Theoretical and Spectroscopic Study of a Series of Styryl-Substituted Terthiophenes

Molecular structures of a series of 3‘-[1E-2-(4-R-phenyl)ethenyl]-2,2‘:5‘,2‘ ‘-terthiophenes have been modeled using ab initio calculations. The potential energy surfaces of three important dihedral angles were calculated using the HF/3-21G(d) method. Each dihedral angle is represented by a distinct potential energy surface, while the identity of the R group has only a modest influence. DFT methods (B3LYP/6-31G(d)) were used to calculate the geometry and vibrational spectra of each molecule. Analysis of the theoretical vibrational data reveals numerous conserved modes that are localized on the terthiophene or phenyl groups. There is good agreement between the observed and calculated vibrational spectra of the molecules. Conformational changes have only a minor effect on the spectra. The calculated molecular orbitals, which are supported by electronic absorption measurements, suggest that the first excited state should have charge-transfer features for the molecules with strongly electron withdrawing or donating substituents.