Characterization and reactivities of transient species generated by laser flash photolysis of N-hydroxypyridine-4-thione

The reaction intermediates produced by the nanosecond-laser-flash photolysis of N-hydroxypyridine-4-thione (4-HOPyT) have been studied in aqueous and nonaqueous solutions. The pyridlyl-4-thiyl radical (4-PyS. at 420 nm) and (OH)-O-. are formed by the homolytic N-OH bond cleavage of 4-HOPyT. The 720-nm band was assigned to the pyridyl-N-oxyl radical (4-S=PyO.) generated by the photodissociation of the O-H bond and by H abstraction from 4-HOPyT through (OH)-O-.. The triplet state of 4-HOPyT [(3)(4-HOPyT)*], confirmed by energy transfer with triplet quenchers, is also produced concomitantly with the radical species. Diffusion controlled electron-transfer reactions occur between (3)(4-HOPyT)* and donors (or accepters), which may be caused by the relatively high triplet state energy (E-T1 = 60.1 kcal mol(-1)) of (3)(4-HOPyT)*. In aqueous alkaline solution, the solvated electron (e(aq)(-)) and the one-electron semioxidized radical of 4-HOPyT (4-S=PyO.) are produced by photoionization of the anionic form of 4-HOPyT. Thus, it is possible to control the generation of (OH)-O-. and e(aq)(-) by changing the pH of the solution.