Theoretical description of the Raman spectrum of a vinylene-bridged quaterthiophene oligomer

The Raman spectrum of a quaterthiophene oligomer incorporating a central vinylene spacer has been investigated using density functional theory B3LYP/6-31G** calculations. The spectrum has been fully assigned with the aid of the calculations and in comparison with unsubstituted quaterthiophene. The spectrum preserves most of the vibrational features of linear oligothiophenes. The vibrations of the vinylene spacer are clearly differentiated from those of the rest of the chain. The vinylene spacer increases the conjugation length of the molecule and induces a frequency downshift of approximate to20 cm(-1) for the normal modes associated with the nu(as)(C=C) and nu(s)(C=C) vibrations of the thiophene rings. The molecule is predicted to have a structure in which both the thiophene rings and the vinylene spacer are in an all-anti conformation. (C) 2003 Elsevier Science B.V. All rights reserved.