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

ABSTRACT Hot exciton (HE) materials possessing a locally excited (LE) S 1 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 EUE max (≤ 50%) and consequently low external quantum efficiency (EQE max : 2%). Herein, we introduce a “multi‐functional subunit” triad strategy to circumvent this limitation by engineering the luminescent core to maintain the LE– S 1 state while simultaneously participating in the formation of multiple near‐degenerate T n states that exhibit substantial spin–orbit coupling with the S 1 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– S 1 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 EUE max and a record EQE max 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– S 1 HE emitters.