Spectroscopy and Electronic Structure

• Applicants: Professor Dr. Thomas Fauster; Privatdozent Dr. Ole Lytken; Professor Dr. Hans-Peter Steinrück
• Subject Area: Physical Chemistry of Solids and Surfaces, Material Characterisation ; Experimental Condensed Matter Physics ; Solid State and Surface Chemistry, Material Synthesis ; Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2013 to 2022
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 214951840

In close collaboration with the other funCOS projects, funCOS 2 focusses on electronic structure of porphyrins adsorbed on the different oxide surfaces by using the full range of electron spectroscopies at our disposal. Specifically, we will investigate porphyrins with and without carboxylic, phosphonate and hydroxamic acid anchor groups, adsorbed on magnesium oxide, titanium dioxide and cobalt oxides. We will also investigate different relevant reactions of the adsorbed porphyrin molecules with the oxide surfaces, such as metalation, metal-center exchange and dehydrogenation. To allow sufficient conductivity for electron spectroscopy, magnesium oxide will be grown as MgO(100) thin films on Ag(100), titanium dioxide will be investigated in the form of rutile-TiO2(110) single crystals, and cobalt oxide will be grown as CoO(100), CoO(111), Co3O4(111) thin films on Ir(100). Whenever possible, the porphyrins will be evaporated onto the oxides under ultrahigh vacuum. However, if for some of the anchor groups this is not possible we will use the pulse-spray deposition system, applied for in funCOS 3. During the first funding period, our focus was on the funCOS showcase system MgO(100) and simple porphyrins without anchor groups. During the next funding period, our focus will shift towards titanium dioxide and the cobalt oxide surfaces and porphyrins with anchor groups. The electronic structure of the pristine oxides and adsorbed porphyrin molecules will be investigated with laboratory-based two-photon photoemission and photoluminescence, in addition to synchrotron-based ultraviolet photoemission spectroscopy. We are specifically interested in the occupied and unoccupied states and the role they play in bond formation and charge transfer. Self-metalation, which we observed on both MgO(100) and TiO2(110) during the first funding period, is a reaction where the two central protons of the porphyrin molecule are exchanged with an ion from the surface. During the reaction, the two non-equivalent nitrogen pairs of the free-base molecule becomes equivalent, which is easy to observe in X-ray photoelectron spectroscopy. In the next funding period, we want to investigate if self-metalation also occurs for the cobalt oxide surfaces. Recently, we also observed hints of metal-center exchange of the metal centers of cobalt 5,10,15,20-tetraphenylporphyrin with magnesium ions from MgO(100). Photoluminescence measurements will be used to answer if this is the case. From metal surfaces, we also know that tetraphenylporphyrin undergoes dehydrogenation leading to the fusion of phenyl and pyrrole rings. Near-edge X-ray-absorption fine structure and X-ray standing wave measurements, sensitive to the molecular geometry, will be able to confirm if a similar reaction occurs on oxides. Finally, the diffusion of porphyrin molecules in porphyrin multilayers will be investigated with a combination of X-ray photoelectron spectroscopy and temperature-programmed desorption.

• DFG Programme: Research Units
• Subproject of: FOR 1878: funCOS - Functional Molecular Structures on Complex Oxide Surfaces
• International Connection: Argentina, China, USA
• Cooperation Partners: Professor Robert Bartynski; Professor Federico Williams; Professor Junfa Zhu