Indigo dye

Indigo dyeis anorganic compoundwith a distinctiveblue color. Indigo is anatural dyeobtained from the leaves of some plants of theIndigoferagenus, in particularIndigofera tinctoria. Dye-bearingIndigoferaplants were once common throughout the world.  It is now produced via chemical routes.  Blue colorants are rare.  Since indigo is insoluble, it is also referred to as a pigment (C.I. Pigment Blue 66, C.I.).
Most indigodyeproduced today issynthetic, constituting  around 80,000 tonnes each year, as of 2023.It is most commonly associated with the production ofdenimcloth andblue jeans, where its properties allow for effects such asstone washingandacid washingto be applied quickly.
The primary use for indigo is as a dye for cotton yarn, mainly used in the production of denim cloth suitable for blue jeans; on average, a pair of blue jeans requires 3 grams (0.11 oz) to 12 grams (0.42 oz) of dye. Smaller quantities are used in the dyeing of wool and silk.
Indigo carmine, also known as indigo, is an indigo derivative which is also used as a colorant. About 20,000 tonnes are produced annually, again mainly for the production of blue jeans.It is also used as a food colorant, and is listed in the United States asFD&CBlue No. 2.
A variety of plants have provided indigo throughout history, but most natural indigo was obtained from those in the genusIndigofera, which are native to the tropics, notably the Indian Subcontinent. The primary commercial indigo species in Asia was true indigo (Indigofera tinctoria, also known asI. sumatrana). A common alternative used in the relatively colder subtropical locations such as Japan'sRyukyuIslands and Taiwan isStrobilanthes cusia.
Until the introduction ofIndigoferaspecies from the south,Persicaria tinctoria(dyer's knotweed) was the most important blue dyestuff in East Asia; however, the crop produced less dyestuff than the average crop of indigo, and was quickly surpassed in favour of the more economicalIndigofera tinctoriaplant. InCentraland South America, the species grown isIndigofera suffruticosa, also known asanil, and in India, an important species wasIndigofera arrecta, Natal indigo. In Europe,Isatis tinctoria, commonly known as woad, was used for dyeing fabrics blue, containing the same dyeing compounds as indigo, also referred to as indigo.
Several plants contain indigo, which, when exposed to an oxidizing source such as atmospheric oxygen, reacts to produce indigo dye; however, the relatively low concentrations of indigo in these plants make them difficult to work with, with the color more easily tainted by other dye substances also present in these plants, typically leading to a greenish tinge.
The precursor to indigo isindican, a colorless, water-soluble derivative of the amino acidtryptophan, andIndigoferaleaves contain as much as 0.2–0.8% of this compound. Pressing cut leaves into a vat and soaking hydrolyzes the indican, releasing β-D-glucoseandindoxyl. The indoxyl dimerizes in the mixture, and after 12–15 hours offermentationyields the yellow, water-solubleleucoindigo. Subsequent exposure to air forms the blue, water-insoluble indigo dye.The dye precipitates from the fermented leaf solution upon oxidation, but may also be precipitated when mixed with a strong basesuch aslye. The solids are filtered, pressed into cakes, dried, and powdered. The powder is then mixed with various other substances to produce different shades of blue and purple.
Natural sources of indigo also include mollusks: theMurexgenus of sea snails produces a mixture of indigo and6,6'-dibromoindigo(red), which together produce a range of purple hues known asTyrian purple. Light exposure during part of the dyeing process can convert the dibromoindigo into indigo, resulting in blue hues known as royal blue, hyacinth purple, ortekhelet.
Given its economic importance, indigo has been prepared by many methods. TheBaeyer–Drewsen indigo synthesisdates back to 1882. It involves an aldol condensation of o-nitrobenzaldehyde with acetone, followed by cyclization and oxidative dimerization to indigo. This route was highly useful for obtaining indigo and many of its derivatives on the laboratory scale, but proved impractical for industrial-scale synthesis. Johannes PflegerandKarl Heumann[de]eventually came up with industrial mass production synthesis fromanilineby usingmercuryas a catalyst. The method was discovered by an accident by Karl Heumann in Zurich which involved a broken thermometer.
The first commercially practical route of producing indigo is credited to Pfleger in 1901. In this process,N-phenylglycineis treated with a molten mixture ofsodium hydroxide,potassium hydroxide, andsodamide. This highly sensitive melt producesindoxyl, which is subsequently oxidized in air to form indigo. Variations of this method are still in use today. An alternative and also viable route to indigo is credited to Heumann in 1897. It involves heatingN-(2-carboxyphenyl)glycine to 200 °C (392 °F) in an inert atmosphere with sodium hydroxide. The process is easier than the Pfleger method, but the precursors are more expensive. Indoxyl-2-carboxylic acid is generated. This material readily decarboxylates to give indoxyl, which oxidizes in air to form indigo.The preparation of indigo dye is practised in college laboratory classes according to the original Baeyer-Drewsen route.
The oldest known fabric dyed indigo, dated to 6,000 years ago, was discovered inHuaca Prieta, Peru.Many Asian countries, such asIndia, China, Japan, andSoutheast Asiannations have used indigo as a dye (particularly forsilk) for centuries. The dye was also known to ancient civilizations inMesopotamia,Egypt,Britain,Mesoamerica,Peru,Iran, andWest Africa. Indigo was also cultivated in India, which was also the earliest major center for its production and processing.TheIndigofera tinctoriaspecies was domesticated in India.Indigo, used as a dye, made its way to theGreeksand theRomans, where it was valued as a luxury product.
In Mesopotamia, a neo-Babyloniancuneiformtablet of the seventh century BC gives a recipe for the dyeing of wool, wherelapis-colored wool (uqnatu) is produced by repeated immersion and airing of the cloth.Indigo was most probably imported from India. The Romans used indigo as a pigment for painting and for medicinal and cosmetic purposes. It was a luxury item imported to the Mediterranean from India by Arab merchants.
India was a primary supplier of indigo to Europe as early as the Greco-Roman era. The association of India with indigo is reflected in the Greek word for the dye,indikón(Ἰνδικόν, Indian).The Romanslatinizedthe term toindicum, which passed intoItalian dialectand eventually into English as the word indigo.
InBengalindigo cultivators revolted against exploitative working conditions created by European merchants and planters in what became known as theIndigo revoltin 1859. The Bengali playNil Darpanby Indian playwrightDinabandhu Mitrawas a fictionalized retelling of the revolt.
The demand for indigo in the 19th century is indicated by the fact that in 1897, 7,000 km(2,700 sq mi) were dedicated to the cultivation of indican-producing plants, mainly inIndia. By comparison, the country ofLuxembourgis 2,586 km(998 sq mi).
In Europe, indigo remained a rare commodity throughout the Middle Ages. A chemically identical dye derived from the woad plant(Isatis tinctoria)was used instead. In the late 15th century, thePortugueseexplorerVasco da Gamadiscovered a sea route to India. This led to the establishment of direct trade with India, theSpice Islands, China, and Japan. Importers could now avoid the heavy duties imposed byPersian,Levantine, and Greek middlemen and the lengthy and dangerous land routes which had previously been used. Consequently, the importation and use of indigo in Europe rose significantly. Much European indigo from Asia arrived through ports in Portugal, the Netherlands, and England. Many indigo plantations were established by European powers in tropical climates. Spain imported the dye from its colonies in Central and South America, and it was a major crop inHaitiand Jamaica, with much or all of the labor performed by enslaved Africans and African Americans. In the Spanish colonial era, intensive production of indigo for the world market in the region of modern El Salvador entailed such unhealthy conditions that the local indigenous population, forced to labor in pestilential conditions, was decimated.Indigo plantations also thrived in theVirgin Islands. However, France and Germany outlawed imported indigo in the 16th century to protect the local woad dye industry. In central Europe, indigo resist dyeing is a centuries-old skill that has received UNESCO Intangible Cultural Heritage of Humanity recognition.
Newtonused "indigo" to describe one of the two newprimary colorshe added to the five he had originally named, in his revised account of the rainbow inLectiones Opticaeof 1675.
Because of its high value as a trading commodity, indigo was often referred to as blue gold.
Throughout West Africa, Indigo was the foundation of centuries-old textile traditions. From theTuaregnomads of theSaharatoCameroon, clothes dyed with indigo signified wealth. Women dyed the cloth in most areas, with theYorubaofNigeriaand theMandinkaofMaliparticularly well known for their expertise. Among theHausamale dyers, working at communal dye pits was the basis of the wealth of the ancient city ofKano, and they can still be seen plying their trade today at the same pits.The Tuareg are sometimes called the "Blue People" because theindigopigment in the cloth of their traditional robes and turbans stained their skin dark blue.
In Japan, indigo became especially important during theEdo period. This was due to a growing textiles industry,and because commoners had been banned from wearing silk,leading to the increasing cultivation of cotton, and consequently indigo – one of the few substances that could dye it.
In North America, indigo was introduced into colonial South Carolina byEliza Lucas, where it became the colony's second-most important cash crop (after rice).As a major export crop, indigo supported plantation slavery there.In the May and June 1755 issues ofThe Gentleman's Magazine,there appeared a detailed account of the cultivation of indigo, accompanied by drawings of necessary equipment and a prospective budget for starting such an operation, authored by South Carolina planterCharles Woodmason. It later appeared as a book.By 1775, indigo production in South Carolina exceeded 1,222,000 pounds.WhenBenjamin Franklinsailed to France in November 1776 to enlist France's support for theAmerican Revolutionary War, 35 barrels of indigo were on board theReprisal, the sale of which would help fund the war effort.In colonial North America, three commercially important species are found: the nativeI. caroliniana, and the introducedI. tinctoriaandI. suffruticosa.
In 1865 the German chemistAdolf von Baeyerbegan working on the synthesis of indigo. He described his first synthesis of indigo in 1878 (fromisatin) and a second synthesis in 1880 (from2-nitrobenzaldehyde). (It was not until 1883 that Baeyer finally determined the structure of indigo.) The synthesis of indigo remained impractical, so the search for alternative starting materials atBadische Anilin- und Soda-Fabrik(BASF) and Hoechst continued. Johannes PflegerandKarl Heumanneventually came up with industrial mass production synthesis.
The synthesis of N-(2-carboxyphenyl)glycine from the easy to obtainanilineprovided a new and economically attractive route. BASF developed a commercially feasible manufacturing process that was in use by 1897, at which time 19,000 tons of indigo were being produced from plant sources. This had dropped to 1,000 tons by 1914 and continued to contract. By 2011, 50,000 tons of synthetic indigo were being produced worldwide.
Indigo is a challenging dye because it is notsolublein water. To be dissolved, it must undergo a chemical change (reduction). Reduction converts indigo into "white indigo" (leuco-indigo). When a submerged fabric is removed from the dyebath, the white indigo quickly combines withoxygenin the air and reverts to the insoluble, intensely colored indigo. When it first became widely available in Europe in the 16th century, European dyers and printers struggled with indigo because of this distinctive property. It also required several chemical manipulations, some involving toxic materials, and presented many opportunities to injure workers. In the 19th century, English poetWilliam Wordsworthreferred to the plight of indigo dye workers of his hometown of Cockermouth in his autobiographical poemThe Prelude. Speaking of their dire working conditions and the empathy that he felt for them, he wrote:
A pre-industrial process for production of indigo white, used in Europe, was to dissolve the indigo in stale urine, which contains ammonia. A more convenient reductive agent is zinc. Another pre-industrial method, used in Japan, was to dissolve the indigo in a heated vat in which a culture ofthermophilic,anaerobicbacteria was maintained. Some species of such bacteria generatehydrogenas a metabolic product, which convert insoluble indigo into soluble indigo white. Cloth dyed in such a vat was decorated with the techniques ofshibori(tie-dye),kasuri,katazome, andtsutsugaki. Examples of clothing and banners dyed with these techniques can be seen in the works ofHokusaiand other artists.
Two different methods for the direct application of indigo were developed in England in the 18th century and remained in use well into the 19th century. The first method, known as 'pencil blue' because it was most often applied by pencil or brush, could be used to achieve dark hues.Arsenic trisulfideand a thickener were added to the indigo vat. The arsenic compound delayed the oxidation of the indigo long enough to paint the dye onto fabrics.
The second method was known as 'China blue' due to its resemblance to Chinese blue-and-white porcelain. Instead of using an indigo solution directly, the process involved printing the insoluble form of indigo onto the fabric. The indigo was then reduced in a sequence of baths ofiron(II) sulfate, with air oxidation between each immersion. The China blue process could make sharp designs, but it could not produce the dark hues possible with the pencil blue method.
Around 1880, the 'glucose process' was developed. It finally enabled the direct printing of indigo onto fabric and could produce inexpensive dark indigo prints unattainable with the China blue method.
Since 2004,freeze-driedindigo, or instant indigo, has become available. In this method, the indigo has already been reduced, and then freeze-dried into a crystal. The crystals are added to warm water to create the dye pot. As in a standard indigo dye pot, care has to be taken to avoid mixing in oxygen. Freeze-dried indigo is simple to use, and the crystals can be stored indefinitely as long as they are not exposed to moisture.
Indigo dye is a dark blue crystalline powder thatsublimesat 390–392 °C (734–738 °F). It is insoluble in water,alcohol, orether, but soluble inDMSO,chloroform,nitrobenzene, and concentratedsulfuric acid. Thechemical formulaof indigo isC16H10N2O2.
The molecule absorbs light in the orange part of the spectrum (λmax=613 nm).The compound owes its deep color to the conjugation of thedouble bonds, i.e. the double bonds within the molecule are adjacent and the molecule is planar. In indigo white, the conjugation is interrupted because the molecule is non-planar.
The benzene rings in indigo can be modified to give a variety of related dyestuffs.Thioindigo, where the two NH groups are replaced by S atoms, is deep red.Tyrian purpleis a dull purple dye that is secreted by a common Mediterranean snail. It was highly prized in antiquity. In 1909, its structure was shown to be6,6'-dibromoindigo(red). 6-bromoindigo (purple) is a component as well.It has never been produced on a commercial basis. The relatedCiba blue(5,7,5',7'-tetrabromoindigo) is, however, of commercial value.
Indigo and its derivatives featuring intra- and intermolecular hydrogen bonding have very low solubility in organic solvents. They can be made soluble using transientprotecting groupssuch as thetBOC group, which suppresses intermolecular bonding.Heating of the tBOC indigo results in efficient thermal deprotection and regeneration of the parent H-bonded pigment.
Treatment with sulfuric acid converts indigo into a blue-green derivative calledindigo carmine(sulfonated indigo). It became available in the mid-18th century. It is used as a colorant for food, pharmaceuticals, and cosmetics.
Indigo and some of its derivatives are known to be ambipolarorganic semiconductorswhen deposited as thin films by vacuum evaporation.
Indigo has a low oral toxicity, with anLD50of 5 g/kg (0.5% of total mass) in mammals.In 2009, large spills of blue dyes had been reported downstream of a blue jeans manufacturer inLesotho.
The compound has been found to act as anagonistof thearyl hydrocarbon receptor.