Photo-responsive multi-bilayered film consisting of azobenzene polymer liquid crystals (PAzo) and polyvinylalcohol
(PVA) was prepared on a glass substrate by spin coating of the polymer solutions alternately. The reflectivity of the
multi-bilayered film disappeared by annealing at 80 °C. The disappearance of the reflection by the annealing was related
to the thermal out-of-plane molecular orientation of PAzo even in the multi-bilayered film, leading to a very small
difference in refractive indices between PAzo and PVA. The reflectance of the multi-bilayered film was increased again
by UV irradiation because of the transformation from the
out-of-plane orientation to the in-plane random orientation,
resulting in the restoration of difference in the refractive indices. In this way, the on-off switching of the reflection was
achieved by combination of the thermally spontaneous out-of-plane molecular orientation and following
photoisomerization of PAzo comprising the multi-bilayered film.
Photochemically tunable photonic band gap materials were prepared by infiltration of liquid crystal polymers having
azobenzene groups into voids of SiO2 inverse opal films. Linearly polarized light irradiation resulted in transformation
from a random to an anisotropic molecular orientation of azobenzene side chains in the voids of the SiO2 inverse opal
film, leading to the reversible and stable shift of the reflection band to longer wavelength more than 15 nm. In order to
improve switching properties, we used copolymers with azobenzene monomer and tolane monomer, which indicate
higher birefringence, as infiltration materials into the voids. The azo-tolane copolymers were found to show the higher
birefringence than azobenzene homopolymers by the linearly polarized light irradiation. Thus, the reflection band of the
SiO2 inverse opal film infiltrated with the azo-tolane copolymers was shifted to long wavelength region more than 55 nm
by the irradiation of linearly polarized light.
A photochemically tunable structural color material was prepared by infiltration of the polymer liquid crystal (LC)
having azo-chromophores in a SiO2 inverse opal structure. The SiO2 inverse opal film infiltrated with the polymer LC
reflected a light, which is called a structural color, corresponding to the periodicity as well as the refractive indices of the
inverse opal structure. Linearly polarized light irradiation caused the shift of the structural color band to longer
wavelength more than 15 nm. This is caused by the formation of uniaxially anistorpic molecular orientation of the
polymer LC. The switched state was stable under interior light, and reversible switching of the reflection band can be
achieved by the linearly and circularly polarized light irradiation. This photoswitching property will be suitable for
various optical materials such as memory, display so on.
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