PHOTOREDUCTION OF METHYLENE BLUE BY WATER*

Abstract— The photoreduction of methylene blue in acid solution has been studied electrochemically. It has been found that methylene blue photoreduction proceeds in red light, with only water available as reducing agent. Reaction energy considerations require two photons to reduce each methylene blue molecule. A two-photon mechanism involving a long-lived dimer intermediate is proposed, and is shown to lead to the observed kinetics, including the dependence of rate on light intensity and methylene blue concentration.