Photochemical Reactions of Azo Dyes in Solution with Different Substrates

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Abstract
Solutions of azo dyes iii water containing different soluble substrates have been exposed to ultraviolet and visible radiation. It has been found that many compounds react photochemically as hydrogen donors and cause reduction of azo dyes present in the same solution. Fading of such solutions takes place only when the radiation is absorbed by the substrate. Fading is evidently the result of interaction of excited substrate molecules with dye molecules or excited dye molecules. Substrates containing carboxyl, hydroxyl, or aryl groups attached to methyl, methylene, or aliphatic CH‐ groups show high photochemical activity when the compounds absorb light of long wavelength.Kinetic measurements have shown that the fading reactions are of first order with respect to the concentration of the excited substrate molecules at low substrate concentrations. Fading of azo dyes in solutions containing different substrates is partly inhibited by the presence of oxygen. The results indicate that oxygen can act as a hydrogen acceptor in the same way as the azo dyes. Kinetic experiments carried out for one system suggest that the inhibition is partly caused by reoxidation of reduced dye radicals to the original dye by peroxide radicals. A mechanism for the inhibition is proposed.