Water-soluble, diamagnetic metalloporphyrins have been prepared which contain either zinc@), palladium(@ or tin(1v) 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(m), may result in formation of the metalloporphyrin n-radical cation in quite high yield. These n-radical cations are powerful oxidants, in some cases Eo > 1 y us NHE, but they undergo secondary reactions that lead to formation of n-dications and isoporphyrins. Despite the high redox potentials, it has not been possible to couple the one-electron reduction of the n-radical cations to the four-electron oxidation of water to molecular oxygen, even in the presence of a redox catalyst such as RuO,. Therefore these compounds appear to possess little promise as water oxidants in homogeneous photosystems.
Irradiation of tris(2,2'-bipyridyl)ruthenium(11j in aqueous solution containing a sacrificial electron acceptor results in formation of bipy3Ru3+, a powerful oxidant which is able, in the presence of a redox catalyst, to oxidise water to 02. Both colloidal RuOz and simple cobalt(r1) ions are effective catalysts for O2 production and, with [ C O ( N H ~) ~C ~] ~' as sacrificial electron acceptor, the quantum efficiency for formation of O2 was ca. 12%. However, when the sacrificial electron acceptor was replaced with Fe3 -, which is a reversible redox couple, no O2 was observed upon prolonged irradiation. The absence of O2 in this latter system can be explained by kinetic considerations.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.