It is observed that the lowest unoccupied molecular orbital (LUMO) always corresponds to the π * -orbital of the NDI core, whereas the highest occupied molecular orbitals (HOMOs) are located at the various polycyclic aromatic groups. As a result, LUMOs and HOMOs are well separated on the chemical structures. Also, the LUMO remains nearly constant in all oligomers, whereas the HOMO level varies according to the -conjugation degree of the polycyclic aromatic groups. Consequently, the HOMO-LUMO energy gaps follow the trend of Bp > Fluo > Naph > Ant Py, which is consistent with the photophysical properties observed experimentally (Figure ). In addition, consistent results were obtained for the end-functionalized NDI oligomers with 2 styrene units and 2 polycyclic aromatic groups (Table and Figure ). Notably, linear correlations were discovered between the energy gap and the conjugation electron number of the polycyclic aromatic monomer or alcohol that was introduced (Figure ). These results indicate a general tunability of the NDI-based TSCT polymer platform via manipulation of the -conjugation degree of donor structures.