Controlling the intersystem crossing/reverse intersystem crossing (ISC/RISC) competition to achieve efficient red metal-free phosphorescent molecules

Abstract

In this study, we have systematically modulated the intersystem crossing/reverse intersystem crossing (ISC/RISC) competition in organic molecules (PhODCB, PhSDCB and PhSeDCB) with a D–A–D configuration to achieve emission from fluorescence to thermally activated delayed fluorescence (TADF) and then to room-temperature phosphorescence (RTP) by incorporating the chalcogen atoms oxygen (O), sulfur (S) and selenium (Se), respectively. Their distinct photophysical behaviors can be ascribed to both the enhanced heavy-atom effect and n → π* transition from O to Se atoms, which can enhance the quantum yield and effectively promote radiative decay of the triplet excited states to the ground state. Notably, with o-carborane as a strong electron acceptor, PhSeDCB can represent an unprecedented red-emitting RTP molecule with a very impressive photoluminescence quantum yield (PLQY) of 0.48 and a short lifetime of 14.7 µs. In addition, the first red organic light-emitting diodes (OLEDs) prepared with PhSeDCB show a high electroluminescence efficiency of 18.9%. All these encouraging results have indicated the great potential of metal-free phosphorescent materials in the field of OLEDs.