Enhanced Reverse Intersystem Crossing Promoted by Triplet Exciton-Photon Coupling.

Polaritons are hybrid light-matter states formed via strong coupling between excitons and photons inside a microcavity, leading to upper and lower polariton (LP) bands splitting from the exciton. The LP has been applied to reduce the energy barrier of the reverse intersystem crossing (rISC) process from T, harvesting triplet energy for fluorescence through thermally activated delayed fluorescence. The spin-orbit coupling between T and the excitonic part of the LP was considered as the origin for such an rISC transition. Here we propose a mechanism, namely, rISC promoted by the light-matter coupling (LMC) between T and the photonic part of LP, which is originated from the ISC-induced transition dipole moment of T. This mechanism was excluded in previous studies. Our calculations demonstrate that the experimentally observed enhancement to the rISC process of the erythrosine B molecule can be effectively promoted by the LMC between T and a photon. The proposed mechanism would substantially broaden the scope of the molecular design toward highly efficient cavity-promoted light-emitting materials and immediately benefit the illumination of related experimental phenomena.