Analysis of sulphur‐containing indigoid dyes as leuco forms by NMR spectroscopy

The NMR spectra of different types of sulphur‐containing indigoid dyes as leuco forms (i.e. reduced forms or ‘vats’) in deuterium oxide are recorded and discussed. NMR spectroscopy provides an expeditious technique for identification, structure elucidation and purity control of these dyes. The regio‐selective exchange of halogen for deuterium or hydrogen in indigo derivatives was successfully adapted to thioindigos and [indole‐(2′)]‐[thionaphthene‐(2)]‐indigos. The reaction is useful as supplementary method for structure analysis and renders access to new selectively deuterated dyes. Care must be observed with the concentration of reducing agents and pH values in vat dyeing process in order to avoid further reduction of the leuco forms. The ‘overreduction’ of leuco forms of [acenaphthene‐(1′)]‐[thionaphthene‐(2)]‐indigos was followed by NMR spectroscopy and interpreted.