Multi-Method Investigation of the Mechanism of Surfactant-Induced Chromatotropism of Metal-Chelometric Dye Complexes

Abstract

The mechanism by which certain surfactants change the ultraviolet-visible (UV-vis) spectroscopic properties of certain metal-dye complexes has been largely ignored in the chemical literature. In this manuscript a model is developed to describe this mechanism. Intramolecular forces among surfactant molecules and host molecules are described to emphasize the importance of specific interactions to the mechanism. A review of the theoretical basis of UV-vis spectroscopic changes in dyes (chromatotropism) is given followed by application to the dyes of interest in this study. UV-vis spectra of the dyes, metal-dye complexes, and metal-dye-surfactant systems are analysed and a relationship between the symmetry of the chromophore and the UV-vis spectroscopic properties is described. This is the basis of the mechanistic model which postulates that the metal ion of the complex migrates from an asymmetric binding site to a symmetric binding site of the dye upon interaction with surfactant. Evidence in support of the molecular structures of the metal-dye complexes and metal-dye-surfactant systems proposed by the model is given in the analysis of infrared (IR) and Raman (R) spectra of these species. Structures of two metal-dye surfactant systems are proposed on the basis of the vibrational data. Thermodynamics of the rearrangements proposed by the model are investigated using ab-initio Hartree-Fock molecular orbital calculations.