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, Delta E-ST, with increasing carbazole content. OLED devices made using the donor- acceptor compounds exhibiting the lowest Delta E-ST 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).