Azo dye

Physical properties, structure, and bonding

As a consequence of π-delocalization, aryl azo compounds have vivid colors, especially reds, oranges, and yellows. An example is Disperse Orange 1. Some azo compounds, e.g., methyl orange, are used as acid-base indicators. Most DVD-R/+R and some CD-R discs use blue azo dye as the recording layer.

Azo dyes are solids. Most are salts, the colored component usually being the anion, although some cationic azo dyes are known. The anionic character of most dyes arises from the presence of 1-3 sulfonic acid groups, which are fully ionized at the pH of the dyed article:

RSO₃H → RSO₃⁻ + H⁺

Most proteins are cationic, thus dyeing of leather and wool corresponds to an ion exchange reaction. The anionic dye adheres to these articles through electrostatic forces. Cationic azo dyes typically contain quaternary ammonium centers.

Reversible photoisomerization

A unique property of azo dyes is their ability to undergo reversible photoisomerization between trans and cis configurations. In the trans isomer, the aromatic rings (or other substituents) are on opposite sides of the double bond, resulting in a more extended and linear conformation. Upon exposure to ultraviolet (UV) light, typically at 365 nm, the molecule absorbs energy and converts to the cis isomer, where the substituents are on the same side of the N=N bond, leading to a bent or kinked structure.

This process is reversible: the cis form can thermally relax back to the trans form over time, or be actively reverted using visible light irradiation (commonly around 450–500 nm), depending on the substituents and molecular environment.

Preparation

Most azo dyes are prepared by azo coupling, which entails an electrophilic substitution reaction of an aryl diazonium cation with another compound, the coupling partner. Generally, coupling partners are other aromatic compounds with electron-donating groups:

ArN⁺
₂ + Ar′H → ArN=NAr′ + H⁺

In practice, acetoacetanilides are widely used as coupling partners:

ArN⁺
₂ + Ar′NHC(O)CH₂C(O)Me → ArN=NCH(C(O)Me)(C(O)NHAr′) + H⁺

Azo dyes are also prepared by the condensation of nitrated aromatic compounds with anilines followed by reduction of the resulting azoxy intermediate:

ArNO₂ + Ar′NH₂ → ArN(O)=NAr′ + H₂O

ArN(O)=NAr′ + C₆H₁₂O₆ → ArN=NAr′ + C₆H₁₀O₆ + H₂O

For textile dying, a typical nitro coupling partner would be disodium 4,4′-dinitrostilbene-2,2′-disulfonate. Typical aniline partners are shown below. Since anilines are prepared from nitro compounds, some azo dyes are produced by partial reduction of aromatic nitro compounds.

Many azo dyes are produced by reactions from pre-existing azo compounds. Typical reactions include metal complexation and acylation.

Azo pigments

Azo pigments are similar in chemical structure to azo dyes, but they lack solubilizing groups. Many so-called azo pigments are not strictly classifiable as azo compounds since they exist as keto hydrazide tautomers, which lack the -N=N- linkage.

Azo pigments are important in a variety of plastics, rubbers, and paints (including artist's paints). They have excellent coloring properties, mainly in the yellow to red range, as well as good lightfastness. The lightfastness depends not only on the properties of the organic azo compound, but also on the way they have been absorbed on the pigment carrier.

Biodegradation

In order for dyes to be useful, they must possess a high degree of chemical and photolytic stability. As a result of this stability, photolysis is not considered to be a degradation pathway for azo dyes. In order to prolong the lifetime of products dyed with azo dyes, it is essential to ensure stability against microbial attack, and tests have shown that azo dyes biodegrade negligibly in short term tests under aerobic conditions. Under anaerobic conditions, however, discoloration may be observed as a consequence of biodegradation.

Safety and regulation

Many azo pigments are non-toxic, although some, such as dinitroaniline orange, ortho-nitroaniline orange, or pigment orange 1, 2, and 5 are mutagenic and carcinogenic.

Azo dyes derived from benzidine are carcinogens; exposure to them has classically been associated with bladder cancer. Accordingly, the production of benzidine azo dyes was discontinued in the 1980s in many western countries.

European regulation

Certain azo dyes degrade under reductive conditions to release any of a group of defined aromatic amines. Since September 2003, the European Union has banned the manufacture or sale of consumer goods which contain the listed amines. Since only a small number of dyes produced those amines, relatively few products were actually affected.