  To identify reliable molecular design principles for energy level tuning in
donor/acceptor conjugated polymers (CPs), we studied the governing factors by
means of ab initio calculations based on density-functional theory (DFT). We
investigated a series of CPs in which we independently and systematically
varied the electron withdrawing power of the acceptor unit and the electron
donating power of the donor unit, while maintaining the same conjugated chain
conformation. We observed that the introduction of a stronger acceptor unit,
while keeping the same donor unit in the CP, lowers the LUMO level, but leaves
the HOMO level almost unchanged. Conversely, enhancing the strength of the
donor unit for the same acceptor unit raises the HOMO level, while maintaining
the LUMO level. We identified strong correlations between the frontier orbital
energy levels and the degree of orbital localization, which depends on the
electron donating or withdrawing power of the molecular groups carrying the
orbitals. Moreover, the HOMO/LUMO gap of the CP is directly proportional to the
charge transfer between donating and accepting units, which provides a robust
design criterion for CPs.
