Strategy for Designing Electron Donors for Thermally Activated Delayed Fluorescence Emitters

Thermally activated delayed fluorescence (TADF) emitters are promising dopants for organic light-emitting diodes, including those containing highly twisted donor-acceptor-type structures. However, highly twisted structures limit the variety of chemical structures applicable as TADF emitters. We present a strategy for designing electron donors that can eliminate this requirement and increase the structural diversity of TADF emitters. Using this strategy, we developed an electron donor containing carbazolyl and diphenylamino groups by carefully controlling its electron-donating ability. By combining this donor with a quinoxaline-based acceptor, we obtained the efficient green TADF emitter, N-3,N-3,N-6,N-6-tetraphenyl-9-(4-(quinoxalin-6-yl)phenyl)-9H-carbazole-3,6-diamine (DACQ), without a highly twisted structure. DACQ exhibits high photoluminescence and electroluminescence efficiencies, comparable to those of a highly twisted TADF emitter containing the same electron-accepting unit. Quantum chemical calculations showed that the diphenylamino groups within the carbazolyl moiety effectively withdraw the HOMO distribution. This reduces the singlet-triplet energy gap, thus inducing TADF. The photophysical properties of TADF compounds depend on the twisting angle between the electron-donating and accepting units. Eliminating the highly twisted structure increases the diversity of potential TADF emitters and allows their photophysical properties to be controlled by changing the twisting angle.