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.

spectra of 6,6'-dibromoindigo

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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