Syntheses and UV/Vis-Spectroscopic Properties of Hydrophilic 2-, 3-, and 4-Pyridyl-Substituted Solvatochromic and Halochromic PyridiniumN-Phenolate Betaine Dyes as New Empirical Solvent Polarity Indicators

Syntheses and negative solvatochromism of nine new hydrophilic 2-, 3-, and 4-pyridyl-substituted pyridinium N-phenolate betaine dyes 3−11 are described. These were produced in order to obtain zwitterionic dyes better soluble in water and other aqueous media (such as binary water/solvent mixtures, 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 ET(30) scale. Betaine dye 8, in which three of the peripheral phenyl groups of 1 are replaced by two 3-pyridyl rings and one 4-pyridyl ring, proved to be particularly suitable for the determination of ET(30) values in aqueous media.