Geometrical and Electronic Structure Characterization of Conducting Hydroquinone Substituted Polypyrrole

Abstract: The fundamental electronic and geometric structures of charged polypyrrole and a hydroquinone substituted polypyrrole, poly(pyrrol-3-ylhydroquinone), were investigated. A single electron was constrained over different sizes of the polymer chains, a polaron, with the purpose of finding the polaron size which minimizes the excitation energy. This was done using constrained density functional theory (CDFT) by the use of a relatively newly developed source code. The polaron size for polypyrrole was not found but could be estimated to exist for a periodic chain of about sixty monomers with the polaron stretching over the entire polymer chain. The polypyrrole with changed head groups, on the other hand, seems to possess a polaron size stretching over two monomers with a periodic structure of two monomers. This indicates that this particular material could have up to thirty times better storage capacity as compared to polypyrrole. The investigation was restricted to single electron constraints, so the possibility of an even better storage capacity exists. Investigation of the band structure indicates that the charged systems possess a similarity to a doped n-type material with an occupied impurity level inside the band gap. Geometrical structures seems to transition from an aromatic form to a quinoid form upon being charged.