Effect of the Number and Substitution Pattern of Carbazole Donors on the Singlet and Triplet State Energies in a Series of Carbazole-Oxadiazole Derivatives Exhibiting Thermally Activated Delayed Fluorescence

Substitution of a varying number of 9H-carbazole donors onto 2-(pentafluorophenyl)-5-phenyl-1,3,4-oxadiazole yields a series of donor–acceptor compounds with similar electrochemical band gaps, but dissimilar optical properties. Each substitution of a carbazole onto the 2- and 4-positions results in a 150–200 meV stabilization of the first singlet excited state, evident in the fluorescent emission. The first triplet excited state is less sensitive to these substitutions, leading to a reduced energy separation between the singlet and triplet states, ΔEST, with increasing carbazole content. OLED devices made using the donor–acceptor compounds exhibiting the lowest ΔEST were fabricated and exhibit sky blue emission with an external quantum efficiency of up to 24.4% at a luminance of 10 cd m–2, and 9.2% at 1000 cd m–2.