Synthesis and characterization of liquid crystalline semiconducting materials, dialkyl-2-phenylnaphthalene derivatives - Mesomorphic behavior and charge carrier transport

Article abstract of DOI:10.1080/15421400490435251

For establishment of high electronic change carrier transport at ambient temperature, dialkyl derivatives of 2-phenylnaphthalene have been synthesized and characterized their liquid crystalline behaviors and photoelectrical properties. These materials exhibited smectic masophases (Smectic A, B and E phases), at fairly low temperature range extended down below ambient temperature. Fast electronic transport of positive carriers was observed in SmE phase, of 2-(4′-octylphenyl)-6-butylnaphthalene at 30°C, whose mobility was over 10^-2 cm²/Vs.

Full text of DOI:10.1080/15421400490435251

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Synthesis and Characterization
of Liquid Crystalline
Semiconducting Materials,
Dialkyl-2-phenylnaphthalene
Derivatives—Mesomorphic
behavior and Charge carrier
transport
Yukiko Takayashiki a & Jun-Ich Hanna a
a Imaging Science and Engineering Laboratory ,
Tokyo Institute of Technology , Yokohama, Japan
Published online: 18 Oct 2010.
To cite this article: Yukiko Takayashiki & Jun-Ich Hanna (2004) Synthesis and
Characterization of Liquid Crystalline Semiconducting Materials, Dialkyl-2-
phenylnaphthalene Derivatives—Mesomorphic behavior and Charge carrier
transport, Molecular Crystals and Liquid Crystals, 411:1, 265-272, DOI:
10.1080/15421400490435251
To link to this article: http://dx.doi.org/10.1080/15421400490435251
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Mol. Cryst. Liq. Cryst., Vol. 411, pp. 265=[1307]–272=[1314], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490435251
SYNTHESIS AND CHARACTERIZATION OF LIQUID
CRYSTALLINE SEMICONDUCTING MATERIALS,
DIALKYL-2-PHENYLNAPHTHALENE DERIVATIVES—
MESOMORPHIC BEHAVIOR AND CHARGE CARRIER
TRANSPORT
Yukiko Takayashiki and Jun-ich Hanna
Imaging Science and Engineering Laboratory, Tokyo Institute of
Technology, Nagatuta 4259, Midori-ku, Yokohama 226-8503, Japan
For establishment of high electronic charge carrier transport at ambient tem-
perature, dialkyl derivatives of 2-phenylnaphthalene have been synthesized
and characterized their liquid crystalline behaviors and photoelectrical
properties. These materials exhibited smectic mesophases (Smectic A, B and
E phases), at fairly low temperature range extended down below ambient tem-
perature. Fast electronic transport of positive carriers was observed in SmE
phase of 2-(40-octylphenyl)-6-butylnaphthalene at 30ꢀC, whose mobility was
over 10ꢁ2 cm2=Vs.
Keywords: calamitic liquid crystal; carrier transport; hole mobility; photoconductivity;
2-phenylnaphthalene; smectic phase
INTRODUCTION
Recently it was discovered that some types of liquid crystals including not
only the discotics [1] but also the calamitics [2] exhibit very fast electronic
carrier mobility over 10ꢁ2 cm2=V, which is 3 to 4 orders of magnitude
higher than that of the conventional amorphous organic semiconductors
practically used. Liquid crystalline molecules self-organization in columnar
or layered structure, in which the molecules are closely packed and aligned
in some orders. Therefore, now, liquid crystals are being recognized as
The authors are grateful to Dr. Masahiro Funahashi for his useful discussion and guidance
of material synthesis. This study was supported partly by NEDO International Joint Research
Grant.
Address correspondence to Yukiko Takayashiki, Imaging Science and Engineering Labora-
tory, Tokyo Institute of Technology, Nagatuta 4259, Midori-ku, Yokohama 226-8503, Japan.
265=[1307]

266=[1308]
Y. Takayashiki and J. Hanna
a new class of organic semiconductors, i.e., self-organizing molecular
semiconductor (SOMS).
We have reported that as a typical example, 6-(40-alkylphenyl)-2-
alkoxynaphthalene derivatives (m-PNP-On), which are calamitic liquid
crystals [3–10] and exhibit preferentially smectic mesophases (SmA,
SmB, and SmE phases), show the fast charge carrier transport character-
ized by bipolar mobility over 10ꢀ4 cm2=Vs and 10ꢀ2cm2=Vs at best. But
temperature ranges of mesophases in these compounds are higher than
55ꢁC, where the fast carrier transport is observed. Therefore, it would be
very attractive for device application that a material exhibits liquid crystal-
linity and high carrier mobility at ambient temperature: in display applica-
tions, a liquid crystalline mixture is very convenient for reduction of
mesophase temperature down below ambient temperature indeed, but it
is not always acceptable in this new property; this is because the guest
material can be a charge carrier trap or a diluent, often resulting in deterio-
ration of the carrier transport properties. In this study, for establishment
of high electronic carrier transport at ambient temperature, we designed
and synthesized several dialkyl-2-phenylnaphthalene derivatives, i.e.,
2-(40-alkylphenyl)-6- alkylnaphthalene derivatives (m-PNP-n) on the basis
of our previous results of 6-alkoxynaphthalene derivatives. In this report,
we have characterized and discussed their mesophase behaviors and
photoconductive and carrier transport properties in comparison with those
of 6-alkoxynaphthalene derivatives.
SYNTHESIS
We used 2-Methoxynaphthalene as the starting material and derived it to
6-alkyl-2-naphthol via Friedel Craft acylation, and successive hydrazine
reduction, and followed by demethyaltion by hydrobromic acid. And the
resulting 2-naphthols were derived to their triflate and exploited for Suzuki
coupling reaction [11,12] with 4-alkylphenylboronic acid in the presence of
palladium catalyst. as shown in Scheme 1. The resulting dialkyl derivatives
of 2-phenylnaphthalene (m-PNP-n) were purified by flush column chroma-
tography with hexane as eluent, and followed by successive recrystalliza-
tion several times.
CHARACTERIZATION
The mesophoric behaviors in these 6-alkyl-2-phanylnaphthalenes were
studied with observation of optical texture under polarized optical micro-
scope, DSC measurements (Seiko Instruments Inc.) and X-ray diffraction
analysis.

Synthesis and Characterization of Liquid Materials
267=[1309]
SCHEME 1 Synthetic route of Dialkylnaphthalene derivatives.
Phase transition temperature of 6-alkyl-2-phanylnaphthalene are sum-
marized in Table 1. These materials exhibited smectic mesophases such
as smectic A (SmA), SmB, and sometimes SmE phase. These behaviors
were similar to alkoxynaphthalene analogues, but their mesophase tem-
perature ranges were fairly low compared with the alkoxynaphthalene ana-
logues. Some of them exhibited the highly ordered smectic phase below
room temperature. The texture profiles of 8-PNP-4, for example, are shown
in Figure 1, and X-ray diffraction data are shown Table 2, respectively.
Judging from these results, it is clear that the distance of intersmectic
layers is determined by the molecular length and the intermolecular dis-
tance in a smectic layer by the molecular order in the smectic layer but
they did not depend on temperature. These data were almost same as those
of Alkoxynaphthalene analogues.
TABLE 1 Phase Transition Behavior of Dialkylnaphthalene Derivatives
8-PNP-4
8-PNP 6
Cryst. -3.5 SmE 75.2 SmB 88.8 SmA 93.0 Iso.
Cryst. 7.6 SmE 77.2 SmB 98.2 SmA 102.5 Iso.
Cryst. 39.1 SmB 92.7 SmA 97.2 Iso.
8-PNP-12
10-PNP-10
10-PNP-12
Cryst. 67.6 SmB 94.7 SmA 100.4 Iso.
Cryst. 67.4 SmB 92.1 SmA 97.8 Iso.
ꢁ
unit: C.

268=[1310]
Y. Takayashiki and J. Hanna
FIGURE 1 Micrographic texture profiles of 8-PNP-4.
TABLE 2 X ray Diffraction Data for Layer Spacing and Intermolecular Distance in
Smectic Layers in the Smectic B Phase of 6-alkylnaphthalene Derivatives
m–n
Smectic A
34.2
Smectic B
Distance of intermolecules
Crystal
8–12
10–10
35.9
36.5
4.6
4.6
33.4
34.0
˚
unit of length: A
PHOTOCONDUCTIVE MEASUREMENT
All the measurements on photo-electronic properties were carried out
with a symmetrical sandwich-type cell consisting of two indium-tin oxide
(ITO)-coated glass plates rubbed mechanically to realize homogeneous
alignment in liquid crystalline phase. The cell was prepared by capillary-
filled with the liquid crystals under isotropic phase temperature. The cell
thickness was 15 mm. The resulting cell exhibit the polydomain texture
consisting of fairly large fan-like domains on the order of 100 mm, so that
it is quite possible that each domain has no boundaries between two
electrodes. The cell was mounted on a hot stage whose temperature was
controlled by thermocontroller (Mettler corp.) within an accuracy of 0.1ꢁC.
To confirm their photoconductivity, photocurrent in steady–state was
measured under UV light illumination from Xe lamp (500 W) equipped with
a band-pass filter (UV33DS, Toshiba Glass Co. Ltd.) and a IR cut filter. The
light intensity was 2.5 mW=cm2. The carrier transport properties were eval-
uated with a transient photocurrent measured by time-of-flight (TOF)
technique with N2 laser (k ¼ 337 nm, 40 mJ=pulse, pulse width ¼ 600 ps).

Synthesis and Characterization of Liquid Materials
269=[1311]
RESULT AND DISCUSSION
The photocurrent response was observed clearly irrespective of bias
polarity of illuminated electrode in all the phases including isotropic phase.
The dark conductivities in mesophases were estimated to be on the
order of 10ꢀ12 ꢀ 10ꢀ13 S=cm from a linear relation in current-voltage
characteristic at low electric field. The photocurrent was little smaller than
6-alkoxynaphthalene analogues at the same condition. Photocurrent
response and current-voltage characteristics are shown in Figure 2 and
Figure 3, respectively.
The typical transient photocurrents as a function of applied bias are
shown in Figure 4. These curves are a little dispersive. We think that this
is due to insufficient purification of the material. In fact, even in a trace
amount of impurity contamination results in dispersive transport in the
smectic liquid crystals. We could determine clearly the transit time from
the inflection point of in their double logarithmic plots. The carrier mobility
of dialkyl 2-phenylnaphthalene derivatives was on the order of 10ꢀ4 cm2=Vs
in SmA phase, 10ꢀ3 cm2=Vs in SmB phase, and 10ꢀ2 cm2=Vs in SmE phase.
It was absolutely independent of temperature and electric field in each
mesophase, as shown in Figure 5 and Figure 6, and was not influenced even
by the domain size in the cell [13,14]. These were the same results with
alkoxynaphthalene analogues.
The fact that the higher molecular order in mesophase becomes, the
faster carrier mobility in these two series of phenylnaphthalene analogues
indicates that the intermolecular distance between calamitic molecules in
FIGURE 2 Photocurrent response at 31ꢁC in the smectic E phase of 8-PNP-4.
(d ¼ 15 mm, S ¼ 16 mm2, ITO=ITO)

270=[1312]
Y. Takayashiki and J. Hanna
FIGURE 3 Photocurrent response at 31ꢁC in the smectic E phase. of 8-PNP-4.
(d ¼ 15 mm, S ¼ 16 mm2, ITO=ITO)
smectic layer determines the carrier mobility primarily. This is well under-
stood in considering that the carrier transport in the smectic is governed by
hopping transport among the molecular hoping sites.
CONCULUSION
2-(40-alkylphenyl)-6-alkylnaphthalene derivatives that have very simple
and stable molecular structure exhibited smectic mesophases even below
FIGURE 4 Transient photocurrent curves at 31ꢁC in the smectic E phase of
8-PNP-4. (d ¼ 15 mm, S ¼ 100 mm2, ITO=ITO)

Synthesis and Characterization of Liquid Materials
271=[1313]
FIGURE 5 Temperature dependence on hole carrier mobility in the smectic E
phase of 8-PNP-4. (d ¼ 15 mm, S ¼ 100 mm2, ITO=ITO)
room temperature, whose positive carrier mobility was on the order of
10ꢀ2 cm2=Vs in SmE phase irrespective of different carbon chains. This
is the highest mobility ever achieved in calamitic mesophases in a single
material at room temperature. No significant difference in mobility of 6-al-
kyl and alkoxy-2-phenylnaphthalene derivatives in each phase indicates
FIGURE 6 Electric field dependence on hole carrier mobility at 31ꢁC in the smectic
E phase of 8-PNP-4. (d ¼ 15 mm, S ¼ 100 mm2, ITO=ITO)

272=[1314]
Y. Takayashiki and J. Hanna
that the carrier transport is governed by molecular alignment in the smec-
tic layer and affected little by chemical modification of core moiety. The
present results are the first step to get a guiding principle for the materials
design of high mobility self-organizing molecular semiconductor.
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