Achieving Efficient Thermally Activated Delayed Fluorescence through Tuning ³LE Energy Levels of Difluoroboron Acceptors by the Isomeric Strategy

The development of thermally activated delayed fluorescence (TADF) materials with highly efficient reverse intersystem crossing (RISC) processes for organic light-emitting diodes (OLEDs) has received significant attention recently. In this study, we reported two D–A-type TADF emitters, DMAC-Pz1BF2 and DMAC-Pz2BF2, based on isomeric difluoroboron acceptors with similar LUMO energy levels and distinct different locally excited triplet (3LE) energy levels. While both emitters had the same energy splitting between their lowest charge-transfer singlet (S1, 1CT) and triplet (T1, 3CT) states (ΔEST = 0.03 eV), DMAC-Pz2BF2 possesses a much smaller S1 (1CT)–T2 (3LE) gap (ΔE(1CT–3LE) = 0.11 eV) compared to that of DMAC-Pz1BF2 (0.38 eV). Consequently, DMAC-Pz2BF2 showed more efficient triplet-to-singlet crossover, along with a larger photoluminescence quantum yield and a higher rate constant of reverse intersystem crossing (kRISC) compared to DMAC-Pz1BF2. As expected, the solution-processed OLED device of DMAC-Pz2BF2 exhibited better EL performance with a maximum external quantum efficiency of up to 12.9% and a maximum brightness reaching 21,452 cd/m2.