Highly birefringent liquid-crystalline polymers for photonic applications: Synthesis of liquid-crystalline polymers with side-chain azo-tolane mesogens and their holographic properties

Article abstract of DOI:10.1039/b506131h

We have synthesized highly birefringent liquid-crystalline polymers (LCPs) containing an azobenzene group directly connected to a tolane moiety (P3AT/m), in which the alkyl spacer length (CH₂)_m was varied as m = 6, 9 and 12. All LCPs showed a nematic phase in a broad temperature range (>170 °C). The homogeneously aligned LCP films showed a high value of birefringence (about 0.4) and a large change in the birefringence could be induced by photoinduced alignment change. Grating formation of LCPs has been explored in a glassy state. The diffraction efficiency (η) showed a maximum when an order-disorder change was completed in the bright areas of the interference patterns. Studies on the effects of light intensity on the grating formation suggested that the diffraction efficiency was high (η = 20%) even though the alignment change of the azotolane moieties was small. Furthermore, the behavior of the grating formation was found to be dependent on the alkyl spacer length of P3ATm. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b506131h

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Highly birefringent liquid-crystalline polymers for photonic applications:
synthesis of liquid-crystalline polymers with side-chain azo-tolane mesogens
and their holographic properties
Kunihiko Okano, Atsushi Shishido, Osamu Tsutsumi,{ Takeshi Shiono{ and Tomiki Ikeda*
Received 3rd May 2005, Accepted 24th June 2005
First published as an Advance Article on the web 12th July 2005
DOI: 10.1039/b506131h
We have synthesized highly birefringent liquid-crystalline polymers (LCPs) containing an
azobenzene group directly connected to a tolane moiety (P3ATm), in which the alkyl spacer length
(CH2)m was varied as m 5 6, 9 and 12. All LCPs showed a nematic phase in a broad temperature
range (.170 uC). The homogeneously aligned LCP films showed a high value of birefringence
(about 0.4) and a large change in the birefringence could be induced by photoinduced alignment
change. Grating formation of LCPs has been explored in a glassy state. The diffraction efficiency
(g) showed a maximum when an order–disorder change was completed in the bright areas of the
interference patterns. Studies on the effects of light intensity on the grating formation suggested
that the diffraction efficiency was high (g 5 20%) even though the alignment change of the azo-
tolane moieties was small. Furthermore, the behavior of the grating formation was found to be
dependent on the alkyl spacer length of P3ATm.
improved by inducing a large periodical change in refractive
Introduction
index, which is caused by trans–cis photoisomerization of
azobenzene molecules,3–5 followed by a phase transition or an
Recently, the demand for higher storage capacity in optical
data storage is rapidly increasing, and holographic technology
alignment change of LC phases. Furthermore, to obtain a
has been applied for data storage, display, and optical
larger change in refractive index in LCPs, we prepared a
copolymer of tolane and azobenzene.8 Tolane derivatives are
elements, because the information packing densities can be
increased drastically by using 3-D storage techniques. In
of polarizability.9 The effect of birefringence (Dn) of LCs on
holography, diffraction efficiency (g) is one of the most
known as highly birefringent LCs due to their large anisotropy
important factors to evaluate the ability of the materials. The
the formation of gratings was studied and the results revealed
diffraction efficiency of phase-type gratings is higher than that
that a considerable enhancement of g can be achieved in the
of amplitude-type ones because the former are formed by a
Raman–Nath regime by using LCs with high birefringence
change in refractive index. Therefore, optical materials, in
because the change in alignment of mesogens induced by
which one can induce a large change in refractive index by
photoisomerization of azobenzene moieties gives rise to the
change in birefringence.8 However, the photosensitivity was
light, are most desirable for holography.
While a number of materials for holography have been
not high enough due to the low content of the azobenzene
developed, polymers functionalized with azobenzene moieties
moieties in these LCPs.
are effective for modulating holograms due to their photo-
To improve the photosensitivity, we prepared LCPs contain-
sensitivity and reversibility.1,2 Among these materials, liquid-
ing both tolane and azobenzene moieties; but unlike conven-
crystalline polymers (LCPs) containing azobenzene moieties
tional LC copolymers, in which azobenzene and tolane are
in the side chain are known as high-performance materials
incorporated into their side chains separately, both moieties
for optics due to a large change in birefringence caused by
were attached directly through a central benzene ring and
photochemical processes.3–6 Furthermore, the polymer films
the combined azo-tolane moiety was incorporated into every
have many advantages for practical applications such as
monomer unit. We also changed the alkyl spacer length
stability of optical storage and good mechanical properties.
We previously reported the formation of gratings by
means of photochemical phase transition of LCPs containing
azobenzene moieties.7 In general, diffraction efficiency can be
between the polymer main chain and the azo-tolane moiety,
and explored not only the grating formation behavior of the
resulting LCPs but also the photoinduced alignment change of
the azo-tolane moiety.
Chemical Resources Laboratory, Tokyo Institute of Technology, R1-11,
4259 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan.
E-mail: tikeda@res.titech.ac.jp
{ Present address: Ebara Research Co., 4-2-1 Honfujisawa 251-8502,
Japan.
Experimental
Characterization
NMR spectra were recorded in CDCl3 with a Lambda-300.
Mass spectra were obtained with a JMS-700 spectrometer
with fast atom bombardment (FAB). Molecular weights of the
{ Present address: Department of Applied Chemistry, Graduate
School of Engineering, Hiroshima University, Higashi-Hiroshima
739-8527, Japan.
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polymers were determined by gel permeation chromatography
(GPC; JASCO DG-980-50; column, Shodex GPC K802 +
K803 + K804 + K805; eluent, chloroform) calibrated with
standard polystyrenes. LC behavior and phase transition
behavior were examined on an Olympus Model BH-2
polarizing microscope equipped with Mettler hot-stage models
FP-90 and FP-82. Thermotropic properties of LCPs were
determined with a differential scanning calorimeter (Seiko I&E
SSC-5200 and DSC220C) at a heating rate of 10 uC min21
.
At least three scans were performed for each sample to
check reproducibility. The thermodynamic properties and the
molecular weights of the LCPs are given in Table 1.
Absorption spectra were recorded with a UV-vis absorption
spectrometer (JASCO, V-550).
Synthesis of polymers P3ATm
The synthesis of monomers P3ATm is outlined in Scheme 1.
1 and 2 were prepared using a procedure similar to the
literature.10,11 The experimental details are described only for
6-[4-[2-methyl-4-(4-ethoxyphenylethynyl)phenylazo]phenoxy]-
hexyl methacrylate (3AT6) as an example. P3AT9 and P3AT12
were synthesized similarly.
4-(6-Hydroxyhexyloxy)-29-methyl-49-(4-ethoxyphenylethynyl)-
azobenzene (3)
4-Bromo-2-methyl-49-(6-hydroxyhexyloxy)azobenzene
(2)
(7.83 g, 20 mmol), 1-ethynyl-4-ethoxybenzene (2.92 g,
20 mmol), PdCl2(PPh3)2 (0.70 g, 1.0 mmol), CuI (0.70 g,
3.7 mmol), PPh3 (1.30 g, 5.0 mmol) were dissolved in
triethylamine (20 ml) and THF (20 ml). The mixture was
stirred under nitrogen at 60 uC for 8 h. The mixture was cooled
to room temperature and extracted with ethyl acetate. After
the organic layer was dried with anhydrous magnesium sulfate,
the solvent was removed by evaporation. The product was
purified by column chromatography (silica gel, ethyl acetate :
chloroform 5 1 : 1 as eluent) to yield 7.60 g (70%) of orange
powder. 1H-NMR (CDCl3, d/ppm): 1.24–1.63 (m, 6H), 1.41 (t,
J 5 6.9 Hz, 3H), 1.61–1.83 (m, 2H), 2.67 (s, 3H), 3.66 (t,
J 5 6.3 Hz, 2H), 4.01–4.05 (m, 4H), 6.86 (d, J 5 6.9 Hz, 2H),
6.97 (d, J 5 8.1 Hz, 2H), 7.37 (d, J 5 6.6 Hz, 1H), 7.46–7.48
(m, 3H), 7.59 (d, J 5 8.4 Hz, 1H), 7.89 (d, J 5 5.1 Hz, 2H).
6-[4-[2-Methyl-4-(4-ethoxyphenylethynyl)phenylazo]phenoxy]-
hexyl methacrylate (3AT6)
Scheme 1
A solution of methacryloyl chloride (2.09 g, 20 mmol) in THF
(30 ml) was added dropwise at 0 uC to a mixture of 3 (5.00 g,
11 mmol), triethylamine (2.02 g, 20 mmol) and a trace amount
of hydroquinone, and the reaction mixture was stirred at room
temperature for 24 h. The solution was poured into saturated
aqueous sodium hydrogen carbonate, and the product was
extracted with ethyl acetate. After the organic layer was dried
with anhydrous magnesium sulfate, the solvent was removed
by evaporation. The crude solid was purified by column
chromatography (silica gel, chloroform as eluent) and finally
recrystallized from methanol to yield 3.40 g (68%) of orange
solid. 1H-NMR (CDCl3, d/ppm): 1.39–1.83 (m, 8H), 1.93 (s,
3H), 2.68 (s, 3H), 4.01–4.14 (m, 6H), 5.34 (s, 1H), 6.09 (s, 1H),
Table 1 Thermodynamic properties and molecular weights of
P3ATma
Phase transition
Polymer temperature/uC
DHNI/kJ mol21 Mn
Mw/Mn
P3AT6 G 47 N 220 I
P3AT9 G 39 N 212 I
P3AT12 G 34 N 229 I
0.5
1.0
1.6
23000 1.7
21000 3.9
34000 3.7
a
G, glassy; N, nematic; I, isotropic; DHNI, change in enthalpy at the
N–I phase transition.
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6.86 (d, J 5 8.4 Hz, 2H), 6.98 (d, J 5 8.7 Hz, 2H), 7.37
(d, J 5 8.1 Hz, 1H), 7.44–7.47 (m, 3H), 7.59 (d, J 5 8.4 Hz,
1H), 7.89 (d, J 5 9.0 Hz, 2H). 13C-NMR: 14.66, 17.33,
18.24, 25.70, 28.47, 29.00, 63.41, 64.53, 65.03, 68.03,
88.18, 91.44, 114.45, 114.59, 115.35, 124.81, 125.17, 125.37,
129.49, 133.04, 133.96, 136.39, 137.63, 147.31, 149.80, 159.08,
161.50, 167.40. MS (FAB): 525 (MH+). Anal. Calcd for
C33H36N2O4: C, 75.54; H, 6.92; N, 5.34. Found: C, 75.78; H,
7.14; N, 5.29%.
Polymerization of monomers 3ATm
The experimental procedure for the polymerization of mono-
mer 3AT6 is given below.
3AT6 (0.76 g, 1.4 mmol) and AIBN (3.5 mg, 14 mmol) were
dissolved in dry DMF (7.6 ml) and placed in a polymerization
tube. After several freeze–pump–thaw cycles, the tube was
sealed under high vacuum. Then, the tube was kept at 60 uC
for 48 h. The resulting solution was cooled to room tem-
perature and poured into 400 ml of methanol with vigorous
stirring to precipitate the polymer. The polymer obtained was
purified by repeated reprecipitation from DMF into a large
excess of methanol and dried under vacuum for 48 h to yield
0.50 g of P3AT6 in 66% conversion. 1H-NMR (CDCl3,
d/ppm): 0.88–1.83 (m, 14H), 2.55 (s, 3H), 3.91 (m, 6H), 6.77
(m, 4H), 7.24–7.49 (m, 5H), 7.76 (d, 2H).
Fig. 1 Polarizing optical micrograph of the texture of P3AT6 at
200 uC on heating.
Results and discussion
LC behavior of polymers
It was observed with a polarizing microscope that all the
polymers used in this study showed an LC phase (Fig. 1). The
LCPs exhibited LC states in a broad temperature range
(.170 uC) compared with conventional LCPs. We assigned the
LC phases of these polymers as nematic phases by POM and
DSC measurements. From the DSC curves of P3ATm (Fig. 2),
the phase transition temperature, glass transition temperature
(Tg), and the change in enthalpy from the nematic phase to
the isotropic phase (DHNI) were obtained and are summarized
in Table 1. The DHNI of P3ATm increased with the alkyl
spacer length. In general, LCPs showing large values of DHNI
exhibit highly packed states of mesogens. A similar depen-
dence of the DHNI on the spacers was already reported by
Craig and Imrie.12
Preparation of films
To obtain an oriented film, P3ATm was dissolved in chloro-
form, and then a small portion of the resultant solution was
cast on a polyimide-coated glass substrate, which had been
rubbed to align mesogens. Homogeneously aligned films were
obtained after annealing. Thickness of the sample films was
measured as 1.2 mm with a surface profiler (Veeco Instruments
Inc., Dektak 3ST). This film was used for g measurements. For
the investigation of the photoinduced alignment change, the
sample films with thickness of about 400 nm were prepared in
a similar manner.
Change in alignment of LCP
The photoinduced alignment change was brought about in a
monodomain state: the direction of alignment of mesogens in a
uniaxially aligned LCP film was changed to another direction
upon exposure to s-polarized actinic light. As shown in Fig. 3,
in the initial state (a), the probe light was observed through
crossed polarizers, with the sample film between them, because
Optical setup and measurements
Photoinduced alignment change of LCPs was investigated
by the following procedure. The polymer film was irradiated
with linearly polarized light at 488 nm from an Ar+ laser in
a glassy state. The direction of polarization of the pumping
light was parallel to the aligned direction of mesogens, i.e.,
s-polarization. Intensity of the probe light at 633 nm from a
He–Ne laser (NEC, GLC5370, 1 mW) transmitted through a
pair of crossed polarizers, with the sample film between them,
was measured with a photodiode. The formation of gratings
was performed by a procedure similar to the literature.7
Writing beams were two linearly s-polarized beams from an
Ar+ laser. The incident angle of the writing beams was fixed at
h 5 7u. In this study, the value of g was defined as the ratio of
the intensity of the first-order diffraction beam to that of the
transmitted beam through the film in the absence of the
writing beams. After grating formation, the surface structure
of the LCP films was investigated with an AFM (Shimazu,
SPM-9500 J2).
Fig. 2 DSC curves of P3ATm on the third heating at a rate of
10 uC min21
.
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Fig. 3 A typical profile of transmittance as a function of irradiation
time in P3AT6. The film was exposed to an s-polarized beam at
488 nm.
of the birefringence of the LCP. When the film was exposed to
the pump beam, the initial order of homogeneously aligned the
azo-tolane moieties was destroyed by trans–cis isomerization
cycles (b in Fig. 3). Upon further irradiation, the transmittance
gradually increased and finally became saturated (c in Fig. 3).
This result indicates that a change in alignment of azo-tolane
moieties is induced photochemically. To discuss the alignment
change behavior quantitatively, the order parameter (S) of the
LCP was evaluated by using the following equation:
S 5 (A// 2 A )/(A// + 2A )
(1)
H
H
Fig. 4 Polarized absorption spectra of the P3AT6 film. Spectra (A),
where A// and A are absorbance measured with light
H
(B) and (C) correspond to the states shown in Fig. 3: (A) before
irradiation (a in Fig. 3); (B) disruption of the initial order by trans–cis
photoisomerization (b in Fig. 3); (C) induction of the photoinduced
alignment change (c in Fig. 3).
polarized parallel and perpendicular to the rubbing direction,
respectively. Polarized absorption spectra of the P3AT6 film
measured before and after irradiation with linearly polarized
light at 488 nm at room temperature are shown in Fig. 4 where
spectra (A), (B) and (C) correspond to the states (a), (b) and (c)
in Fig. 3, respectively. Before irradiation, the value of S
was 0.41, which indicates a typical aligned nematic LC phase
(Fig. 4(A)). In Fig. 4(B), the value of S was 0.11. This value
was smaller than that of the initial state because the
homogeneously aligned azo-tolane moieties are disorganized
by the trans–cis isomerization process. In Fig. 4(C), the value
of S changed to 20.25, indicating that the alignment of the
azo-tolane moieties in P3AT6 has become perpendicular to the
electric vector of the irradiation light. In this study, it has
been observed that the transmittance change is caused by the
photoinduced alignment change.
Fig. 5 Change in transmittance of the P3AT6 film as a function of
irradiation time. Films were exposed to an s-polarized beam at 488 nm.
Light intensity: a, 3 mW cm22; b, 50 mW cm22; c, 150 mW cm22; d,
Change in birefringence of LCP
Fig. 5 shows the change in transmittance of P3AT6 at various
light intensities. From this result, we evaluated the values of
birefringence by using eqn. (2):13–15
250 mW cm22
.
moieties are separately incorporated into side chains of
LCPs, because azo-tolane moieties could have large anisotropy
of polarizability. When the intensity of the pumping beam was
250 mW cm22 and 150 mW cm22, the transmittance decayed
upon irradiation of s-polarized light in the glassy state and
reached very low values (c and d in Fig. 5). This result indicates
that a photoinduced alignment change destroys the initial
order of homogeneously aligned azo-tolane moieties and
T 5 sin2(pdDn/l)
(2)
where d is the film thickness, Dn is the birefringence of the
P3AT6 film, T is the transmittance, and l is the wavelength of
the probe light (633 nm). In the initial state, the calculated
value of birefringence was 0.39. This value is much higher than
those of the previous studies,8 in which two-ring tolane
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Fig. 6 Change in intensity of diffracted light in the P3AT6 film
as a function of irradiation time. Films were exposed to (s + s)-
polarized writing beams at 488 nm. Light intensity: a, 3 mW cm22; b,
50 mW cm22; c, 150 mW cm22; d, 250 mW cm22
.
Fig. 7 AFM three-dimensional and slice views of the grating
recorded in P3AT6. The gratings showed g # 25% with a surface
modulation of ca. 20 nm.
induces a large change in birefringence (#0.35). Upon
irradiation of the actinic light at 3 mW cm22, the change in
birefringence was 0.15. It was found that one can induce a
large change in birefringence in this LCP even though the
intensity of the pumping light is several mW cm22. This
observation gives an idea that one can induce a large change in
refractive index upon exposure to actinic light if LC materials
with high birefringence are employed.
beams. Among them, two s-polarized (s + s) configuration
gives rise to a very small surface modulation, nearly negligible
(,a few nanometers), and shows the smallest value of g
(,0.4%) due to SRG.18 In this study, we explored the surface
modulation of the P3AT6 films under the (s + s) configuration.
Since the periodic structure of the surface relief was about
2.0 mm wide in each case while its height was about 20 nm,
the contribution of the surface modulation to the spatial
change in refractive index is almost negligible in this study. So
we conclude that the gratings are formed by the change in
refractive index caused by an order–disorder change of the
azo-tolane moieties. From this result, the difference in
refractive index between the bright and the dark areas (Dn9)
could be evaluated by eqn. (3):19
Grating formation in LCP
We attempted the formation of gratings in P3AT6 at various
light intensities (Fig. 6). When the intensity of the writing
beams was 250 mW cm22 and 150 mW cm22, we observed
maximum diffraction efficiency (gmax) of above 25% within 30 s
(c and d in Fig. 6). The rate of the grating formation was much
higher than that observed in the previous study.8 The intensity
of the diffracted light increased first with irradiation time, and
then it decreased. This behavior could be due to an increase at
first, followed by a decrease, of difference in the refractive
index between bright and dark areas of the interference
patterns. Similar behavior has been reported when high-
intensity writing beams were employed.7 It is worth noting that
we observed a value of g of 20% even at writing beam intensity
as low as 3 mW cm22. This result is explicable in terms of the
change in transmittance shown in Fig. 5. Namely, a large
change in refractive index in the bright areas of the interference
patterns is induced even though the change in alignment by
light is small. Therefore, it has become apparent that P3AT6
shows high sensitivity as holographic material. Generally, low-
molecular-weight LCs exhibit high sensitivity in the formation
of gratings, but possess low stability of stored information.16,17
However, the LCP film also shows high stability of gratings
below the glass transition temperature (Tg).
g 5 (pdDn9/l)2
(3)
where d and l are the film thickness and the wavelength of the
reading beam, respectively. The maximum value of Dn9 was
estimated as 0.09.
Mechanism for formation of gratings
We explored the relation between the diffracted light intensity
and the photoinduced alignment behavior. Fig. 8 shows the
dynamics of the change in transmittance by the alignment
change as well as the diffraction efficiency of the P3AT6 film.
As is apparent from the change in the diffraction efficiency
and the transmittance, the value of g showed a maximum when
the transmittance of probe light was minimum. This result
indicates that when the destruction of the initial order of the
azo-tolane moieties is completed in the bright areas of the
interference patterns, the value of g shows a maximum since
the difference in refractive index between the bright and the
dark areas reaches a maximum (Fig. 9(A)). Upon further
irradiation, the value of g gradually decreases due to the
order–disorder change of azo-tolane moieties in the dark areas
(Fig. 9(B)), which leads to a smaller difference in refractive
index between the bright and dark areas. Similar mechanisms
have already been reported when high-intensity writing beams
were employed.7
Modulation of surface structure
Evaluation of photoinduced surface modulation by AFM also
demonstrated the formation of gratings. Fig. 7 shows an AFM
image which was recorded at a light intensity of 250 mW cm22
for 30 s. After exposure to the writing beams, the g value was
about 25%. The formation of surface relief grating (SRG)
depends on the polarization configuration of the writing
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Fig. 10 Change in intensity of diffracted light as a function of
Fig. 8 Typical profiles of the grating formation and the photo-
chemical alignment change in the P3AT6 film. The intensity of the
irradiation time for the P3ATm films. a, P3AT6; b, P3AT9, c, P3AT12.
Films were exposed to (s + s)-polarized writing beams (488 nm) at
writing beams and the pumping beam was adjusted to 250 mW cm22
.
150 mW cm22
.
Conclusion
In summary, we synthesized highly birefringent LCPs contain-
ing an azobenzene group directly connected with a tolane
moiety. One can induce a large change in birefringence
(#0.35) in three LCPs by the photochemical alignment
change. The grating formation in these LCPs could be
attributed to the photoinduced order–disorder change of the
azo-tolane moieties. The formation of gratings takes place
faster in the LCPs having shorter alkyl spacers because the
alignment change in the LCPs possessing short spacer length
could be easily generated by the order–disorder change.
Furthermore the LCP films exhibited high sensitivity as
holographic materials. Therefore it might be possible to use
these LCPs in compact optical systems.
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Effect of spacer length on grating formation
The effect of the spacer length of P3ATm on the diffraction
efficiency was evaluated (Fig. 10). We observed a tendency
that the formation of gratings takes place faster in the LCPs
having shorter alkyl spacers (P3AT6 . P3AT9 . P3AT12).
This tendency could be attributed to the packing density of
the azo-tolane moieties because the grating is formed by the
order–disorder change of the azo-tolane moieties. Thus the
least ordered azo-tolane moieties are likely to be influenced by
an external stimulus causing an order–disorder change. It may
be assumed that LCPs with low-packing density exhibit high
ability to undergo the order–disorder change. From the
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DHNI (Table 1), P3AT12 with the largest value of DHNI
shows the highest resistance to the stimuli that could destroy
the ordered structure. The mobility of the azo-tolanes in the
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This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 3395–3401 | 3401