Electric-field Induced Birefringence in Azobenzene Thin Films
Paulo M. Zagalo, Gonçalo Magalhães-Mota, Susana Sério, Paulo A. Ribeiro and Maria Raposo
CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, UNL,
Campus de Caparica, 2829-516 Caparica, Portugal
Keywords: Birefringence, Azobenzene, PAZO, Thin Films, Energy Harvesting.
Abstract: It has been recently shown that solar light is able to induce a small birefringence in azo-benzene chromophore
containing thin films, parallel to its surface. In order to enhance this effect, towards the development of energy
harvesting devices, poly{1-(4-(3-carboxy-4-hydroxy-phenylazo)benzenesulfonamido)-1,2-ethanediyl,
sodium salt} (PAZO) cast films were thermally polarized to achieve a net dipole moment in the medium.
Therefore, creation and relaxation kinetics curves of PAZO cast films were obtained in terms of poling at
different temperatures and applied voltages. Results show that the maximum birefringence induced is
proportional to both temperature and applied external electrical field while the relaxation curves reveal that
the residual birefringence increases with the temperature, behaviour which is indicative of cooperative
orientation processes between the chromophores which in turn guarantees the stability of chromophores
orientation.
1 INTRODUCTION
The growing requirements for optical signal
processing in current optical fibre based
telecommunications is seeking for the development
of novel photonic devices, capable of processing
optical signals, towards higher processing rates
capabilities and lower energy consumption. Among
other functionalities of interest to be addressed are
light modulation, optical amplification, optical
multiplexing/ demultiplexing, optical selective
filtering, optical storage and energy harvesting, all to
be integrated in an all-optical based architecture.
The development of novel optical devices for
integrated optics requires the addressing of both novel
materials and material processing procedures.
Photonic materials of particular interest are those
containing highly polarizable chromophore
molecules. Among these the azo-benzene based
chromophores have been arousing much attention
from the scientific community as a result of their
photochromic features. These are formed by a pair of
benzene rings chemically bound together via two
double bonded nitrogen atoms and having a donor
group in one of the benzene ring and an acceptor on
the other. The main interest for the azo-chromophores
comes from their photoisomerization capabilities
which induces spatial rearrangement of the
chromophore molecules, as result of trans-cis-trans
conformation interchange (Hartley,1937) (Natansohn
and Rochon, 2002). This process under certain light
conditions, light wavelength and polarization state,
can give rise to anisotropy creation within the
medium containing the azo-chromophores and resulte
in a net birefringence (Kasap, 2013). This feature can
be of particular interest for the creation of energy
harvesting devices, based on the photoelectret
concept (Farinha, 2016). These devices consist of a
medium having oriented dipoles, thus with a net
polarization, which can be changed by an external
stimulus, in a process that can give to the delivery of
electrical current to an external circuit. Changes in
device polarization can be achieved either by
mechanical stress, temperature, chemical reaction, or
in the case of phtotoisomerizable azochromophores
by light. For device production, generally the
azochromophores are incorporated in a polymeric
matrix and processed in electrode thin film form. The
orientation of electrical dipoles can be achieved by
optical poling or by the application of an external
electric field at temperatures close to that of the glass
transition temperature and then cooled down to room
temperature with the electrical field applied.
In this work the birefringence induced by external
poling electric filed will be investigated in thin films
of the azo-polymer poly{1-(4-(3-carboxy-4-hydroxy-
Zagalo P., MagalhÃ
ˇ
ces-Mota G., SÃl’rio S., Ribeiro P. and Raposo M.
Electric-field Induced Birefringence in Azobenzene Thin Films.
DOI: 10.5220/0006321603810385
Copyright
c
2017 by SCITEPRESS – Science and Technology Publications, Lda. All rights reserved
phenylazo) benzenesulfonamido)-1,2-ethanediyl,
sodium salt} (PAZO) in terms of poling temperature
and polarization voltage, having in view its use in
energy harvesting devices.
2 EXPERIMENTAL
2.1 Materials and Methods
Poly{1-(4-(3-carboxy-4-hydroxy-phenylazo)benzene
sulfonamido)-1,2-ethanediyl, sodium salt} (PAZO),
figure 1, was acquired from Sigma Aldrich. Cast
films of PAZO were prepared by spilling drops of
PAZO solution with 10
-2
M concentration using
methanol as solvent onto BK7 glass substrates with
two regions of FTO 1 mm apart. For this, glass
supports with a FTO layer with a thickness of 414± 6
nm were used, and, in the middle, a 1 mm strip of
FTO was removed from the glass support by
depositing a mash of zinc metallic powder with a few
droplets of hydrochloric acid on it. The cast films
were then dried by leaving the solid supports with
PAZO solution in a desiccator for about two hours.
Figure 1: Chemical structure of the PAZO.
2.2 Birefringence Measurements
The samples were placed in a sample holder with
heating facilities and two electrical spring metallic
probes were used to connect the FTO electrodes with
a high voltage power supply HCN14 (0-20 kV) for
sample poling. The spring probes were placed in
contact with the opposite ends of the FTO conducting
surfaces. A laser beam was then made go impinge the
sample, in the gap between electrodes,and hit a
photodetector that was connected to a National
Instruments SCB-68 tracer. The sample was placed
between crossed polarizers, as depicted in Figure 2
(Monteiro-Timóteo, 2016). The laser and
photodetector used were a Melles Griot with 6,35
mW and 632.8 nm and a Newport Corporation 818-
UV/DB (200-1100 nm), respectively. The samples
were heated to different temperatures while keeping
the electric field applied. Under these conditions, the
transmitted signal intensity can be related to
birefringence by the expression:
Δ=
(1)
where λ is the wavelength of the probe beam, l the
film thickness,
the incident beam intensity, and I
the intensity after the analyzer. The transmitted light
through crossed polarizers reaching the photodetector
was monitored during polarization process and during
polarization relaxation after electrical field removal.
Figure 2: Schematic of the configuration implemented to
measure the birefringence in the films.
0 500 1000 1500 2000 2500
0.08
0.10
0.12
25
o
C
80
o
C
90
o
C
120
o
C
Birefringence Signal (a.u.)
Time (s)
Figure 3: Birefringence creation and relaxation kinetics
curve at room temperature obtained in PAZO cast films
with a voltage of 600V applied.
3 RESULTS AND DISCUSSION
The birefringence creation and relaxation kinetics
curves obtained for PAZO cast films are shown in
Figure 3. The birefringence creation curves
correspond to the increase of transmitted light signal
until the applied voltage of 600V be turned off; while
the birefringence relaxation curve is obtained
immediately after the voltage be turned off and
corresponds to a decrease of the transmitted signal.
The obtained curves present a similar behaviour of
the photoinduced birefringence build-up (Ferreira,
2007) (Ferreira, 2012), increasing the value of
birefringence with the time of application of the
electrical field.
The photoinduced birefringence creation kinetics
curves can be analysed fitting the experimental data
to a sum of two exponential functions, which
indicates the presence of two distinct processes in
accordance with literature (Ferreira, 2012). One of
these process is a fast process usually assigned to the
birefringence induced by instantaneous polarization,
which depends on the free local volume available and
on interactions between chromophores. While the
second is a slower process attributed to the main chain
mobility, which relies on chain size and interactions
between polymeric chains. In the present case, the
birefringence creation,
, is relatively slow so
that the experimental data can be fitted by a single
exponential function:
=
1−−
(2)
where
is the pre-exponential factor that represents
the magnitude of the process and
is the
characteristic time constant. The values of writing
characteristic times obtained from birefringence
induced by electrical field are displayed in table 1.
The activation energy calculated from these values,
suing the method described by (Ferreira, 2012), takes
a value of 53
±3kJ/mol.
Table 1: Writing and relaxation characteristic times
obtained from fitting of curves of figure 3.
Temperature
(ºC)
w
(s)
r1
(s)
25 670 ±80 88 ± 7
80 20 ±1 36 ± 2
90 18 ±1 24 ± 1
120 3.5 ±0.5 31 ± 1
From the obtained birefringence creation curves
one can observe that the induced birefringence
increases, as temperature increases. Opposite results
have been obtained by Ferreira et al (Ferreira, 2012)
in poly(allylamine hydrochloride) (PAH)/PAZO
Layer-by-Layer (LbL) films when irradiated with the
488 and 514 nm lines of a tunable Ar
+
laser, used as
writing beams for inducing birefringence. In fact, it
has been shown that birefringence decreases linearly
with the increase of temperature. For the present case,
the maximum attained birefringence increases
linearly with temperature, as can be seen in graph of
figure 4, which indicates that the temperature
promotes dipolar orientation.
20 40 60 80 100 120
0.076
0.080
0.084
0.088
Birrefringence Signal (arb. units)
Temperature (
o
C)
Figure 4: Maximum intensity of birefringence for PAZO
cast films attained at different temperatures with a constant
voltage of 600V applied.
The birefringence decay curves can be fitted with
two exponential Debye like processes as follows:
=
−
+
−
(3)
where
and
are the pre-exponential factors for
the birefringence normalized intensity,
and
are the characteristic time constants of the
processes. The process with shorter characteristic
time is usually associated with dipole disorientation
and the one with longer characteristic time the long-
term relaxation related to disorientation arising from
the movement of polymer chains. In the present case,
the second process revealed to be slow enough so that
the relaxation kinetics curves were fitted with an
exponential decay curve plus a constant. The
relaxation characteristics are also displayed in table 2.
It should be also worth to refer that the residual
birefringence after the relaxation of orientated
chromophores, which follows the fast relaxation
process, increases as the temperature increases as
shown in graph figure 5, where the residual
birefringence signal is plotted as a function of
temperature. This result indicates that temperature
promotes the chromophores orientation and the
alignment is contributing to the cooperative
aggregation which was proven to be more effective in
accordance with photoinduced birefringence results
obtained in PAH/PAZO LbL films (Ferreira, 2012).
It should be also referred that the calculated value of
activation energy of 53
±3kJ/mol is very close of the
value obtained by (Ferreira, 2012) for PAH/PAZO
LbL films prepared with pH=8 and where it was
proved that cooperative aggregation has a major
contribution.
20 40 60 80 100 120
0.076
0.080
0.084
0.088
Residual Birefringence (a.u)
Temperature (
o
C)
Figure 5: Residual birefringence after the fast relaxational
process for PAZO cast films firstly subjected at different
temperatures with a constant voltage of 600V applied. The
solid line is a guideline.
Figure 6 shows the birefringence creation and
relaxation kinetics curves obtained for PAZO cast
films by applying different poling voltages, while
maintaining the temperature constant at 120ºC. From
these results, one can conclude that the induced
birefringence increases with the applied electrical
field stablished in the gap between electrodes.
0 500 1000150020002500
0.08
0.10
0.12
0.14
0.16
400V
600V
800V
1000V
Birrefingence Signal (a.u)
Time (s)
Figure 6: Birefringence creation and relaxation kinetics
curve at room temperature obtained in PAZO cast films
with different voltages applied. The temperature was
maintained constant at 120ºC.
By plotting the maximum birefringence attained
versus the applied voltage, as shown in figure 7 one
can conclude that induced birefringence is
proportional to the applied external electrical field.
These results show that it possible to orientate the
chromophore of the PAZO molecules and to create a
net birefringence in the medium by applying an
external electrical field. If the medium is now
subjected to polarized light, one expects that azo-
chromophores will be able to experience successive
cis-trans-cis processes, which induce changes in
sample dipole moment and thus can deliver current to
an external circuit.
400 500 600 700 800 900 1000
0.08
0.09
0.10
0.11
Birrefringence Signal (arb. units)
Applied Voltage (V)
Figure 7: Maximum intensity of birefringence for PAZO
cast films attained with different voltages applied. The
temperature was maintained constant at 120ºC.
4 CONCLUSIONS
This work demonstrates that birefringence can be
induced in PAZO cast films by electrical poling close
the glass transition temperature. The birefringence
kinetics curves revealed that the maximum
birefringence induced is proportional to both
temperature and applied electrical field. Moreover,
the obtained values of characteristic times of
birefringence creation and relaxation kinetics curves
are strongly dependent of temperature, allowing to
conclude the poling process is much more effective at
high temperatures and high applied voltage. The
relaxation curves reveals that the residual
birefringence also increases with temperature which
indicates that cooperative processes between the
chromophores are taking place in such a way that
guarantees the stability of chromophores orientation.
One expects that after electrical poling, if the medium
is subjected to polarized light, more azo-
chromophores are now be able to experience
successive cis-trans-cis processes, which induce
changes in sample dipole moment and thus can
deliver current to an external circuit.
ACKNOWLEDGEMENTS
This work was supported by the Portuguese research
Grant UID/FIS/00068/2013 through FCT-MEC, the
"Plurianual" financial contribution of "Fundação para
a Ciência e Tecnologia" (Portugal) and by the project
POCTI/FAT/47529/2002 (Portugal).
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