Extended theoretical modeling of reverse intersystem crossing for thermally activated delayed fluorescence materials
Masaya Hagai, Naoto Inai, Takuma Yasuda, Kazuhiro J. Fujimoto, Takeshi Yanai
Abstract
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. Our extended modeling offers in-depth descriptions of k RISC .