Photochemical Modulation of Color and Transmittance in Chiral Nematic Liquid Crystal Containing an Azobenzene as a Photosensitive Chromophore

Abstract

All-optical switching of selectively reflected colors based on isomerization of an azobenzene (Azo) and subsequent change in transmittance was investigated in cholesteric liquid crystals (LCs) which reflect light in the visible region. Photoirradiation at 366 nm, which causes an efficient trans-cis isomerization of Azo, led to change in color of cholesteric LCs toward shorter wavelengths with a concomitant lowering of phase transition temperature from a cholesteric to an isotropic phase ( T Ch-I ). Reversible change in color was induced all-optically by irradiating alternatively at effective wavelengths for reversible isomerization of Azo. A remarkable change in transmittance was also observed when the photoinduced change in colors was measured by a probe light with the same handedness as the helical sense of the cholesteric LCs. In the wavelength regions of reflected colors before photoirradiation, the spectral position played an important role in producing a normal-mode and a reverse-mode switching in photoinduced modulation of transmittance of the cholesteric samples with respect to the probe light.