Pyridinium <i>N</i>‐Phenoxide Betaines and Their Application to the Characterization of Solvent Polarities, XX. – Synthesis, Solvatochromism, and Halochromism of Chromoionophoric Crown Ether‐substituted Pyridinium <i>N</i>‐Phenolate Betaine Dyes

Abstract

Abstract 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 longwavelength 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.