# Reflection Band Control of Inverse Opal Film  with Photoresponse Properties of Push-Pull Type  Azobenzene LC Polymers

Authors: Sunnam Kim; Takahiro Shiozawa; Masaki Moritsugu; Tomonari Ogata; Takamasa Nonaka; Seiji Kurihara
Journal: Research Letters in Materials Science
 (2008)

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## Abstract

DcAz2Mc and DR1Mc molecules having push-pull type of azobenzene groups are synthesized, and their photo-orientational behaviors are investigated in the polymer system. In order to understand the relationship between a molecular structure and its physical properties, electronic structure calculation is computationally carried out. Regarding to their photo response properties, the copolymers ofpoly(DcAz2Mc-co-M6PBMe) and poly(DR1Mc-co-M6PBMe) are infiltrated into inverse opal films, and Bragg reflection shifts are observed under photostimuli. When the linearly polarized light is irradiated, Bragg reflection bands are shifted to the longer wavelength region as reflective index increases.


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## Body

## 1. Introduction

Recently, in the fields of telecommunication and optical in-dustry, a way to
tailor the propagation of light has attracted considerable attention for
high-speed and wide-bandwidth information processing. Photonic crystals (PCs) having a three-dimensionally (3D)
ordered structure with a periodicity in the optical wavelength range have been
proposed as a feasible material to confine light, to guide light, or to inhibit
or amplify spontaneous emission properties [1, 2]. The self-assembly method is simple and
cost-effective method for fabricating 3D PCs such as an opal structure consisting of colloidal spheres and air voids.
In addition, an inverse opal structure can be prepared
by using
the opal structure as template. With the opal
and inverse opal structures, a pho-tonic bandgap (PBG), providing a wavelength range of
for-bidden propagation in all directions, can
be realized. It is known that the PBG mainly depends on the
periodicity of spheres and the refractive
indices of components, as can be expressed by the following Bragg’s
equation:
(1)λ=223dna2f+nb2(1−f),
where λ is the wavelength of the Bragg reflection under normal incident condition, d is the distance of periodic units in dielectrics, f is the volume fraction of dielectrics, na and nb are refractive indices of dielectrics. Therefore, a change in either spacing or
refractive index is expected to cause a shift of the Bragg
reflection band. There are many studies for
control of Bragg reflection band by incorporation of low-molecular-weight liquid crystals (LMWLCs) in the inverse
opal system, because LMWLCs
have a large optical anisotropy, and consequently change in the refractive index can be induced through the change in molecular orientation by applying electric field or
photoreaction [3].On the other hand, we reported that the
photochemical switching of Bragg reflection band of a silica inverse opal structure
infiltrated with polymer LC bearing azobenzene chromophores. In our previous works [4–7], stable and reversible shift of Bragg reflection band was achieved by irradiation of linearly and circularly polarized light, causing the
change in the birefringence through the reversible transformation of azobenzene
chromophores between uniaxial and random states. However, the response time was slow. The
response property is closely related not only to less re-sponse property of
polymeric materials, but also to the electronic property of azobenzene dyes. It
is well known that electronically push-pull
type azobenzene dyes show
better photoalignment property compared with nonpush-pull type azobenzene dyes.
Therefore, in this study, we synthesized two kinds of push-pull type azobenzene
dyes,DcAz2Mc
and DR1Mc, and explored
their photoresponse
properties experimentally and theoretically. In addition, Bragg reflection band shift was
investigated with the
inverse opal films infiltrated with the push-pull type azobenzene polymers.


## 2. Experimental

### 2.1. Materials

All the chemicals were purchased from commercial
suppliers and used without further purification unless otherwise noted.
DcAz2Mc, DR1Mc, and M6PBMe were synthesized according to
the manner reported earlier [8–11] and were
confirmed by elemental analysis, IR and NMR spectroscopic methods. Synthesis of polymers and copolymers, poly(DR1Mc),
poly(DcAz2Mc),
poly(DcAz2Mc-co-M6PBMe),
and poly(DR1Mc-co-M6PBMe), was
carried out by radical polymerization of monomers with the use of
N, N′-azobis(isobutyronitrile) as initiator at 60°C for 48 hours in dimethylformamide, and their molecular structures are
shown in Figure 1.
The molecular weights and distributions were
measured by gel permeation chromatography (GPC),
and the thermal properties were studied by differential scanning calorimetry
(DSC).Figure 1
Molecular structures ofpoly(DR1Mc-co-M6PBMe) and
poly(DcAz2Mc-co-M6PBMe) containing nitro- and dicyano-end
groups, respectively. (x = 1, 0.05).DR1Mc: m.p. 83°C. Anal. calc. for C17H19O2N2:
C, 62.82; H, 5.80; N, 14.65. Found: C, 62.85; H, 5.80; N, 14.44.DcAz2Mc: m.p.
135°C. Anal. calc. for C24H23O2N5:
C, 69.88; H, 5.38; N, 16.98. Found: C, 70.00; H, 5.73; N, 16.93.M6PBMe: m.p. 45°C. Anal. calc. for C24H28O6:
C, 66.64 H, 7.23. Found: C, 66.51 H, 7.21.


### 2.2. Photochemical Properties

Photochemical molecular orientation
behavior of polymers in the solid state was investigated by using a pump-probe
method [4]. Thin solid films were prepared by spin coating
method from THF solution. An Ar+ laser (LASER GAPHICS 165 LGS, λ = 488 nm, 250 mW/cm2) and Xenon lamp
(Ushio SX-UI500XQ) equipped with band-pass filter (λ = 500–600 nm) were used
as a pumping light source, and an He-Ne laser (633 nm) was
used as the probe light. The thin solid film of the polymers was placed between
two crossed polarizers where the polarizing directions of the polarizers were set at an angle of 45° with respect to the polarizing direction of a pumping laser
light. Transmitted light intensity of the He-Ne laser light was
measured under irradiation of linearly or circularly polarized Ar+ laser light. The photoinduced birefringence, Δn, was estimated [12].The SiO2 inverse opal was prepared by the same method
described in our previous work [4]. The polymers wereinfiltrated into the
SiO2 inverse opal film by annealing.
Changes in optical properties of the SiO2 inverse opal/azo-polymer
composite films were explored by measuring reflectionspectra
for incident white light with an Ocean Optics USB2000. Reflection spectra were
measured with/without irradiation of Ar+ laser or Xenon lamp.


### 2.3. Calculation

The
geometry optimizations of DR1Mc and DcAz2Mc were carried out using Gaussian 98
with B3LYP/6-31G** basis set.  Dipole
moment, aspect ratio, and polarizability were also calculated with the
same level of theory using the standard Gaussian 98 keyword “Polar”. Each
birefringence was estimated from polarizability [13].


## 2.1. Materials

All the chemicals were purchased from commercial
suppliers and used without further purification unless otherwise noted.
DcAz2Mc, DR1Mc, and M6PBMe were synthesized according to
the manner reported earlier [8–11] and were
confirmed by elemental analysis, IR and NMR spectroscopic methods. Synthesis of polymers and copolymers, poly(DR1Mc),
poly(DcAz2Mc),
poly(DcAz2Mc-co-M6PBMe),
and poly(DR1Mc-co-M6PBMe), was
carried out by radical polymerization of monomers with the use of
N, N′-azobis(isobutyronitrile) as initiator at 60°C for 48 hours in dimethylformamide, and their molecular structures are
shown in Figure 1.
The molecular weights and distributions were
measured by gel permeation chromatography (GPC),
and the thermal properties were studied by differential scanning calorimetry
(DSC).Figure 1
Molecular structures ofpoly(DR1Mc-co-M6PBMe) and
poly(DcAz2Mc-co-M6PBMe) containing nitro- and dicyano-end
groups, respectively. (x = 1, 0.05).DR1Mc: m.p. 83°C. Anal. calc. for C17H19O2N2:
C, 62.82; H, 5.80; N, 14.65. Found: C, 62.85; H, 5.80; N, 14.44.DcAz2Mc: m.p.
135°C. Anal. calc. for C24H23O2N5:
C, 69.88; H, 5.38; N, 16.98. Found: C, 70.00; H, 5.73; N, 16.93.M6PBMe: m.p. 45°C. Anal. calc. for C24H28O6:
C, 66.64 H, 7.23. Found: C, 66.51 H, 7.21.


## 2.2. Photochemical Properties

Photochemical molecular orientation
behavior of polymers in the solid state was investigated by using a pump-probe
method [4]. Thin solid films were prepared by spin coating
method from THF solution. An Ar+ laser (LASER GAPHICS 165 LGS, λ = 488 nm, 250 mW/cm2) and Xenon lamp
(Ushio SX-UI500XQ) equipped with band-pass filter (λ = 500–600 nm) were used
as a pumping light source, and an He-Ne laser (633 nm) was
used as the probe light. The thin solid film of the polymers was placed between
two crossed polarizers where the polarizing directions of the polarizers were set at an angle of 45° with respect to the polarizing direction of a pumping laser
light. Transmitted light intensity of the He-Ne laser light was
measured under irradiation of linearly or circularly polarized Ar+ laser light. The photoinduced birefringence, Δn, was estimated [12].The SiO2 inverse opal was prepared by the same method
described in our previous work [4]. The polymers wereinfiltrated into the
SiO2 inverse opal film by annealing.
Changes in optical properties of the SiO2 inverse opal/azo-polymer
composite films were explored by measuring reflectionspectra
for incident white light with an Ocean Optics USB2000. Reflection spectra were
measured with/without irradiation of Ar+ laser or Xenon lamp.


## 2.3. Calculation

The
geometry optimizations of DR1Mc and DcAz2Mc were carried out using Gaussian 98
with B3LYP/6-31G** basis set.  Dipole
moment, aspect ratio, and polarizability were also calculated with the
same level of theory using the standard Gaussian 98 keyword “Polar”. Each
birefringence was estimated from polarizability [13].


## 3. Results and Discussion

### 3.1. Photresponse Properties

The
cyano group is one of strong electrowithdrawing groups so that side chain type
azo polymer having the cyano group,poly(DcAz2Mc),
was synthesized and its photoresponse be-havior was
compared with the polymer having DR1 group as the azo-side chains,
poly(DR1Mc). The molecular weights and
glass transition temperatures were
8.8 × 103 and 145°C for poly(DcAz2Mc) and 6.1 ×
103 and 122°C for poly(DR1Mc),
respectively, as given in Table 1.Table 1
Thermal properties and molecular weights of polymers and absorption peaks of their films.
HomopolymersTg(C°)(a)Mn(b)Mw/Mn(b)λmax (nm)(c)Thickness of films (nm)(c)poly(DR1Mc)1226.1×1031.7465172poly(DcAz2Mc)1458.8×1031.7495152poly(DR1Mc-co-M6PBMe)411.5×1043.148597poly(DcAz2Mc-co-M6PBMe)451.2×1043.3495100Determined by DSC(a),
GPC(b) and UV-absorption(c).Solid polymer films were prepared by spin
coating method. The thickness of the films was
estimated spectroscopically,
152 nm forpoly(DcAz2Mc) and 172 nm
for poly(DR1Mc), respectively. The
photo-orientation behavior was investigated with linearly polarized light
irradiation of Ar+ laser. The irradiation wavelength of 488 nm is
comparable to the absorption band peaks (λmax., 465–495 nm) of polymer
films as can be seen in Table 1. When the linearly polarized light was irradiated onto
the polymer thin films, the uniaxially molecular orientation perpendicularly to
the polarization direction of the incident light was confirmed for both
films of poly(DcAz2Mc) and poly(DR1Mc), and the photoinduced birefringences of thin films, Δn, are plotted in Figure 2.Figure 2
Changes in birefringence ofpoly(DR1Mc) (solid line) and
poly(DcAz2Mc) (dotted line) by irradiation of linearly polarized
light (LP) and circularly polarized (CP) light. Ar+ laser (488 nm) is
used as light source.The birefringences ofpoly(DcAz2Mc) and poly(DR1Mc)
were 0.24 and
0.14, respectively, at the oriented states after prolonged
irradiation of linearly polarized light. Poly(DcAz2Mc) showed higher birefringence than that of poly(DR1Mc). The stability of uniaxial orientation states was also examined under dark condition by monitoring transmitted light intensity after turnoff the irradiation. Theoriented state
of poly(DcAz2Mc) was preserved for
hours, while poly(DR1Mc) was disordered
about 20% within 3 minutes after
light off. Thus, it was found that orientation stability
for poly(DcAz2Mc) film was
superior to that of poly(DR1Mc). On the other hand, with the irradiation
of circular polarized light, the
oriented state of both polymers was disordered at once. Based on the experimental results indicating well-controlled photoresponse properties, calculations of geometry optimizations for DR1Mc and DcAz2Mc molecules
were computationally carried out in order to explore the relationship between a
molecular structure and its orientation behaviors.


### 3.2. Calculation

The molecular
structures and the results of electronic structure calculations for DR1Mc and
DcAz2Mc molecules are shown in Figure3. According to the calculations, the
dipole moment values of DcAz2Mc and DR1Mc molecules are 5.31 and 4.08, respectively. DcAz2Mc has larger
dipole moment value as 1.23 debye than that of DR1Mc. The result indicates that dicyano electron
withdrawing group affords
stronger electronically
push-pull property to azobenzene dyes, leading to larger
dipole moment. Therefore, it should be expected that DcAz2Mc can undergo more rapid cis-trans isomerization and rapid photoresponse property. On the other hand, the molecular structure geometries are described by the
dipole moment distribution and aspect ratio. The dipole moment distributions of
both molecules are concentrated along the x-axis of the (x; y; z) coordinates in the Gaussian function distribution, and the aspect ratios (L/D) of
DcAz2Mc and DR1Mc were,
respectively, 2.7 and 2.3. These results indicate that DcAz2Mc molecule has more thin and
longish molecular shape than DR1Mc, although both
azobenzene dyes are in the rod like molecular shape. In addition, the birefringence of
the molecules was calculated
from their molecular polarizabilities. A density of 103 kg/m3 and an orientational order
parameter S of 0.7 were assumed for the calculation. The birefringences of DcAz2Mc and DR1Mc were 0.58 and 0.43, respectively. Consequently, the
photochemically rapid and larger change in the birefringence for DcAz2Mc can be interpreted in terms of the
molecular structure geometries and intrinsic optical properties.Molecular structure designs of DR1Mc and DcAz2Mc and
their electronic calculation results of dipole moment distribution,
aspect ratio, and birefringence.
(a)(b)


### 3.3. Infiltration of Azo Polymers intoSiO2 Inverse Opal

In the procedure
of infiltration of polymers into inverse opal, these
push-pull type azobenzene polymers were found to have a drawback with their short of
fluidity. Since the polymers are generally rigid due to their high Tg and high viscosity compared with low-molar-mass materials. In particular, electronic intermolecular interaction due to
push-pull groups substituted in the dyes is considered to attribute to short of
fluidity. It brings about a limitation to infiltration into every gap in the inverse
opal film or even destroys the silica inverse opal skeleton during cooling down
of infiltrated polymers from melted state at high temperature.In
addition, the use of materials containing higher con-tent of azobenzene chromophores may lead to
the incomplete photoisomerization over the whole silica inverse opal film infiltrated with the
polymers, because of strong absorption of incident light for the photoisomerization near the surface of the inverse
opal film. Therefore, in order to induce
photoresponse all over the films as well as to provide fluidity, copolymers were synthesized by copolymerization of DcAz2Mc or DR1Mc with M6PBMe for dilution of the content of azobenzene
chromophores.In our study, 5% of push-pull type of
azo-group containing polymers,poly(DcAz2Mc-co-M6PBMe) and poly(DR1Mc-co-M6PBMe), were prepared by radical
polymerization. The
structures are shown in Figure 1. Tg
of poly(DcAz2Mc-co-M6PBMe)
and poly(DR1Mc-co-M6PBMe) were 45°C and
41°C, respectively, as shown in Table 1. The
photo-orientation behavior was investigated with linearly polarized light
irradiation with Ar+ laser (488 nm). Birefringences of poly(DcAz2Mc-co-M6PBMe) and poly(DR1Mc-co-M6PBMe) were, respectively, corresp-onding to
0.11 and 0.09 at the oriented states, as shown in Figure 4. A little content of azo moiety also leads to photo orientation in the copolymer films, and poly(DcAz2Mc-co-M6PBMe) shows the higher birefringence than
poly(DR1Mc-co-M6PBMe).Figure 4
Time courses of photo orientation ofpoly(DR1Mc-co-M6PBMe) (solid line) and poly(DcAz2Mc-co-M6PBMe) (dotted
line) by irradiation of linearly polarized (LP) light. Thicknesses are
172 nm and 152 nm. Ar+ laser (488 nm) is used as light source.


### 3.4. Bragg Reflection Behaviors

Poly(DcAz2Mc-co-M6PBMe)
and poly(DR1Mc-co-M6PBMe) were readily infiltrated into the inverse opal films by heating to the melting state. Bragg
reflection bands of inverse opal films were measured
after cooling, and their reflection peaks were observed at 701 nm for Poly(DcAz2Mc-co-M6PBMe)
and 697 nm for poly(DR1Mc-co-M6PBMe), respectively. Change in Bragg
reflection band were explored under photo irradiation. When Ar+ laser was used as light source,
no significant change of Bragg reflection band was observed. It is assumed that the narrow wavelength of Ar+ laser (488 nm), which is corresponding to the
maximum peaks of azobenzene polymers, induces partial photoisomerization only near the surface of the films. In contrast, irradiation of the light with wavelengths
ranging from 500 to 600 nm with Xenon lamp was found to cause the change in Bragg reflection band. With linearly polarized light of
Xenon lamp (500–600 nm), reflection band peaks of poly(DcAz2Mc-co-M6PBMe) and poly(DR1Mc-co-M6PBMe) were, respectively, shifted to the longer wavelength region by 18.5 nm and 14.5 nm after irradiation for 1hour as shown in Figure 5.Figure 5
Reflection band shifts forpoly(DR1Mc-co-M6PBMe) (solid
line) and poly(DcAz2Mc-co-M6PBMe) (dotted line) under linearly
polarized light.


## 3.1. Photresponse Properties

The
cyano group is one of strong electrowithdrawing groups so that side chain type
azo polymer having the cyano group,poly(DcAz2Mc),
was synthesized and its photoresponse be-havior was
compared with the polymer having DR1 group as the azo-side chains,
poly(DR1Mc). The molecular weights and
glass transition temperatures were
8.8 × 103 and 145°C for poly(DcAz2Mc) and 6.1 ×
103 and 122°C for poly(DR1Mc),
respectively, as given in Table 1.Table 1
Thermal properties and molecular weights of polymers and absorption peaks of their films.
HomopolymersTg(C°)(a)Mn(b)Mw/Mn(b)λmax (nm)(c)Thickness of films (nm)(c)poly(DR1Mc)1226.1×1031.7465172poly(DcAz2Mc)1458.8×1031.7495152poly(DR1Mc-co-M6PBMe)411.5×1043.148597poly(DcAz2Mc-co-M6PBMe)451.2×1043.3495100Determined by DSC(a),
GPC(b) and UV-absorption(c).Solid polymer films were prepared by spin
coating method. The thickness of the films was
estimated spectroscopically,
152 nm forpoly(DcAz2Mc) and 172 nm
for poly(DR1Mc), respectively. The
photo-orientation behavior was investigated with linearly polarized light
irradiation of Ar+ laser. The irradiation wavelength of 488 nm is
comparable to the absorption band peaks (λmax., 465–495 nm) of polymer
films as can be seen in Table 1. When the linearly polarized light was irradiated onto
the polymer thin films, the uniaxially molecular orientation perpendicularly to
the polarization direction of the incident light was confirmed for both
films of poly(DcAz2Mc) and poly(DR1Mc), and the photoinduced birefringences of thin films, Δn, are plotted in Figure 2.Figure 2
Changes in birefringence ofpoly(DR1Mc) (solid line) and
poly(DcAz2Mc) (dotted line) by irradiation of linearly polarized
light (LP) and circularly polarized (CP) light. Ar+ laser (488 nm) is
used as light source.The birefringences ofpoly(DcAz2Mc) and poly(DR1Mc)
were 0.24 and
0.14, respectively, at the oriented states after prolonged
irradiation of linearly polarized light. Poly(DcAz2Mc) showed higher birefringence than that of poly(DR1Mc). The stability of uniaxial orientation states was also examined under dark condition by monitoring transmitted light intensity after turnoff the irradiation. Theoriented state
of poly(DcAz2Mc) was preserved for
hours, while poly(DR1Mc) was disordered
about 20% within 3 minutes after
light off. Thus, it was found that orientation stability
for poly(DcAz2Mc) film was
superior to that of poly(DR1Mc). On the other hand, with the irradiation
of circular polarized light, the
oriented state of both polymers was disordered at once. Based on the experimental results indicating well-controlled photoresponse properties, calculations of geometry optimizations for DR1Mc and DcAz2Mc molecules
were computationally carried out in order to explore the relationship between a
molecular structure and its orientation behaviors.


## 3.2. Calculation

The molecular
structures and the results of electronic structure calculations for DR1Mc and
DcAz2Mc molecules are shown in Figure3. According to the calculations, the
dipole moment values of DcAz2Mc and DR1Mc molecules are 5.31 and 4.08, respectively. DcAz2Mc has larger
dipole moment value as 1.23 debye than that of DR1Mc. The result indicates that dicyano electron
withdrawing group affords
stronger electronically
push-pull property to azobenzene dyes, leading to larger
dipole moment. Therefore, it should be expected that DcAz2Mc can undergo more rapid cis-trans isomerization and rapid photoresponse property. On the other hand, the molecular structure geometries are described by the
dipole moment distribution and aspect ratio. The dipole moment distributions of
both molecules are concentrated along the x-axis of the (x; y; z) coordinates in the Gaussian function distribution, and the aspect ratios (L/D) of
DcAz2Mc and DR1Mc were,
respectively, 2.7 and 2.3. These results indicate that DcAz2Mc molecule has more thin and
longish molecular shape than DR1Mc, although both
azobenzene dyes are in the rod like molecular shape. In addition, the birefringence of
the molecules was calculated
from their molecular polarizabilities. A density of 103 kg/m3 and an orientational order
parameter S of 0.7 were assumed for the calculation. The birefringences of DcAz2Mc and DR1Mc were 0.58 and 0.43, respectively. Consequently, the
photochemically rapid and larger change in the birefringence for DcAz2Mc can be interpreted in terms of the
molecular structure geometries and intrinsic optical properties.Molecular structure designs of DR1Mc and DcAz2Mc and
their electronic calculation results of dipole moment distribution,
aspect ratio, and birefringence.
(a)(b)


## 3.3. Infiltration of Azo Polymers intoSiO2 Inverse Opal

In the procedure
of infiltration of polymers into inverse opal, these
push-pull type azobenzene polymers were found to have a drawback with their short of
fluidity. Since the polymers are generally rigid due to their high Tg and high viscosity compared with low-molar-mass materials. In particular, electronic intermolecular interaction due to
push-pull groups substituted in the dyes is considered to attribute to short of
fluidity. It brings about a limitation to infiltration into every gap in the inverse
opal film or even destroys the silica inverse opal skeleton during cooling down
of infiltrated polymers from melted state at high temperature.In
addition, the use of materials containing higher con-tent of azobenzene chromophores may lead to
the incomplete photoisomerization over the whole silica inverse opal film infiltrated with the
polymers, because of strong absorption of incident light for the photoisomerization near the surface of the inverse
opal film. Therefore, in order to induce
photoresponse all over the films as well as to provide fluidity, copolymers were synthesized by copolymerization of DcAz2Mc or DR1Mc with M6PBMe for dilution of the content of azobenzene
chromophores.In our study, 5% of push-pull type of
azo-group containing polymers,poly(DcAz2Mc-co-M6PBMe) and poly(DR1Mc-co-M6PBMe), were prepared by radical
polymerization. The
structures are shown in Figure 1. Tg
of poly(DcAz2Mc-co-M6PBMe)
and poly(DR1Mc-co-M6PBMe) were 45°C and
41°C, respectively, as shown in Table 1. The
photo-orientation behavior was investigated with linearly polarized light
irradiation with Ar+ laser (488 nm). Birefringences of poly(DcAz2Mc-co-M6PBMe) and poly(DR1Mc-co-M6PBMe) were, respectively, corresp-onding to
0.11 and 0.09 at the oriented states, as shown in Figure 4. A little content of azo moiety also leads to photo orientation in the copolymer films, and poly(DcAz2Mc-co-M6PBMe) shows the higher birefringence than
poly(DR1Mc-co-M6PBMe).Figure 4
Time courses of photo orientation ofpoly(DR1Mc-co-M6PBMe) (solid line) and poly(DcAz2Mc-co-M6PBMe) (dotted
line) by irradiation of linearly polarized (LP) light. Thicknesses are
172 nm and 152 nm. Ar+ laser (488 nm) is used as light source.


## 3.4. Bragg Reflection Behaviors

Poly(DcAz2Mc-co-M6PBMe)
and poly(DR1Mc-co-M6PBMe) were readily infiltrated into the inverse opal films by heating to the melting state. Bragg
reflection bands of inverse opal films were measured
after cooling, and their reflection peaks were observed at 701 nm for Poly(DcAz2Mc-co-M6PBMe)
and 697 nm for poly(DR1Mc-co-M6PBMe), respectively. Change in Bragg
reflection band were explored under photo irradiation. When Ar+ laser was used as light source,
no significant change of Bragg reflection band was observed. It is assumed that the narrow wavelength of Ar+ laser (488 nm), which is corresponding to the
maximum peaks of azobenzene polymers, induces partial photoisomerization only near the surface of the films. In contrast, irradiation of the light with wavelengths
ranging from 500 to 600 nm with Xenon lamp was found to cause the change in Bragg reflection band. With linearly polarized light of
Xenon lamp (500–600 nm), reflection band peaks of poly(DcAz2Mc-co-M6PBMe) and poly(DR1Mc-co-M6PBMe) were, respectively, shifted to the longer wavelength region by 18.5 nm and 14.5 nm after irradiation for 1hour as shown in Figure 5.Figure 5
Reflection band shifts forpoly(DR1Mc-co-M6PBMe) (solid
line) and poly(DcAz2Mc-co-M6PBMe) (dotted line) under linearly
polarized light.


## 4. Conclusion

DR1Mc and DcAz2Mc, having push-pull type of azobenzene group,
are prepared and their photo-orientational behaviors are compared. It
is found that DcAz2Mc molecule is theo-retically and experimentally advantageous structure for rapid photo
response and orientation behavior than that of
DR1Mc.
In the inverse opal films infiltrated with their
copolymers, Bragg reflection bands are shifted to the longer wavelength
region under irradiation of linearly polarized light. The shift width forpoly(DcAz2Mc-co-M6PBMe) is larger than that
for poly(DR1Mc-co-M6PBMe).



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*Source: 962160-2008-07-20.xml*