Influence of the backbone on photoinduced birefringence in a poly(malonic ester) containing p-Cyanoazobenzene

Article abstract of DOI:10.1021/jp0144808

The influence of the backbone on the photoinduced birefringence was investigated in a poly(malonic ester) containing a p-cyanoazobenzene group as a side-chain. The photoinduced birefringence was successively measured in the fifteen cycles of the recording, relaxation, and erasing processes. The linearly polarized light, with a polarization direction of 45?° relative to the analyzer, was used as the pumping beam during the first five cycles, and the same pumping beam at negative 45?° was used during the next 10 cycles. When the polarization direction of the pumping beam was changed perpendicularly, a very interesting birefringence curve was observed in the recording and relaxation processes, which could be well explained by our model considering the reorientation of the backbone caused by the elastic force between the backbone and side-chain. Consequently, we confirmed the fact that the relaxation behavior of the photoinduced birefringence is closely associated with the reorientation of the backbone.

Full text of DOI:10.1021/jp0144808

5378
J. Phys. Chem. B 2002, 106, 5378-5381
Influence of the Backbone on Photoinduced Birefringence in a Poly(malonic ester)
Containing p-Cyanoazobenzene
Won-Jae Joo and Cha-Hwan Oh*
Department of Physics, Hanyang UniVersity, Seoul 133-791, Korea
Yang-Kyoo Han
Department of Chemistry, Hanyang UniVersity, Seoul 133-791, Korea, and Center for AdVanced and
Functional Polymers, KAIST, Taejeon, Korea
ReceiVed: December 10, 2001; In Final Form: February 27, 2002
The influence of the backbone on the photoinduced birefringence was investigated in a poly(malonic ester)
containing a p-cyanoazobenzene group as a side-chain. The photoinduced birefringence was successively
measured in the fifteen cycles of the recording, relaxation, and erasing processes. The linearly polarized
light, with a polarization direction of 45° relative to the analyzer, was used as the pumping beam during the
first five cycles, and the same pumping beam at negative 45° was used during the next 10 cycles. When the
polarization direction of the pumping beam was changed perpendicularly, a very interesting birefringence
curve was observed in the recording and relaxation processes, which could be well explained by our model
considering the reorientation of the backbone caused by the elastic force between the backbone and side-
chain. Consequently, we confirmed the fact that the relaxation behavior of the photoinduced birefringence is
closely associated with the reorientation of the backbone.
Introduction
Recently, polymeric materials containing an azobenzene
group have been recognized to be promising materials for
holographic applications due to their large optical nonlinearity
and reversibility.1-4 In general, in this type of the polymer, the
optical data can be recorded by photoinduced birefringence or
dichroism via the alignment of the mesogenic azobenzene
groups perpendicular to the polarization direction of irradiating
light.4-6 As is well-known, the alignment of mesogenic azoben-
zene groups occurs as a result of the trans-cis photochemical
isomerization process.4-7
In the previous paper, we observed an abnormal relaxation,
positive relaxation of the induced birefringence in the poly-
(malonic esters) containing p-cyanoazobenzene as a side-chain
and we presented a novel model of the fast relaxation of the
photoinduced birefringence to explain this abnormal phenom-
enon.8 In the model, the side-chain is first aligned by the
illumination of the linearly polarized light and then the backbone
Figure 1. Synthesis route for poly(malonic esters), PCN.
is reoriented by means of the elastic force between the aligned
side-chain and the backbone. Consequently, the fast relaxation
containing p-cyanoazobenzene. Especially, as the polarization
direction of the pumping beam was changed perpendicularly,
is a result of the elastic force, and the remnant birefringence
we observed very interesting recording and relaxation curves
after the fast relaxation is closely associated with the reorienta-
of the photoinduced birefringence, which proved our model
tion of the backbone occurring in the recording process.
experimentally for the reorientation of the backbone. Moreover,
Therefore, if the reorientation of the backbone can be controlled
we could find out the concept of the influence of the reorienta-
by adjusting the molecular weight, the spacer length, or other
tion of the backbone on the relaxation behavior of the induced
variables, it can be practically applied to improve the optical
birefringence.
properties of polymer film, such as long-term stability and fast
response.
Experimental Section
In this paper, we have investigated the reorientation of the
backbone in the recording, relaxation, and erasing processes of
the photoinduced birefringence in the poly(malonic esters)
As the optical recording medium, novel liquid crystalline-
(LC) side-chain poly(malonic ester) was prepared according to
the reported procedure shown in Figure 1.9 A new thermotropic
LC malonic ester compound (MCN) was first synthesized by
* Author to whom correspondence should be addressed.
10.1021/jp0144808 CCC: $22.00 © 2002 American Chemical Society
Published on Web 05/03/2002

Photoinduced Birefringence in a Poly(malonic ester)
J. Phys. Chem. B, Vol. 106, No. 21, 2002 5379
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 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 temperatures were 95
and 55 °C, respectively. For the phase transition temperature
of the MCN monomer, the degree of supercooling between the
heating and cooling cycles has a large value from 27 to 40 °C.
Such a phenomenon 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 mono-
mers. It, however, is not clear yet and needs to be examined 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. The polymeric thin film (ca. 0.3 µm) was
cast from the polymer solution (5 wt %) in CHCl3 onto a glass
plate for 30 s using a spin coater.
We employed a standard crossed polarizers setup for mea-
surement of the photoinduced birefringence in the polymeric
film.8 The polarization axes of the polarizer and the analyzer
were 0° and 90° with respect to the incident plane, respectively.
The linearly polarized beams from an Ar+ ion laser of 496 nm
and from a He-Ne laser of 633 nm were used as the pumping
and probe beams, respectively. The wavelength of the probe
beam, 633 nm, is far from the absorption band of the PCN film.
The polarization direction of the pumping beam was adjusted
to 45° and -45° relative to the analyzer by using a λ/2
waveplate. The transmittance of the probe beam through crossed
polarizers and the polymer film placed between them was
measured with a photodiode.
Results and Discussion
In Figure 2a, repetitive recording, relaxation, and erasing
curves of photoinduced birefringence in the PCN film are shown
for five cycles. The polarization direction of the pumping beam
was set to 45° in the recording process, the circularly polarized
pumping beam was used as the erasing beam and the time
interval between each repeated cycles was fixed at 3 min. As
shown in Figure 2a, as the number of the repeated cycles
increased, the induced birefringence in the recording process
increased gradually and the initial birefringence before switching
on the pumping beam also increased slightly. Since these cycles
were successively carried out, the initial birefringence at each
cycle was the remnant birefringence of the previous erasing
process, and therefore it can be eliminated by increasing the
erasing time or the erasing beam intensity. However, we can
obtain information about the reorientation of the backbone from
the remnant birefringence after a finite erasing process, which
will be dealt with in detail later in Figure 4. In Figure 2b, the
recording, relaxation, and erasing processes of the photoinduced
birefringence were continued with the linearly polarized pump-
ing beam at -45°, following the experiments of Figure 2a. As
can be seen in Figure 2b, the initial birefringence remained,
more or less, before the first recording process. As soon as the
pumping beam was turned on at -45°, the birefringence
decreased initially to zero and then increased gradually to
0.0025, which is smaller than the induced birefringence, 0.0045,
Figure 2. Photoinduced birefringence curves of PCN film in the
recording, relaxation, and erasing processes, which were successively
measeured fifteen times in total; (a) for the first five cycles, the
polarization direction of the pumping beam was 45° relative to the
analyzer, and (b) for the next 10 cycles, the polarization direction was
-45°. The inset shows the first photoinduced birefringence curve after
changing the polarization direction of the pumping beam.
in the first recording process of Figure 2a. However, similarly
to the results of Figure 2a, as the number of the repeated cycles
increased, the induced birefringence gradually increased. In the
first relaxation curve of Figure 2b, it is a remarkable result that
the remnant birefringence nearly became zero after the fast
relaxation of the photoinduced birefringence. As the number
of the repeated cycles increased, the remnant also increased
gradually to the value that is shown in Figure 2a.
Figure 3 shows the maximum photoinduced birefringence
with the number of repeated cycles, where the pumping beam
with a polarization direction of 45° was used during first five
cycles, and the pumping beam at -45° was used during the
following 10 cycles. The photoinduced birefringence increased
gradually for the first five cycles and then decreased greatly as
soon as the polarization direction was changed perpendicularly.
This can be explained well by the reorientation of the backbone
according to our model.8

5380 J. Phys. Chem. B, Vol. 106, No. 21, 2002
Joo et al.
Figure 5. Ratio of the remnant birefringence of the fast relaxation to
the maximum photoinduced birefringence with the number of repeated
cycles.
Figure 3. Maximum photoinduced birefringence with the number of
repeated cycles.
remnant birefringence after the whole relaxation. During the
first five cycles, the ratio has a constant value of 0.43, but as
soon as the polarization direction of the pumping beam was
changed perpendicularly, the ratio dramatically decreased to 0.1.
Then, as the cycle was repeated, the ratio increased gradually
to 0.35. By fitting exponentially the growth, the ratio is expected
to approach 0.38. From these results, we can conclude that the
remnant birefringence is mainly determined by the reorientation
of the backbone in the relaxation process, which is expected in
our model. In this kind of polymer, the long-term stability is
one of most important properties required for holographic
applications such as optical data storage. Moreover, the fast
response in the recording and erasing processes is also necessary
for optical switching. If the reorientation of the backbone can
be controlled in any way, it will be useful for improving the
optical properties mentioned above in a polymer containing an
azobenzene group.
Figure 4. Initial birefringence with the number of repeated cycles.
Conclusion
Figure 4 shows the variation of the initial birefringence before
the recording process with the number of repeated cycles. As
mentioned before, the initial birefringence is due to the remnant
birefringence of the previous erasing process. In the erasing
process, the reoriented side-chain can be randomized much more
easily than the reoriented backbone, because of the photo-
responsive property of the side-chain containing an azobenzene
group. Namely, we could obtain information about the reori-
entation of the backbone from the remnant birefringence of an
erasing process. Until the polarization direction of the pumping
beam was changed, the initial birefringence increased to 4.8 ×
10-4 with the number of repeated cycles. At the next cycle (7th
cycle), it decreased dramatically to almost zero, but the backbone
was still reoriented slightly. At the 8th cycle, the initial
birefringence became zero, and the initial value then increased
gradually with the number of repeated cycles. At the 6th cycle,
the initial birefringence was smaller slightly than that of the
5th cycle due to taking several extra minutes to change the
polarization direction of the pumping beam.
The reorientation of the backbone was experimentally inves-
tigated in a poly(malonic ester) containing a p-cyanoazobenzene
group as a side-chain, when the birefringence was induced by
a linearly polarized pumping beam. After five cycles of
recording, relaxation, and erasing processes with the linearly
polarized pumping beam at 45°, the cycles are repeated 10 times
with the pumping beam at -45°. In these repeated cycles, we
could observe very interesting recording and relaxation curves
of the photoinduced birefringence, which proved our previous
model experimentally for the reorientation of the backbone.
Consequently, we confirmed the fact that the relaxation behavior
of the photoinduced birefringence is closely associated to the
reorientation of the backbone in a polymer containing an
azobenzene group. This might be practically applied for the
improvement of optical properties, such as the long-term stability
of the recorded optical data and the fast response time in a
polymer containing an azobenzene group.
Acknowledgment. This work was supported by Grant R01-
2000-00338 from the Basic Research Program of the Korea
Science & Engineering Foundation as well as the Center for
Advanced Functional Polymers, KAIST in part.
Figure 5 shows the ratio of remnant birefringence after the
fast and slow relaxation to the maximum induced birefringence
of the recording process. The remnant birefringence was
determined by fitting the relaxation curve to the biexponential
function, f(t) ) Af exp(-t/τf) + As exp(- t/τs) + c, where the
first and second terms are assignable to the fast and slow
relaxation processes, respectively, and constant “c” denotes the
References and Notes
(1) Rasmussen, P. H.; Ramanujam, P. S.; Hvilsted, S.; Berg, R. H. J.
Am. Chem. Soc. 1999, 121, 4738.

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J. Phys. Chem. B, Vol. 106, No. 21, 2002 5381
(2) Todorov, T.; Nikolova, L.; Stoyanova, K.; Tomova, N. Appl. Opt.
1985, 24, 785.
(3) Anderle, K.; Birenheide, R.; Eich, M.; Wendorff, J. H. Makromol.
Chem. Rapid Commun. 1989, 10, 477.
(6) Joo, W.; Oh, C.; Song, S.; Kim, P.; Han, Y. J. Phys. Chem. B
2001, 105, 8322.
(7) Natansohn, A.; Rochon, P.; Gosselin, J.; Xie, S. Macromolecules
1992, 25, 2271.
(4) Ahuja, R. C.; Maack, J.; Tachibana, H. J. Phys. Chem. 1995, 99,
9221.
(8) Joo, W.; Oh, C.; Song, S.; Kim, P.; Han, Y. J. Chem. Phys. 2000,
113, 8848.
(5) Eich, M.; Wendorff, J. J. Opt. Soc. Am. B 1990, 7, 1428.
(9) Han, Y.; Kim, K. M. Bull. Korean Chem. Soc. 1999, 20, 1421.