The contribution of singlet oxygen to the photofading of triphenylmethane and related dyes

The contribution of singlet oxygen (¹Δ_gO₂) to the photofading of Crystal Violet in some solvents is examined. The rates of photofading in dichloromethane or acetone are accelerated in the presence of singlet oxygen sensitizers, e.g. Methylene Blue. The rates are retarded by adding effective singlet oxygen quenchers such as β-carotene or nickel dimethyldithiocarbamate. It seem shtat Crystal Violet is mainly photooxidized to give Michler's ketone and p-dimethylaminophenol via the reaction with singlet oxygen, which can be generated by the dye itself or by an added sensitizer. The effect of various singlet oxygen quenchers on the photostability of coloured materials derived from colour formers, such as Crystal Violet Lactone and 3-diethylamino-6-methyl-7-anilinofluoran, is examined also on silica gel.     

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.     

Factors influencing the photofading of commercial anthraquinone dyes in solution

The photofading of two anthraquinone dyes has been studied in aqueous solution using ultraviolet-visible absorption spectroscopy and flash photolysis. The influence of alcohol concentration, pH, atmosphere, photosensitisers and stabilisers have been examined and these lead to some important conclusions on the mechanism of dye fading. Essentially, the results indicate the photoexcited triplet state of the dye undergoes a process of either electron or hydrogen-atom abstraction depending on the nature of the environment. Other factors such as aggregation and singlet oxygen also appear to play an important role in solution photofading. The relevance of these results to photofading in a polymeric phase is discussed.     

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.     

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.     

Analysis of photofading of phenylazo-indole and phenylazo-N-ethanolaniline disperse dyes on poly(ethylene terephthalate) fabric using the PM5 method

The photofading behaviors of phenylazo-N-(ethanol)aniline and phenylazo-indole, nitrohetarylazo-N-substituted aniline disperse dyes on poly(ethylene terephthalate) (PET) substrate were analyzed using the Kubelka-Munk (K/S) parameters of the dyed fabrics exposed to a carbon arc in air. The initial experimental slopes (K_PET) of fading on PET were estimated from the time profiles of the K/S values at the λ_max. The rates (k_0,i) of reaction of these dyes with ¹O₂ were estimated by frontier orbital theory using the PM5 method. The photosensitivities (f_i) of the dyes were estimated from the K_PET assuming that the K_PET values are proportional to the product of k_0,i and f_i. Dyes with small f_i values, irrespective of their k_0,i values, possess excellent lightfastness (LF), while dyes with larger f_i values possess poorer LF. The validity of estimating k_0,i values by the MO method was confirmed experimentally by analyzing the mutually photosensitized fading behaviors of combination dyeings.     

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.     

The role of azo-hydrazone tautomerism in the photofading of 1-phenylazo-4-naphthol in polymer substrates

The photofading of 1-phenylazo-4-naphthol has been studied in films of cellulose acetate and cellulose triacetate. The relative rates of fading of the azo and hydrazone tautomers were monitored by resolving the visible absorbance spectrum of the dye into two overlapping bands corresponding to each tautomer. Permanent photofading of 1-phenylazo-4-naphthol was found to be due to oxidative attack on the ground state hydrazone tautomer of the dye by singlet excited oxygen. The lowest excited triplet state of the hydrazone tautomer was responsible for sensitizing singlet oxygen. The azo tautomer was resistant to permanent photofading, but its concentration was depleted during irradiation while it temporarily existed in the hydrazone form. As well as thermal tautomeric interchange, tautomeric conversion following trans → cis isomerization of the azo tautomer could have been responsible for this temporary existence. Relative fading rates were influenced by the nature of the polymer concerned, and the method used to incorporate the dye. Temporary fading occurred as a result of isomeric and conformational changes in each tautomer.     

Prediction of the photofading of 1-H or 1-ethyl derivatives of 3-cyano-6-hydroxy-4-methyl-5-(p-X-phenylazo)-2-pyridone dyes and their azo-hydrazone tautomerism: A theoretical study

The lightfastness calculations for photofading of monoazo pyridone dyes were performed. For this purpose, all-valence molecular orbital method PM3 was applied. Frontier electron density distribution on the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) was examined. The obtained parameters seem to reflect the tendency for an electrophilic attack with a singlet oxygen (¹O₂) atom or a nucleophilic attack with superoxide anion (O₂^●─) on a particular atom in a molecule. Reactivity indicators for superdelocalisability ($S_r^{\mathrm{E}\left(\mathrm{N}\right)}$) and electron density distribution in ground and excited state were calculated. Superdelocalisabilities enable the fastness values to be explained in different chemical molecules depending on tautomeric forms in which they may occur.     

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.     

Effect of uv absorbers containing singlet oxygen quenching groups on the photochemical stability of crystal violet lactone and related colours

The photofading behaviour of Crystal Violet Lactone was investigated in solution. The contribution of singlet oxygen to that fading was suggested. UV absorbers containing nickel sulphonate quenching groups were prepared and the protecting effect of these compounds towards the photofading of colour materials derived from colour formers was examined in solution and in solid state. It was proposed that nickel salts of 2,2′-dihydroxy-4,4′-dimethoxybenzophenone-5-sulphonic acid and its derivative can be applied as effective stabilisers against the photofading of indicator dyes in pressure (or heat) sensitive recording systems.     

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.     

Thermal cis-trans isomerization and photofading of some azo dyes in poly(ethylene terephthalate) matrices: A kinetic study

The thermal cis-trans isomerization of azo dyes XC₆H₄N=NC₆H₄N(C₂H₅)₂ (where X is H, OCH₃, CN, and NO₂) in poly(ethylene terephthalate) matrices, below the glass transition temperature, and in dibutylphthalate solutions has been studied kinetically in the temperature range 20–60°C. Kinetic behavior in the polymer is characterized by two or three simultaneous first-order processes with similar activation enthalpies. These processes reflect an unequal distribution of “tie-molecules,” “folds,” and “cilia” in the amorphous region of the polymer. Lightfastness of the dyes in poly(ethylene terephthalate) films has been investigated. Polymer morphology strongly affected the photofading, but no significant influence was observed in the kinetic experiments.     

Photostabilisation of colour formers for heat-sensitive recording systems: Effect of counter-ions on the photofading of fluoran dyes

Fluorans are given considerable attention as functional dyes for heat-and/or pressure-sensitive recording media. Numerous potential metal complexes were synthesised to prolong the life of coloured species. The protecting effect of these compounds against the photofading was examined on cellulose. It was found that the zinc or nickel salts of 3,5-dihydroxy-2-naphthoic acid and its derivatives can be applied as effective stabilisers against the fading of fluoran dyes.     

Photodegradation of some 1:2 metal-complexed formazan dyes in an amide environment

The photodegradation of some representative metallised formazan dyes has been investigated. Photofading was examined in NN-dimethylformamide (DMF) at wavelengths of 300 and 350 nm, and in daylight. The measured quantum yield of photodegradation depended on the metal present in the dye molecule and the photoysis wavelength. It was found that with UV light under anerobic conditions the presence of ketone sensitisers such as benzophenone accelerated photofading, confirming that hydrogen abstraction is responsible for degradation. Oxygen also played an important role in the photolytic behaviour under daylight exposure conditions. Co(III) complexed formazan dyes, unlike their Fe(III) analogues, were found to be effective singlet oxygen quenchers. Photofading of the metallised formazan dyes on polyamide fabric was also examined. Fading rates were directly related to the reciprocal of the light fastness of polyamide dyeings. The expected direct relationship between log quantum yield and the reciprocal of light fastness was also observed.     

The effect of polycation on the light fastness of CI Acid Red 249

Cationic compounds are frequently used in direct dyeing processes and in ink-jet printing as dyefixing agents. In some cases, however, they can reduce the light fastness of dyes used. This report deals with the effect of a polycationic compound, Polyfix 601, on the light fastness of CI Acid Red 249, an anionic 2-phenylazo-1-naphthol dye, in aqueous solutions and in PVA films. As a comparison, the much smaller ammonium cation was used, which had no observable effect on hue or dye light fastness. However, some changes to the dye's physical properties were observed. The effect of the polycation is explained as a consequence of dye aggregation, facilitated by presence of polycation. Self-sensitised singlet oxygen seemed to play no part in dye photofading, but other oxygen species could participate in the process.     

The synthesis of novel,visible-wavelength oxidizable polymerization sensitizers based on the 5,12-dihydroquinoxalino[2,3-b]pyridopyrazine skeleton

Novel dyes based on the 5,12-dihydroquinoxalino[2,3-b]pyridopyrazine skeleton were synthesized and subsequently characterized using ¹H NMR. Their electrochemical and spectral properties, such as absorption and emission spectra, quantum yield of fluorescence, and quantum yield of singlet oxygen generation, were also measured. The dyes were useful as photoinitiators for sensitizing compounds to photooxidation. Photoredox pairs comprising dyes and onium salts were found to be effective visible-wavelength initiators of free radical or cationic polymerization.     

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

The initial rates of photofading for 30 disperse azo dyes on nylon fabric upon exposure to a carbon arc in air have been analysed by formulating a kinetic equation that describes azo scission via the disproportionation reactions and intramolecular H‐transfer by two kinds of hydrazinyl radicals and the conversion of a nitro group to a nitroso group via the disproportionation reaction of nitrosyl hydroxide radicals. The five reaction rate constants are discussed in terms of the heats of reaction by calculating the heats of formation for the reactants, intermediates and products of each reaction using the PM5 method. Phenylazo‐ and thiazoleazo‐N,N‐substituted anilines and phenylazo‐pyridones exhibited large rate constants of multiple terms, while phenylazo‐phenols had the highest light fastness and very small rate constants for the disproportionation reactions of hydrazinyl radicals. Photofading on nylon fabric was primarily controlled by the thermal reactions of photo‐induced monohydrogenated dye radicals, which occurred via one or two primary multiple routes.     

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.     

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.