Pyridine-substituted triazine as an acceptor for thermally activated delayed fluorescence emitters showing high efficiency and low roll-off in organic light-emitting diodes

In thermally activated delayed fluorescence (TADF) molecular design, triazine (TRZ) is of the most widely used acceptor core with three phenyl rings protected. In this work, by substituting the peripheral phenyl group with electrophilic pyridine rings, a new acceptor 2-phenyl-4,6-di(pyridin-3-yl)-1,3,5-triazine (PyTRZ) was designed and two novel TADF emitters 10-(4-(4,6-di(pyridin-3-yl)-1,3,5-triazin-2-yl) phenyl)-10H-phenoxazine (PXZ-PyTRZ) and 10-(4'-(4,6-di(pyridin-3-yl)-1,3,5-triazin-2-yl)-[1,1'-biphenyl]-4-yl)-10H-phenoxazine (PXZ-Ph-PyTRZ) were accordingly exploited. Both compounds exhibit obvious TADF emitters, while their kinetic processes related to exciton radiation exhibit some obvious differences and deeply affect their overall performances in organic light-emitting diodes (OLEDs). The optimized device by using PXZ-Ph-PyTRZ as the emitter achieves a superior maximum external quantum efficiency (EQE) of 22.2% than that of PXZ-PyTRZ (18.5%), whereas their device efficiency roll-offs show different tendencies with increasing current densities. Particularly, owing to a shorter decay lifetime of 1.9 ms, the PXZ-PyTRZ-based device achieves an impressive EQE of 13.1% at an ultra-high luminance of 10,000 cd/m(2), which is comparable with state-of-the-art high-performance OLEDs. These results demonstrate a bright prospect of the PyTRZ moiety in exploiting TADF emitters; more importantly, we provide a new method of deriving TRZ cores for TADF molecular design. (C) 2020 Elsevier Ltd. All rights reserved.