2,6-Diphenylbenzo[1,2-b:4,5-b′]dichalcogenophenes: A New Class of High-Performance Semiconductors for Organic Field-Effect Transistors

Article abstract of DOI:10.1021/ja0496930

2,6-Diphenylbenzo[1,2-b:4,5-b′]dichalcogenophenes including thiophene, selenophene, and tellurophene analogues as organic semiconductors for field-effect transistors were effectively synthesized in three steps from commercially available 1,4-dibromobenzen

Full text of DOI:10.1021/ja0496930

Published on Web 04/03/2004
2,6-Diphenylbenzo[1,2-b:4,5-b′]dichalcogenophenes: A New Class of
High-Performance Semiconductors for Organic Field-Effect Transistors
Kazuo Takimiya,*,† Yoshihito Kunugi,‡ Yasushi Konda,† Naoto Niihara,† and Tetsuo Otsubo*,†
Graduate School of Engineering, Hiroshima UniVersity, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-8527, Japan,
and Faculty of Integrated Arts and Sciences, Hiroshima UniVersity,
1-7-1 Kagamiyama, Higashi-Hiroshima 739-8521, Japan
Received January 16, 2004; E-mail: ktakimi@hiroshima-u.ac.jp
Scheme 1. Synthesis of 2,6-Diphenylbenzodichalcogenophenes
Recent high performances of organic field-effect transistors
(OFETs) have been accomplished from two approaches: the
improvement of device fabrication techniques, as represented by
pentacene-based OFETs,1 and the development of new molecular
semiconductors for active OFET materials.2 Such materials exhibit-
ing high performances are mainly composed of thiophene-compris-
ing molecules, such as oligothiophenes,3 thienylene-phenylene co-
oligomers,4 thienylene-vinylene co-oligomers,5 and fused thiophene
aromatics.6 From the theoretical point of view, it is pointed out
that transfer integral between neighboring molecules is the basic
parameter to determine field-effect mobilities.7 According to this
guideline, one may naturally expect that high-performance OFET
materials can be developed by replacement of sulfur atoms in the
thiophene-comprising molecules with heavy chalcogen atoms such
as selenium and tellurium with enhanced overlap integrals. How-
ever, selenophene- and tellurophene-comprising organic semicon-
ductors have not been reported thus far. Very recently, we reported
OFETs using quaterselenophene as an active layer, which showed
a relatively high mobility (3.6 × 10-3 cm2 V-1 s-1) comparable
with that of quaterthiophene.8 Here we report a new prototypical
class of FET materials, 2,6-diphenylbenzo[1,2-b:4,5-b′]dichalco-
genophenes (1-3), and their remarkably high FET performances
(Figure 1).
Figure 2. (A) XRD pattern of an evaporated thin film of 2 (Tsub ) 60 °C).
(B) Packing diagram of 2 viewed along a-axis in a bulk single crystal.
of the films reveals highly ordered structures for all the cases. Figure
2A shows an X-ray diffraction of the film of 2 deposited at 60 °C.
A series of sharply resolved peaks assignable to multiple (00l)
reflections can be seen, and from the first-layer line, the monolayer
thickness is determined to be ca. 18.0 Å. This value corresponds
well to the molecular length of 2 determined by X-ray crystal-
lographic analysis, indicating nearly perfect orthogonal orientation
of molecules onto the substrate. Comparing the monolayer thickness
in the thin film and the lattice parameter determined by the single-
crystal X-ray analysis, one can notice a similarity between these
two; the a-axis in the unit cell (36.83 Å) is just twice that of the
monolayer thickness, implying that the molecular assembly in the
thin film is basically the same as that in bulk crystals. Figure 2B
shows a crystal packing of 2 projected along the crystallographic
a-axis direction, which corresponds to the molecular long axis of
2. The crystal packing of 2 is classified into a herringbone-type,
reminiscent of the crystal structure of pentacene12 and sexithiphene13
showing high FET mobilities. Although the single-crystal structures
of 1 and 3 were not elucidated, we assume that they take similar
thin-film structures as seen for 2, because the monolayer thicknesses
of 1 (17.9 Å) and 3 (18.5 Å) determined by X-ray diffractions are
similar to that of 2.
Figure 1. Molecular structures of 2,6-diphenylbenzo[1,2-b:4,5-b′]dichal-
cogenophenes.
The conventional methods for the synthesis of benzo[1,2-b:4,5-
b′]dithiophenes using thiophene derivatives as key starting materials6a,9
are unlikely to be applicable to the selenium and tellurium
homologues. Alternatively, we have successfully developed an
efficient synthetic method of benzo[1,2-b:4,5-b′]dichalcogenophenes,
which can be applicable to all the chalcogen series (1-3). This
method takes advantage of double heterocycle formation on the
central benzene ring, as shown in Scheme 1.10 Thus, 1,4-dibromo-
2,5-bis(phenylethynyl)benzene (4), readily prepared from 1,4-
dibromobenzene via a two-step conversion,11 was treated with
t-BuLi followed by an addition of powdery chalcogen (sulfur,
selenium, or tellurium) to give 1-3 in moderate to good yields.
These new compounds were purified by vacuum sublimation to
give yellow powder or fine crystals. Thermal properties of 1-3
investigated by differential scanning calorimetry (DSC) indicated
that the present compounds are thermally stable: neither phase
transition nor thermal decomposition was observed up to 350 °C.
Thin films of 1-3 can be deposited on oxidized silicon substrates
by thermal evaporation under vacuum. The X-ray diffraction (XRD)
Field-effect transistors were made in a top contact device
configuration.14,15 All the compounds performed as good p-type
transistors. As a representative, Figure 3 shows the drain-source
current (IDS) characteristics with different voltages (VG) for a FET
device using 2 deposited at 60 °C. It demonstrates a typical FET
response; IDS scales up with increasing VG. The FET mobility
calculated using the IDS in the saturation regions is 0.17 cm2 V-1
s-1, and the on/off ratio of the IDS between VG of 0 and -100 V is
† Graduate School of Engineering.
‡ Faculty of Integrated Arts and Sciences.
9
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J. AM. CHEM. SOC. 2004, 126, 5084-5085
10.1021/ja0496930 CCC: $27.50 © 2004 American Chemical Society

C O MMU N I C A T I O N S
Education, Science, Sports, and Culture, Japan. We are grateful to
Professor S. Yamanaka (Hiroshima University) for the XRD
measurements.
Supporting Information Available: Details for the synthesis and
the device fabrication of 1-3, XRDs and AFM images of the thin films
of 1-3, crystallographic information files (CIF) for 2. This material is
available free of charge via the Internet at http://pubs.acs.org.
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)
1 (DPh-BDT)
2 (DPh-BDS)
3 (DPh-BDTe)
mobility/cm2 V-1 s-1
(on/off ratio)
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(on/off ratio)
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(on/off ratio)
Tsub/°C
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3.7 × 10-3
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60
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100
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Acknowledgment. This work was partially supported by
Industrial Technology Research Grant Program in 2003 from New
Energy and Industrial Technology Development Organization
(NEDO) of Japan and Regional Science Promotion Program
Evolutional Research. We are also grateful for financial support
from a Grant-in-Aid for Scientific Research on Priority Areas of
Molecular Conductors (No. 15073218) from the Ministry of
JA0496930
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