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?