Wavelength-tuning and band-broadening of a cholesteric liquid crystal induced by a cyclic chiral azobenzene compound

Abstract

A new cyclic chiral azobenzene compound Azo-o-Bi was synthesized, which exhibits photochemically reversible cis-trans isomerization in both organic solvents and liquid crystal hosts. When doping into a nematic liquid crystal, it displays high helical twisting power (HTP, β) and a large change in HTP (Δβ) due to photoisomerization. Therefore, we are able to reversibly tune the reflection colors from near-IR to blue using irradiation with light. For weak UV light irradiation, a broadband reflection film with pitch gradients can be obtained with superior stability in the dark. This stability is associated with the cyclo-shape of the Azo-o-Bi and the high viscosity of the host liquid crystal. The latter reduces the diffusion contribution of isomers to a negligible level. The spatial and temporal control of the light irradiation can be used to photoaddress the CLC with RGB reflection colors and white-on-black reflective displays.