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

Substituents are widely used to modify the properties of conducting polymers. To study substituent effects on energy levels and energy gaps systematically, CH3-, OH-, NH2-, CN-, and CCH-substituted bithiophenes were examined with density functional theory and NBO analysis. Total charges and π-electron densities were analyzed separately to examine π- and σ-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 π-donating and π-accepting abilities are most important; electronegativity mainly influences the σ-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.