The Concept of `Doping' of Conducting Polymers: The Role of Reduction Potentials

The conductivity of certain organic polymers can be raised to metallic levels by chemical or electrochemical `p-doping' (oxidation), or `n-doping' (reduction). Polyacetylene, (CH)_x, the prototype conducting polymer, has been studied more extensively than any other conducting polymer and the doping concepts involved appear to be applicable to other polymer systems. The doping of an organic polymer to achieve certain metallic properties is phenomenologically similar to the doping of a classical inorganic semiconductor in that very large increases in conductivity are observed when the material takes up very small amounts of certain chemical species. However, mechanistically it is different in that the doping of an organic polymer involves simply the partial oxidation or reduction of the polymer, each oxidation state exhibiting its own characteristic reduction potential. The dopant ion incorporated may be derived from the chemical dopant species or it may be completely unrelated to it. The reduction potentials of neutral trans-(CH)_x and its various oxidized or reduced states, and also the band gap of cis- and trnas-(CH)_x have been determined electrochemically. The reduction potentials have been used, together with known standard reduction potentials of a variety of redox couples, to rationalize the doping of (CH)_x to achieve metallic conductivity by using a number of dopant species, including I₂, Li, AgClO₄, gaseous O₂, H₂O₂ or benzoquinone (the last three species in aqueous HBF₄ and aqueous HClO₄, etc. The stability of p-doped polyacetylene in aqueous acidic media is ascribed to the fact that a positive charge on a CH unit in trans-(CH)_x is delocalized over approximately fifteen carbon atoms in what is termed a `positive soliton'. This reduces the ease of nucleophilic attack of the partly oxidized polymer chain. The O₂-doping of (CH)_x permits the use of (CH)_x as an electrocatalytic electrode for the spontaneous reduction of oxygen at one atmosphere pressure and at room temperature in strong aqueous HBF₄ solutions. It is concluded that reduction potentials can be used to rationalize the ability of certain dopants to increase the conductivity of selected organic polymers by many orders of magnitude and that they may also be used to predict new chemical species that are thermodynamically capable of acting as p- or n-dopants.