Highly efficient red fluorescent OLEDs based on diphenylacridine-naphthothiadiazole derivatives with upper level intersystem crossing

[Display omitted] •Three novel red emitters are obtained with strong charge transfer characters.•Small efficiency roll-offs are realized in both nondoped devices and doped devices.•The red-emitting doped device shows the maximum EQE of 8.7% with high EUE over 70%. Energy ordering of the lowest singlet and triplet excited states dominates the photophysical properties of conjugated molecules, especially emitters utilizing triplet energy through some special pathways such as triplet–triplet annihilation (TTA), thermally activated delayed fluorescence (TADF) and “hot exciton” process. Here, three novel D-A type red light-emitting compounds, DPACNZP, DPACNZPCN and DPACNZPPI, are designed and synthesized, in which the lowest triplet states are all dark states and the high-level triplet states are enabled to realize the reverse intersystem crossing (RISC) process to successfully harvest triplet excitons via “hot exciton” approach. Further, unique molecular architecture of naphthothiadiazole (NZ) group with dual-side substitutions leads to the “wrapped” state of the acceptors, which is favorable for suppressing the intermolecular interactions in solid states. As a result, DPACNZP, DPACNZPCN and DPACNZPPI achieve high PLQYs, excellent thermal stabilities and appropriate energy levels of the highest occupied molecular orbitals (HOMOs) and the lowest unoccupied molecular orbitals (LUMOs). Both nondoped and doped devices using the three compounds as active layers achieve high external quantum efficiencies (EQEs) with reduced efficiency roll-offs. The doped device based on DPACNZPPI achieves the highest EQE of 8.7% with CIE coordinates of (0.604, 0.392) and EQE of 8.3% at 100 cd m−2. The exciton utilization efficiency (EUE) is as high as 74%, which is among the best results of red electroluminescence to our knowledge, and is the best performance for OLEDs based on reported NZ-derivatives.