Polythiophenes A Chance for Maximum Conductivity

By integrating the polyacetylene moiety into a polyheterocyclic system, the electron donating function of N or S ring atoms stabilizes the conjugated system, especially in the doped, positively charged highly conductive state (bipolaron state). 

Mesomeric charge delocalization in O-substituted, doped polythiophene moiety. 

The first mention of polythiophenes as potential conducting polymers can be found in the literature in 1967. A. G. Davies and coauthors investigated the acid catalyzed polymerization of furan, pyrrole, and thiophene hetero-cycles. Although this polymerization reaction was known before, several new and interesting facts could be elucidated. Besides the confirmation of the cyclic structure of the polymer units (instead of ring opened), the electric conductivity was investigated. In contrast to the already known types of electronically conducting polymers (polyaniline and polypyrrole, see Chapter 1), the conductivity of all polyheterocycles in Armour s paper, isolated as trichloro- or trifluoroacetates (triflate), is ionic. In the case of the polythiophene, ion pairs formed by the reaction of polythiophene with trifluoro acetic acid dissociated in methylene chloride solution to the protonated polymer and triflate anions. The polythiophene triflate decomposed at relatively low temperatures (60°C-80°C).  

Tourillon and Gamier first observed true electronic conductivity in polythiophenes in 1982. Thiophene was electropolymerized on platinum electrodes in acetonitrile with perchlorate or tetrafluoroborate counterions. A remarkable conductivity of 10-100 S/cm was obfained. By this experiment, a new era of polyfhiophene chemistry had started, and within a few years polythiophene(s) belonged to the most prominent classes of polymers with electric conductance. Polythiophenes obviously had an enormous potential for technical applications. 

We will see in the following section that some of these findings can be transferred, with several constraints, to oxygen-bearing substituted thiophenes. But besides various analogies there is a remarkable exception. 

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