Novel thermally activated delayed fluorescence (TADF) materials (PCD-TRZ and PCCD-TRZ) with 9,9-dimethyl-9,10-dihydroacridine derivatives (PCD and PCCD) as electron donors and a triphenyl-triazine (TRZ) as an electron acceptor were designed, and the electronic and optical properties were theoretically investigated for blue OLED emitters. Using DFT and TDDFT calculations, the large separation between the HOMO and LUMO gives a small energy difference (E-ST) between the S-1 and T-1 state and the HOMO of PCD and PCCD are largely delocalized, which considerably enhances the rate of radiative decay by inducing an oscillator strength (F) for fluorescence even when there is a small overlap between the HOMO and LUMO. The calculated E-ST values of PCD-TRZ (0.022 eV) were small enough for TADF because of the large dihedral angles between the donor and accepter moieties. The PCD-TRZ would thus provide highly efficient TADF properties with both a large F and a low E-ST for the blue OLED emitter.