Why the thio analogue of indigo is red? What is the reason behind change of colour from indigo to red just by change of N to S. What I get is that there will be electronegativity, lone pair availability, MO energy factors playing here but I am not getting exact answer.

  • $\begingroup$ The colour of a compound is a very complex thing and hardly ever easily explained. All three of the factors you mentioned play a role, ultimately it is a change in the energy difference HOMO-LUMO. But that is all you can say. $\endgroup$ – Jan Nov 8 '15 at 14:59

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