Potential-driven Conductivity of Polypirroles, Poly-N-alkylpyrroles and Polythiophenes: the Role of the Pyrrole NH Moiety in the Doping-charge Dependence of Conductivity

The in situ conductivity vs p-doping charge of low-defect polypyrroles, N-substituted polypyrroles, and polythiophenes has been investigated in acetonitrile in the presence of the weakly coordinating perchlorate ion as supporting electrolyte. In-situ ESR and EQCM measurements have given supporting information on polymer structure and conduction carriers. The structures of the polymers cover a wide range of conjugative, geometrical, and solvation conditions, but the conductive pattern follows simply the polymer ring type (pyrrole, N-substituted pyrrole, or thiophene). In polythiophenes an initial region of low conductivity, due to strongly spin-dimerized polarons, is followed by an increase of conduction to a plateau of high conductivity. N-substituted polypyrroles display a linear increase of conductivity with charge followed by a plateau of conductivity. Polypyrroles without N-substitution show an increase of conductivity to a maximum followed by a symmetrical decrease to zero at a charge corresponding to one bipolaron per tetrapyrrole unit. A redox-type bipolaron model of conduction, based on stabilization of the bipolaron positive charge by H-bonding with the counteranion, is suggested. The parent polypyrrole shows the uncomplicated conductivity pattern (increase of conductivity to a plateau) due to a uniquely strong stabilization of the pi-stacked polymer chains.