Photoreduction of Azo Dyes by Reducing Agents Formed upon Photo-excitation of some Simple Aliphatic and Aromatic Compounds

Many organic compounds that absorb ultraviolet radiation can act as powerful reducing agents when irradiated in the ultraviolet. Such irradiation in the presence of azo dyes can lead to the photoreduction of the dyes. It is shown that simple ketyl radicals have strong reducing properties and can reduce azo dyes. It is demonstrated that similar radicals produced from glucose also act as reducing agents. The structural relation between glucose and cellulose suggests that the photoreduction can also play a role in the photofading of azo dyes on cellulosic textile materials.     

Formation of Hydrazyl Radicals and Hydrazo Compounds by Photoreduction of Azo Dyes

The photoreduction of eighteen different monoazo dyes in aqueous and ethanolic solutions, containing d, l -mandelic acid or acetone as hydrogen donors, to the corresponding amines has been investigated by flash-photolysis and rapid-flow techniques. The photoreduction of azo dyes proceeds in stages involving hydrazyl radicals and hydrazo compounds. The former are formed in a rapid reaction of the azo dyes with the active reducing agent produced upon absorption of light by the hydrogen donor. These radicals are unstable and disproportionate with formation of hydrazo compounds and regeneration of the original azo dye. The hydrazo compounds are also unstable and decompose with formation of amines and regeneration of the dye. The reaction schemes presented are supported by kinetic evidence and electron spin resonance measurements.     

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.     

Azo polymers — syntheses and reactions

Summary Polycarbonates containing different amounts of thermolabile azo functions have been synthesized. The resulting polymeric initiators have been used to start the polymerization of methyl methacrylate. The kinetic results are explained on the basis of a high degree of termination reactions by primary radicals.     

Room temperature polymerization of vinylic monomers by azo initiator

The azo initiator, sodium‐{4‐(4‐hydroxy‐phenylazo)‐benzenesulfonate (SHPBS)}, has been synthesized by coupling reaction with sodium‐(p‐sulfonate benzene diazonium chloride) and phenol. Unsymmetrical azo group leading to weak azo bond in case of SHPBS resulted in dissociation at lower temperature (25°C) to produce free radicals. Its versatile solubility (soluble in aqueous and organic solvents), generation of free radicals at 25°C, and its application without mechanical stirring are the added greater advantage over commercially available azo initiators (AIBN, etc.). The SHPBS has been characterized by FTIR, ¹H‐NMR spectroscopies, and thermal analysis. It has been observed that various monomers such as AM, AA, IPAM, and AMPS which have been polymerized with SHPBS have led to high molecular weight polymers at shorter reaction time. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Photo‐reductive fading of disperse azo dyes on nylon substrate: thermochemical analyses of reactions of photo‐induced radicals using the semiempirical molecular orbital method†

When the excited triplet states of disperse azo dyes with nitro groups abstract hydrogen to generate hydrazinyl (from azo groups) and nitrosyl hydroxide radicals (from nitro groups), both the radicals as H‐acceptors carry out azo scission, conversion to nitrogen dioxide via disproportionation reactions and self‐decomposition via rearrangement. A kinetic equation was formulated by the sum of these reactions, which describes the initial rates (K_PA) of reductive fading. The K_PA values were controlled by the rate constants of the reactions of hydrazinyl and nitrosyl hydroxide radicals as H‐acceptors, which were estimated by thermochemical analyses of the reactants, intermediates and end products using the semiempirical molecular orbital PM5 method, and by the concentrations of the reactants: H‐acceptors and H‐donors. The K_PA values observed for 12 dyes were explained semi‐quantitatively by multiple routes of reactions depending upon to what extent each radical reaction was thermochemically favoured.     

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

The photofading of phenylazo‐aniline, ‐pyridone and ‐quinolone disperse dyes on nylon fabric was analysed using the Kubelka–Munk (K/S) spectra of fabrics exposed to a carbon arc in air. The exposure of dyed fabric through ultraviolet and coloured filters, which do not shield the main absorption band, showed a large decrease in the initial rate (K_PA) of fading. Compared with the K_PA values without filters, the values using filters were half as large for dyes without the nitro groups and a quarter as large for dyes with the nitro groups. The K_PA values may be qualitatively explained by the sum of azo scission (decrease of K/S value at λ_max) and the conversion of nitro groups to nitroso groups. These phenomena occur, respectively, via thermal disproportionation reactions between hydrazinyl radicals (from the azo group) and the reaction between hydrazinyl and N‐centred nitrosyl hydroxide radicals (from the nitro group). The azo scission is promoted by N‐centred nitrosyl hydroxide radicals via the latter reactions.     

Products and intermediates formed during the biacetyl-sensitised photo-oxidation of some azo dyes

The flash irradiation for a few microseconds of aqueous aerobic solutions of biacetyl(2,3-butanedione) and a number of different o-arylazonaphthols resulted in the formation of the corresponding diazonium ions and α-naphthoquinones. This was verified from the intermediate spectra obtained after the flash. Verification for the formation of diazonium ions was also obtained from flow experiments, in which the diazonium ion was identified by coupling it to H-acid, forming a new azo compound. Less than equimolar formation of the diazonium ion from one of the azo dyes investigated indicated the occurrence of side reactions in which diazenyl radicals could play a role.     

Azo disperse dyes with polyfluoroalkyl groups attached to the nitrogen atom of the coupling component

A number of new azo disperse dyes with polyfluoroalkyl groups attached to the nitrogen atom of the coupling component have been prepared. The influence of polyfluoroalkyl groups on the colour and properties of azo dyes has been studied. The replacement of hydrogen atoms of the methyl group of the coupling component by polyfluoroalkyl radicals causes a hypsochromic shift of absorption maximum in a neutral solution and deepening of the dye colour in an acid solution. All tested dyes showed a high lightfastness on acetate and polyamide fibres.     

Surface grafting of polymer onto carbon thin film

The surface grafting of polymers onto carbon thin film deposited on a glass plate was achieved by two methods: the graft polymerization initiated by initiating groups introduced onto the surface; and the trapping of polymer radicals by surface aromatic rings of the thin film. It was found that the radical and cationic graft polymerization of vinyl monomers are initiated by azo and acylium perchlorate groups introduced onto the surface, respectively, and the corresponding polymers are grafted onto the surface: the surface grafting of polymers were confirmed by the contact angle of the surface with water. In addition, the anionic ring-opening alternating copolymerization of epoxides with cyclic acid anhydrides was found to be initiated by potassium carboxylate groups on the carbon thin film to give the corresponding polyester-grafted carbon thin film. On the other hand, polymer radicals formed by the decomposition of azo polymer, such as poly(polydimethylsiloxane-azobiscyanopentanoate) and poly(polyoxyethylene-azobiscyanopentanoate), were successfully trapped by the surface aromatic rings of carbon thin film and polydimethylsiloxane and polyoxyethylene were grafted onto the surface. © 1995 John Wiley & Sons, Inc.     

超声联合臭氧在水处理中的反应机理及应用

超声通过提高臭氧的分解速率、传质速率以及产生更多具有活性的自由基来强化臭氧的氧化能力,提高污染物的降解速率.利用超声与臭氧结合处理含氮染料废水以及水中难降解有机物如硝基芳香烃非常有效.处理过程中,臭氧投加量、超声的声能密度和环境的pH是主要的影响因素.     

Further discussion on the estimation of primary polymer chain length of vinyl-type network polymers based on the thermal decomposition of azo crosslinks

2,2′-Azobis[N-(2-propenyl)-2-methylpropionamide] (APMPA) having two carbon-carbon double bonds and an azo group was copolymerized with vinyl benzoate (VBz) at 60 °C, providing an azo groups containing VBz/APMPA prepolymer or crosslinked polymer which acts as a soluble or insoluble polymeric azo initiator, respectively. The gelation in VBz/APMPA (90/10 mol/mol) copolymerization was discussed briefly in order to reveal the characteristic polymerization behavior of APMPA as a novel crosslinker. Then, the resulting poly(VBz-co-APMPA)s as prepolymers or crosslinked polymers were thermally decomposed at elevated temperatures in the presence of lauryl mercaptan (LM) as a chain transfer agent; thus, the polymeric radicals generated through the cleavage of azo crosslinks would undergo the chain transfer reaction with LM to give primary polymer chains. This will provide a new approach to estimate the primary polymer chain length of vinyl-type network polymers by cutting all crosslinks in the network at the gel point as defined for the derivation of Stockmayer's equation.     

Sonochemical destruction of textile dyestuff in wasted dyebaths

Ultrasonic degradation at 300 kHz of basic azo and reactive anthraquinone dyes was investigated in dyebath wastewater at acidic and near-acidic pH. For all conditions, it was found that colour decay was exponential, levelling off at a non-zero plateau and slowing down with increased acidity. It was also found that the bleaching rate of a dyebath was more sensitive to the dye structure than to the dyebath matrix. The rate of azo dye bleaching was slower than that of anthraquinone dyes at both test pH levels regardless of the unfavourable conditions in the latter dyebath against oxidation by hydroxyl radicals. The addition of hydrogen peroxide to the dyebaths rendered a slight enhancement in the degree of colour decay and a significant increase in the overall degradation of the dyestuff.     

Photocatalytic Discoloration of Dyes: Relation between Effect of Operating Parameters and Dye Structure

The process of TiO₂/UV photocatalytic discoloration of wastewaters containing organic dyes is influenced by a number of variables. In this paper, the effects of some of these parameters, i.e., mass of catalyst, initial dye concentration, and pH, were related to the structural nature of the dyes investigated, azo and anthraquinone dyes, two families that are representative of about 75 % of the dyes marketed nowadays. Both bibliographical and own experimental data were used to analyze and discuss the photocatalytic discoloration in terms of dye structure and degradation pathways. It is shown that azo dyes (Methyl Orange, Metanil Yellow, Acid Orange 7, and Reactive Red 120) are more easily discolored than Reactive Blue 19 (anthraquinone dye), due to the higher molecular stability of the latter. It is also shown that azo dye discoloration is more influenced by the initial dye concentration, whereas recalcitrant anthraquinone dyes (Reactive Blue 19) mainly demand for basic pH values to generate enough OH^• radicals. Concerning TiO₂ loading, both the bibliographical and current research data showed no effect of this variable in the experimental range analyzed.     

Action of 1-(11-selenadodecyl)-glycerol and 1-(11-selenadodecyl)-3-trolox-glycerol against lipid peroxidation

The antioxidant action on lipid peroxidation of the synthesized selenium compounds 1-(11-selenadodecyl)-glycerol (SeG) and 1-(11-selenadodecyl)-3-Trolox-glycerol (SeIrG, where Trolox=6-hydroxyl-2,5,7,8-tetramethylchroman-2-carboxylic acid) was investigated. We compared the reactivity of the selenium compounds toward peroxyl radicals and their inhibitory effect on lipid peroxidation, induced by several kinds of initiating species such as azo compounds, metal ions, and superoxide/nitric oxide in solution, micelles, membranes, and rat plasma. SeTrG, but not SeG, scavenged peroxyl radicals. SeG reduced methyl linoleate hydroperoxides in organic solution and in methyl linoleate micelles oxidized by ferrous ion (Fe²⁺)/ascorbic acid. In rat plasma SeG and SeTrG decreased the formation of lipid hydroperoxides generated by hydrophilic azo compounds. SeG and SeTrG spared α-tocopherol (α-TOH) consumption in multilamellar vesicle membranes oxidized by hydrophilic or lipophilic initiators, and only SeTrG spared α-TOH in superoxide/nitric oxide oxidized membranes. In rat plasma oxidized by radical initiators (either hydrophilic or lipophilic) or superoxide/nitric oxide, SeTrG suppressed α-TOH consumption, but SeG had no effect. The two selenium-containing compounds showed inhibitory effects on lipid peroxidation that depended on their structure, the medium where they acted, and the oxidant used.     

Some Aspects of the Light Fastness of Reactive Azo-dyes on Cellulose under Wet Conditions

The photochemical behaviour of four reactive-azo dyes, (two dyes with poor wet light fastness and two dyes with normal wet light fastness) was examined to elucidate the mechanisms by which some reactive azo dyes, applied to cellulose, fade more rapidly when wet. In one case it was found that the major primary photochemical reaction is photoejection of electrons, which is enhanced in the aqueous environment, while in the second case, a back reaction between hydroxyl radicals and dye molecules is facilitated by the aqueous environment.     

Graft polymerization of vinyl monomers from initiating groups introduced onto polymethylsiloxane-coated titanium dioxide modified with alcoholic hydroxyl groups

The grafting of vinyl polymers onto the surface of polymethylsiloxane-coated titanium dioxide modified with alcoholic hydroxyl groups (Ti/Si–R–OH) were investigated. The introduction of azo and trichloroacetyl groups onto the surface of Ti/Si–R–OH was achieved by the reaction of the surface alcoholic hydroxyl groups with 4,4′-azobis(4-cyanopentanoic acid) and trichloroacetyl isocyanate, respectively. The radical polymerizations of vinyl monomers were successfully initiated by the azo groups introduced onto the surface and by the system consisting of Mo(CO)₆ and Ti/Si–R–COCCl₃. During the polymerization, the corresponding polymers were effectively grafted onto the titanium dioxide surface through propagation from surface radicals formed by the decomposition of azo groups and by the reaction of Mo(CO)₆ with trichloroacetyl groups on the surface. The percentage of grafting and grafting efficiency in the graft polymerization initiated by the system consisting of Ti/Si–R–COCCl₃ and Mo(CO)₆ were much larger than those initiated by azo groups. The polymer-grafted titanium dioxide was found to produce a stable colloidal dispersion in good solvents for the grafted polymer. The dispersibility of poly(N,N-diethylacrylamide)-grafted titanium dioxide in water was controlled by temperature. In addition, the wettability of the surface of titanium dioxide to water was readily controlled by grafting of hydrophilic or hydrophobic polymers.     

An Electron Spin Resonance Study of the Photolysis of Dyed Wool

By means of temperature-dependence studies of e.s.r. spectra it has been found possible to distinguish between the wool signal and the dye signal when dyed wool is subjected to ultra-violet irradiation. Two different observable patterns of behaviour were noted, allowing the dyes to be divided into two classes. The Class I dyes (e.g. azo, triphenylmethane dyes) gave e.s.r. spectra characteristic of the wool alone, despite the fact that there was evidence of both dye fading and dye-radical formation. Class II dyes gave e.s.r. responses characteristic of radicals in the dye and the wool. The two types of radicals were formed independently after the dye and wool had competed for the light energy available. These findings have been explained in terms of the photoconductivity behaviour of the dyes.