Electrochromism of Transition Metal Phthalocyanine Thin Films

Abstract

Thin films (500-4500 (ANGSTROM) thick) of Fe(II), Co(II), Ni(II), and Cu(II), and Zn(II) phthalocyanines on gold or indium oxide electrodes undergo stoichiometric oxidation and rereduction. Excepting FePc and CoPc, the process is essentially reversible. Chronocoulometry showed that ZnPc films oxidized to the extent of 1.21 electrons per ZnPc molecule; CoPc required 1.92 electrons per molecule. Charge compensation is attained upon oxidation by uptake of anions from the electrolyte and by expulsion of anions upon reduction. Auger electron spectrometry allowed detection of the ions and characterization of their distributions. In partially oxidized films, the anions are homogeneously distributed. Oxidation seems to proceed at all grains with equal probability, with anions entering and departing along grain boundaries. Smaller anions allow full oxidation at rapid rates; larger ones inhibit the oxidation with respect to rate. Optical spectroscopy showed evidence for reorganization of the crystalline lattices. The rereduced form is not the same as the original material, but it can be restored to the original form by annealing at 125(DEGREES). In cyclic oxidations and rereductions, there is a gradual loss of charge-consuming ability, apparently related to electrical isolation of small domains, perhaps grains. The oxidations and rereductions are electrochromic, and the various color changes are described.