The Photofading of 1-Arylazo-2-naphthols in Solution. Part II – Contribution of Photoreduction to the Anomalous Photofading of 1-(p- and o-Nitrophenylazo)-2-naphthols in Alcoholic Solvents

1-(Substituted phenylazo)-2-naphthols with a nitro group positioned para- or ortho- to the azo group, show anomalous photofading behaviour in methanol, i.e., their fastness to light is very much lower than that of similar compounds. Photochemical reaction of 1-(p-nitrophenylazo)-2-naphthol (Ia) in methanol, ethanol or 2-propanol produces not only oxidative but also reductive products, while photochemical reaction of Ia in acetone gives only oxidative products. From these results and earlier observations, it is suggested that the anomalous photofading of la is a substrate-specific phenomenon, and may be caused by photo-reduction of the nitro and azo groups to amino groups by the substrate, instead of the normal photo-oxidation of the hydrazone tautomer by singlet oxygen. On the other hand, although 1-(o-nitrophenylazo)-2-naphthol (II) in alcoholic solvents faded to give similar products via photooxidative and/or photoreductive reactions, little photoreduction of the nitro group was detected, and the rate of photo fading of II was lower than that of Ia. The contribution of intramolecular interaction, such as intramolecular bifurcated hydrogen bonding, involving the o-nitro group, azo group and o’-hydroxy group of 1-(o-nitrophenylazo)-2-naphthol, are also discussed