Revealing structure–property relationships and charge transfer dynamics in host–guest phosphorescent organic light-emitting diodes

Host materials are widely employed in organic light-emitting diodes (OLEDs) to achieve a high external quantum efficiency, initially presumed to function solely through molecular motion restriction. Recent experiments suggest that the host matrix may also facilitate energy transfer processes, yet theoretical understanding remains limited. Here, we employ non-adiabatic molecular dynamics to investigate excited state dynamics in a host-guest system comprising 2,7-dibromophenanthrene-9,10-dione (27PNDO) as the emitter and 6,11-dibromodibenzo-[f,h]quinoxaline (27QNX) as the host material. Our simulations reveal that 27QNX enables phosphorescence of 27PNDO at room temperature, a phenomenon that is absent in 27PNDO films due to inefficient singlet-to-triplet conversion. The binary system establishes two phosphorescence pathways: direct intersystem crossing in 27PNDO and energy transfer from the S1 state of 27QNX to a higher-lying triplet state of 27PNDO, followed by relaxation to T1. Molecular flexibility strongly influences exciton dynamics, with excessive conformational freedom in the 27PNDO dimer inhibiting intersystem crossing. Furthermore, molecular packing geometry proves crucial: antiparallel configuration facilitates S1 → T2 → T1 conversion, while parallel configuration induces molecular distortions that impede triplet state population. These findings emphasize the importance of both emitter selection and host material design in the development of efficient phosphorescence materials.