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Fundamental experimental and theoretical studies on the lightfastness of azo dyes. / Owen J Guy

Swansea University Author: Owen J Guy

Abstract

Photo-degradation reactions of donor acceptor azo dyes in methanol solution have been investigated. Irradiation of the second absorption band was confirmed to be the predominant cause of permanent photo-fading under anaerobic and oxygenated conditions. Under anaerobic conditions, 2'-nitro subst...

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Published: 2001
Institution: Swansea University
Degree level: Doctoral
Degree name: Ph.D
URI: https://cronfa.swan.ac.uk/Record/cronfa42255
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Abstract: Photo-degradation reactions of donor acceptor azo dyes in methanol solution have been investigated. Irradiation of the second absorption band was confirmed to be the predominant cause of permanent photo-fading under anaerobic and oxygenated conditions. Under anaerobic conditions, 2'-nitro substituted azobenzenes and azothiophene dyes were the least lightfast, with half-lives of under one hour, whilst 4'-nitro substituted dyes had half-lives of between 1 and 1.5 hours. Photo-degradation was retarded in the presence of oxygen by between 4 and 16 times relative to fading under anaerobic conditions. The photo-fading of dyes deposited on various substrates exhibited similar behaviour to dyes in methanol solution. UV/visible spectra showed that 2'-nitro substituted dyes underwent complete loss of intensity of the visible absorption peak and a corresponding increase in the absorption in the UV region of the spectrum when irradiated under anaerobic conditions, indicating cleavage at the azo bridge and subsequent formation of mono-phenyl derivatives. In contrast, under oxygenated conditions, a gradual loss of intensity at the visible absorption maximum was observed, with no notable newly formed peaks are detected in the UV region suggesting that cleavage at the azo bridge is less significant. The photoproducts of the reaction are suggested to include the reduced form of azo dyes containing nitro groups. Theoretical semi-empirical AMI and ab initio calculations using the 3-21G basis set, predicted reasonable structures for the ground states of dyes. Spectroscopic calculations using a version of the CNDO/S method gave good correlations between calculated transition energies and experimental data obtained in cyclohexane. The results of a multi electron configuration interaction treatment of AMI structures in the gas phase were inconsistent with experimental spectral data obtained in cyclohexane but improved correlations were obtained between calculated transition energies and experimental data in methanol. The structures and energies of excited singlet and triplet states have also been calculated by the AMI method but no apparent correlation of these energies with the lightfastness of azo dyes could be identified. A tentative relationship between calculated distributions of the unpaired electrons in the second excited triplet state and the site of reactivity in the dyes was proposed.
Keywords: Organic chemistry.;Physical chemistry.
College: Faculty of Science and Engineering

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