The State of Indigoid and Thioindigoid Dyes in Dyed Materials

The structural composition of indigo and its ring‐substituted derivatives in the solid state has been considered. It is concluded that, except for indigo derivatives containing bulky substituents in the 4,4′‐ and 7,7′‐positions, these compounds are associated in the trans configuration by means of intermolecular hydrogen bonds. Unlike indigo, some derivatives of thioindigo (e.g. methylchloro‐ and ethoxy‐substituted compounds and the hemi‐thioindigoid dye, 1′‐acenaphthene‐2‐thianaphtheneindigo) exist in the solid state as mixtures of cis and trans forms. Thioindigo, 6,6′‐dichlorothioindigo and bis‐4,5‐benzothioindigo, however, exist solely in the trans configuration in the solid state. It is suggested that, in the solid state, thioindigo is stabilised in the trans configuration by electrostatic attraction between partial negative charges on the oxygen atoms and slight positive charges on the sulphur atoms. Where the thioindigoid dye exists in the solid state as a mixture of cis and trans forms, the application of the dye by conventional dyeing methods followed by oxidation may not result in the formation of a stable equilibrium mixture, and changes may occur on subsequent swelling treatments, e.g. soaping, which lead to a change in the absorption spectra of dyed Cellophane (and of the colour of dyed fabrics). The nature of these changes is illustrated by absorption and reflection spectra.