Forcing dimethylacridine crooking to improve the efficiency of orange-red thermally activated delayed fluorescent emitters

Exploiting effective orange-red thermally activated delayed fluorescent (TADF) emitters is still challenging. In this work, we proposed a method by using a crooked donor (D) moiety instead of its conventional planar form to balance the opposite influences of frontier molecular orbital overlap on the reverse intersystem crossing and the singlet radiative decay rate. A novel orange-red TADF emitter, named 2-(4-(9,9-dimethyl-4-phenylacridin-10(9H)-yl)phenyl)anthracene-9,10-dione (AQ-PhDMAC), was accordingly designed and synthesized. Owing to the steric effect of the α-phenyl ring in 9,9-dimethyl-4-phenyl-9,10-dihydroacridine (α-PhDMAC), a relatively small singlet-triplet energy splitting of 0.22 eV and a high fluorescence quantum yield of 0.89 can be realized simultaneously. The orange-red organic light-emitting diode based on AQ-PhDMAC exhibited a maximum external quantum efficiency of 18.1%, much improved compared with the 13.7% efficiency of the α-phenyl ring-free control emitter (AQ-DMAC). These results propose a new viewpoint that enforcing crooked D segments by large steric hindrance actually has a great potential for applications in orange-red TADF molecular design, to improve their electroluminescence efficiency.