Detuning Effects on the Reverse Intersystem Crossing from Triplet Exciton to Lower Polariton

The lower polariton (LP) can reduce the energy barrier of the reverse intersystem crossing (rISC) process from T₁ to harvest triplet energy for fluorescence. Based on a Tavis-Cummings model including both singlet and triplet excitons, both coupled with quantized photons, we derive here a comprehensive rISC rate formalism. We found that the latter consists of three contributions: the one originated from spin-orbit coupling as first obtained by Martinez-Martinez et al. ( J. Chem. Phys. 2019, 151, 054106), the one from light-matter coupling of Ou et al. ( J. Am. Chem. Soc. 2021, 143, 17786), and the cross-term first reported here. We apply the formalism to investigate the experimentally observed barrier-free rISC (BFrISC) process in cavity devices with DABNA-2 molecular thin film. We found it can be attributed to the detuning effect. The rISC rates can be increased by orders of magnitude through changing the detuning energy to realize the BFrISC process. In addition, the BFrISC rates exhibit a maximum as a function of the incident angle and the doping concentration. The formalism provides a solid ground for molecular design toward highly efficient cavity-promoted light-emitting materials.