Contribution of Singlet Oxygen to the Photofading of Indigo

The photofading of indigo in solution is examined. This photofading is accelerated in the presence of sensitizers of singlet oxygen, e. g. Methylene Blue or Rose Bengal. The fading is retarded by adding effective singlet oxygen quenchers, such as β–carotene, DABCO or nickel di–methyldithiocarbamate. In irradiations of both filtered (λ > 390 nm) and nnfiltered types, isatin is formed as the main photofading product. It is suggested that the photofading of indigo in solution may be due to a self–sensitized photo–oxidation involving singlet oxygen. By carrying out competitive photo–oxidation experiments in solution, the relative rate constant for the reaction with singlet oxygen is determined.