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Aqueous solutions of the herbicidally active bipyridylium salts, paraquat dichloride and diquat dichloride, containing formate, oxalate, and benzilate anions undergo photoreduction to the bipyridylium cation radicals with high quantum efficiencies. Photoactivity occurs with light of wavelengths longer than can be absorbed by either oxidant or reductant, taken separately, and it is concluded that charge-transfer interaction between bipyridylium ion and carboxylate ion is the major activating influence. The three carboxylate anions are effective quenchers for the fluorescence of diquat suggesting a possible additional mechanism for photoreduction of the bipyridylium salts via interaction with their excited singlet states. SEVERAL types of bipyridylium salts have been shownto have interesting herbicidal properties 1 and the most important of these are exemplified by paraquat dichloride (I) (1,l '-dimethyl-4,4'-bipyidylium dichloride) and diquat dibromide (111) (6,7-dihydrodipyrido[1,2a : 2',1'-c]pyrazinedi--ium dibromide). Herbicidal activity appears to depend, in part, on the ease of (reversible) one-electron reduction of (I) and ( 111) to form stable but air-sensitive cation-radicals ( 11) and (IV) respectively.2 One-electron reduction of bipyridylium salts inay be achieved electro~hemically,~ by a variety of chemical reducing and also, as we have recently shown,6 by photolysis in the presence of primary and secondary alcohols. A particularly interesting observation from the latter type of reaction was that photoexcited bipyridylium dications react with alcohols by a primary electron-transfer process leading, in turn, to development of bipyridylium salts and their cation-radicals as new types of radical scavenger^.^^^

t-Butoxy radicals generated by photolysis of di-t-butyl peroxide, or thermolysis of di-t-butyl hyponitrite, are quantitatively reduced to t-butoxide ion by electron transfer from paraquat cation radical; the reaction affords a novel and convenient method of studying rates of homolytic fission in organic molecules.

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