Abstract
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.
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