MOLECULAR FILMS OF THIOL-DERIVATIZED TETRAPHENYLPORPHYRINS ON GOLD - FILM FORMATION AND ELECTROCATALYTIC DIOXYGEN REDUCTION

A tetraphenylporphyrin derivative with four thiol moieties, 5,10,15,20-tetrakis[o-(2-mercaptoethoxy)-pheyl]porphyrin, H-2(o-TMEPP), has been synthesized and is shown to form molecular films via irreversible chemisorption on a variety of gold surfaces. The properties of these molecular films have been investigated by X-ray photoelectron spectroscopy (XPS) and electrochemical methods. XPS confirms the formation of a porphyrin film on the electrode and the presence of thiolate moieties bound to the gold surface. Electrochemical surface coverage determinations, although not conclusive, suggest near monolayer coverage with preferential binding of the porphyrin ring coplanar to the gold surface. Molecular films of Co(o-TMEPP) reduce dioxygen at potentials consistent with two-electron reduction to hydrogen peroxide. These films retain catalytic activity for more than 10(5) turnovers and are more active and durable than nonspecifically adsorbed cobalt tetraphenylporphyrin. Production of hydrogen peroxide was verified with a new approach based on use of gold/platinum interdigitated array electrodes where collection efficiency of electrogenerated hydrogen peroxide is as high as 90%. The feasibility of employing thiolate binding to gold to study dioxygen reduction by related, thiolated cofacial diporphyrins is discussed.