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Substituents are widely used to modify the properties of conducting polymers. To study substituent effects on energy levels and energy gaps systematically, CH(3)-, OH-, NH(2)-, CN-, and CCH-substituted bithiophenes were examined with density functional theory and NBO analysis. Total charges and pi-electron densities were analyzed separately to examine pi- and sigma-effects. Second-order perturbation theory was used to quantify conjugation in terms of orbital interactions. NBO orbital energies were employed to investigate the effect of alternating donor-acceptor substitution. Substituents in 3- and 4-positions shift HOMO and LUMO levels in parallel and hardly influence HOMO-LUMO gaps. For level shifting the pi-donating and pi-accepting abilities are most important; electronegativity mainly influences the sigma-orbitals and is less crucial in determining energy gaps. Alternating donor-acceptor substitution leads to HOMO and LUMO energies that are average between those of the parent systems and has little effect on energy gaps.

Theoretical Analysis of Substituent Effects on Building Blocks of Conducting Polymers: 3,4‘-Substituted Bithiophenes