Halochromic dyes

Reichardt, C. (1992). Solvatochromism, thermochromism, piezochromism, halochrom-ism, and chiro-solvatochromism of pyridinium IV-phenoxide betaine dyes. Chem. Soc. Rev. 21, 147-153... 

Carbenium ions (84) (R are heteroaryl substituents) are dyes showing halochromic properties <83DP101>. 

Jaung JY (2006) Synthesis and halochromism of new quinoxaline fluorescent dyes. Dyes Pigments 71 245-250. doi 10.1016/j.dyepig.2005.07.008... 

It is well known that azobenzenes containing amino or dialkylamino groups undergo a pronounced color change in solution or in polymer film under protonation [45]. This phenomenon, called halochromism, is the basis of their use as optical sensors and pH indicators [46].The relative effect of the color changes in such dyes is associated with the existing ammonium-azonium (bt a) tautomerism and the tautomeric ratio, which depends, as usual, on the solvent, temperature, and existence of additional substituents in the protonated molecule. 

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