Solvent Polarity and Solvatochromic Shifts

The variations in the absorption energies of various dyes have been used to characterise the polarity of various media and create empirical scales. For this purpose, the most widely used dye is the highly, negatively solvatochromic betaine (1.98), known as Reichardt s dye, whose transition energy f .j,(30) in kcal mol, measured in a particular solvent, characterises the polarity of that solvent. 

This method of deriving the f .j.(30) has the advantage of providing polarities for a wide range of solvents due to the solubility of (1.98) in both polar and non-polar solvents (see Table 1.15). However, this scale is an over-simplification as there are other interactions occurring between solvents and solutes, e.g. hydrogen bonding effects, and this needs to be borne in mind. 

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