Manganese(II) Mixed Halide: Enhanced Thermal/UV Stability and Humidity History Memory

Applications of stimuli‐responsive metal halides are often constrained by their inherent trade‐off between high response sensitivity and long‐term environmental stability. In this work, by partial incorporation of Cl − into C 5 H 11 N 3 MnBr 4 (C 5 H 11 N 3 2+ = histamine cation), overall structural rigidity and local structural flexibility are successfully integrated within one metal halide, thus leading to the dual optimization of photophysical and responsive performance. The obtained mixed halide, C 5 H 11 N 3 MnCl 3 Br, exhibits markedly improved resistance to both thermal treatment and continuous UV irradiation, as well as a 1.5‐fold increase in photoluminescence quantum yield. Concurrently, the mixed halide exhibits a humidity‐triggered emission color transition from green to red. This transition cannot be reversed unless heated to at least 80°C, demonstrating a unique humidity history memory function, which is highly suitable for humidity contamination monitoring in long‐distance shipping. This work highlights the role of halide ions mixing in activating responsive performance, which has been rarely reported, and provides a new structural regulation strategy to harmonize luminescence efficiency, environmental stability, and stimuli‐responsive functionality in metal halides.