40. Read the paper entitled Synthesis and Properties of Low-Bandgap Zwitterionic and Planar Conjugated Pyrrole-Derived Polymeric Sensors. Reversible Optical Absorption Maxima from the UV to the Near-IR by T. W. Brockmann and J. M. Tour (J. Am. Chem. Soc., 1995, 117, 4437-4447). Give an explanation about why polypyrrole is a semiconductor. Why does zwitterion formation have such an important effect on the semiconducting properties?
Polypyrrole has extended π conjugation along the polymer backbone. The overlap of the π orbitals forms an energy band that is filled while overlap of the π* orbitals creates an empty conduction band at only modestly higher energies. The band gap is relatively small, so polypyrrole is a semiconductor. Oxidative doping can remove electrons from the valence band, significantly improving the electrical conductivity by creating positive charge cariers, i.e., a p-type semiconductor.
Zwitterion formation puts positive and negative charges along the polymer backbone. This alters the degree of overlap between π and π* orbitals, thereby modifying the band properties of the polymer.