Optimization of the ground state geometries of the three emitters was carried out by DFT calculation at the PBE0/6-31G(d,p) level of theory, starting from the X-ray crystal structures. The Tamm-Dancoff approximation (TDA) to TD-DFT was applied to investigate the character of both the excited singlet and triplet states as well as to estimate ΔEST. The spin-orbital coupling matrix element (SOCME) values between excited states were obtained using PySOC based on the optimized triplet state geometries. The HOMO of 4CzPyz is distributed across the whole molecule while the LUMO is mainly localized on the pyrazine, whereas for 3CzBPz and 2CzBPz, the HOMOs are located on the carbazole moieties and the pyrazine, while LUMOs are delocalized on pyrazine and the biphenyl substituent. The HOMOs for the three emitters are predicted to be around -5.50 eV, while the LUMOs for 3CzBPz and 2CzBPz are destabilized to -1.62 eV and -1.60 eV, respectively, compared to -1.72 eV for 4CzPyz, which hints that the delocalization of the pyrazine weakens the electron-withdrawing character of the acceptor. The TDA-DFT calculations predict the three emitters to have similar, moderately large ΔEST values of around 0.40 eV. For 4CzPyz, the S1 and T1 are 3.09 eV and 2.56 eV, respectively, and the intermediate T2 (2.99 eV) is energetically close to S1 (Figure ). Moreover, the nature of T2 is of HLCT character and different from the CT dominant character of S1 and T1, which leads to a much higher SOCME value (𝐻 !"# !$%& = 0.47 cm -2 ) between T2 and S1 than 2×10 -4 cm -2 for direct SOC between T1 and S1 (Figure ). The S1 and T1 of 3CzBPz are destabilized, respectively, to 3.17 eV and 2.65 eV, both of which show dominant CT character, while the T2 is 2.99 eV and T3 is degenerated to S1 at 3.19 eV, both possessing HLCT character. T3 and S1 exhibits the highest SOC with 𝐻 !"# !$%' of 1.21 cm -2 compared to 1.18 cm -2 and 0.45 cm -2 for 𝐻 !"# !$%& and 𝐻 !"# !$%$ , respectively (Figure ). The high SOC and small energy gap (0.02 eV) between S1 and T3 indicate that RISC in 3CzBPz can occur between T3 and S1 via high-energy triplet state mechanism. The S1 and T1 of 2CzBPz are 3.18 eV and 2.67 eV, respectively, both of which show dominant CT character. The T2 and T3 of 2CzBPz are 3.01 eV and 3.28 eV, and the energy offset between S1/T2 (0.17 eV) or S1/T3 (0.10 eV)