Cation radicals: photochemically induced electron-transfer reactions between bipyridylium dications and formate, oxalate, and benzilate anions

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.