Light control of cholesteric liquid crystals using azoxy-based host materials

The characteristics of cholesteric liquid crystals can be controlled by light irradiation if conformationally photo-active molecules are present. Recently, control of the selective reflection band (spiral pitch) in nemato-chiral mixtures was demonstrated when photosensitive molecules, namely nematic azoxy-based compounds, were used as the host material. In this report, the investigation of light induced effects in cholesterics with azoxy-based host materials is continued to highlight the mechanisms of the response. Different non-photosensitive chiral materials were added to different azoxy-nematic liquid crystals and the pitch change caused by UV irradiation was investigated. A change in the pitch of 50– 210nm was observed depending on the exposure time and the intensity of the light. This effect is reversible: under illumination at wavelengths greater than 410 nm, the pitch shifts in the opposite direction. The dependence of the selective reflection band and the full-width-at-half-maximum of the band on the exposure time and the temperature dependence of the selective reflection band were investigated. The lowering of the phase transition temperature and narrowing of the width of the selective reflection band can be explained by a decrease in the orientational order parameter. The blue shift of the selective reflection band is due to a decrease in both the order parameter and the concentration of linear nematogenic molecules.