Electrochemical solution and solid-state investigations on conjugated oligomers and polymers of the alpha-thiophene and the p-phenylene series

A study of the solution and solid-state electrochemical properties of defined oligomers of the alpha-thiophene and the p-phenylene series with chain lengths 2 less than or equal to N less than or equal to 16 is presented. Upon p-doping of the oligomers in the solid state their solid-state polymerization on the electrode was observed. The investigations clearly demonstrate that the important steps of the electropolymerization of conducting polymers take place exclusively in the solid state on the electrode. The degree of polymerization strongly depends on the anodic electrode potential. At low potentials polymers with long, ideally-linked chains are formed, whereas at high potentials strongly cross-linked polymers are generated. A new charge storage mechanism for conjugated oligomers and polymers is presented taking into account the morphology of the materials. We propose a two-step mechanism to explain hysteresis effects observed in voltammograms for charging processes on these materials. In the first stage of the charging process we assume that the originally twisted segments become more planar, enabling a better delocalization of charges along the chain (intramolecular stabilization). This is the prerequisite for the second step, an intermolecular stabilization through interactions between neighbouring charged segments, leading to the delocalization of the excess charges over several segments and, therefore, to further stabilization of the system. Copyright (C) 1996 Elsevier Science Ltd