FEMTOSECOND PICOSECOND LASER PHOTOLYSIS STUDIES ON THE MECHANISMS OF FLUORESCENCE QUENCHING INDUCED BY HYDROGEN-BONDING INTERACTIONS - 1-PYRENOL-PYRIDINE SYSTEMS

The mechanisms of strong fluorescence quenching of 1-pyrenol (PyOH) caused by hydrogen-bonding interaction with pyridine (P) and various methyl-substituted pyridines (MeP's) have been investigated by means of fluorescence and femtosecond-picosecond laser spectroscopic measurements and compared with those of 1-aminopyrene (AP)-P and-MeP systems studied previously.7,10,11 Femtosecond-picosecond time-resolved transient absorption spectral measurements on PyOH-P and -MeP systems in hexane show rapid decay of locally excited (LE) state (D*-H...A) without formation of detectable intermediate state, in contrast to the case of AP-P and -MeP systems where rapid establishment of equilibrium between LE and electron transfer (ET), (D+-H...A-), states occurs in the excited state. On the basis of the fact that the decay time of the LE state of PyOH-P and -MeP systems depends on the free energy gap for the LE --> ET reaction, it has been concluded that the decay of the LE state is due to the reaction LE --> ET, which is realized even in a nonpolar solvent assisted by hydrogen-bonding interactions and by a slight shift of the D-H proton toward A, further facilitating ET. Moreover, immediately after ET a large scale and ultrafast proton shift takes place which induces a large destabilization of the ground state (GS) leading to an ultrafast nonradiative ET --> GS ''switchover'', contrary to the case of AP-P and -MeP systems where the proton shift in the excited state remains moderate.