Self-assembly of multiligand supramolecular architectures at metal surfaces

Abstract

Self-assembly of organic molecules represents an efficient and convenient bottom-up approach for the structural functionalization of surfaces at the nanometer scale. The great potential of assembling supramolecular architectures from organic molecules lies in the vast choice of building blocks that are accessible. This allows to predefine and to direct the molecular organization through the steric and electronic information stored in the molecules. A large variety of homotopic supramolecular structures can be achieved, however limited in complexity. The full realization of highly developed surface architectures for designable chemical functions and physical properties may depend critically on a higher level of complexity, which could be satisfied by utilizing a mixture of different molecular building blocks. Within this thesis, the self-organization of multi-component systems at well-defined metal surfaces is investigated by scanning tunneling microscopy. The mixtures consist of different representatives of linear polyaromatic ligands with carboxylate or pyridyl derived functional groups. The assembly is directed by hydrogen bonding, metal-organic complex formation or a combination of both, i.e. a hierarchical interaction scheme. The aim of this thesis is threefold: First, the homotopic self-assembly of different pyridyl ligands is investigated as a basis for the multi-component systems. One-dimensional chains as well as two dimensional (2D) networks can be formed by copper-pyridyl coordination motifs. The influence of the substrate on these structures will be discussed in detail. Moreover, the self-organization process of multi-components is investigated at the fundamental level. Since functional groups primarily discriminate the different molecular species during the assembly and with that assure a distinguished arrangement, selective bonding schemes are essential for the design of highly ordered supramolecular architectures. Two suitable selective coordination motifs are identified, an iron-carboxylate and a copper-pyridyl complex node. In ligand mixtures, strong preference of one metal species is observed at surfaces, which would not be expected in solution. In addition, redundant mixtures (i.e. ligands of different size but same functional group) serve as model systems to investigate the dynamic self-organization process of modular multicomponent systems, e.g. self-selection or error tolerance, directly with nanometer accuracy. Self-selection is observed if the molecular recognition is steered by a highly reversible coordination bond, while a more robust bonding fosters structural adaption and tolerance to the introduced error (i.e. redundant mixture). These experiments underline the importance of reversibility of the interactions steering the self-assembly process into highly ordered structures. Finally, the extended possibilities due to the multi-component approach for the construction of advanced architectures and structural control are explored: Complex and sophisticated supramolecular networks can be directly synthesized out of simple molecules at the surface. A hierarchical assembly is presented, where a coordination motif defines a primary unit, while hydrogen bonding steers the 2D organization. The secondary level of interaction (hydrogen bonding) can be exclusively addressed, leading to distinct 2D arrangement of the undisturbed primary coordination units. A concept of structural control by rational design is presented, where geometric network parameters can be modified by the replacement of the appropriate molecular species. Due the multi-component nature of the systems, a finer tuning of the structure is enabled (independent substitution of each component) compared to homotopic assemblies. Moreover, a continuation of the multi-component concept is demonstrated, where organizational control is imposed to a homotopic system by adding an additional molecular species. It is demonstrated that a long range order and thermal stability is imposed onto meta-stable Cu-pyridyl coordination chains by cooperative assembly with a carboxylate species.