Selective photoexcitation of the thione and thiol forms of N-hydroxypyridine-4(1H)thione:: A tautomeric heteroaromatic system

The strong solvent-dependence of the tautomeric equilibrium exhibited by N-hydroxypyridine-4(1H)thione (4-NHPT) and the distinct absorption properties of the thione and thiol tautomers offer the rare opportunity to investigate the photochemistry of each form independently. Laser flash photolysis studies have revealed that triplet state formation was the main photoprocess undergone by the thione tautomer of 4-NHPT in protic solvents (Phi(T) = 0.90, lambda(exc) = 355 nm), whereas pulsed excitation (lambda(exc) = 308 nm) of the thiol form in apolar media resulted in a less efficient intersystem crossing (Phi(T) = 0.19) accompanied by homolytic S-H bond cleavage (Phi(S-H) = 0.24). The latter process leads to the production of the N-oxy-4-pyridinethiyl radical (4-PyNOS .). In most organic solvents, however, the thione and thiol tautomers of 4-NHPT coexist. Under these conditions, the form absorbing the excitation light was observed to undergo the expected primary photochemistry followed by secondary processes involving reaction with the ground state of the nonexcited tautomer (i.e., electron transfer from the thione triplet state to the thiol form and addition of 4-PyNOS to the carbon-sulfur double bond of the tautomeric thione). Contrary to the closely related N-hydroxypyridine-2(1H)-thione, 4-NHPT was not found to be a primary photochemical precursor of hydroxyl radicals ( OH) in organic solvents. The results obtained in this work are discussed in terms of structure/photoreactivity and with regard to the reported photobiological effects of N-hydroxypyridinethiones.