Laser flash photolysis of 2,2'-dithiobis(pyridine N-oxide): Reactivity of N-oxypyridyl-2-thio radical

Reaction kinetics of radicals produced by the nanosecond laser flash photolysis of 2,2'-dithiobis(pyridine N-oxide) and related compounds have been studied. The transient absorption band at 360 nm was attributed to the radical in which the unpaired electron mainly localizes on the S atom (N-oxypyridyl-2-thio radical). The reactivities of the radical for conjugated dienes are lower than those of the pyridyl-2-thio radical, suggesting that a considerable unpaired electron density on the S atom delocalizes onto the N-oxypyridine moiety. The addition reaction rate of the radical to the conjugating diene was accelerated with hydrogen-bonding solvents and with addition of the cation, which may stabilize the N+-O- canonical structure, increasing the unpaired electron density on the S atom. By the photolysis of N-hydroxypyridine-2-thione, the NO bond was predominantly dissociated producing a pyridyl-2-thio radical, By the photolysis of its anion, photoejection took place followed by the N-O bond fission, yielding pyridine-2-thione.