Charge carrier transport properties in liquid crystalline 2-phenylbenzothiazole derivatives

Article abstract of DOI:10.1080/15421400903069384

We synthesized liquid crystalline 2-phenylbenzothiazole derivatives and investigated their charge carrier transport properties by time-of-flight experiments. These materials show less ordered phases such as smectic A, smectic C, and nematic phases at temperature range lower than 100°C, and their mobility was relatively small, from 10^-5cm²/Vs to 10 ^-4cm²/Vs. In addition, the mobility depends on temperature, while it did not depend on electric fields. According to these results, we estimated the eneregy distribution of density of states responsible for condition, σ of Gaussian width, to be 78 ～ 118meV.

Full text of DOI:10.1080/15421400903069384

This article was downloaded by: [University of California, San Diego]
On: 17 February 2013, At: 20:39
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Molecular Crystals and Liquid
Crystals
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/gmcl20
Charge Carrier Transport
Properties in Liquid Crystalline
2-Phenylbenzothiazole
Derivatives
Keiji Tokunaga a c , Hiroaki Iino a b & Jun-ichi Hanna
a b
a Imaging Science and Engineering Laboratory, Tokyo
Institute of Technology, Yokohama, Japan
b JST CREST, Yokohama, Japan
c Dai Nippon Printing Co. Ltd., Kashiwa, Chiba,
Japan
Version of record first published: 05 Oct 2009.
To cite this article: Keiji Tokunaga , Hiroaki Iino & Jun-ichi Hanna (2009): Charge
Carrier Transport Properties in Liquid Crystalline 2-Phenylbenzothiazole Derivatives,
Molecular Crystals and Liquid Crystals, 510:1, 241/[1375]-249/[1383]
To link to this article: http://dx.doi.org/10.1080/15421400903069384
PLEASE SCROLL DOWN FOR ARTICLE
Full terms and conditions of use: http://www.tandfonline.com/page/terms-
and-conditions
This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden.

The publisher does not give any warranty express or implied or make any
representation that the contents will be complete or accurate or up to
date. The accuracy of any instructions, formulae, and drug doses should be
independently verified with primary sources. The publisher shall not be liable
for any loss, actions, claims, proceedings, demand, or costs or damages
whatsoever or howsoever caused arising directly or indirectly in connection
with or arising out of the use of this material.

Mol. Cryst. Liq. Cryst., Vol. 510, pp. 241=[1375]–249=[1383], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903069384
Charge Carrier Transport Properties in Liquid
Crystalline 2-Phenylbenzothiazole Derivatives
Keiji Tokunaga1,3, Hiroaki Iino1,2, and Jun-ichi Hanna1,2
1Imaging Science and Engineering Laboratory, Tokyo Institute
of Technology, Yokohama, Japan
2JST CREST, Yokohama, Japan
3Dai Nippon Printing Co. Ltd., Kashiwa, Chiba, Japan
We synthesized liquid crystalline 2-phenylbenzothiazole derivatives and investigated
their charge carrier transport properties by time-of-flight experiments. These
materials show less ordered phases such as smectic A, smectic C, and nematic
phases at temperature range lower than 100ꢀC, and their mobility was relatively
small, from 10ꢁ5 cm2=Vs to 10ꢁ4 cm2=Vs. In addition, the mobility depends on
temperature, while it did not depend on electric fields. According to these results,
we estimated the eneregy distribution of density of states responsible for condition,
r of Gaussian width, to be 78 ꢂ 118 meV.
Keywords: benzothiazole derivatives; benzoxazole derivatives; charge carrier mobility;
electronic transport; ionic transport; smectic phase; the density of states
INTRODUCTION
After a long historical quest for the electronic conduction in the liquid
crystals, which was initiated in late 1960s [1,2], the electronic conduc-
tion was discovered in the columnar phase of a discotic liquid crystal-
line triphenylene derivative, i.e., hexapentyloxytriphenylene (H5T) in
1993 [3]. As for calamitic liquid crystals, the electronic conduction was
first established in the SmA phase of a 2-phenylbenzothiazole
derivative, i.e., 2-(40-heptyloxyphenyl)-6-dodecylthiobenzothiazole (7O-PBT-
S12) [4,5]. Since then, the charge carrier transport properties in
various liquid crystals were characterized [6–12], leading to a general
recognition about the intrinsic nature of charge carrier transport in
Address correspondence to Jun-ichi Hanna, Imaging Science and Engineering
Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta Midori-ku, Yokohama,
226-8503, Japan. E-mail: hanna@isl.titech.ac.jp
241=[1375]

242=[1376]
K. Tokunaga et al.
the smectic liquid crystals. This let us call a question about the charge
carrier transport properties of the 7O-PBT-S12 reported originally in
terms of exceptionally high hole mobility in SmA phase and a shape
of transient photocurrent measured by time-of-flight experiments
[4]. Then, we re-investigated the charge carrier transport properties
of 7O-PBT-S12 and clarified that the high mobility was attributed to
a mobility of the crystalline phase appearing next to the SmA phase,
and that the carrier transport in SmA phase was ambipolar and
characterized by a mobility on the order of 10ꢁ4 cm2=Vs [5], which is
comparable to that of SmA phase in various liquid crystals reported
so far. However, because of a very limited temperature range of the
SmA phase in 7O-PBT-S12, it was very difficult to know whether
the mobility depend on temperature.
In this report, in order to characterize a general carrier transport
properties of 2-phenylbenzothiazole derivatives [4,5], we synthesized
a variety of 2-phenylbenzothiazole derivatives and studied their
carrier transport properties by time-of-flight experiments in detail.
This would be available for understanding a relation between carrier
transport properties and the chemical structure of liquid crystals.
EXPERIMENTAL
We synthesized various 2-phenylbenzothiazole derivatives whose
chemical structure is shown in Figure 1 according to the synthetic
routes as shown in Figure 2(a): 2-(40-heptyloxyphenyl)-6-thioalkylben-
zothiazole (7O-PBT-S12, 7O-PBT-S4) (5) were synthesized in 60 ꢂ 70%
FIGURE 1 Chemical structure of 2-phenyl benzothiazole derivatives.

Charge Carrier Transport Properties in Liquid Crystalline
243/[1377]
FIGURE 2 Synthetic route of 2-phenylbenzothiazole and 2-phenylbenzoxa-
zole derivatives.

244/[1378]
K. Tokunaga et al.
yield in 2 steps from a key intermediate of 2-amino-5-bromothiophenol
(3) with p-heptyloxybenzaldehyde in DMSO, which was prepared
by hydrolysis of 2-amino-6-bromobenzothiazole(2) in potassium
hydroxide solution at 100ꢀC. 2-Amino-6-bromobenzothiazole(2) was
synthesized by oxidative cyclization into 2-aminobenzothiazole of
p-bromoaniline(1) with ammonium thiocyanate and bromine in acetic
acid at room temperature; 2-(40-alkoxyphenyl)-6-alkylbenzothiazole
(9O-PBT, 9O-PBT-12, 7O-PBT-4, 7O-PBT-8, 7O-PBT-12, 5O-PBT-12)
(9) were synthesized in 50 ꢁ 60% yield from 2-amino-5-alkyl-thiophe-
nol(8) with p-alkoxybenzaldehydes in DMSO at 150ꢀC, as shown in
Figure 2 (b); A dialkoxy-phenylbenzothiazole derivative, i.e., 2-(40-
octyloxyphenyl)-6-butoxybenzothiazole (8O-PBT-O4, 15) was synthe-
sized in 5 steps from p-nitrophenol(10) as shown in Figure 2(c).
In addition, for comparison of a core structure , we synthesized
2-phenybenzoxazole derivative, i.e., 2-(40-decyloxyphenyl)-5-butoxy-
benzoxazole (10O-PBO-O4) and 2-(40-decyloxyphenyl)-5-octyloxyben-
zoxazole (10O-PBO-O8) as indicated by the synthetic route shown in
Figure 2(d).
The benzothiazole derivatives isolated were purified with flush
chromatography and recrystallization from ethanol and n-hexane
several times. Their phase transition behaviors were determined by
differential scanning calorimetry (DSC, Shimazu DSC-60) and X-ray
diffraction (XRD, RigakuRAD-2B diffractometer with Cu Ka radiation)
studies. The charge carrier transport properties were studied by
time-of-flight experiments, whose set-up was described elsewhere in
detail [13]. The samples for TOF experiments were prepared by filling
2-phenylbenzothiazole derivatives purified into liquid crystals cells in
capirally action at their isotropic temperatures. The resulting molecu-
lar alignment of the cells was homogeneous and it was stable at entire
electric field applied in TOF experiments.
RESULTS AND DISCUSSION
The phase transition behaviors of 2-phenylbenzothiazole derivatives
synthesized, which are basically 2-(40-alkoxyphenyl)benzothiazoles
6-substituted with an alkyl, alkoxy, or thioalkyl group, are sum-
marized in Figure 3. These materials tend to exhibit less ordered
mesophases such as nematic, SmA, and SmC phases at a relatively
low temperature range, i.e., lower than 100ꢀC. Two 2-phenylbenzox-
azole derivatives also follow this trend in spite of different
alkoxy-substituted position in benzoxazole ring. The typical texture
images observed by polarized optical microscope are shown in
Figure 4.

Charge Carrier Transport Properties in Liquid Crystalline
245/[1379]
FIGURE 3 Phase transition temperature of various benzothiazole and
benzoxazole derivatives in cooling process.
We observed only slow transit of positive carriers characterized by a
small mobility of 10ꢁ5 cm2=Vs in SmA and SmC phases of 2-phenylben-
zothiazole derivatives, when the purity of the samples was low, as we
FIGURE 4 Typical texture images of benzothiazoles under polarized optical
microscope at (a) nematic phase of 8O-PBT-O4, (b) SmA phase of 7O-PBT-
S12, and (c) SmC phase of 8O-PBT-O4.

246/[1380]
K. Tokunaga et al.
FIGURE 5 Typical transient photocurrents for positive carriers in the
smectic C phase of benzothiazole derivative (8O-PBT-O4). The cell thickness
was 10 mm. (a) linear plot (b) double logarithmic plot.
reported in the previous paper [5]. However, we obsevered a fast tran-
sit characterized by a high mobility on the order of 10À4 cm2=Vs when
the purity was sufficiently high after purification by flush chromato-
graphy and recrystallization from ethanol and n-hexane several times.
In fact, the fast transit was accompanied with the slow transit when the
purity was not enough. We attributed to these fast and slow transits of
carriers to the hole conduction and inoic conduction, respectively [5].
The typical transient photocurrents for positive carriers in the smec-
tic C phase of benzothiazole derivative (8O-PBT-O4) in Figure 5. The
mobility in the SmA or SmC phases of these 2-phenylbenzothiazole deri-
vatives are plotted as a function of temperature in Figure 6 with a mobi-
lity of 2-phenylnapthalene derivative, i.e., 8-PNP-O12 for a reference
[7,8], whose carrier transport properties are well characterized. Interest-
ingly, the mobility of 2-phenylbenzothiazole derivatives depends on tem-
perture irrespective of materials, which is different from that of
8-PNP-O12, and we obtained 0.16 ꢀ 0.44 eV as its activation energy from
the Arrhenius polt. Interestingly, the mobility, however, does not
depend on electric field at the range from 104 to 105 V=cm.
The mobility of 2-phenylbenzoxazole derivaties (10O-PBO-O4 and
10O-PBO-O8) showed a similar trend to the benzothiazole derivatives,
and were slightly lower than those of the benzothiazole derivatives.
Figure 7 shows the typical transient photocurrents for positive car-
riers in the smectic A phase of benzoxazole derivative (10O-PBO-O8).
Judging from these coincident results for benzothiazole and benzox-
azole derivatives, it is plausible tht relatively low mobility that
depends on temperature is basically responsible for benzothiazole or
benzoxazole core moieties, rather than contamination of trace amount

Charge Carrier Transport Properties in Liquid Crystalline
247/[1381]
FIGURE 6 Temperature dependence of carrier mobility at variety of
benzothiazole and benzoxazole derivatives with a reference of 2-phenyl-
naphthalene derivative (8-PNP-O12).
impurities that cause temperarue dependence of mobility. In order to
conclude the origin of this temperature dependence, we need more
examples of the mobilities in the smectic mesophases of both
benzothiazole and benzoxazole derivatives.
FIGURE 7 Typical transient photocurrents for positive carriers in the
smectic A phase of benzoxazole derivative (10O-PBO-O8). The cell thickness
was 10 mm. (a) linear plot (b) double logarithmic plot.

248/[1382]
K. Tokunaga et al.
TABLE 1 Gaussian Width of Density of States Estimated
for Hole Conduction in Benzothiazole and Benzoxazole
Derivatives with a Reference of Phenylnapthalene
Derivative, 8-PNP-O12
Entry
Liquid crystals
r (meV)
1
2
3
7O-PBT-S12
7O-PBT-S4
9O-PBT-12
9O-PBT
92
97
108
80
4
5
6
7
8
7O-PBT-12
7O-PBT-8
7O-PBT-4
5O-PBT-12
8O-PBT-O4
10O-PBO-O8
10O-PBO-O4
8-PNP-O12
104
100
114
78
118
109
111
60
9
10
11
12
We estimated the energetic distribution of density of states for
hole conduction in 2-phenylbenzothiazole derivatives according to
following Eq. (1) formalized by 2-domensional disoder model for
smectic mesophases [14].
!
ꢀ
ꢁ
2
A
0:67r
lsmectic
¼
exp ꢁ
ð1Þ
kT
kT
where A=kT is the prefactor and k is Boltzmann constant and T is
temperature.
As is indicated from the temperature dependence of the mobility,
the Gaussian width of the density of states, r for was estimated to
be about 78 ꢂ 118 meV as shown in Table 1. This value is higher than
that of phenylnaphtalene and terthiophene derivatives of 40 ꢂ 60 meV.
[14]. The reason why therefore phenylbenzothiazole derivatives are
large is not clear because the origin of energetic disorder in the meso-
phases has not been clarified yet. The polar core structure may be
responsible for enhanced distribution of density of states.
CONCLUSION
We synthesized liquid crystalline 2-phenylbenzothiazole and 2-phenyl-
benzoxazole derivatives and characterized their charge carrier trans-
port properties by time-of-flight experiments. These materials shows
less ordered phases of SmA or SmC phases at temperatures lower than

Charge Carrier Transport Properties in Liquid Crystalline
249/[1383]
100ꢀC. We found that their mobility in these mesophases was
relatively small, i.e., on the order of 10ꢁ5 cm2=Vs to 10ꢁ4 cm2=Vs and
depended on temperature. We attributed this temperature depen-
dence of the mobility to the energetic disorder of the density of states
and estimated the Gaussian with of the density of states to be
78 ꢂ 118 meV. This value is relatively large compared with those of
phenylnaphtalene and terthiophene derivatives. The origin of this
large distribution is not clear, and we need further investigation on
the charge carrier transport of benzothiazole derivatives in order to
clarify it.
REFERENCES
[1] Heilmeir, G. H. & Heyman, P. M. (1967). Phys. Rev. Lett., 18, 583.
[2] Kusabayashi, S. & Labes, M. M. (1967). Mol. Cryst. Liq. Cryst., 7, 395.
[3] Adam, D., Closs, F., Frey, T., Funhoff, D., Haarer, D., Ringsdorf, H., Schuhmacher, P.,
& Siemensmeyer, K. (1993). Phys. Rev. Lett., 70, 457.
[4] Funahashi, M. & Hanna, J. (1997). Phys. Rev. Lett., 78, 2184.
[5] Tokunaga, K., Iino, H., & Hanna, J. (2007). J. Phys. Chem. B, 111, 12041.
[6] Funahashi, M. & Hanna, J. (1996). Jpn. J. Appl. Phys., 35, L703.
[7] Funahashi, M. & Hanna, J. (1997). Appl. Phys. Lett., 71, 602.
[8] Funahashi, M. & Hanna, J. (2000). Appl. Phys. Lett., 76, 2574.
[9] Kurotaki, K., Haruyama, A., Takayashiki, Y., & Hanna, J. (2006). Chem. Lett.,
35, 1194.
[10] Ahn, H., Ohno, A., & Hanna, J. (2005). Jpn. J. Appl. Phys., 44, 3764.
[11] Ahn, H., Ohno, A., & Hanna, J. (2007). J. Appl. Phys., 102, 093718.
[12] Funahashi, M. & Hanna, J. (2004). Chem. Phys. Lett., 397, 319.
[13] Iino, H. & Hanna, J. (2006). Jpn. J. Appl. Phys., 45, L867.
[14] Ohno, A. & Hanna, J. (2003). Appl. Phys. Lett., 82, 751.