Combined electric and photocontrol of selective light reflection by oblique helicoidal cholesteric doped with azobenzene derivative*

A cholesteric liquid crystal (ChLC) composed of flexible dimeric molecules shows an oblique helicoidal state (Ch_OH) in the presence of an electric field. By controlling the Ch_OH pitch P by the electric field, one can shift the wavelength of the selective reflection (transmission) of light in a broad spectral range, from UV to IR. In this work, we demonstrate that a combined action of the electric field and UV irradiation could continuously tune the pitch of a Ch_OH material doped with photosensitive azobenzene molecules capable of trans-cis isomerization. At a fixed voltage, UV irradiation causes a gradual redshift of the reflection wavelength by about 100 nm for the explored composition. The dynamic scenarios of the radiation-induced changes are described by kinetic equations. The phenomenon can find applications such as smart windows, lasers, optical filters, and sensors of UV intensities/doses.