Pyridinium-N-phenoxide betaines

Reichardt, C., Pyridinium N-phenoxide betaine dyes and their application to the determination of solvent polarities, part 29— Polarity of ionic liquids determined empirically by means of solvatochromic pyridinium N-phenolate betaine dyes. Green Chem., 7, 339-351, 2005. 

PYRIDINIUM-N-PHENOXIDE BETAINE DYES AS SOLVENT POLARITY INDICATORS, SOME NEW FINDINGS... 

Abstract - Empirical parameters of solvent polarity seem now-days 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 E.p-scale. In addition, the influence of solvents and substituents on the chemical reactivity of the pyridinium-N-phenoxides has now also been studied. 

The pyridinium-N-phenoxide betaine dye shown in Figure 1 exhibits a negative solvatochromic absorption with intramolecular... 

It is believed that the use of the pyridinium-N-phenoxide betaine dyes as solvent polarity indicators follows from some peculiar properties of these dyes, summarized in Figure 3. 

Recently, Tamura and Imoto(23) as well as Kelm and coworkers (24) have observed pressure effects on the solution spectra of our pyridinium-N-phenoxide betaines, which are illustrated in Figure 6. 

Fig. 9. Taft correlation between a -values and the modified transition energies of the longest wavelength n -7r -absorption band of the pyridinium-N-phenoxide betaine, measured at 25°C in different aliphatic alcohols R-OH (Erj,...

In conclusion, it can be said that the outstanding sensitivity of the spectral absorption and chemical reactivity of our pyridinium-N-phenoxide betaine dyes to small changes in solvent, temperature, pressure, and substituents makes these dyes a very useful class of compounds. They are not only solvatochromic compounds, but exhibit also the phenomena of thermo and piezo-solvatochromism. Their use for setting up different reaction series in order to get Linear Free-Energy Relationships has been demonstrated by the fact that the same betaine dye can be used not only for the introduction of a spectroscopic solvent polarity scale, the so-called E i-scale, but also for the establishment of kinetic and spectroscopic scales of substituents. 

Reichardt, C. (1992) Solvatochromism, thermochromism, piezochromism, halochromism, and chiro-solvatochromism of pyridinium N-phenoxide betaine dyes. Chemical Society Reviews, 21,147-153. 

Sander, W. and Hintze, F., A new synthetic approach to pyridinium N-phenoxide betaine dyes, Chem. Ber., 127, 267, 1994. 

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