Study of the complexation and solvation of polar molecules in binary solvents by differential spectroscopy

A method of application of differential absorption spectroscopy for determining the absorption spectra of primary 1: 1 solvated complexes between polar molecules of an organic dye and the active component of a binary solvent whose neutral component is a nonpolar liquid is proposed. The method was tested on diluted solutions of 4-dimethylaminochalcone (4-DMC) (which is one of the most efficient spectral-luminescent probes used in present-day medical and biological investigations) in mixtures of n-hexane with acetone at extremely small concentrations of the polar component. It is shown that the experimentally found absolute shift of the long-wavelength absorption band of 4-DMC is in satisfactory quantitative agreement with the analogous value obtained independently on the basis of the theory describing the joint effect of nonlinear (complexation) and linear (solvation) dipole-dipole interactions on the spectral band positions.