Platinum porphyrins as ionophores in polymeric membrane electrodes

A comparative study of Pt(II)- and Pt(IV)-porphyrins as novel ionophores for anion-selective polymeric membrane electrodes is performed. Polymeric membranes of different compositions, prepared by varying plasticizers, cationic and anionic additives and Pt porphyrins, have been examined by potentiometric and optical techniques. Pt porphyrin-based devices were found to exhibit enhanced potentiometric selectivity toward iodide ion compared to electrodes based on a typical anion-exchanger (e.g. tridodecylmethylammonium chloride). It is shown that Pt(II)-porphyrins function as neutral anion carriers within the electrode membranes, while those based on Pt(IV)TPPCl(2) operate via a mixed mode carrier mechanism, evidencing also a partial reduction of the starting ionophore to Pt(II)TPP. Spectrophotometric measurements of thin polymeric films indicate that no spontaneous formation of hydroxide ion bridged porphyrin dimers occurs in the membrane plasticized both with high or low dielectric constant plasticizer, due to a low oxophilicity of central Pt. The computational study of various anion-Pt(IV)TPPCl(2) complex formation by means of semi-empirical and density functional theory (DFT) methods revealed a good correlation between calculated and measured ionophore selectivity.