Polythiophene and Related Polymers

Some film-cycling experiments were conducted on the thicker polythiophene films grown at higher potentials, and these were notable for the observation that film thickness decreased markedly on oxidation. This behavior is remarkably consistent with that discussed earlier for polypyrrole, and the reasons for it are doubtlessly similar. 

Other workers have studied polythiophene electropolymerization with two-parameter ellipsometry. The importance of experimental conditions in the 

SIMON J. HIGGINS, PAULA. CHRISTENSEN, AND ANDREW HAMNETT 

Although much of the earlier work on the infrared spectroscopy of conducting polymer films was carried out on polythiophene in situ work has been rather sparser. The main conclusion from this latter work was identification of carrier type from an examination of the types of electronic and IRAV band seen on oxidation of the polymer. Interestingly the intensities of IRAV and electronic bands do not generally track each other on oxidation, in spite of the fact that theoretical analysis of the intensity of the IRAV band shows that  

It is clear that a plot of l/ i versus / e should be linear with slope and an 

Sulfur-ojqrgen interactions have been proposed as a possible cause for the considerable stability of polyalkojQfthiophenes, based on ex situ investigations of a large selection of substituted thiophenes and their polymers [843]. The amplification of chemical and electrical signals has been applied in a microelectrochemical transistor based on poly(3-methylthiophene) [844]. 

Polymer blends and composites of polythiophenes with other nonconducting polymers, which are outside the scope of this review, have been studied elsewhere [864-866]. The formation of copolymeric gels (usually considered to be an inconvenience in polymer formation) has been studied with 3-n-octylthiophene, and various substituted benzenes were reported by Pepin-Donat et al. [867]. 

An interesting bridge between charge transfer salts and conducting polymers was suggested by Miller et al. [868]. They investigated a substituted terthiophene cation radical that forms tt stacks, which show electric conductivity without being a true polymer. 

General reviews on PT and related derivatives have been provided elsewhere [869-871]. 

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Properties of Polythiophene and Related Polymers and their Electrochemically... 

Related Polymer Systems and Synthetic Methods. Figure 12A shows a hypothetical synthesis of poly (p-phenylene methide) (PPM) from polybenzyl by redox-induced elimination. In principle, it should be possible to accomplish this experimentally under similar chemical and electrochemical redox conditions as those used here for the related polythiophenes. The electronic properties of PPM have recently been theoretically calculated by Boudreaux et al (16), including bandgap (1.17 eV) bandwidth (0.44 eV) ionization potential (4.2 eV) electron affinity (3.03 eV) oxidation potential (-0.20 vs SCE) reduction potential (-1.37 eV vs SCE). PPM has recently been synthesized and doped to a semiconductor (24). 

As explained in the introduction, the polysilanes (and related polygermanes and poly-stannanes) are different from all other high polymers, in that they exhibit sigma-electron delocalization. This phenomenon leads to special physical properties strong electronic absorption, conductivity, photoconductivity, photosensitivity, and so on, which are crucial for many of the technological applications of polysilanes. Other polymers, such as polyacetylene and polythiophene, display electron delocalization, but in these materials the delocalization involves pi-electrons. 

The preparation of polypyrrole, polythiophene, polyaniline, and related conducting polymers demonstrates principles of electrochemical synthesis that are more widely applicable, and it is instructive to examine these in detail. 

The conditions for polymerization were also foimd to be crucial in relation to polythiophene and polybithiophene films [58,80,84,114-121], The relatively high potential required for the oxidation prevents the use of many metallic substrates. The electrochemical oxidation of substituted thiophenes and thiophene oligomers yields conducting polymers, and these compounds can be electropolymerized at less positive potentials, so it is a good strategy to use these derivatives instead of thiophene (see Sect. 2.2.6). Another approach is the deposition of a thin polypyrrole layer that ensures the deposition of polythiophene on these substrates (eg., Ti, Au) [115]. Interestingly, other polymers as well as copolymers and composites (see Chap. 2) can also be synthesized. 

The redox behavior of polythiophene and substituted polythiophenes (mainly 3-alkyl substituted) is closely related to that of polypyrrole, as might be expected. The cyclic voltammogram of polythiophene [42] shows that oxidation of the polymer occurs at 1.0 V versus SCE whereas reduction occurs at 0.9 V. Past 1.71 V, another peak appears, and if the potential of the film is taken beyond this value, deactivation of the film occurs. But in another aspect polythiophene differs from polypyrrole. It shows better redox activity when there is a substituent in the ring. In fact, the processibility (ability to spin cast films etc.) also improves, especially if there are hexyl or octyl groups substituted at the 3-position. 

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