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.