7 Photochromism
267
UV
Δ
Fig. 7.5 Shrinkage of poly(oxymethylsilylene) crosslinked with azobenzene by photoirradiation
this way, molecules can be aligned by light, dichroism and birefringence appear in
the film. This phenomenon is called optical alignment of liquid crystal and is utilized
as a technique for producing the oriented film of liquid crystal in the electronics field
(Ichimura 2000).
Attempts are also made to develop mechanical functions leading to macroscopic
deformation associated with photoisomerization of azobenzene. It has been found
that shrinkage of the polymer form occurs due to light-induced phase transition
between order and disorder of liquid crystal polymer having poly(oxymethylsilylene)
as the crosslinking site of azobenzene (Fig. 7.5) (Finkelmann et al. 2001). Thus,
with the photoisomerization, the nematic phase changes to an isotropic phase, and
shrinkage of about 20% occurs in the orientation direction of the mesogen. If
shrinkage occurs near the surface of the film, the film can bend toward the incident light. When the film containing azobenzene is irradiated with polarized light,
the bending direction can be changed according to the direction of polarization (Yu
et al. 2003).
In photoisomerization of azobenzene, a mass transfer phenomenon has been found
where molecules are moved more greatly by repeatedly reversible isomerization. This
example is known as a photoinduced surface relief formation (Rochon et al. 1995;
Kim et al. 1995). Interferential exposure from two directions using polarized light
produces periodic intensity, and irregularities of micrometer size are periodically
formed accordingly. The irregularity can be eliminated by heating at a temperature
higher than the glass transition temperature of the polymer or irradiating uniform
light. The phenomenon occurs due to the large movement of molecules by photoirradiation. It has been observed not only in polymer systems but also in low molecular
amorphous systems. These azobenzene type materials are expected to be applied to
photo-alignment of liquid crystals, holographic memory, and so on.
The crystals of 3,3’-dimethylazobenzene exhibited the crawling motion on a glass
surface upon irradiation with UV and visible light simultaneously from the left and
267
UV
Δ
Fig. 7.5 Shrinkage of poly(oxymethylsilylene) crosslinked with azobenzene by photoirradiation
this way, molecules can be aligned by light, dichroism and birefringence appear in
the film. This phenomenon is called optical alignment of liquid crystal and is utilized
as a technique for producing the oriented film of liquid crystal in the electronics field
(Ichimura 2000).
Attempts are also made to develop mechanical functions leading to macroscopic
deformation associated with photoisomerization of azobenzene. It has been found
that shrinkage of the polymer form occurs due to light-induced phase transition
between order and disorder of liquid crystal polymer having poly(oxymethylsilylene)
as the crosslinking site of azobenzene (Fig. 7.5) (Finkelmann et al. 2001). Thus,
with the photoisomerization, the nematic phase changes to an isotropic phase, and
shrinkage of about 20% occurs in the orientation direction of the mesogen. If
shrinkage occurs near the surface of the film, the film can bend toward the incident light. When the film containing azobenzene is irradiated with polarized light,
the bending direction can be changed according to the direction of polarization (Yu
et al. 2003).
In photoisomerization of azobenzene, a mass transfer phenomenon has been found
where molecules are moved more greatly by repeatedly reversible isomerization. This
example is known as a photoinduced surface relief formation (Rochon et al. 1995;
Kim et al. 1995). Interferential exposure from two directions using polarized light
produces periodic intensity, and irregularities of micrometer size are periodically
formed accordingly. The irregularity can be eliminated by heating at a temperature
higher than the glass transition temperature of the polymer or irradiating uniform
light. The phenomenon occurs due to the large movement of molecules by photoirradiation. It has been observed not only in polymer systems but also in low molecular
amorphous systems. These azobenzene type materials are expected to be applied to
photo-alignment of liquid crystals, holographic memory, and so on.
The crystals of 3,3’-dimethylazobenzene exhibited the crawling motion on a glass
surface upon irradiation with UV and visible light simultaneously from the left and