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 noncontact and non-destructive feature. In this paper, we report a photo-induced 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 first synthesized and characterized. P-(Z)-345 with three alkoxy tails exhibits hexagonal columnar structure. P-(Z)-4, P-(Z)-34, and P-(Z)-35 with one or two alkoxy tails possess lamellar structure. The substituted position and number of alkoxy tails also influence the photoluminescence behaviour, giving rise to different fluorescent color in bulk. Both monomers and polymers perform photo-induced E/Z isomerization in solution. Under the UV light of 365 nm and 254 nm, polymer thin film or monomer thin film at noncrystalline state could change their fluorescent color accordingly, which is mainly due to the photo-induced reversible Z/E isomerization and [2+2] dimerization in film.