Theoretical Insight into Herzberg-Teller Spin-Vibronic Coupling Enhanced Reverse Intersystem Crossing in Hybrid MR-TADF Emitters.

Hybrid multiple-resonance thermally activated delayed fluorescence (MR-TADF) emitters can combine narrowband emission with efficient triplet harvesting, but the factors governing these properties remain unclear. Here, we investigate 12 BO-acceptor MR-TADF emitters using quantum-chemical calculations and the thermal vibration correlation function formalism. Our results reveal distinct origins of color purity and reverse intersystem crossing (RISC) kinetics. High color purity is governed by the collective structural-vibronic response rather than charge-transfer character alone, with para-linkage favoring narrow emission through reduced overall vibronic reorganization. The equilibrium-limit effective RISC descriptor reflects channel-specific rates and higher-triplet thermal accessibility. For emitters with complete channel-resolved data, the fastest intrinsic RISC pathway consistently lies within the higher-triplet T2/T3 manifold, while Herzberg-Teller (HT) coupling selectively enhances specific higher-triplet channels relative to Franck-Condon values.