This question asks about the color change of 7-hydroxyphenoxazone, the active compound in litmus paper, and the relation of that color change to the conjugated system in the molecule.
In Klaus Warzecha's answer, he mentions 4-nitrophenol as another, simpler indicating system, analogous to 7-hydroxyphenoxazone.
In both of these species, as noted by Klaus, deprotonation of the hydroxyl group to an -olate moiety enables a resonance between the substituents, through the molecular cores, which is not possible in the protonated species:
I'm interested in knowing whether this property of an "opened-up resonance on deprotonation" and the concomitant influence on the excitation spectrum are necessary characteristics for indicating compounds.
So: Are there any known examples of acid-base indicators (e.g., compounds exhibiting a change in visible-spectrum absorbance upon deprotonation) where the removal of the proton does not "open up" additional resonance structures in this fashion?
Clicking through some of the links on Wikipedia's $\mathrm{pH}$ indicators page, I would guess that cresol red, bromocresol green, and bromophenol blue potentially fall into this category, but I'm not confident enough in my evaluation of resonance structures to be certain.
Answer
In contrast to 4-nitrophenol, cresol red, bromocresol green, and bromophenol undergo structural changes upon deportonation because the sultone opens up.
But again, you can draw quinoid resonance structures for the resulting anions.
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