ELECTROCHEMISTRY AND STRUCTURE OF THIN-FILMS OF (PROTOPORPHYRINATO(IX))IRON(III) CHLORIDE

The deposition and structure of (protoporphyrinato(IX))iron(III) chlor ide (abbreviated as PP(IX)Fe(III)Cl) films on highly oriented pyrolyti c graphite (HOPG) have been investigated by cyclic voltammetry, scanni ng tunneling microscopy (STM), and electrochemical quartz crystal micr obalance (EQCM) measurements. PP(IX)Fe(III)Cl films are prepared by tw o methods: (1) adsorption, yielding an electrochemically active film; (2) electrooxidative polymerization, yielding a partially inert film. STM images, in conjunction with electrochemical results, indicate that adsorption of PP(IX)Fe(III)Cl from aqueous solutions onto HOPG result s in a thin film (equivalent thickness of similar to 1-2 monolayers) t hat is comprised of 30-Angstrom-diameter molecular aggregates. In cont rast, thicker films (similar to 50 nm) prepared by electrooxidative po lymerization have a dense morphology, consistent with polymerization o ccurring by radical-cation-initiated coupling of the vinyl groups on t he protoporphyrin ring. The number of electrons transferred per molecu le deposited during electrooxidative polymerization has been measured by combined EQCM and coulometric measurements and is shown to be a str ong function of potential, with a maximum value of n similar to 8. The se results are consistent with the previously reported conclusion of M acer and Spiro that radical-cation polymerization is accompanied by me tal-centered oxidation and the six-electron oxidation of the protoporp hyrin ring to the electrochemically inert dioxoporphomethene species.