Why Bindschedler's Green is redder than Michler's Hydrol Blue
It addresses the important and subtle question of what happens in methine dye molecules when the central carbon atom is replaced by a nitrogen atom:
We offer a new physical interpretation of the color shift between diarylmethane dyes and their azomethine analogues. We use an isolobal analogy between state-averaged complete active space self consistent field solutions for corresponding methines and azomethines to show that the shift contains a significant contribution from configuration interaction between a methine-like ππ* excitation and an nπ* excitation out of the azomethine lone pair. The latter does not exist in the corresponding methine systems. This picture is qualitatively inconsistent with traditional models of the shift based on molecular orbital perturbation theory of independent π-electron Hamiltonians. A key prediction is the existence of a dipole-allowed band in the blue/near-UV spectra of the azomethines, which has polarization parallel to the lowest energy band. This forces a revision of past assumptions about the nature of the low-energy spectra of the azomethines. We show that a band at the predicted energies was observed as far back as 1938, but its significance at the time appears to have been unrecognized.