Thermal and Photochemical Control of Molecular Orientation of Azo‐Functionalized Polymer Liquid Crystals and Application for Photo‐Rewritable Paper

Abstract

A photo-responsive multi-bilayered film consisting of azobenzene polymer liquid crystals (PA6Az1) and poly(vinyl alcohol) (PVA) has been prepared on a glass substrate by alternate spin coating of the polymer solutions. The reflectivity of the multi-bilayered film disappears by annealing at 80 °C. The disappearance of the reflection by the annealing is related to the thermal out-of-plane molecular orientation of PA6Az1 even in the multi-bilayered film, which leads to a very small difference in refractive indices between PA6Az1 and PVA. The reflectance of the multi-bilayered film is increased again by UV irradiation because of the transformation from the out-of-plane orientation to an in-plane random orientation. In this way, on-off switching of the reflection is achieved by combination of the thermally spontaneous out-of-plane molecular orientation and following photoisomerization of PA6Az1 comprising the multi-bilayered film.

Optically switchable multi-stopband of non-quarter-wavelength dielectric multilayer using azobenzene polymer liquid crystal

“…3) However, as mentioned above, the strong absorption of azobenzene does not allow switching of the reflection characteristics in a simply RGB-stacked azo-multilayer which has many layers with different thicknesses. In a multilayer having thick azo-layers, UV or visible light for control of the molecular alignment does not reach the bottom of the multilayer.…”

“…[1][2][3] The polymer multilayers can be obtained relatively easily by spin coating and it is easy to produce a large-area thin film. Lightness and scratch resistance are also advantages of the polymer multilayers compared with conventional inorganic multilayers.…”

“…3,[5][6][7][8] The cis-trans isomerization of azobenzenes induces a change in the refractive index or photomechanical effects. 5,6) The refractive index tunability of the azobenzene polymer is utilized for dielectric multilayers 3) and opal photonic crystals.…”

“…3,[5][6][7][8] The cis-trans isomerization of azobenzenes induces a change in the refractive index or photomechanical effects. 5,6) The refractive index tunability of the azobenzene polymer is utilized for dielectric multilayers 3) and opal photonic crystals. 7,8) Such optical devices are known as tunable photonic crystals, which have variable photonic band gaps by controlling optical periodicities and are also expected to have functional applications, for example, tunable optical waveguides and tunable lasers.…”

“…[9][10][11][12] We proposed an azobenzene polymer liquid crystal=poly(vinyl alcohol) (PVA) multilayer as a tunable one-dimensional photonic crystal in which switching of the photonic band gap was demonstrated by ultraviolet (UV) and visible light irradiation. 3) The next challenge for this azo-multilayer is to achieve an optically switchable multistopband. In general, multilayers consisting of low-index-contrast materials can obtain multicolor or broadband reflection by stacking many layers of different thicknesses.…”