Kosower’s Z scale

Another solvent polarity scale, Kosower s Z-scale, is based on the highly solvent-dependent charge transfer absorption of the para-substituted pyridin-ium salt (Scheme 4). The ground state of this dye is best described as a highly polar tight ion-pair, whereas the less polar charge-transfer form is more important in the excited state. Thus, the solvatochromism of the pyridinium salt is negative, Uke Reichardt s dye. In fact, the Ex(30)- and the Z-scale... 

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