Engineering the singlet-triplet energy splitting in a TADF molecule

The key to engineering an efficient TADF emitter is to achieve a small energy splitting between a pair of molecular singlet and triplet states. This work makes important contributions towards achieving this goal. By studying the new TADF emitter 2,7-bis(phenoxazin-10-yl)-9,9-dimethylthioxanthene-S, S-dioxide (DPO-TXO2) and the donor and acceptor units separately, the available radiative and non-radiative pathways of DPO-TXO2 have been identified. The energy splitting between singlet and triplet states was clearly identified in four different environments, in solutions and solid state. The results show that DPO-TXO2 is a promising TADF emitter, having Delta E-ST = 0.01 eV in zeonex matrix. We further show how the environment plays a key role in the fine tuning of the energy levels of the (CT)-C-1 state with respect to the donor (LED)-L-3 triplet state, which can then be used to control the Delta E-ST energy value. We elucidate the TADF mechanism dynamics when the (CT)-C-1 state is located below the (LE)-L-3 triplet state which it spin orbit couples to, and we also discuss the OLED device performance with this new emitter, which shows maximum external quantum efficiency (E.Q.E.) of 13.5% at 166 cd m(-2).