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 (¹CT) and a local exciton triplet (³LE) 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 S₁ energies of 2PQ-based emitters are significantly reduced, and thus, the smaller energy splits (ΔE_ST) of 0.36 eV between ¹CT and ³LE 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%