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Published on the web May 25, 2013
Charge-carrier Transport in 1,4-Bis(phenylethynyl)benzene
Derivatives Exhibiting Crystal Mesophases
Tetsuo Yatabe,* Hajime Okumoto, Yuji Kawanishi, and Takahito Inoue
National Institute of Advanced Industrial Science and Technology (AIST),
Tsukuba Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565
(Received March 13, 2013; CL-130219; E-mail: tetsuo-yatabe@aist.go.jp)
1,4-Bis(phenylethynyl)benzene (BPB) derivatives with alkyl
transport in LC BPBs have an important role to play in research
on self-organizing organic semiconductors, including LC mate-
rials.1,2,20
or alkoxy terminal chains were synthesized, and the mesomor-
phic and charge-carrier-transport properties were investigated.
The BPB derivatives exhibited high charge-carrier mobilities in
the crystal mesophase or crystal phase.
In the present study, we investigated the charge-carrier
transport in two LC BPB materials. One is diheptyloxy
derivative 1, with a phase sequence of Cr1-Cr2-CrM1-CrM2-
N-I, and the other is dioctyl derivative (C8-BPB, 2), a new
mesogen with a Cr-CrM-N-I phase sequence. We found that
C7O-BPB exhibits mobilities of 0.03 and 0.12 cm2 V¹1 s¹1 in the
CrM1 and Cr2 phases, respectively. C8-BPB was found to have
Self-organizing conjugated oligomers have good potential
for use as semiconducting materials in organic thin-film
transistors.1-4 Charge-carrier transport in semiconducting films
depends both on the ³ electrons delocalized over the molecule
and on the ³-³ stacking between the molecules.2b,3b In recent
years, there has been increasing interest in arylacetylene
oligomers, namely, ³ systems containing aromatic rings linked
by carbon-carbon triple bonds, because they are easy to
synthesize and stable in air.5-13 Furthermore, some of these
oligomers exhibit charge-carrier mobilities with values exceed-
ing 0.1 cm2 V¹1 s¹1. One is a phenylacetylene oligomer with
electron donor and acceptor groups, Me2N(C6H4CC)3SiMe3.5
The others contain fused ring(s) (naphthalene,8 anthra-
cene,9,11a,11b pyrene,11c or benzodithiophene12a) or bithiophe-
ne.12c However, due to the existence of such semiconducting
oligomers, there have been few studies of charge-carrier
transport in oligomers without either thiophene and fused rings
or electron donor and acceptor groups.6
¹1
a high value of 0.09 cm2 V¹1 s in the CrM phase. We also
investigated the thermal and structural properties of 1 and 2. The
CrM1 phase of 1 and the CrM phase of 2 were both found to be a
mesophase in which the molecular long axes are tilted with
respect to the layer normal.
BPBs 1 and 2 were synthesized by Sonogashira coupling of
1,4-diiodobenzene with phenylacetylenes 3 and 4, respectively
(Scheme 1).21 Colorless crystals of 1 and 2, obtained by careful
recrystallization, were used to investigate the properties.
The phase-transition behavior of 1 and 2 was investigated
using polarizing optical microscopy (POM) and differential
scanning calorimetry (DSC, Figure S1).21 The thermotropic data
are summarized in Table 1. C7O-BPB formed N and two CrM
phases and two Cr phases enantiotropically. On cooling from the
isotropic phase, a nematic Schlieren texture appeared,22 which
changed to a mosaic texture (Figure 1a). On further cooling,
striped patterns were observed in the CrM1 phase, and very small
scratch-like cracks were found in some domains of the Cr2 phase
(Figures 1b and 1c). Finally, enlarged cracks and defect lines
The phenylacetylene pentamer, H(C6H4CC)4C6H5, exhibits
¹1
mobility of 0.045 cm2 V¹1 s in the vacuum-deposited films.6b
It has also been reported that the tetramer, H(C6H4CC)3C6H5,
¹3
and its dihexyl derivative exhibit values of 1.3 © 10 and
3.5 © 10¹5 cm2 V¹1 s¹1, respectively.6a In general, the charge-
carrier mobility in a homogeneous series of linear oligomers
decreases with decreasing conjugation length.14 Therefore, the
trimer, i.e., 1,4-bis(phenylethynyl)benzene (BPB),15-17 and its
dialkyl or dialkoxy derivatives would be expected to exhibit
mobilities lower than those found for the tetramers.
I
I
R
R
R
[Pd(PPh3)2Cl2], CuI
piperidine, THF
3: R = OC7H15
4: R = C8H17
1: R = OC7H15 (94%)
2: R = C8H17 (78%)
Nevertheless, a dioctyloxy-terminated derivative of BPB
(C8O-BPB) exhibits mesophases, nematic (N) and two smectic
(Sm) phases, and two crystal (Cr) phases.18 The diheptyloxy
derivative (C7O-BPB, 1) also exhibits mesophases.19 Although 1
has been reported to exhibit two or three Cr phases, the thermal
behavior is similar to that of C8O-BPB. Therefore, by utilizing
liquid-crystalline (LC) BPB materials, including the above
derivatives, i.e., by cooling the materials from the isotropic (I)
phase, molecularly ordered thin films would be prepared in the
mesophases, including crystal mesophase (CrM phase) such as
a CrE phase.1,2 Furthermore, although crystallization from a
mesophase generally causes cracks to appear in the film, LC
BPBs that have two or more Cr phases might provide suitable
films with which to investigate mobilities, at least in the highest-
temperature Cr phase. Therefore, studies of charge-carrier
Scheme 1. Synthesis of BPB derivatives 1 and 2.
Table 1. Phase-transition temperatures (°C) and transition
enthalpies (in square brackets, kJ mol¹1) of BPBs 1 and 2
Compound Phase transitiona
1
h: Cr1 72 [16.4] Cr2 131 [13.0] CrM1 177 [2.8]
CrM2 179 [20.3] N 223 [1.7] I
c: I 223 [1.7] N 179 [20.5] CrM2 175 [2.9]
CrM1 127 [13.0] Cr2 70 [16.4] Cr1
2
h: Cr 49 [8.2] CrM 155 [19.9] N 171 [1.0] I
c: I 170 [1.0] N 155 [19.3] CrM 49 [1.4] Cr
ah: on heating; c: on cooling; Cr: crystal phase; CrM: crystal
mesophase; N: nematic phase; I: isotropic phase.
Chem. Lett. 2013, 42, 764-766
© 2013 The Chemical Society of Japan