Excited-state proton transfer of 7-hydroxyquinoline in a non-polar medium: mechanism of triple proton transfer in the hydrogen-bonded system

Excited-state proton transfer (ESPT) of 7-hydroxyquinoline (7HQ) has been investigated in hexane solutions containing a small amount of alcohol (methanol, ethanol, 2-propanol, tert-butanol and 2,2,2-trifluoroethanol) by means of picosecond time-resolved fluorescence spectroscopy. Formation of hydrogen-bonded complexes with 1:2 stoichiometry, 7HQ·(alcohol)2, is concluded from measurements and analysis of steady-state absorption spectra. The formation of cyclic complexes in the ground state is supported by semi-empirical MO calculations. The rate constants of ESPT within the complexes are found to increase with increasing acidity of the alcohols. The mechanism of triple proton transfer is discussed on the basis of the Kamlet–Taft acidity and basicity scales of alcohols.