Indigo Properties

Indigo, also known as indigo blue and indigotin (C.I. Vat Blue 1, C I 7300, CAS number: 82/582-89-3, IUPAC name 3H-indol-3-1, 2-(1,3-dihydro-3-oxo-2H-indol-2-ylidene)-1,2-dihydro-, chemical formula C16H10N2O2), is present at ambient temperature and normal pressure as dark blue-violet needles or prisms with a distinct coppery luster, melting point 300°C. Indigo absorbs light in the orange part of the spectrum (λmax=613nm). The compound owes its deep color to the conjugation of the double bonds. Indigo is insoluble in water and poorly soluble in most of the common solvents. It is more soluble in polar organic solvents than non-polar ones. The poor solubility is most likely due to the strong inter- and intramolecular hydrogen bonds that are formed in indigo crystals. The hydrogen bonding also explains indigo’s relatively high melting point (∼300°C).

The color of indigo is dependent on its environment. In the gas phase, where indigo is in its monomeric form, it is red, and in non-polar solvents it is violet, but in solid form and in polar solvents as well as when it is applied to textiles as a vat dye, it is blue. Indigo is non-biodegradable, has a low mammalian toxicity and there is no indication of sensitization in humans after repeated skin applications. Indigo is classified as a vat dye, although its properties are not typical of the vat dyes.

Indigo has moderate to very high light fastness depending on the substrate it is on or whether it is a pigment or a dye. The light mostly affects the oxidative degradation of indigo to the degradation products such as isatin, isatoic anhydride and anthranilic acid. There are synthetic dyes, especially vat dyes, with better fastness properties particularly to light, washing and chlorine bleaching, than indigo, but it is this fading of color that is so characteristic of indigo that has kept it so popular with jeans-wearing people.