Extended theoretical modeling of reverse intersystem crossing for thermally activated delayed fluorescence materials

Thermally activated delayed fluorescence (TADF) materials and multi-resonant (MR) variants are promising organic emitters that can achieve an internal electroluminescence quantum efficiency of ~100%. The reverse intersystem crossing (RISC) is key for harnessing triplet energies for fluorescence. Theoretical modeling is thus crucial to estimate its rate constant (k_RISC) for material development. Here, we present a comprehensive assessment of the theory for simulating the RISC of MR-TADF molecules within a perturbative excited-state dynamics framework. Our extended rate formula reveals the importance of the concerted effects of nonadiabatic spin-vibronic coupling and vibrationally induced spin-orbital couplings in reliably determining k_RISC of MR-TADF molecules. The excited singlet-triplet energy gap is another factor influencing k_RISC. We present a scheme for gap estimation using experimental Arrhenius plots of k_RISC. Erroneous behavior caused by approximations in Marcus theory is elucidated by testing 121 MR-TADF molecules.