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

Abstract

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 productsdiazo components and iminooxoquinones from the coupling componentas 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. Coloration Technology Society of Dyers and ColouristsAbbreviations and symbols AC = azo cleavage; AT = azo tautomer; HT = hydrazone tautomer; PA = polyamide 6; QSPR = quantitative structure-property relationship; VS = vinylsulphonyl; K PA and K PET = initial rates of fading on PA and PET respectively. Radical moieties attached to or contained in a dye molecule: (N11) = N11-hydrazinyl dye radical; (N12) = N 12-hydrazinyl dye radical; (MHN) = monohydrogenated dye radical. In order to distinguish clearly the position and the position-centred radical, only the Ni-and Oicentred monohydrogenated dye radicals are described, respectively, as (Ni) and (Oi) in parentheses in the text, scheme, chemical equations, and tables; (MHN) is their general name; i describes the number of atoms including carbon: 2 1The numbering of atoms used in molecular orbital (MO) calculations is different from the normal chemical nomenclature (see the 'MO calculations' section).

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 results of thermochemical calculation by the PM5 method led to the same conclusion as calculation by the RM1 method [1], but the numerical Table 9 Chemical structure (based on F0), light fastness, and D r H°(gas) (kcal mol À1 ) of reactants for the substituted phenylazophenol dyes (Class F) on PA reported by Kamel and co-workers [20] and their results of thermochemical analysis Substituents Light fastness [20] D r H°(gas) of (N11) D r H°(gas) of (N12) From the Δ r H°(gas) values by Eqn (1) for F1 and F2 (light fastness = 8) and for F3 and F4 (light fastness = 3-4 and 7 respectively), the threshold value of Δ r H°(gas), which results in limited light fastness of the reduction-fast azo dyes, can be estimated as 46.4 kcal mol À1 .…”

“…Two AC reaction pathways for the reaction of (MHN) with azo dyes without nitro groups According to previous reports [1,4], among photoinduced (MHN)s, only (N11) and (N12) can participate in the secondand first-order AC reaction pathways (1) and (2). Pathway (1) is the second-order redox reaction, in which (N11) or (N12) acts as the H-acceptor, with (MHN) as the H-donor.…”

“…As in previous studies [1,4], the D f H°(gas) values for the reactants, intermediates, and products in Eqns (1-4) were calculated and are listed in Table 4. As in previous studies [1,4], the D f H°(gas) values for the reactants, intermediates, and products in Eqns (1-4) were calculated and are listed in Table 4.…”

“…A general reaction scheme for the AC reaction pathways of an o-sulphophenylazo-J-acid derivative that is applicable to azo dyes including substituted phenylazophenols is illustrated in Scheme 1 [1]. Thermochemical analyses using these equations or their applications to the fading behaviours of individual dyes on PA substrate are described below.…”

“…are inhibited 4. 5 Dyes with Δ r H°(gas) < 46.4 kcal mol À1 (F3 and F4) exhibit photofading behaviours that depend on Eqn(1) and are strongly affected by the population ratio. 5 Dyes with Δ r H°(gas) < 46.4 kcal mol À1 (F3 and F4) exhibit photofading behaviours that depend on Eqn(1) and are strongly affected by the population ratio.…”