Photoinduced processes of 3-substituted 6-fluoro-1,4-dihydro-4-oxoquinoline derivatives: A theoretical and spectroscopic study

Presented computational study of 3-substituted 6-fluoro-1,4-dihydro-4-oxoquinoline derivatives confirmed the existence of two tautomeric (oxo- and hydroxy-) forms of these molecules originating from the presence of 4-oxo substitution on quinoline skeleton. The computed singlet excitation energies at the time-dependent density functional theory (DFT) level and the simulated absorption spectra are in accordance with the experimental UV/vis spectra measured in aprotic solvents (1,4-dioxane, acetonitrile, dimethylsulfoxide). Measured spectra indicate the solvent capability to affect the ratio of tautomeric forms in the solution. Reaction pathways of O2− and singlet oxygen (1Δg) formation and the energetics of these processes were studied using TD-DFT including the solvent effect in terms of polarizable continuum model. EPR spectroscopy confirmed that photoinduced reactions of 3-substituted 6-fluoro-1,4-dihydro-4-oxoquinoline derivatives with molecular oxygen lead to O2− and singlet oxygen (1Δg) formation.