Cis-trans Isomerism of Thioindigo Dyes in the Solid State

Abstract

As the result of some preliminary observations by Stearns¹, an extensive study was made by Wyman and Brode² of the absorption spectra of solutions of several thioindigo dyes in benzene and chloroform under various conditions of illumination. Evidence was obtained that in solution thioindigo and its derivatives exist in equilibrium as two forms (cis and trans), which give rise to separate absorption bands in the visible region. X-ray diffraction studies on crystalline thioindigo by Madame von Eller³ suggest, however, that in the solid state the dye is present in the trans form. It is thought that the electrostatic attraction between slight positive and negative charges on neighbouring sulphur and oxygen atoms, respectively, maintains the thioindigo molecule in the trans configuration and prevents isomerization⁴. Presumably in some cases these attractive forces will be weakened by solvents or by substitution in the thioindigo molecule.