Solid-State Fluorescence Enhancement of Bromine-Substituted Trans-Enaminone Derivatives

Halogen bonding, as a kind of intermolecular interaction, has rarely been used to tune solid-state emission properties of luminescent materials, especially fluorescent materials. Herein, three trans-enaminone (TE) derivatives (nonbrominated TE, monobrominated BrTE, and tribrominated Br3TE) with aggregation-induced emission property have been designed and synthesized. Two types of BrTE crystals (BrTE-B and BrTE-G) with different fluorescence properties were obtained. It was observed that their solid-state fluorescence has been enhanced by the formation of halogen bonding. In particular, the crystal BrTE-G containing Br…π interactions exhibits a fluorescence quantum yield (9.6%) nearly sevenfold higher than BrTE-B, the crystal without halogen bonding (1.4%), and fivefold higher than the nonbrominated TE derivative (2.1%). By careful inspection of the single-crystal data and theoretical calculations, the high fluorescence quantum yield of BrTE-G appears to be due to halogen-bonding interactions as well as multiple stronger intermolecular interactions which may restrain molecular motions, leading to the reduced nonradiative decay rate and the enhanced radiative decay rate. Additionally, increasing the number of bromine substituents may further promote the radiative decay rate, explaining therefore the higher fluorescence quantum yield (12.5%) of Br3TE.