An unsymmetrical thermally activated delayed fluorescence emitter enables orange-red electroluminescence with 31.7% external quantum efficiency

The thermally activated delayed fluorescence (TADF) emitters based on donor–acceptor (D–A) configuration were continuously developed in the past few years, whereas an unsymmetrical TADF emitter with A–D–A′ configuration has never been reported. Herein, an A–D–A′ type TADF emitter of TRZ–SBA–NAI was firstly developed by simultaneously integrating 2-phenyl-1H-benzo[de]isoquinoline-1,3(2H)-dione and 2,4,6-triphenyl-1,3,5-triazine acceptors into a spirobiacridine donor core. Due to the coexistence of double charge-transfer excited states, TRZ–SBA–NAI displayed dual emission containing a dominant orange-red emission and an anti-Kasha's rule sky-blue emission shoulder in solution. As doped into the host matrix, TRZ–SBA–NAI only exhibited an orange-red emission, together with a high photoluminescence quantum yield of 87%. The linear molecular shape imparted TRZ–SBA–NAI with a high horizontal dipole ratio of 88%. As a result, the TRZ–SBA–NAI based devices achieved a record-high external quantum efficiency of 31.7% with an electroluminescence peak at 593 nm. This finding not only enriches the diversity in TADF molecular design, but also unlocks the huge potential of A–D–A′ type TADF emitters for excellent device performance.