Fused Donor–Acceptor TADF Emitter Bearing Ancillary Donor: Toward High‐Performance OLED With Fast Reverse Intersystem Crossing

The fast reverse intersystem crossing (RISC) is critical to ensuring efficient thermally activated delayed fluorescence (TADF). Herein, a D–A fused core (TCXO) and a D|A–D type emitter (TCXO‐BPA) with a covalently attached auxiliary donor are reported. The TCXO core exhibits high photoluminescence quantum yield (PLQY) and enables multiple RISC channels from high‐lying triplet states (T n , n ≥ 1) to S 1 with enhanced spin–orbit coupling (SOC). The green organic light‐emitting diode (OLED) based on TCXO achieves a maximum external quantum efficiency (EQE) of 13.4%. Attaching auxiliary donor induces red‐shifted emission and allows precise modulation of RISC kinetics. TCXO‐BPA, featuring a small twist angle, achieves a smaller Δ E ST and a faster RISC rate as high as 1.73 × 10 7 s −1 . The resulting yellow OLED achieves a maximum EQE of 25.3% and a remarkably retained EQE of 14.3% at 1000 cd m −2 . This work demonstrates that D–A fused architecture combined with conformational engineering of auxiliary donor provides an effective strategy for high‐performance TADF‐based OLEDs.