Single crystals of MeTPA-BQ and tBuTPA-BQ were obtained from the slow evaporation of dichloromethane solution and temperature-gradient vacuum sublimation, respectively. Attempts to grow crystals of TPPA-BQ were unsuccessful. Both MeTPA-BQ and tBuTPA-BQ crystallized in the triclinic space group P1 (Figure , b, Table ). For MeTPA-BQ, the torsion angles of 58.44(16)° and 22.63 (16)° between the acceptor and the phenylene rings are similarly twisted to those in tBuTPA-BQ [(51.9 Density Functional Theory (DFT) calculations at the PBE0 16 /6-31G(d,p) 17 level of theory were performed in the gas phase to provide insight into the electronic structure of the molecules (Figure ). The excited-state properties were calculated using time-dependent density functional theory (TD-DFT) within the Tamm-Dancoff approximation (TDA-DFT) 18 based on the optimized excited S1 state geometries. In the case of MeTPA-BQ and tBuTPA-BQ, the HOMO is located on the triphenylamine (TPA) donor moieties along with some contribution from the pyrazine, while the LUMO is located on the benzo[g]quinoxaline-5,10-dione (BQ) acceptor. In the case of TPPA-BQ, the HOMO is delocalized across both the TPA and the adjacent phenylene spacer and there is no electron density on the pyrazine, while the LUMO is localized on the BQ acceptor. As a result, the HOMO/LUMO energy levels are stabilized in TPPA-BQ (-5.20 /-2.78 eV) in comparison to both MeTPA-BQ (-5.06 /-2.60 eV) and tBuTPA-BQ (-5.06 /-2.61 eV). The computed HOMO-LUMO gaps for MeTPA-BQ, tBuTPA-BQ and TPPA-BQ are 2.46, 2.45 and 2.42 eV, respectively, which are nearly the same, indicating that there is no significant impact of either the donor strength or spacer length on HOMO-LUMO gaps. The S1 and T1 energies of TPPA-BQ are 1.73 and 1.67 eV, respectively, which are stabilized to 1.60 and 1.48 eV for MeTPA-BQ and 1.46 and 1.41eV for tBuTPA-BQ. The calculated ∆EST values for TPPA-BQ (0.06 eV) and tBuTPA-BQ (0.05 eV) are smaller than that of MeTPA-BQ (0.12 eV). Natural transition orbital (NTO) analysis based on the S1-optimized geometry revealed that the hole is located on the TPA segments, and the particle is located on the BQ acceptor for the S1 and T1 states of the MeTPA-BQ (Table ). Similarly, in the case of tBuTPA-BQ and TPPA-BQ, the hole is mostly located on one TPA group with a very small amount on the other TPA, while the particle is located on the BQ. Nearly similar electron distribution patterns at their S1 and T1 states resulted in small spin-orbit coupling matrix elements (<S1|ĤSOC|T1> for MeTPA-BQ (0.13 cm -1 ), tBuTPA-BQ (0.06 cm -1 ) and TPPA-BQ (0.03 cm -1 ) in the gas phase. Thus, relatively slow kRISC from T1 to S1 is expected in all three compounds.