Dye‐sensitized Photoreduction of Methyl Viologen

The kinetics of the proflavine‐sensitized photoreduction of methyl viologen (MV 2+ ) to the blue radical cation (MV . + ) are presented. The triplet excited state of proflavine accepts an electron from EDTA to form the singly‐reduced species of proflavine (PH . ); this species donates an electron to either the oxidized (MV . + ) or the singly–reduced (MV . + ) species of the bipyridyl. MV 2+ can be reoxidized by oxygen to MV 2+ but is decomposed by H 2 O 2 . The doubly‐reduced form can not be reoxidized either by oxygen or by peroxide. Potassium iodide inhibits the photoreduction of MV 2+ by competing with it as reactant for the singly‐reduced form of proflavine (PH . ). The mechanism presented may be analogous to that occurring in the reduction of MV 2+ by illuminated spinach chloroplasts; its herbicidal action can not be ascribed to the formation of peroxides.