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

In this work, side-chain luminescent liquid crystalline polymers (SCLLCPs) that contain di-substituted cyanostilbene (CS) groups with different numbers of substituted terminal alkyl chains, namely PNB-Z-4, PNB-Z-34, and PNB-Z-345, were synthesized. Their mesomorphic structure, luminescence properties and photochromic luminescent behaviour were investigated. PNB-Z-4 containing one alkyl chain in each mesogen possesses a supramolecular oblique columnar (Col_ob) phase with large lattice parameters, which can transform into a lamellar structure at high temperature. PNB-Z-34 and PNB-Z-345 with two and three alkyl tails exhibit a hexagonal columnar phase (Col_h) structure. Their fluorescence quantum yields (Φ_F) in the 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 shows weak fluorescence. Furthermore, their photochromic fluorescence responses are quite different. PNB-Z-4 exhibits an obvious “turn-on” photo-responsive behaviour with Φ_F 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 fluorescence response.