Picosecond dynamics of the photoexcited 6-methoxyquinoline and 6-hydroxyquinoline molecules in solution

The photophysical properties of 6-methoxyquinoline (6MeOQ) and 6-hydroxyquinoline (6HQ) in organic solvents and in neutral, acidic, and alkaline aqueous solutions have been studied by picosecond transient absorption spectroscopy. The dynamics of photoinduced tautomerization by excited state proton transfer(s) arising in the case of 6HQ has been analyzed. Hydroxyl deprotonation of the cationic (quinolinium) form in acidic solution (pH 1.5) is 1 order of magnitude faster (dp ~ 2.2 ps) than imine protonation of the anionic (phenolate) form in 1 M KOH alkaline solution (p ~ 30 ps). In neutral solution, our results suggest a two-step process of initial proton release from the hydroxyl group followed by proton capture by the nitrogen atom with characteristic times close to the dp and p values, respectively.