Following the great success of phosphorescent heavy-metal complexes as efficient emissive materials for high-efficiency organic light-emitting diodes(OLEDs),thermally activated delayed fluorescence(TADF)emitters have recently bloomed as a viable alternative to achieve 100%internal quantum efficiency without involving noble metals.[1] Although promising results have been made on TADF emitters with external quantum efficiencies(EQEs)comparable with phosphorescent OLEDs,further improvement in device efficiency and stability is still in demand,and insight into the correlation between molecular structures and optoelectronic properties for TADF emitters has to be deepened.