Dielectric properties of a binary cryoelectrochemical solvent according to a solvatochromic probe

The application of a solvatochromic dye to determination of the dielectric properties of organic solvent mixtures is described. The dye 2,6-diphenyl-4-(2,4,6-triphenylpyridino)phenolate (Reichardt's dye) features an intramolecular charge transfer band in its visible spectrum with a large associated change in dipole moment between the ground and excited states. The position of the charge transfer band is sensitive to local dielectric environment and so is an indicator of solvation of the dye. Room-temperature spectra of Reichardt's dye in the cryoelectrochemical solvent system 2:1 (v/v) ethyl chloride/butyronitrile and in the neat components are reported and related to the dielectric properties of the mixture. The effective dielectric properties of the mixture scale with the value of the Pekar factor, 1/ε(OP) - 1/ε(S), that is predicted from the mole fraction-weighted (or volume fraction-weighted) sum of the dielectric constants for the pure components, suggesting that strongly selective solvation of the dye by one component of the solvent mixture does not occur. The results are pertinent to a recent study of electrode kinetics in the solvent mixture.