Photosalient effect and reversible photochromic photoluminescence driven by cascade [2 + 2] cycloaddition reaction and water adsorption in a 0D hybrid metal halide

The ionic structure of hybrid metal halides (HMHs) is conducive to the integration of the photoactivity of organic components and mechanical properties of inorganic components. Therefore, HMHs can function as a suitable platform to advance photomechanical performance due to their great structural designability and wide compositional diversity. Herein, a notable photosalient effect was observed in 0D ionic (LH)₂InCl₅ (1, L = 4-styrylpyridine). Upon irradiation with 450 nm light, 1 undergoes a rare single-crystal-to-single-crystal transformation to (L₂H₂)[In(H₂O)Cl₅] (1a), along with distinct photochromic photoluminescence and various photomechanical responses, driven by the cascade photoinduced [2 + 2] cycloaddition of LH⁺ and water sorption by coordinatively unsaturated In³⁺. A detailed mechanistic investigation unraveled the photosalient effect by initiation of the cycloaddition reaction and enhancement by water sorption, which resulted in a phase transformation that is more energetically and kinetically favorable through decreasing Gibbs free energy (1.51 eV) and a kinetical energy barrier (0.29 eV). Additionally, the partially reversible transformation between LH⁺ monomers and dimers endows 1a with excellent photoswitch performance due to the variation in the π electron-conjugated degree for LH⁺. At last, by integrating powdered 1 with polymethyl methacrylate (PMMA), the 1-PMMA hybrid matrix membrane exhibited rapid and controllable responsive behaviors, including bending and curling, through controlling the direction of the concentration gradient. This work demonstrates that ionic HMHs might function as a promising platform for advanced photomechanical and photochromic materials.