Restriction of Twisted Intramolecular Charge Transfer as a Mechanism for the Aggregation-Induced Emission of Cationic Fluorophores

Although cationic units show great promise as building blocks for functional aggregation-induced emission (AIE) materials, their underlying working mechanisms have not been sufficiently elucidated. Here, we present a comprehensive theoretical investigation into the AIE behavior of cationic fluorophores, employing density functional theory (DFT) and time-dependent DFT (TD-DFT). Our findings reveal that methylation of cationic fluorophores substantially lowers their lowest unoccupied molecular orbital (LUMO) levels and narrows the energy gaps. The enhanced electron-withdrawing nature of the cationic unit facilitates an extended intramolecular charge transfer in the excited state, indicative of a pronounced push-pull electronic effect. Notably, we identified a nonemissive twisted intramolecular charge transfer (TICT) state in solution, which accounts for fluorescence quenching. In contrast, enhanced intermolecular interactions in the aggregated state restrict intramolecular rotations, suppressing the TICT pathway and restoring the fluorescence. These insights elucidate the mechanistic basis of cationic AIE systems and offer strategic guidance for designing desired AIE-active materials.