Ultrafast excited-state deprotonation and electron transfer in hydroxyquinoline derivatives

Excited-state proton transfer, electron transfer, and solvent relaxation processes in N-methyl-6-hydroxyquino-linium (NM6HQ(+)) and N-methyl-7-hydroxyquinolinium (NM7HQ(+)) iodides have been investigated in both acidic and basic solutions. The hydroxyl group behaves like a superacid in the excited state, exhibiting deprotonation times in acidic aqueous solution for NM6HQ(+) and NM7HQ(+) of 2.0 and 4.5 ps, respectively. These rates for intermolecular proton transfer to water molecules are among the fastest reported to date. This high photoacidity correlates with ultrafast intramolecular electron transfer from the hydroxylate (-O-) group to the positively charged pyridinium ring, which can be observed in separate measurements in basic solution, where the ground states exist as the deprotonated neutral forms N-methyl-6-oxyquinolinium (NM6OQ) and N-methyl-7-oxyquinolinium (NM7OQ). The most obvious effect in basic solution is a Stokes shift resulting from solvent relaxation (1.0 ps), driven by the dipole moment change resulting from ultrafast excited-state electron transfer, apparently on the time scale <450 fs. Studies of such coupling between proton transfer and electron transfer offer to provide new insights into the interpretation of other phototautomerization processes.