Reactions of organic molecules on metal surfaces studied by STM

Two distinct systems of surface reactions involving complex organic molecules were studied on single crystal metal surfaces by Scanning Tunneling Microscopy (STM), Low Energy Electron Diffraction (LEED) and X-ray Photoelectron Spectroscopy (XPS) under ultra high vacuum conditions. The first system focused on the Pt-cinchona alkaloid system, which is a powerful catalytic system in the field of enantioselective heterogeneous catalysis. Herein, an achiral reactant molecule is hydrogenated over a cinchona alkaloid modified platinum surface. Due to the modification of the surface with the chiral cinchona modifier, the symmetry of the system is broken and enantioselectivity is introduced. The hydrogenation of the reactant therefore is biased and leads to an enantiomeric excess (ee) of one enantio form of the product. The aim of the present STM study was to gain direct insight in the molecular processes which occur during adsorption of such modifiers and reactants. Therefore, the adsorption of cinchonidine (CD), cinchonine (CN) and 2,2,2-trifluoroacetophenone (TFAP) was investigated on both Pt(111) and Pd(111) single crystal surfaces in the presence and absence of hydrogen. Different adsorption modes were identified and a change in adsorption geometry upon addition of hydrogen was followed. The findings are discussed in the light of catalytic, theoretical and spectroscopic data available for this system. Our work showed, that the discrimination of different surface species by their mobility and the time-resolved observation of their surface chemical processes by STM provides a powerful tool for the investigation of complex catalyst systems. These studies can complement other surface analytical methods and quantum chemical calculations in order to gain insight into the mechanistic aspects. The second system is an example how surface reactions can be used to convert large organic molecules subsequent to their deposition in order to efficiently self-assemble larger structures. For this purpose a perylene derivative, 4,9-diaminoperylene-quinone-3,10-diimine (DPDI), was deposited on a Cu(111) single crystal surface and investigated by STM. These highly mobile precursor molecules are then transformed via a thermally induced surface-assisted dehydrogenation reaction by annealing in-situ at 300 ◦ C. The resulting molecules form autocomplementary species of hydrogen-bond donor and acceptors and thereby can interact with each other via H-bonding. Depending on the surface concentration prior to annealing different well-order molecular patterns are created. The most stable structure among these molecular patterns is an open hexagonal structure. Due to an exact register with the Cu surface and due to a strong so-called resonance-assisted hydrogen-bonding (RAHB) this highly robust and porous structure is well-suited for the incorporation or trapping of guest molecules. Trapping and manipulation experiments of C60 and octaethylporphyrins (OEP) inside the network are presented and discussed.