Photoinduced birefringence in poly(malonic ester) containing p-cyanoazobenzene with photoexcitation of cis conformer

Article abstract of DOI:10.1021/jp0102825

Photoinduced birefringence in a poly(malonic ester) containing two symmetrical p-cyanoazobenzene groups has been investigated using two pumping beams with different wavelengths of 365 and 488 nm, which are assignable to the absorption wavelengths of trans

Full text of DOI:10.1021/jp0102825

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322
J. Phys. Chem. B 2001, 105, 8322-8326
Photoinduced Birefringence in Poly(malonic ester) Containing p-Cyanoazobenzene with
Photoexcitation of cis Conformer
Won-Jae Joo, Cha-Hwan Oh,* Seok-Ho Song, and Pill-Soo Kim
Department of Physics, Hangyang UniVersity, Seoul 133-791, Korea
Yang-Kyoo Han
Department of Chemistry, Hangyang UniVersity, Seoul 133-791, Korea, and Center for AdVanced and
Functional Polymers, KAIST, Taejeon, Korea
ReceiVed: January 24, 2001; In Final Form: June 28, 2001
Photoinduced birefringence in a poly(malonic ester) containing two symmetrical p-cyanoazobenzene groups
has been investigated using two pumping beams with different wavelengths of 365 and 488 nm, which are
assignable to the absorption wavelengths of trans and cis conformers of azobenzene, respectively. We observed
unexpected photoinduced birefringence even by the pumping beam of 488 nm as large as that by 365 nm. To
describe such specific phenomenon, we measured the magnitude and direction of the induced birefringence
by means of a new experimental setup, adding λ/4 waveplate between the polymer film and the analyzer of
conventional a pair of crossed polarizers setup. The measurement of the photoinduced phase retardation by
simultaneous illumination with two perpendicularly polarized beams of 365 and 488 nm supports the fact
that there is another process for the photoinduced birefringence besides a typical trans-cis photoisomerization
of azobenzene.
Introduction
explained by the conventional theory for the photochemical or
thermal isomerization. We proposed a new interpretation for
the relaxation process on the basis of elastic force between
Recently, polymeric materials have been suggested as the
most promising materials for optical data storage and optical
information process applications.1-3 There are many organic
photoresponsive materials, including photopolymers, photo-
refractive polymers, LB films, and the polymers containing
azobenzene group in the side-chain or main-chain. Among them,
in particular, the azobenzene containing side-chain polymers
have been studied intensively due to their large nonlinearity
and reversibility in data storage.
In general, storage or processing of an optical information
in the azo-polymers results from the photoinduced birefringence
caused by the alignment of azobenzene mesogenic group. As
is well-known, the alignment occurs as a result of the angular
selective trans-cis and cis-trans photoisomerization of azo-
benzene group or its thermal isomerization.3 The isomerization
from rod-shaped trans conformer of azobenzene to bent-shaped
cis one takes place only by the photochemical excitation,
whereas both photochemical excitation and thermal relaxation
can drive the cis-trans isomerization. The orientational photo-
induced birefringence procedure is represented simply as
follows: The linearly polarized pumping beam illuminating on
the polymer film converts the trans conformer to the cis one,
which is proportional to the square of the cosine of the angle θ
between the polarization axis of pumping beam and the
symmetry axis of rod-shaped trans conformer. The photoexited
cis conformer goes back to trans conformer with nonangular
selectivity by means of thermal relaxation or photochemical
process. As a result of repetitive cycles of trans-cis-trans
isomerization, anisotropic angular distribution of trans conformer
7
neighboring molecules in our previous paper.
In the present article, we reports the photoinduced birefrin-
gence in poly(malonic ester) containing p-cyanoazobenzene
group using two pumping beams with different wavelengths of
3
65 and 488 nm, which are assignable to π-π* transition of
the trans conformer of azobenzene and n-π* transition of the
8,9
cis form, respectively. Unexpected photoinduced birefringence
was observed by the pumping beam of 488 nm. To describe
such specific phenomenon, a novel experimental setup was
introduced and the results were analyzed on the basis of the
conventional photoisomerization theory.
Experiments
-5
As optical recording media, novel liquid crystalline (LC) side-
chain poly(malonic ester) was prepared according to the reported
1
0
procedure shown in Figure 1. New thermotropic LC malonic
ester compound (MCN) was first synthesized by reacting in
chloroform for 48 h at 5 °C malonyl dichloride and mesogenic
alcohol (mp 142 °C) with p-cyanoazobenzene. The yield and
melting temperature of the MCN were 46% and 95 °C,
respectively. It was observed by means of differential scanning
calorimetry for the MCN monomer to show a smectic structure
on both heating and cooling cycles. The phase transition
temperatures to LC state appeared at 63 and 90 °C, respectively,
on the heating and cooling cycles. The isotropization temper-
atures were 95 and 55 °C, respectively. In the phase transition
temperatures, a large value of supercooling between the heating
and cooling cycles might be due to the rapid cooling rate
(average rate: 30 °C/min) of the equipment and also to the
complicated structural change with temperature of the monomer.
6
is induced by the illumination of the linearly polarized light.
We have reported an abnormal phenomenon in the relaxation
process of photoinduced birefringence, which could not be
1
0.1021/jp0102825 CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/10/2001

Photoinduced Birefringence in Poly(malonic ester)
J. Phys. Chem. B, Vol. 105, No. 35, 2001 8323
Temporal evolution of concentration of trans conformer in
the pumping process was obtained by measuring the absorbance
of the polymer film at 365 nm with exposure time. The color-
filtered Xe-lamp was used as both pumping and reading beams.
To avoid any photochemical effect caused by the reading beam
on the film, the intensity of the reading beam was attenuated
+
with ND filter. On the other hand, Ar ion laser of 488 nm
was used as the other pumping beam with different wavelength.
By the conventional two-crossed polarizers setup, only the
magnitude of birefringence can be measured without the
directional information of fast-axis of a induced birefringence,
but the directional information is very important for investigating
the mechanism of photoinduced birefringence in the azo polymer
film. Thus, the modified experimental setup was prepared to
obtain the information of both magnitude and direction. That
is, a quarter waveplate was added between the polymer film
and the analyzer of the conventional two-crossed polarizers
setup, as shown in Figure 2. The optic axis of the quarter
waveplate was adjusted by 5° with respect to the polarization
axis of the analyzer. Two pumping beams with different
wavelengths of 365 and 488 nm were used for simultaneous
illumination on the polymer film.
Figure 1. Synthesis route for poly(malonic ester), PCN.
It, however, is not clear yet and need to examine in more detail.
The LC structure of the MCN was found by means of optical
polarizing microscopy to show a focal-conic texture. The MCN
was then condensed with 1,6-dibromohexane in tetrahydrofuran
in the presence of sodium hydride at 65 °C for 48 h to give
poly(malonic ester) (PCN) with two symmetrical azo dye
mesogens in the side chain. The polymeric thin film (ca. 0.8
µm) was cast from the polymer solution (5 wt %) in CHCl3
onto a glass plate for 30 s using a spin coater.
During illumination of the pumping beam, the change of
absorption spectrum of the prepared polymer film was measured
using optical multichannel analyzer (OMA), which is composed
of monochromator, charge-coupled display (CCD) as a photo-
detector and sufficiently attenuated Xe-lamp as a reading light
source. The spectral resolution of the used OMA was 2 nm.
We employed a standard two-crossed polarizers setup for
Results and Discussion
Figure 3 shows the change of the absorption spectrum of PCN
film during the irradiation of pumping beam at 365 and 488
nm. The absorption spectrum was measured by means of OMA.
The PCN film has two broad absorption bands. As is well-
known, the larger absorption band around 362 nm is due to the
trans conformer of azobenzene and the much smaller one around
4
50 nm is due to the cis one. The color-filtered Xe-lamp with
+
center wavelength of 365 nm and Ar laser beam of 488 nm
were used for pumping trans and cis conformers of azobenzene,
respectively. As shown in Figure 3a, the absorption band of
trans conformer rapidly decreased with the irradiation time of
the pumping beam of 365 nm. However, the band of cis one
increased gradually with increasing the exposure time. Photo-
stationary state of the concentration of trans and cis forms was
approached after irradiating over 30 s. On the other hand, when
the pumping beam of 488 nm was irradiated, the absorption
bands of both trans and cis conformers remained unchanged as
6
measuring the photoinduced birefringence in the polymer film.
The color-filtered Xe-lamp and Ar+ ion laser were used as
pumping beams of 365 and 488 nm, respectively. The model
of the color filter was UG.11 from Newport Company. The
wavelength of the reading beam from He-Ne laser, 633 nm, is
far from the absorption bands of the PCN film. The transmit-
tance of the reading beam passing through two crossed polarizers
and a polymer film placed between them was measured with a
photodiode.
Figure 2. Experimental setup for measurements of the photoinduced phase retardation of PCN thin film. The inset shows the angle δ between the
fast axis of λ/4 waveplate and the polarization axis of analyzer.

8324 J. Phys. Chem. B, Vol. 105, No. 35, 2001
Joo et al.
Figure 4. Time evolution of the absorbance of the PCN film with
angle between the polarization axes of the pumping and reading beams,
during pumping at (a) 365 and (b) 488 nm. The angle increased from
0° to 90° by 10°.
Figure 3. UV absorption spectra of the PCN film after irradiation by
pumping beams of (a) 365 and (b) 488 nm.
shown in Figure 3(b). These results mean that the initial
concentration of unstable cis conformer is very low, and also
that the rate constant of photochemical trans-cis transition by
the pumping beam of 488 nm is fairly small.
Temporal evolution of the concentration of trans conformer
in the pumping process was measured through the absorbance
of the polymer film at 365 nm. The photoinduced birefringence
in our PCN film mainly results from the reorientation of trans
form, so that the angular distribution of the concentration of
trans conformer in pumping process offers a molecular dynamics
of the trans conformer of azobenzene. In other words, we can
easily estimate the angular distribution of the concentration of
trans conformer from Figure 4. The absorbance of PCN film
was measured with the exposure time, while the polarization
axis of the linearly polarized reading beam is rotated discretely
from 0 to 90° with respect to that of the pumping beam. As the
pumping beam of 365 nm was illuminated, the absorbance
curves totally decreased due to the large photochemical trans-
cis isomerization rate and they showed an angular sinusoidal
distribution of trans conformer, which is responsible for the
birefringence of the polymer film. On the other hand, as the
pumping beam of 488 nm was illuminated, the absorbance
curves decreased at the parallel direction to the polarization axis
of the pumping beam, but increased at the perpendicular
direction. The average absorbance of all angles did not change
during illumination of the pumping beam of 488 nm. Hence,
this supports that the photochemical trans-cis isomerization
occurred very weakly at the wavelength of pumping beam.
Figure 5 shows the photoinduced birefringence of the PCN
film measured by pumping beams of 365, 488 and 514.5 nm.
The pumping beams of 488 and 514.5 nm as well as 365 nm
induced the optical birefringence. It is noticeable that the
magnitude of the induced birefringence by the pumping beam
Figure 5. Photoinduced birefringence in pumping at 365, 488 and
514.5 nm onto the PCN film. The intensity of the pumping beams was
2
2
mW/cm .
of 488 nm is comparable to that by the pumping beam of 365
nm: The photoinduced birefringence by the pumping beam of
365 nm matches well with the conventional trans-cis photo-
isomerization theory of azobenzene. However, in the case of
pumping at 488 and 514.5 nm, those results were beyond our
expectation. Since the energy of the pumping beam at 488 and
514.5 nm is lower than the energy required for π-π* transition
of a trans conformer of azobenzene, it is impossible for the trans
form to be excited to the cis one by illuminating with those
8
,9
pumping beams. Nevertheless, the illumination at 488 and
514.5 nm produced the photoinduced birefringence resulting
from the alignment of trans conformer, without the repetitive
cycles of trans-cis-trans isomerization. But we cannot confirm
that any trans-cis photoisomerization did not occur at all by

Photoinduced Birefringence in Poly(malonic ester)
J. Phys. Chem. B, Vol. 105, No. 35, 2001 8325
trans one. Thus, when the two pumping beams were illuminated
simultaneously on the PCN film, the large trans-cis isomer-
ization rate by pumping at 365 nm and the large cis-trans
isomerization rate by pumping at 488 nm could be expected.
In this case, the main direction of the aligned azobenzene group
would be determined by the competition of the optical trans-
cis isomerization rate derived from each pumping beam.
Although the intensity of the pumping beam of 365 nm was
the same as that of 488 nm, the trans-cis isomerization rate
derived from the 365 nm should have a much larger value than
that from the 488 nm, because the absorbance of the trans
conformer of azobenzene at 365 nm is over 10 times higher
than that at 488 nm. Therefore, it is obvious that the main
direction of the aligned azobenzene group has to be perpen-
dicular to the polarization axis of the pumping beam of 365
nm and the magnitude of the phase retardation should remark-
ably increase rather than that induced only by the pumping beam
of 365 nm. However, the photoinduced phase retardation by
the two pumping beams was nearly zero, as shown in Figure 6.
It is impossible to explain the above results utilizing the
mechanism of the repetitive trans-cis-trans isomerization. The
phase retardation for the simultaneous pumping with the two
beams was nearly the same to the numerical sum of the phase
retardations induced independently by each pumping beam.
Unfortunately, it is not clear yet what the photoreorientation
mechanism induced by pumping at 488 nm proceeds exactly.
As far as we know, the mechanism seems to be related with
the permanent and field-induced dipole moment of azobenzene
molecules and the photomolecular resonant interaction. In liquid
crystal molecules, the Freedericksz transition is a typical
photomolecular interaction, but it exhibits large threshold
Figure 6. Photoinduced phase retardation of the PCN thin film. Curve
A is irradiated at 365 nm (45°), curve B at 488 (-45°), curve C at 365
(45°), and 488 nm (-45°), and curve D is calculated from the sum of
curves A and B.
the pumping beam of 488 nm, although the wavelength of the
pumping beam was out of the absorption band of trans
conformer.
Consequently, we are not sure whether the explanation on
the basis of a typical photoisomerization theory of azobenzene
is right or not. To describe the unexpected results, we designed
new experimental setup shown in Figure 2 by adding a λ/4
waveplate and a pumping beam with different wavelength and
polarization into the conventional two-crossed polarizers setup.
When the birefringence was induced by pumping beam with
the polarization axis of 45° or -45°, the transmitted intensity
increases or decreases, respectively. The transmittance can be
represented by11
2
12
pumping intensity of around 150 W/cm .
Recently, it has
been reported that the threshold pumping intensity for the
Freedericksz transition decreases greatly in azobenzene doped
Iout
1
2
Γ
2
13
2
2
polymers. We need to carry out the studies on the interaction
)
2sin + sin Γsin 2δ + sin 2δ cos Γ
(1)
(
)
Iin
between the molecule and optical field to establish the mech-
anism of the birefringence induced by pumping at the absorption
band of cis conformer of azobenzene.
where δ is the angle between the optic axis of the λ/4 waveplate
and the polarization axis of the analyzer, and Γ is the phase
retardation occurred through the polymer film and it is defined
Conclusion
1
1
as the difference of the phase delays in Γ ) (ns - nf)ωl/c.
We investigated the photoinduced birefringence of poly-
(malonic ester) containing p-cyanoazobenzene group by the
pumping beams at 365 and 488 nm, which are responsible for
the photoexcitations of trans and cis conformers of azobenzene,
respectively. From the measurements of the change of absor-
bance in pumping at 365 and 488 nm, the optical trans-cis
isomerization rate by pumping at 488 nm was found to be very
small. Nevertheless, the birefringence was induced by pumping
not only at 365 nm, but also at 488 and 514.5 nm. According
to the trans-cis photoisomerization theory, the trans-cis-trans
cycles could be occurred due to large rate constant of optical
cis-trans isomerization, although trans-cis isomerization is
very weak. In the measurement of the photoinduced phase
retardation by simultaneous illumination with two perpendicu-
larly polarized beams of 365 and 488 nm, the magnitude of
phase retardation was very small and the main direction of the
aligned mesogenic azobenzene group was not perpendicular to
the polarization of the pumping beam of 365 nm unlike our
expectation. These results cannot be explained by the mechanism
of the trans-cis-trans photoisomerization reported so far. Such
an unexpected phenomenon for the reorientation effect of the
mesogenic group is not clear yet, but it seems to be related
with the permanent and field-induced dipole moment of
azobenzene molecules and the photomolecular resonant inter-
action.
The polarization axis of the pumping beam of 365 nm was
45° with respect to the polarization axis of the analyzer and
that of the other pumping beam of 488 nm was -45°. The
pumping beams will reorient the mesogenic azobenzene groups
in the PCN film perpendicularly with respect to their polarization
axis. Thus, if the two pumping beams illuminate simultaneously
on the film, the main direction of the reoriented mesogenic
groups will be determined by the competition reaction between
the two pumping beams.
Figure 6 shows the photoinduced phase retardation curves at
various pumping conditions. When the pumping beam of 365
nm was illuminated on the PCN film, the phase retardation
increased gradually with positive sign, as shown in Figure 6A.
By the pumping beam of 488 nm, the phase retardation
decreased with negative sign (Figure 6B). The sign, positive or
negative, of the phase retardation indicates the direction of the
mesogenic azobenzene groups. As mentioned earlier, it is well-
known that the photoinduced birefringence occurs through the
repetitive trans-cis-trans isomerization cycles. The birefrin-
gence induced by the pumping beam of 488 nm is attributed to
large optical cis-trans isomerization rate and very small trans-
cis isomerization rate. On the other hand, the birefringence by
3
65 nm is attributed to large optical trans-cis isomerization
rate with the angular selectivity and large (or not small) cis-

8326 J. Phys. Chem. B, Vol. 105, No. 35, 2001
Joo et al.
Acknowledgment. This work was supported by the Korea
Research foundation Grant KRF-99-015-DI0046.
(6) Natansohn, A.; Rochon, P.; Gosselin, J.; Xie, S. Macromolecules
992, 25, 2268.
1
(7) Joo W.; Shin, H.; Oh, C.; Song, S.; Kim, P.; Ko, B.; Han, Y. J.
Chem. Phys. 2000, 113, 8848.
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