Using the singles configuration interaction (CIS) theoretical approach, we investigated the nature of the geometric conformations and electronic transitions in pi-conjugated oligo(cyclopentadiene)s and oligo(fulvene)s, and their cyano derivatives. Geometry optimizations were first carried out with the restricted Hartree-Fock (RHF/3-21G*) method which was followed by calculations of excitation energies using CIS (CIS/3-21G*) method. The trends in excitation energies as a function of chain lengths show that the band gaps in the cyano substituted compounds are lowered by approximately 0.5 eV relative to their parent polymers. The results of CIS/3-21G* oligomer calculations also show that delocalized singlet excited states are accompanied by a geometry relaxation in comparison to their ground state (HF) geometries. (C) 2003 Elsevier Ltd. All rights reserved.