Role of redox-active axial ligands of metal porphyrins adsorbed at solid–liquid interfaces in a liquid-STM setup

In a liquid-STM setup environment, the redox behavior of manganese porphyrins was studied at various solid–liquid interfaces. In the presence of a solution of Mn(III)Cl porphyrins in 1-phenyloctane, which was placed at a conductive surface, large and constant additional currents relative to a set tunneling current were observed, which varied with the magnitude of the applied bias voltage. These currents occurred regardless of the type of surface (HOPG or Au(111)) or tip material (PtIr, Au or W). The additional currents were ascribed to the occurrence of redox reactions in which chloride is oxidized to chlorine and the Mn(III) center of the porphyrin moiety is reduced to Mn(II). The resulting Mn(II) porphyrin products were identified by UV–vis analysis of the liquid phase. For solutions of Mn(III) porphyrins with non-redox active acetate instead of chloride axial ligands, the currents remained absent.