- ELECTROCHEMICAL AND CHEMICAL OXIDATION OF SOME AZO DYES
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The oxidation potential of representative azo dyes, viz. a series of N,N-dialkylaminoazo dyes 1 and of tautomeric azonaphthols (2 and 5) has been determined.Anodic oxidation and chemical oxidation have been carried out for some of the dyes.The results are rationalized as involving a single intermediate, the radical cation of the dyes, and its evolution along two main pathways, viz. a) deprotonation whenever possible (from the α position of the N-alkyl group in aminoazo dyes, leading ultimately to N-dealkylation; fromthe naphthols to eventually yield a C-C-coupled dimer) and b) heterolytic cleavage to an aryl radical and a diazonium cation, the latter trapped under favourable conditions to yield a bisazodye.
- Fasani, Elisa,Soldi, Teresa,Albini, Angelo,Pietra, Silvio
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p. 109 - 114
(2007/10/02)
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- Physical Quenching and Chemical Reaction of Singlet Molecular Oxygen with Azo Dyes
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The quenching rate constants of singlet oxygen liefetime by dialkylamino azo dyes 1 and tautomeric azo dyes 2 and 5 (as well as azo- and hydrazone-model methylated derivatives 3 and 4) were determined by measuring the 1.27-μm 1O2 emission lifetime, and the values obtained are in the range 106 - 108 M-1 s-1.The quenching involves a charge-transfer interaction, and different correlations with the oxidation potentials of the two classes of azo dyes are observed.Chemical reactions take place with a rate in the range 103 - 104 M-1 s-1 and are rationalized as a secondary pathway for the charge-transfer complex between 1O2 and the azo dye.Products include dealkylated amino azo dyes 6 from 1; compounds arising from the fragmentation of the aryl-azo bond, viz., benzenes 7 and phenoles 8 from 1 and 2; and 1,4-naphthoquinone 10 as well as the dimer 9 and the oxidized cyclic derivative 11 from 2.The methoxy derivative 3 yields an endoperoxide, which rearranges to a 1,2-naphthoquinone derivative 14.The results are discussed in comparison with the currently accepted mechanism for the reaction of 1O2 with azo dyes.
- Bortolus, Pietro,Monti, Sandra,Albini, Angelo,Fasani, Elisa,Pietra, Silvio
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p. 534 - 540
(2007/10/02)
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- Photochemical Decomposition of 4-Arylazo- and 4-Arylazoxy-N,N-dialkylaniline N-Oxides
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The 4-aryl-N,N-dialkylaniline N-oxides (1a-c) decompose on u.v. irradiation in aprotic solvents, undergoing deoxygenation as well as intramolecular hydrogen abstraction, to give amides and dealkylated products.The latter process is more important for the diethyl derivative (1b) (photoCope elimination).Visible irradiation is ineffective.The 4-arylazoxy-N,N-dialkylaniline N-oxides (2a-d) undergo photochemical deoxygenation from the amino group (major process) as well as from the azo group.The mechanism of these photoprocesses is discussed in general and in relation to the possible involvement of these N-oxide derivatives in photofading of the related azo dyes.
- Albini, Angelo,Fasani, Elisa,Moroni, Micaela,Pietra, Silvio
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p. 1439 - 1444
(2007/10/02)
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