Polythiophene

Polythiophenes are polymerized thiophenes, a sulfur heterocycle. The parent PT is an insoluble colored solid with the formula _n. The rings are linked through the 2- and 5-positions. Polys have alkyl substituents at the 3- or 4-position. They are also colored 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".

Mechanism of conductivity and doping

PT is an ordinary organic polymer, being a red solid that is poorly soluble in most solvents. Upon treatment with oxidizing agents 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 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 PF₆:
In principle, PT can be n-doped using reducing agents, but this approach is rarely practiced.
Image:Polythiophenes Bipolaron.png|frame|Removal of two electrons from a PT chain produces a bipolaron.
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.

Oxidants

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 studies have shown that polys are also partially halogenated by the residual oxidizing agent. Poly 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.

Structure and optical properties

Conjugation length

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 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. Image:Polythiophenes Conjugation.png|frame|Conjugated π-orbitals of a coplanar and a twisted substituted PT. 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.
Determining the maximum effective conjugation length requires the synthesis of regioregular PTs of defined length. The absorption band in the visible region is increasingly red-shifted as the conjugation length increases, and the maximum effective conjugation length is calculated as the saturation point of the red-shift. Early studies by ten Hoeve et al. estimated that the effective conjugation extended over 11 repeat units, while later studies increased this estimate to 20 units. Using the absorbance and emission profile of discrete conjugated oligos prepared through polymerization and separation, Lawrence et al. determined the effective conjugation length of poly to be 14 units. The effective conjugation length of polythiophene derivatives depend on the chemical structure of side chains, and thiophene backbones.
The absorption band of poly in aqueous solutions of poly shifts from 480 nm at pH 7 to 415 nm at pH 4. This is attributed to formation of a compact coil structure, which can form hydrogen bonds with PVA upon partial deprotonation of the acetic acid group.
Shifts in PT absorption bands due to changes in temperature result from a conformational transition from a coplanar, rodlike structure at lower temperatures to a nonplanar, coiled structure at elevated temperatures. For example, poly undergoes a color change from red–violet at 25 °C to pale yellow at 150 °C. An isosbestic point indicates coexistence between two phases, which may exist on the same chain or on different chains. Not all thermochromic PTs exhibit an isosbestic point: highly regioregular polys show a continuous blue-shift with increasing temperature if the side chains are short enough so that they do not melt and interconvert between crystalline and disordered phases at low temperatures.

Optical effects

The optical properties of PTs can be sensitive to many factors. PTs exhibit absorption shifts due to application of electric potentials, or to introduction of alkali ions. Soluble PATs exhibit both thermochromism and solvatochromism in chloroform and 2,5-dimethyltetrahydrofuran.

Substituted polythiophenes

Polythiophene and its oxidized derivatives have poor processing properties. They are insoluble in ordinary solvents and do not melt readily. For example, doped unsubstituted PTs are only soluble in exotic solvents such as arsenic trifluoride and arsenic pentafluoride. Although only poorly processable, "the expected high temperature stability and potentially very high electrical conductivity of PT films still make it a highly desirable material." Nonetheless, intense interest has focused on soluble polythiophenes, which usually translates to polymers derived from 3-alkylthiophenes, which give the so-called polyalkylthiophenes.

3-Alkylthiophenes

Soluble polymers are derivable from 3-substituted thiophenes where the 3-substituent is butyl or longer. Copolymers also are soluble, e.g., poly.
One undesirable feature of 3-alkylthiophenes is the variable regioregularity of the polymer. Focusing on the polymer microstructure at the dyad level, 3-substituted thiophenes can couple to give any of three dyads:

2,5', or head–tail, coupling
2,2', or head–head, coupling
5,5', or tail–tail, coupling

These three diads can be combined into four distinct triads. The triads are distinguishable by NMR spectroscopy.
Regioregularity affects the properties of PTs. A regiorandom copolymer of 3-methylthiophene and 3-butylthiophene possessed a conductivity of 50 S/cm, whereas a more regioregular copolymer with a 2:1 ratio of HT to HH couplings had a higher conductivity of 140 S/cm. Films of regioregular poly with greater than 94% HT content possessed conductivities of 4 S/cm, compared with 0.4 S/cm for regioirregular POPT. PATs prepared using Rieke zinc formed "crystalline, flexible, and bronze-colored films with a metallic luster". On the other hand, the corresponding regiorandom polymers produced "amorphous and orange-colored films". Comparison of the thermochromic properties of the Rieke PATs showed that, while the regioregular polymers showed strong thermochromic effects, the absorbance spectra of the regioirregular polymers did not change significantly at elevated temperatures. Finally, Fluorescence absorption and emission maxima of polys occur at increasingly lower wavelengths with increasing HH dyad content. The difference between absorption and emission maxima, the Stokes shift, also increases with HH dyad content, which they attributed to greater relief from conformational strain in the first excited state.

Special substituents

Water-soluble PT's are represented by sodium polys. In addition to conferring water solubility, the pendant sulfonate groups act as counterions, producing self-doped conducting polymers. Substituted PTs with tethered carboxylic acids also exhibit water solubility. and urethanes
Thiophenes with chiral substituents at the 3 position have been polymerized. Such chiral PTs in principle could be employed for detection or separation of chiral analytes.
Polys is soluble in supercritical carbon dioxide Oligothiophenes capped at both ends with thermally-labile alkyl esters were cast as films from solution, and then heated to remove the solublizing end groups. Atomic force microscopy images showed a significant increase in long-range order after heating.
Fluorinated polythiophene yield 7% efficiency in polymer-fullerene solar cells.