Strategy for Modulating Dihedral Angle Distribution for Enhanced Anti-Kasha TADF Behavior

D-A type thermally activated delayed fluorescence (TADF) materials with aggregation-induced emission (AIE) have attracted extensive research interest for their intense emission and high quantum yield in aggregated state. Herein, TADF molecules with vertical twisted structures are achieved by the methylation strategy which effectively separated the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). With the increase of the dihedral angle, 18 times enhancement of the photoluminescence quantum yield (PLQY) is realized in the aggregated state. The density functional theory (DFT) calculations demonstrate that these TADF molecules exhibit excellent photophysical properties in violation of Kasha's rule: fluorescence originates from the higher energy electronic excited state (S2) rather than the S1 state.