Trends in geometric and electronic properties of thiophene‐ and cyclopentadiene‐based polymers

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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 those of their parent polymers. The geometric and electronic properties of the parent and the derivative polymers were studied with the use of two methodologies: (1) the pseudo-one-dimensional band-structure calculations performed using the semi-empirical molecular orbital theory (MNDO, AM1) and (2) oligomer calculations performed using the ab initio molecular orbital theory both at the Hartree–Fock and configuration interaction levels. In particular, we found that an organic polymer, poly(dicyanomethylene cyclopentadifulvene) (PCNFv), has a comparable (possibly lower) band gap to the one observed in poly(dicyanomethylene cyclopentadithiophene) (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 dicyanomethylene group. The additional bond conjugation (in contrast to PCNCY) perpendicular to the chain axis makes PCNFv very rigid and fully planar. Trends in structural properties indicate that the lower band gaps in the cyano-substituted polymers, in comparison to their parent polymers, are accompanied by a decrease in bond alternations in the aromatic or trans–cisoid forms and by an increase in bond alternations in the quinoid or cis–transoid forms. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 66: 229–240, 1998