TAUTOMERIZATION KINETICS OF 7-HYDROXY-4-METHYLCOUMARIN IN THE LOWEST EXCITED SINGLET-STATE

The biprotonic phototautomerism of 7-hydroxy-4-methylcoumarin, in the lowest excited singlet state, was evaluated fluorometrically, using steady-state kinetics. Rate constants for dissociation of the excited neutral molecule and reprotonation of the excited anion to form the neutral molecule were determined unambiguously. However, the present method allows only composite constants, containing the rate constants for prototropism of the excited zwitterion implicitly, to be evaluated. In the absence of exact information about the excited state equilibrium constant for the reactions involving the zwitterion, only limiting values of the rate constants of the latter can be estimated. However, it was possible to establish from these that, in the lowest excited singlet state, the zwitterion is a weaker acid than the neutral molecule. © 1979 American Chemical Society.