Control of Singlet Emission Energy in a Diphenyloxadiazole Containing Fluorophore Leading To Thermally Activated Delayed Fluorescence

2-(4-(9,9-Dimethylacridin-10(9 H )-yl)phenyl)-5-phenyl-1,3,4-oxadiazole has an energy difference between the lowest excited singlet and triplet states (Δ E ST ) of ca. 0.24 eV. Introduction of two electronegative fluorine atoms onto the acceptor portion of the molecule to give 2-(4-(9,9-dimethylacridin-10(9 H )-yl)-3,5-difluorophenyl)-5-phenyl-1,3,4-oxadiazole lowers the energy of the singlet emission with a negligible effect on the corresponding triplet energy, leading to a donor-acceptor compound with decreased Δ E ST of ca. 0.13 eV that displays thermally activated delayed fluorescence. Organic light-emitting diodes fabricated using the latter compound display high EQE max of 21.9% at a luminance of 10 cd/m 2 and sky-blue emission, however, they suffer from a large efficiency roll-off at increased luminance.