UV-visible spectroelectrochemistry of conducting polymers. Energy linked to conformational changes

Large and reverse hypsochromic and bathochromic shifts on either polaronic (1.29 eV) or bipolaronic (0.68 eV) bands have been determinated from electrochromic polypyrrole films by "in-situ" spectroelectrochemical measurements in the visible region during oxidation/reduction switching. Whatever the rate of the electrochemical reaction, or the electrochemical method used, the energy of the maxima changes linearly with the number of positive charges stored in the chains per monomeric unit. Any possible solvatochromic, ionochromic, or thermochromic effects, related to water or counterion interchange or to thermal heating by the Joule effect, have been experimentally studied and discarded as the origin of this great shift. Only the conformational changes of the polymeric chains during the electrochemically induced swelling/shrinking processes seem to be responsible for this new electrochemical way to store and release molecular energy in a reverse way, observed in polypyrrole.