Yellow thermally activated delayed fluorescence emitters based on a pyridyl-acridone acceptor for organic light-emitting diodes

Abstract

The frontier molecular orbitals (FMOs) of thermally activated delayed fluorescence (TADF) emitters should be sufficiently separated for effective reverse intersystem crossing, but should also have significant overlap to guarantee efficient radiative transition. Two yellow TADF emitters, PXZ-PAO and DPCz-PAO, were developed with a pyridyl-substituted acridone as the acceptor and phenoxazine (PXZ) or diphenylaminocarbazole (DPCz) as the donor. Aiming at enhancing the radiation rate constant k_r and photoluminescence quantum yields (PLQYs), two key design strategies were focused on donors. First, by replacing the six-membered-ring PXZ donor with the five-membered-ring carbazole, the donor–acceptor dihedral angle was decreased from approximately 80° to 50°, leading to an increased FMO overlap. Second, by incorporating two diphenylamino groups at the 3,6-sites of carbazole to enhance donor strength (i.e. DPCz), the HOMO delocalization was widely extended, thereby synergistically enhancing the transition dipole moment, and the FMOs were effectively separated even at such a small dihedral angle of 50°. As a result, DPCz-PAO not only realized a quite small ΔE_ST of 0.03 eV, but also exhibited a threefold increase in k_r relative to PXZ-PAO and a remarkable PLQY of 92.1%. The doped OLEDs of PXZ-PAO and DPCz-PAO exhibited brilliant yellow emission with external quantum efficiencies (EQEs) of 14.4% and 18.5%, respectively, even without light-extraction or a sensitizer.