The uv and visible absorption spectrum of azo dye compounds

 

The four azo dye molecules below all contain the same chromophore, but the presence of other groups in the benzene ring will modify the colour of these azo dyes.

(i) , where C₆H₅-N=N-C₆H₄ is the chromophore, modified by the OH group, orange

(ii) , where C₆H₅-N=N-C₆H₄ is the chromophore, modified by the NH₂ group, yellow

(iii)  , where C₆H₄-N=N-C₆H₄ is the chromophore, modified by the OH group, colour?

(iv) , where C₆H₄-N=N-C₆H₄ is the chromophore, modified by the NH₂ group, colour?

 

The uv and visible absorption spectrum of the dye disperse red 19 azo dye

Image adapted from https://www.semanticscholar.org/paper/Absorption-and-fluorescence-spectra-of-disperse-red-Jaggi-Giri/6e59de87899a3ca3b70cfe6c64ff1bfea6819d28

The uv-visible absorption spectra of the azo dye disperse red 19 azo dye in dilute solution in ethanol.

It absorbs strongly in the blue-green region of the visible region, and appears red in solution.

The two peaks of maximum absorbance occur at 285 nm (ultraviolet) and 495 nm (blue-green visible light)

 

For more details of absorption spectra of dyes and pigments see:

The absorption spectra of selected aromatic compounds e.g. arenes and azo dyes

The absorption spectrum of chlorophyll

The absorption spectrum of haemoglobin

The absorption spectra of the photopigments in the human eye

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