Using the singles configuration interaction (CIS) theoretical approach, we
investigated the nature of the geometric conformations and electronic trans
itions in pi -conjugated oligo(thiophene)s, and their cyano derivatives. Ge
ometry optimizations were carried at the restricted Hartree-Fock (RHF/3-21G
*) level for the ground-state configurations, which were followed by calcul
ations of first (lowest) singlet excitation energies using the CIS (CIS/3-2
1G*) method. In agreement with several previous theoretical/experimental st
udies, this investigation indicates that the cyano substituted thiophene po
lymer has a considerably smaller intrinsic band gap than its parent polymer
. CIS calculations produce singlet excitation energies in excellent agreeme
nt 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. T
he shape and the location of the relaxation depends on the oligomer length.
(C) 2001 American Institute of Physics.