Regulation of Self‐Trapped Excitons in an Organic–Inorganic Hybrid Cu(I) Halide with Mixed Halogens for Use in Advanced Anticounterfeiting

Abstract Low‐dimensional Cu‐based halides exhibit considerable potential as luminescent materials owing to their efficient exciton emissions originating from strongly localized charge carriers. However, the luminescence processes of multiple self‐trapped excitons (STEs) in Cu‐based halides with mixed halogens have not been systematically studied. In this study, a novel single crystal, MACuBrI (MA = methylammonium), with mixed anions and a 1D chain structure is successfully synthesized. MACuBrI displays two different STE luminescences at room and low temperatures, which forms different STE structures through breaking CuBr or CuI bonds. Therefore, MACuBrI is adopted as an ink in an anticounterfeiting strategy to realize multiple encryption effects, which may clearly identify and authenticate the substrate under ultraviolet light irradiation and even after low‐temperature treatment. This discovery may deepen the understanding of the STE emission mechanism and provide a novel pathway for the development of Pb‐free metal halides with remarkable luminescence performances.