Highly Reversible Moisture‐Induced Bright Self‐Trapped Exciton Emissions in a Copper‐Based Organic–Inorganic Hybrid Metal Halide

Abstract Metal halide perovskites are promising optoelectronic materials due to their unique luminescent properties. However, their practical application is limited by their poor chemical stability, especially in humid environments. Moisture can cause phase changes or chemical decomposition, resulting in significant fluorescence quenching. In this study, a copper‐based organic–inorganic hybrid metal halide (t‐BA) 3 Cu 6 I 9 is synthesized (t‐BA + is the tert‐butyl‐ammonium ion (C(CH 3 ) 3 NH 3 + )). This material exhibits water‐induced luminescence, and its chemical stability in humid environments is being investigated. Tetragonal t‐BA 3 Cu 6 I 9 is not luminescent, but it reacts quickly with water in the air. The resultant (t‐BA) 2 Cu 2 I 4 ·H 2 O has a broad green emission peak at 520 nm, high photoluminescence quantum yield of 59.4%. Remarkably, (t‐BA) 2 Cu 2 I 4 ·H 2 O is converted back to t‐BA 3 Cu 6 I 9 at temperatures above 40 °C. This phase conversion is highly repeatable, and the luminescent intensity can be fully recovered after 50 transformation cycles. The mechanism of luminescence is investigated through temperature‐dependent photoluminescence spectra and theoretical calculation, which suggests that (t‐BA) 2 Cu 2 I 4 ·H 2 O has a more localized charge distribution and sufficient polyhedral distortion, resulting in a bright and efficient emission from self‐trapped excitons. This is the first report of water‐induced luminescence in copper‐based metal halides, and it paves the way for stable luminescent materials that are responsive to humidity.