Photorefractive effect of photoconductive-polymer-stabilized ferroelectric liquid crystals

Article abstract of DOI:10.1080/15421400902841361

The photorefractive effect of photoconductive-polymer-stabilized ferroelectric liquid crystals (FLCs) was investigated. A photoconductive acrylate monomer and an electron acceptor compound were mixed with an FLC, and the mixtures were photopolymerized in

Full text of DOI:10.1080/15421400902841361

This article was downloaded by: [UQ Library]
On: 19 September 2013, At: 00:22
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
Photorefractive Effect of
Photoconductive-Polymer-
Stabilized Ferroelectric Liquid
Crystals
Takeo Sasaki a , Hirotada Hazato a , Atsushi Katsuragi
a & Yukihito Nakazawa a
a Department of Chemistry, Faculty of Science,
Tokyo University of Science, Tokyo, Japan
Published online: 03 Jun 2009.
To cite this article: Takeo Sasaki , Hirotada Hazato , Atsushi Katsuragi & Yukihito
Nakazawa (2009) Photorefractive Effect of Photoconductive-Polymer-Stabilized
Ferroelectric Liquid Crystals, Molecular Crystals and Liquid Crystals, 503:1, 81-98,
DOI: 10.1080/15421400902841361
To link to this article: http://dx.doi.org/10.1080/15421400902841361
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the
information (the “Content”) contained in the publications on our platform.
However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness,
or suitability for any purpose of the Content. Any opinions and views
expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the
Content should not be relied upon and should be independently verified with
primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages,
and other liabilities whatsoever or howsoever caused arising directly or

indirectly in connection with, in relation to or arising out of the use of the
Content.
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. Terms & Conditions of access and use can be found at
http://www.tandfonline.com/page/terms-and-conditions

Mol. Cryst. Liq. Cryst., Vol. 503, 81–98, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400902841361
Photorefractive Effect of Photoconductive-
Polymer-Stabilized Ferroelectric Liquid Crystals
Takeo Sasaki, Hirotada Hazato, Atsushi Katsuragi,
and Yukihito Nakazawa
Department of Chemistry, Faculty of Science, Tokyo University of
Science, Tokyo, Japan
The photorefractive effect of photoconductive-polymer-stabilized ferroelectric
liquid crystals (FLCs) was investigated. A photoconductive acrylate monomer
and an electron acceptor compound were mixed with an FLC, and the mixtures
were photopolymerized in the ferroelectric phase. A photoconductive polymer net-
work was formed in the FLC medium, and a polymer-stabilized FLC was obtained.
The photorefractive effect was evaluated by a two-beam coupling experiment. A
large gain coefficient was obtained in the polymer-stabilized FLCs.
Keywords: ferroelectric liquid crystals; optical hysteresis; photorefractive effect;
polymer stabilization; two-beam coupling
1. INTRODUCTION
The photorefractive effect is a nonlinear optical effect which induces
a change in the refractive index of a material by illuminating it with
two interfering laser beams [1–4]. This effect enables the formation
of dynamic volume holograms. The photorefractive effect in organic
materials has been studied actively since the 1990s. Liquid crystals
are attractive materials for the development of high performance
photorefractive materials [5]. A liquid crystal mixed with photo-
charge generating compounds has been shown to exhibit the photore-
fractive effect [6–13]. The mechanism of the photorefractive effect in
liquid crystals is as follows. The interference of two laser beams in a
Address correspondence to Takeo Sasaki, Department of Chemistry, Faculty of
Science, Tokyo University of Science, 1-3 Kaguraztaka, Shinjuku-ku, Tokyo 162-8601,
Japan. E-mail: sasaki@rs.kagu.tus.ac.jp
81

82
T. Sasaki et al.
photorefractive liquid crystal induces electron transfer between
donor molecules and acceptor molecules, thereby positive and
negative charges (ions or hole=electron pairs) are generated at the
light positions of the interference fringe. In liquid crystalline photo-
refractive materials, it is believed that photogenerated ions mainly
contribute to the photorefractive effect [11–13]. These photogener-
ated ions are transported by diffusion and=or by an externally
applied electric field, leading to separation of cations and anions as
a result of the difference in the mobilities of the ions. While ions with
small mobilities remains at the light positions of the interference
fringes, counter ions with larger mobilities diffuse to all parts of
the material. As a result, the light positions and the dark positions
are charged in opposite polarizations, yielding a periodic space-
charge field (internal electric field) along the interference fringes.
The directors of the liquid crystal are altered by this internal electric
field. Thus, an orientational grating of liquid crystal molecules is
formed. This grating is capable of diffracting laser beams. The
apparent change in the refractive index of liquid crystals produced
by the photorefractive effect is large compared to polymers and
inorganic materials.
In recent studies, the photorefractive effect in ferroelectric liquid
crystals (FLCs) has attracted significant attention [8–13]. In FLCs,
spontaneous polarization responds to the internal electric field and a
refractive index grating is formed. The response of FLCs to the
induced internal electric field is very fast, and refractive index grating
formation times of 10–20 ms were obtained in FLCs [9,10]. FLCs are
essentially liquid crystals that form the chiral smectic C phase (SmCꢀ)
[14,15]. The SmCꢀ possesses a helical structure so that the SmCꢀ
phase itself does not exhibit ferroelectricity. However, when the FLC
is sandwiched between glass plates to give a film thickness of
ꢀ
2–10 mm, the helical structure of the SC phase unravels and forms a
surface-stabilized state. The ferroelectricity of FLCs appears only in
the surface-stabilized state. The preparation of a defect-free surface
stabilized state for FLCs is difficult and requires sophisticated
techniques. The presence of a defect however leads to a smaller
photorefractive effect [9]. It has been reported that a defect-free FLC
material was obtained in a polymer-stabilized FLC (PS-FLCs)
[16,17]. PS-FLC materials are prepared by photopolymerization of a
mixture of FLC, LC monomer, and photoinitiator with the application
of a monopolar electric field at a temperature, where the FLC material
is in the SmCꢀ phase. Polymer-stabilized FLCs exhibit unique electro-
optical properties. The direction of the spontaneous polarization is
stabilized to the direction of the applied electric field during the

PR Effect of Polymer-Stabilized FLCs
83
photopolymerization through interaction between FLC molecules and
the polymer networks.
In this study, the photorefractive effect in a photoconductive polymer-
stabilized FLC was investigated. A series of photoconductive methacry-
late monomers was synthesized. The photoconductive monomer and an
electron acceptor compound were mixed with an FLC and photopoly-
merized. The effects of polymer stabilization on the photorefractive
effect were investigated.
2. EXPERIMENTAL
2.1. Materials
A commercially available FLC, FELIX-M4851=050 (Clariant Co.), was
used in this study. The physical properties of this FLC are listed in
Table 1. The structures of the photoconductive monomers (mTPA-n),
photoconductive polymers (pTPA-n), and the electron acceptor com-
pound (TNF) used in the study are shown in Fig. 2. The synthetic
route for mTPA-3 is shown in Fig. 3. IRGACURE-369 (Chiba Specialty
Chem.) was used as a photoinitiator. The polymer-stabilized FLC was
prepared as follows. A mixture of FLC, mTPA-n (2–10 wt%), photoini-
tiator (1 wt%), and TNF (0.1 wt%) was injected into a 5 mm–gap glass
cell equipped with an ITO electrode and a polyimide alignment layer.
In order to form a highly homogeneous surface-stabilized state, the
samples were heated to the isotropic phase and deliberately cooled
ꢀ
to the SC phase at a rate of 0.1ꢁC=min using a hot plate (Mettler
FP-80 and FP-82). The monomer=FLC mixture was then photopoly-
merized; 365 nm light from an ultra high pressure mercury lamp
(10 mW=cm2) was irradiated on to the sample for 5 minutes. An
electric field of ꢂ2.0 V=mm was applied to the mixture during the
photopolymerization. The polarity of the electric field is defined as
shown in Fig. 4.
TABLE 1 Physical Properties of the FLC Used in this Study
Phase transition
temperaturea
(ꢁC)
Rotational
viscosity
(mPas)
Cone
angle
(deg.)
Ps at 25ꢁC
(nC=cm2)
FLC
FELIX-M4851=050
ꢂ14
ꢂScꢀ 60 SA 80 Nꢀ 104 I
65
56
aScꢀ ¼ chiral smectic C phase; SA ¼ smectic A phase; Nꢀ ¼ chiral nematic phase;
I ¼ isotropic phase.

84
T. Sasaki et al.
FIGURE 1 Schematic illustration of the mechanism of the photorefractive
effect: (a) two laser beams interfere in the photorefractive material; (b) charge
generation occurs in the bright areas of the interference fringes; (c) a charge
separated state is formed between the light and the dark positions of the inter-
ference fringes because of the difference in the mobilities of the positive and
the negative charges, thereby generating an internal electric field between
the bright and dark positions; (d) change in the refractive index is induced
by the internal electric field.

PR Effect of Polymer-Stabilized FLCs
85
FIGURE 2 Structures of the photoconductive monomers, polymers, and
electron acceptor (TNF).

86
T. Sasaki et al.
FIGURE 3 Synthetic route for mTPA-3.
FIGURE 4 Schematic illustration of the beam incidence condition, definition
of the direction of the externally applied electric field, and the set-up for the
two-beam coupling experiment.

PR Effect of Polymer-Stabilized FLCs
87
Synthesis of Photoconductive Monomer mTPA-3
2-Methyl-acrylic acid 6-chloro-hexyl ester. 6-Choloro-1-hexanol
(10 g, 90 mmol) was dissolved in 50 mL of chloroform and 1 mL of pyr-
idine was added to this. A chloroform solution of methacrylchloride
(9.2 g, 88 mmol) was added dropwise to the solution, which was stirred
for 3 hr at ambient temperature. The solution was then washed with
an aqueous solution of NaHCO3. The chloroform solution was concen-
trated and the product was purified by silica gel chromatography
(eluent: chloroform). A colorless liquid was consequently obtained with
a yield of 78%.
1H-NMR (CDCl3 400 MHz): d6.08 (s, 1H, –C¼CH–H), 5.55 (s, 1H,
ꢂC¼CH–H), 4.14 (t, 2H, –CH2–OOC–), 3.53 (t, 2H, Cl–CH2–), 1.94
(s, 3H, CH2¼C–(CH3)–), 1.82–1.37 (m, 8H, –CH2–).
2-Methyl-acrylic acid 6-(40-hydroxy-biphenyl-4-yloxy)-hexyl ester.
4,40-Biphenol (9.3 g, 50 mmol) was dissolved in 100 mL of dry DMF.
2-Methyl-acrylic acid 6-chloro-hexyl ester (5 g, 24 mmol) and 12 g of
potassium carbonate were added to the solution, which was stirred
for 10 hr at 50ꢁC. After filtration of the potassium carbonate, DMF
was evaporated. The resultant product was dissolved in 200 mL of
chloroform and the insoluble component was removed by filtration.
The chloroform solution was evaporated and the product was purified
by silica gel chromatography (eluent: 3:1 solution of chloroform and
ethyl acetate). A colorless liquid was obtained. yield: 45%.
1H-NMR (CDCl3 400 MHz): d7.43 (d, 2H, Ar–H), 7.29 (d, 2H, Ar–H),
6.92 (d, 2H, Ar–H), 6.88 (d, 2H, Ar–H), 6.11 (s, 1H, –C¼CH–H), 5.55 (s,
1H, –C¼CH–H), 4.17 (t, 2H, Ar–O–CH2–), 3.98 (t, 2H, –CH2–OOC–),
1.95 (s, 3H, CH2¼C(CH3)–), 1.84–1.41 (m, 8H, –CH2–).
4-Diphenylamino-benzoic
acid
40-[6-(2-methyl-acryloyloxy)-
hexyloxy]-biphenyl-4-yl ester (mTPA-3). 4-Diphenylamino-benzoic
acid (3.5 g, 11 mmol), 2-methyl-acrylic acid 6-(4-hydroxy-biphenyl-
4-yloxy)-hexyl ester (2.6 g, 7.3 mmol), N,N0-dicyclohexylcarbodiimide
(2.5 g, 11 mmol), and 4-dimethylamino-pyridine (0.5 g, 4.1 mmol) were
dissolved in 100 mL of chloroform, and the solution was stirred for
16 hr at 25ꢁC. A 1 N aqueous solution of HCl was added to this, and
the precipitant was removed by filtration. The solution was washed
with an aqueous solution of NaHCO3 and dried over Na2SO4.
After removal of the solvent, the product was purified by silica gel
chromatography (eluent: 3:1 solution of hexan and ethyl acetate)
and recrystallized from 2-propanol. Yield: 40%.
1H-NMR (CDCl3 400 MHz): d8.05 (d, 2H, Ar–H), 7.56 (d, 2H, Ar–H),
7.50 (d, 2H, Ar–H), 7.33 (m, 4H, Ar–H), 7.24 (m, 4H, Ar–H), 7.03 (d, 2H,

88
T. Sasaki et al.
Ar–H), 6.96 (d, 2H, Ar–H), 6.89 (d, 2H, Ar–H), 6.09 (s, 1H, –C¼CH–H),
5.54 (s, 1H, –C¼CH–H), 4.17 (t, 2H, Ar–COO–CH2–), 4.00 (t, 2H,
–CH2–OOC–), 1.94 (s, 3H, CH2 ¼ C(CH3)–), 1.86–1.45 (m, 8H, –CH2–).
Anal. Calcd for C41H39NO5 calc. C, 78.70; H, 6.28; N, 2.24. Found: C,
78.42; H, 6.22; N, 2.41.
2.2. Measurements
The photopolymerization of the monomer in the FLC medium was con-
firmed by gel permeation chromatography (GPC; TOSOH HLC-8220
with Super Multipore HZ-M, eluent, THF). The textures of the FLC
mixtures were observed by a polarizing optical microscope (Olympus,
BX-50, with Mettler, FP-80 and FP-82 hot stage). The spontaneous
polarization (Ps) was measured by a triangular waveform voltage
method (10 Vp-p, 100 Hz). The electro-optical hysteresis of the FLC
sample was measured using a polarizing optical microscope with
application of 20 Hz, 10 Vp-p triangular waveform voltage. The photo-
refractivity was measured by a two-beam coupling experiment, which
was performed using a p-polarized Arþ laser (Laser Graphics,
165LGS-S, 488 nm, continuous wave, 2.5 mW, 1 mm diameter); the
experimental set-up is shown in Fig. 4. The incident beam angles to
the glass plane were 40ꢁ and 60ꢁ, respectively. An interval of the inter-
ference fringe was 1.87 mm. The photorefractive effects were measured
with DC voltage applied to the sample on account of promotion of
charge separation efficiency.
3. RESULTS AND DISCUSSION
3.1. Photorefractive Effect in Polymer Mixed FLC Samples
The photorefractivity of the mixture of FLC and photoconductive
polymers pTPA-n was investigated. The molecular weights of the
pTPA-n are listed in Table 2. The polymers, FLC, and TNF were dis-
solved in dichloromethane, and the solvent was evaporated. The
TABLE 2 Molecular Weights of the Photoconductive
Polymers Used in this Study
Polymer
Mn
Mw
Mw=Mn
pTPA-1
PTPA-2
PTPA-3
10000
10000
13000
19000
22000
21000
1.9
2.2
1.6

PR Effect of Polymer-Stabilized FLCs
89
mixtures were injected into a 5 mm–gap cell equipped with a 1 cm2 ITO
electrode and a polyimide alignment layer (Fig. 4). The sample was
heated to 130ꢁC (isotropic phase) and deliberately cooled to ambient
temperature at a rate of 0.1ꢁC=min. Figure 5 shows the polarizing
optical microscopy photographs of the polymer=FLC mixtures in a
5 mm–gap cell. The samples formed a surface-stabilized state, however,
several defects were observed under the polarizing microscope. Since
no defect was observed in the samples without the polymer, the defects
observed were considered to be produced by the incorporation of the
polymers. The insolubility of polymers in the FLC medium may be
the cause for the presence of the defects in the surface stabilized state.
Comparing the homogeneity of the surface stabilized state in samples
of pTPA-1, pTPA-2, and pTPA-3, there were fewer defects in the
samples mixed with pTPA-3. The introduction of a mesogen-like struc-
ture (biphenyl) likely contributed to the increase in solubility into the
FLC medium.
Since the phase of the refractive index grating formed through the
photorefractive effect is shifted from the interference fringe, the
interfering laser beams have a unique mode of propagation. That is,
the intensity of one beam increases while that of the other beam
decreases. This phenomenon is known as asymmetric energy exchange
in photorefractive two-beam coupling. The photorefractive effect of the
mixtures of FLC and pTPA-n was measured by a two-beam coupling
experiment. Interference of the two beams in the sample resulted in
increased transmitted intensity of one of the beams and decreased
transmittance of the other beam. Since Q ¼ 2pkL=nK2 ¼ 3–6 (k ¼ wave
wave length; L ¼ interaction path length; n ¼ refractive index;
K ¼ grating spacing), the diffraction observed in this experiment is
predominantly but not entirely in the Bragg regime, though Raman–
Nath type diffraction also partly contributes. However, since we could
not observe higher order diffraction, the two-beam coupling gain
coefficient C was calculated assuming only Bragg diffraction, as
follows [1–3]:
ꢀ
ꢁ
1
C ¼ ln
D
gm
;
1 þ m ꢂ g
where D ¼ L=cos(h) is the interaction path for the signal beam
(L ¼ sample thickness, h ¼ propagation angle of the signal beam in
the sample), g is the ratio of intensities of the signal beam behind
the sample with and without a pump beam, and m is the ratio of the
beam intensities (pump=signal) in front of the sample.

90
T. Sasaki et al.
FIGURE 5 POM photographs of mixtures of FLC and photoconductive
polymers. The concentration of the polymer was 0.5 wt%.

PR Effect of Polymer-Stabilized FLCs
91
Figure 6 shows the electric field dependence of the gain coefficients
of the polymer=FLC samples. The gain coefficients of the FLC mixed
with pTPA-1, 2, and 3 were measured to be 2–3 cmꢂ1. This was much
smaller than the values reported for the gain coefficient of FLCs doped
with low molecular weight photoconductive compounds (20–50 cmꢂ1).
The presence of many defects in the surface-stabilized state of FLC
in the polymer mixed sample leads to a lower gain coefficient. The
defects strongly scatter light and prevent the formation of clear
interference fringes. It can be concluded that in the photoconductive
polymer mixed FLC system, as long as photoconductive polymers
that have high solubility to FLC medium are not prepared, large gain
coefficients cannot be obtained.
FIGURE 6 Electric field dependence of the gain coefficients of mixtures of the
photoconductive polymer, TNF, and FLC. The concentration of the polymers
was 0.5 wt%, and that of TNF was 0.1 wt%.

92
T. Sasaki et al.
3.2. Photorefractive Effect in Photoconductive-Polymer-
Stabilized FLCs
The photorefractive effect in the photoconductive polymer stabilized
FLC was investigated. Mixtures of mTPA-3=TNF=FLC containing
1 wt% of IRGACURE-369 (photoinitiator) were irradiated with
366 nm light along with application of an electric field of ꢂ2.0 V=mm.
Polarizing microscope photographs of the textures of the FLC mixed
with photoconductive monomer mTPA-3 before and after ultraviolet
(UV) light irradiation are shown in Fig. 7. The SS-state of the
mTPA-3=TNF=FLC mixture was almost monodomain, and a highly
homogeneous SS-state was retained even after photopolymerization.
GPC curves of the mTPA-3=TNF=FLC before and after UV irradiation
are shown in Fig. 8. The generation of the polymer was confirmed after
irradiation. The conversion of the monomer to a polymer was investi-
gated as a function of the concentration of the monomer (Fig. 9). The
extent of this conversion increased with the concentration of the mono-
mer. Because of the high viscosity of the FLC medium, the conversion
was suppressed at lower concentrations of the monomer.
The electro-optical hysteresis of the mTPA-3=TNF=FLC mixture
before and after photopolymerization was measured. Since the FLC
exhibits a bistable state, the electro-optical response of the FLC shows
a hysteresis curve (optical hysteresis curve). Transmittance through a
pair of crossed polarizers sandwiching a FLC sample changes with the
applied voltage and yields a hysteresis curve. When the FLC is comple-
tely bistable, the center of the optical hysteresis curve is 0 V. The
deviation of the center of the hysteresis curve from 0 V means that
the FLC exhibits a monostable state [14–17]. In the monostable state,
the direction of spontaneous polarization is stabilized in one direction.
The monostability of the direction of the spontaneous polarization is
considered to be produced by the strong interaction between the FLC
molecules and the mesogenic part of the polymer [16,17]. The electro-
optical hysteresis of the mTPA-3=TNF=FLC mixture before and after
UV irradiation was measured using the polarizing microscope
(Fig. 10). The mTPA-3=TNF=FLC mixtures were photopolymerized
with application of ꢂ2.0 V=mm DC electric field. After photopolymeriza-
tion, the center of the hysteresis curve was shifted from 0 V to a positive
voltage. The deviation of the threshold voltage indicates that the mTPA-
3=TNF=FLC mixture exhibits a monostable state. The direction of the
Ps (and the alignments of the FLC molecules) was stabilized to the
direction of the electric field applied during the photopolymerization.
The electric field dependence of the gain coefficients and the refrac-
tive index grating formation times of the mTPA-3=TNF=FLC mixture

PR Effect of Polymer-Stabilized FLCs
93
FIGURE 7 POM photographs of the mTPA-3=TNF=FLC mixture before and
after photopolymerization. (a), (c), measured in a 5 mm-gap cell and (b), (d),
measured in a 10 mm gap cell. The concentration of mTPA-3 was 8 wt%.
before and after photopolymerization were investigated (Fig. 11). The
two-beam coupling experiments were conducted with the application
of an electric field with the same polarity as the electric field applied
during the photopolymerization (negative polarity). The gain coeffi-
cient of the 5 mm sample before photopolymerization decreased with
increasing electric field as a result of the increased defects in the SS-
state. It has been reported that the magnitude of the gain coefficient

94
T. Sasaki et al.
FIGURE 8 GPC curves of mTPA-3=TNF=FLC mixture before (a) and after (b)
366 nm photoirradiation (3.6mW=cm2) of 5 min. The concentration of mTPA-3
was 8 wt% and the concentration of the photoinitiator was 1 wt% to the mTPA-3.
in FLCs doped with a low-molecular-weight photoconductive com-
pound is generally reduced with increasing strength of the electric
field higher than 0.5–1 V=mm. However, the gain coefficient did not
decrease in the sample after photopolymerization. This indicates that
FIGURE 9 Conversion of mTPA-3 in the mTPA-3=TNF=FLC mixture as a
function of the monomer concentration. The concentration of the photoinitia-
tor was 1 wt% to the mTPA-3. 3.6 mW=cm2 of 366 nm UV light was irradiated
for 5 min.

PR Effect of Polymer-Stabilized FLCs
95
FIGURE 10 Optical hysteresis of the mTPA-3=TNF=FLC mixture before (a)
and after (b) photopolymerization with application of a ꢂ2.0 V=mm electric
field. The concentration of the mTPA-3 was 8 wt%. The light transmitted
through the parallel polarizers without the ITO glass cell was defined as
100 % transmittance. (a), (d), measured in a 2 mm-gap cell, (b), (e), measured
in a 5 mm gap cell and (c), (f), measured in a 10 mm gap cell.
the photoconductive polymer formed in the FLC prevented the
generation of defects. The gain coefficient of the photopolymerized
sample increased with increasing electric field and reached a constant

96
T. Sasaki et al.
FIGURE 11 Electric field dependence of the gain coefficients and the forma-
tion times of mTPA-3=TNF=FLC mixture before and after photopolymeriza-
tion measured in the 5 mm–gap cell. The concentration of the mTPA-3 was
8 wt%. The concentration of the photoinitiator was 1 wt% to the mTPA-3.
3.6 mW=cm2 of 366 nm UV light was irradiated for 5 min with application of
a ꢂ2.0 V=mm electric field.
value. The refractive index grating formation time of the FLC mixture
after photopolymerization was longer than that of the mixture before
photopolymerization (Fig. 11(b)). The mobility of the FLC molecules
was likely reduced by the polymer network and led to a slower
response.
The two-beam coupling gain coefficients of the mTPA-3=TNF=FLC
mixture was investigated as a function of the concentration of TNF
(electron acceptor). As shown in Fig. 12(a), in the samples before photo-
polymerization, though the gain coefficient was affected by the TNF
concentration, the change in the gain coefficient was not very large.
On the other hand, when the samples after photopolymerization were
measured, the gain coefficient was strongly affected by the
concentration of TNF (Fig. 12(b)). Ionic conduction possibly plays a
major role in the formation of the space-charge field. When laser beams
are illuminated on to the sample, photoinduced charge transfer occurs
between the TPA-3 chromophore and TNF. The mobility of the cation of
the TPA-3 polymer is much smaller than that of the TNF anion, and
this difference in mobility is thought to be the origin of the charge
separation. In this case, the magnitude of the internal electric field is
dominated by the concentration of the ionic species. On the other hand,
the difference in the mobilities of the TPA-3 monomer and TNF is
small, and thus, less effective charge separation is induced, resulting
in small dependence of the internal electric field on the concentration
of the ionic species. The refractive index grating formation times of

PR Effect of Polymer-Stabilized FLCs
97
FIGURE 12 Electric field dependence of the gain coefficients of mTPA-
3=TNF=FLC mixtures with several TNF concentrations before (a) and after
(b) photopolymerization. The concentration of mTPA-3 was 8 wt%. The concen-
tration of the photoinitiator was 1 wt% to the mTPA-3. 3.6 mW=cm2 of 366 nm
UV light was irradiated for 5 min with application of a ꢂ2.0 V=mm electric field.
the mTPA-3=TNF=FLC mixtures with several TNF concentrations
before and after photopolymerization are shown in Fig. 13. Different
from the gain coefficient, the formation time did not depend on the
TNF concentration regardless of the photopolymerization. This result
can be understood by assuming the ionic conduction mechanism. Con-
sidering that the electric field response of the FLCs is in the range of ms
to ms, and the refractive index grating formation time is in the range of
few tens of ms, the rate determining step of the photorefractive effect of
FIGURE 13 Electric field dependence of the formation time of mTPA-
3=TNF=FLC mixtures with several TNF concentrations before (a) and after
(b) photopolymerization. The concentration of mTPA-3 was 8 wt%. The concen-
tration of the photoinitiator was 1 wt% to the mTPA-3. 3.6 mW=cm2 of 366 nm
UV light was irradiated for 5 min with application of a ꢂ2.0 V=mm electric field.

98
T. Sasaki et al.
the FLCs is the formation of the internal electric field. The magnitude
of the internal electric field is dominated by the concentration of the
ions. However, the rate of transportation of ions does not depend on
the concentration of ions. Thus, the index grating formation time did
not depend on the concentration of TNF.
4. CONCLUSION
The photorefractivity of a polymer-stabilized FLC by a photocon-
ductive polymer (electron donor) was investigated using two-beam
coupling experiments. The photorefractive effect in the polymer
stabilized FLC was enhanced by mixing larger amounts of a low mole-
cular weight electron acceptor compound. A large difference in the
mobilities of photogenerated cations and anions leads to larger photo-
refractivity. The use of a pair of a photoconductive polymer and a low
molecular weight electron donor=acceptor compound yields large
photorefractivity.
REFERENCES
[1] Yeh, P. (1993). Introduction to Photorefractive Nonlinear Optics, John Wiley:
New York.
[2] Moerner, W. E., & Silence, S. M. (1994). Chem. Rev., 94, 127.
[3] Solymar, L., Webb, J. D.,
& Grunnet-Jepsen, A. (1996). The Physics and
Applications of Photorefractive Materials, Oxford: New York.
[4] Ostroverkhova, O., & Moerner, W. E. (2004). Chem. Rev., 104, 3267.
[5] Khoo, L. C., Li, H., & Liang, Y. (1994). Opt. Lett., 19, 1723.
[6] Wiederrecht, G. P., & Wasielewski, M. R. (1998). J. Am. Chem. Soc., 120, 3231.
[7] Ono, H., Kawamura, T., Frias, N. M., Kitamura, K., Kawatsuki, N., & Norisada, H.
(2000). Adv. Mater., 12, 143.
[8] Wiederrecht, G. P., Yoon, B. A., & Wasielewski, M. R. (2000). Adv. Materials,
12, 1533.
[9] Sasaki, T., Katsuragi, A., Mochizuki, O., & Nakazawa, Y. (2003). J. Phys. Chem. B,
107, 7659.
[10] Sasaki, T., Mochizuki, O., Nakazawa, Y., & Noborio, K. (2004). J. Phys. Chem. B,
108, 17083.
[11] Talarico, M., Termine, R., Prus, P., Barberio, G., Pucci, D., Ghedini, M., &
Goelemme, A. (2005). Mol. Cryst. Liq. Cryst., 429, 65.
[12] Talarico, M., & Goelemme, A. (2006). Nature Mater., 5, 185.
[13] Sasaki, T., Moriya, N., & Iwasaki, Y. (2007). J. Phys. Chem. C, 111, 17646.
[14] Skarp, K., & Handschy, M. A. (1988). Mol. Cryst. Liq. Cryst., 165, 439.
[15] Fukuda, A., & Takezoe, H. (1990). Structure and Properties of Ferroelectric Liquid
Crystals, Corona: Tokyo.
[16] Furue, H., Miyama, T., Iimura, Y., Hasebe, H., Takatsu, H., & Kobayashi, S. (1997).
Jpn. J. Appl. Phys., 36, L1517.
[17] Fujikake, H., Murashige, T., Sato, H., Fujisaki, Y., Kawakita, M., Kikuchi, H., &
Kurita, T. (2003). Jpn. J. Appl. Phys., 42, L186.