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 electr ochromic polypyrrole films by "in-situ" spectroelectrochemical measurements in the visible region during oxidation/reduction switching. Whatever the r ate of the electrochemical reaction, or the electrochemical method used, th e energy of the maxima changes linearly with the number of positive charges stored in the chains per monomeric unit. Any possible solvatochromic, iono chromic, or thermochromic effects, related to water or counterion interchan ge or to thermal heating by the Joule effect, have been experimentally stud ied and discarded as the origin of this great shift. Only the conformationa l changes of the polymeric chains during the electrochemically induced swel ling/shrinking processes seem to be responsible for this new electrochemica l way to store and release molecular energy in a reverse way, observed in p olypyrrole.