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

Abstract: 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 bandgaps, but dissimilar optical properties. Each substitu-tion of onto the 2- and 4-positions result in a 150-200 meV stabilization of the first singlet excited state, evident in the fluorescent emission. The first triplet excited state was less sensitive to this effect, leading to a reduced energy separation between the singlet and triplet states, ？EST, as the singlet state decreased in energy. OLED devices made using the donor-acceptor compounds exhibiting the lowest ？EST were fabricated, and exhibit sky blue emission with external quantum efficiency of up to 24.4％. at a luminace of 10 cd/m2 (9.2％ at 1,000 cd/m2)