Ab initio study of polarizabilities of oligothiophene, oligocyclopentadiene, oligofulvene and their cyano substituted oligomers

Ab Initio study of polarizabilities and first order hyperpolarizabilities of thiophene, fulvene and cyclopentadiene based conducting oligomers and polymers and their cyano derivatives have been performed using the Hartree-Fock (HF), configuration interaction singles (CIS) and density functional (DFI) theories with 3-21G* basis using Gaussian 94, 98 and 03 softwares. The main motivation for this investigation is to determine the correlation between the excitation energies and polarizabilities and hyperpolarizabilities for the conjugated systems studied. It has been found that HF and DFT approaches give similar magnitudes for polarizabilities whereas CIS theory provides results that are considerably different. All three methods predict similar trends in polarizabilities as a function of oligomer length and bond alternation along the backbone of the oligomers. It has also been observed that the end groups and the number of 'double' bonds have a significant effect on the magnitude of polarizability per C-C bond. Comparison with experimental results has been made where possible