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How is indigo related to indole rings?

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heterocyclic compound There are many different sulfur-containing heterocycles. One of the best known is thiophene, C₄H₄S, derivatives of which occur as plant pigments and as other natural products such as biotin.

Indigo is chemically related to indole because it is classified as a compound that contains the indole molecular structure.1

Specifically, indigo is a member of the indigoid group of pigments, which are structurally derived from indole. The natural precursor to indigo is indican, which is hydrolyzed into indoxyl. Indoxyl is a derivative of indole that serves as the immediate building block for indigo; upon exposure to air, indoxyl undergoes mild oxidation to form the blue dye known as indigo.213

While indole itself is a heterocyclic organic compound found in substances like coal tar and certain flower oils, indigo represents a more complex application of this structural framework. The relationship is fundamental to the chemistry of these pigments, as the indole ring system provides the core scaffold for both indigo and related compounds like Tyrian purple.13