Cyanostilbene‐Based AIEgen Smart Film: Optical Switching by Engineering Molecular Packing Structure and Molecular Conformation Through Thermal and Photoinduced Monotropic Phase Transition

Abstract The optical properties of newly synthesized cyanostilbene‐based AIEgen (D4CS) exhibiting a unique monotropic phase transition are modulated by engineering molecular packing structure and molecular conformation through temperature change and light irradiation. D4CS exhibits only a low‐ordered smectic liquid crystal (SmA) phase over a wide temperature range during cooling and subsequent heating processes, while D4CS crystallizes after a significantly long annealing time at room temperature. Crystallized D4CS exhibits a complex phase transition (SmCr–SmB–SmA–Iso) in the subsequent heating process. In addition, the photoisomerization behavior of D4CS from trans to cis conformer can induce a phase transition to the isotropic (Iso) state, and the reverse isomerization can occur by high‐temperature heat treatment. This unique thermal and photoinduced phase transition of D4CS is accompanied by changes in optical properties including transmittance and luminescence. Based on the optical switching behavior of D4CS, photopatternable temperature and time‐dependent optical films are demonstrated. The newly developed cyanostilbene‐based AIEgen is a smart optical material with great potential for advanced optical sensors, anti‐counterfeiting ink and smart display system.