5-Methoxyquinoline Photobasicity is Mediated by Water Oxidation.

Abstract

A mechanism explaining the photobasicity of 5-methoxyquinoline (5-MeOQ) is proposed on the basis of nonadiabatic molecular dynamics simulations using time-dependent hybrid density functional theory (TDDFT) and fewest switches surface hopping (SH) and analysis of existing ultrafast spectroscopy experiments. According to the TDDFT- SH simulations, the rate-determining step is hole transfer from photoexcited 5-MeOQ to adjacent water molecules within ∼ 5 ps, followed by rapid electron-coupled pro- ton transfer and deactivation to the ground state. This fast redox-catalyzed proton transfer mechanism is consistent with simple thermodynamic arguments, correlated wavefunction calculations, and recent isotope substitution time-resolved fluorescence experiments. Although energetically feasible, direct protonation of 5-MeOQ in the S1 state appears to be too slow to contribute significantly to 5-MeOQ photobasicity in aqueous solution.