PHOTOCHEMISTRY OF N-HYDROXYPYRIDINE-2 THIONE DERIVATIVES - INVOLVEMENT OF THE 2-PYRIDYLTHIYL RADICAL IN THE RADICAL-CHAIN REACTION-MECHANISM

The primary and subsequent photochemistry of four N-hydroxypyridine-2-thione esters has been investigated by laser flash photolysis (lambda(exc) = 355 nm). A simple, high-yield synthetic method is given for their preparation with high purity. UV irradiation of these ester compounds was shown to lead specifically to the formation of benzyl, diphenylmethyl, tert-butyl, and benzoyloxyl radicals in addition to the 2-pyridylthiyl radical. in all cases, the initial photoinduced nitrogen-oxygen bond cleavage was found to occur in high quantum yield (Phi(N-O) approximate to 0.5) The radical species generated by this process (2-pyridylthiyl radical and carbon-centered or oxygen-centered radicals) were characterized and their reactivity toward several radical scavengers has been studied. An efficient delayed depletion of the N-hydroxypyridine-2-thione esters was also observed, leading to overall bleaching quantum yields, (Phi(B1), close to unity. We have demonstrated that the delayed consumption of ground-state ester was due to the reaction of the 2-pyridylthiyl radical with its precursor, occurring with a rate constant, k(r), of 3-4 x 10(9) M(-1) s(-1). This reaction, hitherto never proposed, leads to the formation of 2,2'-dipyridyl disulfide and further release of propagating radicals.