Why is INDIGOTINE BLUE "blue" in colour? Explain with a O-QUINONOID charged structure.

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ncchemist | eNotes Employee

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Indigotine blue is a chemical dye that is based on the classic dye indigo.  It is also called indigo carmine.  It is a symmetrical dimer that is composed of two monomers.  The monomer is based on the compound indole with a carbonyl on the five membered ring.  The two monomers are attached to each other via a carbon carbon double bond.  The quinonoid structure you are talking about involves intramolecular hydrogen bonding.  The hydrogen on the nitrogen can form a hydrogen bond with the carbonyl on the other side of the molecule.  Since the compound is a dimer, there are two such hydrogen bonds in each indigotine blue molecule.  It's hard to explain all of this verbally, so look at the first link below to see the structure with the hydrogen bonds drawn out.  This type of structure is what gives indigotine blue its distinctive blue color.