What is "thioindigo"

Thioindigo is an organosulfur compound that is used to dye polyester fabric. A synthetic dye, thioindigo is related to the plant-derived dye indigo, replacing two NH groups with two sulfur atoms.

Thioindigo is generated by the alkylation of the sulfur in thiosalicylic acid with chloroacetic acid. The resulting thioether cyclizes to 2-hydroxy thianaphthene, which is easily converted to thioindigo. The related compound 4,7,4',7'-tetrachlorothioindigo, also a commercially important dye, can be prepared by chlorination of thioindigo.