Piezo-solvatochromism

Tamura and Imoto [74] and Hendrickson, Drickamer et al. [182] observed pressure effects on the solution spectra of (44). In all solvents used they found a hyp-sochromic shift of the longest-wavelength absorption band with increasing pressure. The observed hypsochromic shift of a betaine solution in methanol amounts to Ad = — 15 nm (Av = +600 cm AEj = +1-7 kcal/mol) on raising the pressure from 1 to 10 kbar [182]. On the supposition that this piezo-solvatochromism results from better solute/ solvent interactions with increasing pressure, it can be stated that, the higher the pressure, the more polar the solvent, and the higher the t(30) value. 

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. 

On the supposition that this "piezo-solvatochromism" results from a better ground state stabilization of the betaine dye with increasing pressure, it can be stated that, the higher the pressure, the higher the E -value, and the more polar the solvent. In other words, solvent polarity is also pressure-dependent ... 

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. Solvatochromism, thermochromism, piezo-chromisni, halochromism and chiro-solvatochromism of pyridinium A -phenoxide betaine dyes. Chem. Soc. Rev. 1992. 21. 147-153. 

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