ESR investigation of the radical intermediates formed in the photoreduction of azo dyes

Abstract

83In general, these experiments show the importance of the pretreatment of the CaA zeolites for the adsorption of CO. Moreover, one must be very careful in comparing CO adsorption results, because the quantity adsorbed depends largely on the pretreatment conditions of the CaA zeolite. Small changes in pretreatment can result in large adsorption J. A. Michelena acknowledges a grant from the EEC differences. Therefore, we suggest that for adsorption (European Economic Community). E. F. Vansant thanks measurements on CaA zeolites, always fresh CaA zeolite samples, dehydrated at identical temperatures for the same period of time, should be used. Abstract. ESR-spectra of hydrazyl and aminonaphthoxy radicals formed upon photoreduction of azo dyes have been measured. For the hydrazyl radicals the results obtained further confirm previous investigations of the mechanism of photoreduction of azo dyes. The direct identification in photoreduced dye solutions of aminonaphthoxy radicals, which are formed in the oxidation reduction equilibrium of aminonaphthols and iminoquinones provides strong evidence for previously proposed mechanisms for the disproportionation of hydrazyl radicals Acknowledgement

Photofading of phenylazo‐aniline, ‐pyridone and ‐quinolone disperse dyes on a nylon 6 substrate

“…The rates of reductive fading are considerably different from each other as indicated by the K PA values in Table 1. As for the photoreduction of the azo groups, a multistep mechanism or a photo‐excitation followed by a dark radical reaction has been proposed by van Beek and co‐workers [30–36]. The photoreduction of monoazo dyes in solution via irradiation with UV light generates hydrazinyl radicals of the dyes, which undergo disproportionation reactions in the dark, resulting in the formation of the corresponding amines and iminoquinones, as well as the original dyes via different mechanisms, depending upon the concentrations of the reactants [32,33].…”

“…Thus, hydrazinyl or NOH radicals act as hydrogen acceptors, accepting hydrogen from the hydrogen donor of MHN radical, a disproportionation reaction. The hydrogen donor is converted to the original azo dye, while the hydrogen acceptor performs AS to generate the corresponding amino compounds and iminyl‐aniline biradicals depending upon which Nα or Nβradical acts as a hydrogen acceptor [reaction route via intramolecular hydrogen transfer (S2A), proposed by van Beek and co‐workers [30–36], has not been discussed in this paper].…”

Relationship between photofading and chemical structure of disperse azo dyes on nylon fabric†

“…Reaction routes (R1) and (R3) have been proposed by van Beek and co‐workers [8–15], while AS via the radical reaction between a hydrazinyl radical (as H‐acceptor) and (MHN) from a photosensitiser such as benzophenone has been proposed by Řehák and co‐workers [16]. The present authors previously reported the marked promotion effect on the disproportionation reaction (R2) between a hydrazinyl radical as H‐acceptor and (NO 2 H) as H‐donor [6,7].…”

Light‐induced colour changes in dyes and materials

“…The mechanism of these reductions was later completely elucidated. [26][27][28] In subsequent investigations it was found that ketyl radicals can also be generated efficiently by irradiation of compounds which decompose into oxidizing radicals, or of which the excited state has oxidizing properties, in the presence of simple hydrogen-containing compounds. Examples are the photoreduction of azo dyes in ethanol in the presence of acetone, halogen compounds, or hydrogen p e r o~i d e .~~.~~ In these investigations it was found separately that simple alkyl radicals also attack azo dyes and convert them into substituted amines.…”