Mesomorphic and photochromic luminescent behaviour of side-chain liquid crystalline polymers containing di-substituted cyanostilbene groups with the different number of alkyl tail chain

In this work, side-chain luminescent liquid crystalline polymers (SCLLCPs) that contain di-substituted cyanostilbene (CS) groups with the different substituted number of terminal alkyl chain, which are PNB-Z-4, PNB-Z-34, and PNB-Z-345, were synthesized. Their mesomorphic structure, luminescent propereties and photochromic luminescent behaviour were investigated. PNB-Z-4 containing one alkyl chain in each mesogen possesses supramolecular oblique columnar (Colob) phase with large lattice parameters, which could transform to lamellar structure at high temperature. PNB-Z-34 and PNB-Z-345 with two and three alkyl tails exhibit hexagonal columnar phase (Colh) structure. Their fluorescent quantum yields (ΦF) in solid state are improved by increasing the number of alkyl tails as well. PNB-Z-34 and PNB-Z-345 exhibit aggregation induced emission (AIE) properties, while PNB-Z-4 show weak fluorescence. Furthermore, their photochromic fluorescence responses are quite different. PNB-Z-4 exhibits obvious “turn-on” photo-responsive behaviour with ΦF increasing from 0.70% to 7.73% after UV irradiation with the existence of [2+2] cycloaddition, while PNB-Z-34 and PNB-Z-345 show Z/E isomerization induced blueshift fluorescent response .