Thermodynamic characterization of the dimerization equilibrium of an asymmetric dye by spectral titration and chemometric analysis

The monomer-dimer equilibrium of an asymmetric cyanine dye has been investigated by means of UV-Vis spectroscopy. The data have been processed by a recently developed chemometric method for quantitative analysis of undefined mixtures, that is based on simultaneous resolution of the overlapping bands in the whole set of absorption. In this work the dimerization constant of 1-carboxydecyl-4-{3-3-methyl-3H-benzothiazol-2-ylidene]-propenyl}-quinolinium (TO-3) has been determined by studying the dependence of absorption spectrum on temperature in the range 25-72.5 °C at different total concentrations of dye (8.5×10⁻⁶ to 2.87×10⁻⁵ M). Utilizing the van’t Hoff relation, which describes the dependence of the equilibrium constant on temperature, as constraint we determine the spectral responses of the monomer and dimer species as well as the enthalpy and entropy of the dimerization equilibrium.