Ion transfer and adsorption of water-soluble metal complexes of 8-hydroxyquinoline derivatives at the water|1,2-dichloroethane interface

The transfer mechanism and adsorption state of water-soluble 8-quinolinolate complexes were studied at the water|1,2-dichloroethane interface by electrochemical and spectroelectrochemical techniques. The interfacial affinities of the metal complexes of 8-hydroxyquinoline-5-sulfonate (QS) were estimated as AlQS33−, CuQS22− > ZnQS22−. Potential modulated fluorescence spectroscopy revealed the potential-driven process of fluorescent QS complexes, where Al(III) and Zn(II) complexes were transferred across the interface accompanied by the adsorption at the aqueous side of the interface. The adsorption state and preferential molecular orientation of these complexes were analyzed in detail by polarization-modulation total internal reflection fluorescence (PM-TIRF) spectroscopy. The PM-TIRF results showed that the square-planar 1:2 complexes, AlQS2− and ZnQS22−, were oriented relatively in parallel to the interface and approximately identical to the aqueous species. The adsorption behavior of the Zn(II) complex of tridentate 8-hydroxyquinoline-2-carboxylate (QC) ligand was also investigated, and ZnQC22− exhibited a strong interfacial affinity with intermediate spectral features between the aqueous and organic species.