A strategy for designing highly efficient thermally activated delayed fluorescence (TADF) emitters was reported. TADF emitters with donor–acceptor–donor (D–A–D)-type structures showed highly efficient TADF because of their small singlet–triplet energy gap and large oscillator strength. An organic light-emitting diode containing a D–A–D-type TADF emitter, cis-BOX2, exhibited a high external electroluminescence quantum efficiency of 17.6%.
A spiro compound DPAA-AF combining an electron-accepting diazafluorene unit and an electron-donating bis(diphenylamino)acridane unit has been synthesized and used as an emitter in organic light-emitting diodes. DPAA-AF exhibits efficient thermally activated delayed fluorescence, and a device based on this compound emits blue-greenish electroluminescence with a maximum external quantum efficiency of 9.6%.