Polythiophene and derivatives

In general there are many different structures observed in polythiophenes, depending on the monomer unit, the polymerization method, and, for electrochemical preparation, the counterion and solvent. 

Polythiophenes usually form disordered layers, in particular if electrochemical deposition is used [15, 202-204]. Preparation of plasma-polymerized polythiophene leads to disordered layers with comparable structure to electrochemically prepared films [205]. 

Under the assumption that the first hyperpolarizability / (compare section 4.3.3.1) is along the chain axis of polythiophene, Kurata et al. also found by SHG (second harmonic generation) measurements, that the polymer strands stand upright on a glass substrate surface. The angle between surface normal and the chain axes is decreased by increasing the substrate temperature with 0 = 14° at Tsub = 473K. 

Spinning of end-substituted poly(3-hexylthiophene) and poly(3-octylthiophene) on HOPG leads to aggregates and to single molecules with different conformations with anti and syn arrangements of the thiophene units leading to flat lying loops, rods, and coils, as determined by STM measurements [130]. 

Side-chain substituted polythiophenes in stretched films show a lamellar structure whereby the alkyl-chains act as spacers between two-dimensional sheets formed by stacking the polymer main chains one above another. The alkyl-chains have zig-zag conformation and their planes are slightly tilted against the thiophene backbone [209]. Precipitation of poly(3-octylthiophene) from solvents with strong temperature dependent solubility leads to an orientation of the alkyl-chains perpendicular to a glass surface [210]. 

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Polyacetylene has good inert atmospheric thermal stability but oxidizes easily in the presence of air. The doped samples are even more susceptible to air. Polyacetylene films have a lustrous, silvery appearance and some flexibility. Other polymers have been found to be conductive. These include poly(p-phenylene) prepared by the Freidel-Crafts polymerization of benzene, polythiophene and derivatives, PPV, polypyrrole, and polyaniline. The first polymers commercialized as conductive polymers were polypyrrole and polythiophene because of their greater stability to air and the ability to directly produce these polymers in a doped form. While their conductivities (often on the order of 10" S/m) are lower than that of polyacetylene, this is sufficient for many applications. 

G. Tourillon and F.Garnier, Structural effect on the electrochemical properties of polythiophene and derivatives, y. Electroanal. Chem., 161 51 (1984). 

Tourillon, G., and F. Gamier. 1983. Stability of conducting polythiophene and derivatives. 

S. Glenis, G. Tourillon, F. Gamier, Photoelectrochemical properties of thin films of polythiophene and derivatives doping level and structure effects. Thin Solid Films, 122, 9-17 (1984)... 

Several attempts to use otganic polymeric semiconductors as the active component in photovoltaic devices have been reported during the last two decades. Interest in the photovoltaic properties of conjugated polymers like polyacelylcne, various derivatives of polythiophenes and poly(para-phenylene vinylene)s arose from... 

The critical role of the latter process was clearly shown in the extremely elegant work of Wegner and Riihe (1989) who measured the temperature dependence of the DC conductivity of a range of polythiophene and polypyrrole derivatives as a function of the interchain separation. The derivatives... 

Considerable attention is presently devoted to heterocyclic polymers, such as polypyrrole, polythiophene and their derivatives. The kinetics of the electrochemical doping processes of these polymers has been extensively studied in electrochemical cells using non-aqueous electrolytes. 

Rudge et al. believe that polythiophene and its derivatives are suitable materials for type HI supercapacitors. Especially, the electrochemically prepared poly-... 

The second method is the synthesis of copolymers or derivatives of a parent conjugated polymer with more desirable properties. This method is the more traditional one for making improvements to a polymer. It modifies the structure of the polymer to increase its processibility without compromising its conductivity or its optical properties. All attempts to do this on polyacetylene have failed as they always significantly reduced its conductivity. However, such attempts on polythiophenes and polypyrroles proved more fruitful. 

A major goal of the research on conducting polymers has been the development of a rechargeable plastic battery. Cells based on polypyrrole and lithium electrodes have been developed in which the energy per unit mass and discharge characteristics are comparable to nickel-cadmium cells. Current interest appears to center around stable, processable polymers, such as polythiophene and its derivatives, and polyaniline. 

Oxide, flouride, and polymeric films, as well as certain others, are used as protective coatings for HTSC materials (for example, see [505]). The electrodeposition of conducting polymers such as polypyrrole [433,491, 493, 506], polythiophene and its derivatives [493, 507], and polyaniline [478] is the most effective process. Anodic electropolymerization in acetonitrile solutions proceeds without any degradation of the HTSC substrate and ensures continuous and uniform coatings. Apparently, this method is promising not only for the fabrication of compositions with special properties based on HTSC [50, 28,295] as mentioned above, but also for the creation of junctions with special characteristics [507]. 

The performance of OFETs has continuously improved since they were first reported in 1987 [8, 9]. The rate of the progress can be visualized in Figure 14-13, where we have plotted the field-effect mobility of five prominent organic com-pounds as a lunclion of the publication date. The data include one polymer, polythiophene and its derivatives and four small molecules (three oligothiophenes, plus pentacene). Note that the highest mobility of small molecules was reported on single crystals. 

Gadisa A, Svensson M, Andersson MR, Ingantls O (2004) Correlation between oxidation potential and open-circuit voltage of composite solar cells based on blends of polythiophenes/fullerene derivative. Appl Phys Lett 84 1609... 

Besides PANI, PPy and polythiophene and their derivatives are very important families of conductive polymers due to their good properties, great chemical stability and large number of potential applications. However, although their enzymatic syntheses are not as straightforward as that of PANI some reactions pathways have been developed. 

This chapter will focus on some recent developments in the synthesis of polyacetylene itself, (CH) , and of substituted derivatives, (CR) f(CH) [24, 25]. While the great variety of other conductive polymers, including poly(phenylene vinylenes), polythiophenes and polypyrroles, may be thought of as annulated derivatives of polyacetylene [26-28], (Fig. 10-4), their syntheses and properties differ enough to put them outside the scope of this chapter. The reader is referred to a number of reviews covering both these polymers and polyacetylene [15,17, 29]. 

There were however few exceptions—the most important being polythiophene and poly(p-phenylene vinylene). It was demonstrated [4,5] that in the case of polythiophene, the substitution of hydrogen in the 3 position of the thiophene ring, by an alkyl group longer than the propyl group, renders the polymer soluble in common solvents and the decrease of conductivity in the doped state, caused by the functionalization is rather minor. Since the initial work of Elsenbaumer [4,5] hundreds of papers devoted to soluble polythiophene derivatives have been published. 

Since the initial discovery of polyacetylene doping by FeClj [92], ferric chloride still remains one of the most popular dopants for conjugated polymers. In addition, it is a very efficient oxidizing/polymerizing agent for the preparation of polypyrrole, polythiophene and their derivatives [32,84]. The most extensive Mbssbauer effect studies have therefore been carried out for FeCl, doped polymers. 

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