Porphyrin bi-layer formation induced by a surface confined reduction on an iodine-modified Au(100) electrode surface

A controlled electron transfer can be employed as a tool to drive the crafting of long-range self-assembled nanostructures of electro-active species on surfaces. In this work, we show this approach with the formation of an ordered bi-layer of tetra(N-methyl-4-pyridyl)-porphyrin molecules (TMPyP) on an iodine-modified Au(100) electrode studied by means of Cyclic Voltammetry (CV) and in situ Electrochemical Scanning Tunneling Microscopy (EC-STM). This bi-layer exists only towards negative electrode potentials between the first reduction potential of the adsorbed molecules and the desorption potential of the iodine. Starting a cathodic potential sweep at positive potentials, where the surface is covered with an adsorbed monolayer of TMPyP molecules which did not undergo any previous electrochemical reaction yet (Ref. Surfaces 2018, 1, 3–17), the formation of the ordered bi-layer coincides with the first reduction step of the adsorbed molecules; and it disappears again in a joint desorption process with the iodine. Reversal of the potential sweep leads to the opposite observations. Depending on the actual potential value as well as the scan direction up to five different competing ordered bi-layer phases can be identified. Synergistic effects between the modified substrate and the adsorbate are determined to be responsible of the corresponding structures, and at the same time surface-mediated catalytic effects shifting the first reduction step of the adsorbed molecules to a more positive potential in comparison to the reduction potential of the species from the bulk solution are observed. Moreover, based on detailed potentiodynamic STM measurements, a mechanism is also proposed to explain these observations.