents like a
mesityl (Mes) unit around the boron center for steric protection
to enhance the stability. These molecules have weak intermolec-
ular interactions and are unfavorable for FETs since carrier
transportation requires strong intermolecular interactions. As
stable boron-containing compounds with planar structures, we
have paid attention to boronates and their analogues.14 Although
boronic acids and esters are well-known reagents for the Suzuki–
Miyaura coupling reaction, they have never been used as semi-
conductors for OFETs. We have now prepared arylboronates
and related compounds with extended ꢀ conjugation and exam-
ined their physical properties and structures. The FET devices
based on them were successfully fabricated here for the first
time.
2
H
N
S
B
B
N
H
S
3
Chart 1.
Table 1. Optical properties and redox potentials of 1–3
Compound ꢁabc/nm
ꢁ
em/nm
337a
Eox/Vc Ered/Vc
+1.10 ꢃ1:36
1
2
291a
268, 278, 332 277, 389, 409b +1.22 ꢃ0:85
347, 376b
270, 341b
3
443b
+0.95
aIn CH2Cl2. bIn DMF. c0.1 M n-Bu4NPF6 in DMF, Pt electrode,
scan rate 100 mV/s, V vs. SCE.
mined by the spectral data along with elemental analysis.
The optical properties and redox potentials of 1–3 are shown
in Table 1. They have no absorptions in the visible region and
their transparent films could be formed. They showed efficient
fluorescence even in the solid state. The fluorescence spectrum
of 3 in the solid state is shown in Figure S1.15 The emission is
blue with a maximum at 446 nm, and the fluorescence quantum
yield was 20% in the solid state. Compounds 1 and 2 showed
both the oxidation and reduction potentials, whereas compound
3 showed only the oxidation potential. The observation of the
reduction potentials can be attributed to the presence of the
electron-deficient boron atoms.
The single crystals of 1–3 suitable for the X-ray structure
analysis were obtained by sublimation.16 All of the molecules
were found to be almost planar as expected. In molecule 1, the
dihedral angles between the phenyl and boronate rings are 8.9
and 9.4ꢂ, and the angle in the biphenyl unit is 0.4ꢂ. This molecule
forms a ꢀ-stacking structure, where the electron-donating ben-
zene ring is overlapped with the electron-deficient boron atom
(Figure S2). In the crystal of 2, there exsist two crystallograph-
ically independent molecules. These also form a ꢀ-stacking
structure (Figure S3). The dihedral angle between the pyrene
and boronate rings is 8.1ꢂ (molecule I) and 10.9ꢂ (molecule II).
In molecule 3, the dihedral angles between the phenyl and
heterocyclic rings are 6.3 and 5.3ꢂ, and the angle between the
Compounds 1–3 (Chart 1) were synthesized by reaction of
the corresponding boronic acids with catechol, 1,2-dihydroxy-
naphthalene and 2,5-diamino-1,4-benzenedithiol dihydrochlo-
ride in the presence of p-toluenesulfonic acid in toluene in 5–
34% yields. Compound 3 was designed because the nitrogen
and sulfur atoms were expected to enhance the stability of the
boron-containing heterocycle by conjugation. The reaction prod-
ucts were purified by sublimation. Their structures were deter-
Copyright ꢀ 2008 The Chemical Society of Japan
Kojima, Takahiro
