Aggregation-Induced TADF Mechanism of Ortho-Phenylene-Linked Donor–Acceptor BF2 Compounds Elucidated via Multiscale Simulations

Abstract Aggregation-induced emission thermally activated delayed fluorescence (AIE-TADF) materials have received widespread attention due to having both high exciton utilization efficiency and excellent solid-state luminescence performance. Nevertheless, the underlying effects of aggregate packing configurations on molecular luminescence behaviors and corresponding luminescence mechanisms remain insufficiently elucidated. In this work, the quantum mechanics/molecular mechanics (QM/MM) method, thermal vibration correlation function (TVCF) theory, and Marcus energy transfer model are used to theoretically investigate the luminescence properties of these two boron heterocycle-based TADF molecules (M1 and M2) in toluene, crystal, and doped film. Calculations show that both molecules have a small singlet–triplet energy gap (ΔEST) in different phase states, capable of realizing effective reverse intersystem crossing (RISC). Phase state changes can regulate orbital transition components and excited-state transition modes and effectively restrict geometric structure relaxation, thereby realizing an increase in radiative rate and a decrease in nonradiative rate. M1 has a faster RISC rate and higher luminescence efficiency in the aggregated state. In addition, stronger electronic coupling makes M1 have a higher singlet exciton energy transfer rate, while relatively balanced reorganization energy and Gibbs free energy make M2 exhibit a higher triplet exciton energy transfer rate. This study reveals the key impact of different phase state changes on the highly efficient luminescence of AIE-TADF, providing a theoretical basis for the design of OLED materials with excellent solid-state luminescence performance.