Pyridinium-N-phenoxide betaine dyes as solvent polarity indicators. Some new findings

Abstract

The position of the long-wavelength, intramolecular charge-transfer UV/Vis, absorption band of pyridinium-N-phenoxide betaine dyes of the general formula 1 is very sensitive to changes in solvent polarity (negative solvatochromism) [1], temperature (thermo-solvatochromism) [2], external pressure (piezo-solvatochromism) [3], as well as to the introduction of substituents R 1 and R 2 and in the 4-phenyl ring [4]. This extreme sensitivity of 1 (R 1 = R 2 = H) to small changes in the surrounding medium has been used to establish an empirical scale of solvent polarity, called the E T(30)-scale ▪ [1, 5]. E T(30)-values are simply defined as the transition energy of 1 (R 1 = R 2 = H), measured in kcal/mol (or kJ/mol). They are known for more than 150 organic solvents and numerous binary organic solvent/water mixtures, and they have found wide application [5, 6]. It will be shown that empirical parameters of solvent polarity such as the E T(30)-values can be understood within the framework of Liner Free-Energy (LFE) Relationships [6]. Examples for the application of E T(30)-values to correlate spectral, thermodynamic, and kinetic solvent-dependent data taken from differed areas of chemistry will be give. Finally, the improvement of the LFE-correlations of singular empirical parameters of solvent polarity such as the E T(30)-values by means of multiparametric equations will be described.