FUSION OF HETEROCYCLIC POLYMEROGENIC UNITS ONTO A CENTRAL RING - A FRUITFUL APPROACH TO THE INVESTIGATION AND SPECIFIC TAILORING OF THE DEPENDENCE OF ELECTRICAL-PROPERTIES ON MONOMER STRUCTURE IN CONDUCTIVE POLYHETEROCYCLES

In this review the 'spacer' strategy is described and evaluated in detail. According to this principle, the monomer contains, as terminal units, two polymerogenic rings (pyrrole or thiophene) linked to a central pi-conjugatively-active frame; as spacers, we have investigated ethenylic, sulphide, and aromatic moieties. In this way the redox potentials, E0, of the polymers derived from such monomers can be calibrated by the substituent effect exerted by the spacer. This control is beneficial because it is possible to increase the E0 value of polypyrrole-type systems and decrease the E0 value of polythiophene-type systems. Also, the 'spacer' may be further functionalised and, being remote from the polymerisation site, cannot alter the conductivity characteristics typical of the polymer derived from the parent heterocycle (pyrrole or thiophene): such a functionalisation may provide the final, tailored, conductive polymer with special properties (e.g., solubility). If the central ring, acting as a 'spacer', is formed linking two positions of a di-heterocycle with a saturated chain, it is possible partially to control the twist angle between the heterocyclic units. Crystal and molecular structures have shown that dipyrrole units, further linked through the nitrogen atoms, are quite sensitive to the central ring size. With regard to the conductivity of unsubstituted polypyrrole, the conductivity of the polymers derived from such monomers is dependent upon the twist angle between the rings. This result is relevant to a description of conjugation conditions between the heterocyclic units of a monomer required to produce a conductive polymer on doping.