Double Emission and Reversible Thermochromic Luminescence Based Sn ²⁺ :(NH ₄ ) ₄ CdCl ₆ Single Crystal for Anti-Counterfeiting

Multimodal optical responses metal halide materials are valuable in advanced anticounterfeiting field. However, most of them depend on the ions codoped, rare-earth ions, or fluorescent organic groups to achieved, which inevitably reduce their cost-effectiveness and stability, thereby hindering their practical applications. To overcome these challenges, we proposed a new strategy to achieve multimodal optical responses. By Sn²⁺ doping into the (NH₄)₄CdCl₆ crystal constructed two distinct emission centers─Self-Trapped Excitons (STE) and Defect-Bound Excitons (DBE). Taking advantage of the different optical response behaviors of STE and DBE, we designed a metal halide single crystal with double emission and reversible thermochromic luminescence. In addition, benefiting from the unique ns² electronic configuration of Sn²⁺, the material exhibits a high photoluminescence quantum yield (PLQY) of 41.23% and structural stability. Even after being stored under ambient conditions for more than six months, it still retain structural integrity. With its high PLQY and multimodal optical responsiveness, we further fabricated light-emitting diode (LED) and PDMS film devices to demonstrate its promising performance in lighting and optical anticounterfeiting applications. This study not only expands the application prospects of metal halide materials in optical anticounterfeiting but also provides a new design concept for developing multimodal optical response materials.