Using the singles configuration interaction (CIS) theoretical approach, we investigated the nature of the geometric conformations and electronic transitions in pi -conjugated oligo(thiophene)s, and their cyano derivatives. Geometry optimizations were carried at the restricted Hartree-Fock (RHF/3-21G*) level for the ground-state configurations, which were followed by calculations of first (lowest) singlet excitation energies using the CIS (CIS/3-21G*) method. In agreement with several previous theoretical/experimental studies, this investigation indicates that the cyano substituted thiophene polymer has a considerably smaller intrinsic band gap than its parent polymer. CIS calculations produce singlet excitation energies in excellent agreement with the experimental values (similar to0.1-0.5 eV). They also show that delocalized first singlet excited states (CIS) are accompanied by a strong geometry relaxation in comparison to their ground-state (HF) geometries. The shape and the location of the relaxation depends on the oligomer length.