Paraquat photolysis

The hydroxybenzyl radical formed is a good electron donor radical and can be oxidized by suitable oxidants. The effectiveness of such a redox process was evidenced by a series of flash photolysis experiments in which the biradical was trapped by paraquat (l,r-dimethyl-4,4 -bipyridinium) dications, PQ2+, according to the following reaction ... 

It is interesting that, on the other hand, paraquat does not undergo photolysis in aqueous solution (Slade, 1966), but adsorbed on silica gel or at the leaf surface, depending on the intensity of light, 25-50% is photolysed into 4-carboxyl-1-methylpyridinium chloride and methylamine within three weeks (Calderbank, 1968) ... 

Laser flash photolysis of o-tolyl vinyl ketone-methyl vinyl ketone copolymers [e,g., (17)] has produced some interesting observations. The triplet state of the aromatic carbonyl decays to give 1,4-biradicals that are capable of transferring an electron to an acceptor such as paraquat. In the absence of an acceptor the macroradicals decay to yield photoenols, which then tautomerize to regenerate the original polymer (Scheme 30). 

Patterson, L. K., Small Jr., R. D., and Scaiano, L. C., 1977, Reaction of paraquat radical cations with oxygen A pulse radiolysis and laser photolysis study, Radiat. Res. 72 218-225. 

Flash photolysis experiments show that [Ru(bipy)3] + ion once excited dispro-portionates into [Ru(bipy)3] + and [Ru(bipy)3]+ ions. The reaction is catalysed by oxidative quenchers like the paraquat dication, and also by reductive quenchers such as iV-dimethylaniline. ... 

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