Q.Write the chromogen, chromophore and auxochrome present in para hydroxy azo benzene.

In para-hydroxyazobenzene the azo linkage is the chromophore that confers colour, the aromatic azobenzene framework carrying it is the chromogen, and the hydroxyl substituent is the auxochrome that deepens/intensifies the colour.

Structure: Para-hydroxyazobenzene is $HO-C_6H_4-N=N-C_6H_5$ — a benzene ring bearing a $-OH$ group at the para position, joined through an azo ($-N=N-$) linkage to a second, unsubstituted benzene ring.

Chromophore: A chromophore is an unsaturated group capable of exciting electrons (via $\pi \rightarrow \pi^*$ or $n \rightarrow \pi^*$ transitions) with visible/near-UV light, thereby imparting colour to the compound. In this molecule, the chromophore is the azo group, $-N=N-$, which conjugates the two aromatic rings and is responsible for the compound being coloured.

Chromogen: A chromogen is an aromatic compound containing a chromophore (the colour-bearing group is present, but the compound need not yet be a true dye). Here, the azobenzene system — the two benzene rings joined through the $-N=N-$ chromophore — constitutes the chromogen.

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