2,7-Diphenyl[1]benzoselenopheno[3,2-b][1]benzoselenophene as a stable organic semiconductor for a high-performance field-effect transistor

Article abstract of DOI:10.1021/ja057641k

[1]Benzoselenopheno[3,2-b][1]benzoselenophene (BSBS) and its 2,7-diphenyl derivative (DPh-BSBS) were readily synthesized from diphenylacetylene and bis(biphenyl-4-yl)acetylene, respectively, with a newly developed straightforward selenocyclization protoco

Full text of DOI:10.1021/ja057641k

Published on Web 02/11/2006
2,7-Diphenyl[1]benzoselenopheno[3,2-b][1]benzoselenophene
as a Stable Organic Semiconductor for a High-Performance
Field-Effect Transistor
Kazuo Takimiya,*,† Yoshihito Kunugi,‡,¶ Yasushi Konda,† Hideaki Ebata,†
Yuta Toyoshima,‡ and Tetsuo Otsubo†
Contribution from the Department of Applied Chemistry, 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 November 9, 2005; E-mail: ktakimi@hiroshima-u.ac.jp
Abstract: [1]Benzoselenopheno[3,2-b][1]benzoselenophene (BSBS) and its 2,7-diphenyl derivative (DPh-
BSBS) were readily synthesized from diphenylacetylene and bis(biphenyl-4-yl)acetylene, respectively, with
a newly developed straightforward selenocyclization protocol. In contrast to the parent BSBS that has poor
film-forming properties, the diphenyl derivative DPh-BSBS formed a good thin film on the Si/SiO2 substrate
by vapor deposition. X-ray diffraction examination revealed that this film consists of highly ordered molecules
that are nearly perpendicular to the substrate, making it suitable for use in the fabrication of organic field-
effect transistors (OFETs). When fabricated at different substrate temperatures (room temperature, 60 °C,
and 100 °C) in a “top-contact” configuration, all the DPh-BSBS-based OFET devices exhibited excellent
p-channel field-effect properties with hole mobilities >0.1 cm2 V-1 s-1 and current on/off ratios of ∼106.
This high performance was essentially maintained over 3000 continuous scans between Vg ) +20 and
-100 V and reproduced even after storage under ambient laboratory conditions for at least one year.
substrate,3 and 3.0 cm2 V-1 s-1 on a modified alumina
Introduction
substrate4) and rubrene as the best single-crystal material (∼20
Organic field-effect transistors (OFETs) have attracted much
attention as low-cost alternatives to conventional silicon-based
transistors, and recent intensive research efforts in this field have
led to the development of a number of superior p-channel
organic semiconductors showing mobilities higher than 0.1 cm2
V-1 s-1 in FET devices, almost comparable to that of amorphous
silicon.1-14 Showing remarkable properties are pentacene as the
best thin-film material (0.3-0.7 cm2 V-1 s-1 on a Si/SiO2
substrate,2 1.5 cm2 V-1 s-1 on a chemically modified Si/SiO2
cm2 V-1 s-1).5 One of the remaining key challenges in the
practical application of such OFETs is to enhance device
durability. In this regard, considerable attention has been paid
to the stability of organic semiconducting materials under
ambient conditions.7-13 The main reason for the low durability
is sensitivity to air of the organic semiconducting materials used
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Kunugi, Y.; Toyoshima, Y.; Otsubo, T. J. Am. Chem. Soc. 2005, 127,
3605-3612. (c) Merlo, J. A.; Newman, C. R.; Gerlach, C. P.; Kelley, T.
W.; Muyres, D. V.; Fritz, S. E.; Toney, M. F.; Frisbie, C. D. J. Am. Chem.
Soc. 2005, 127, 3997-4009. (d) Payne, M. M.; Parkin, S. R.; Anthony, J.
E.; Kuo, C.-C.; Jackson, T. N. J. Am. Chem. Soc. 2005, 127, 4986-4987.
(e) Xiao, K.; Liu, Y.; Qi, T.; Zhang, W.; Wang, F.; Gao, J.; Qiu, W.; Ma,
Y.; Cui, G.; Chen, S.; Zhan, X.; Yu, G.; Qin, J.; Hu, W.; Zhu, D. J. Am.
Chem. Soc. 2005, 127, 13281-13286.
† Graduate School of Engineering.
‡ Faculty of Integrated Arts and Sciences.
¶ Present address: Department of Applied Chemistry, Faculty of
Engineering, Tokai University, 1117 Kitakaname, Hiratsuka 259-1292,
Japan.
(7) Sirringhaus, H.; Wilson, R. J.; Friend, R. H. Science 1998, 280, 1741-
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369. (c) Katz, H. E. J. Mater. Chem. 1997, 7, 369-376. (d) Rogers, J. A.;
Bao, Z.; Katz, H. E.; Dodabalapur, A. In Thin-Film Transistors; Kagan,
C. R., Andry, P., Eds.; Marcel Dekker: New York, 2003; p 377.
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J. AM. CHEM. SOC. 2006, 128, 3044-3050
10.1021/ja057641k CCC: $33.50 © 2006 American Chemical Society

DPh-BSBS-Based OFETs
A R T I C L E S
Chart 1. Selenophene-Based Organic Semiconductors
Table 1. Calculated HOMO and LUMO Levelsa and
HOMO-LUMO Gapsb
compound
HOMO/eV
LUMO/eV
∆E/eV
R-4S
DPh-BDS
1a
-4.93
-5.21
-5.46
-5.38
-2.05
-1.73
-1.26
-1.52
2.88
3.48
4.20
3.86
1b
in the fabrication or operation of OFETs. Since most of the
p-channel semiconductors have relatively high-lying energy
levels for the highest occupied molecular orbitals (HOMOs) and
relatively low excitation gaps to the lowest unoccupied molec-
ular orbitals (LUMOs), they tend to be readily photo- or thermo-
oxidizable, leading to the degradation of the resulting OFETs
under ambient conditions. When processed in an inert atmo-
sphere, regioregular poly(3-hexylthiophene)-based FET devices
exhibited higher mobilities and larger current on/off ratios than
those in ambient conditions.7 The annealing of fabricated FET
devices based on poly(3,3′′′-dialkylquaterthiophene) and poly-
(3′,4′-dialkyl-2,2′:5′,2′′-terthiophene) films resulted in not only
high mobilities but also increased stability against oxidative
doping by atmospheric oxygen.10 Furthermore, 2,5′-bis(2-
fluorenyl)bithiophenes,11 5,11-diarylindolo[3,2-b]carbazoles,12
2,6-bis(5-hexylthiophen-2-yl)anthracene,13 and 5,5′-bis(2-an-
thracenyl)-2,2′-bithiophene6c were recently reported to be high-
performance OFET materials with reasonable stability during
operation under ambient conditions. However, there are still very
few organic semiconductors endowed with both high environ-
mental stability and high OFET performance. In our search for
such organic semiconductors, we found recently that 2,6-
diphenylbenzo[1,2-b:4,5-b′]diselenophene (DPh-BDS) behaves
as a high-performance FET material.14 In this system, the
selenium atoms of DPh-BDS act to enhance intermolecular
overlapping in the solid state, thereby facilitating charge carrier
migration. As another novel potential semiconductor, we have
focused our efforts on 2,7-diphenyl[1]benzoselenopheno[3,2-
b][1]benzoselenophene (DPh-BSBS, 1b) because of its structural
similarity to DPh-BDS. Whereas the parent BSBS (1a) is long
known,15,16 the diphenyl derivative (1b) remains unknown. In
this article, we report a newly developed synthetic approach to
BSBS (1a) and DPh-BSBS (1b) (Chart 1) and the excellent
field-effect characteristics as well as high environmental stability
of 1b-based FET devices.
a Calculated at the B3LYP/6-31G(d) level. b Defined as |HOMO -
LUMO|.
Chart 2. Structure of R-Quaterselenophene (R-4S)
semiconductors, because such molecules can enhance the
overlapping of frontier molecular orbitals between adjacent
molecules through strong heteroatomic interactions. To obtain
insight into the selenium effect of BSBS (1a) and DPh-BSBS
(1b), we first carried out MO calculations by the DFT method
at the B3LYP-6-31G(d) level using the PCGAMESS program.18
The HOMO and LUMO energy levels of 1a and 1b are
summarized in Table 1, together with those of two selenophene-
based p-type semiconductors previously studied, DPh-BDS14
and R-quaterselenophene (R-4S, Chart 2),19 for comparison.
Table 1 indicates that 1a and 1b have lower HOMO energy
levels and larger HOMO-LUMO gaps than R-4S and DPh-
BDS. Although lowering of the HOMO levels may be disad-
vantageous to charge carrier injection, the MO calculations
suggest that 1a and 1b can behave as air-stable semiconductors.
In addition, it should be noted that the four selenium-containing
molecules have quite different orbital coefficients on selenium
atoms in the HOMOs, as depicted in Figure 1. In the HOMO
of R-4S, there are virtually no coefficients on selenium atoms,
well explaining the fact that R-4S-based OFETs showed
relatively low hole mobility (10-3 cm2 V-1 s-1),19 comparable
to that of R-quaterthiophene.20 In contrast, the HOMO of DPh-
BDS has large coefficients on selenium atoms, suggesting the
effective contribution of polarizable selenium atoms to the
intermolecular interactions for charge migration. Actually, this
assumption was corroborated by the fact that FET devices made
of DPh-BDS showed higher hole mobility (0.17 cm2 V-1 s-1
)
than that observed for the sulfur counterpart, 2,6-diphenylbenzo-
[1,2-b:4,5-b′]dithiophene (0.08 cm2 V-1 s-1).14 The HOMOs
of 1a and 1b, similar to that of DPh-BDS, have large coefficients
on selenium atoms. From this result, we expected that 1a and
1b can function as high-performance p-channel semiconductors.
Results and Discussion
MO Calculations. A number of effects are important to
carrier transport in thin films of semiconducting molecules at
both molecular and intermolecular levels.17 Although it is very
difficult to predict such intermolecular effects as molecular
packing and phonon energies from a molecular structure itself,
we considered that polycyclic heteroaromatics with large orbital
coefficients on heteroatoms are especially promising as organic
Synthesis. In 1972, Faller and Mantovani reported the first
synthesis of BSBS (1a) as only a byproduct upon treating
inaccessible o-methylselenobenzaldehyde with bromoacetic
acid.15 Much later, in 1988, Sashida and Yasuike reported that
1a was obtained in 52% yield by successive one-pot reactions
(14) (a) Takimiya, K.; Kunugi, Y.; Konda, Y.; Niihara, N.; Otsubo, T. J. Am.
Chem. Soc. 2004, 126, 5084-5085. (b) Zeis, R.; Kloc, C.; Takimiya, K.;
Kunugi, Y.; Konda, Y.; Niihara, N.; Otsubo, T. Jpn. J. Appl. Phys. Part 1
2005, 44, 3712-3714.
(15) Faller, P.; Mantovani, F. Bull. Soc. Chim. Fr. 1972, 1643-1645.
(16) Sashida, H.; Yasuike, S. J. Heterocycl. Chem. 1988, 35, 725-726.
(17) (a) Cornil, J.; Beljonne, D.; Calbert, J.-P.; Bre´das, J.-L. AdV. Mater. 2001,
13, 1053-1066. (b) Bre´das, J.-L.; Calbert, J. P.; da Silva Filho, D. A.;
Cornil, J. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 5804-5809. (c) Bre´das,
J.-L.; Beljonne, D.; Coropceanu, V.; Je´roˆme, C. Chem. ReV. 2004, 104,
4971-5003. (d) Newman, C. R.; Frisbie, C. D.; da Silva Filho, D. A.;
Bre´das, J.-L.; Ewbank, P. C.; Mann, K. R. Chem. Mater. 2004, 16, 4436-
4451.
(18) MO calculations were carried out by DFT methods at the B3LYP-6-31G(d)
level using the PC GAMESS program. (a) Granovsky, A. A., http://
classic.chem.msu.su/gran/gamess/index.html. (b) Schmidt, M. W.; Bald-
ridge, K. K.; Boatz, J. A.; Elbert, S. T.; Gordon, M. S.; Jensen, J. J.; Koseki,
S.; Matsunaga, N.; Nguyen, K. A.; Su, S.; Windus, T. L.; Dupuis, M.;
Montgomery, J. A. J. Comput. Chem. 1993, 14, 1347-1363. See Supporting
Information for details.
(19) Kunugi, Y.; Takimiya, K.; Yamane, K.; Yamashita, K.; Aso, Y.; Otsubo,
T. Chem. Mater. 2003, 15, 6-7.
(20) (a) Katz, H. E.; Torsi, L.; Dodabalapur, A. Chem. Mater. 1995, 7, 2235-
2237. (b) Hajlaoui, R.; Horowitz, G.; Garnier, F.; Arce-Brouchet, A.; Laigre,
L.; Kassmi, A. E.; Demanze, F.; Kouki, F. AdV. Mater. 1997, 9, 389-391.
9
J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006 3045

A R T I C L E S
Takimiya et al.
Figure 2. Molecular structures of (a) 1a and (b) 1b. The molecular lengths,
which are defined as the distance between edge hydrogen atoms, are 10.4
and 19.0 Å for 1a and 1b, respectively.
Scheme 1. Synthesis of 1a and 1b
Figure 1. HOMOs of (a) R-4S, (b) DPh-BDS, (c) BSBS (1a), and (d)
DPh-BSBS (1b) at the B3LYP/6-31G(d) level.
of o,o′-dibromodiphenylacetylene with tert-butyllithium and then
with elemental selenium.16 This method for synthetis of 1a is
quite straightforward and looks attractive; however, considering
that the starting bis(3-bromobiphenyl-4-yl)acetylene is still
unknown and hardly accessible, we thought that the application
of the Sashida method to the synthesis of DPh-BSBS (1b) would
be impractical. Since the Sashida reaction proceeds via o,o′-
dilithiodiphenylacetylene, we first examined the generation of
the same metalated species (to be exact, o,o′-dilithio(potassio)-
diphenylacetylene) by direct metalation of commercially avail-
able diphenylacetylene with the LICKOR superbase (BuLi/t-
BuOK) according to the protocol of Kowalik and Tolbert.21
Successive treatment of the reaction mixture with selenium
powder allowed for the isolation of 1a in 28% yield, which is
nearly half the yield obtained in the Sashida reaction. We then
attempted the synthesis of 1b by a similar treatment of readily
available bis(biphenyl-4-yl)acetylene (2b),22 but the reaction
gave only a complex mixture. This forced us to develop an
alternative synthetic method for the BSBS framework, as
follows. The mixture obtained by reacting diphenylacetylene
with the LICKOR superbase followed by selenium powder was
quenched with methyl iodide to afford o,o′-bis(methylseleno)-
diphenylacetylene (3a) in 68% yield. Subsequently, refluxing
the chloroform solution of 3a in the presence of excess iodine
allowed for ring formation to give 1a in 90% isolated yield
after purification by column chromatography. This approach
turned out to be also effective for the synthesis of diphenyl
derivative 1b via bis(3-methylselenobiphenyl-4-yl)acetylene (3b)
from 2b (Scheme 1).
Molecular and Crystal Structures. 1a and 1b were fully
characterized by spectroscopic and elemental analyses. Single-
crystal X-ray analyses revealed the exact molecular structures
of 1a and 1b, where the BSBS cores are almost planar, with
the maximum deviation from the mean plane being ca. 0.03 Å
for 1a and ca. 0.05 Å for 1b (Figure 2). The attached phenyl
rings in 1b are twisted out of the central plane with a torsion
angle of ca. 30°. The crystal structures of 1a and 1b are
characterized by the “layer-by-layer” stacking mode (Figure 3).
In both crystals, the unit cell contains two layers along the
a-axis, and the molecular arrangement in each layer is classified
as a herringbone type, common for crystal structures of excellent
organic semiconductors such as pentacene,23 R-sexithiophene,24
and tetramethylpentacene.25 In the layer of 1a, the molecules
are stacked nearly upright, and very short Se-Se contacts (3.76
(23) Cornil, J.; Calbert, J.-P.; Bre´das, J.-L. J. Am. Chem. Soc. 2001, 123, 1250-
1251.
(24) (a) Horowiz, G.; Bachet, B.; Yassar, A.; Lang, P.; Demanze, F.; Fave, J.
L.; Garnier, F. Chem. Mater. 1995, 7, 1337-1341. (b) Siegrist, T.; Fleming,
R. M.; Haddon, R. C.; Laudise, R. A.; Lovinger, A. J.; Katz, H. E.;
Bridenbaugh, P.; Davis, D. D. J. Mater. Res. 1995, 10, 2170-2173.
(25) Meng, H.; Bendikov, M.; Mitchell, G.; Helgeson, R.; Wudl, F.; Bao, Z.;
Siegrist, T.; Kloc, C.; Chen, C. H. AdV. Mater. 2003, 15, 1090-1093.
(21) Kowalik, J.; Tolbert, L. M. J. Org. Chem. 2001, 66, 3229-3231.
(22) (a) Nakasuji, K.; Akiyama, S.; Nakagawa, M. Bull. Chem. Soc. Jpn. 1972,
45, 883-891. (b) Mio, M. J.; Kopel, L. C.; Braun, J. B.; Gadzikwa, T. L.;
Hull, K. L.; Brisbois, R. G.; Markworth, C. J.; Grieco, P. A. Org. Lett.
2002, 4, 3199-3202.
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DPh-BSBS-Based OFETs
A R T I C L E S
Figure 4. UV-vis spectra of 1b in thin film (solid line) and in THF solution
(dotted line).
Figure 3. Crystal structures of (a) 1a and (b) 1b.
Å) exist. In contrast, in the layer of 1b, the molecules are stacked
with large slipping in the molecular long-axis direction. This
loose packing is responsible for the layer width of ca. 15 Å
and increases the Se-Se distance to 4.53 Å, meaning no
selenium interaction.
Voltammetric Properties. Cyclic voltammetry of 1a and 1b
showed irreversible oxidation waves (for the charts, see Figure
S1, Supporting Information). The oxidation peaks of 1a and
1b were found at +1.06 and +0.86 V (vs Fc/Fc+), respectively,
which were much higher than the potentials of R-4S (+0.41 V)
and DPh-BDS (+0.80 V), as expected by their calculations. On
the premise that the energy level of Fc/Fc+ is 4.8 eV below the
vacuum level,26 the HOMO levels of 1a and 1b were estimated
using the oxidation onsets (+0.80 V for 1a and +0.74 V for
1b) to be 5.60 and 5.54 eV, respectively. These values are fairly
consistent with the respective calculated HOMO energy levels
(5.46 eV for 1a and 5.38 eV for 1b).
Figure 5. (a) XRD pattern of evaporated thin film of 1b (Tsub ) 60 °C)
and (b) simulated powder pattern based on the single-crystal X-ray structure
of 1b.
from the loosely stacked structure determined by the above
X-ray crystallographic analysis. The optical band gap of the
thin film estimated from the absorption edge is ca. 3.0 eV, which
is fairly large compared with those of typical organic semicon-
ductors utilized for OFETs.2-6
To obtain further information on the film structure, the thin
film of 1b was examined by X-ray diffraction (XRD), which
showed a series of peaks assignable to multiple (h00) diffrac-
tions, meaning that the film has a highly ordered structure
(Figure 5a). On the basis of the diffraction pattern, the
monolayer thickness (d-spacing) was determined to be 19 Å,
which corresponds well to the molecular length of 1b determined
by X-ray crystallographic analysis. From this result as well as
the above electronic absorption spectra, we conclude that the
molecules of 1b in the film phase are nearly perpendicular to
the substrate and are aligned with close interactions. A
comparison of the XRD diffraction pattern with a powder pattern
simulated from the single-crystal analysis of 1b (Figure 5b) also
supports the finding that the molecular arrangement of 1b in
the film phase is different from that in the crystal phase. The
tight stacking structure can increase not only π-overlapping but
also Se-Se contacts between the stacked molecules, allowing
for the effective contribution of polarizable selenium atoms to
Thin Films. Parent BSBS 1a formed no continuous thin film
with good quality by thermal evaporation, because of its high
crystallinity. In contrast, 1b was readily obtained as a good thin
film not only on a glass substrate but also on an n-doped Si
wafer with 200 nm thermally grown SiO2. The UV-vis
absorption spectrum of 1b in THF exhibited a structured π-π*
band at 343 nm, as shown in Figure 4. In contrast, the absorption
band of 1b in the film phase was markedly red-shifted to 398
nm. In addition, it is remarkable that the 0-0 transition of the
longest wavelength band in the film spectrum was markedly
intensified to the strongest peak, in contrast to that in the solution
spectrum. This observation suggests that 1b adopts a strongly
interactive stacked structure in the film phase. Presumably, the
stacked structure of 1b in the film phase is fairly tight, different
(26) (a) Bre´das, J.-L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R. J. Am. Chem.
Soc. 1983, 105, 6555-6559. (b) Pommerehne, J.; Vestweber, H.; Guss,
W.; Mark, R. F.; Ba¨ssler, H.; Porsch, M.; Daub, J. AdV. Mater. 1995, 7,
551-554.
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1/2
Figure 6. (a) Ids-Vds plots as a function of Vg. (b) Ids and Ids versus Vg
plots at Vds ) -100 V (Tsub ) 60 °C).
Table 2. FET Characteristics of DPh-BSBS-Based Devices
1
Tsub
/
°
C
µ
FET/cm2 V-1 s-
I
on/Ioff
Vth/V
Figure 7. Ids versus Vg plots at Vds ) -100 V (Tsub ) 60 °C); first, 2000th,
and 3000th scans.
RT
60
100
0.19-0.20
0.24-0.31
0.12-0.17
106
106
-18
-20
-19
5 × 105
and large current on/off ratios of ∼106. In addition, the devices
are highly durable, undergoing no significant deterioration under
long-term continuous operation and during long-term storage
of at least 12 months under ambient laboratory conditions. This
high stability is speculated to arise primarily from the elevated
oxidation potential and the increased HOMO-LUMO energy
gap inherent in DPh-BSBS (1b). Thus, it can be concluded that
DPh-BSBS is a valuable semiconducting material for practical
applications and that the present successful development of DPh-
BSBS supports the validity of molecular designs based on fused
selenophenes in the search for air- and light-stable novel organic
semiconductors.
charge migration. Such a highly ordered structure in the film
phase is also the case for previously studied DPh-BDS.14a
OFET Devices. OFET devices using the thin film of 1b
vacuum-deposited on the Si/SiO2 substrate at different temper-
atures (Tsub)sroom temperature, 60 °C, and 100 °Csshowed
typical p-channel FET characteristics. Figure 6 demonstrates
the FET characteristics of the device fabricated at Tsub ) 60
°C: a high mobility of 0.24-0.31 cm2 V-1 s-1 and a large
current on/off ratio of 106 were obtained. The FET performances
of the other devices fabricated at Tsub ) room temperature and
100 °C are shown in Figure S2 (Supporting Information). As
summarized in Table 2, the FET characteristics of these devices
vary slightly with Tsub. Atomic force microscopy (AFM) images
showed essentially no significant differences among the surface
morphologies of the three films (Figure S3, Supporting Informa-
tion).
It is emphasized that the high performance of 1b-based OFET
was maintained even when the device was operated in the
presence of air and light. Figure 7 demonstrates a continuous
operation test in air with repeated scanning of Vg between +20
and -100 V at constant Vd ) -100 V. Although there was a
slight shift of the threshold voltage (Vth) after 2000 continuous
scans, there were no significant changes in mobility and Ion/
Ioff, even after 3000 continuous scans, indicating that 1b is a
very stable organic semiconductor. In addition, the device
showed almost no deterioration in both mobility and current
on/off ratio, even after it was stored under ambient laboratory
conditions for more than one year (Figure S4, Supporting
Information). This durability term exceeds the previously
reported two months for 2,5′-bis(2-fluorenyl)bithiophenes11 and
is comparable to the very recently reported best records for 5,11-
diarylindolo[3,2-b]carbazoles (more than one year)12 and 2,6-
bis(5-hexylthiophen-2-yl)anthracene (15 months).13
Experimental Section
Synthesis. All chemicals and solvents were of reagent grade unless
otherwise indicated. All reactions were carried out under nitrogen
atmosphere. THF was purified by distillation from sodium benzophe-
none ketyl under nitrogen prior to use. Column chromatography was
carried out with silica gel (Daisogel IR-60, 63-210 µm). Melting points
are uncorrected. Nuclear magnetic resonance spectra were obtained in
deuterated chloroform with a JEOL Lambda 400 spectrometer operating
at 400 MHz for 1H and 100 MHz for 13C with TMS as internal
reference; chemical shifts (δ) are reported in parts per million. EI-MS
spectra were obtained on a Shimadzu QP-5050A spectrometer using
an electron impact ionization procedure (70 eV). The molecular ion
peaks of the selenium-containing compounds showed a typical isotopic
pattern, and all the mass peaks are based on 80Se. Cyclic voltammo-
grams (CVs) were recorded on a Hokuto Denko HA-301 potentiostat
and a Hokuto Denko HB-104 function generator in benzonitrile
containing tetrabutylammonium hexafluorophosphate (Bu4NPF6, 0.1 M)
as supporting electrolyte at a scan rate of 100 mV/s. Counter and
working electrodes were made of Pt, and the reference electrode was
Ag/AgCl. All the potentials were calibrated with the standard ferrocene/
ferrocenium redox couple (E1/2 ) +0.46 V measured under identical
conditions). UV-vis spectra in THF solution or thin films were
recorded on a Shimadzu UV-3100 spectrometer.
Bis(2-methylselenophenyl)acetylene (3a). To a suspension of
t-BuOK (0.8 g, 7.3 mmol) in THF (15 mL) was slowly added
butyllithium (1.59 M in hexane, 4.6 mL, 7.3 mmol) at -78 °C, and
the resulting mixture was stirred for 20 min at the same temperature.
A solution of diphenylacetylene (0.5 g, 2.8 mmol) in THF (10 mL)
was added to the solution, and the mixture was stirred for another 40
min at -78 °C and then at -25 °C for 1.5 h. Selenium powder (0.44
In summary, we have succeeded in the facile synthesis of
DPh-BSBS (1b) by developing a straightforward selenocycliza-
tion method from bis(biphenyl-4-yl)acetylene. The thin-film
devices fabricated on the basis of 1b showed excellent p-type
FET performances, with hole mobilities of ∼0.3 cm2 V-1 s-1
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3048 J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006

DPh-BSBS-Based OFETs
A R T I C L E S
g, 5.6 mmol) was added in several portions to the mixture at -78 °C,
and the resulting mixture was stirred for 40 min at -78 °C. Methyl
iodide (0.37 mL, 5.7 mmol) was then added, and the mixture was
allowed to warm to room temperature. Insoluble precipitate in the
reaction mixture was filtered off, and the filtrate was extracted with
chloroform (20 mL × 2). The combined extracts were washed with
water (20 mL) and dried (MgSO4). After evaporation of the solvent,
the crude product was subjected to column chromatography on silica
gel eluted with dichloromethane-hexane (1:3, v/v). The desired product
was obtained from the eluent of Rf ) 0.4 and further purified by
recrystallization from chloroform to give colorless needles (0.69 g,
from chloroform gave 3b as yellow needles (0.28 g, 18%, conversion
yield 45%): mp 165-167 °C; H NMR (CDCl3) δ 7.62 (d, J ) 8.0
1
Hz, 2H), 7.61-7.57 (m, 4H), 7.52 (d, J ) 1.2 Hz, 2H), 7.47 (t, J )
7.2 Hz, 4H), 7.40 (dd, J ) 1.2 Hz, 8.0 Hz, 2H), 7.39 (t, J ) 7.2 Hz,
2H), 2.45 (s, 6H); 13C NMR (CDCl3) δ 141.86, 140.32, 136.66, 132.92,
128.89, 127.82, 127.13, 126.28, 124.24, 122.54, 93.84, 6.49; MS (EI)
m/z ) 518 (M+), 503 (M+ - CH3), 488 (M+ - 2 × CH3). Anal. Calcd
for C28H22Se2(CHCl3)0.5: C, 59.42; H, 3.94. Found: C, 59.21; H, 3.70.
2,7-Diphenyl[1]benzoselenopheno[3,2-b][1]benzoselenophene (DPh-
BSBS, 1b). To a solution of 3b (0.9 g, 1.7 mmol) in refluxing
chloroform (20 mL) was added powdery iodine (6.9 g, 27 mmol), and
the resulting mixture was further refluxed for 12 h. After cooling, the
resulting solid was successively washed with aqueous saturated sodium
sulfate solution (20 mL × 2), methanol (20 mL), and hot hexane (20
mL) to give 1b as a white solid (0.75 g, 91%). Recrystallization from
chlorobenzene gave an analytically pure sample as colorless plates. For
device fabrication, the recrystallized sample was further purified by
gradient sublimation at 320 °C (source temperature) under vacuum (10-3
Pa): mp > 300 °C; 1H NMR (CDCl3) δ 8.14 (d, J ) 1.5 Hz, 2H), 7.83
(d, J ) 8.3 Hz, 2H), 7.65-7.68 (m, 6H), 7.46 (t, J ) 7.6 Hz, 4H),
1
68%): mp 108-110 °C; H NMR (CDCl3) δ 7.53 (dd, J ) 1.2 Hz,
7.6 Hz, 2H), 7.29 (dd, J ) 1.2 Hz, 7.6 Hz, 2H), 7.26 (dt, J ) 1.2 Hz,
7.6 Hz, 2H), 7.17 (dt, J ) 1.2 Hz, 7.6 Hz, 2H), 2.37 (s, 6H); 13C NMR
(CDCl3) δ 136.18, 132.47, 128.85, 127.29, 125.02, 123.32, 93.15, 6.14;
MS (EI, 70 eV) m/z ) 366 (M+), 351 (M+ - CH3), 336 (M+ - 2 ×
CH3). Anal. Calcd for C16H14Se2: C, 52.76; H, 3.87. Found: C, 52.75;
H, 3.81.
[1]Benzoselenopheno[3,2-b][1]benzoselenophene (BSBS, 1a). To
a solution of 3a (0.5 g, 1.4 mmol) in refluxing chloroform (20 mL)
was added powdered iodine (5.7 g, 22.4 mmol), and the resulting
mixture was further refluxed for 12 h. After cooling, the solution was
washed with aqueous saturated sodium sulfate solution (20 mL × 2)
and water (20 mL × 2), and the chloroform layer was dried (MgSO4).
Evaporation of the solvent gave a yellow solid that was subjected to
column chromatography on silica gel eluted with hexane-dichlo-
romethane (3:1, v/v) to give pure BSBS (1a, Rf ) 0.6) as a white solid
(0.42 g, 90%). Recrystallization from chloroform gave colorless needles
suitable for X-ray structural analysis: mp 207-209 °C (lit.16 207-
208 °C); 1H NMR (CDCl3) δ 7.95 (dd, J ) 0.8 Hz, 8.0 Hz, 2H), 7.79
(dd, J ) 0.8 Hz, 8.0 Hz, 2H), 7.44 (dt, J ) 0.8 Hz, 8.0 Hz, 2H), 7.32
(dt, J ) 0.8 Hz, 8.0 Hz, 2H); 13C NMR (CDCl3) δ 141.27, 137.81,
134.28, 126.80, 125.38, 125.13, 123.84; MS (EI, 70 eV) m/z ) 336
(M+); CV, Epa ) +1.06 V vs Fc/Fc+; UV-vis (THF) λmax (ꢀ) ) 261
(24 875), 314 (25 532), 300 (21 368), 314 (25 532), 343 (11 260) nm.
7.36 (t, J ) 7.6 Hz, 2H); MS (EI, 70 eV) m/z ) 488 (M+); CV, Epa
)
+0.86, +1.33 V vs Fc/Fc+; UV-vis (THF) λmax (ꢀ) ) 283 (13 271),
343 (39 282), 366 (24 875) nm. Anal. Calcd for C26H16Se2: C, 64.21;
H, 3.32. Found: C, 64.60; H, 3.32.
X-ray Crystallographic Analysis. A single crystal of 1a suitable
for X-ray crystallographic analysis was grown by recrystallization from
chloroform, whereas that of 1b was grown by horizontal physical vapor
transport in flowing argon gas.27 X-ray crystal structure analyses were
performed on a Rigaku AFC7R four-circle diffractometer (Mo KR
radiation, λ ) 0.71069 Å, graphite monochromator, T ) 296 K, ω
scan, 2θmax ) 55.0°). The structure was solved by direct methods.28
Non-hydrogen atoms were refined anisotropically.29 Hydrogen atoms
were included but not refined. All calculations were performed using
the crystallographic software package teXsan.30 Crystal data for BSBS
(1a): C14H8Se2, M ) 334.14, colorless plate, 0.50 × 0.20 × 0.03 mm3,
monoclinic, space group P21/c (No. 14), a ) 12.037(2) Å, b ) 6.025(1)
Bis(biphenyl-4-yl)acetylene (2b). To a deaerated solution of 4-bro-
mobiphenyl (5.0 g, 20 mmol) in diisopropylamine (40 mL) and dry
benzene (20 mL) were consecutively added trimethylsilylacetylene (1.4
mL, 10 mmol), PdCl2(PPh3)2 (0.84 g, 1.2 mmol), CuI (0.38 g, 2.0
mmol), DBU (1.83 g, 0.84 mmol), and water (0.14 mL, 7.8 mmol),
and the resulting mixture was stirred for 18 h at 60 °C. After cooling,
the mixture was diluted with water (50 mL) to precipitate a solid that
was collected by filtration and washed successively with water,
methanol, and hot hexane. The crude product was recrystallized from
carbon disulfide to give colorless plates (1.01 g, 31%): mp 256-258
°C (lit.22a 253-254 °C); 1H NMR (CDCl3) δ 7.64-7.59 (m, 12H), 7.46
(t, J ) 7.6 Hz, 4H) 7.44 (tt, J ) 1.2 Hz, J ) 7.6 Hz, 2H); 13C NMR
(CDCl3) δ 140.97, 140.34, 132.02, 128.86, 127.63, 127.02 (× 2),
122.19, 89.98; MS (EI, 70 eV) m/z ) 330 (M+).
Bis(3-methylselenobiphenyl-4-yl)acetylene (3b). To a mixture of
t-BuOK (0.89 g, 8.0 mmol) in THF (15 mL) was slowly added
butyllithium (1.54 M in hexane, 5.1 mL, 8.0 mmol) at -78 °C, and
the resulting mixture was stirred for 10 min at the same temperature.
Bis(biphenyl-4-yl)acetylene (2b, 1.0 g, 3.0 mmol) was then added, and
the resulting mixture was stirred for 30 min at -78 °C and then at
-30 °C for 3 h. To the mixture cooled at -78 °C was slowly added
selenium powder (0.47 g, 6.0 mmol) over 10 min, and the mixture
was stirred at -78 °C and gradually warmed to -20 °C over a period
of 4 h. At this temperature, methyl iodide (0.5 mL, 8.0 mmol) was
added, and the mixture was gradually warmed over the period of 10 h.
The starting material that precipitated from the mixture was removed
by filtration (0.6 g, 60%), and the filtrate was extracted with chloroform
(20 mL × 3). The combined extracts were washed with water (30 mL
× 3), dried (MgSO4), and concentrated in vacuo. The residue was
subjected to column chromatography on silica gel eluted with carbon
disulfide to give crude 3b (Rf ) 0.3) as a yellow solid. Recrystallization
Å, c ) 8.420(1) Å, â ) 108.45(1)°, V ) 579.2(2) Å3, Z ) 2, Dcalc
)
1.916 g cm-3, R ) 0.035 for 1083 observed reflections (I > 2σ(I)) and
85 variable parameters, Rw ) 0.111 for all data. Crystal data for DPh-
BSBS (1b): C26H16Se2, M ) 486.30, colorless plate, 0.40 × 0.20 ×
0.01 mm3, orthorhombic, space group Pbca (No. 61), a ) 30.008(7)
Å, b ) 8.400(4) Å, c ) 7.701(3) Å, V ) 1940(2) Å3, Z ) 4, Dcalc
)
1.650 g cm-3, R ) 0.043 for 1009 observed reflections (I > 2σ(I)) and
128 variable parameters, Rw ) 0.169 for all data.
Device Fabrication. OFETs were fabricated in a “top-contact”
configuration on a heavily doped n+-Si (100) wafer with 200-nm-thick
thermally grown SiO2 (Ci ) 1.73 × 10-8 F cm-2). A thin film (50 nm
thick) of DPh-BSBS (1b) as the active layer was vacuum-deposited
on the Si/SiO2 substrate maintained at various temperatures (Tsub) at a
rate of 1-2 Å s-1 under a pressure of ∼2 × 10-3 Pa. On the top of the
organic thin film, gold films (80 nm) as drain and source electrodes
were deposited through a shadow mask. For a typical device, the drain-
source channel length (L) and width (W) are 50 µm and 1.5 mm,
respectively. The characteristics of the OFET devices were measured
at room temperature under vacuum or in air with an Agilent 4155C
semiconductor parameter analyzer. Field-effect mobility (µFET) was
calculated in the saturation regime of the Ids using the equation Ids
)
(WCi/2L) µFET(Vg - Vth)2, where Ci is the capacitance of the SiO2
insulator and Vg and Vth are the gate and threshold voltages, respectively.
(27) Laudise, R. A.; Kloc, Ch.; Simpkins, P. G.; Siegrist, T. J. Crystal Growth
1998, 187, 449-454.
(28) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.; Giacovazzo, C.;
Guagliardi, A.; Polidori, G. J. Appl. Crystallogr. 1994, 27, 435-436.
(29) Sheldrick, G. M. Program for the Refinement of Crystal Structures;
University of Goettingen: Goettingen, Germany, 1997.
(30) teXsan: Single Crystal Structure Analysis Software, Version 1.11; Molecular
Structure Corporation and Rigaku Corporation, 2000.
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J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006 3049

A R T I C L E S
Takimiya et al.
Current on/off ratio (Ion/Ioff) was determined from the Ids at Vg ) 0 V
(Ioff) and Vg ) -100 V (Ion). The mobility and Ion/Ioff data summarized
in Table 2 are typical values obtained from several devices.
XRD and AFM Images of DPh-BSBS Film. X-ray diffraction
patterns of DPh-BSBS thin films deposited on the Si/SiO2 substrate
were obtained with a Maxscience M18XHF diffractometer with a Cu
KR source (λ ) 1.541 Å) in air. AFM images of DPh-BSBS thin films
on Si/SiO2 were obtained with a Shimadzu SPM-9500 microscope in
air (see Figure S3 in Supporting Information).
Development Organization (NEDO) of Japan. K.T. is indebted
to The Furukawa Foundation for financial support. We are also
grateful to Professor S. Yamanaka (Hiroshima University) for
XRD measurements.
Supporting Information Available: Cyclic voltammograms
of 1a and 1b; FET characteristics of 1b-based OFETs fabricated
at Tsub ) room temperature and 100 °C; AFM images of thin
films of 1b; transfer characteristics of typical devices in saturated
regimes as a function of time; details of DFT calculations of
1a, 1b, R-quaterselenophene (R-4S), and DPh-BDS. (PDF).
Crystallographic information file for 1a and 1b (CIF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. This work was partially supported by
Grants-in-Aid for Scientific Research on Priority Areas of
Molecular Conductors (Nos. 15073218 and 16038219) and
Scientific Research (Nos. 15550162 and 16750162) from the
Ministry of Education, Culture, Sports, Science and Technology,
Japan, and an Industrial Technology Research Grant Program
in 2005 from the New Energy and Industrial Technology
JA057641K
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3050 J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006