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

## TL;DR

Changing free halide ions in a manganese-based material enables it to switch light colors in response to heat or water, useful for sensing and encryption.

## Contribution

Replacing Cl− with Br− in a metal halide compound introduces stimuli-responsive photoluminescence switching.

## Key findings

- Br− substitution weakens hydrogen bonding, allowing water molecules to escape under heat.
- Structural reorganization after water loss causes a reversible red-to-green photoluminescence switch.
- Halide/polymer films show practical use in temperature sensing and encryption.

## Abstract

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.