Photo-switchable spiropyran composites via liquid-assisted grinding method for multilevel information encryption

Hydrogen-bonded organic frameworks (HOFs) hold promises for improving solid-state photochromism of spiropyran (SP), yet conventional physical adsorption methods are limited by procedural complexity and host-guest size-matching issues. To address these challenges, this work employs a ligand-integration strategy by combining 2,4,6-tris(4-carboxyphenyl)-1,3,5-triazine (H₃TATB) with SP derivatives to synthesize photochromic composites via straightforward liquid-assisted grinding (LAG) and solvent evaporation. By systematically regulating ratios of the two components, a series of complexes were prepared. It was found that a 1:2 M ratio of H₃TATB to SP derivatives maximizes intermolecular interactions, leading to optimal photochromic performance-specifically, a rapid white-to-deep purple color transition under UV irradiation. Deviations from this ratio result in excessive HOF self-assembly and subsequent physical adsorption of SP, which diminishes synergistic effects. To clarify the influence of intermolecular forces, various SP derivatives were examined, confirming their crucial role in determining photochromic efficiency. Furthermore, the composite was successfully applied in information encryption, demonstrating its practical utility. This approach offers two key advantages: (1) a simplified synthesis process that improves reproducibility, and (2) elimination of pore-size mismatch issues typical of physical adsorption, thereby enabling stable and homogeneous integration. Here, LAG promotes direct binding between H₃TATB and SP, while solvent evaporation ensures structural uniformity.