Tautomerism of 1-hydroxy-2-pyridone and 1-hydroxypyridine-2-thione in the excited triplet state

Abstract

Both the title compounds show dual phosphorescence depending on the excitation wavelength and the nature of the solvents at 77K. 1-Hydroxy-2-pyridone (HP) gives dual emission only in a nonpolar matrix, whereas dual emission of 1-hydroxypyridine-2-thione (HPT) was observed even in polar and hydrogen-bonding solvents as well as in a nonpolar matrix. From a comparison with the phosphorescence behaviour of 2-ethoxypyridine 1-oxide and 2-ethylthiopyridine 1-oxide, which have structures corresponding to the enol and enethiol isomers of the tautomeric HP and HPT, respectively, it is concluded that tautomerization taking place in the excited triplet state is responsible for the dual phosphorescence of the title compounds. Further supporting evidence for this conclusion is obtained from the measurements of the phosphorescence lifetimes and phosphorescence excitation spectra. HPT is much more subject to tautomerization to its enethiol isomer form than HP. This is explained on the basis of the fact that the thiocarbonyl group is more polar and more polarizable than the carbonyl group.