Dyeing is the process of importing colour to a textile material in loose fibre, yarn, cloth or garment form by treatment with a dye.

Dye can generally be described as a coloured substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and may require a mordant to improve the fastness of the dye on the fibre.

Dyestuffs are generally large group substances mostly organic used for colouring of textiles, inks, food products and other substances.

The colour properties of organic compounds depend on their structure. In general, the coloured compounds used as dyes are unsaturated organic chemicals. The quality of possessing colour is particularly noticeable in compounds containing certain well-defined chemically unsaturated groupings. These groups, known as chromophores (colour bearers), are not all equally effective in producing colour.

Chemical radicals, known as auxochromes, have the property of anchoring the desired dye effectively. They are acidic or basic and give rise to acid and basic dye salts. In the case of some compounds the addition of an auxochrome group also changes a colourless compound into a coloured one.

The basic raw materials of synthetic dyes are compounds, such as benzene, that are derived from the destructive distillation of coal. From these materials, intermediates are manufactured by a number of chemical processes that in general involve the substitution of specific elements or chemical radicals for one or more of the hydrogen atoms in the basic substance.

Dyestuffs can be classified according to their use or by their chemical structure. The chemical classification is generally made according to the nucleus of the compound. Among the more important dye groups are the azo dyes, which include butter yellow and Congo red; the triphenylmethane dyes, which include magenta and methyl violet; the phthalein dyes; the azine dyes, which include mauve; and the anthraquinone dyes, which include alizarin. Indigo is a vat dye, occurring in nature in the crystalline glucoside indican. An important new group of dyes is the phthalocyanine dyes, which are blue or green in colour and resemble chlorophyll in chemical structure. Of all the groups of dyes the azo dyes are the most generally useful and widely employed. See Diazo Compounds.

ORGANIC SYNTHESIS

A major unit of dyestuff is the Diazonium compound.

These are a group of organic compounds sharing a common functional group with the characteristic structure of R-N2+ X- where R can be any organic residue such alkyl or aryl and X is an inorganic or organic anion such as a halogen. Diazonium salts have been developed as important intermediates in the organic synthesis of dyes.

PREPARATION

The process of forming diazonium compounds is called diazotation, diazoniation, or diazotization.

The most important method for the preparation of diazonium salts is treatment of aromatic amines such as aniline with sodium nitrite in the presence of a mineral acid. In aqueous solution these salts are unstable at temperatures higher than +5 ?°C; the -N+≡N group tends to be lost as N2, i.e. nitrogen gas. One can isolate diazonium compounds as tetrafluoroborate salts, which are stable at room temperature. Typically diazonium compounds are not isolated and once prepared, used immediately in further reactions. For example, in the preparation of an aryl sulfonyl compound.

Reactions

The most important aromatic diazonium salt reactions are azo coupling with anilines and phenols to azo compounds (azo dyes) in electrophilic aromatic substitution.

Nitrogen replacement reactions by halogens take place in nucleophilic aromatic substitution such as the Sandmeyer Reaction, the Gomberg-Bachmann reaction and the Schiemann reaction. In the so-called Craig method, 2-aminopyridine reacts with sodium nitrite, hydrobromic acid and excess bromine to 2-bromopyridine [1]

In Meerwein arylation the salt also decomposes and the aryl residue reacts with an electron-deficient alkene in an addition reaction

In the Bamberger triazine synthesis and the Widman-Stoermer synthesis a diazonium salt reacts as an electrophile through its terminal nitrogen atom with an activated double bond.

Hydrolysis of diazonium salts yields alcohols

Reduction with hypophosphorous acid replaces the nitrogen by hydrogen, which allows amino and nitro groups to be removed easily from rings

Formation by Azo Coupling

An azo compound is formed by a reaction known as an azo coupling. It is an organic reaction between a diazonium compound and an aniline or a phenol. The reaction with phenol may be written as follows:

Mechanism: This reaction is called an electrophilic aromatic substitution. The diazonium salt acts as an electrophile, and the activated arene, a nucleophile. The reaction mechanism may be written as follows:

Synthesis of an Azo Dye – the Coupling Reaction of Benzenediazonium Ion with Naphthalen-2-ol

Dyes are used in almost every commercial product such as food, clothing, pigments and paints, etc. There are many different classes of dyes in which azo dyes are certainly one of the most important classes. About half of the dyes used in industry are azo dyes. Azo dyes have the basic structure, Ar??’N=N??’Ar’, where Ar and Ar’ are two aromatic groups.

The unit containing the nitrogen-nitrogen double bond is called an azo group. The

nature of the aromatic substituents on both sides of the azo group controls the colours of the azo compounds as well as the water-solubility of the dyes and how well they bind to a particular fabric.

Preparation of the target azo dye involves the conversion of 4-aminophenol to the diazonium ion intermediate 4-hydroxybenzenediazonium ion followed by the reaction with naphthalen-2-ol.

Naphthalen-2-ol dissolves poorly in acidic aqueous solutions. To prevent naphthalen-2-ol from precipitating out prematurely, the addition of the acidic benzenediazonium solution to the naphthalen-2-ol solution should be slow. The mixture forms a thick paste during addition.

Azobenzene

Azobenzene is a chemical compound composed of two phenyl rings linked by a N=N double bond. It is the best known example of an azo compound. The term ‘azobenzene’ or simply ‘azo’ is often used to refer to a wide class of molecules that share the core azobenzene structure, with different chemical functional groups extending from the phenyl rings. These azo compounds are considered as derivatives of diazene (diimide),[1] and are sometimes referred to as ‘diazenes’. The diazenes strongly absorb light and are used as dyes in a variety of industries.

Synthesis of Azobenzene

Azobenzene was first described in 1856 as "gelblich-rote krystallinische Blättchen" ("yellowish-red crystalline flakes" in German).[2] Its original preparation is similar to the modern one. According to the 1858 method, nitrobenzene is reduced by iron filings in the presence of acetic acid. In the modern synthesis, zinc is the reductant in the presence of a base.

Trans-cis isomerisation

One of the most intriguing properties of azobenzene (and derivatives) is the photoisomerization of trans and cis isomers. The two isomers can be switched with particular wavelengths of light: ultraviolet light, which corresponds to the energy gap of the π-π* (S2 state) transition, for trans-to-cis conversion, and blue light, which is equivalent to that of the n-π* (S1 state) transition, for cis-to-trans isomerization. For a variety of reasons, the cis isomer is less stable than the trans (for instance, it has a distorted configuration and is less delocalized than the trans configuration). Thus, cis-azobenzene will thermally relax back to the trans via cis-to-trans isomerization. The trans isomer is more stable by approximately 50 kJ/mol, and the barrier to photo-isomerization is approximately 200 kJ/mol.

Photophysics of isomerisation

The photo-isomerization of azobenzene is extremely rapid, occurring on picosecond timescales. The rate of the thermal back-relaxation varies greatly depending on the compound: usually hours for azobenzene-type molecules, minutes for aminoazobenzenes, and seconds for the pseudo-stilbenes.

The mechanism of isomerization has been the subject of some debate, with two pathways identified as viable: a rotation about the N-N bond, with disruption of the double bond, or via an inversion, with a semi-linear and hybridized transition state. It has been suggested that the trans-to-cis conversion occurs via rotation into the S2 state, whereas inversion gives rise to the cis-to-trans conversion. It is still under discussion which excited state plays a direct role in the series of the photoisomerization behavior. However, the latest research on femtosecond transition spectroscopy has suggested that the S2 state undergoes internal conversion to the S1 state, and then the trans-to-cis isomerization proceeds. Recently another isomerization pathway has been proposed by Diau , the "concerted inversion" pathway in which both CNN bond angles bend at the same time.

Photoinduced motions

The photo-isomerization of azobenzene is a form of light-induced molecular motion [5] [6] [7]. This isomerization can also lead to motion on larger length scales. For instance, polarized light will cause the molecules to isomerize and relax in random positions. However, those relaxed (trans) molecules that fall perpendicular to the incoming light polarization will no longer be able to absorb, and will remain fixed. Thus, there is a statistical enrichment of chromophores perpendicular to polarized light (orientational hole burning). Polarized irradiation will make an azo-material anisotropic and therefore optically birefringent and dichroic. This photo-orientation can also be used to orient other materials (especially in liquid crystal systems)[8]. For instance, it has been used to selectively orient liquid crystal domains, and used to create nonlinear optical (NLO) materials. Azo isomerization can also be used to photo-switch the liquid crystal phase of a material from cholesteric to nematic[9][10] or to change the pitch of a cholesteric phase.[11]

In 1995, it was reported that exposing a thin film of azo-polymer to a light intensity (or polarization) gradient leads to spontaneous surface patterns. In essence, the polymer material will reversibly deform so as to minimize the amount of material exposed to the light. This phenomenon is not laser ablation, since it readily occurs at low power and the transformation is reversible. This detailed mechanism of this surface holography is still unresolved, although it is clearly related to the azobenzene isomerization.

Bulk expansion and contraction of azobenzene materials have also been observed. In one report, a thin film was made to bend and unbend by exposing it to polarized light. The direction of the macroscopic motion could be controlled by the polarization direction. The bending occurred because the free surface of the material contracted more than the inside of the thin film (due to absorption of laser light as it passes through the film).

Azobenzene molecules can be incorporated into polymer matrices as stabilizers. It is also interesting to note that the rigid rod-like structure of azo molecules allows them to behave as liquid-crystal mesogens in many materials.

The large geometry change associated with azobenzene photoisomerization has also been used to control protein activity with light. Azobenzene has been attached to ligands (drug) to photo-modulate their affinity for proteins. Azobenzene has been employed as a photoswitchable tether between a ligand and the protein: one end of the azobenzene is substituted with a reactive group that attaches to the target protein. The other end displays a ligand for the protein. Depending on the where the azobenzene is attached, either the cis or trans isomer will present the ligand to the ligand-binding site, while the other isomer prevents the drug from reaching the site. Again, photoswitching between isomers turns the protein on and off. When applied to ion channels in the nervous system, this approach affords optical control of electrical activity in neurons.

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