24 августа, 2015 
Pokraskin The question of the contribution of the benzene rings to the indigo chromogen led to a search for the basic chromophore. Consecutively removing parts of the mesomeric system leaves a cross-conjugated structure 7 that, as the basic chromophore, still exhibits the typical deep color and redox properties of indigo.
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A Pariser-Parr-Pople (PPP) calculation indicates that the long-wavelength absorption band is what would be expected for the trans-s-cis-s-cis configuration shown by crystal-structure analyses to occur in the indigo crystal [2].
It was possible to test the various theoretical structures proposed for the basic chromophore 7 by synthesizing urindigo [4,4,4′,4′-tetramethyl(2,2′-bipyrrolidiny — lidene)-3,3’dione] [7].
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