Enhanced Solid-State Triplet-Triplet Annihilation Upconversion Steered by AIE-Active Isomers

A red-to-blue solid-state triplet-triplet annihilation upconversion (TTA-UC) molecular crystal with a significantly improved upconverted photoluminescence intensity was first achieved via a controlled crystallization pathway. Cyano-substituted stilbene derivatives and transition metal complexes were coupled for TTA-UC systems. The photophysical properties of the two annihilators and their TTA-UC systems in solution and aggregate were comprehensively studied. Particularly, UC crystals were simply prepared under different crystallization conditions resulting in different morphological and structural features. It turned out that the UC crystal prepared in the surfactant-assisted crystallization method demonstrated a 100-fold higher UC intensity than that in the evaporation crystallization method. The morphological and structural study indicated small nanograins with intact crystalline lattice would facilitate the triplet energy migration leading to a boosted UC efficiency. This work provides a novel perspective for the facile construction of high-efficient solid-state TTA-UC systems by utilizing crystals with appropriate morphology, which significantly promotes the practical applications of TTA-UC.