Decoupling Intersystem Crossing and Radiative Rate in MR-TADF Emitters via High-Lying Triplet Channels

Conventional thermally activated delayed fluorescence (TADF) emitters face an intrinsic trade-off for theranostic applications: enhancing charge transfer to accelerate intersystem crossing (ISC) inevitably diminishes spatial orbital overlap, thereby sacrificing the fluorescence radiative rate required for imaging. Herein, using multiple resonance TADF emitters (CzBNPh and DABNA-2), we theoretically demonstrate that the active participation of high-lying triplet states elegantly circumvents this fundamental dilemma. By employing nonadiabatic transition state theory with the Landau-Zener model, we reveal that the S1 → T2 crossing pathway features vanishingly small activation barriers (e.g., 0.09 kcal/mol), enabling exceptionally fast ISC rates (∼107 s-1). Crucially, because this upper-level mechanism avoids the need for substantial frontier orbital separation, it fully preserves the large orbital overlap required for rapid prompt fluorescence, successfully decoupling the spin-flip efficiency from fluorescence. These foundational insights establish a robust paradigm for theranostic agents: synergizing high-lying triplet channels to concurrently maximize fluorescence imaging and photodynamic efficacy.