Conductivity in polyacetylene. II. Ab initio and tight-binding calculations of soliton structure and reorganization energy in ordered and disordered structures

Abstract

Alkali-doped polyacetylene is considered as an electron-transfer system. To estimate the reorganization energy due to bond-length changes when electrons are added or subtracted, we applied the (U)MP2 and CASSCF methods to small systems of the type H(CH)NH. The simple tight-binding (Hückel) model with bond-length-dependent resonance integrals has been applied to the same and larger polyenes. The bond lengths are obtained via the bond orders for the various oxidation states. The results agree very well with the ab initio results and experiments for small polyenes. Odd-N and even-N systems behave differently. In odd-N systems, a structural “soliton” exists in the neutral molecule. An electron can be added or removed without bond-length change. In even-N chains, with perfect bond alternation in the neutral molecule, the bond length changes when an electron is accepted occur over about 20 carbon atoms. The reorganization energy tends to a constant value (0.22 eV) as the chain length is increased. Soliton structure is studied as a function of out-of-plane torsional defects and it is found that an additional electron is localized primarily on a segment with an odd number of carbon atoms. In the presence of positively charged ions, electrons are attracted toward this charge and positive solitons formed at some distance from the perturbing ion. © 1996 John Wiley & Sons, Inc.