Investigation of geometric and electronic properties of conducting polymers using semiempirical approach

Abstract

In this work, we present theoretical evidence illustrating that cyano derivatives of conducting polymers such as polythiophene, polycyclopentadiene and polyfulvene have smaller intrinsic band gaps than their parent polymers. The geometric and electronic properties of the parent and the derivative polymers are studied with the use of two methodologies: (1) the pseudo one-dimensional band structure calculations performed at the level of the semiempirical molecular orbital theory (Modified Neglect of Diatomic Overlap (MNDO) and Austin Model 1 (AM1)) and (2) oligomer calculations performed at the level of the ab initio molecular orbital theory. In particular, we have found that an organic polymer, poly-(dicyano-methylene-cyclopenta-difulvene) (PCNFv) has a comparable (possibly lower) band gap to the one observed in poly-(dicyano-methylene-cyclopenta-dithiophene) (PCNTH) (which has a band gap of 0.8 eV). The precursor of PCNFv is poly-(dicyano-methylene-cyclopenta-dicyclopentadiene) (PCNCY) in which two cyclopentadiene rings are connected by a dicyanomethylene group. Trends in structural properties indicate that the lower band gap in the cyano substituted polymers is accompanied by greater charge delocalization in the aromatic or trans-cisoid forms and by greater charge localization in the quinoid or cis-transoid forms in comparison to their parent polymers. Other important factors that contribute to band gap lowering are maximum planarity, weak interactions of the chain backbone with the bridging groups, enhanced π character of the highest occupied and lowest unoccupied bands and the additional stabilization of the conduction bands due to the charge transfer phenomena. The comparison of the heats of formation for the compounds indicates that polymers in planar-anti conformation are more stable than those in twisted-syn conformation.