Development of highly efficient long wavelength thermally activated delayed fluorescent organic light-emitting diodes using an auxiliary acceptor as reverse intersystem crossing promoter

To achieve high efficiency for long wavelength thermally activated delayed fluorescent emitter is difficult because small singlet emission energy and high fluorescence radiative rate are contradictory to each other, as per energy gap law. We designed and synthesized two yellow-red TADF emitters namely, Ac-DCQX-1 and Ac-DCQX-2 based on a molecular design concept of inserting auxiliary acceptor to the main acceptor unit. Auxiliary acceptor was introduced at the ortho position of the acridine donor for the intensified charge transfer character from donor to acceptor, increased HOMO-LUMO overlap and reduced singlet-triplet energy gap. Highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of Ac-DCQX-1 and Ac-DCQX-2 were -5.87/-3.49 and -5.89/-3.87 eV respectively and the singlet-triplet energy gap (AEST) calculated were 0.08/0.05 eV. As a result, high EQE values of 27.6 and 23.8 % with color coordinate of (0.42, 0.55) and (0.55, 0.44) were obtained for Ac-DCQX-1 and Ac-DCQX-2 respectively.