Photorefractive effect of liquid crystalline polymers possessing azobenzene chromophores

Article abstract of DOI:10.1080/10587250210541

A series of liquid crystalline polymers possessing azobenzene chromophores were synthesized, and in each polymer the photorefractive effect of the isotropic phase was investigated. The photorefractive effect was evaluated by a four-wave mixing experiment.

Full text of DOI:10.1080/10587250210541

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Photorefractive Effect of Liquid
Crystalline Polymers Possessing
Azobenzene Chromophores
Takeo Sasaki a , Ryutaro Kai b , Atsushi Sato b
Yuichi Ishikawa b & Takeshi Yoshimi b
,
a Department of Chemistry , Science University of
Tokyo , Shinjuku-ku, Tokyo, Japan
b Department of Applied Chemistry, Faculty of
Engineering , Oita University , Oita, Japan
Published online: 18 Oct 2010.
To cite this article: Takeo Sasaki , Ryutaro Kai , Atsushi Sato , Yuichi Ishikawa &
Takeshi Yoshimi (2002) Photorefractive Effect of Liquid Crystalline Polymers Possessing
Azobenzene Chromophores, Molecular Crystals and Liquid Crystals, 373:1, 53-70, DOI:
10.1080/10587250210541
To link to this article: http://dx.doi.org/10.1080/10587250210541
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Mol. Cryst. and Liq. Cryst., 2002, Vol. 373, pp. 53-70
# 2002 Taylor and Francis, Inc.
Photorefractive Effect of Liquid
Crystalline Polymers Possessing
Azobenzene Chromophores
TAKEO SASAKI
Department of Chemistry, Science University of Tokyo, Shinjuku-ku,
Tokyo, Japan
RYUTARO KAI, ATSUSHI SATO, YUICHI ISHIKAWA and
TAKESHIYOSHMI I
Department of Applied Chemistry, Faculty of Engineering,
Oita University, Oita, Japan
A series of liquid crystalline polymers possessing azobenzene chromophores were synthesized,
and in each polymer the photorefractive effect of the isotropic phase was investigated. The
photorefractive effect was evaluated by a four-wave mixing experiment. The diffraction effi-
ciency of a side-chain liquid crystalline polymer possessing chiral moieties was measured and
compared with that of a polymer possessing achiral chromophores. Chiral polymers exhibited
unusual electric-field dependence on diffraction efficiency. The effects of liquid crystallinity and
chiral substituents on photorefractivity are discussed.
Keywords Photorefractive effect, polymer liquid crystal, four-wave mixing
INTRODUCTION
The photorefractive effect is defined as an optical modulation of the re-
fractive index [1, 2], and the photorefractive effect of polymer materials has
recently attracted great interest. When laser beams interfere in a photo-
refractive material, a photovoltaic effect occurs between the bright and dark
areas of the interference. The resultant electric field causes a change in the
refractive index via the electrooptic effect and produces a refractive index
Received 23 February 2001; accepted 15 June 2001.
We would like to express our gratitude for the financial support of The Mazda Foundation.
Address correspondence to Takeo Sasaki, Department of Chemistry, Faculty of Science,
Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601, Japan. E-mail:
sasaki@rs.kagu.sut.ac.jp
53

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T. SASAKI et al.
grating. It is a characteristic of the photorefractive effect that the phase of
the index grating is shifted by p=2 from the interference pattern. This phe-
nomenon enables us to create various photonic applications, such as optical
image processing, parallel optical logic, pattern recognition, and phase
conjugators. The photorefractive effect of organic materials was first re-
ported in 1990 [3]. Since then, many studies on organic materials have been
reported, and extremely high photorefractivity has been achieved in organic
polymer materials [4–16]. The high photorefractivity of polymer materials
arises from a change in chromophoric orientation induced by an electric
field, termed orientational enhancement [4, 15, 17].
Recently, we investigated the photorefractive effects of liquid crystalline
polymers and compared the effects with those of amorphous polymers [18].
Our investigation revealed that the photorefractive effect is significantly
enhanced in the isotropic phase of the liquid crystalline polymers, and that
the diffraction efficiency of the isotropic phase of the liquid crystalline
polymers is much greater than that of amorphous polymers, even though
these polymers have almost identical molecular structures. Although the
exact mechanism of the enhancement effect has not yet been elucidated, the
enhancement is considered to arise from the presence of the microscopically
ordered structure of component chromophores in the isotropic phase. If the
microscopically ordered structure plays an important role in the enhance-
ment, the effect of the introduction of a chiral structure into the component
chromophores is interesting. The chiral liquid crystals tend to form a specific
phase structure [19, 20], which suggests that the chiral moiety may affect the
microscopic structures that exist in the isotropic phase, thereby influencing
photorefractivity. In this study, we investigated the photorefractivity of a
liquid crystalline polymer that possesses a series of azobenzene chromo-
phores. The azobenzene chromophore with an electron withdrawing sub-
stituent exhibits a large hyperpolarizability, so that a large Pockels constant
of the polymer film can be expected. The effects of liquid crystallinity and
chiral moiety on photorefractivity are discussed.
EXPERIMENTAL
Preparation of Samples
The structures of the liquid crystalline polymers and photoconductive
compounds used in this study are shown in Figure 1, and the thermal
properties of these polymers are listed in Table 1. The polymers AZO*-8 and
AZO-8 were synthesized according to Figure 2. The photoconductive

PHOTOREFRACTIVE EFFECT OF LC POLYMERS
55
FIGURE 1 Structures of compounds used in this study.

56
T. SASAKI et al.
TABLE 1 Molecular weights and phase transition temperatures of the polymers used in this
study
Abbrev.
Mn
Mw=Mn
Phase transition temperaturea
ꢀC
NitroAzo6
DMAzo6
AZO-8
9000
8500
8500
8300
3.3
3.4
2.2
2.5
g
g
g
g
91
80
71
67
N
169
I
N
N*
103
82
I
I
AZO*-8
ag, glassy state; N, nematic phase; I, isotropic phase.
compound, CDH, was synthesized via reaction of disphenylhydrazine and
and N-ethlycarbazolylaldehyde in pyridine. TNF (sensitizer=electron trap
reagent) was obtained from Tokyo Kasei Co. and purified by re-
crystallization from a mixture of hexane-ethylacetate. The photorefractive
sample was prepared by casting on an ITO-patterned glass a dichloroethane
solution of the polymer mixed with CDH and TNE. The ratio of CDH in
the film was 40 wt%, and the concentration of TNF was 1 wt%. No phase
separation was observed under the polarizing microscope or during DSC
measurements. In order to obtain a flat film, the casting solution was pre-
pared at rather high concentration (200 mg in 1 mL). The film was dried
under vacuum for a week and sandwiched between two ITO glasses at 70ꢀC.
The film was softened at this temperature and completely filled the space
between the ITO electrodes. The film thickness was 100 mm, as governed by
a Teflon spacer.
Synthesis of Azobenzene Monomer AZO*-8
1) 4-((S)-2-methylbutyl)sulfonyl nitrobenzene
4-Nitrothiophenol (14.5 g, 0.093 mol) and (S)-1-chloro-2-methylbutane
(Tokyo Chemical Co. 10.0 g, 0.093 mol) were dissolved in 200 mL of N,N-
dimethlyformamide (DMF), and 0.5 g of potassium iodide and 40 g of
potassium carbonate were added. The mixture was stirred vigorously at
80ꢀCfor 6 h. After the solution was cooled to ambient temperature, 400 mL
of water was added to the solution, and the sulfide was extracted with
diethylether. After the diethylether solution was dried over sodium sulfate
the solvent was evaporated, and the resultant solid was dissolved in 400 mL
of methanol. Sodium tungstate dihydrate (Na2WO4 ꢁ 2H2O, 0.6 g) in 30 mL
of water was added to the solution. The solution was stirred and heated to
40ꢀC, and 50 mL of 30% H2O2 was added dropwise over 2 h. The solution
was then heated to 80ꢀC , 25 mL of HO2 was added, and the resultant
2
mixture was refluxed for 2 h. The solution was cooled to ambient

57

58
T. SASAKI et al.
temperature, and slightly yellow crystals were deposited and collected by
filtration. The product was purified by recrystallization from methanol-
water (3:1) (yield: 61%). Anal. Calcd for C11H15NO4S: C, 51.35; H, 5.88; N,
1
5.44. Found: C, 51.30; H, 5.91; N, 5.40. H NMR (CDCl3) d 0.83 (t, CH3),
1.03 (d, CH3), 1.24–1.47 (m, CH2), 1.95 (m, CH), 2.86–3.07 (m, SO2CH2),
6.98 and 7.71 (aromatic, 4H).
2) 4-((S)-2-methylbutyl) sulfonyl aminobenzene
4-((S)-2-Methylbutyl)sulfonyl nitrobenzene (14.8 g, 0.057 mol) was dis-
solved in 300 mL of methanol, and 1.0 g of Pt-Al catalyst (5% Pt on alu-
mina) was suspended under N2 atmosphere. The flask was flushed with
hydrogen and the solution was stirred at ambient temperature for 6 h. After
the Pt-Al was removed by filtration, the solvent was evaporated, and the
resultant product was purified by column chromatography on silica gel
(eluent, dichloromethane) (yield: 89%). Anal. Calcd for C11H15NO4S: C,
58.12; H, 7.54; N, 6.16. Found: C, 58.05; H, 7.57; N, 6.13. 1H NMR
(CDCl3) d 0.85 (t, CH3), 1.05 (d, CH3), 1.18–1.50 (m, CH2), 1.95 (m, CH),
2.83–3.24 (m, SO2CH2), 4.19 (s, NH2), 6,70 and 7.64 (aromatic, 4H).
3) 4-Hydroxy-40-((S)-2-methylbutyl)sulfonyl azobenzene
4-((S)-2-Methylbutyl)sulfonyl aminobenzene (11.0 g, 0.048 mol) was dis-
solved in 200 mL of a 1:1 mixture of acetone and 1N HCl. The solution was
cooled to 0ꢀC, and 3.8 g (0.055 mol) of sodium nitrite in 100 mL of water
was added slowly. The resultant diazonium solution was added dropwise to
an aqueous solution of phenol (3.6 g, 0.048 mol) and sodium hydroxide
(4.0 g, 0.10 mol) at 0ꢀC. After stirring for 2 h, the solution was brought to
ambient temperature and neutralized with HCl, and the resultant precipitate
was collected by filtration. The product was purified by column chroma-
tography on silica gel (eluent, dichloromethane), followed by recrystalliza-
tion from methanol (yield: 68%). Anal. Calcd for C11H15NO4S: C, 61.42; H,
6.06; N, 8.43. Found: C, 61.39; H, 6.10; N, 8.41. 1H NMR (CDCl3) d 0.85 (t,
CH3), 1.05 (d, CH3), 1.18–1.50 (m, CH2), 2.05 (m, CH), 2.95–3.16 (m,
SO2CH2), 6.97, 7.91 and 7.98–8.11 (aromatic, 8H).
4) 4-(8-Hydroxyoctyloxy)-40-((S)-2-methylbutyl)sulfonyl azobenzene
4-Hydroxy-40-((S)-2-methylbutyl)sulfonyl azobenzene (6.8 g, 0.02 mol) and
2.2 g (0.02 mol) of 8-bromooctanol were dissolved in 100 mL of DMF, 10 g
of potassium carbonate were added, and the mixture was vigorously stirred
at 60ꢀCfor 12 h. The solution was poured into 500 mL of water, and the
resultant precipitate was collected by filtration. The product was purified by

PHOTOREFRACTIVE EFFECT OF LC POLYMERS
59
column chromatography on silica gel (eluent, dichloromethane), followed
by recrystallization from methanol (yield: 51%). Anal. Calcd for
C11H15NO4S: C, 65.19; H, 7.88; N, 6.08. Found: C, 65.15; H, 7.89; N, 6.05.
1H NMR (CDCl3) d 0.85 (t, CH3), 1.05 (d, CH3), 1.25–1.76 (m, CH2, 14H),
2.05 (m, CH), 2.95–3.16 (m, SO2CH2), 3.65 (t, HO-CH2), 4.15 (t, -O-CH2),
7.01, 7.94 and 8.00–8.05 (aromatic, 8H).
5) 4-(8-Acryloyloxyoctyloxy)-40-((S)-2-methylbutyl)sulfonyl
azobenzene (AZO*-8 monomer)
4-(8-Hydroxyoctyloxy)-40-((S)-2-methylbutyl)sulfonyl azobenzene (4.7 g,
0.01 mol) and 3 mL of pyridine were dissolved in 100 mL of di-
chloromethane, and 2.3 g (0.02 mol) of methacryloyl chloride in 50 mL of
dichloromethane was added dropwise and stirred for 5 h. The solution was
washed with an aqueous solution of saturated sodium bicarbonate and dried
over sodium sulfate. After the solvent was evaporated, the product was
purified by column chromatography on silica gel (eluent, dichloromethane),
followed by recrystallization from 2-propanol (yield: 61%). Anal. Calcd for
C11H15NO4S: C, 65.88; H, 7.63; N, 5.30. Found: C, 65.85; H, 7.60; N, 5.26.
1H NMR (CDCl3) d 0.85 (t, CH3), 1.05 (d, CH3), 1.25–1.76 (m, CH2, 14H),
2.05 (m, CH), 2.95–3.16 (m, SO2CH2), 3.99 (t, HO-CH2), 4.15 (t, -O-CH2),
5.55 and 6.10 (s, C¼CH2), 7.01, 7.94 and 8.00–8.05 (aromatic, 8H).
Polymerization
Thermal radical polymerization of AZO*-8 was carried out in a 10 wt%
benzene solution with azobis(isobutylonitrile) (AIBN, 3 mol%) serving as
an initiator. All solutions were degassed by a freeze-pump-thaw cycle and
heated at 70ꢀCfor 20 h. The resulting polymers were purified by repeated
reprecipitation from chloroform solution into methanol.
Measurement
The number-average molecular weight (Mn) of the polymers was de-
termined by gel permeation chromatography (GPC; Toyo-soda TSK-GEL
G3000HHR; eluent, THF), and the glass transition temperature (Tg) and
phase transition temperature were determined by a differential scanning
calorimeter (DSC; Seiko I&E SSC-5000) and by microscopic observation
(Mettler FP-80, FP-82 and Olympus BX-50 polarizing microscope).
The photorefractive property was evaluated via a four-wave mixing
experiment. Figure 3 shows a schematic illustration of the experimental

60
T. SASAKI et al.
FIGURE 3 Schematic illustration of experimental setup used in this study.
setup. Four-wave mixing experiments were performed using s-polarized
writing beams and a p-polarized probe beam counterpropagating to one of
the writing beams. A He-Ne laser (Mellesgriot, 633 nm, 7 mW output) was
used as a light source. Beam 1 and beam 2 interfered in a sample film, and
the intensity of the diffraction of beam 3 (phase conjugate wave, beam 4)
was measured by a photodiode (E-O TECH ET2010, response < 1 ns) and
recorded by a storage-oscilloscope. The angle between the two writing
beams was 72ꢀ inside the film (refractive index ¼ 1.75), and the grating
period was 2.9 mm. The diffraction efficiency is defined as the ratio of in-
tensity of the diffracted beam and the intensity of the probe beam that is
transmitted when no grating is being written in the sample by the writing
beams. The temperature of the sample was controlled by a thermo-con-
troller (Chino Co. DB1000). An electric field of 10 ꢂ 40 V=mm was applied
to the sample by a high voltage supply unit (Glassman High Voltage, Inc.,
PS=EL1OR4.0-10). For two-beam coupling experiments, the transmitted
intensities of beams 1 and 2 were measured by power meters. SHG in-
tensities were measured by the Maker fringe method (Tokyo Instruments
NL-100 NLO evaluation system). A 1064 nm Nd:YAG laser was used as a

PHOTOREFRACTIVE EFFECT OF LC POLYMERS
61
fundamental source. The generated 532 nm signals were detected by a
photomultiplier and integrated by a boxcar averager.
RESULTS AND DISCUSSION
Photorefractivity of the Isotropic Phase of a Liquid Crystalline
Polymer NitroAzo6 and Amorphous Polymer DMAzo6 Doped
with Photoconductive Compounds
A 1:1 copolymer of methylmethacrylate and 4-methacryloyloxyhexyloxy-40-
nitroazobenzene (NitroAzo6) exhibits a nematic liquid crystal phase,
whereas a 1:1 copolymer of methylmethacrylate and 40-methacryloyloxy-
hexyloxy-2050-dimethyl-4-nitroazobenzene (DMAzo6) does not exhibit a
liquid crystal phase. The diffraction efficiencies of polymers NitroAzo6 and
DMAzo6 mixed with photoconductive compounds (CDH and TNF)
were investigated. The sample composition was TNF:CDH:polymer ¼
1:40:59 wt%. The photoconductive dopant, CDH, is not a liquid crystal,
and therefore the mixture of NitroAzo6 and 40 wt% of CDH did not ex-
hibit a liquid crystal phase. The sample film was isotropic and transparent.
A two-beam coupling experiment was performed on the sample. An
asymmetric energy exchange was observed as applying an external electric
field (Figure 4). The diffraction efficiencies were evaluated by the four-wave
mixing experiments. When an external electric field was applied to the
sample, the diffraction of beam 3 (phase conjugate wave 4) was clearly
observed (Figure 5). When the field was turned off, the diffraction dis-
appeared. The diffraction efficiency is defined as DI=I0, where I0 represents
the transmitted intensity of beam 3 through the sample without beams 1 and
2, and DI represents the intensity of the diffracted beam. The dependence of
diffraction efficiency on the magnitude of the electric field is shown in Figure
6, where we can see that diffraction efficiency increases with the intensity of
the applied electric field.
The four-wave mixing experiment was also performed on the amorphous
polymer DMAzo6. The concentrations of CDH and TNF were 40 wt% and
1 wt%, respectively. Both samples induced less scattering of the laser beam
and neither sample produced any XRD peaks. The diffraction efficiency of
DMAzo6 was found to be much smaller than that of NitroAzo6, although
their molecular weights are almost the same and DMAzo6 exhibits a lower
glass transition temperature than does NitroAzo6. As compared with the
amorphous polymer, the isotropic phase of the liquid crystalline polymer

62
T. SASAKI et al.
FIGURE 4 Typical example of asymmetric energy exchange in two-beam coupling experi-
ment for NitroAzo6 doped with 40 wt% CDH and 1 wt% TNF. (a) An electric field of
20 V=mm was applied to the sample and the shutter was open at t ¼ 22 s and closed at t ¼ 86 s.
(b)An electric field of 20 V=mm was applied to the sample at t ¼ 20 s and cut off at t ¼ 115 s.

PHOTOREFRACTIVE EFFECT OF LC POLYMERS
63
FIGURE 5 Diffracted beam observed in four-wave mixing experiment conducted on
NitroAzo6 doped with 40 wt% CDH and 1 wt% TNF. An electric field of 40 V=mm was
applied to the sample at t ¼ 50 s and cut off at t ¼ 100 s.
exhibits higher photorefractivity, which supports the enhancement effect in
the isotropic phase of liquid crystalline polymers recently reported [18]. The
enhancement of photorefractivity in the isotropic phase of liquid crystalline
polymer is considered to arise from the microscopic ordering of mesogens
(chromophores) in the isotropic phase. The mesogens of the liquid crystal-
line polymer NitroAzo6 tend to align with each other even in the isotropic
phase, producing a temporal microscopic ordering of chromophores. The
existence of a small domain is considered to enhance the change in the
refractive index through an orientational effect.
The Photorefractive Effect of AZO-8 and AZO*-8
We also investigated the photorefractivity of liquid crystalline polymers
possessing alkylsulfon-substituted azobenzene poly 4-(8-methacryloylox-
yoctyloxy)-40-butylsulfonylazobenzene (AZO-8) and poly 4-(8-methacry-
loyloxyoctyloxy)-40-((s)-2-methylbutyl)sulfonylazobenzene (AZO*-8), by
measuring the diffraction efficiencies of the isotropic phase of each of these
polymers. The sample composition was TNF:CDH:polymer ¼ 1:40:59 wt%.
Figure 7 shows the diffraction efficiency of AZO-8 as a function of the

64
T. SASAKI et al.
FIGURE 6 Electric field dependence of diffraction efficiency of NitroAzo6 and DMAzo6
doped with 40 wt% CDH and 1 wt% TNF. r, NitroAzo6; &, DMAzo6.
magnitude of an applied electric field. Diffraction efficiency showed sin2
functional dependence on the electric field, which can be related to
Kogelnik’s theory of a coupled wave in a thick medium [21]. The mechanism
of the photorefractive effect of AZO-8 is considered to consist primarily of
the photovoltaic effect and the succeeding electrooptic effect. Figure 8 shows
the diffraction efficiency of the chiral polymer AZO*-8 as a function of the
magnitude of an applied electric field. The diffraction efficiency of AZO*-8
showed no clear dependence on the magnitude of the applied electric field,
which indicates that for AZO*-8 the polarization of the film (and thus the
Pockels constant of the film) is not important in four-wave mixing. The
diffraction was observed even when the external electric field was not ap-
plied. If the diffraction at an electric field of 0 V=mm arises from the chiral
structure of the microscopic domain, the second order nonlinear optical
properties of AZO*-8 must be active in the absence of the electric field.
However, no SHG was observed in AZO*-8 films when no external electric
field was applied. Thus, the change in the refractive index of AZO*-8 at
0 V=mm is not via the Pockels effect. Although the asymmetric energy

PHOTOREFRACTIVE EFFECT OF LC POLYMERS
65
FIGURE 7 Electric field dependence of diffraction efficiency of AZO-8 doped with 40 wt%
CDH and 1 wt% TNF.
exchange was observed in AZO*-8 at 0 V=mm in the two-beam coupling
experiment, the intensity of the signal was unstable and fluctuated with time.
The azobenzene chromophore can exhibit trans-cis photoisomerization. If
the chromophore absorbs the laser beam, the refractive index grating can be
created by photoisomerization. However, distinct from the photorefractive
effect, the phase of the grating created by photoisomerization is not shifted
from the interference pattern. The asymmetric energy exchange cannot
be observed if the index grating was made by the photoisomerization.
In addition, the azobenzene chromophore of AZO*-8 does not exhibit an
absorption band at 633 mn, and therefore the diffraction at 0 V=mm is
considered not to be induced by trans-cis photoisomerization. Figure 9
shows the SHG signal measured by the Maker-fringe method under appli-
cation of an electric field of 40 V=mm. The SHG signal of AZO*-8 was

66
T. SASAKI et al.
FIGURE 8 Electric field dependence of diffraction efficiency of AZO*-8 doped with 40 wt%
CDH and 1 wt% TNF.
larger than that of AZO-8 under identical conditions. This indicates that the
AZO*-8 films can be polarized more easily than can the AZO-8 films,
because the films exhibit almost identical hyperpolarizabilities [22, 23]. Also,
the glass transition temperature of AZO*-8 is lower than that of AZO-8.
The chromophores of AZO*-8 may be able to move more easily than those
in AZO-8.
The data on distribution of free volume can be obtained by investigating
trans-cis photoisomerization behavior in the film [24–28]. Figure 10 shows
the absorption spectra of the polymer films before and after irradiation of
366 nm UV light. Reversible changes in UV spectra are induced by irra-
diation in both films. These polymers differ only in the structure of the alkyl
tail unit of the mesogens; no difference is observed in photoisomerization
behavior in dichloroethane solution. Figure 11 shows the change in

PHOTOREFRACTIVE EFFECT OF LC POLYMERS
67
FIGURE 9 Second harmonic signal intensities of AZO-8 and AZO*-8 films doped with
40 wt% CDH and 1 wt% TNF measured by the Maker-fringe method. An electric field of
40 V=mm was applied to the films.
absorption of the films at 350 nm as a function of irradiation time.
Measurement of the two films was conducted under the same conditions.
AZO*-8 exhibited a faster rate of photoisomerization and a larger change in
absorption spectra, and AZO*-8 films are considered to possess larger inner
free volume than AZO-8 films. The chiral group in the alkyl tail unit gen-
erates a larger free volume in the film and enhanced the mobility of the
chromophores. The directions of the chromophores of AZO*-8 are more
responsive to the electric field than are those in AZO-8. The diffraction in
four-wave mixing in AZO*-8 may be partially due to thermal gratings.
However, the asymmetric energy exchange observed in two-beam coupling
indicates the occurrence of the orientational photorefractive effect. The
orientations of azobenzene chromophores of AZO*-8 were affected by the
photoinduced internal electric field and produced an orientational grating.

68
T. SASAKI et al.
FIGURE 10 Absorption spectra of AZO-8 and AZO*-8 films before and after irradiation
with 366 nm light.
The existence of both thermal grating and orientational grating may have
caused the unstable fluctuation of the signal intensities. In contrast, the
AZO-8 polymer is rather rigid, so that the orientations of chromophores
were less affected by the internal electric field.

PHOTOREFRACTIVE EFFECT OF LC POLYMERS
69
FIGURE 11 Absorption at 350 nm of AZO-8 and AZO*-8 films as a function of irradiation
time.
CONCLUSION
The photorefractive effects of a series of side-chain polymers that possess
azobenzene chromophores were investigated. Diffraction efficiency was
found to be significantly higher in the isotropic phase of the liquid crys-
talline polymer than in an amorphous polymer of almost identical molecular
structure. AZO*-8 exhibited unusual behavior in the photorefractive effect.
Chromophores of the AZO*-8 film are considered to exhibit large mobility
and exhibit an orientational photorefractive effect.
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