Photorearrangement of quinoline 1-oxides: relevance of ground and excited state basicity and effect of heavy atom quenchers

Abstract

The photoisomerization of some quinoline 1-oxides to 2-quinolones has been studied in aqueous solution as a function of pH. Ground state pK values have been measured. There is a decrease in quantum yield of reaction for the protonated molecules. Evidence is given for the setting up of a prototropic equilibrium in the excited state of 4-methoxyquinoline 1-oxide. Fluorescence and reaction quenching by inorganic anions, together with emission spectra at low temperature, showed that the protonated molecule is sensitive to a heavy atom effect. Taken together, the results indicate that the photoisomerization occurs from the n,π* singlet state of the non-protonated molecule.