Quantitative structure–property relationship modelling for photoreduction‐fast azo dyes on a nylon substrate: a methodology for thermochemical analysis of the photocleavage of azo dyes using the RM1 semi‐empirical molecular orbital method

Abstract

Editor-in-chief's recommendation: Thanks to decades of research and development, dyers of polyester can choose from a palette of industrial colorants that possess excellent technical characteristics. These properties extend to high photostability. However, many azo dyes optimised for polyester struggle to survive the photoreductive conditions prevalent in a polyamide microenvironment. They often exhibit poor light fastness in nylon. In this Feature article, the authors not only argue that much more could be done to develop light resistant colorants for polyamide, but they also arrive at novel conclusions as to how this goal might be achieved. Using computational chemistry as a tool to rationalise the ways in which commercial colorants photodegrade, the authors build on their extensive previous investigations into the photofading of azo dyes. Consequently, this paper provides even more detailed insight into the breakdown of industrial colorants in polyamide substrates, which is welcome given that the manner of their photochemical decomposition in real-world conditions is still not well understood. In addition, following meticulous analysis of data relating to several series of azo dyes, the authors propose criteria to aid the creation of a new generation of more robust colorants for nylons and other polyamides. For example, they identify the reaction amongst the photoreductive pathways that leads to the most rapid azo bridge destruction and then specify a limit for heat of reaction which will suppress that particular pathway and thus maximise photostability. Since such values are calculable for candidate structures drawn up during dye design, this Feature article presents for the first time a mechanistically grounded framework enabling dye chemists to hunt down photoreduction-resistant azo colorants in silico.The rates of fading of 18 reactive azo dyes on polyamide fabrics and films upon exposure to a carbon arc in air have been determined from the initial slopes. The rates have been thermochemically analysed by calculating the heats of formation of the reactants, intermediates, and products by chemical equations describing (1) the second-order disproportionation (redox reaction) between the photoinduced hydrazinyl radicals and (2) the intramolecular H-transfer (self-decomposition) of the radicals, using the RM1 semi-empirical molecular orbital method. The rates of azo cleavage (or reductive fading) were studied to correlate the molecular structures with three molecular descriptors: (a) the heats of reaction for two reaction pathways, (b) the thermodynamic stability of the photoinduced hydrazinyl radicals, and (c) the quantum yields of generation. The possibility of structure optimisation of the examined azo dyes is discussed from the perspectives of (a) and (b). The light fastness of several previously reported substituted phenylazophenol (model) dyes on a polyamide substrate has been analysed by the same procedure. Two of the latter dyes exhibit typical photoreduction-fast properties, which determine the thresho...