The excited-state properties were then calculated using time-dependent DFT (TD-DFT) within the Tamm-Dancoff approximation (TDA) based on the optimized ground-state geometry. The S1 energies are 2.17 eV for DMACBP, to 2.04 eV for DMACPyBP, 2.05 eV for DMACBPN and 1.91 eV for DMACPyBPN, while the T1 energies likewise decrease from 2.15 eV, to 2.00 eV, 2.03 eV and 1.89 eV, respectively, following a trend of increasingly stabilized excited states with increasing N atom content in the acceptor, although DMACPyBP and DMACBPN are close in energy. The corresponding DEST values are all relatively small at 0.02, 0.04, 0.02 and 0.03 eV, respectively, due to the large dihedral angles between the DMAC and the acceptor that effectively minimize the electronic coupling [DMACBP (82.2°), DMACPyBP (82.7°), DMACBPN (83.8°) and DMACPyBPN (86.1°) based on the optimized S0 geometries] (Figure ). To gain insight into the photophysical properties of the S1 excited states, we performed natural transition orbital (NTO) analyses at the S0 geometry (Figure ). The S1 state of each of the compounds possesses an evident intramolecular charge transfer (ICT) character. Hole and electron distributions mirror HOMO/LUMO localization, respectively. Root-mean-square displacements (RMSDs) between S0, S1 and T1 were calculated (Figure and S27) using VMD program to evaluate the structural changes that occur upon excitation and intersystem crossing. The RMSD values of DMACBP, DMACPyBP, DMACBPN and DMACPyBPN between S0 and S1 were simulated to be 0.17, 0.14, 0.13 and 0.07 Å, respectively. The smaller RMSD of DMACPyBPN indicates that the introduction of N at 10-position or 4/5-position reduces the magnitude of structural relaxation. At the relaxed S1 geometry, there is a larger S1-T1 spin-orbit coupling (SOC) matrix element of 0.05 cm -1 in DMACPyBP and of 0.04 cm -1 in DMACPyBPN than that in DMACBP (0.01 cm -1 ) and DMACBPN (0.01 cm -1 ) (Figure ), indicating that the rate constants of intersystem crossing (kISC) in DMACPyBP and DMACPyBPN will be faster than in the latter two compounds. As shown in the electrostatic potential maps (Figure ), the nitrogen-rich components have more negative electrostatic potential while the other parts have neutral and positive potential, which correlates with the trend in increasing electron-withdrawing ability of the acceptor with the inclusion of an increasing number of nitrogen atoms within the acceptor moiety.