Tunable Phosphorescence in Metastable CuI Assemblies Prepared by Acid‐Fueled Crystal‐to‐Crystal Conversion

Abstract Acid acts as a “fuel” in driving kinetic self‐assembly, with crystals of the Cu II complex ([Cu II Cl 2 (BH) 2 ], BH=benzothiadiazole helicene derivative) being successfully converted to crystals of 1 ([H 2 BH][CuBr 3 ]) and 2 ([H 2 BH][CuBr 3 ⋅2H 2 O]). The metastable features of 1 and 2 enable Cu I guest release, with transformation into a thermodynamically favorable form 3 (H 2 BH⋅2Br). Compound 1 exhibits dual phosphorescence at 452 nm ( 3 MC) and 709 nm ( 3 CT) at room temperature. At lower temperatures, the 3 CT band disappears and the 3 MC band weakens, while a new emission band at 621 nm ( 3 ππ*) is enhanced. In 1 , the subtle structural change and significant emission change depending on temperature is reversible. In contrast, 2 exhibits only 3 MC emission at room temperature, with less efficient triplet‐triplet energy transfer (TTET) below 250 K, and with residual 3 ππ* emission occurring upon returning to room temperature, even after one week.