Photochromic fluorescence switching in liquid crystalline polynorbornenes with α-cyanostilbene side-chains

Photoresponsive materials have been drawing much attention due to their fast switching properties and non-contact and non-destructive features. In this paper, we report a photoinduced reversible fluorescence switching based on the α-cyanostilbene containing liquid crystalline polynorbornenes. Z- and E-stereoisomers of α-cyanostilbene monomers with different flexible alkoxy tails and their corresponding Z- and E-polymers were synthesized and characterized for the first time. P-(Z)-345 with three alkoxy tails exhibits a hexagonal columnar structure. P-(Z)-4, P-(Z)-34, and P-(Z)-35 with one or two alkoxy tails possess a lamellar structure. The substitution position and number of alkoxy tails also influence the photoluminescence behaviours, giving rise to different fluorescence colours in bulk. Both monomers and polymers show photoinduced E/Z isomerization in solution. Under the UV light of 365 nm and 254 nm, polymer thin films or monomer thin films at the noncrystalline state could change their fluorescence colours accordingly, which is mainly due to the photoinduced reversible E/Z isomerization and [2+2] dimerization in film.