In this work, we report new metastable Cu^I assemblies prepared by crystal-to-crystal conversion that exhibit temperature-dependent phosphorescence complexity.

Abstract

Acid acts as a “fuel” in driving kinetic self-assembly, with crystals of the Cu^II complex ([Cu^IICl₂(BH)₂], BH=benzothiadiazole helicene derivative) being successfully converted to crystals of 1 ([H₂BH][CuBr₃]) and 2 ([H₂BH][CuBr₃⋅2H₂O]). The metastable features of 1 and 2 enable Cu^I guest release, with transformation into a thermodynamically favorable form 3 (H₂BH⋅2Br). Compound 1 exhibits dual phosphorescence at 452 nm (³MC) and 709 nm (³CT) at room temperature. At lower temperatures, the ³CT band disappears and the ³MC band weakens, while a new emission band at 621 nm (³ππ*) is enhanced. In 1, the subtle structural change and significant emission change depending on temperature is reversible. In contrast, 2 exhibits only ³MC emission at room temperature, with less efficient triplet-triplet energy transfer (TTET) below 250 K, and with residual ³ππ* emission occurring upon returning to room temperature, even after one week.