Excited-state Reactions of Coumarins in Aqueous Solutions. I. The Phototautomerization of 7-Hydroxycoumarin and Its Derivative

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Abstract
The kinetics and mechanism of the reaction in the photo-excited state of 7-hydroxycoumarin and its derivative have been studied in aqueous solutions by means of measuring the fluorescence spectra and the fluorescence lifetime. The four fluorescence bands have been identified as emissions from the neutral, anionic, tautomeric, and cationic forms of 7-hydroxycoumarins. The quantitative analysis was performed by using the rate equation which included the term for the direct conversion of the excited neutral molecule to its tautomer in addition to the usual reaction term of dissociation. The rate constants of the tautomerization and dissociation were discussed in detail, and it was revealed that the tautomerization in aqueous solutions was mainly caused by the transfer of a proton between two active sites in the molecule. Such an intramolecular proton-transfer of the coumarins, found first in this paper, is considered to be promoted by water molecules with a hydrogen bond surrounding the excited molecule.