Singlet-triplet splitting energy management for thermally activated delayed fluorescence emitters: up-conversion of acceptor triplet into charge transfer singlet state

Energetic proximity between charge transfer singlet ((CT)-C-1) and a local exciton triplet ((LE)-L-3) is critical to facilitate reverse intersystem crossing rate and thus ultimately TADF. In this study, a new TADF emitter, namely 2PQ-OCz, was successfully designed by systematically regulating the HOMO level of donors. Using 2PQ-Cz as a template molecule, shallower HOMO levels of the tCz and OCz moieties are utilized to replace the Cz unit, their S-1 energies of 2PQ-based emitters are significantly reduced, and thus, the smaller energy splits (Delta E-ST) of 0.36 eV between (CT)-C-1 and (LE)-L-3 was achieved. Consequently, 2PQ-OCz exhibits a high photoluminescence quantum yield (PLQY) of 40.8% and obvious TADF character in air, achieving an efficient rISC giving rise to TADF through a SOC mechanism. As expected, the 2PQ-OCz-based OLED device exhibits excellent OLED performances with an EQE of 9.3% and a maximum brightness of up to 14 481 cd m(-)(2).