The Role of Anthraquinonoid Dyes in the Catalytic Fading'of Dye Mixtures– Substituent–dependent Triplet Yield of Diaminoanthraquinones

Taking various diaminoanthraquinones as model compounds for commercial anthraquinonoid dyes it was found that the ability of initiating catalytic fading via singlet oxygen mechanism is closely related to the quantum yield of triplet formation in these dyes. The quantum yields of triplet formation show a marked difference between the various diaminoanthraquinones. Whereas 1, 4– and 1, 2–diaminoanthraquinones have very small quantum yields, the 1, 5– and 1, 8–derivatives pass into the triplet state with a fairly high efficiency. From this it was concluded that catalytic fading occuring via singlet oxygen mechanism must be expected in dye mixtures consisting of 1, 5– or 1, 8–diamino substituted anthraquinonoid dyes and easily oxidizable azo–compounds.     

Mechanism of the photofading of Dye. Contribution of singlet oxygen in the catalytic fading of anthraquinone dye mixtures

The mechanism of the catalytic fading of some dyes has been investigated. The relative fading of quinophthalone in solution increases when C.I. Disperse Red 15 was added. The mechanism of catalytic fading proceeds via the singlet oxygen mechanism, since this phenomenon is suppressed by adding singlet oxygen quencher as DABCO or nickel dimethyldithiocarbamate (NMC). However, when quinophthalone mixed with C.I. Disperse Blue 14 was irradiated under the same conditions (λ > 520 nm), no catalytic fading occurred. Competitive photo-oxidation experiments for these dyes with NMC, giving evidence for intermediacy of singlet oxygen by the self-sensitised process, were carried out. The relative rate constants for reactions in solution were examined to predict the possibility of the catalytic fading in these dye mixtures.     

Flash photolysis of azo dyes in aqueous solutions of biacetyl

Irradiation by microsecond flash of aqueous aerobic solutions of biacetyl (2, 3-butanedione) and a number of different azo dyes resulted in oxidative fading of the dyes. The fading kinetics of all tautomeric dyes could only be described properly by assuming three oxidative species. The first one is believed to be singlet oxygen. Due to its short lifetime in aqueous systems, the dye fraction consumed by singlet oxygen was confined to a maximum of about 30%. Second order reaction rate constants for the second oxidator were in the range of 1.4–4.7 × 10⁸ l/(mol s). A radical mechanism is proposed, in which the adduct of oxygen and triplet biacetyl is the second oxidising species. A third oxidator formed from the second was necessary to describe adequately the fading curves of four azo dyes with slow fading rates. This oxidator could be an acylperoxy-radical. The non-tautomeric dye Cl Acid Red 37 was recycled partially after very fast fading during the first 0.5 ms.     

Improvement of the light fastness of natural dyes: the action of singlet oxygen quenchers on the photofading of red carthamin

While the contribution of singlet oxygen quenchers to synthetic dyes has been investigated previously by several workers, there have been few studies pertaining to the use of singlet oxygen quenchers as a means of improving the light fastness of natural dyes. In this paper, the influence of various additives on the photofading of carthamin has been examined in cellulose acetate film. The rate of photofading of the colour was remarkably suppressed in the presence of nickel hydroxyarylsulphonates, while the addition of UV absorbers afforded little retardation in the rate of fading.     

Action of light on dyes in polymer materials

Some of the general features of the way in which the nature of the polymer substrate influences the fading of dyes by light are briefly surveyed. The effects of oxygen, water vapour and atmospheric contaminants are considered in particular. It is suggested that singlet oxygen and hydrogen peroxide vapour play an important role in the fading of many dyes.     

HPLC analysis of the natural scale insect,madder and indigoid dyes

A linear gradient elution method has been applied to the HPLC analysis of plant and scale insect red anthraquinonoid mordant dyes and molluscan blue and red-purple indigoid vat dyes. The method enables the use of the same elution program for the determination of different chemical classes of dyes. In addition, it significantly shortens the retention times of natural anthraquinonoid dyes over those previously published. For the first time a new dye, probably dibromoindirubin, has been detected in the Murex trunculus sea snail. The dye families investigated include the ones most often found on ancient textiles and shards from dyeing vessels in Israel and other regions.     

Photostability of a range of azobenzene dyes and their benzothiazolyl analogues in the presence of air

Irradiation of a range of azobenzene dyes and their benzothiazolyl analogues with visible light in the presence of air has been shown to give rise to the conversion of a triplet oxygen to the singlet state. As proof of the presence of a singlet oxygen, its reaction with tetraphenylcyclopentadienone was studied. The formation of the oxidation product cis-α,α'-dibenzoylstilbene by a singlet oxygen has been confirmed by chemical ionisation mass spectrometry, both in ethyl benzoate solution and on polyester fabric. Inhibition of photodegradation of the dyes by a singlet oxygen quencher (DABCO) has also been demonstrated.     

Photodecomposition of Aminoanthraquinone Disperse Dyes on Poly(ethylene terephthalate)

The rapid fading on polyester fibres of certain blue anthraquinone disperse dyes under normal conditions of exposure is due to the ease of photochemical de-alkylation of their substituted-amino groups. The products of the photoelimination reaction for a series of dyes have been identified, and a mechanism for the reaction is proposed. The higher photochemical stability of these dyes on cellulose acetate and nylon is perhaps due to hydrogen bonding of the alkylamino groups to the polymer. The influence of wavelength of irradiation, oxygen, moisture and the physical structure of the polymer is discussed.     

The effects of fluorescent substances on the photofading of colours. Part 4-photochemical interaction between fluorescent brightening agent of a pyrazoline type and 1,4-diaminoanthraquinone

The catalytic fading of 1,4-diaminoanthraquinone by a fluorescent agent of a pyrazoline type was demonstrated on irradiation with 366 nm light in aerobic acetonitrile solution. The quenching study with a singlet oxygen quencher and the chromatographic analysis of the photoproducts revealed a singlet oxygen intermediate. The fluorescent agent generated singlet oxygen in cellulose acetate films as well, causing a small amount of enhanced fading of the dye. No acceleration of the fading was observed either in ethanol or in nylon films. On 253 nm light irradiation the dye and the fluorescent agent suppressed each other's fading.     

Contribution of Singlet Oxygen to the Photofading of Indigo

The photofading of indigo in solution is examined. This photofading is accelerated in the presence of sensitizers of singlet oxygen, e. g. Methylene Blue or Rose Bengal. The fading is retarded by adding effective singlet oxygen quenchers, such as β–carotene, DABCO or nickel di–methyldithiocarbamate. In irradiations of both filtered (λ > 390 nm) and nnfiltered types, isatin is formed as the main photofading product. It is suggested that the photofading of indigo in solution may be due to a self–sensitized photo–oxidation involving singlet oxygen. By carrying out competitive photo–oxidation experiments in solution, the relative rate constant for the reaction with singlet oxygen is determined.     

The Role of Intramolecular Quenching in the Catalytic Fading of Dye Mixtures

The catalytic fading of dye mixtures has been investigated, and the influence of a nickel sulphonate group, an intramolecular singlet oxygen quencher, was examined. Dyes containing the quenching group showed improved resistance to catalytic fading in solution and in the solid state.     

Improvement of light fastness of natural dyes. Part 2: Effect of functional phenyl esters on the photofading of carthamin in polymeric substrate†

The photofading behaviour of carthamin in cellulose acetate film was investigated. A contribution from singlet oxygen to that fading was suggested. Phenyl esters containing nickel sulphonate quenching groups were prepared and the protecting effect of these compounds towards the photofading of carthamin was examined in cellulose acetate film. It is proposed that nickel sulphonate derivatives of phenyl esters can be applied as effective stabilisers against the fading of natural dyes.     

II—Evidence for the Formation of Singlet Oxygen by Phototendering Vat Dyes

The reaction of tetraphenylcyclopentadienone (TPC) with singlet oxygen has been used as a probe for the photochemical formation of singlet oxygen by phototendering vat dyes. The dyes C.I. Vat Yellow 1, 2 and 26 and C.I. Vat Orange 9 sensitised the oxidation of TPC in phenol-methylene chloride solution by the action of visible light. In each case the sole photo-oxidation product was cis-dibenzoylstilbene, indicating the intermediacy of singlet oxygen. Similar results were obtained when the dyes were applied to cotton fabric, although the relative efficiencies of the dyes as sensitisers differed from those observed in solution. The singlet-oxygen-sensitising efficiencies of the dyes on cotton parralleled their relative phototendering activities towards cellulose, providing evidence in favour of the singlet-oxygen theory of phototendering. Quenching studies of the dye-sensitised oxidation of TPC confirmed that the reaction proceeded by a singlet-oxygen mechanism rather than a radical process.     

Synthesis and photochemical stability of 1-phenylazo-2-naphthol dyes containing insulated singlet oxygen quenching groups

Although 1-arylazo-2-naphthols undergo direct and sensitised photochemical fading in solution by a singlet oxygen mechanism, the introduction of singlet oxygen-quenching substituents (N,N-dialkylaminomethyl-) affords no protection against fading, and in many cases accelerates photodegradation. This has been attributed to the high reactivity of the dialkylaminomethyl-groups towards hydrogen abstraction, thus rendering free radical modes of decomposition more favourable. In the presence of peroxide-radical scavengers the latter reactions are suppressed, and the dialkylaminomethyl-groups then show a protecting effect against fading by singlet oxygen.     

Kinetics of the biacetyl-sensitised photo-oxidation of CI Acid Orange 7

A kinetic model for the biacetyl-sensitised photo-oxidation of Cl Acid Orange 7 in aqueous solution during continuous irradiation at 365 nm is presented. Biacetyl was taken as a model for naturally occurring carbonyl species in polymers such as oxidised celluloseand lignin involved in photosensitised degradation of azo dyes. Quenching of triplet biacetyl by the dye and by oxidation products as well as quenching of singlet oxygen by waterand by oxidation products was accounted for. The equations derived from the model describe the relationship between the measured quantum yield of dye fading and dye concentration, as well as that between measured quantum yield of oxygen demand and dye concentration. Reaction rate parameters were calculated by a nonlinear least squares method, and were in accordance with literature values and values measured by flash photolysis. The relative amount of dye faded by singlet oxygen was estimated as 1.4–14%, indicating that singlet oxygen was of minor importance as an oxidative species.     

Mechanism of accelerated photofading of thioether anthraquinone dyes in styrene butadiene resins

Many anthraquinone dyes fade in visible light up to 100 times faster in styrene butadiene copolymers than in polyester resins or polystyrene. A mechanistic investigation of the photofading of 1,5-dihydroxy-2,6-diisobutyl-4-thiophenylanthraquinone has established that the dye sensitizes formation of singlet oxygen, a very reactive species, which attacks the double bonds in the styrene butadiene, resulting in extensive chain cleavage and peroxide formation. The major process is the oxidation of the polymer, and the dye fading is a minor process whereby the dye is apparently attacked by polymeric peroxides. The mechanism appears to be general for anthraquinone dyes, especially those with thioether, amino, hydroxy, or double-bond functionality. Unfortunately the fading is unaffected by most anticxidant stabilizers: the best result achieved was a doubling of half-life with 1 wt% Spinuvex A-36. This is still insufficient to give colored styrene butadiene copolymer resins the intrinsic dye stability available in other polymers, rendering styrene butadiene unsuitable for applications requiring high photostability of anthraquinone dyes.     

Fading of Phenylazo–β–naphthol Dyes on Polypropylene*

The fading of a series of phenylazo–β–naphthol dyes on polypropylene by exposure to either long– or short–wavelength ultraviolet radiation and under nitrogen and oxygen atmospheres, respectively, is reported. The half–lives and relative rates of fading of the dyes were calculated at the wavelength of maximum absorption of the hydrazone band and the hydrazone–azo ratios of the dyes were determined on polypropylene and in ethanol. The relative rates of fading were plotted against the appropriate Hammett constants for the substituents. In general, a linear relation was observed. The graphs had positive slopes, but negative slopes were also obtained with the dyes bearing electropositive groups. Graphs of relative rates of fading against hydrazone?azo ratio were also drawn. The results were generally similar to those obtained with the plots of Hammett constants, but correlation was better. The results indicate that, on polypropylene, the most important fading mechanism for the hydrazone form of the dye is reductive, although there is evidence that oxidation may occur, especially in the presence of oxygen.     

Azomethinfarbstoffe - Sensibilisatoren,Löscher und Reaktionspartner für Singulettsauerstoff

Azomethine Dyes — Sensitizers, Quenchers and Reaction Partner for Singlet Oxygen Azomethine dyes show photofading in photographic materials, possibly due to oxidation by singlet oxygen. The azomethine dyes 1a–h , 2a–h , 3c , d , f are inefficient photosensitizers of singlet oxygen formation. In tetrachloromethane the quantum yield ϕ¹O₂ is in the range between 8 · 10⁻⁴ and 4 · 10⁻³. Contrary to the literature we found that azomethine dyes react with ¹O₂ (k_r). The reaction constants were measured by a competetive technique using 9, 10-diphenyl anthracene and methylene blue as ¹O₂ sensitizer. The values of k_r show that the reaction of the dyes with ¹O₂ can not be ignored. Besides this reaction, ¹O₂ is also quenched by the dyes (k_q). The values of k_q point to quenching of ¹O₂ by the tertiary amino group of the dyes via a CT-mechanism. This study shows that in photofading of azomethine dyes the oxydation by ¹O₂ must be taken into consideration.     

Mechanism of the photofading of dye: Contribution of singlet oxygen to the photofading of aminoanthraquinone dyes

The contribution of singlet oxygen (¹Δ_gO₂) to the photofading of some animoanthraquinone dyes in solution is examined. The rates of the photofading are accelerated in the presence of sensitizers of singlet oxygen, e.g. Rose Bengal or Methylene Blue. The rates are retarded by adding effective singlet oxygen quenchers, such as DABCO or nickel dimethyldithiocarbamate (NMC). Additional evidence for the intermediacy of singlet oxygen is obtained by examining the effect of deuterium solvent, the effect of chemical sources of singlet oxygen, and the competitive photo-oxygenation with the dyes and NMC.     

Untersuchungen zum photochemischen Ausbleichverhalten von tautomeriefähigen Azofarbstoffen. VI. Zur photochemischen Oxydation von Phenylazo-acetylacetonen und Phenylazopyrazolonen durch Singulettsauerstoff

Investigations of the Photo-Fading of Tautomeric Azo Dyes. VI. The Photochemical Oxidation by Singlet Oxygen of Phenylazoacetylacetones and Phenylazopyrazolones The dye-sensitized photo-oxidation by singlet oxygen of phenylazoacetylacetones ( 1 ) and Phenylazopyrazolones ( 2 ) has been investigated. The photo-fading of dyes is accelerated in the presence of Methylene Blue, a sensitizer of singlet oxygen, in airsatured methanol An excellent relationship exists between the fading-rate of a series of dyes ( 1 ) and Hammett-σ-constants. An unexpected trend was found for photo-fading of a series of dyes ( 2 ). The Hammett plot consists of two parts, with a negative slope derived from electron-releasing substituents and a positive slope derived from electron-withdrawing substituents. The anomalous photo-fading is due to an attack of singlet oxygen on the more reactive anion of the phenylazopyrazolones in the case of electron-withdrawing substituents. In methanol containing acetic acid the dyes ( 2 ) exist only as the hydrazone tautomers. Under these conditions the fading-rate show a linear relationship with σ-constants.