The relation of solvatochromism and thermochromism to the solvent dielectric constant: The basis of the ET and E'T polarity scales

We have measured the thermochromism of pyridinium‐N‐phenoxide betaine and its penta‐tert‐butyl substituted derivative dissolved in MTHF together with the solvent dielectric properties in order to relate polarity scales to the dielectric constants of the solvent. Both thermochromic and solvatochromic shifts of the absorption band of the dyes can be rationalized on the basis of the MSA theory for relating the solvation free energy to the static dielectric constant. Deviations between theory and experiment in the regime of higher viscosities can be attributed to changes in the offset between absorption maximum and electronic transition energy.