Pyridinium-N-Phenoxide Betaine Dyes as Solvent Polarity Indicators. Some New Findings

Empirical parameters of solvent polarity seem nowdays as useful for the prediction of solvent effects as Hammett’s substituent constants for the estimation of substituent effects on chemical reactions. The extreme sensitivity of the position of the long-wavelength UV/VIS absorption band of pyridinium-N-phenoxide betaine dyes not only changes in temperature (thermo-solvatochromism), external pressure (piezo-solvatochromism), and the introduction of substituents, but also to small changes in solvent polarity (negative solvatochromism) has been used to establish a comprehensive empirical scale of solvent polarity, called the ET-scale. In addition, the influence of solvents and substituents on the chemical reactivity of the pyridinium-N-phenoxides has now also been studied.