TAUTOMERISM OF 1-HYDROXY-2(1H)-PYRIDINETHIONE - EFFECTS OF TRIPLET QUENCHERS AND DEUTERIUM SUBSTITUTION ON THE PHOSPHORESCENCE BEHAVIOR

In order to clarify the mechanism of tautomerization of 1-hydroxy-2(1 H)-pyridinethione (HPT) giving 1-pyridinethiol 1-oxide, we investigated the effects of triplet quenchers and deuterium substitution on the phosphorescence behavior of a tautomeric HPT molecule at 77 K. The finding that both 2,4-hexadien-1-ol and naphthalene quench the phosphorescence of enethiol isomer while the thione isomer phosphorescence is not affected by these quenchers (Figs. 1 and 2) demonstrates that the tautomeric equilibrium between the thione and enethiol isomers cannot be established during the triplet-state lifetime of the thione isomer. This finding coupled with a consideration of the relative triplet-state energy of both tautomers makes the tautomerization in the excited triplet state (Scheme 2), proposed in our previous study (Ref. 2), less likely. Deuterium substitution of the N-hydroxyl hydrogen in HPT suppresses the tautomerization to the enethiol isomer (Figs. 4 and 5), whereas this substitution affects the UV spectrum of HPT to a negligible extent. Although this observation is consistent with the mechanism in which the ground-state enethiol isomer is generated photochemically from the thione isomer in excited states (Scheme 3), it is difficult for this mechanism to explain the effect of photoirradiation on the phosphorescence intensity ratio of two tautomers. From both an analysis of temperature and deuterium isotope effects on the UV spectrum of HPT and a comparison of the UV and fluorescence spectra of HPT with those of 2-ethylthiopyridine 1-oxide, it was found that the phosphorescence quantum yield of enethiol isomer should be much larger than that of thione isomer, and thus that the ground-state tautomeric equilibrium (Scheme 4), which lies practically completely in favor of the thione isomer, is reflected much more remarkably in the phosphorescence behavior than in the fluorescence behavior.