Excited State Properties of Aggregation-Induced Delayed Fluorescence Molecules: A Microscopic Insight

Purely organic fluorescence emitters with ultrahigh exciton utilization, such as thermally-activated delayed fluorescence (TADF) materials, are pursued as the cornerstone of the new-generation organic light-emitting diodes (OLEDs). However, most TADF emitters suffer from the dilemma of the aggregation-caused fluorescence quenching effects, severely limiting their applications. Excitingly, the recently proposed aggregation-induced delayed fluorescence (AIDF) strategy holds the potential to tackle this problem thoroughly. Here, the recently reported AIDF emitters CP-BP-PXZ and its halogenated counterparts, 3-CCP-BP-PXZ and 3-BCP-BP-PXZ, are investigated with a united theoretical and experimental insight. Based on first-principles calculations combined with the polarizable continuum model (PCM) and the quantum mechanics/molecular mechanics (QM/MM) model, the photophysical mechanisms of these novel materials are proposed. It is found that both reverse intersystem crossing (RISC) and the subsequent radiative transition of CP-BP-PXZ are promoted via aggregation, owing to the increased effective RISC channels and boosted oscillator strength. Meanwhile, the unfavorable internal conversion rate is greatly slumped, ascribed to the suppressed Duschinsky rotation effect (DRE). The unique heavy-atom effect is disclosed in 3-CCP-BP-PXZ and 3-BCP-BP-PXZ that the halogens act as steric blockers in crystal, rather than spin-orbit coupling enhancers. It is hoped that this work can shed new light on the designation of high-efficient solid-state fluorescence emitters.