PYRIDINIUM N-PHENOXIDE BETAINES AND THEIR APPLICATION TO THE CHARACTERIZATION OF SOLVENT POLARITIES .20. SYNTHESIS, SOLVATOCHROMISM, AND HALOCHROMISM OF CHROMOIONOPHORIC CROWN ETHER-SUBSTITUTED PYRIDINIUM N-PHENOLATE BETAINE DYES

Synthesis, negative solvatochromism (Table 1, Figure 1), and negative halochromism (Tables 2 - 3, Figures 2 - 3) of the new crown ether-substituted pyridinium N-phenolate betaine dyes 3 - 6 are described. In contrast to other known chromoionophores, 3 - 5 exhibit a cation-selective negative halochromism, which manifests itself as a hypsochromic shift of the long-wavelength intramolecular charge-transfer (CT) absorption band of the betaine chromophore on addition of salts. As this CT absorption band lies within the visible spectral region, the cation-induced colour change can be followed easily by eye, which suggests an application of these new chromoionophoric betaine dyes as cation indicators. The halochromism of 3 - 6 constitutes a new type of true halochromism, in contrast to the trivial halochromism first defined by Baeyer and Villiger in 1902.