The colour of dibromoindigo
6,6'-Dibromoindigo in solution is blue, but when used as a dye on wool the colour is purple. Below are spectra of the absorption in solution, tetrachloroethane, showing a maximum absorption at 590 nm and of the wool-dyed reflectance spectrum showing a maximum absorption at 520 nm.
The wider range of absorption at both higher and lower wavelengths is due to the solid state aggregation of the dye on the surface of the wool.
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