MECANISME D'OXYDATION PHOTOSENSIBILISEE DANS UN MILIEU RIGIDE MACROMOLECULAIRE

Abstract— The photosensitized oxidation of 4-methoxynaphthol by erythrosin is studied in a film of ethylcellulose which acts in this reaction as a binder, permeable to oxygen. The rate of the oxidation of the naphthol is measured as a function of the average intermolecular distance. It is shown that sensitized photo-oxidation occurs in such rigid medium with a rather high quantum yield (0·2-0·3) and that the quantum yield remains high when the intermolecular distance increases up to a value which is large (80 A) compared to the molecular size. As a consequence, it seems that such a reaction cannot proceed without the intervention of an oxidizing entity able to diffuse in the rigid binder. Such an entity can be O2, as proposed by Kautsky, as well as H2O2 or the ion O2−. A strong effect of deactivation of the sensitizer by a resonance energy transfer process has been shown to take place in the film when the average distance between dye molecules is less than about 120 A. This deactivation of the sensitizer appears as fluorescence quenching and as a decrease in the rate of the reaction when the intermolecular distance decreases. At very short distances another mechanism seems to occur which results in a higher quantum yield.