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

A tetraphenylporphyrin derivative with four thiol moieties, ,20-tetrak is[o-(2-mercaptoethoxy)-pheyl]porphyrin, H-2(o-TMEPP), has been synthe sized and is shown to form molecular films via irreversible chemisorpt ion on a variety of gold surfaces. The properties of these molecular f ilms 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, alt hough not conclusive, suggest near monolayer coverage with preferentia l binding of the porphyrin ring coplanar to the gold surface. Molecula r films of Co(o-TMEPP) reduce dioxygen at potentials consistent with t wo-electron reduction to hydrogen peroxide. These films retain catalyt ic activity for more than 10(5) turnovers and are more active and dura ble than nonspecifically adsorbed cobalt tetraphenylporphyrin. Product ion of hydrogen peroxide was verified with a new approach based on use of gold/platinum interdigitated array electrodes where collection eff iciency of electrogenerated hydrogen peroxide is as high as 90%. The f easibility of employing thiolate binding to gold to study dioxygen red uction by related, thiolated cofacial diporphyrins is discussed.