Polythiophene

Polythiophenes (PTs) are polymerized thiophenes, a sulfur heterocycle. They are white solids with the formula (C4H2S)n for the parent PT. The rings are linked through the 2- and 5-positions. Poly(alkylthiophene)s have substituents at the 3- or 4-position. They are also white solids, but tend to be soluble in organic solvents. Polythiophenes (PTs) are polymerized thiophenes, a sulfur heterocycle. They are white solids with the formula (C4H2S)n for the parent PT. The rings are linked through the 2- and 5-positions. Poly(alkylthiophene)s 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. Heeger, Alan MacDiarmid, and Hideki Shirakawa 'for the discovery and development of conductive polymers'. 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 optimmally 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 PF6: 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. A variety of reagents have been used to dope PTs. Iodine and bromine produce highly conductive materials, which are unstable owing to slow evaporation of the halogen. Organic acids, including trifluoroacetic acid, propionic acid, and sulfonic acids produce PTs with lower conductivities than iodine, but with higher environmental stabilities. Oxidative polymerization with ferric chloride can result in doping by residual catalyst, although matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) studies have shown that poly(3-hexylthiophene)s are also partially halogenated by the residual oxidizing agent. Poly(3-octylthiophene) dissolved in toluene can be doped by solutions of ferric chloride hexahydrate dissolved in acetonitrile, and can be cast into films with conductivities reaching 1 S/cm. Other, less common p-dopants include gold trichloride and trifluoromethanesulfonic acid. The extended π-systems of conjugated PTs produce some of the most interesting properties of these materials—their optical properties. As an approximation, the conjugated backbone can be considered as a real-world example of the 'electron-in-a-box' solution to the Schrödinger equation; however, the development of refined models to accurately predict absorption and fluorescence spectra of well-defined oligo(thiophene) systems is ongoing. Conjugation relies upon overlap of the π-orbitals of the aromatic rings, which, in turn, requires the thiophene rings to be coplanar. The number of coplanar rings determines the conjugation length—the longer the conjugation length, the lower the separation between adjacent energy levels, and the longer the absorption wavelength. Deviation from coplanarity may be permanent, resulting from mislinkages during synthesis or especially bulky side chains; or temporary, resulting from changes in the environment or binding. This twist in the backbone reduces the conjugation length, and the separation between energy levels is increased. This results in a shorter absorption wavelength.