Indigo dye#Chemical properties

File:Indigo plant extract sample.jpg

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 {{convert|3|g}} to {{convert|12|g}} 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.{{Ullmann|doi=10.1002/14356007.a14_149.pub3|title=Indigo and Indigo Colorants|year=2021|first1=Roshan|last1=Paul|first2=Richard S. |last2=Blackburn|first3=Thomas |last3=Bechtold|isbn=3527306730}}

Indigo is also used to dye the skin in Egypt, Kurdistan,{{Cite book |last=Brauer |first=Erich |title=The Jews of Kurdistan}} and West Africa.{{Cite web |last=Jakobsen |first=Mads |date=2016-09-07 |title=An Introduction to the Indigo Dye Styles of Western Africa |url=https://www.heddels.com/2016/09/an-introduction-to-the-indigo-dye-styles-of-western-africa/ |access-date=2025-02-26 |website=Heddels}}{{Cite book |last=McKinley |first=Catherine E. |title=Indigo: In Search of the Color that Seduced the World|publisher=Bloomsbury | isbn=9781608195053 | year=2011}}

{{see also|#Indigo derivatives}}

A variety of plants have provided indigo throughout history, but most natural indigo was obtained from those in the genus Indigofera, which are native to the tropics, notably the Indian Subcontinent. The primary commercial indigo species in Asia was true indigo (Indigofera tinctoria, also known as I. sumatrana). A common alternative used in the relatively colder subtropical locations such as Japan's {{lang|ja-Latn|Ryukyu|italic=no}} Islands and Taiwan is Strobilanthes cusia.

Until the introduction of Indigofera species 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 economical Indigofera tinctoria plant. In Central and South America, the species grown is Indigofera suffruticosa, also known as anil, and in India, an important species was Indigofera 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 is indican, a colorless, water-soluble derivative of the amino acid tryptophan, and Indigofera leaves contain as much as 0.2–0.8% of this compound. Pressing cut leaves into a vat and soaking hydrolyzes the indican, releasing β-{{sc|D}}-glucose and indoxyl. The indoxyl dimerizes in the mixture, and after 12–15 hours of fermentation yields the yellow, water-soluble leucoindigo. Subsequent exposure to air forms the blue, water-insoluble indigo dye.{{cite book |last=Schorlemmer |first=Carl |author-link=Carl Schorlemmer |title=A Manual of the Chemistry of the Carbon compounds; or, Organic Chemistry |url=https://archive.org/details/amanualchemistr02schogoog |location=London |publisher=Macmillan and co. |year=1874}} Quoted in the Oxford English Dictionary, second edition, 1989{{cite book |last1=Freeman |first1=H. S. |title=Colorants for non-textile applications |last2=Peters |first2=A. T. |publisher=Elsevier |year=2000 |isbn=978-0-444-82888-0 |location=Amsterdam [Netherlands] New York |pages=382–455 |chapter=9 - Natural Dyes}} The dye precipitates from the fermented leaf solution upon oxidation, but may also be precipitated when mixed with a strong base{{cite web |url=https://www.coyuchi.com/the-naturalista/indigo_dyeing/ |title=Indigo Dyeing |website=Coyuchi Inc. |language=en |access-date=2019-05-24 |archive-date=2019-05-24 |archive-url=https://web.archive.org/web/20190524001631/https://www.coyuchi.com/the-naturalista/indigo_dyeing/ |url-status=dead}} such as lye. 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: the Murex genus of sea snails produces a mixture of indigo and 6,6'-dibromoindigo (red), which together produce a range of purple hues known as Tyrian 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, or tekhelet.

Given its economic importance, indigo has been prepared by many methods. The Baeyer–Drewsen indigo synthesis dates 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 Pfleger and {{ill|Karl Heumann|de}} eventually came up with industrial mass production synthesis from aniline by using mercury as 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-phenylglycine is treated with a molten mixture of sodium hydroxide, potassium hydroxide, and sodamide. This highly sensitive melt produces indoxyl, 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 heating N-(2-carboxyphenyl)glycine to {{convert|200|°C}} 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.{{cite journal |last1=McKee |first1=James R. |last2=Zanger |first2=Murray |year=1991 |title=A microscale synthesis of indigo: Vat dyeing |journal=Journal of Chemical Education |volume=68 |issue=10 |page=A242 |doi=10.1021/ed068pA242 |bibcode=1991JChEd..68..242M}}

File:Indigo-Historische Farbstoffsammlung.jpg, Germany]]

The oldest known fabric dyed indigo, dated to 6,000 years ago, was discovered in Huaca Prieta, Peru.{{cite journal |vauthors=Splitstoser JC, Dillehay TD, Wouters J, Claro A |date=2016-09-14 |title=Early pre-Hispanic use of indigo blue in Peru |journal=Science Advances |volume=2 |issue=9 |pages=e1501623 |doi=10.1126/sciadv.1501623 |pmid=27652337 |pmc=5023320 |bibcode=2016SciA....2E1623S}} Many Asian countries, such as India, China, Japan, and Southeast Asian nations have used indigo as a dye (particularly for silk) for centuries. The dye was also known to ancient civilizations in Mesopotamia, Egypt, Britain, Mesoamerica, Peru, Iran, and West Africa. Indigo was also cultivated in India, which was also the earliest major center for its production and processing.Kriger & Connah, page 120 The Indigofera tinctoria species was domesticated in India. Indigo, used as a dye, made its way to the Greeks and the Romans, where it was valued as a luxury product.

In Mesopotamia, a neo-Babylonian cuneiform tablet of the seventh century BC gives a recipe for the dyeing of wool, where lapis-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.

File:Indian indigo dye lump.jpg

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 ({{lang|grc|Ἰνδικόν}}, Indian).{{cite book |title=The Secret Lives of Colour |last=St. Clair |first=Kassia |publisher=John Murray |year=2016 |isbn=9781473630819 |location=London |page=189 |oclc=936144129}} The Romans latinized the term to indicum, which passed into Italian dialect and eventually into English as the word indigo.

In Bengal indigo cultivators revolted against exploitative working conditions created by European merchants and planters in what became known as the Indigo revolt in 1859. The Bengali play Nil Darpan by Indian playwright Dinabandhu Mitra was a fictionalized retelling of the revolt.

File:Ridpath's history of the world; being an account of the ethnic origin, primitive estate, early migrations, social conditions and present promise of the principal families of men (1897) (14597001160).jpg, India, drawn by {{Interlanguage link|Émile Thérond|fr|3=Émile Thérond}}, 19th century]]

The demand for indigo in the 19th century is indicated by the fact that in 1897, {{convert|7000|sqkm|sqmi|abbr=on}} were dedicated to the cultivation of indican-producing plants, mainly in India. By comparison, the country of Luxembourg is {{convert|2,586|sqkm|sqmi|abbr=on}}.

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, the Portuguese explorer Vasco da Gama discovered a sea route to India. This led to the establishment of direct trade with India, the Spice Islands, China, and Japan. Importers could now avoid the heavy duties imposed by Persian, 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 in Haiti and 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.Fowler, Walter (6 August 1991). The Formation of Complex Society in Southeastern Mesoamerica. CRC Press. Indigo plantations also thrived in the Virgin 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.{{cite web |last=Denisyuk |first=Yulia |title=Europe's secret dyeing formula |url=https://www.bbc.com/travel/article/20230420-blaudruck-europes-secret-dyeing-formula |access-date=2023-04-21 |website=bbc.com |language=en}}

Newton used "indigo" to describe one of the two new primary colors he added to the five he had originally named, in his revised account of the rainbow in Lectiones Opticae of 1675.Quoted in {{cite book |last=Hentschel |first=Klaus |title=Mapping the spectrum: techniques of visual representation in research and teaching |publisher=Oxford University Press |location=Oxford, England |year=2002 |page=28 |isbn=978-0-19-850953-0}}

Because of its high value as a trading commodity, indigo was often referred to as blue gold.{{cite web |title=History of Indigo & Indigo Dyeing |url=http://www.wildcolours.co.uk/html/indigo_history.html |website=wildcolours.co.uk |publisher=Wild Colours and natural Dyes |access-date=30 December 2015 |quote=Indigo was often referred to as Blue Gold as it was an ideal trading commodity; high value, compact and long lasting}}

In the early days of Islam, Christians were expected to wear a blue turban or mantle to identify them. In Egypt, which was majority Christian and remained so for generations, this created a high demand for blue dye, particularly indigo. Even though regulation of the distinguishing dress relaxed from the 10-16th centuries, indigo remained a significant part of the Egyptian economy. It lost its Christian connotations and became the color of the people's dress, because it was plentiful and cheap. It remained so until the 19th century. The British taxed farmers who grew indigo to encourage people to grow cotton, and restricted water use in favor of cotton. This also affected flax, which had been a popular local fiber for indigo dyers. This, paired with the import of cheap black cotton fabrics from Britain, caused blue to fall out of favor in Egyptian women's dress in the early 20th century. Egyptian men continued to wear indigo for some time, but the total preference for the dye waned as well.{{Cite book |last=Mehrez |first=Shahira |title=Costumes of Egypt: The Lost Legacies}} Blue was also a mourning color, and mourning women would dye their faces, arms, and hands blue for the week after the death and periodic grave visitations. Blue cloths were worn and waved around at these times as well to accompany the women's wailing.{{Cite book |last=Blackman |first=Winifred |title=The Fellahin of Upper Egypt}}

File:Touaregs at the Festival au Desert near Timbuktu, Mali 2012.jpg wearing the indigo-dyed tagelmust]]

Throughout West Africa, Indigo was the foundation of centuries-old textile traditions. From the Tuareg nomads of the Sahara to Cameroon, clothes dyed with indigo signified wealth. Women dyed the cloth in most areas, with the Yoruba of Nigeria and the Mandinka of Mali particularly well known for their expertise. Among the Hausa male dyers, working at communal dye pits was the basis of the wealth of the ancient city of Kano, and they can still be seen plying their trade today at the same pits.Kriger, Colleen E. & Connah, Graham (2006). Cloth in West African History. Rowman Altamira. {{ISBN|0-7591-0422-0}}. The Tuareg are sometimes called the "Blue People" because the indigo pigment in the cloth of their traditional robes and turbans stained their skin dark blue.Gearon, Eamonn, (2011) The Sahara: A Cultural History Oxford University Press, p. 239

Palestine and Iran also historically produced large amounts of indigo. In Palestine, it was a major industry since at least the 17th century. It was used to dye southern women's dresses, coats in Galilee, pants all over, and men's cloaks. A similar blue-to-black shift occurred due to British colonialism, though blue is still seen on traditional Palestinian dresses with some frequency in synthetic form. Widows would dye their dresses with indigo to cover the other colors, and wear blue veils and belts. Blue was also associated with unmarried girls, whose dresses were embroidered with it.{{Cite book |last=Weir |first=Shelagh |title=Palestinian Costume}}

File:Arimatsu Shibori手蜘蛛絞.jpg}} dying technique traditionally used in Japan; cloth ready for dyeing in indigo, and after dyeing]]

In Japan, indigo became especially important during the Edo period. This was due to a growing textiles industry,{{cite book |author=Eiko Ikegami |title=Bonds of Civility: Aesthetic Networks and the Political Origins of Japanese Culture |url=https://books.google.com/books?id=KsN81J1s70kC&pg=PA284 |date=28 February 2005 |publisher=Cambridge University Press |isbn=978-0-521-60115-3 |page=284}} and because commoners had been banned from wearing silk,{{cite book |author=John H. Sagers |title=Confucian Capitalism: Shibusawa Eiichi, Business Ethics, and Economic Development in Meiji Japan |url=https://books.google.com/books?id=1-1lDwAAQBAJ&pg=PA27 |date=20 July 2018 |publisher=Springer |isbn=978-3-319-76372-9 |page=27}} leading to the increasing cultivation of cotton, and consequently indigo – one of the few substances that could dye it.{{cite book |author=Trudy M. Wassenaar |title=Bacteria: The Benign, the Bad, and the Beautiful |url=https://books.google.com/books?id=YGtR43I93iMC&pg=PA105 |date=3 November 2011 |publisher=John Wiley & Sons |isbn=978-1-118-14338-4 |page=105}}

In North America, indigo was introduced into colonial South Carolina by Eliza Lucas, where it became the colony's second-most important cash crop (after rice).{{cite web |title=Eliza Lucas Pinckney: Indigo in the Atlantic World |author=Eliza Layne Martin |url=http://cwh.ucsc.edu/SocialBiog.Martin.pdf |access-date=2013-08-24 |url-status=dead |archive-url=https://web.archive.org/web/20100607061823/http://cwh.ucsc.edu/SocialBiog.Martin.pdf |archive-date=2010-06-07}} As a major export crop, indigo supported plantation slavery there.Andrea Feeser, Red, White, and Black Make Blue: Indigo in the Fabric of Colonial South Carolina Life (University of Georgia Press; 2013) In the May and June 1755 issues of The 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 planter Charles Woodmason. It later appeared as a book.{{cite journal |last1=Jones |first1=Claude E. |year=1958 |title=Charles Woodmason as a Poet |journal=The South Carolina Historical Magazine |volume=59 |issue=4 |pages=189–194}}{{cite book |last=Shields |first=David S. |title=Oracles of Empire: Poetry, Politics, and Commerce in British America, 1690-1750 |year=2010 |publisher=University of Chicago Press |pages=69, 249}} By 1775, indigo production in South Carolina exceeded 1,222,000 pounds.{{cite book |editor-last=Edgar |editor-first=Walter B. |title=The South Carolina Encyclopedia |year=2006 |publisher=University of South Carolina Press |page=9}} When Benjamin Franklin sailed to France in November 1776 to enlist France's support for the American Revolutionary War, 35 barrels of indigo were on board the Reprisal, the sale of which would help fund the war effort.{{cite book |last=Schoenbrun |first=David |date=1976 |title=Triumph in Paris: The Exploits of Benjamin Franklin |publisher=Harper & Row |location=New York |isbn=978-0-06-013854-7 |page=51}} In colonial North America, three commercially important species are found: the native I. caroliniana, and the introduced I. tinctoria and I. suffruticosa.{{cite journal |journal=Economic Botany |volume=33 |issue=2 |year=1979 |pages=128–134 |title=The indigo of commerce in colonial North America |author=David H. Rembert Jr. |doi=10.1007/BF02858281 |s2cid=2488865}}

File:Indigoproduktion BASF 1890.JPG plant (1890)]]

In 1865 the German chemist Adolf von Baeyer began working on the synthesis of indigo. He described his first synthesis of indigo in 1878 (from isatin) and a second synthesis in 1880 (from 2-nitrobenzaldehyde). (It was not until 1883 that Baeyer finally determined the structure of indigo.Adolf Baeyer (1883) [https://gallica.bnf.fr/ark:/12148/bpt6k90697p/f512.image.langEN "Ueber die Verbindungen der Indigogruppe"] [On the compounds of the indigo group], Berichte der Deutschen chemischen Gesellschaft zu Berlin, 16 : 2188-2204; see especially p. 2204.) The synthesis of indigo remained impractical, so the search for alternative starting materials at Badische Anilin- und Soda-Fabrik (BASF) and Hoechst continued. Johannes Pfleger{{cite web |url=https://history.evonik.com/sites/geschichte/en/personalities/pfleger-johannes/pages/default.aspx |title=Johannes Pfleger - Das Evonik Geschichtsportal - Die Geschichte von Evonik Industries |website=history.evonik.com |access-date=7 June 2020 |archive-date=1 August 2020 |archive-url=https://web.archive.org/web/20200801202949/https://history.evonik.com/sites/geschichte/en/personalities/pfleger-johannes/pages/default.aspx |url-status=dead}} and Karl Heumann eventually came up with industrial mass production synthesis.{{cite web |url=http://www.ingenious.org.uk/site.asp?s=RM&Param=1&SubParam=1&Content=1&ArticleID=%7BCBDF1082-9F5C-498F-A769-B33A7DA83B30%7D&ArticleID2=%7B3C4444FC-FC4D-4498-B0B4-8B8A47C5BA76%7D&MenuLinkID=%7BA54FA022-17E2-483C-B937-DEC8B8964C33%7D |title=The Synthesis of Indigo |access-date=2015-01-05 |archive-url=https://web.archive.org/web/20160304084155/http://www.ingenious.org.uk/site.asp?s=RM&Param=1&SubParam=1&Content=1&ArticleID=%7BCBDF1082-9F5C-498F-A769-B33A7DA83B30%7D&ArticleID2=%7B3C4444FC-FC4D-4498-B0B4-8B8A47C5BA76%7D&MenuLinkID=%7BA54FA022-17E2-483C-B937-DEC8B8964C33%7D |archive-date=2016-03-04 |url-status=dead}}

The synthesis of N-(2-carboxyphenyl)glycine from the easy to obtain aniline provided 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.{{cite journal |title=Chemists go green to make better blue jeans |journal=Nature |year=2018 |volume=553 |issue=7687 |page=128 |doi=10.1038/d41586-018-00103-8 |bibcode=2018Natur.553..128. |doi-access=free}}

File:Leucoindigo.svg

File:IndigoDyedYarn.JPG

Indigo is a challenging dye because it is not soluble in 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 with oxygen in 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 poet William Wordsworth referred to the plight of indigo dye workers of his hometown of Cockermouth in his autobiographical poem The Prelude. Speaking of their dire working conditions and the empathy that he felt for them, he wrote:

{{Poem quote|

Doubtless, I should have then made common cause

With some who perished; haply perished too

A poor mistaken and bewildered offering

Unknown to those bare souls of miller blue}}

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 of thermophilic, anaerobic bacteria was maintained. Some species of such bacteria generate hydrogen as a metabolic product, which convert insoluble indigo into soluble indigo white. Cloth dyed in such a vat was decorated with the techniques of shibori (tie-dye), kasuri, katazome, and tsutsugaki. Examples of clothing and banners dyed with these techniques can be seen in the works of Hokusai and 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 trisulfide and 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.{{Citation needed|date=May 2024}}

File:IndigoDyePotOnStove.JPG

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 of iron(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-dried indigo, 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.{{cite web |title=Directions for Instant Indigo |author=Judith McKenzie McCuin |url=http://www.paradisefibers.com/instantindigo.htm |access-date=2008-05-06 |url-status=dead |archive-url=https://web.archive.org/web/20041116195957/http://paradisefibers.com/instantindigo.htm |archive-date=2004-11-16}}

File:Indigo3D.png

Indigo dye is a dark blue crystalline powder that sublimes at {{convert|390|–|392|°C}}. It is insoluble in water, alcohol, or ether, but soluble in DMSO, chloroform, nitrobenzene, and concentrated sulfuric acid. The chemical formula of indigo is C₁₆H₁₀N₂O₂.

The molecule absorbs light in the orange part of the spectrum (λ_max=613 nm).{{cite journal |last1=Wouten |first1=J. |last2=Verhecken |first2=A. |year=1991 |title=High-performance liquid chromatography of blue and purple indigoid natural dyes |journal=Journal of the Society of Dyers and Colourists |volume=107 |issue=7–8 |pages=266–269 |doi=10.1111/j.1478-4408.1991.tb01351.x}} The compound owes its deep color to the conjugation of the double 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.

File:Tyrian-Purple.svg

File:Indigo carmine.svg

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 purple is 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 be 6,6'-dibromoindigo (red). 6-bromoindigo (purple) is a component as well.{{cite journal |last1=Ramig |first1=Keith |last2=Lavinda |first2=Olga |last3=Szalda |first3=David J. |last4=Mironova |first4=Irina |last5=Karimi |first5=Sasan |last6=Pozzi |first6=Federica |last7=Shah |first7=Nilam |last8=Samson |first8=Jacopo |last9=Ajiki |first9=Hiroko |last10=Massa |first10=Lou |last11=Mantzouris |first11=Dimitrios |last12=Karapanagiotis |first12=Ioannis |last13=Cooksey |first13=Christopher |title=The nature of thermochromic effects in dyeings with indigo, 6-bromoindigo, and 6,6'-dibromoindigo, components of Tyrian purple |journal=Dyes and Pigments |date=June 2015 |volume=117 |pages=37–48 |doi=10.1016/j.dyepig.2015.01.025}} It has never been produced on a commercial basis. The related Ciba 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 transient protecting groups such as the tBOC group, which suppresses intermolecular bonding.{{cite journal |last1=Głowacki |first1=Eric Daniel |last2=Voss |first2=Gundula |last3=Demirak |first3=Kadir |last4=Havlicek |first4=Marek |last5=Sünger |first5=Nevsal |last6=Okur |first6=Aysu Ceren |last7=Monkowius |first7=Uwe |last8=Gąsiorowski |first8=Jacek |last9=Leonata |first9=Lucia |last10=Sariciftcia |first10=Niyazi Serdar |display-authors=5 |date=2013 |title=A facile protection–deprotection route for obtaining indigo pigments as thin films and their applications in organic bulk heterojunctions |journal=Chemical Communications |volume=49 |issue=54 |pages=6063–6065 |doi=10.1039/C3CC42889C |pmid=23723050}} 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 called indigo carmine (sulfonated indigo). It became available in the mid-18th century. It is used as a colorant for food, pharmaceuticals, and cosmetics as FD&C Blue No. 2.{{cite web |title=Regulatory Status of Color Additives: FD&C Blue No. 2 |url=https://hfpappexternal.fda.gov/scripts/fdcc/index.cfm?set=ColorAdditives&id=FDCBlue2 |website=U.S. Department of Health and Human Services |access-date=29 April 2025}}

Indigo and some of its derivatives are known to be ambipolar organic semiconductors when deposited as thin films by vacuum evaporation.{{cite journal |doi=10.1002/adma.201102619 |pmid=22109816 |title=Indigo - A Natural Pigment for High Performance Ambipolar Organic Field Effect Transistors and Circuits |year=2012 |last1=Irimia-Vladu |first1=Mihai |last2=Głowacki |first2=Eric D. |last3=Troshin |first3=Pavel A. |last4=Schwabegger |first4=Günther |last5=Leonat |first5=Lucia |last6=Susarova |first6=Diana K. |last7=Krystal |first7=Olga |last8=Ullah |first8=Mujeeb |last9=Kanbur |first9=Yasin |last10=Bodea |first10=Marius A. |last11=Razum |first11=Vladimir F. |last12=Sitter |first12=Helmut |last13=Bauer |first13=Siegfried |author14-link=Niyazi Serdar Sarıçiftçi |last14=Sarıçiftçi |first14=Niyazi Serdar |journal=Advanced Materials |volume=24 |issue=3 |pages=375–380 |bibcode=2012AdM....24..375I |s2cid=205241976|hdl=11511/68594 |hdl-access=free }}

Indigo has a low oral toxicity, with an {{LD50}} of 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 in Lesotho.{{cite web |title=Gap alarm |date=2009-08-09 |newspaper=The Sunday Times |url=http://www.timesonline.co.uk/tol/news/world/africa/article6788728.ece |archive-url=https://archive.today/20100528081251/http://www.timesonline.co.uk/tol/news/world/africa/article6788728.ece |url-status=dead |archive-date=May 28, 2010 |access-date=2011-08-16}}

The compound has been found to act as an agonist of the aryl hydrocarbon receptor.{{cite journal |vauthors=Denison MS, Nagy SR |title=Activation of the aryl hydrocarbon receptor by structurally diverse exogenous and endogenous chemicals |journal=Annu. Rev. Pharmacol. Toxicol. |volume=43 |pages=309–334 |year=2003 |pmid=12540743 |doi=10.1146/annurev.pharmtox.43.100901.135828}}

• Champaran Satyagraha
• Haint blue
• Indigo revolt
• Red, White, and Black Make Blue

{{Reflist}}

• {{cite book |last=Balfour-Paul |first=Jenny |title=Indigo: Egyptian Mummies to Blue Jeans |publisher=British Museum Press |year=2016 |isbn=978-0-7141-1776-8 |location=London |pages=264 pages}}
• {{cite journal |last1=Ferreira |first1=E.S.B. |last2=Hulme A. N. |author2-link=Alison Hulme |last3=McNab H. |last4=Quye A. |year=2004 |title=The natural constituents of historical textile dyes |url=http://eprints.gla.ac.uk/109139/1/109148.pdf |journal=Chemical Society Reviews |volume=33 |issue=6 |pages=329–36 |doi=10.1039/b305697j |pmid=15280965}}
• Paul, Jenny Balfour. 2020. "Indigo and Blue: A Marriage Made in Heaven." Textile Museum Journal 47 (January): 160–85.
• {{cite journal |last=Sequin-Frey |first=Margareta |year=1981 |title=The chemistry of plant and animal dyes |url=http://jchemed.chem.wisc.edu/Journal/Issues/1981/Apr/jceSubscriber/JCE1981p0301.pdf |journal=Journal of Chemical Education |volume=58 |issue=4 |pages=301 |bibcode=1981JChEd..58..301S |doi=10.1021/ed058p301}}

• [https://web.archive.org/web/20080509184103/http://www.plantcultures.org/plants/indigo_landing.html Plant Cultures: botany, history and uses of indigo]
• [https://www.accessdata.fda.gov/scripts/cdrh/cfdocs/cfcfr/CFRSearch.cfm?fr=74.102 FD&C regulation on indigotine]

{{Dyeing}}

{{Aryl hydrocarbon receptor modulators}}

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Category:Enones

Category:Indigo structure dyes

Category:Indolines

Category:Organic pigments

Category:Organic semiconductors