Organic conductor

(materials) A two-component material containing anion and cation charged species originating from a charge transfer between two inorganic molecules or between one organic molecule and one inorganic ion.

An organic substance with low electrical resistance. Two major classes of organic conductors are charge-transfer compounds and conducting polymers.

Charge-transfer compounds

The search for organic conductors in the early 1970s led to the observation of metallic-like electrical conduction in well-ordered molecular crystals and the discovery of many new phenomena such as the stabilization of charge-density waves and spin-density waves, new mechanisms for electronic transport, organic superconductivity, and new states of matter produced under strong magnetic fields. Most of these phenomena are due to the low dimensionality (one or two dimensions) of the electron gas in the charge-transfer compounds where they were observed. Among these properties, superconductivity has created much interest since the zero-resistance state is now observed at temperatures as high as 10 K (−442°F) in certain organic superconductors.

Charge-transfer compounds are two-component materials containing anionic and cationic species originating by charge transfer between donor and acceptor entities; these may be two organic molecules or an organic molecule with an inorganic ion. Tetrathiafulvalene-tetracyanoquinodimethane (TTF-TCNQ) is the prototype of charge-transfer organic crystals. Its crystal structure exhibits piled-up segregated columns of donor TTF and acceptor TCNQ molecules (illus. a). In the solid state, the amount of charge transferred from donor to acceptor is determined by the overall crystal stability.

Stacking of charge-transfer compounds, (a) Segregated stacking of TTF-TCNQ-like materials, (b) Typical zigzag stacking of the (TMTSF)₂X series.

Another class of organic conductors is exemplified by radical cation salts such as (TMTSF)₂X where the organic molecule is tetramethyltetraselenafulvalene and X is an inorganic anion. In this class of materials (Bechgaard salts) the molecules display a zigzag packing along the stacking axis (illus. b), where one positive charge (hole) is shared between two organic molecules.

The strong overlap between electron clouds of neighboring molecules along the stacks spreads the partially filled molecular electronic states into an energy band 0.5–1 eV wide. This bandwidth is large enough to allow electron delocalization among all molecules on a given stack and to promote electrical conduction similar to that in metal crystals. See also Delocalization; Molecular orbital theory.

Conducting polymers

Polymeric materials are typically considered as insulators. However, research since the late 1970s has led to the discovery of polymeric materials with extremely high conductivity, approaching that of copper. The prospect of materials combining the properties of plastics and metals or semiconductors has led to a search for applications, made attractive because improved polymers no longer suffer from such drawbacks as low stability, processing difficulties, and brittleness. Most conducting polymers can be switched reversibly between conductive and nonconductive states, with the result that their conductivities can span an enormous range. This switching is accomplished through oxidation-reduction (redox) chemistry, the conductivity being sensitive to the degree of oxidation of the polymer backbone. This property distinguishes conducting polymers from metals and semiconductors and is the basis of many existing and potential applications. In addition, certain polymers become conducting upon oxidation or reduction and thus can exhibit p- or n-type conduction. See also Oxidation-reduction; Polymer.

In a conducting polymer an oxidant removes electrons from the π-electron system of the polymer, creating radical cations that, at high concentrations, dimerize to form cation pairs known as bipolarons. Charge-balancing counterions are concomitantly incorporated between polymer chains. The overall process is referred to as doping, and the counteranion (or countercation in the case of reduction) is the dopant.

Considerable progress has been made on the theory of important parameters such as oxidation potentials, band gaps, and band widths, often with good agreement with experiment. Such work is important for the design of new conductive polymers with specific properties.