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

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 threshold Δ_rH°(gas) value of the N11‐hydrazinyl dye radical for (1), indicating that the chemical structures of azo dyes may, in principle, be modified to include reduction‐fast azo groups. The current state of reactive azo dyes is considerably below the level required to achieve reduction‐fast dye. The chemical structures of current reactive azo dyes appear not to have been optimised on the basis of the molecular descriptors (a) and (b), and only insufficiently on the basis of (c).