Cationic Effect on the Equilibria and Kinetics of the Excited-State Proton Transfer Reaction of a Photoacid in Aqueous Solutions

Dissolved ions have a significant effect on the chemical equilibria and kinetics in aqueous solutions by changing the physical properties and hydrogen bond network of water. In this work, the ionic effects on the excited-state proton transfer (ESPT) reactions of Coumarin 183 (C183) in aqueous ionic solutions are comprehensively studied in terms of pK_a, pKa ∗, activation energies, and kinetic isotope effect (KIE). The acid dissociation constants (pK_a and pKa^∗) of C183 on the ground and excited states are determined by UV-visible absorption and steady-state fluorescence spectroscopy. The activation energies (E_a) and KIE for the ESPT reaction of C183 are directly obtained by time-resolved fluorescence spectroscopy. The changes in pK_a, pKa^∗, E_a, and KIE values of C183 are found to be dependent on the charge density of cations. The secondary KIE is more substantially influenced by the dissolved ions than the primary KIE. Furthermore, the ionic effects on the equilibrium (pK_a and pKa^∗) and kinetic (E_a and KIE) parameters of C183 are found to be well-explained by the free-energy reactivity relation. Our current results are very important in understanding the ionic effects on the equilibria and ESPT kinetics of photoacids in aqueous ionic solutions.