Mechanism of a photoinduced solvent-assisted transfer of a proton to a specified remote target

Stationary and dynamic properties of the fluorescence of liquid solutions of 7-hydroxy-8-(N-morpholinomethyl)-quinoline (HMMQ), 2-hydroxy-1-(N-morpholinomethyl)-naphthalene (HMMN), and 7-hydroxyquinoline (7-HQ) have been studied as a function of temperature, solvent composition, and deuteration of the OH group in the solute. The solvents were neat n-hexane, neat 1,4-dioxane, neat tetrahydrofurane, or binary mixtures with a proton-accepting component. The analysis of the observations leads to a scheme for the photochemical and photophysical behavior of HMMQ in liquid solutions that differs from previous ones. It is now clear how solvents act in the catalysis of the multistep photoinduced enol-keto tautomerization of HMMQ. There are two forms of HMMQ in the ground state when the solvent contains proton accepters (S), namely a bare form with an intramolecular H-bond and a complex in which the proton in the intramolecular H bond is also H-bonded to S. Only the latter exhibits photoinduced enol-keto tautomerization. Both forms are converted to a zwitterionic form by proton tunneling that proceeds only prior to vibrational relaxation of the excess energy deposited viith the excitation. Part of the excess energy in the complex with S is used to break the intramolecular H bond and to set the protonated side group into rotational motion. The main conclusions are summarized.