Redox-activity and self-organization of iron-porphyrin monolayers at a copper/electrolyte interface

The electrochemical behaviour and molecular structure of a layer of water-soluble 5,10,15,20-Tetrakis-(N-methyl-4-pyridyl)-porphyrin-Fe(III) pentatosylate, abbreviated as FeTMPyP, on a chloride modified Cu(100) electrode surface were investigated by means of cyclic voltammetry (CV) and in-situ electrochemical scanning tunneling microscopy. Voltammetric results of HOPG in an electrolyte containing FeTMPyP molecules indicate three distinguishable redox steps involving both the central iron metal and the π-conjugated ring system. However, only the first two reduction steps are observable within the narrow potential window of CVs of Cu(100) measured in the same electrolyte. In the potential range below the first reduction peak, at which the [Fe(III)TMPyP](5+) molecules are reduced to the corresponding [Fe(II)TMPyP](4+) species, in-situ scanning tunneling microscopy (STM) images revealed, for the first time, a highly ordered adlayer of this reduced porphyrin species on the chloride terminated Cu(100) surface. The ordered adlayer exhibits a (quasi)square unit cell with the lattice vectors |a→2|=|b→2|=1.53±0.1 nm and an angle of 93° ± 2° between them. A model is proposed based on the STM observation illustrating the arrangement of the [Fe(II)TMPyP](4+) molecules at the electrolyte/copper interface.