Potential Driven Non-Reactive Phase Transitions of Ordered Porphyrin Molecules on Iodine-Modified Au(100): An Electrochemical Scanning Tunneling Microscopy (EC-STM) Study

Abstract

The modelling of long-range ordered nanostructures is still a major issue for the scientific community. In this work, the self-assembly of redox-active tetra(N-methyl-4-pyridyl)-porphyrin cations (H2TMPyP) on an iodine-modified Au(100) electrode surface has been studied by means of Cyclic Voltammetry (CV) and in-situ Electrochemical Scanning Tunneling Microscopy (EC-STM) with submolecular resolution. While the CV measurements enable conclusions about the charge state of the organic species, in particular, the potentio-dynamic in situ STM results provide new insights into the self-assembly phenomena at the solid-liquid interface. In this work, we concentrate on the regime of positive electrode potentials in which the adsorbed molecules are not reduced yet. In this potential regime, the spontaneous adsorption of the H2TMPyP molecules on the anion precovered surface yields the formation of up to five different potential-dependent long-range ordered porphyrin phases. Potentio-dynamic STM measurements, as a function of the applied electrode potential, show that the existing ordered phases are the result of a combination of van der Waals and electrostatic interactions.

Unveiling the Interplay between a Au(100) Electrode, Adsorbed TTMAPP Porphyrin Cations, and Iodide Anions: An EC-STM and CV Study

“…Both the (2√2 × p√2) and the pseudohex-rot structures result from a reversible electro-compression of the iodine adlayer. 45−47 Our results of this phenomenon have been described in detail in our previous publications 13,14 and are briefly summarized here in the Note S1.…”

“…In order to understand the relationship between the porphyrin adlayer of phase I and the substrate lattice beneath it, a series of STM images (Figure d,e) were taken at the same surface area. It is a known fact that by altering the tunneling conditions, such as the tunneling bias, it is possible for enhanced resonant tunneling to occur. ,, At high bias voltage conditions, such as −606 mV in Figure c and in the upper half of Figure d, almost all electrons flow through the potential well using the quantized energy levels that are defined by molecular orbitals or bands formed by the periodic arrangement of the molecules. Therefore, tunneling through these states results in STM images that emphasize the contribution of the organic molecules.…”

“…−620 and −400 mV, preventing the creation of an ordered structure from the third orange regime on the I-free Au(100) surface, which in turn would hinder the readsorption of the iodine anions. At this point, comparing the results of the self-assembly of TTMAPP to our previous investigations of 5,10,15,20tetrakis(N-methyl-4-pyridyl)-porphyrin tetrakis(p-toluenesulfonate) molecules (TMPyP) on the iodine-modified Au(100) surface, 13,14 we can deduce that the interaction between TTMAPP and the iodine-modified Au surface is stronger than that of TMPyP. This is evidenced by the reduced mobility of TTMAPP molecules and the stabilization of the iodine phases by the presence of the molecule over a wider potential range.…”

“…Instead, they will be caused by alterations in the interactions between the porphyrin molecules and the substrate. 13,14 This phenomenon will be broadly discussed in the next subchapter, as well as in the context of the semireversibility of the orange regime in the anodic direction. 100) Surface-Orange Regime.…”

“…In contrast to the assembly of supramolecular architectures in three-dimensional (3D) bulk environments, such as solutions, which relies on specific and directional intermolecular interactions between the molecules, a complex interplay between adsorbate–adsorbate and adsorbate–substrate interactions controls the two-dimensional (2D)-phase formation behavior on surfaces. − This interplay makes it possible to create highly ordered and functional structures at the nanoscale. Furthermore, if the molecular building-blocks are deposited electrochemically in the form of ions, the electrochemical (EC) potential becomes a very useful parameter, which allows control of the self-assembly process, − which can even lead to the formation of different structures of the same building blocks. Ultimately, the aim is to exploit this controlled self-assembly of organic molecules on suitable solid substrates to form functional surfaces for applications in important technological fields like, for instance, nanoscale electronics, green energy catalysis, or novel light-stimulated sensors. − …”