Geometry relaxation in singlet excited states of oligomers containing cyclopentadiene and fulvene and their cyano derivatives

Using the singles configuration interaction (CIS) theoretical approach, we investigated the nature of the geometric conformations and electronic transitions in π-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.