Sensitized Photo-Reduction of Methyl Viologen by Metalloporphyrins.

We report the sensitized photo-reduction of methyl viologen by a series of porphyrins and metalloporphyrins, by visible light, in the presence of triethanolamine. Electrochemical studies and absorption and emission spectra are used to identify the redox potentials of the relevant photo-excited states and to calculate thermodynamic driving forces for quenching and subsequent thermal reactions. Electron spin resonance data are used to confirm the occurrence of oxidative andor reductive quenching in these various systems. In general, the quantum yields for methyl viologen reduction parallel the thermodynamic driving force in a regular manner. The complex L OAc RhIIITPP L equal solvent molecule is shown to be an effective catalyst, photoreducing methyl viologen via an oxidative pathway with a quantum yield as high as 51. LIfetimes and quenching constants are also reported for these species. Author