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Organic Dyes

Colours have immense effect on human population and it has psychological aesthetic functional and economic impact on society. The colorants are characterised by their ability to absorb or emit light in the visible range of 400 nm to 700 nm. The beginning in synthetic dye was made in the year 1856 by Perkin and over a period of almost one and half century several million various colour compounds are synthesised and produced on an industrial scale.

In terms of chemical structures the colorants can either have inorganic or organic compounds and both groups are further categorised into natural and synthetic forms. The colorants are either dyes or pigments and organic synthetic dyes remains a basic tool to get these. Pigments are attached to substrate whereas dyes are applied to various substrates.

 

Organic Dyes Chemistry

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The strong influence of synthetic organic chemistry on dye preparation is evident from the fact that many shades are obtained from first three reactive dyes for cellulose. And although thousands of different colorants commercially available yet new dyes and pigments are developed on a regular basis. The reaction mechanism of such dyes are studied and optimization of the processes along with diversification of application is also carried out.

The textile, leather, paper and hair colorants are prepared from a liquid in which dyes are partly or completely soluble. This is very important taking into account that dyes must have a specific affinity to a given substrate.

 Class of dye 
 Substrate 
 Disperse  Polyester and acetate 
 Reactive
 Cotton and viscose
 Acid
 Nylon wool and silk
 Direct
 Cotton and viscose
 Cationic
 Acrylic
 Others   Various

These includes sulfur, mordant and azoic dyes. The commercially available organic dyes for any optical recording media absorb very strongly near IR light close to 800 nm. The absorption characteritics of near IR light are the most important factors for development of organic dyes for such media.

There are almost 50 types of organic dyes that selectively absorb light in the range of 700 nm to 840 nm. The molecular structures of such dyes have polymethine, porphine, indanthrene, quinone or triphenylmethane as basic kinds.

Various research work has been carried out on polymethine cyanine dyes and the effect of heterocyclic moeity of cyanine dyes on absorption spectra and solubility of such compounds in organic solvents.

The characteristics of cyanine dyes are as follows:
  • A thin film can be easily produced by the spin coating process without using any binders.
  • Structures can be easily modified at the methine moiety and peripheral organic radicals.
  • Thin film has metallic lustre and monolayer thin film are available.
  • These are non-toxic.
  • Cyanine dyes can be decomposed by light and nascent oxygen
  • Not durable for repeat use.

Organic Chemistry Azo Dyes

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The azo dyes are the most important class of dyes and comprises over 50% of dye materials used world over. The prominence for the azo dye is mainly due to factors which worked for these category of dyes.
  • Azo dyes are twice as strong the strength of Anthroquinone and are reasonably stronger than any dye materials available. Hence tinctorially strong
  • They are produced easily and can be produced from cheap readily available materials.
  • These categories of dyes cover whole range of colour shade.
  • The azo dyes are quite resistant to fading or are fast and hence preferred over other dyes.
These are cost effective compared to any class of dyes and the easiness of preparation is what makes these category of dye the most sought after compounds.

But the azo dyes have their share of deficiencies as well. Especially compared to anthraquinone category these colour shades are dull and generally cannot equal the excellent light fastness of such dyes. These characterises more in the shade of blue. 

As far the theoretical organic chemistry is concerned the study of azo compounds have been a big revelation and hence azo dyes are widely used for development and the theories to check for colour and ingredients, the tautomerism, the indicator action and also the acid base equilibria.
 All azo dyes contain at least one or sometimes two aromatic residues which are attached to these azo group.

The more stable form of azo compound is the trans- form instead of the cis- form and both the N atoms are hybridised in $sp^{2}$  which eventually results in the carbon nitrogen bonding angles coming to 120o

Organic Chemistry Azo Dyes trans - form
The trans- form. 

Organic Chemistry Azo Dyes cis - form
The cis- form.
 
In the trans- azo benzene the basic azo chromogen is essentially a planar structure in both the physical states of solid and solution. It is although a non-planar structure in vapour phase. 

The bond length determination indicate some contribution from resonance forms, so that the carbon nitrogen bonding leads to shorter lengths than the expected length as compared to the nitrogen " nitrogen bond length.

Both the phenyl rings show the character of quinonoid. The electron donor group in one ring and electron acceptor groups in the other during the conjugation with azo linkage. These linkages help in the resonance contributions. Example: crystallography of methyl orange.

Preparation of Organic Dyes

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The coloured glasses such as stained glass are prepared from molten glass which contains a coloured transition metal ion like ferric $Fe^{3+}$, cobalt $Co^{2+}$ and Nickel $Ni^{2+}$.

The hues of glass plate are caused by the oxidation number of transition metal ion which easily transforms by the application of temperature and atmosphere.

The coloration of thin glass films by such inorganic compounds d-d transition of the transition metal ion.

On the other hand organic colorants have higher molar absorptivities due to $\pi - \pi$ transition of chromophore.

Hence they are capable of coloration at the same concentration in one hundredth to one thousandth of the thickness of material coloured by inorganic compounds.

The overall preparation of the polymer bound nitrite is as follows:

$Ar – NH_{2} + HNO_{2} \rightarrow Ar- N\equiv N \rightarrow Ar \rightarrow Ar - N = N-N$ 

The sol gel process has also become very popular in preparation of advanced glass and ceramics. This is also important in thin glass substrate coatings.

Organic Dyes Synthesis

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Although there is huge diversity of polymethine dyes like different heterocyclic donor and acceptor substituents, functional groups and variable chain length, yet there are only a few synthetic principles involved in their preparation.

The electrophilic and nucleophilic substitutions precede or follow the deprotonation. 

Organic Dyes Synthesis
 
The symmetric Z = Z and the asymmetric where Z ≠Z tri methine cyanine of the final product type can be easily synthesized from two heterocyclic salts that can bear a methyl group (Me) and a leaving group Y in their respective positions. 

This is first deprotonated with a tertiary aliphatic amine before the resulting C nucleophile.

In this reaction a variety of heterocyclic systems can be used. Indolins or (1,3,3 tri methyl 2 methyl idenindoline) benzoxazoles, benzoselenazoles or even quinoline derivatives. In all of these reactions the preferred leaving group is thioalkyl groups.

If two heterocyclic coupling reagents are both quinoline derivatives then cyanine with seven methine C atoms between the donor and the acceptor are obtained in such a compound.

The mechanism if this type of reaction is carried out with NaOH as base. 

In the synthesis of cyanine dyes with three methine groups, the standard method s to react the 2 equivalent of triethyl ortho formate which get replaced the $R - N + = C - Z$. 

Organic Dyes Synthesis

Apart from the orthoformate another two EtO groups also gets replaced. The third initiates the formation of the central $C = C$ bond by means of elimination of ethanol (EtOH). 

Organic Dyes Synthesis Mechanisms

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From the structural viewpoint there are three basic types of aromatic or highly conjugated systems that constitute the molecular architecture of dyes.

The first basic type consist of substituted benzene and fused polycyclic ring system like anthraquinone.

The second basic type comprises of di, tri, and poly-aryl ring systems such as biphenyl and stilbenes.

The third basic type consist of various heterocyclic ring system such as benzothiazoles, xanthenes quinolines etc.

The incorporation of these conjugated ring system and chemical species into dye molecules for effecting changing or improving fibre quality along with chromoprotic character and also the colour fastness requires synthetic strategies in conjugation of organic reaction mechanism.

Electrophilic aromatic substitution reactions in dye synthesis:
The most important electrophilic aromatic substitution or replacement of hydrogen atom in an aromatic or hetero-aromatic ring system by other atoms or functional groups or replacement of functional groups by hydrogen atom.

The presence of substituents of aromatic and hetero-aromatic rings influences reactivity towards electrophilic aromatic substitution and also the orientation of electrophilic agents into the aromatic nucleus.

If the substituent is already present in ring (Y) then it increases the reactivity of the ring towards an electrophilic reagent or species (E+) compared to unsubstituted ring. This is known as the activating group. 

If Y causes the reactivity of the ring towards an electrophilic agent to decrease compared to unsubstituted ring it is termed as deactivating group. 

Organic Dyes Synthesis Mechanisms
 
The increase or decrease in reactivity towards electrophiles is caused mainly by combination of inductive and resonance effects and less frequently by the steric effects.

If the inductive effect and resonance effect of functional group Y in the ring tends to make the ring more reactive then these are written as +I and +R effects.

In case Y substituents deactivate the ring towards electrophiles then they are designated as -I and "R effects.

The electrophilic aromatic substitution reactions are frequently employed for synthesis of dyes and their intermediates and such representative reactions include nitration, halogenation, sulfonation, Friedel craft acylation, carboxylation and formylation.
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