Halogen Engineering and Solvent-Induced Strategies Regulate Structural Transitions and Luminescence Switching of Hybrid Copper(I) Halides

Copper-based halides have attracted significant attention due to their unique photophysical properties and diverse coordination configurations. However, enhancing water stability and modulating structural transitions in cuprous halide materials remain challenging. In this work, we successfully synthesized three copper(I) halides, (C24H28P)CuBr2 (L1, [C24H28P]+ = hexyltriphenylphosphonium), (C24H28P)2Cu4Br6 (L2), and (C24H28P)2Cu4I6 (L3), via solvent volatilization, demonstrating exceptional water stability even after 27 days of submersion. Crystals L1 and L2 emit yellow and orange light, respectively, under ultraviolet excitation, while L3 emits green light when excited by blue light. A rapid phase transition between L1 and L2 occurs when stimulated with methanol at room temperature. Further exposure of L2 to a NaI solution can result in substitution of Br– ions by I– in [Cu4Br6]2– to form L3. Multimodal anticounterfeiting luminescent labels with high-security levels have been successfully manufactured based on sequential L1 → L2 → L3 phase transition. Moreover, a test paper designed based on L1 can be applied for low-level methanol detection. This work successfully accomplishes the coregulation of structural transformations and luminescence switching in copper(I) halides by combining methanol simulations with halogen substitution strategies, which not only deepens the understanding of the structure–property relationships but also broadens the optical applications of these materials.