Observing Cooperative Behavior with Molecular Surface Structures

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In the work at hand the interplay between molecules as well as molecules and substrate is studied in ultrahigh vacuum (UHV).

In Chapter 3 hydrogen-bond (H-bond) recognition, which is based on an H-bonding motif well-known in supramolecular chemistry, is investigated on Ag(111) for a three component system. Moreover, for one of the molecules a thermally induced phase transition from a porous network to a close-packed structure is found. Scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) show that both surface structures exhibit long-range order and are commensurate with the Ag-substrate. In particular, this transition involves a conformational change of each molecule that is not observed in solution.

Chapter 4 deals with the study of intermolecular interactions of the perylene derivative 1,3,8,10-tetraazaperopyrene (TAPP) on Cu(111). Different surface structures are observed which exhibit different types of intermolecular interactions. Surface structures formed by TAPP molecules interacting via weak van-der-Waals forces are transformed into a long-range-ordered porous network upon annealing. For this network, which is commensurate to the substrate, the intermolecular interactions are based on the coordination of Cu-adatoms to the N-atoms of TAPP. Upon further annealing covalent C-C couplings between TAPP molecules result in the formation of chains. X-ray photoelectron spectroscopy (XPS) and computational studies using density functional theory (DFT) show that the N-atoms of the chains can coordinate to Cu-adatoms as well. Moreover, in a preliminary study the attempt was undertaken to create polymerized structures in two dimensions. However, the formation of ordered structures is a challenging task since self-correction that is inherent to weak reversible interactions is missing.

The molecule-surface interaction of a long-range-ordered porous network formed by the perylene derivative 4,9-diaminoperylene-quinone-3,10-diimine (DPDI) on Cu(111) is studied in Chapter 5. This network whose molecular building-blocks interact via H-bonds exhibits an extraordinary stability. In particular, the molecular adsorption height above the Cu surface determined by X-ray standing wave experiments (XSW) indicates that the molecular perylene core is not directly involved in a strong interaction with the surface. Thus, it is assumed that Cu-adatoms interact with the molecular N-atoms in order to explain the stability of the network. Moreover, scanning tunneling spectroscopy (STS) and angle-resolved photoemission spectroscopy (ARPES) reveal the formation of an electronic band which is induced by the periodic influence of the molecular network on the surface state of Cu(111).