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 i nvestigated in hexane solutions containing a small amount of alcohol (metha nol, ethanol, 2-propanol, tert-butanol and 2,2,2-trifluoroethanol) by means of picosecond time-resolved fluorescence spectroscopy. Formation of hydrog en-bonded complexes with 1:2 stoichiometry, 7HQ.(alcohol)(2), is concluded from measurements and analysis of steady-state absorption spectra. The form ation of cyclic complexes in the ground state is supported by semi-empirica l MO calculations. The rate constants of ESPT within the complexes are foun d to increase with increasing acidity of the alcohols. The mechanism of tri ple proton transfer is discussed on the basis of the Kamlet-Taft acidity an d basicity scales of alcohols. (C) 2000 Elsevier Science B.V. All rights re served.