Novel thermally activated delayed fluorescence (TADF) emitters with carbazole (Cz) as an electron donor and thianthrene 5,5-dioxide (TTDO) and 9,9-dimethyl-9H-thioxanthene 10,10-dioxide (DMTA) as an electron acceptor (DCz-TTDO and DCz-DMTA) were designed, and their electronic and optical properties were investigated theoretically for applications to blue organic light-emitting diodes (OLEDs). By using density functional theory (DFT) and time-dependent density functional theory (TD-DFT) calculations, we obtained the electron distribution of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) and the energy of the lowest singlet (S-1) and lowest triplet (T-1) excited states. We show that DCz-DMTA would be a suitable blue OLED emitter because it has sufficiently small E-ST value, which is favorable for a reverse intersystem crossing (RISC) process from the T-1 to S-1 states and an emission wavelength of 432.7nm with sufficiently large F values.