Organic polymorphic crystals with multi-stimuli response: excellent mechanical elasticity, novel two-stage heterotropic photochromism and self-reversible acidichromism

Smart multi-stimuli responsive organic materials have become increasingly favored due to their advantages of high stability and easy regulation. In this study, a cyanostilbene derivative ((αZ)-2-chloro-α-[(4-methoxyphenyl)methylene]-5-pyridinehyde, TAR) was successfully synthesized, and its polymorphs (TAR-1 and TAR-2) exhibited multiple responsive behaviors under different stimulus conditions. TAR-1 is supposed to have excellent mechanical properties, while TAR-2 shows fascinating heterotropic photochromism (green-cyan-yellow) under different intensities of ultraviolet (UV) irradiation, which has never been reported in previous studies. Further investigation revealed that this was attributed to different degrees of [2+2] cycloaddition induced by UV light. In addition, both polymorphs could be protonated during Trifluoroacetic acid fuming and deprotonate automatically after fuming, accompanied by the significant variation in fluorescence. Finally, two scenarios were designed based on their unique stimuli-responsive properties, which proved their promising potential in fields of information security and anti-counterfeiting encryption.