Enhancing reverse intersystem crossing through molecular design: insights from strong light–matter coupling in an optical microcavity

Abstract

To mitigate the efficiency roll-off in organic light-emitting diodes (OLEDs), an effective strategy is to accelerate the reverse intersystem crossing (RISC) process of triplet excitons. Strong light–matter coupling (LMC) within an optical microcavity enables the formation of upper (UP) and lower polariton (LP) states through the hybridization of singlet excitons and photons. The participation of LP can reduce the energy barrier between the lowest triplet excited state (T₁) and the emissive singlet state, thereby promoting the RISC process. In this work, we systematically investigated the RISC process of a purely N-embedded multi-resonance thermally activated delayed fluorescence (MR-TADF) molecule, BisICz, and its thiophene derivatives under both isolated and LMC conditions. Three key electronic descriptors—the singlet–triplet energy gap, transition dipole moment, and reorganization energy—were employed to characterize the RISC process. Our results demonstrate that the three electronic parameters can be synergistically optimized within the molecular orbital framework. Guided by this understanding, we further designed thiophene- and phenyl-substituted BisICz derivatives, which exhibit a markedly increased RISC rate (from 10² s⁻¹ in the isolated condition to 10⁶ s⁻¹ in the LMC condition), offering a practical route to mitigate the efficiency roll-off in OLEDs.