Multiple Photochemical Processes in Liquid Crystalline Azo Dye-Doped Liquid Crystals

We present a detailed study on the multiple photochemical processes that call occur in a nematic liquid crystal (BL006) doped with a liquid crystalline azobenzene compound (LC azo dye) as a result of the photoinduced isomerization of the chromophore. On the basis of the photo-optical response (transmittance change upon illumination) and the electro-optical response (transmittance change upon application of a voltage) as well as polarizing microscopic observations, two regimes of behaviors can be distinguished upon UV exposure inducing the trans-cis isomerization. In the regime of low LC azo dye concentrations (less than or similar to 7 wt%), where the achievable amount of cis isomers are miscible with the LC host, only a photochemical disordering of the LC takes place at room temperature, with a precisely controllable degree of disordering. At elevated temperatures, but below the clearing temperature T-ni of the LC, the photochemical disordering can lead to the photochemical nematic-isotropic phase transition. In the regime of high LC azo dye concentrations, when the amount of cis isomers passes a certain concentration, they become immiscible with the LC, which triggers a photochemical phase separation process. Depending on the rate at which cis isomers are formed, the phase separation process can either follow the disordering or appear simultaneously with and compete against the disordering and the nematic-isotropic phase transition. In all cases, the mixture evolves over time toward two separated LC phases, with one disordered rich in cis isomer. All these photochemical processes under UV light can be reversed by visible light inducing the cis-trans back-isomerization of the chromophore. This study shows the richness of the photo-optical and electro-optical behaviors of LC azo dye-doped liquid crystals and provides a comprehensive understanding oil the multiple reversible photochemical processes.