Ternary donor-acceptor phosphine oxide hosts with peculiar high energy gap for efficient blue electroluminescence

Ternary donor (D)-acceptor (A)-acceptor (A) molecules are commonly considered as low triplet (T₁) energy systems for specific applications. In this work, exception to this behavior was observed in a triangle-shaped D-A-A molecule PCImbPO with unusually high triplet energy of 3.0 eV. Profiting from the enhanced D-A electronic coupling, electron injecting and transporting ability of PCImbPO was dramatically improved with negligible influences on its highest occupied molecular orbital (HOMO) characteristics. Its particular T₁ configuration adjustment further gives rise to the separated frontier MO and T₁ locations, beneficial to suppress quenching effects. By utilizing PCImbPO as host in blue phosphorescent organic light-emitting diodes (PHOLEDs) and thermally activated delayed fluorescence devices, impressively high external quantum efficiency of 22% and 12% were achieved, respectively. This work established a new understanding of high-energy-gap complicated D-A systems.