  Novel thermally activated delayed fluorescence (TADF) material with diphenyl
sulfone (DPS) as an electron acceptor and 3,6-dimethoxycarbazole (DMOC) and
1,3,6,8-Tetramethyl-9H-carbazole (TMC) as electron donors were investigated
theoretically for a blue organic light emitting diode (OLED) emitter. We
calculate the energies of the first singlet (S1) and first triplet (T1)-excited
states of TADF materials by performing density functional theory (DFT) and
time-dependent DFT (TD-DFT) calculations on the ground state using a dependence
on charge transfer amounts for the optimal Hartree-Fock percentage in the
exchange-correlation of TD-DFT. The calculated {\Delta}EST values of TMC-DPS
(0.094 eV) was smaller than DMOC-DPS (0.386 eV) because of the large dihedral
angles between the donor and accepter moieties. We show that TMC-DPS would have
a suitable blue OLED emitter, because it has a large dihedral angle that
creates a small spatial overlap between the HOMO and the LUMO and,
consequently, the small {\Delta}EST and the emission wavelength of 2.82 eV
(439.9 nm).
