Dynamic Exciton‒Color Center Switching and Multicolor/Dark Luminescence in Cs 3 Cu 2 Cl 5 for Multilevel Sensing

Lead‐free perovskite‐like copper‐halide compounds are emerging as promising candidates for next‐generation optoelectronics, but the mechanisms underlying their dynamic photophysical responses to ambient stimuli remain elusive. Here, we demonstrate that Cs 3 Cu 2 Cl 5 microplatelets exhibit reversible, environment‐controlled switching between distinct luminescence mechanisms and emission states, spanning green, blue, and non‐emissive (dark) phases. Upon heating to 500 K, Cs 3 Cu 2 Cl 5 undergoes a phase transition from a 1D Cmcm structure that exhibits robust green emission via self‐trapped excitons to a 0D Pnma phase with suppressed photoluminescence, forming a reversible dark state. In humid conditions, ion migration drives a phase transformation to Cu‐doped CsCl, resulting in the emergence of blue emission (∼2.6 eV) from Cu + color centers. This unique, ambient‐triggered exciton‐to‐color center transition enables dynamic multicolor and dark‐state luminescence switching in a single material platform. Our findings distinguish copper halides from their lead‐based counterparts and suggest their significant potential for multilevel sensing, dynamic anti‐counterfeiting, and other advanced optoelectronic applications.