Synthetic Metals

Chemical reduction of polyacetylene with incorporation of divalent dopant cations

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A chemical method has been developed which makes possible the divalent doping of polyacetylene, (CH)x. Solvated electrons, formed by dissolving alkaline earth metals in liquid ammonia, provide the reducing power needed to partially reduce polyacetylene to a polycarbanion [My+2(CH)-2y]x M = Ca, Ba or Sr. The divalent dopant cations serve to maintain electrical neutrality. This new method of doping polyacetylene can be used to study some interesting predictions of the soliton theory of conductivity. This theory, which offers an explanation of the spinless conductivity observed in polyacetylene doped with monovalent ions, predicts that coulomb interactions between the divalent dopant and a pair of charged solitons should result in a bound state of the soliton pair. Visible spectra show midgap absorptions characteristic of both solitons, which could form by interchain interactions, and bipolarons, which could form by intrachain coupling of a pair of solitons to a single divalent dopant. Infrared data confirms dopant induced soliton-like resonances. Preliminary magnetic measurements indicate that the carriers are spinless, at least at low doping levels. The high observed conductivities (σ = 10-20 Ω-1cm-1), which are within one order of magnitude of polyacetylenes doped with alkali metal ions to comparable levels, likely involve some localization of charge (bipolarons) as well as some interchain interactions (solitons). This method has been extended to include polyparaphenylene as a conducting polymer host and the lanthanides Eu+2 and Yb+2 as dopant ions. © 1989.