Photoresponsive behavior of two well-defined azo polymers with different electron-withdrawing groups on push-pull azo chromophores

Article abstract of DOI:10.1016/j.dyepig.2013.05.006

Two well-defined methacrylate-based azo polymers bearing strong push-pull azo chromophores were synthesized by RAFT polymerization. The polymers contain cyano and carboxyl as electron-withdrawing groups at 4-positions of the azo chromophores. The polymers

Full text of DOI:10.1016/j.dyepig.2013.05.006

Accepted Manuscript
Photoresponsive behavior of two well-defined azo polymers with different electron-
withdrawing groups on push-pull azo chromophores
Yu Zhu, Yuqi Zhou, Xiaogong Wang
PII:
S0143-7208(13)00171-X
10.1016/j.dyepig.2013.05.006
DYPI 3943
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 30 January 2013
Revised Date: 6 May 2013
Accepted Date: 7 May 2013
Please cite this article as: Zhu Y, Zhou Y, Wang X, Photoresponsive behavior of two well-defined
azo polymers with different electron-withdrawing groups on push-pull azo chromophores, Dyes and
Pigments (2013), doi: 10.1016/j.dyepig.2013.05.006.
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ACCEPTED MANUSCRIPT
Photoresponsive behavior of two well-defined azo polymers with
different electron-withdrawing groups on push-pull azo chromophores
Yu Zhu, Yuqi Zhou, Xiaogong Wang
Department of Chemical Engineering, Key Laboratory of Advanced Materials (MOE),
Tsinghua University, Beijing 100084, P. R. China
Corresponding author: Tel: 86-10-62784561, Fax: 86-10-62770304, email: wxg-
dce@mail.tsinghua.edu.cn.
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Abstract
Two well-defined methacrylate-based azo polymers bearing strong push-pull azo
chromophores were synthesized by RAFT polymerization. The polymers contain cyano
and carboxyl as electron-withdrawing groups at 4-positions of the azo chromophores.
The polymers exhibited significantly different photoresponsive properties on all light-
driven motion levels. When irradiated with a linearly polarized laser beam at 532 nm, the
cyanoazobenzene-containing polymer showed rapid birefringence growth relative to that
of the carboxylazobenzene-containing polymer. When the light was switched off, a much
lower orientation decay was observed for the carboxylazobenzene-containing polymer
compared with its counterpart. Upon irradiation with the interfering laser beams at these
two wavelengths, surface-relief-grating amplitude formed on the film of the
cyanoazobenzene-containing polymer was substantially larger than that for the other.
When the polymer films were irradiated with a homogenous laser beam at 488 and 532
nm, self-structured surface pattern was only detected for the cyanoazobenzene-containing
polymer with the 532 nm light irradiation.
Key Words: Azobenzene, RAFT, Hydrogen-bonding, Photoinduced birefringence,
Surface-relief-grating, Self-structured surface pattern.
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1. Introduction
Polymers containing aromatic azo chromophores (azo polymers for short) have
attracted considerable attention in recent years for their unique photoresponsive properties
and potential applications [1-7]. Those properties that have been extensively explored
include photoinduced phase transition [8], photoinduced dichroism and birefringence [9],
surface-relief-grating (SRG) formation [10, 11], photo-mechanical thin film contraction
and bending [12-14], spontaneous surface pattern formation [15], and photoinduced
colloidal deformation among others [16, 17]. Azo polymers are expected to be applied in
areas such as optical data-storage, photo-switching, holography, photomechanical devices,
sensors and actuators. The above-mentioned interesting properties are closely related
with the trans-cis photoisomerization of the azo chromophores. According to the spectral
feature and isomerization behavior, azo chromophores have been classified into
azobenzene type, aminoazobenzene type, and pseudo-stilbene type [18]. The pseudo-
stilbene type azo chromophores usually contain strong electron donating and withdrawing
substituents at 4 and 4' positions on the aromatic rings. The push-pull type azo
chromophores possess strong absorption bands in visible light region caused by π→π*
electronic transition. Different from azobenzene type compounds, the 1(π, π*) state might
have lower energy than that for 1(n, π*) state, i.e. the levels of 1(n, π*) and 1(π, π*) states
are reversed on the state energy diagram [18]. This type of azo chromophores shows fast
trans-to-cis isomerization and thermal relaxation. The polymers containing push-pull
type azo chromophores can show a variety of photoresponsive functions [1-4, 9-11, 15-
17].
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Light-driven motions at different levels are the most important photoresponsive
properties that have been intensively investigated in recent years [2]. At the molecular
level, repeated cycles of trans-cis-trans isomerization will force the azo chromophores of
the polymers to take preferential orientation in a direction perpendicular to the light
polarization. The orientation can also cause some correlative alignments of adjacent
groups or segments in the azo polymers. The irradiation-driven orientation will result in
the photoinduced anisotropy (PIA) of solid thin films, such as optical dichroism and
birefringence. The photoisomerization can induce massive motion of azo polymers at the
micrometer level [2, 10, 11, 15-17]. Upon irradiation with interfering laser beams,
surface-relief-gratings (SRGs) will be formed on the azo polymer films [10, 11]. SRGs
are usually formed at a temperature well below the glass transition temperature (Tg) of the
polymers and can be erased by heating sample to a temperature above its Tg. More
recently, another interesting photoinduced effect occurring on azo polymer film surfaces
has been observed, which is known as spontaneous surface pattern formation through
light irradiation [15]. It is characterized by the self-structured surface patterns such as
submicrometer hexagonal pillar arrays formed on the azo polymer films after irradiation
with a uniform single laser beam at normal incidence. To understand the structure-
property relationship and optimize those useful properties, azo polymers with different
structures have been prepared and studied by different methods [1-7, 19-22]. The
influence of experimental parameters, such as polarization of the laser beam, irradiation
time and intensity, has been investigated [1-3]. In comparison with the extensive
exploration of the PIA and SRG formation, there are relatively few studies devoted to the
photoinduced self-structured pattern formation.
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It has been known for years that electron-withdrawing groups on azo chromophores
can significantly influence the absorption band position and molar extinction coefficient
of azo dyes and pigments [23, 24]. In recent years, effects of the electron-withdrawing
groups on photoresponsive properties of azo polymers have been investigated, which
include their influences on the SRG formation [25, 26], dichroism and birefringence [2,
27] and self-structured surface pattern formation [28]. The studies show that there exists
a close correlation between the electron-withdrawing groups on the push-pull azo
chromophores and the photoresponsive behavior of azo polymers. However, most of the
studies were carried out by using azo polymers obtained from the common radical
polymerization or polycondensation, which was difficult to control the molecular weight
and usually resulted in a broad molecular weight distribution. Moreover, the studies were
mainly concentrated on one or two of the photoresponsive properties, which could not
give a unified picture of the effect of the electron-withdrawing groups.
In this study, two azo polymers with narrow molecular weight distributions are used to
study a series of the photoresponsive properties including photoisomerization,
photoinduced birefringence and dichroism, self-structured surface pattern formation and
SRG formation. One of the azo polymers, poly(6-(N-methyl-N-(4-(4'-carboxylphenylazo)
phenyl)amino)hexyl methacrylate) (PAzoCA-6), was synthesized in this study. For
another azo polymer, poly(6-(N-methyl-N-(4-(4'-cyanophenylazo)phenyl)amino)hexyl
methacrylate) (PAzoCN-6), its synthesis has been reported by us previously [29]. This
polymer is used here to study the properties for the comparison. The two azo polymers
were synthesized by reversible addition fragmentation chain transfer (RAFT)
polymerization to possess narrow molecular weight distributions. The structural
difference of the polymers only exists in the electron-withdrawing substituents at the 4-
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position of the azo chromophores. The photoresponsive properties of the azo polymers
were investigated to understand their correlation with the electron-withdrawing
substituents on the azo chromophores.
2. Experimental Section
2.1 Materials
Anisole and THF were distilled from sodium prior to use and the other solvents were
flash distilled before use. All other reagents and chemicals were purchased from
commercial sources and used as received without further purification. The synthetic
route to obtain the monomers is shown in Scheme 1. The synthetic route to synthesize
the two azo polymers is shown in Scheme 2. The synthetic detail of poly(6-(N-methyl-N-
(4-(4'-cyanophenylazo)phenyl)amino)hexyl methacrylate) (PAzoCN-6) has been given in
our previous report [29]. The chain transfer agent 2-(2-cyanopropyl)dithiobenzoate
(CPDB) and the intermediate products to obtain the monomer 6-(N-methyl-N-(4-(4'-
carboxylphenylazo)phenyl)amino)hexyl methacrylate (AzoCA-6) were synthesized
1
according to the literature method [30, 31]. H NMR of the intermediate products and the
monomers are given in the Supporting Information.
2.2 Syntheses
6-(N-Methyl-N-(4-(4'-carboxylphenylazo)phenyl)amino)hexyl
methacrylate)
(AzoCA-6). 4-Aminobenzoic acid (6.85 g, 0.05 mol) was dissolved in a mixture of HCl
(15 mL) and deionized water (30 mL). An aqueous solution (20 mL) of NaNO2 (4.14 g,
0.06 mol) was slowly dripped into the mixture cooled with an ice bath. The reaction
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o
mixture was stirred at 0 C for 5 min and then filtrated to obtain the solution of the
diazonium salt. The solution of 6-(N-methyl-N-phenylamino)hexyl methacrylate (11.01 g,
0.04 mmol) in DMF (50 mL) was added into a reaction flask and cooled with an ice bath
for 30 min. The diazonium salt solution was added dropwise into the DMF solution
while the temperature was controlled to be below 5 oC. After stirring for 3 h, the solution
was poured into an excess of deionized water. The precipitate was collected by filtration
and washed with plenty of water. After drying, the product was purified by
recrystallization from ethanol and dried under vacuum. Yield: 89%. 1H NMR (300 MHz,
CDCl3): δ (ppm): 8.20 (d, J = 6.9 Hz, 2H), 7.91-7.86 (overlap, 4H), 6.73 (d, J = 9.3 Hz,
2H), 6.09 (s, 1H), 5.55(s, 1H), 4.15 (t, J = 6.5 Hz, 2H), 3.43 (t, J = 7.4 Hz, 2H), 3.07 (s,
3H), 1.94 (s, 3H), 1.74-1.64 (m, 4H), 1.48-1.37 (m, 4H). FT-IR (KBr, ν, cm-1): 2935,
2654, 2542, 1716, 1684, 1635, 1599, 1516, 1421, 1379, 1290, 1246, 1136. MS (MALDI-
TOF): calcd. for C24H30N3O4 [M + H]+: m/z = 424.22; found: 424.22.
Poly(6-(N-methyl-N-(4-(4'-carboxylphenylazo)phenyl)amino)hexyl methacrylate)
(PAzoCA-6). AzoCA-6 (0.846 g, 2 mmol), 2,2'-azobis(2,4-dimethyl) valeronitrile
(ABVN, 2.5 mg, 0.01 mmol), and CPDB (11.1 mg, 0.05 mmol) were added into a 50 mL
Schlenk flask, followed by the addition of THF (3 mL). The mixture was degassed by
three freeze-pump-thaw cycles and sealed under Argon. The flask was then placed in an
o
oil bath of 55 C. After reaction for 20 h, the flask was immersed in ice water to
1
terminate the reaction. The reaction solution (100 µL) was extracted for H NMR
measurement to determine the conversion of the monomer. The reaction solution was
then diluted with THF and precipitated in a large amount of hexane to obtain the crude
product. The purification procedure was repeated three times and the polymer was dried
in a vacuum oven for 24 h. The conversion of the monomers was calculated by Eq. 1 on
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1
the basis of the integration areas of the H NMR peaks at 6.09 ppm (I6.09) and 6.55 ppm
(I6.55), which represents the vinyl proton of the methacrylate of the remaining monomer
and the two protons of the phenyl at the ortho-position to the amino group.
conv(%)=(1-2× I6.09 / I6.55 )×100%
(1)
1
Conversion of the monomer was estimated to be 95 mol % obtained from the H NMR
1
analysis. Mn (GPC) =13,000, Mw/Mn (GPC) = 1.21. H NMR (300 MHz, DMSO-d6): δ
(ppm): 8.11-7.95 (m), 7.84-7.59 (m), 6.79-6.37 (m), 4.09-2.55 (m), 1.52-0.48 (m). FT-IR
(KBr, ν, cm-1): 2935, 2858, 2659, 2542, 1720, 1686, 1601, 1516, 1419, 1381, 1290, 1248,
1138.
2.3 Characterization
The molecular weights and molecular weight distributions were measured using gel
permeation chromatography (GPC) equipped with a PLgel 5 ꢀm mixed-D column and a
refractive index (RI) detector (Wyatt Optilab rEX). The measurements were carried out
at 35 oC and the molecular weights were calibrated with polystyrene standards. THF was
used as the eluent and the flow rate was 1.0 mL/min. 1H NMR spectra were recorded on a
JEOL JNM-ECA300 spectrometer (300 MHz for proton) by using CDCl3 and DMSO-d6
as the solvent and tetramethylsilane as the internal standard. Fourier transform infrared
spectroscopic (FT-IR) measurements were carried out on a Nicolet 560-IR
spectrophotometer by incorporating the samples in the KBr tablets. UV-vis spectra were
measured on an Agilent 8453 UV-vis spectrophotometer. Thermal properties of the
polymers were tested by using TA Instrument DSC 2920 with a heating rate of 10 oC/min
in the nitrogen atmosphere.
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2.4 Sample preparation
Solid thin films of the azo polymers were prepared by spin-coating and used for the
following experiments. To obtain solutions for spin-coating, PAzoCN-6 was dissolved in
chloroform and PAzoCA-6 was dissolved in DMF with weight fractions ranging from 1.0
-2.5 %. The homogeneous solutions were filtered through 0.45 µm syringe filters and
spin-coated on clean substrates (quartz and glass slides) at a speed of 1500 rpm. The
films obtained from the spin-coating were first dried under the ambient condition for 12 h
and then annealed at 100 oC under vacuum for 24 h. The film thicknesses were estimated
by scratching the films with a razor blade and measuring the step heights by the scanning
probe microscopy (Nanoscope− A) in the tapping mode.
2.5 Photoresponsive property measurements
Photoinduced trans-cis isomerization was measured by irradiating the solid films on
quartz slides at 25 °C for different time periods. The irradiation was conducted by using
Asahi Spectra model MAX-302 xenon light source (300 W), which was equipped with
band pass filters (Asahi Spectra) to obtain the light of
λ = 500 ± 2 nm. The spectroscopic
variations caused by the irradiation were recorded using a UV-vis spectrophotometer
(Agilent 8453).
The photoinduced birefringence of the azo polymers was measured in-situ using an
optical setup similar to those reported before [32, 33]. Both a diode-pumped frequency-
doubled solid-state laser at 532 nm and an Ar+ laser at 488 nm were used as the light
source. The polymer films spin-coated on glass slides were irradiated with the linearly p-
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polarized laser beam at these two wavelengths. The intensity of the laser beam was
adjusted to be 25 mW/cm2. The laser beam was expanded through a spatial filter and
collimated to provide a homogeneous intensity profile over the irradiated area on the
films. The photoinduced change in the refractive index was probed using an unpolarized
beam from a He-Ne laser (632.8 nm, 2.3 mW/cm2). The intensity of the probe beam that
transmitted through the sample set between a pair of crossed polarizers was recorded by a
photodiode. The polarization directions of the polarizers were set at ±45o angle to the
polarization of the excitation beam. When the intensity variation of the probe beam was
saturated, the excitation beam was switched off to investigate the relaxation process.
Photoinduced dichroism of the azo polymer films was evaluated using polarized FT-IR
spectroscopy. Films spin-coated on CaF2 plates were used for the polarized FT-IR
measurements, respectively. The light irradiation condition was the same as those
described above for the photoinduced birefringence experiments.
SRG and self-structured surface pattern were inscribed using the polymer films spin-
coated on glass slides. A linearly polarized laser beam at 488 nm or 532 nm was used as
the light source. The experimental set-up for inscribing SRGs was similar to those
reported before [10, 11]. The laser beam was spatially filtered, expanded and collimated
to provide homogeneous intensity profile. Half of the laser beam was incident on the
films directly and the other half of the beam was reflected onto the films from a mirror.
The space periods of the SRGs were controlled by selecting the proper incident angle.
The time evolution of the gratings was monitored using a He-Ne laser (633 nm) as the
probe beam and the first-order diffraction efficiency was recorded in a real time manner.
Self-structured surface patterns on the azo polymer films were induced by irradiation with
linearly polarized and circularly polarized laser beams at the normal incidence for both
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wavelengths. The surface patterns formed on the films after light irradiation were probed
using a Nanoscope-Ⅲ A scanning probe microscope in the tapping mode.
3. Results and Discussion
3.1 Synthesis and characterization
The two azo polymers poly(6-(N-methyl-N-(4-(4'-cyanophenylazo)phenyl)amino)-
hexyl methacrylate) (PAzoCN-6), and poly(6-(N-methyl-N-(4-(4’-carboxylphenylazo)
phenyl)amino)hexyl methacrylate) (PAzoCA-6), were synthesized by RAFT
polymerization. The synthetic routes of the monomers and polymers are given in the
Scheme 1 and 2. The polymerizations needed to be carried out in good solvents for the
azo polymers, which were different for these two polymers. AzoCN-6 was polymerized
by using anisole as the solvent at 75 oC and AIBN was used as the initiator. As PAzoCA-
6 is almost insoluble in a low-polar solvent, the polymerization of AzoCA-6 was carried
o
out in THF at 55 C. Owing to the lower polymerization temperature, which was
determined by the low boiling point of THF, 2,2'-azobis(2,4-dimethyl) valeronitrile
(ABVN) with a lower Td compared to AIBN was selected as the initiator.
GPC analysis indicates that PAzoCN-6 and PAzoCA-6 with narrow molecular weight
distributions were obtained under the conditions (Figure 1). The number average
molecular weights obtained from the GPC measurements are 13,000 and 15,400 for
PAzoCA-6 and PAzoCN-6 with the polydispersities of 1.21 and 1.18, respectively. It
shows that the azo polymers with narrow molecular weight distributions can be obtained
from the RAFT polymerization. On the other hand, the conversions of the two monomers
are obviously different under the conditions. Compared with the low conversion of 45%
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for AzoCN-6, the conversion of the AzoCA-6 can nearly reach 100%. By adjusting the
polymerization condition, such as increasing the polymerization time and using similar
polymerization condition to PAzoCA-6, the conversion of AzoCN-6 cannot be obviously
improved. The foregoing result indicates that AzoCA-6 is a monomer suitable for the
RAFT polymerization. Moreover, as the carboxylic groups are ionizable functional
groups, PAzoCA-6 might show advantages to be used as polyelectrolytes or in
hydrophilic materials.
1
Figure 2 shows the H NMR spectra of PAzoCN-6 and PAzoCA-6 in CDCl3 and
DMSO-d6, respectively. The resonance signals of the spectra can be readily assigned to
the protons of the polymers. The resonances in the low magnetic field range are the
contribution from the push-pull azo chromophores. The obvious difference can be seen
for the resonances of the protons on the aromatic rings, which is related to the different
electron-withdrawing groups on the azo chromophores. Optical microscopic observation
shows that the polymers functionalized with the strong push-pull azo chromophores do
not form a liquid crystalline phase. Figure 3 gives DSC curves of the two azo polymers
from the second heating and cooling scans under N2 protection. PAzoCN-6 shows a
thermal phase transition behavior of an amorphous polymer and has a glass transition
temperature (Tg) of 58 oC. No obvious glass transition or crystalline transition can be
identified for PAzoCA-6, which is attributed to the intermolecular hydrogen-bonding
formed between the carboxyl groups.
3.2 Spectral characteristics and photoisomerization
Figure 4 gives the UV-vis spectra of the solid films of the two azo polymers before
and after the light irradiation. PAzoCN-6 and PAzoCA-6 possess similar absorption
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spectra before the light irradiation. The strong absorption bands with the maxima around
460 nm can be seen for both polymers. The absorption bands in the visible light range
are attributed to the π-π* electronic transition of the azo chromophores. The weak n-π*
transition band is overlapped with the π-π* band, which cannot be identified in the
spectra. The spectra exhibit typical characteristics of the pseudo-stilbene type azo
chromophores [18]. The two electron-withdrawing substituents at 4-positions of the azo
chromophores do not cause discernible variance in their UV-vis spectra.
Upon light irradiation (λ
= 500 nm, I = 5 mW/cm2), both azo polymers show the
spectral changes related to the trans-cis photoisomerization. When the irradiation light is
turned off, the electronic absorption spectra show reversal changes and recover the
original features. Figure 4 shows the UV-vis spectra of the two azo polymers after the
light irradiation for 1 min. For PAzoCN-6, the absorbance band at 460 nm decreases
significantly and a new absorption band (λmax = 383 nm), corresponding to π-π* transition
of the cis isomer, appears at the same time. The spectral variation evidences the
significant trans-cis isomerization occurring for the polymer. On the other hand,
PAzoCA-6 shows a much lower decrease for the absorbance band at 460 nm after the
irradiation. It indicates that although both azo polymers can reach the photo-stationary
states quickly upon the irradiation, the ratios between the cis and trans isomers are
obviously different at the steady-states for the polymers. The quantum yields of the two
isomerization processes (Φtc and Φct) could not be obtained because of the ultrafast
photoisomerization and fast thermal relaxation of push-pull substituted azobenzenes [18,
34, 35]. The much lower cis content for PAzoCA-6 at the photo-stationary state can be
attributed to the low
Φtc, high Φct, and rapid thermal relaxation. The relative significance
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of these factors to cause this result is unclear at this stage. The hydrogen-bonding
between the carboxyl groups, especially for the solid film, may also play an important
role.
3.3 Photoinduced birefringence and dichroism
Photoinduced birefringence of the two polymers was investigated in a real-time
manner by irradiating the solid thin films of the polymers with linearly polarized laser
beams (25 mW/cm2) at 488 nm and 532 nm as described in the experiment section. The
photoinduced birefringence was calculated from the detected transmittance of the probing
laser by Equation 3, where IT is the intensity of the transmitted probe beam when the
sample film is irradiated with the writing beam, I0 is the intensity of the probe beam that
transmitted through the first polarizer, λ is the wavelength of the probing beam (632.8
nm), d is the film thickness, and ꢀn is the birefringence.
λ
IT
I0
∆n =
sin−1
(2)
π
d
Figure 5(a) shows a typical writing-relaxing-erasing sequence for the film of PAzoCN-
6 observed in a real-time manner. Before the excitation laser was switched on, no
transmittance of the probing laser beam could be detected by the photodiode, because the
sample placed between the crossed polarizers was isotropic. When the writing laser beam
was switched on (point A), the intensity of the transmitted probing laser beam increased
rapidly and then gradually reached the saturated value after a certain period of time.
When the incident laser beam was turned off (point B), ꢀn slowly decayed and
maintained about 67% of the saturated value after the relaxation. When the film was then
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irradiated with a circularly polarized laser beam at the same wavelength (point C), the
photoinduced anisotropy was completely erased. The photoinduced birefringence is a
result of the repeated cycles of trans-cis-trans isomerization caused by the light
irradiation, which forces the azo chromophores of the polymer to orientate preferentially
in a direction perpendicular to the electric vector of the light.
The photoinduced birefringence is closely related to the wavelengths of the excitation
light and polymer structures. Figure 5(b) compares the birefringence growth and
relaxation profiles of PAzoCN-6 and PAzoCA-6, when the polymer films were irradiated
with the 488 nm and 532 nm laser beams (25 mW/cm2) for 125 s and then the excitation
laser was switched off. The correlation with the excitation wavelength can be seen by
comparing the birefringence growth profiles of the polymers under the irradiation of
different wavelengths. For PAzoCN-6, upon irradiation with a 532 nm laser, a typical
birefringence growth curve can be obtained with a saturated ꢀn of 0.036. However, the
photoinduced birefringence instantaneously jumps upon irradiation with a 488 nm laser
and then stabilizes at a ꢀn value about 0.0055. When PAzoCA-6 films were irradiated
with laser beams at these two wavelengths, the birefringence growth rates are much
slower compared to those of PAzoCN-6. For irradiation with these two wavelengths, no
obvious difference can be observed for the photoinduced birefringence growth behavior.
The result indicates that although the absorption spectra of these two polymers are similar,
their photo-responsive orientation behavior is quite different.
Another important point that can be seen from the figure is the significantly different
relaxation behavior of these two polymers. After switching off the excitation laser beam,
the residual birefringence after relaxation appears to be much higher for PAzoCA-6 (91%)
compared to PAzoCN-6 (67%). The disparity of the birefringence decay between
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PAzoCA-6 and PAzoCN-6 can be attributed to the hydrogen-bonding formed between the
carboxyl groups on the side chains of PAzoCA-6. The above observation related to the
hydrogen-bonding effect is consistent with results reported for epoxy-based azo polymers
[27]. The effect of the hydrogen-bonding can prohibit the relaxation after switching off
the irradiation light, which will be further discussed in Section 3.6.
The birefringence growth curve can be fitted by the biexponential relationship (Eq. 3),
∆n = A {1− exp(−kat)}+ B {1− exp(−kbt)}
(3)
where ꢀn is the birefringence measured at time t, ka and kb are the rate constants for
birefringence growth, and A and B are two constants [32, 33]. The sum of A and B
represents the maximum induced birefringence and the proportions of A and B to the sum
represent the relative contribution of each term to the induced birefringence. The ka and
kb represent the characteristic rate constants for the slow and fast growth processes. The
fast process is linked to orientations of the chromophores and the slow process is related
to the motion of the backbone, which are both caused by the repeated trans-cis-trans
isomerization cycles [32]. The fitting parameters for the birefringence growth are listed
in Table 1, except the aberrant result for PAzoCN-6 irradiated with 488 nm laser, which
cannot be fit by the equation. Table 1 shows that the slow process plays the predominant
role for PAzoCA-6, whilst the fast process is dominated for PAzoCN-6. The
birefringence values (ꢀnmax) of these two polymer films after an irradiating period of 120s
are given in the same table, which are almost the same for the two polymers when
irradiated with 532 nm light and show no obvious difference when irradiated at the two
wavelengths for PAzoCA-6.
The photoinduced dichroisms of the PAzoCA-6 and PAzoCN-6 films were
characterized using polarized FTIR spectra. Figure 5(c) gives the polar plots of the
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relative infrared absorption intensities of stretching vibration bands of the N=N bond
(1379 cm-1) of the two polymer films. The polymer films were irradiated using linearly
polarized laser beam (532 nm) till saturation and then measured with a polarized FTIR
spectrophotometer. PAzoCA-6 shows a much higher dichroic ratio than PAzoCN-6,
which can be rationalized by the small relaxation of PAzoCA-6 after switching off the
light. The orientation order parameter (S) was calculated by the following equation
S = (A - A||)/(A +2 A||)
(4)
where A䐩 and A|| are the absorbance perpendicular and parallel to the polarization
direction of the laser beam. The orientation order parameters derived from the angular-
dependent absorption are 0.023 and 0.044 for PAzoCN-6 and PAzoCA-6, respectively.
3.4 Photoinduced SRG formation
The surface relief gratings (SRGs) were obtained by exposing spin-coated films of the
azo polymers to the interference patterns formed by the two p-polarized laser beams. The
laser beams with the wavelengths of 488 and 532 nm and an intensity of 150 mW/cm2
were used as the excitation light. Under the conditions, SRG formation was observed on
films of both azo polymers, whilst the SRGs on the films of the two polymers showed
different modulation amplitudes under the same condition. Figure 6 shows typical AFM
images of the SRGs formed on the films of PAzoCA-6 and PAzoCN-6 after the films
were irradiated using interfering laser beams with the wavelength of 488 nm for 1 h. The
sinusoidal surface patterns with a period of about 1 ꢀm can be seen from the images. The
average amplitude of the SRG inscribed on the surface of the PAzoCN-6 film can reach
400 nm, whilst the amplitude of SRG on the PAzoCA-6 film is only 40 nm.
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The first order diffraction efficiency of the SRGs was measured in the real-time
manner to characterize the grating growth rate. Figure 7 presents the first order
diffraction efficiency as a function of the irradiation time in the process of SRG formation
on the films of PAzoCN-6 and PAzoCA-6. Although the absorption maxima of the two
polymers approximately appear at the same wavelength, PAzoCN-6 and PAzoCA-6
exhibit different dependences of the grating growth rates on the excitation wavelengths.
For PAzoCN-6 film, the 1st order diffraction efficiency shows an obviously higher
growth rate when irradiated at 532 nm than that obtained by the irradiation at 488 nm
with the same light intensity (Figure 7A). This wavelength dependence is similar with
those observed for the photo-induced birefringence (Figure 5(b)). On the contrary, for
PAzoCA-6 film, this sequence of the wavelength dependence is reversed, where the
irradiation with 488 nm light can more efficiently induce the SRG formation (Figure 7B).
The first order diffraction efficiency of the SRG on PAzoCN-6 is much higher than that
for PAzoCA-6, which is consistent with the AFM observation. After being exposed to
the interfering laser beams at 532 nm for 1600 s, the first order diffraction efficiency of
the SRG on PAzoCN-6 film can reach as high as 22%, whilst the value is only 0.41% for
PAzoCA-6. The low efficiency for the SRG formation can also be attributed to the
hydrogen-bonding formed between the carboxyl groups for PAzoCA-6. As the SRG
formation needs a larger scale motion compared with photoinduced orientation [2], the
restraint caused by the hydrogen-bonding appears more severe in this case.
3.5 Self-structured surface pattern formation
The ability of these two polymers to form self-structured surface pattern was
investigated by the irradiation with a uniform laser beam. For comparison, the azo
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polymer films were irradiated with a homogeneous normal-incident laser beam (200
mW/cm2) at different wavelengths (488 nm and 532 nm) for 30 min. The self-structured
surface patterns can only be observed on PAzoCN-6 films when irradiated with the 532
nm laser as the incident beam. No discernible well-ordered surface pattern can be formed
on the PAzoCA-6 films when irradiated using laser beams of these two wavelengths,
neither can PAzoCN-6 films when irradiated using 488 nm laser. Figure 8 shows typical
AFM images and their two-dimensional Fourier transform (2D-FT) images of the
photoinduced surface patterns on the PAzoCN-6 film after irradiation at 532 nm for 30
min. The surface patterns are composed of regularly spaced pillar-like structures with
certain localized hexagonal arrangements. The periodic orders of the surface structures
on a large scale can be witnessed from the 2D-FT images. The surface patterns obtained
after irradiation using linearly and circularly polarized laser beam are different in the
arrangement of the self-structured pillar-like structures. In the case of irradiation with the
linearly polarized laser beam, the relief structures are observed to be elongated with their
elongation axis aligning along ±60o with respect to the polarization direction of the
excitation beam, which can be seen from the two dimensional Fourier transform image.
On the contrary, no such correlation is observed for the irradiation with circularly
polarized light. The above observations on the self-structured pattern formation are
consistent with the previous report [15, 36]. In contrast to the well-organized surface
pattern formation, irradiating PAzoCA-6 films with a homogeneous beam with the same
light intensity (200 mW/cm2) can only result in the bleaching of the irradiated area. The
bleaching is caused by the chromophore orientation along the direction perpendicular to
the film surface [2]. AFM height image of the irradiated area indicates that the surface of
the films is smooth within the detect limit (see supporting information). The observation
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further indicates that due to the restraint of the hydrogen-bonding, only small scale
motion can be induced by the light irradiation.
3.6 Effect of hydrogen-bonding
Above results indicate that the hydrogen-bonding in PAzoCA-6 could play a
significant role to cause the differences observed for these two polymers. To further
confirm this point, we prepared another azo polymer PAzoCE-6, which is a polymer of 6-
(N-Methyl-N-(4-(4’-ethoxycarbonylphenylazo)phenyl)amino)hexyl methacrylate). The
synthesis of this polymer was suggested by the referees during the paper reviewing
process. The synthetic route, structure and analytical results of this polymer are given in
the Supporting Information (Scheme S1, Figure S2 and S3). The main difference
between PAzoCA-6 and PAzoCE-6 is that the carboxyl group of PAzoCA-6 is replaced
by ethoxycarbonyl group, which can avoid the hydrogen-bonding formation in PAzoCA-6.
The UV-vis spectrum of PAzoCE-6 before and after UV light irradiation is given in
Figure S4 (in the Supporting information). The spectral profile of this polymer is similar
to those of PAzoCN-6 and PAzoCA-6. Distinct from PAzoCA-6, PAzoCE-6 shows a
more significant spectral change caused by trans-cis isomerization upon light irradiation.
Figure 9 compares the photoinduced birefringence growth and relaxation behavior of
these three azo polymers. The birefringence (ꢁn) was induced by the irradiation with a
linearly polarized laser beam at 532 nm (25 mW/cm2) for 120 s and the relaxation was
recorded after switching off the light. The photoinduced ꢁn growth of PAzoCE-6 shows
an obviously slower rate compared with PAzoCN-6, but is similar to PAzoCA-6. On the
other hand, after switching off the light, the relaxation decay of PAzoCA-6 is much
smaller compared with those of the other two polymers.
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The surface-relief-grating formation behavior of PAzoCE-6 was also studied and
compared with the other two azo polymers under the same conditions. The experimental
details can be seen in the Experimental Section and Supporting Information. After the
irradiation with the interfering laser beams at 532 nm (150 mW/cm2) for 1 h, the 1st order
diffraction efficiency of the SRG formed on PAzoCE-6 film can reach 5.1 %, whilst that
for PAzoCA-6 is only c.a. 0.4 %. The AFM height images of the SRGs on the films of
these two polymers are given in Figure 10. The average amplitude of the grating on
PAzoCE-6 film can reach 150 nm in comparison to 20 nm on PAzoCA-6 film.
Above results indicate that both electronic interaction and hydrogen-bonding ability of
the electron-withdrawing groups can play an important role in the photoresponsive
processes. The difference between PAzoCE-6 and PAzoCN-6 can be attributed to the
different electronic interaction of the substituents. This effect on the photoinduced
orientation can be seen from the results given in Figure 5(b) and Figure 9. On the other
hand, the PAzoCA-6 show significant differences in the orientation decay and SRG
formation behavior compared with the other two polymers. These differences can be
attributed to the effect of hydrogen-bonding. It means that the hydrogen-bonding can
most significantly affect the orientation relaxation after light irradiation and SRG
formation that is related to the large-scale mass-transfer.
4. Conclusion
The two azo polymers (PAzoCN-6 and PAzoCA-6) synthesized by RAFT
polymerization show significant difference in their photoresponsive properties and
photomechanical properties, which is related to the electron-withdrawing groups on the
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azo chromophores. Compared with PAzoCA-6, PAzoCN-6 shows a rapid growth rate for
photoinduced birefringence, much higher efficiency for the SRG formation, and ability to
form self-structured surface pattern when irradiated with the laser beam of the appropriate
wavelength. On the other hand, much lower orientation decay is observed for PAzoCA-6
compared with its counterpart after switching off the irradiation light. The difference in
the orientation decay and SRG formation can be attributed to the hydrogen-bonding
between the carboxyl groups in PAzoCA-6. The hydrogen-bonding can restrain the large-
scale motion of the azo chromophore and associated groups. Although these two
polymers possess similar UV-vis spectra, their responses to the irradiation at these two
wavelengths are different. To understand the mechanism of the wavelength effect, further
theoretical and experimental investigations are required.
Acknowledgement. The financial support from the NSFC under Projects 51233002 and
91027024 is gratefully acknowledged.
Supporting Information Available: More characterization results are given there. This
information is available free of charge via the Internet at http://www.sciencedirect.com.
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Captions of Tables, Schemes and Figures
Table 1.
Parameters obtained from curve-fitting of the photoinduced birefringence
kinetics for PAzoCN-6 and PAzoCA-6.
Scheme 1. Synthetic route for the monomers AzoCN-6 and AzoCA-6.
Scheme 2. Synthetic route for the azo polymers PAzoCN-6 and PAzoCA-6.
Figure 1. GPC curves of PAzoCN-6 and PAzoCA-6.
Figure 2. 1H NMR spectra of PAzoCN-6 in CDCl3 (A) and PAzoCA-6 in DMSO-d6
(B).
Figure 3. DSC curves of the PAzoCN-6 and PAzoCA-6 on the second heating scan
and second cooling scan with a heating/cooling rate of 10 oC/min.
Figure 4. UV-vis spectra of PAzoCN-6 (A) and PAzoCA-6 (B) films before and upon
irradiation at 500 nm (I = 5 mW/cm2).
Figure 5. (a) Typical writing, relaxing and erasing sequence for the film of PAzoCN-6
irradiated with incident laser beam (25 mW/cm2) at 532 nm. Point A: The
linearly polarized writing beam is switched on; Point B: The writing beam is
switched off; Point C: The circularly polarized beam as the erasing beam is
switched on. (b) Photoinduced growth and relaxation of birefringence as a
function of time for the films of PAzoCN-6 and PAzoCA-6 under
irradiation of laser at 488 nm and 532 nm. (c) Polar plots of the infrared
absorbance at 1379 cm-1 due to N=N stretching vibration for the films of
PAzoCN-6 and PAzoCA-6 recorded upon rotation of the polarizer at every
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10o. The arrow denotes the direction of electric vector of the writing laser
beam.
Figure 6. AFM images (10 µm × 10 µm) of the surface relief grating (SRG) formed
on the films of PAzoCA-6 (a, b) and PAzoCN-6 (c, d) with irradiation at
488 nm.
Figure 7. The first order diffraction efficiency of the SRGs as a function of time in the
course of the SRG formation on the films of PAzoCN-6 (A) and PAzoCA-6
(B) with excitation laser beam at 488 nm and 532 nm.
Figure 8. Typical AFM images (10 µm × 10 µm) and 2D-FT images of the
photoinduced surface patterns formed on the films of PAzoCN-6 after
irradiation at 532 nm with different polarizations: (a) linearly polarized, 2D-
view; (b) linearly polarized, 3D-view; (c) linearly polarized, 2D-FT image;
(d) circularly polarized, 2D-view; (e) circularly polarized, 3D-view; (f)
circularly polarized, 2D-FT image.
Figure 9. Photoinduced growth and relaxation of birefringence as a function of time
for the films of PAzoCA-6, PAzoCN-6 and PAzoCE-6 under irradiation of
laser at 532 nm.
Figure 10. AFM images (10 µm × 10 µm) of the surface relief grating (SRG) formed
on the films of PAzoCA-6 (a, b) and PAzoCE-6 (c, d) with irradiation of
laser at 532 nm.
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