Syntheses and negative solvatochromism of t wo 2,6‐di(carbamoyl)‐substituted pyridinium N ‐phenolate betaines dyes, 3 and 4 , are described for the purpose of obtaining more hydrophilic zwitterionic dyes which are better soluble in water and other aqueous media (such as binary water–solvent mixtures or aqueous ionophore solutions) than the rather hydrophobic standard betaine dyes 1 and 2 , which have been used to establish an empirical scale of solvent polarity, called the E T ( 30 ) or ETN scale.
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Reichardt et al. (2003) studied this question.
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