Radical mechanism of azo cleavage for monoazo reactive dyes, a QSPR study: the similarity between photoreduction on polyamide substrates and thermal reduction in aqueous dithionite solutions

The same radical mechanism occurs in reductive azo cleavage via monohydrogenated radicals generated as the starting reactants in photo-and thermal (by dithionite) reduction on a nylon 6 substrate and in aqueous and alcoholic solutions. The rates of photo-and thermal azo cleavage in the solid and liquid phases have been thermochemically analysed by calculating the heats of formation of the reactants, intermediates, and products for (1) intramolecular hydrogen transfer (self-decomposition via rearrangement) in the photo-and radically induced hydrazinyl radicals as well as for (2) the disproportionation (redox reaction) between the radicals via a molecular orbital method. The rates of reductive fading (or azo cleavage) are controlled by the heats of reaction via reaction pathways (1) and (2) as well as by rearrangement, depending on the reaction system. Photoreduction of these dyes on polyamide generated the same end products - diazo components and iminooxoquinones from the coupling component - as the products of thermal reduction in aqueous sodium dithionite and of photoreduction in the liquid phase. Photofading on polyamide substrates owing to continuous irradiation is controlled by three factors: the quantum yield of photoinduced hydrazinyl radicals, its rearrangement processes, and the heats of reaction for the thermal azo cleavage reactions via the two pathways. These factors may become molecular descriptors that correlate the rates of fading with molecular structures. The present study may become an introductory trial to construct quantitative structure-property relationship modelling for reduction-fast azo dyes.