Spectroscopic and density functional theory study of functionalized thiophene-benzene derivatives

A group of diethenylbenzene-linked oligothiophenes were investigated using vibrational spectroscopy in conjunction with quantum calculations. Geometry calculations revealed dihedral angles that are sensitive to both the presence of cyano substituents on the ethylene linkages and the identity of the attached oligothiophene. In particular, cyano substitution in the presence of terthiophene groups leads to a significant loss of planarity. The resultant loss of conjugation has been observed in the experimental Raman spectra on the basis of a band at ∼1520 cm−1, the wavenumber and intensity of which are dependent on conjugation length. In the case of the diethenylbenzene-linked terthiophene, the wavenumber of this band suggests a conjugation that extends across both terthiophene and the diethenylbenzene species. The calculated vibrational spectra successfully predict the wavenumber shifts upon cyano substitution in both the infrared and Raman spectra. The presence of the cyano groups has the greatest influence on the Mulliken charge distribution, whereas the identity of the oligothiophene has little effect. This study shows that quantum calculations, particularly when combined with experimental parameters such as vibrational spectroscopy, can be informative in understanding the electronic structure of molecules of this type. Copyright © 2005 John Wiley & Sons, Ltd.