Connect – Catch – Couple

In this project, the chemical interaction between molecules and metallic nanostructures will be investigated. This is of great importance in heterogeneous catalysis, where chemical reactions are accelerated by a metallic catalyst, the catalyst being in the solid phase and the reacting molecules in the gaseous or liquid phase, adsorbed on the catalyst. To gain insight into fundamental processes, scanning tunneling microscopy will be used in this project, allowing imaging and characterization of single molecules on a surface. It is important that the high spatial resolution of the instrument gives access to exact position, orientation and internal structure of individual molecules and metal clusters. By combining organic molecules and nanostructures made of palladium (Pd) on surfaces, this project will investigate in detail how individual molecules interact with the nanostructures. The structure and size of the metallic structures will be varied systematically, from stepped crystalline sur faces to size-selected clusters and individual atoms. The coupling process between molecule and metal consists of three steps, which will be examined separately: (1) CONNECT: formation of metal-molecule bonds on the Pd nanostructures; (2) CATCH: study the influence of the metal-molecule interaction on the shape and dynamics of the nanostructure; (3) COUPLE: analyze the reaction products resulting from two molecules and the Pd nanoparticles. This project aims to answer key questions that are important in heterogeneous catalysis: How do molecules interact with Pd catalysts of different structures? How are the molecules activated so that they can be transferred into the desired products? How can the various parameters, such as particle size or solvent, be syste matically adjusted in order to increase the catalyst stability? The experiments in this project will be carried out in a variety of different environments: From measurements under ultra-high vacuum, where model systems can be examined under idealized and highly controlled conditions, to investigations in liquids, which approximate realistic conditions for chemical reactions. Comparing the resulting complementary results will give fundamental insights into the chemical processes. By producing precisely defin ed Pd nanostructures and thereby controlling metal-molecule interaction, it should become possible to steer basic catalytic properties through targeted design, with impact in various fields, from sensors to molecular electronics and beyond.

The formation and dissociation of chemical bonds is fundamental in any chemical reaction. In this project these transformations were studied on solid surfaces, which are relevant for instance in heterogeneous catalysis or on-surface polymerization. This was done at the single-molecule level by using scanning tunneling microscopy (STM) under ultrahigh vacuum conditions and at cryogenic temperatures. First, Heck cross-coupling of specific precursor molecules adsorbed on metallic surfaces was studied. In solution, Heck cross-coupling is catalyzed by Pd complexes, while here Pd was used in nanostructured shapes. A main result of this project is the homo-coupling of molecules on the surface, catalyzed by different Pd nanostructures, controlling their density and size. After deposition of dibromoterfluorene molecules and subsequent heating, various species were found: in addition to de-brominated molecules, connected to the edges of the Pd islands, metal-organic dimers and also dibromohexaphenyl molecules were found - a clear proof that C-C bond formation has occurred. Moreover, the Pd density on the surface was varied systematically. Minimal amounts of Pd, when actually Pd islands are not even formed, but only Pd substitution at defects of the gold surface are present, turned out to be still active to catalyze the coupling reaction, which occurs at much higher temperatures without Pd nanostructures. Statistical analysis of the population of species and Pd islands morphology at each reaction step and for different Pd nanostructures from single atoms to large islands, which is still ongoing, will help to understand where the molecular coupling occurs. Beside coupling, i.e. bond formation, also the breaking of interatomic bonds was studied on the level of single molecules. To gain insight into the details of the bond dissociation processes, it is advantageous to investigate single molecules that are adsorbed on crystalline surfaces with well-known positions, conformations and orientations. Bond breaking at selected positions in single molecules was triggered by tunnelling electrons of the STM. So far, bond dissociation dynamics have been studied only in small molecules, but not in larger ones that exhibit distinct rotational degrees of freedom. Here, a bromine atom was dissociated from a single dibromo-terfluorene molecule with an elongated chemical structure on a Ag(111) surface, allowing to identify not only the displacement, but also the rotation of the molecular fragment. It turns out that the molecular excitation that causes bond breaking can propagate through the backbone of the molecule to dissociate also a remote bromine atom. Moreover, the role of the metal substrate is evident as the molecular fragment quickly binds to the nearest silver atom after dissociation and dissipates its energy in rotational motion. These findings open new ways towards the control of chemical reactions on surfaces, of interest for improved synthesis of 2D materials and molecular architectures.