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I have been trying to understand the colour change in litmus paper on a more detailed level than "the chemical identity changes, and therefore also physical properties". I have knowledge of the particle in a box model of quantum mechanics, and i would like to apply this to the conjugated system in the active indicator-component of litmus dye. In other indicators I have looked at, you can clearly see that when the dye molecule dissociates or is protonated, there is change to the length of the conjugated system (number of alternating double bonds in a row), which explains the colour change. An example is given below for BTB (bromo thymol blue) BTB colour change

However, i can't find any explanation like this as to why the indicator dye in litmus paper (7-hydroxyphenoxazone) changes colour when protonated/deprotonated. From what I can see, the length of the conjugated system is the same in both versions of the molecule. Neutral 7-hydroxyphenoxazone

Edit: If someone knows of a model that more easily explains this colour change than conjugation, feel free to share that as well.

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  • $\begingroup$ Did you search for the reaction of litmus/other indicators in different mediums on the Net ? Usually a chemical reaction takes place according to medium. $\endgroup$
    – user14857
    Oct 28 '16 at 19:02
  • $\begingroup$ I think that's already addressed in the question. The conjugated system extends throughout the molecule regardless of whether or not you protonate on the carbonyl... $\endgroup$
    – Zhe
    Oct 28 '16 at 19:12
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You have shown the structural change in bromothymol blue upon deprotonation.

For the anion of 7-hydroxyphenoxazone, the following resonance structures involving a p-quinone imine are conceivable:

litmus anion

The assumption that the $\pi$-system of the neutral species and the corresponding anion are identical is not justified.

A smaller system, in which a similar effect can be observed, is 4-nitrophenol. A solution of the phenol is colourless, whereas the solution of the phenolate is yellow ($\lambda_{\mathrm{max}}$ = 405 nm).

Again, loss of a proton leads to tautomerism with contribution of a quinoid structure.

tautomerism of 4-nitrophenolate

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  • $\begingroup$ I think the OP's question is that this system is just as conjugated as the original. $\endgroup$
    – Zhe
    Oct 28 '16 at 20:56
  • $\begingroup$ Yes, these are the resonance structures I thought would arise from the deprotonation of 7-hydroxyphenoxazone. But, as @Zhe pointed out, I dont understand why this structure gives rise to a different colour than neutral 7-hydroxyphenoxazone. From what i can see the conjugated system is similar in both. $\endgroup$
    – Adroit
    Oct 28 '16 at 21:12
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    $\begingroup$ @Adroit Similar, but not identical. Change in uv-vis absorption due to protonation/deprotonation without breaking further bonds is a rather common effect. Think in methyl red, methyl violet, or the anthocyane dyes. This is about electron distribution in the HOMO, isn't it? $\endgroup$
    – Klaus-Dieter Warzecha
    Oct 28 '16 at 21:20
  • $\begingroup$ @KlausWarzecha Hmm, not sure I understand. Does the dissociation add another electron to the same conjugated system, and so the energy of the HOMO is higher => HOMO-LUMO gap corresponds to lower energy light => more red light absorbed => appears blue? If this is correct, can you explain why the dissociation adds one electron to the conjugated system? $\endgroup$
    – Adroit
    Oct 29 '16 at 9:51
  • $\begingroup$ @Adroit In my (somewhat limited) experience, deprotonated oxygens (for example) with lone pairs and a negative formal charge have a tendency to withdraw a much larger fraction of electron density from the structure they're attached to than they do when they're protonated. So, even though the structure remains conjugated, the bond system has a different density distribution -- different enough to have distinguishable absorbance behavior. $\endgroup$
    – hBy2Py
    Dec 3 '16 at 13:37

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