Isomerization enhanced quantum yield of dibenzo[a,c]phenazine-based thermally activated delayed fluorescence emitters for highly efficient orange OLEDs

Photoluminescence quantum yield (PLQY) and the reverse intersystem crossing (RISC) process are critical for next-generation highly efficient organic light-emitting diodes (OLEDs). However, it is not easy to simultaneously obtain high PLQY and fast RISC, especially in TADF emitters with twisted donor-acceptor structures due to their conflicting requirements for wave function overlapping. Herein, based on a rigid planar dibenzo[a,c]phenazine (DPPZ) unit as an acceptor, four emitters (isomers of DMAC-11-DPPZ and DMAC-10-DPPZ, and isomers of PXZ-11-DPPZ and PXZ-10-DPPZ) with 9,9-dimethylacridine (DMAC) and 10H-phenoxazine (PXZ) as donors substituted via 11- and 10-positions were designed and synthesized to balance the wave function overlapping and explore the relationship between their structures and luminescence properties. Photophysical characterization and theoretical calculations suggest small energy gaps (ΔE_ST) featuring fast RISC rates in all these compounds. Meanwhile, DMAC-11-DPPZ and PXZ-11-DPPZ achieve higher PLQYs due to the largely suppressed non-radiative transition as revealed by the decreased Huang-Rhys factors. As a result, high external quantum efficiencies (EQE_max) of 23.8% for DMAC-11-DPPZ and 13.7% for PXZ-11-DPPZ are obtained. This work provides a promising strategy to improve the PLQY of TADF materials by adjusting the substituted site, and also shows the potential of phenazine derivatives for OLED applications.