The short material lifetime of thermally activated delayed fluorescence (TADF) technology is a major obstacle to the development of economically feasible, highly efficient, and durable devices for commercial applications. TADF devices are also hampered by insufficient operational stability. In this paper, we report the design, synthesis, and evaluation of new TADF molecules possessing a sterically twisted skeleton by interlocking donor and acceptor moieties through a C-C bond. Compared to C-N-bond TADF molecules, such as CPT2, the C-C-bond TADF molecules showed a large dihedral angle increase by more than 30 times and a singlet-triplet energy-gap decrease to less than 0.22 eV because of the steric hindrance caused by the direct C-C bond connection. With the introduction of a dibenzofuran core structure, devices comprising BMK-T317 and BMK-T318 exhibited a magnificent display performance, especially their external quantum efficiencies, which were as high as 19.9% and 18.8%, respectively. Moreover, the efficiency roll-off of BMK-T318 improved significantly (26.7%). These results indicate that stability of the material can be expected through the reduction of their singlet-triplet splitting and the precise adjustment of dihedral angles between the donor-acceptor skeletons.
Keywords: C–C bond; blue emitter; efficiency roll-off; external quantum efficiency (EQE); organic light-emitting diode (OLED); synthesis; thermally activated delayed fluorescence (TADF).