Density functional theory (DFT) calculations predict the ground-state (S0) dihedral angles between the donor groups and pyrazine in 4tCzPz, and 4αCbPz to be greater than 50°, which are close to the angles of 4CzPz obtained from single crystal. The HOMOs of 4tCzPz, 4CzPz and 4αCbPz are distributed across the whole molecule while the LUMOs are mainly localized on the pyrazine cores. The HOMO values of 4tCzPz, 4CzPz, and 4αCbPz are progressively stabilized from -5.25 eV to -5.52 eV, and -5.89 eV, respectively, reflecting the decreasing electron-donating strength of the donor across the series. The LUMO levels are stabilized from -1.54 eV to -1.72 eV, and -2.02 eV, respectively, across 4tCzPz, 4CzPz, and 4αCbPz, reflecting that there is electronic coupling between donor and acceptor groups. The corresponding HOMO-LUMO gaps of 4tCzPz, 4CzPz, and 4αCbPz increased from 3.71 eV to 3.80 eV, and 3.82 eV, respectively. The S1 states of 4tCzPz, 4CzPz, and 4αCbPz are progressively destabilized from 2.99 eV to 3.09 eV, and 3.12 eV, respectively, while the corresponding T1 states are to a lesser extent destabilized from 2.51 eV to 2.56 eV, and 2.57 eV, respectively. The corresponding ΔEST values increase from 0.43 eV, to 0.53 eV, and 0.55 eV, respectively. The TDA-DFT calculation result from our previous work on 4CzPz, predicted that RISC could proceed between T2 and S1 due to the small energy gap, (ΔES1T2 = 0.10 eV) between these two states, and the strong SOC (𝐻 !"# !$%& = 0.47 cm -2 ) due to the different nature of the transitions involved to T2 and S1. Indeed, 4CzPz does show TADF character with τd of 150 µs in 10 wt% doped PPT film and the delayed component is temperature dependent. As expected, higher energy intermediate triplet states exist in 4tCzPz and 4αCbPz as well. For 4tCzPz and 4αCbPz, their T2 states (2.92 eV and 2.99 eV, respectively) are energetically close to their corresponding S1 states (2.99 eV and 3.12 eV, respectively). Moreover, the T2 states of 4tCzPz and 4αCbPz also exhibit hybridized local and charge-transfer (HLCT) character with the corresponding highest occupied natural transition orbitals (HONTOs) and the lowest unoccupied natural transition orbitals (LUNTOs) distributed over the whole molecules. By contrast the S1 and T1 states of 4tCzPz and 4αCbPz are of charge-transfer (CT) character, and the corresponding HONTOs and LUNTOs are located on the donor moieties and pyrazine, respectively (Figure ). The different nature of T2/S1 explains the large spin-orbital coupling matrix element (SOCME) values between T2/S1, with 𝐻 !"# !$%& = 1.01 cm -2 and 0.94 cm -2 for 4tCzPz and 4αCbPz, respectively, while the corresponding 𝐻 !"# !$%$ are only 1×10 -4 cm -2 and 0.02, respectively. The small T2/S1 energy gaps and high SOCMEs values result indicate that RISC may occur between T2/S1 for 4tCzPz and 4αCbPz as 4CzPz. ) level of theory in the gas phase (isovalue = 0.02).