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 (ΦT = 0.90, λexc = 355 nm), whereas pulsed excitation (λexc = 308 nm) of the thiol form in apolar
media resulted in a less efficient intersystem crossing (ΦT = 0.19) accompanied by homolytic S−H bond
cleavage (ΦS
-
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.
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