Spectroscopy and Electronic Structure

Objectives

In funCOS 2, XPS, UPS and 2PPE will be used to characterize the electronic structure and chemical properties of tetraphenylporphyrins adsorbed on MgO(100) thin films. We will focus on the preparation and characterization of well-ordered porphyrin layers, the reactivity of the metal center and the changes induced by the adsorption of small molecules on the metal center. We will link ultrahigh vacuum studies on well-defined surfaces to high surface area materials. We aim to:

• Study the electronic properties of thin MgO(100) films on Ag(100) as a function of thickness.
• Create, identify, and quantify defects on the MgO(100) films.
• Investigate the interaction of tetraphenylporphyrin, platinum and cobalt tetraphenylporphyrin, and a functionalized platinum tetraphenylporphyrin with the MgO(100) surface. Focus will be on changes in the N 1s, Pt 4f, Co 2p and valence band regions as the coverage is increased from submonolayers to multilayers.
• Investigate simple reactions with porphyrins on MgO(100) and extract kinetic parameters. Focus will be on metalation and adsorption of CO, NO, O₂, O₃, and H₂S on the metal center and the influence this has on the valence band.
• Characterize selected high surface area systems from funCOS 5 and investigate to which extent they can be described by the results obtained from the model systems.

Systems and strategy

Whereas funCOS 1 and funCOS 3 initially focus on the adsorption of small linker molecules from the funCOS Molecular Toolbox, we will focus on the metallotetrapyrole unit itself, especially the properties of the metal center and the four nitrogen atoms coordinating the metal center. We will first look at the adsorption of simple tetraphenylporphyrins on MgO(100), before we increase the complexity by studying a functionalized tetraphenylporphyrin. The specific functionalization could be a carboxylic acid group, but will be chosen based on the results with small linker molecules from funCOS 1 and 3.

The studies will be carried out under UHV on well-defined MgO(100) thin films grown on single-crystalline Ag(100). Well-defined surfaces, with well-defined sites that can be modified in a well-defined manner are ideal substrates to develop a fundamental understanding of how metallotetrapyroles interact with different sites on oxide surfaces, such as terraces, steps, oxygen vacancies and surface hydroxyl groups. This understanding will later prove critical in WP 3, where we approach the much more complicated nanostructured materials prepared in funCOS 5. Since most of the funCOS groups will initially use MgO(100) thin films as substrate, the growth studies will very much be a collaborative effort between the groups.

For catalysis and sensor applications the adsorption of small molecules on the metal centers of porphyrins plays an important role, and we will study how the adsorption of CO, NO and H₂S affect the electronic structure of the porphyrins. The binding of O₂ and CO to iron porphyrin in hemoglobin is well known, and it has been observed that H₂S can induce a hibernation-like state in mice, possibly through the binding of H₂S to heme in complex IV (cytochrome c oxidase) [1], [2], [3]. We will furthermore extract the adsorption energies of the small molecules to the metal center, using TPD, and even attempt to run simple reactions such as CO oxidation.

The detailed characterization of homogeneous molecular layers on different oxidic substrates sets the stage for investigating, in great detail, electronically excited molecular states that play an important role in photochemistry and photovoltaics. While on metal substrates the strong coupling to the substrate and the continuous electronic density of states lead to lifetimes of electronic excitations generally below 10 fs, the coupling and electron transfer to the substrate can be reduced by introducing insulating spacer layers such as oxides and much longer lifetimes are expected. TR-2PPE provides experimental information on the electronic structure of adsorbed molecules and the lifetimes of excited electrons. The detailed knowledge of the electronic states and electron transfer processes is important for an understanding of the chemical and catalytic activity of functional molecular structures on complex oxide surfaces, which rely on the adsorption/reaction of small molecules at the metal center of the metallotetrapyrroles. These results will complement the spectroscopic information obtained in funCOS 4.