TRENDS IN GEOMETRIC AND ELECTRONIC-PROPERTIES OF THIOPHENE-BASED AND CYCLOPENTADIENE-BASED POLYMERS

In this work, we present theoretical evidence illustrating that cyano derivatives of conducting polymers such as polythiophene, polycyclopen tadiene, and polyfulvene have smaller intrinsic band gaps than those o f their parent polymers. The geometric and electronic properties of th e parent and the derivative polymers were studied with the use of two methodologies: (1) the pseudo-one-dimensional bandstructure calculatio ns performed using the semi-empirical molecular orbital theory (MNDO, AM1) and (2) oligomer calculations performed using the ab initio molec ular orbital theory both at the Hartree-Fock and configuration interac tion levels. In particular, we found that an organic polymer, poly(dic yanomethylene cyclopentadifulvene) (PCNFv), has a comparable (possibly lower) band gap to the one observed in poly(dicyanomethylene cyclopen tadithiophene) (PCNTH) (which has a band gap of 0.8 eV). The precursor of PCNFv is poly(dicyanomethylene cyclopentadicyclopentadiene) (PCNCY ) in which two cyclopentadiene rings are connected by a dicyanomethyle ne group. The additional bond conjugation (in contrast to PCNCY) perpe ndicular to the chain axis makes PCNFv very rigid and fully planar. Tr ends in structural properties indicate that the lower band gaps in the cyano-substituted polymers, in comparison to their parent polymers, a re accompanied by a decrease in bond alternations in the aromatic or t rans-cisoid forms and by an increase in bond alternations in the quino id or cis-transoid forms. (C) 1998 John Wiley & Sons, Inc.