Chapter 39 - Excited-state proton transfer dynamics of 6-hydroxyquinoline in acidic and alkaline aqueous solutions

Abstract

The amphoteric 6-hydroxyquinoline (6HQ) molecule has a weak acidic hydroxyl function (pKA = 9.2) and a weak basic imine function (pKA = 5.1) in the ground state. A cationic (quinolinium) form 6HQ(C), an anionic (quinolinate) form 6HQ(A), and a neutral form 6HQ(N) are thus predominantly present in acidic, alkaline, and neutral aqueous solutions. Steady-State fluorescence measurements as a function of the pH revealed that, in the lowest excited state, both the acidity of the hydroxyl group and the basicity of the nitrogen atom are considerably enhanced: phenol deprotonation upon excitation of 6HQ(C) occurs in 10 M HClO4 whereas imine protonation upon excitation of 6HQ(A) arises in 12 M NaOH. In neutral aqueous solution, simultaneous proton release and proton capture was suggested to arise on both sites of the excited 6HQ(N) molecule. The essential stable form in a large range of pH is thus the excited tautomer, 6HQ(T*). These proton transfer processes are coupled to an intramolecular electron redistribution yielding a neutral ketonic configuration, which strongly lowers the probability of the back proton transfers in the excited state. Recent investigations by picosecond time-resolved fluorescence led to opposed conclusions concerning the chronology of the double proton transfer process in neutral water. This chapter presents a sub-picosecond transient absorption investigation of the photo-induced tautomerization of 6HQ in acidic and alkaline aqueous solutions.