Editing Indigo dye (section)

==Chemical properties==
[[File:Indigo3D.png|thumb|170px|Indigo, space-filling]]
Indigo dye is a dark blue crystalline powder that [[Sublimation (chemistry)|sublimes]] at {{convert|390|–|392|°C}}. It is insoluble in water, [[ethanol|alcohol]], or [[diethyl ether|ether]], but soluble in [[dimethyl sulfoxide|DMSO]], [[chloroform]], [[nitrobenzene]], and concentrated [[sulfuric acid]]. The [[chemical formula]] of indigo is [[carbon|C]]<sub>16</sub>[[hydrogen|H]]<sub>10</sub>[[nitrogen|N]]<sub>2</sub>[[oxygen|O]]<sub>2</sub>.

The molecule absorbs light in the orange part of the spectrum (''λ''<sub>max</sub>=613&nbsp;nm).<ref>{{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}}</ref> The compound owes its deep color to the conjugation of the [[double bond]]s, 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.

===Indigo derivatives===
[[File:Tyrian-Purple.svg|thumb|class=skin-invert-image|200px|Structure of Tyrian purple]]
[[File:Indigo carmine.svg|thumb|class=skin-invert-image|230px|Structure of indigo carmine.]]
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.<ref name="indigo-bromo">{{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}}</ref> 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 group]]s such as the [[Tert-butyloxycarbonyl protecting group|tBOC group]], which suppresses intermolecular bonding.<ref>{{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}}</ref> 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.<ref>{{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}}</ref>