Dopant polythiophenes

As summarized in Table 8.3, both the electrochemical capacity and electroconductivity are markedly increased when the polypyrrole and polythiophene films are prepared in the ionic liquid. This may be attributable to the extremely high concentration of anions as the dopants, which results in a much higher doping level. 

For polypyrroles and polythiophenes, n is usually ca. 3 for optimal conductivity, ie. there is a positive charge on every third or fourth pyrrole or thiophene along the polymer chain, near which the dopant anion A is electrostatically attached. For polyanilines, the ratio of reduced (amine) and oxidised (imine) units in the polymer is given by the y/( 1 - y) ratio. The conducting emeraldine salt form of polyaniline has y = 0.5, i.e. there are equal numbers of imine and amine rings present. 

A characteristic feature of the parent polypyrroles, polythiophenes and polyanilines is their insolubility in water and common organic solvents (although the EB form of polyaniline is soluble in NMP, DMSO and several other solvents). This intractability and consequent difficulties in processing have until recently limited their exploitation. However, the introduction of substituents onto the aromatic rings of the polymers, the use of surfactant-like dopant anions and the generation of colloidal dispersions have markedly enhanced the processability of ICPs (see Section 8 below). 

The direct oxidative polymerization of a thiophene monomer has also been successfully employed to prepare polythiophene, for example, using FeCl3 in chloroform solvent.32 MoC15 may be similarly employed as both the oxidant and dopant.33 Subsequent reduction with ammonia of the doped polythiophene products from these FeCl3 and MoC15 oxidations provides the neutral polymer. 

The dopant molecules may furthermore enter the hose lattice in an organized fashion, giving rise to new, enlarged repeating units and corresponding new reflection peaks in the diffraction patterns. Ordered intercalation is a well known phenomenon for graphite compounds (see for instance [113]) and has also been observed for dopants in polyacetylene [114]. In the present section we shall discuss the actual situation for polythiophenes. 

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. 

Completely different monomers were called for. Before long, three of today s workhorses had been identified pyrrole, aniline and thiophene. In Japan, Yamamoto [38] and in Germany, Kossmehl [39] synthesized polythiophene doped with pentafluoroarsenate. At the same time, the possibilities of electrochemical polymerization were recognized. At the IBM Lab in San Jose, Diaz used oxidative electrochemical polymerization to prepare polypyrrole [40] and polyaniline. [41] Electrochemical synthesis forms the polymer in its doped state, with the counter-ion (usually an anion) incorporated from the electrolyte. This mechanism permits the selection of a wider range of anions, including those which are not amenable to vapor-phase processes, such as perchlorate and tetra-fluoroborate. Electrochemical doping also overcomes an issue associated with dopants... 

If the three positions of the aromatic rings in PPy or PTh are substituted with alkyl (or alkoxy) chains, polymers are obtained that are structurally similar to the parent polypyrrole or polythiophene. An example of this substitution is the series of poly(3-alkylthiophenes) (P3 AT) which exhibit the useful properties of solubility in common solvents and of enhanced environmental stability. Kawai et al who investigated the changes in the lattice vectors of poly(3-alkylthiophenes) with doping levels of BFJ, AsF and toluene sulphonyl anions, conclude that the dopant ions assume sites between the chains and are coplanar with them. In relation to polymers in adjacent planes the anions constitute intercalants, although their sites are not between neighboring pairs of thiophene rings. 

FIGURE 5.21. The minimum-energy migrational transition or jump of a BF4 dopant ion in (a) polypyrrole and (b) polythiophene. The short arrows show the positions of the structurally equivalent initial and final sites of the jump, and the defect energy curves and the r.m.s. distortions are drawn to the same scale for the two polymers in order to show their contrasting profiles. (From Ref. 132 by permission of the publishers.)... 

Some polymers (e.g., polyacetylene and polythiophene ) exhibit an additional morphological heterogeneity consisting of fibrils of dense material dispersed in a void, such that the polymer material and dopants occupy only about 50% of the volume. Although the sample is formally heterogeneous within the solid material, constituent phases are in such close proximity that not only may it be difficult to distinguish a boundary between them, but the constituents of the microdomains themselves are uncertain. Even single crystals which have been... 

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