Torsional effects on excitation energies of thiophene derivatives induced by beta-substituents: comparison between time-dependent density functional theory and approximated coupled cluster approaches.

The influence of methyl or phenyl substitution in beta-position of dioxygenated terthiophene and diphenylthiophene on the optical properties is investigated by first-principles calculations. We compare the approximated singles and doubles coupled cluster (CC2) approach with time-dependent density functional theory methods. CC2 reproduces experimental excitation energies with an accuracy of 0.1 eV. We find that the different substituents modify the inter-ring torsional angle which in turn strongly influences the excitation energies. The steric contribution to the excitation energies have been separated from the total substituent effects.