Synthesis of a novel smectic liquid crystalline glass and characterization of its charge carrier transport properties

Article abstract of DOI:10.1039/c1jm10890e

A new organic semiconductor material with both a self-organized molecular alignment in smectic liquid crystals and a solid nature in amorphous materials, i.e., a glassy liquid crystalline organic semiconductor has been proposed, and its model compound, 1,

Full text of DOI:10.1039/c1jm10890e

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PAPER
Synthesis of a novel smectic liquid crystalline glass and characterization of its
charge carrier transport properties†
a
Jiang Wu, Takayuki Usui and Jun-ichi Hanna
b
ab
*
Received 1st March 2011, Accepted 5th April 2011
DOI: 10.1039/c1jm10890e
A new organic semiconductor material with both a self-organized molecular alignment in smectic
liquid crystals and a solid nature in amorphous materials, i.e., a glassy liquid crystalline
organic semiconductor has been proposed, and its model compound, 1,3,5-benzenetricarboxylic acid,
tris{12-[60-(400-octylphenyl)-20-naphthyloxy]-1-dodecyl ester} (LCG-triester 8-PNP-O12), was
synthesized, and we characterized its phase transition behaviors in slow cooling and rapid cooling. It
exhibited smectic A (SmA) and smectic B (SmBhex) phases in slow cooling, and a SmBhex glassy phase
with a high Tg close to 60 ꢀC when cooled rapidly from the SmA and SmBhex phases. Its charge carrier
transport properties in the smectic mesophases, including the liquid crystal glassy phase, were first
studied by the time-of-flight (TOF) technique, which revealed that the hole mobility in the SmBhex
glassy phase was about 3.5 ꢁ 10ꢂ4 cm2 vꢂ1 sꢂ1, comparable to that in the SmBhex liquid crystal phase.
We concluded that the high charge transport properties of the smectic mesophase are successfully
maintained in the solid glassy state.
polycrystalline thin films, which is comparable to that of amor-
Introduction
phous silicon—used for TFT arrays for liquid crystal displays,
renewed attention has been focused on organic polycrystalline
thin films for TFT applications such as e-papers and radio
frequency identification (RFID) tags.
In the past decades, amorphous films of organic photo-
conductors, including molecularly doped polymers, e.g., triaryl-
amine derivatives, dispersed in a polymer matrix have been used
as important components of photoreceptors for electrophoto-
graphic copiers and laser printers.1 Their application has been
extended for charge transport layers for organic light emitting
diodes (OLEDs).2 In these amorphous materials, which are
characterized as molecular aggregates without any orientational
or positional order, charge transport properties are described by
hopping conduction in energetically and spatially distributed
localized states consisting of aromatic molecular moieties of the
materials.3 Therefore, the mobility is strongly field and
temperature dependent, and is typically in the order of 10ꢂ6 to
On the other hand, there has also been considerable interest in
carrier transport properties in liquid crystals,8–14 which comes
from the basic idea that the molecular alignment of liquid crys-
tals should be beneficial to the fast carrier transport.15 In fact, the
charge carrier transport properties of discotic hexagonal liquid
crystalline phases (Dh) of phthalocyanine and porphyrin deriv-
atives were studied by microwave conductivity measurement.16
In the 1990s, the electronic conduction was established in both
the discotic columnar phase of hexakis n-pentyloxy-
triphenylene17,18 and a smectic phase of 2-phenylbenzothiazole
derivatives, these discoveries gave us a new insight into the fact
10ꢂ5 cm2 vꢂ1 sꢂ1 4,5
This mobility is 6 orders of magnitude smaller
.
than that of their single crystals, in which all the molecules are
closely packed in a crystal lattice and aligned in a long-range
order and the charge carrier transport properties are described by
the band conduction, especially at low temperatures.6,7 Because
of the high mobility of around 0.5 cm2 vꢂ1 sꢂ1 in organic
that the liquid crystal is
conductor exhibiting self-organization and a high mobility over
10ꢂ3 cm2 vꢂ1 sꢂ1 19,20
Indeed, liquid crystalline materials are quite
a new type of organic semi-
.
attractive as an organic semiconductor because of their high
mobility, which is superior to amorphous materials,21 easy
control of the molecular orientation22 and their electrically less
active domain boundaries and disclinations23 which are superior
to polycrystalline materials. Thus, we have investigated the
charge carrier transport properties in various liquid crystals in
order to establish their firm basis for industrial applications to
organic devices after the discovery of their electronic conduction
in the smectic mesophases.24,25
aImaging Science and Engineering Laboratory, Tokyo Institute of
Technology, Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan.
E-mail: hanna@isl.titech.ac.jp; Fax: +81-45-924-5176; Tel: +81-45-924-
5176
bJST-CREST, Kawaguchi, Saitama, Japan
† Electronic supplementary information (ESI) available: sample
preparations for TOF measurement are shown in Figs. 1 & 2. The
TOF experimental setup is shown in Fig. 3. The molecular alignment
in liquid crystal cell is described in Fig. 4. See DOI: 10.1039/c1jm10890e
However, there remain a few questions to be answered about
the charge carrier transport properties in the liquid crystals: how
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does the molecular disorder, which is caused as a result of
molecular motion in a liquid-like phase, affect the charge carrier
transport in the mesophases? How does the dynamic motion of
the molecules affect it? Thus, we are interested in a new material
that has molecular alignment in the liquid crystal mesophase and
frozen molecular motion, i.e., a liquid crystal glass (LCG).
Bearing this in mind, we have designed a smectic liquid crystal
glass based on a 2-phenylnaphthalene derivative as a model.
In this report, we describe the design and synthesis of the new
smectic glass and its phase transition behavior, in addition to
preliminary results on its charge carrier transport properties.
Material design
Scheme 2 Synthetic route of LCG-triester 8-PNP-O12 (I).
Liquid crystal mesophases have a higher degree of sophistication
in the molecular order than amorphous glasses, where all the
molecules sit randomly without any molecular orientational or
positional order, so they crystalize easily when the temperature
goes down, particularly small molecules. Therefore, we need
a subsidiary structure to suppress the crystallization and induce
a smectic glassy phase, except for nematic glasses of p-conju-
gated main chain polymers and oligomers. In fact, the dimer
structure was introduced in order to have a glassy columnar
mesophase in a triphenylene derivative,26 and a trimer structure
for a nematic glassy phase in a triester of 1,3,5-benzene-
tricarboxylic acid.27
carbonate solution (3 mL) was purged with argon gas before
adding 4-octylbenzeneboronic acid (737 mg, 3.15 mmol),
compound 3 (1143 mg, 3 mmol) and tetrakis(triphenyl-phos-
phine)palladium(0) (104 mg, 0.09 mmol). The reaction mixture
was heated to 100 ꢀC under argon for 6 h and the aqueous layer
was extracted by CHCl3. The combined organic layers were
washed with water, dried over MgSO4 and filtered. The filtrate
was evaporated and the crude product was purified by column
chromatography (silica gel) using CHCl3 (Rf ¼ 0.4) as an eluent,
followed by recrystallization from hexane to give 5 as a colorless
crystal (774 mg, 50% yield).
1H NMR (500 MHz, CDCl3, TMS), d (ppm) ¼ 7.12–7.97
(m. 10H), 4.07 (t, 2H), 3.63 (t, 2H), 2.65 (t, 2H), 1.97 (quint, 2H),
1.82 (quint, 2H), 1.63 (quint, 2H) 1.20–1.50 (m, 26H), 0.88 (t,
3H). 13C NMR (125 MHz, CDCl3, TMS), d (ppm) ¼ 157.19,
141.89, 138.55, 136.25, 133.70, 129.16, 128.88, 127.11, 127.01,
125.94, 125.27, 119.38, 106.45, 68.09, 63.09, 35.64, 32.83, 31.91,
31.53, 29.61, 29.58, 29.52, 29.44, 29.43, 29.41, 29.29,26.13, 25.75,
22.69, 14.10.
Thus, we adopted a trimer structure based on benzene tricar-
boxylic acid as the subsidiary structure unit and the 2-(40-octyl-
phenyl)-6-dodecyloxy-naphthalene (8-PNP-O12) moiety as
a mesogen unit for a model material, 1,3,5-benzenetricarboxylic
acid,
tris{12-[60-(400-octylphenyl)-20-naphthyloxy]-1-dodecyl
ester} (LCG-triester 8-PNP-O12), shown in Scheme 1.
Experimental
ESI-TOF-MS: Calcd. For C36H52O2 [M]+: 516.3857; Found:
1. Material synthesis
516.3860.
All the solvent and reagents were obtained from Tokyo Chemical
Industry. Co. Ltd or Sigma-Aldrich Chemical Co., and used
1,3,5-benzenetricarboxylic acid, tris{12-[60-(400-octylphenyl)-20-
naphthyloxy]-1-dodecyl ester} (I). A solution of 1,3,5-benzene-
tricarbonyl trichloride (46 mg, 0.175 mmol), compound 5
(280 mg, 0.543 mmol), and 4-dimethylaminopyridine (DMAP)
(86 mg, 0.706ꢀmmol) in 20 mL of anhydrous dichloroethane was
heated at 60 C for 3 h. The reaction mixture was then poured
into cold water. The precipitate was filtered and dried in
a vacuum oven. Further purification was accomplished by silica
gel column chromatography, with CHCl3 (Rf ¼ 0.5) as the eluent,
followed by recrystallization from dichloromethane to give I as
a colorless solid (147 mg, 49% yield).
without further purification. Compounds
1 and 2 were
purchased. Compounds 328 and 529 were prepared by following
the literature. Our model compound, LCG-triester 8-PNP-O12
(I) was prepared from the ester condensation of benzene tricar-
boxylic chloride with an alcohol of 8-PNP-O12, as shown in the
Scheme 2. The isolated product (I) was purified carefully by flash
column chromatography and recrystallized from dichloroform.
All 1H and 13C NMR were measured on JEOL-JNM-WinLa -500
and HRMS were measured on a Bruker micro TOFII at ESI
mode.
1H NMR (500 MHz, CDCl3, TMS), d (ppm) ¼ 8.84 (s, 3H),
7.12–7.95 (m, 30H), 4.36 (t, 6H), 4.06 (t, 6H), 2.65 (t, 6H), 1.83
(quint, 6H), 1.78 (quint, 6H), 1.65 (quint, 6H), 1.20–1.50 (m,
78H), 0.88 (t, 9H). 13C NMR (125 MHz, CDCl3, TMS),
d (ppm) ¼ 165.12, 157.13, 141.86, 138.48, 136.17, 134.41, 133.64,
131.47, 129.54, 129.09, 128.86, 127.09, 126.98, 125.91, 125.24,
119.36, 106.30, 68.02, 65.85, 35.63, 31.90, 31.55, 29.58, 29.51,
29.41, 29.29, 29.27, 29.26, 28.65, 26.12, 25.98, 22.68,14.12.
12-[6-(4-octylphenyl)-2-naphthyloxy]-1-dodecanol
(5).
A
biphasic mixture of toluene (11 mL), and a 2 M sodium
ESI-TOF-MS: Calcd. For
Found: 1705.1642.
C
117H156O9 [M]+:1705.1596;
Scheme 1 The molecular structure of LCG-triester 8-PNP-O12.
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2. Measurements
The phase transition behavior was determined by differential
scanning calorimetry (DSC, Seiko Inst. DSC 5400C) experiments
calibrated with standard samples of indium.
The polarizing optical microscopy (POM) study was carried
out on a Nikon optical polarizing microscope equipped with
a Mettler Toledo FP900 hot stage. Each powder-like sample was
placed on a glass slide and covered with another glass cover slip.
The X-ray diffractometry (XRD) experiment was carried out
on a Rigaku RAD-2B to identify the phase structure at different
temperatures.
Transient photocurrents were measured by a time-of-flight
(TOF) technique with an N2 pulse laser for photoexcitation.
Briefly, the compound was filled by capillary action into a liquid
crystal cell which was pre-coated by indium tin oxide (ITO) as the
electrode. A DC power supply was used to provide the necessary
bias voltage to the sample for the relevant carrier detection. A N2
laser (l ¼ 337 nm, pulse duration ¼ 600 ps) was used for the
photogeneration of free charges near one side of the sample, and
the resulting transient photocurrent was recorded in a digital
oscilloscope (Nicolet Accura 100). A current sensing resistor R in
series with the sample for converting and magnifying the
photocurrent to voltage readings was adjusted between 1 and
1 MU so that the RC time constant should be at least 20 times less
than the carrier transient time in order to suppress distortion of
the transient photocurrent.
Fig. 2 DSC traces of LCG-triester 8-PNP-O12 under fast cooling and
heating at 40 ꢀCminꢂ1. Symbols: Tg: glass transition temperature; G:
glassy state; SmA; smectic A phase; SmB: smectic B phase; K: crystal
phase; I: isotropic phase.
near 1ꢀ08 ꢀC, followed by melting into an isotropic liquid
at 138 C.
On the other hand, if we treated this sample with different
cooling rates, we observed different phase transition behaviors.
For fast cooling with a rate of 40 C minꢂ1 from the isotropic
ꢀ
ꢀ
phase, there also are two mesophases, from 127 to 101 C and
from 101 to 68 ꢀC, which was shown in Fig. 2. These phases
correspond to the two smectic mesophases described above. By
continuously cooling the sample at a high rate of 40 ꢀC minꢂ1, it
was frozen into the glass state without crystallization. A second
order transition obtained for the heating run at approximately
60 ꢀC shows a small change in the heat capacity Cp for the glass
transition, crystallization near 100 ꢀC and melting into an
Results and discussion
Thermotropic property
ꢀ
isotropic liquid at 140 C.
The phase transition temperature of LCG-triester 8-PNP-O12
was studied by DSC measurement, as shown in Fig. 1 and Fig. 2.
We also studied the phase transition behaviors of its corre-
sponding monomer, 2-(40-octylphenyl)-6-dodecyloxy-naphtha-
lene (8-PNP-O12), which exhibits an SmA and SmBhex liquid
crystalline phase between 79 and 100 ꢀC, 100 and 121 ꢀC,
respectively,30 although no glass behavior was observed in this
monomer. However, the LCG-triester 8-PNP-O12, in which the
mesogenic pendant 2-phenylnaphthalene was chemically bonded
to a central core, is able to form a glass state and is morpho-
logical stable for over two years under ambient atmosphere.
In the slow cooling with a rate of 5 C minꢂ1, therꢀe were two
ꢀ
ꢀ
mesophases from 131 to 106 C and from 106 to 77 C, respec-
tively, which was shown in Fig. 1. Judging from the heat capacity
at these transitions, the two mesophases are smectic phases,
which are identified as smectic A (SmA) and SmBhex phases by
the XRD measurement described later on. In the slow heating
cycle, it does not show any mesophase transition and crystallized
Optical texture and phase identification
As the results of DSC measurement, LCG-triester 8-PNP-O12
exhibits two mesophases when it was cooled from isotropic
liquid. To realize the texture in each phase, we loaded it into
a liquid crystal cell and observed its texture under POM, as
shown in Fig. 3.
Firstly, we melted the material to the isotropic phase by
Mettler Toledo FP900 hot stage with an accuracy of 1 K, and
then cooled it to room temperature at different cooling rates.
Fig. 3(a) shows a POM image at 120 ꢀC. It exhibits a focal conic
defect with a fan-like texture for the SmA phase, which is
a typical so-called smectic texture. (b) is the following smectic
mesophase at 90 ꢀC not showing any big changes in the texture.
When we quenched the sample from 90 ꢀC to room temperature,
we got a glassy phase without encountering crystallization (c),
Fig. 1 DSC traces of LCG-triester 8-PNP-O12 under slow cooling and
heating at 5 ꢀC minꢂ1. Symbols: SmA; smectic A phase; SmB: smectic B
phase; K: crystal phase; I: isotropic phase.
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Fig. 3 (a) POM image of LCG-triester 8-PNP-O12 loaded into a liquid
crystal cell at 120 ꢀC. (b) POM image at 90 ꢀC. (c) POM image at 25 ꢀC by
fast cooling. (d) POM image at 25 ꢀC by slow cooling.
it is clear that the texture was almost the same as in (b), which
indicated that the texture was successfully kept unchanged
through glass transition. However, if we cooled the sample
slowly with a cooling rate of 1 ꢀC minꢂ1, the sample became
crystallized and exhibit clear cracks among grains (d).
The phase structures were also examined by an X-ray
diffraction study (XRD) at different temperatures, as shown in
Fig. 4. For (a), the broad diffraction peak at the high diffraction
angle region reveals that the mesophase at 120 ꢀC is a SmA
phase. For (b), A sharp diffraction peak at 19.2ꢀ with a lattice
ꢀ
constant of 4.6 A indicates the existence of a hexagonal lattice in
the mesophase at 90 ꢀC, which is a typical pattern for a SmBhex
liquid crystal phase. By quenching the sample to room temper-
ature from the SmBhex phase, it formed a glass state and the
XRD pattern was shown in Fig. 4(c), where a relatively sharp
diffraction peak exits at 19.5ꢀ. The molecular alignment in the
SmBhex phase was well maintained in the solid state, i.e., a glassy
state of SmBhex. We tried to form isotropic and SmA glassy states
by cooling the cell from either isotropic or SmA phases at various
cooling rates up to 500 ꢀC minꢂ1. However, all the trials resulted
in the SmBhex glassy state. It is worth noting that only the SmBhex
glassy state appeared when rapidly cooled, irrespective of cooling
rates. This suggests the possible appearance of even higher
ordered smectic glassy states in a better structure of liquid
crystals.
On the other hand, when the sample was cooled very slowly
from the SmBhex phase to room temperature, the sample
crystallized to form a polycrystal with an XRD pattern, as shown
in (d).
To the best of our knowledge, this is the first example of glassy
SmBhex. The glassy SmA phase was reported elsewhere.31
Fig. 4 XRD patterns of LCG-triester 8-PNP-O12 (a) at 120 ꢀC, (b) at
90 ꢀC, (c) at 25 ꢀC by fast cooling. (d) at 25 ꢀC by slow cooling.
Charge carrier transport properties
In order to characterize the charge carrier transport properties of
LCG-triester 8-PNP-O12, transient photocurrents were
measured by TOF experiments. The LCG-triester 8-PNP-O12
was placed between two glass plates which were precoated with
ITO electrodes. The resulting cell showed planar alignment
where the long axis of the pendant moiety of 2-phenyl-
naphthalene is planar to the glass surface (see ESI†). The
thickness of the cell was 15 mm. Pulsed (600 ps) nitrogen laser
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irradiation (337 nm wavelength) in the absorption band of the
LCG-triester 8-PNP-O12 leads to the generation of free electrons
and holes in the electrically biased cell. Depending upon the
polarity of the external electric field, electrons or holes will drift
across the sample cell, causing displacement currents which can
be recorded in an external circuit as shown in Fig. 5. For details
of the sample preparation and of the experimental setup see
ESI.†
The charge carrier mobility m, which characterizes the
macroscopic charge transport properties, is calculated by the
well-known eqn (1), where d is the sample thickness and E is the
electric field, V is a given applied voltage and tT is the transit
time.
d
d2
m ¼
¼
(1)
EtT VtT
Transient photocurrents for hole transport of LCG-triester
8-PNP-O12 in SmA and SmBhex phase are shown in Fig. 5. We
could not observe any transient photocurrent attributed to the
drift of photogenerated negative charges, i.e., electron transport.
Judging from the constant photocurrent before transit time and
its simple decay down to a few ms shown in the inset of Fig. 5(a)
for the SmA phase, the LCG-triester 8-PNP-O12 is highly puri-
fied and the concentration of electrically active impurities in it is
quite small, i.e., less than a few ppm.32,33 All the transient
currents in Fig. 5 are so-called ‘‘non-dispersive’’, where the
dispersion of carrier velocity is very small, so that the transit
time, tT, can be well defined and easily determined from
a shoulder in each photocurrent.
On the other hand, the initial photocurrent goes down sharply
after illumination by a light pulse and then the current increases
before the transit time again. This is probably due to charges
trapped at the interface of the electrode and the thermal
detrapping of trapped charges at the trap states. The hole
mobility of SmA and SmBhex phase is determined to be 1.25 ꢁ
10ꢂ4 cm2 Vꢂ1 sꢂ1 and 5 ꢁ 10ꢂ4 cm2 Vꢂ1sꢂ1 at 25 V, respectively.
The mobility in the SmBhex phase is about 4 times higher than
that for the SmA phase, which is attributed to the higher
molecular alignment i.e., shorter intermolecular distance in the
SmB phase, supported by XRD study.
Fig. 6(a) shows transient photocurrents of holes in the SmBhex
glassy phase (55 ꢀC) which was formed by rapid cooling from
Fig. 5 Transient photocurrent of hole transport (a) in SmA phase
(120 ꢀC); (b) in SmB phase (80 ꢀC) of LCG-triester 8-PNP-O12 in a 15 mm
thick glass cell precoated by an ITO electrode at various applied voltages.
The inset is the double logarithmic plot of respective transient photo-
currents as a function of time.
Fig. 6 Transient photocurrents of holes (a) in the SmBhex glassy phase
(55 ꢀC); (b) in the polycrystalline phase (55 ꢀC), of LCG-triester 8-PNP-
O12 in a 15 mm thick glass cell pre-coated by an ITO electrode at various
applied voltages. The inset shows the double logarithmic plot of respec-
tive transient photocurrents as a function of time.
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reported elsewhere soon. These properties indicate that the
smectic liquid crystal glass provides us with solid self-organized
molecular aggregates while keeping easy control over the
molecular alignment in smectic liquid crystals, which shows
promising advantages over both amorphous and crystalline
materials as a new type of organic semiconductors.
Table 1 Hole mobility of LCG-triester 8-PNP-O12 in different phases
Hole mobility
cmꢂ2 vꢂ1 sꢂ1 (at 25 V)
Phase
SmA
SmBhex
SmBhex glass
Polycrystal
1.25 ꢁ 10ꢂ4
5 ꢁ 10ꢂ4
3.5 ꢁ 10ꢂ4
—
Acknowledgements
The authors would like to thank Dr Hiroaki Iino and Dr Akira
Ohno for helpful discussions regarding experimental analysis,
and to thank Center for Advanced Materials Analysis (Suzuka-
kedai), Technical Department, Tokyo Institute of Technology,
for MS analysis. The authors are also grateful for the financial
supports from a Core Research for Evolutional Science and
Technology (CREST) program sponsored by Japan Science and
Technology Agency.
SmBhex phase. Each photocurrent is also non-dispersive and
shows a clear shoulder. The hole mobility is estimated to be 3.5 ꢁ
10ꢂ4 cm2 Vꢂ1 sꢂ1 at 25 V. This mobility is almost the same as that
in SmBhex phase, indicating that the charge transport properties
of the SmB phase are successfully maintained in solid state, i.e.,
glassy SmBhex phase.
On the other hand, the transient photocurrents in the poly-
crystalline phase decay rapidly as shown in Fig. 6(b). It indicates
that there are lots of defects or deep trap states in the bulk to stop
the carrier transport, which are possibly caused by grain
boundaries among the polycrystallites. Therefore, the carrier
transport properties were dramatically decreased in the poly-
crystal phase. We summarize the charge carrier transport prop-
erties of LCG-triester 8-PNP-O12 in Table 1.
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