I'd like to ask about how the color differences between the dyes methyl violet and malachite green can be explained. Looking at the conjugated bonds of each, methyl violet seems to be more conjugated than malachite green with two $\ce{NMe_2}$ auxochromes, which causes a bathochromic shift, suggesting that methyl violet's absorption peak to have a longer wavelength compared to malachite green's absorption peak.

However, after having collected data using a spectrophotometer, methyl violet appears to absorb at a shorter wavelength ($\pu{593 nm}$) compared to malachite green ($\pu{623 nm}$), with both dyes solvated in $50~\%$ ethanol.

How can this color difference be explained?

Methyl violet:
structural formula of methyl violet

Malachite green:
structural formula of malachite green

  • 1
    $\begingroup$ The auxochromes in MG are $\ce{NMe2}$ groups (as in MY). Conjugation is not the only factor, the HOMO-LUMO gap is also affected by the presence of lone pairs that can take part in the conjugation. Keep in mind that the structures presented are only one of many resonance structures. $\endgroup$ – TAR86 Oct 25 '17 at 4:40
  • $\begingroup$ What does MY stand for? Also, you say that lone pairs can also affect the HOMO-LUMO gap because lone pairs "can take part in the conjugation", but if you take them into account, wouldn't that also point to Methyl Violet having an absorption peak at a longer wavelength? $\endgroup$ – oswinso Oct 25 '17 at 7:16
  • $\begingroup$ I meant to write MV (as in methyl violet). I am not sure which way the N lone pair would influence the gap. $\endgroup$ – TAR86 Oct 25 '17 at 8:19
  • $\begingroup$ Oh, I see. I think amines produce a bathochromic shift, reducing HOMO-LUMO gap by extending the length of the conjugated system. $\endgroup$ – oswinso Oct 25 '17 at 10:21

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