Comparison of ground and excited state polarizabilities of thiophene, cyclopentadiene, and fulvene oligomers and their cyano substituted derivatives -: Ab initio study

Ab Initio study of the ground and excited state polarizabilities of thiophene, fulvene, and cyclopentadiene based conducting oligomers and their cyano derivatives have been performed using the restricted Hartree-Fock (RHF) and the configuration interaction singles (CIS) approaches, respectively, with 3-21G* basis set. For comparison purposes, for some small oligomers (monomers and dimers), higher basis sets (6-31G*, 6-31+G*, aug-cc-pVTZ) were also employed in the computations of polarizabilities. The trends in polarizability as a function of oligomer length were investigated. For all systems, the RHF polarizability increases as n(1.2-1.3) as n gets larger and the CIS polarizability increases as n(1.4-1.6) for n less than seven or eight rings and then increases approximately linearly with n for larger n. For the thiophene based systems the dependence of the polarizabillity on bond length alternation (BLA) along the backbone of the oligomers was also investigated using the RHF, density functional (DFT), and CIS theories (with 3-21G* basis set). For thiophene dimer, we also performed RHF/aug-cc-pVTZ calculations of polarizabilities versus BLA. We found that the polarizability is largest when BLA is near zero (for both ground and excited states), which correlates with the lowest excitation energy. Comparison with experimental results has been made where possible. (c) 2007 Wiley Periodicals, Inc.