Excitation‐Wavelength‐Induced Dual‐Band Fluorescence of Copper Halides for Multi‐Mode Encryption and Anti‐Counterfeiting Applications

Abstract Low‐dimensional metal halides have emerged as promising anti‐counterfeiting materials. However, achieving a multi‐mode and multi‐color anti‐counterfeiting system in metal halides remains challenging. In this study, copper‐halide (TBP)₂Cu₄Br₆ (TBP⁺ = C₁₆H₃₆P⁺) single crystals are synthesized using a cooling crystallization method, which exhibits efficient dual‐band emissions (542 and 708 nm), large Stokes shifts (282 and 330 nm), and a high photoluminescence quantum yield (PLQY) of 92.7% for 542 nm. These exceptional properties are attributed to the unique 0D structure of (TBP)₂Cu₄Br₆ single crystals, which facilitates the formation of two different self‐trapped excitons (STEs). Furthermore, based on (TBP)₂Cu₄Br₆, a multi‐mode and multi‐color digital anti‐counterfeiting system integrated is designed with Morse code information encryption, demonstrating promising applications in information security and anti‐counterfeiting. This work not only illustrates an emitter in copper halides but also paves the way for achieving multi‐mode and multi‐color anti‐counterfeiting systems.