Singular thermochromic effects in dyeings with indigo, 6-bromoindigo, and 6,6′-dibromoindigo

Fundamental dyeing properties of three dyes of historical importance – indigo, 6-bromoindigo, and 6,6′-dibromoindigo – on 13 fabrics are examined. The hues of the dyed fabrics vary from blue-green to blue for indigo, blue to violet for 6-bromoindigo, and violet–blue to purple for 6,6′-dibromoindigo, as determined by reflectance measurements. Many of the dyed fabrics change color markedly with application of gentle heat. For indigo and 6-bromoindigo, the color changes are predominantly towards absorption of longer wavelengths (giving a bluer fabric color), while for 6,6′-dibromoindigo, the color changes are towards absorption of shorter wavelengths (giving a redder fabric color). The greatest thermochromic effects are seen for dyed wool: with 6-bromoindigo, the color changes from violet to blue, while with 6,6′-dibromoindigo, the color changes from violet to red–purple upon brief boiling in water. Transmission electron microscopy analysis of 6-bromoindigo-dyed carbon nanotubes, used as surrogates for dyed fabric fibers, suggests that the basis of the thermochromic effect is a change in the size distribution of dye aggregates.

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► We compare the colors of 13 fabrics dyed with indigo and two brominated derivatives.
► We attempt to explain the physical basis of color changes seen upon dilution and heating.
► Reflectance spectra indicate two types of dye aggregates.
► Carbon nanotubes are used as surrogate fabrics in transmission electron microscopy analysis.
► The color changes may be due to size- and possibly shape-related redistribution of dye aggregates.