Polarity of ionic liquids determined empirically by means of solvatochromic pyridinium N-phenolate betaine dyes

Solutions of the zwitterionic betaine dye 2,6-diphenyl-4-(2,4,6-triphenylpyridinium-1-yl)phenolate are solvatochromic, thermochromic, piezochromic, and halochromic. That is, the position of the longest-wavelength intramolecular CT absorption band depends on the solvent polarity, solution temperature, external pressure, and the nature and concentration of added salts. The outstanding large negative solvatochromism of this standard betaine dye has been used to introduce spectroscopically an empirical scale of solvent polarity, called the ET(30) or ENT scale, which includes meanwhile a large number of molecular solvents and solvent mixtures. In this review, efforts to determine empirically the polarity of room-temperature ionic liquids by means of this betaine dye are summarized and discussed , because ionic liquids have gained importance as environmentally more benign new reaction media in the framework of Green Chemistry.