SOLVATOCHROMISM, THERMOCHROMISM, PIEZOCHROMISM, HALOCHROMISM, AND CHIRO-SOLVATOCHROMISM OF PYRIDINIUM N-PHENOXIDE BETAINE DYES

The position of the longest-wavelength, intramolecular charge-transfer absorption band of the solution UV/Vis spectra of pyridinium N-phenoxide betaine dyes depends on the polarity of the solvent, the temperature of the solution, the external pressure, the nature and concentration of added salts, and possibly, in the case of homochiral betaine dyes, on the use of chiral solvents. A survey of these peculiar spectral properties of pyridinium N-phenoxide betaine dyes is presented. The reasons for this extraordinary behaviour and possible applications (e.g. empirical determination of solvent polarities) are discussed.