page_title: Polythiophene
section_title: Mechanism of conductivity and doping
hierarchical_section_title: Polythiophene / Mechanism of conductivity and doping
context_page_description: Polythiophenes are polymerized thiophenes, a sulfur heterocycle. They are white solids with the formula ₙ for the parent PT. The rings are linked through the 2- and 5-positions. Polys have substituents at the 3- or 4-position. They are also white solids, but tend to be soluble in organic solvents.
  PTs become conductive when oxidized. The electrical conductivity results from the delocalization of electrons along the polymer backbone. Conductivity however is not the only interesting property resulting from electron delocalization. The optical properties of these materials respond to environmental stimuli, with dramatic color shifts in response to changes in solvent, temperature, applied potential, and binding to other molecules. Changes in both color and conductivity are induced by the same mechanism, twisting of the polymer backbone and disrupting conjugation, making conjugated polymers attractive as sensors that can provide a range of optical and electronic responses.
  The development of polythiophenes and related conductive organic polymers was recognized by the awarding of the 2000 Nobel Prize in Chemistry to Alan J.
context_section_description: PT is an ordinary organic polymer, being a white solid that is poorly soluble in most solvents. Upon treatment with oxidizing agents (electron-acceptors), however the material takes on a dark color and becomes electrically conductive.  Oxidation is referred to as "doping".  Around 0.2 equivalent of oxidant is used to convert PTs (and other conducting polymers) into the optimally conductive state.  Thus about one of every five rings is oxidized.  Many different oxidants are used.  Because of the redox reaction, the conductive form of polythiophene is a salt.  An idealized stoichiometry is shown using the oxidant [A]PF₆:
  (C₄H₂S)ₙ  + 1/5n [A]PF₆  →   (C₄H₂S)ₙ(PF₆)0.2n  +  1/5 nA
  In principle, PT can be n-doped using reducing agents, but this approach is rarely practiced.
  Upon "p-doping", charged unit called a bipolaron is formed. The bipolaron moves as a unit along the polymer chain and is responsible for the macroscopically observed conductivity of the material. Conductivity can approach 1000 S/cm. In comparison, the conductivity of copper is approximately 5×10⁵ S/cm. Generally, the conductivity of PTs is lower than 1000 S/cm, but high conductivity is not necessary for many applications, e.g. as an antistatic film.
caption: Removal of two electrons (p-doping) from a PT chain produces a bipolaron.
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