Structural studies of polythiophenes: An ab-initio evaluation of coupling through α and β carbons

Abstract

Extensive studies have been done both fundamentally and practically on conducting polymers like polythiophene (PT). Once positively doped, PT is relatively stable in air which is due at least in part to the resonance effect ofthe sulfur which acts to stabilize a carbonium ion on the polymer chain. The drastic changes in electrical, optical and magnetic properties upon doping have attracted much attention from both theoretical and practical viewpoints.
In the undoped state, polythiophene is an insulator with a band gap of about 2.0 eV. And can be used as heterojunction together with polypyrrole (PPY). Their combination exhibits excellent rectifying characteristics. PT stable films can be prepared by either an electrochemical or a chemical method.
Like polypyrrole (PPY), neutral polythiophene as observed in the IR and Carbon 13 NMR spectra has shown that α−α carbon linkages predominate. Thus, the commonly accepted structure of PT is a linear chain of monomers bonded by α−α carbons.
The following paper is focused on the investigation of the most probable α−α carbon coupling during polymerization. The thermodynamical probabilities of linking α and β carbons in monomers through pentamers of polythiophenes are determined employing an ab-initio method.