Photochemical decomposition of 2,4-dinitro-4′-dimethylaminoazobenzene in aerated solution

Abstract

The irradiation of various ketones in the presence of 2,4-dinitro-4′-dimethylaminoazobenzene (1) causes decomposition of the latter. The chemical process actually observed depends on the solvent and on the ketone structure. In benzene the dye (1) undergoes demethylation in the presence of 2,2-dimethoxy-2-phenylacetophenone (DMPA) and benzil. This is due to the reactions of benzoylperoxyl radicals formed from the ketone sensitizers in the presence of oxygen. Benzophenone and anthraquinone cause the same reaction, in this case through direct interaction of the nπ* triplet with (1). In alcohols the situation is more varied, as with DMPA, benzil, and benzophenone in methanol, demethylation of (1)via the same mechanisms as above is observed, while with anthraquinone and acetone in methanol and with benzophenone in propan-2-ol, 2,4-dinitro-4′-dimethylamino-NNO-azoxybenzene, (4), is formed. The latter reaction is initiated by attack of ketyl radicals, formed in the primary photoreaction of the ketone with the solvent, onto (1). The relevance of this model for the fading of azo dyes under applicative conditions is discussed.