Theoretical description of the Raman spectrum of a vinylene-bridged quaterthiophene oligomer Pedro M. Viruela a , Rafael Viruela a , Enrique Ortı ´ * ,a , Juan Casado b , Victor Herna ´ndez b , Juan T. Lo ´pez Navarrete b, * a Institut de Cie `ncia Molecular, Universitat de Vale `ncia, E-46100 Burjassot, Vale `ncia, Spain b Departamento de Quı ´mica Fı ´sica, Facultad de Ciencias, Universidad de Ma ´laga, E-29071 Ma ´laga, Spain Received 3 September 2002; accepted 26 September 2002 Abstract 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 < 20 cm 21 for the normal modes associated with the n as (CyC) and n s (CyC) 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. q 2003 Elsevier Science B.V. All rights reserved. Keywords: Conjugated oligomers; Oligothiophenes; Raman spectrum; Density functional theory calculations; Normal modes 1. Introduction Conjugated polymers and oligomers are the focus of an intense research activity as functional materials for a variety of electronic applications [1]. Polymeric materials however, present a number of character- istics (polydispersity, low crystallinity and solubility, chemical defects, etc.) that makes difficult to establish a precise relationship between structure and electronic properties. In this sense, conjugated oligomers with perfectly controlled chemical structures have acquired a growing importance both as model compounds for the parent polymers and as a novel class of molecular materials [2,3]. The unique electronic properties of conjugated polymers and oligomers originate from the delocali- zation of the p-electrons. For systems involving aromatic units such as oligophenylenes (n Ps) and oligothiophenes (nTs), the effectiveness of delocali- zation along the conjugated chain is limited by the resonance energy of the aromatic rings and the rotations around the inter-ring single bonds [4]. These two factors tend to confine the p-electrons within the rings. To reduce the effect of these two factors, vinylene groups are incorporated bridging 0022-2860/03/$ - see front matter q 2003 Elsevier Science B.V. All rights reserved. PII: S0022-2860(02)00690-7 Journal of Molecular Structure 651–653 (2003) 657–664 www.elsevier.com/locate/molstruc * Corresponding authors. Tel.: þ 34-95-213-2018; fax: þ 34-952- 132000. E-mail addresses: teodomiro@uma.es (J.T. Lo ´pez Navarrete), enrique.orti@uv.es (E. Ortı ´).