Boosting Triplet Exciton Harvesting via Multi-Channel High-Lying Reverse Intersystem Crossing in a Hot Exciton Material Featuring Locally Excited-State Emission.

Hot exciton (HE) materials possessing a locally excited (LE) S1 state are ideal for achieving narrow emission and high exciton utilization efficiency (EUE) in organic light-emitting diodes (OLEDs). However, constrained by a single high-lying reverse intersystem crossing (hRISC) channel, the currently established donor-bridge-acceptor (D-B-A) design suffers from low EUEmax (≤ 50%) and consequently low external quantum efficiency (EQEmax: 2%). Herein, we introduce a "multi-functional subunit" triad strategy to circumvent this limitation by engineering the luminescent core to maintain the LE-S1 state while simultaneously participating in the formation of multiple near-degenerate Tn states that exhibit substantial spin-orbit coupling with the S1 state, thus activating multiple efficient hRISC pathways. This concept is validated using a newly designed molecule, P-Cz-SO, which exhibits a well-defined LE-S1 state with narrow blue emission. Transient spectroscopy reveals two distinct delayed fluorescence (DF) components, providing the first direct experimental evidence for multi-channel hRISC processes. The resulting OLED demonstrates near-unity EUEmax and a record EQEmax of 15.5% among deep-blue HE-OLEDs (CIEy ≤ 0.1). Comparative studies with a reference compound, P-Cz-Ph, confirm the critical role of the multi-channel hRISC design. This work provides a general paradigm for achieving highly efficient LE-S1 HE emitters.