Microheterogeneities in binary solvent mixtures studied by electronic absorption spectra of a solvatochromic merocyanine dye

Electronic absorption spectra of a solvatochromic merocyanine dye (5-[2-(3-ethyl-2-benzothiazolylidene)-ethylidene]-3-ethyl-2-thioxo-4-thiazolidinone) were measured in two binary solvent mixtures at various mixing ratios. In cyclohexane–trichloromethane mixtures, the absorption maximum of the dye was found to shift continuously as the mixing ratio was changed gradually, showing that the mixtures are almost homogeneous. On the other hand, the absorption bands of the dye in cyclohexane–ethanol mixtures were found to be the overlaps of two components, i.e. the band due to the dye in cyclohexane and that due to the dye in ethanol, suggesting the existence of inhomogeneous microstructures in the mixtures.