Substitution of free halide ions unlocks responsive photoluminescence switching in manganese-based metal halides

Stimuli-responsive organic-inorganic metal halides hold great promise for emerging information-related applications. In this work, replacing the free halide ion Cl^- with Br^- in C₅H₁₁N₃(MnCl₃·H₂O)X (where C₅H₁₁N₃²⁺ represents histamine cation, X represents free halide ions) converts the non-responsive hybrid C₅H₁₁N₃(MnCl₃·H₂O)Cl into a stimuli-responsive C₅H₁₁N₃(MnCl₃·H₂O)Br. The latter exhibits reversible photoluminescence color switching between red and green upon thermal or water exposure. Extensive experimental and theoretical analyses reveal that the responsive property primarily stems from weakened hydrogen bonding surrounding H₂O molecules after Br^- substitution, which facilitates the initial escape of H₂O molecules under heating. Subsequent structural reorganization and coordination transformation then induce the change in photoluminescence. Furthermore, the fabricated halide/polymer luminescent films are demonstrated to be highly applicable in multiple scenarios, such as planar temperature sensing, thermal stamping, and encryption/decryption. This study highlights the crucial yet often overlooked role of free halide ions in metal halides and offers new insights into their structure-property relationships.