Highly conducting acetylene-CO copolymers and limitations of the soliton model

Abstract

There is much scientific and technological interest in conductive polymers, of which polyacetylene is the simplest and best example, and in the mechanism of carrier transport. Trans-[CH]x with C2h symmetry is said1–4 to have an electronically degenerate ground state with very slight alternation of backbone bond lengths, that is, a high degree of conjugation. Radicals or radical ions in the chain are referred to as soliton5,6 and polaron7 excitations with moving domains, but these rigorous concepts are quite restrictive and exclusive. For instance, moving-domain excitations cannot exist in cis-[CH]x because much energy is required for the interconversion of cis-transoid and trans-cisoid structures. However, many physical and spectroscopic properties of undoped and doped trans-[CH]x have been explained by these models8. An intersoliton electron-hopping model had been proposed for carrier transport in lightly doped trans-[CH]x9. Our purpose here is to test the general validity of the soliton/polaron models. Our present results cast strong doubts on the relevance of these theoretical models to carrier transport in conductive polymers, thereby allowing a search for possible new conductive polymers that is free of the constraints imposed by these models.