The ground-state (S0) geometries of TPAPyAP, TPAPyBP and TPAPyBPN were optimized using density functional theory (DFT) at the PBE0 /6-31G(d,p) level of theory in the gas phase starting from a geometry generated in Chem3D. At the optimized S0 geometries, the dihedral angles between the bridging phenylene of the TPA and acceptor moieties are around 31 o for TPAPyAP, 39 o for TPAPyBP and 41 o for TPAPyBPN (Figure ), slightly larger than those found in the crystal structures of the latter two (Figure ). The calculated energy levels of the highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) are shown in Figure and S17 and the results are summarized in Table . The HOMOs are localized on the TPA donor, with some minor contribution onto the proximal pyridine ring of the acceptor moiety. The LUMOs of all three compounds are localized on the acceptor group, with some contribution also located on the bridging phenylene of the TPA donor. As the acceptor strength increases along the series from TPAPyAP to TPAPyBP and TPAPyBPN both the HOMO and LUMO are stabilized, with the stabilization more significant for the latter. The HOMO-LUMO gap, DEHOMO-LUMO, thus decreases from 3.21 eV for TPAPyAP to 3.00 eV for TPAPyBP and 2.90 eV for TPAPyBPN (Figure ). The excited-state properties were calculated using time-dependent density functional theory (TD-DFT) within the Tamm-Dancoff approximation (TDA-DFT) based on the optimized ground-state geometries. The oscillator strength, f, for the S0→S1 transition is high at 0.47, 0.39 and 0.36 for TPAPyAP, TPAPyBP and TPAPyBPN, respectively, reflecting a significant overlap of the electron density between the HOMO and LUMO, a result of the relatively small torsions that exist between the TPA and the acceptor moieties. The S1 energies are 2.82 eV for TPAPyAP to 2.59 eV for TPAPyBP and 2.48 eV for TPAPyBPN, while the T1 energies likewise decrease from 2.44 eV, 2.25 eV, and 2.17 eV, respectively, following a similar trend to that observed for DEHOMO-LUMO. The degree of spatial separation of the frontier orbitals in TPAPyBPN is reflected in a ΔEST of 0.31 eV, while the larger overlap between HOMO and LUMO for TPAPyAP and TPAPyBP lead to ΔEST values that are slightly larger at 0.37 eV and 0.34 eV, respectively. Natural transition orbital (NTO) analyses at the optimized S1 and T1 geometries calculated at the TDA-DFT-PBE0/6-31G(d,p) level are shown in Figures and, respectively. For all three compounds, the S1 states are of charge transfer (CT) character from the TPA donor to the acceptor. However, the T1 states possess mixed CT and locally excited (LE) character on the acceptor. At the relaxed S1 geometry, there is a decreasing S1 -T1 spin-orbit coupling matrix element (SOCME) from 0.27 cm -1 in TPAPyAP to 0.21 cm -1 in TPAPyBP and 0.16 cm -1 in TPAPyBPN (Figure ), while at the relaxed T1 geometry, the T1 -S1 SOCME are 0.17, 0.22, 0.22 cm -1 for TPAPyAP, TPAPyBP and TPAPyBPN, respectively.