Solute–solvent interaction in the photoinduced tautomerization of 7-azaindole in various alcohols and in mixtures of cyclohexane and ethanol

Abstract

The evolution of the fluorescence of 7-azaindole in several alcohols, in mixtures of cyclohexane and etheanol and in mixtures of 1,4-dioxane and hexafluoropropanol has been investigated by excitation with a 25 ps (u.v.) laser pulse, followed by detection with a streak camera. We show that adiabatic keto–enol tautomerization of the compound in its lowest electronically excited singlet state in these solvents proceeds via double proton transfer in a well defined complex of a single alcohol molecule and the excited solute molecule. Unlike previous authors, we conclude that viscosity and solvent aggregation are not the important factors determining the rate of intramolecular adiabatic excited-state double proton transfer in this compound. Within the series of alcohols the variation of the rate constant for this process can be attributed satisfactorily to the variation in the acidity of the alcohols. A temperature-independent kinetic isotope effect has been observed for the complexes of 7-azaindole with ethanol (OH) and ethanol(OD). Our previous explanation for such kinetic behaviour also applies here. It involves thermally activated isotope independent formation of a specific (cyclic doubly hydrogen bounded) solute–solvent complex from an (open singly hydrogen bonded) precursor complex, followed by thermally unassisted proton transfer in the specific complex.