Reversible electrochromism in polymeric metal phthalocyanine thin films

Abstract Thin polymeric films of cobalt, nickel, and palladium phthalocyanines have been prepared on optically transparent indium/tin oxide-coated glass electrodes by anodic electropolymerization. In the absence of oxygen, the electrochemical oxidation and reduction of such films is rapid, reversible, and virtually quantitative. Further, redox switching of these devices is accompanied by significant changes in their electronic absorption spectra, and the electrochromic response is readily apparent to the naked eye. For example, the first reduction of the Co derivative is a metal-centered process, resulting in the appearance of a new metal-to-ligand charge transfer transition and a striking change in color from blue-green to yellow-brown. The Ni and Pd-containing films can be easily switched between green and blue in a similar fashion. In all cases, electrochromism is essentially completely reversible in the potential range from 0 to −1.6 V vs. SCE. The magnitude of the response, as well as its speed, depends critically on the size of the electrolyte cation, suggesting that counterion motion has a strong influence on the behavior of these polymers. Other variables, including film thickness and electrolyte concentration, are also important. Switching times on the order of 1–2 s can be achieved under favorable conditions, even for large area electrodes.