Photo-oxidation of metalloporphyrins in aqueous solution

Water-soluble, diamagnetic metalloporphyrins have been prepared which contain either zinc(II), palladium(II) or tin(IV) ions as the central metals and their photophysical properties have been measured in dilute aqueous solution. All the compounds undergo efficient intersystem crossing to form long-lived excited triplet states that can participate in electron-transfer reactions. Thus excitation of the metalloporphyrin in dilute aqueous solution containing an appropriate electron acceptor, such as iron(III), may result in formation of the metalloporphyrin π-radical cation in quite high yield. These π-radical cations are powerful oxidants, in some cases E° > 1 V vs NHE, but they undergo secondary reactions that lead to formation of π-dications and isoporphyrins. Despite the high redox potentials, it has not been possible to couple the one-electron reduction of the π-radical cations to the four-electron oxidation of water to molecular oxygen, even in the presence of a redox catalyst such as RuO2. Therefore these compounds appear to possess little promise as water oxidants in homogeneous photosystems.