One-dimensional to three-dimensional electronic conduction in liquid crystalline mesophases

Article abstract of DOI:10.1080/15421400903069459

We have established the electronic conduction in the nematic phase of a small molecule of a 2-phenylbenzothiazole derivative, i.e., 2-(4'- octyloxyphenyl)-6-butoxybenzothiazole (8O-PBT-O4). This gives a new insight into the quest for the electronic conduction in liquid crystals, which was initiated by Kusabayashi and Labes in late 1960s and had succeeded over several decades, leading it to the end. In addition, it is clarified that the ionic conduction often observed in less ordered mesophases is induced with trace amounts of chemical impurities due to its low viscosity. The present result indicates that the charge carrier transport in the mesophase is electronic in its intrinsic nature irrespective of mesophases and molecular sizes, i.e., 1D-electronic conduction in columnar phase, 2D-electronic conduction in smectic mesophases, and 3D-electronic conduction in the nematic phase.

Full text of DOI:10.1080/15421400903069459

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One-Dimensional to Three-
Dimensional Electronic
Conduction in Liquid
Crystalline Mesophases
a c
a b
Keiji Tokunaga
, Yukiko Takayashiki
, Hiroaki
a b
a b
Iino
& Jun-ichi Hanna
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 , Yukiko Takayashiki , Hiroaki Iino & Jun-ichi
Hanna (2009): One-Dimensional to Three-Dimensional Electronic Conduction in
Liquid Crystalline Mesophases, Molecular Crystals and Liquid Crystals, 510:1, 250/
[1384]-258/[1392]
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Mol. Cryst. Liq. Cryst., Vol. 510, pp. 250=[1384]–258=[1392], 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903069459
One-Dimensional to Three-Dimensional Electronic
Conduction in Liquid Crystalline Mesophases
1
,3
1,2
Keiji Tokunaga , Yukiko Takayashiki
,
1,2
1,2
Hiroaki Iino , and Jun-ichi Hanna
Imaging Science and Engineering Laboratory, Tokyo Institute
of Technology, Yokohama, Japan
JST CREST, Yokohama, Japan
Dai Nippon Printing Co. Ltd., Kashiwa, Chiba, Japan
1
2
3
We have established the electronic conduction in the nematic phase of a small
0
molecule of a 2-phenylbenzothiazole derivative, i.e., 2-(4 -octyloxyphenyl)-6-butox-
ybenzothiazole (8O-PBT-O4). This gives a new insight into the quest for the
electronic conduction in liquid crystals, which was initiated by Kusabayashi
and Labes in late 1960s and had succeeded over several decades, leading it to
the end. In addition, it is clarified that the ionic conduction often observed in less
ordered mesophases is induced with trace amounts of chemical impurities due to
its low viscosity. The present result indicates that the charge carrier transport in
the mesophase is electronic in its intrinsic nature irrespective of mesophases and
molecular sizes, i.e., 1D-electronic conduction in columnar phase, 2D-electronic
conduction in smectic mesophases, and 3D-electronic conduction in the nematic
phase.
Keywords: charge carrier transport; discotics; electronic conduction; ionic conduction;
nematics; smectics
INTRODUCTION
Recent years, liquid crystals have attracted new attention as a new
type of organic semiconductors exhibiting self-organization, which
was initiated by discovery of electronic conduction in liquid crystals
in 1990s [1,2]. The first attention to the electrical properties of liquid
crystal, however, was paid in late 1960s by two groups: Heilmeier
Address correspondence to Jun-ichi Hanna, Imaging Science and Engineering
Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta Midori-ku, Yokohama,
2
26-8503, Japan. E-mail: hanna@isl.titech.ac.jp
2
50=[1384]

1
-D to 3-D Electronic Conduction
251/[1385]
et al. reported ionic conduction in the nematic phase of p-azoxyaniole,
which was associated with his invention of liquid crystal displays
based on so-called dynamic scattering mode [3]; Kusabayashi and
Labes extended their interest in the electrical properties of molecular
crystals [4] to liquid crystals and reported those of various liquid
crystals. Heilmeier’s invention called high attention, so that the
electrical properties of various calamitic, that is, rod-like liquid
crystals were characterized in 1970s [5,6]. As a result, this led to a
general recognition naturally that the charge carrier transport in
liquid crystals was governed by ionic conduction. The liquid-like
nature of liquid crystalline materials supports this idea easily.
After the synthesis of discotic liquid crystals in 1977 [7], the major
interest in the electrical properties of liquid crystals was focused on
the discotic liquid crystals such as triphenylene and porphirine deriva-
tives because of their solid-like nature compared with calamitic liquid
crystals. In late 1980s, Leeds university group indicated that the
electronic conduction took place in columnar phase of a chemically doped
triphenylene derivative by AC conductivity measurement [8], and Simon
and his co-workers also suggested the electronic conduction in a colum-
nar phase of porphirine derivative [9]. Finally, in 1993, the first electro-
nic conduction was established in the hexagonal columnar phase of
triphenylene derivative, H5 T by evaluating carrier mobility by means
of time-of-flight technique. Hole mobility was determined to be about
ꢀ
3
2
1
0
cm =Vs, which is two to three orders of magnitudes higher than
those of conventional amorphous semiconductor practically used [1].
After the discovery of electronic conduction in the discotic liquid
crystals, however, the electrical conduction in the calamitic liquid
crystals had been still believed to be ionic as a result of historical
studies in 1970s and 1980s [10,11]. In 1995, we found that fast hole
conduction took place in the smectic phase of 2-phenylbenzothiazole
derivative as well [2,12], and since then the electronic conduction have
been reported in the smectic mesophases of various calamitic liquid
crystals including phenylnaphthalene, terthiophene, biphenyl, and
so on [13–15].
The electronic conduction has been reported also in solid-like or
highly viscous nematic phases such as liquid crystalline polymers
including polyfluorenes [16,17] and the nematic glasses including
oligofluorenes [18]. However, as for the low viscous nematic phase of
small molecules, the charge carrier transport has been considered to
be ionic so far, as a result of historical studies in 1970s and 1980s.
In fact, the ionic conduction was reported in the nematic phase of
0
0
4
,4 -cyanopentylbiphenyl (5CB) and 4,4 -cyanooctylbiphenyl (8CB)
recently [19,20]. Furthermore, in a recent report that describes

2
52/[1386]
K. Tokunaga et al.
electronic conduction in the cholesteric phase of phenyl-substituted
quaterthiophene derivative, it is emphasized that a large p-conjugated
moiety of phenyl-substituted quaterthiophene favors hopping conduc-
tion over ionic conduction due to an increase in the intermolecular
transfer integral [21].
In fact, the ionic conduction in the nematic phase is suspected to be
extrinsic due to contamination of ionic impurities, but it has not been
clarified yet what the intrinsic charge carrier transport properties is in
the low viscous nematic phase of small molecules.
In this report, in order to answer this question, we have carefully
investigated the charge carrier transport properties in the nematic
phase of small molecules, e.g., 2-phenylbenzothiazole derivatives
exhibiting the nematic phase.
EXPERIMENTAL
0
We selected a 2-phenylbenzothiazole derivatives of 2-(4 -octyloxyphenyl)-
6-butoxybenzothiazole(8O-PBT-O4(6) in Fig. 1), as a model nematic
liquid crystal because of our accumulated experience in purification of
this type of materials and synthesized it according to a synthetic route
0
as shown in Figure 1: We synthesized 2-(4 -octyloxyphenyl)-6-
butoxybenzothiazole(8O-PBT-O4) (6) with a condensation reaction of
2-amino-6-butoxybenzenethiole(5) prepared from p-nitrophenol (1) in 4
steps as described in the synthetic scheme in Figure 1, with p-octyloxy-
ꢁ
benzaldehyde in DMSO at 150 C in 72% yield.
8O-PBT-O4 was purified by flush chromatography and recrystalli-
zation from ethanol and n-hexane several times. The phase transition-
behavior was 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
0
FIGURE 1 Synthetic route of 2-(4 -octyloxyphenyl)-6-butoxybenzothiazole
(
8O-PBT-O4(6)).

1
-D to 3-D Electronic Conduction
253/[1387]
FIGURE 2 Texture images under polarized optical microscope in nematic
phase.
transport properties were studied by time-of-flight experiments. The
purified 8O-PBT-O4 was capillary-filled into liquid crystal cells having
transparent ITO electrodes without a alighment layer. The resulting
liquid crystal cells exhibited a polydomain texture of homogeneous
alignment. A typical cell thickness was 10 mm. The cell was set on a
sample stage maintained at a constant temperature with a PID
controller. The transient photocurrents were measured by the con-
ventional time-of-flight set-up equipped with a nitrogen laser (k ¼
3
37 nm, pulse duration. 600 ps) as an excitation light and recorded
by a digital oscilloscope (Nicolet Pro92. The bias voltage was applied
to the sample with a power supply unit (Keithley 237). The transit
time of carriers, s , was determined from a kink point of the transient
T
photocurrent measured.
RESULT AND DISCUSSION
The phase transition behavior of 8O-PBT-O4 was identified by the
observation of optical texture under polarized microscope and Xray
diffraction study. 8O-PBT-O4 exhibited a nematic phase (N phase)
ꢁ
ꢁ
between 101 C and 124 C. Figure 2 shows a typical texture of
O-PBT-O4 at the temperature range described above, indicating a
8
very texture of the nematic phase. As shown in Figure 3, an X-ray dif-
fraction pattern of 8O-PBT-O4 shows no sharp peak at entire diffrac-
tion angles measured, supporting the nematic phase.
Figure 4 shows the transient photocurrents observed in the nematic
ꢁ
phase of samples in different purification steps at 115 C: in the sample
‘‘not purified enough’’, the transient photocurrent shows a shoulder

2
54/[1388]
K. Tokunaga et al.
FIGURE 3 XRD patterns of 8O-PBT-O4 in nematic phase.
indicating slow transit of positive carriers at around 548.1 ms as shown
ꢀ
5
2
in Figure 4(c), which corresponds to a slow mobility of 10 cm =Vs; in a
‘moderately purified’’ sample as shown in Figure 4(b), the photocurrent
shows two shoulders at different time ranges of 76.4 ms and 546.4 ms,
‘
ꢀ
5
2
which correspond to the slow and fast mobilities of 10 cm =Vs and
ꢀ4
2
10
cm =Vs; in an ‘‘extensively purified’’ sample, the photocurrent
FIGURE 4 Transient photocurrents for positive carriers in the nematic phase
ꢁ
5
of 8O-PBT-O4 at 115 C and 1 ꢂ 10 V=cm. The cell thickness was 10 mm.

1
-D to 3-D Electronic Conduction
255/[1389]
shows a fast transit at around 57.8 ms as shown in Figure 4(a), which is
the same as the fast transit as shown in Figure 4(b) and corresponds to
ꢀ
4
2
ꢀ4
2
a high mobility of 10 cm =Vs. The mobility of 10 cm =Vs that we
found in the nematic phase of 8O-PBT-O4 is rather fast, comparable
to those of less ordered smectic mesophases such as SmA and SmC
phases [12,13], while it is one order of magnitude higher than those
ꢀ
5
ꢀ6
2
of ions in the nematic phases reported to be 10 to 10 cm =Vs.
As for the negative carriers, we observed a fast transit of carriers
accompanied by a slow transit only when 8O-PBT-O4 was further pur-
ified and a thinner cell of 4.8 mm was employed for the measurements.
ꢀ
5
2
The mobility for the fast transit was estimated to be 9.5 ꢂ 10 cm =Vs
close to the fast mobility of positive carriers, while the mobility for the
ꢀ
5
2
slow transit was to be 5.0 ꢂ 10 cm =Vs.
In order to clarify the origin of the fast and slow mobilities, we con-
ducted TOF experiments in diluted samples with a low viscous solvent
such as n-tetradecane (n-C H ) freshly distilled. At the concentra-
1
4
30
tions of n-tetradecane up to 12.5 mol%, neither macrophase separation
of n-tetradecane in the diluted samples nor any significant change of
phase transition temperatures was observed, which was confirmed
by texture observation under a polarized optical microscope : the
phase transition temperature of 5 mol% and 10 mol% diluted
ꢁ
ꢁ
8
O-PBT-O4 with n-tetradecane were at c.a. 122.1 C and 120.7 C in
ꢁ
cooling, which was not so big difference from 124 C in non-doped
8
O-PBT-O4 in cooling. In a diluted sample, the mobility becomes
higher compared with that of non-diluted sample, indicating that
the conduction mechanism is ionic as is expected in the Walden rule
[
22]. On the other hand, the carrier mobility for the fast transit
becomes smaller in the diluted samples, indicating that the conduction
mechanism is electronic as is expected in the hopping transport within
smectic layers. Table 1 shows the summary of the transit times and
mobilities in non-diluted and diluted samples ‘‘moderately purified’’.
It is very clear that the fast transit time becomes longer with an
increase in the concentration of n-tetradecane, while the slow transit
TABLE 1 The Charge Carrier Mobility for Fast and Slow Positive Carriers in
Non-Diluted and Diluted 8O-PBT-O4 at 5 and 10 mol% with n-tetradecane
n-tetradecane
non-diluted
n-tetradecane
5 mol%
n-tetradecane
10 mol%
Fast transit time (msec)
Fast mobility (cm =Vs)
76.4
82.4
89.2
2
1.31 ꢂ 10ꢀ4
1.21 ꢂ 10ꢀ4
1.12 ꢂ 10ꢀ4
Slow transit time (m sec)
546.4
1.83 ꢂ 10ꢀ5
526.4
1.90 ꢂ 10ꢀ5
500.2
1.99 ꢂ 10ꢀ5
2
Slow mobility (cm =Vs)

2
56/[1390]
K. Tokunaga et al.
time becomes shorter. That is, the fast mobility is decreased from
ꢀ
4
2
ꢀ4
2
1
.31 ꢂ 10 cm =Vs to 1.12 ꢂ 10 cm =Vs, while the slow mobility is
ꢀ5 2 ꢀ5 2
increased from 1.83 ꢂ 10 cm =Vs to 1.99 ꢂ 10 cm =Vs. These
results support that the fast mobility is attributed to the electronic
conduction, and the slow mobility to the ionic conduction. According
to the present results, it is concluded that the nematic phase of small
molecules exhibits ambipolar electronic conduction in its intrinsic
nature, irrespective of viscosity and a size of p-conjugated system in
the core moiety.
The ionic conduction is often observed in low viscous liquid crystals.
According to our previous studies on the impurity effect on the charge
carrier transport in the smectic mesophases, we indicate that the ionic
species responsible for the ionic conduction are not always ionic
impurities, but ionized impurity molecules are most possible, which
are formed by photo-ionization of impurity molecules and=or their car-
rier trapping. It should be noted that the electronic conduction in
mesophases is extremely sensitive to chemical impurities responsible
for carrier traps and trace amounts of them, e.g., tens ppm, are good
enough to destroy the electronic conduction in the mesophases. In
addition, there is another reason why the ionic conduction takes place
so easily in the mesophases. That is a microphase-separated structure
of mesophase: in the columnar and smectic mesophases, the
microphase-separated structure provides two conduction channels;
one is a solid-like region consisting of stacked p-conjugated core
moieties for hole and electrons and the other is a liquid-like region
consisting of flexible hydrocarbon chains for ions [23,24]. As for the
nematic phase, it is due to its low viscosity that favors the ionic
conduction.
In the electronic conduction of mesophases, the conduction becomes
anisotropic because of microphase separated structure of the
mesophases: in discotic columnar phases, stacked core moieties having
aromatic p-conjugate systems of a column, which is responsible for
charge carrier transport of electrons and holes, are separated with long
hydrocarbon chains from neighboring columns, so that inter-columnar
conduction of the charge carriers is negligible compared with
intra-columnar conduction of them because of a big difference in the
transfer integral of neighboring molecules for intra- and inter-colums;
this is true for the smectic mesophases also, and inter-smectic layer
conduction of charge carriers is quite negligible compared with the
intra-smectic layer conduction of them. Therefore, the electronic con-
duction become one dimensional for the discotic columnar phases and
two dimensional for smectic mesophases, as illustrated in Figure 5.
In fact, it was demonstrated by various experiments [8,25,26].

1
-D to 3-D Electronic Conduction
257/[1391]
FIGURE 5 The carrier transport properties in various liquid crystal phases
a) discotic phase (b) smectic phase, (c) nematic phase. An intermolecular dis-
(
tance in neighboring molecules dominates a hopping rate of carriers in meso-
phase. Therefore, the dimension of conduction in mesophases is decided by
anisotropy of the intermolecular distance in each mesophase: therefore, it
becomes one-dimension for discotics, two-dimension for smectics, and three-
dimension for nematics.
For the electronic conduction in the nematic phase, however, the
anisotropy of conduction is considered to be considerably small,
especially in small molecules compared with those for columnar and
smectic mesophases, because of moleculare alignment of nematic
phase, i.e., no positional order and a low order parameter. Therefore,
it is quite plausible that the conduction is three dimensional as in
the case of that in the amorphous solids. This should be confirmed
by comparison of the charge carrier mobilities in the homogeneous
and homeotropic alignment cells.
CONCLUSION
We investigated the charge carrier transport in the nematic phase of
small molecules such as 2-phenylbenzothiazole derivatives by time-of-
flight experiments. As a result, we revealed that the intrinsic
conduction mechanism was electronic and ambipolar irrespective of

2
58/[1392]
K. Tokunaga et al.
ꢀ4
2
molecular size, and its mobility was around 10 cm =Vs for electrons
and holes. This discovery gives a new insight into the quest for the
electronic conduction in liquid crystals after continuous efforts to clar-
ify the intrinsic conduction mechanism in liquid crystals over several
decades, leading it to the end.
In conclusion, the intrinsic charge carrier transport in liquid
crystals is electronic and ambipolar irrespective of mesophases and
molecular sizes, which is one dimensional in columnar phases,
two-dimensional in smectic phases, and three-dimensional in nematic
phases.
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