Light-induced on-surface synthesis of molecular chains

The coupling of molecules at single-crystal surfaces can be used to get insight into chemical reactions in a two-dimensional confinement and to form well-defined molecular nanostructures that can be designed via the chemical structures of the initial building blocks. The aim of this project is to form one-dimensional molecular structures on ultrathin insulator films that are adsorbed on metal surfaces. It will be done by chemical reactions between specific molecular building blocks, under ultrahigh vacuum conditions, using photons as stimulus. This photochemical approach represents a promising alternative to thermally induced reactions and could open new opportunities towards controlled formation of molecular architectures of desired structure and properties on insulator surfaces. So far, on-surface chemistry has relied almost exclusively on mechanisms that employ metallic surface as catalysts and heat to form covalent organic frameworks. Despite remarkable success, there is quest for new reaction mechanisms that avoid high temperature and metal catalysts. For example, high temperatures can lead to molecular desorption or decomposition. On the other hand, the electrons of a metallic surface can couple with molecular orbitals, hampering characterization of their electronic properties. One possibility to circumvent this problem is to synthetize molecular nanostructures by light and free of metal agents. We want to study the photochemically (i.e. non-thermal) driven on-surface synthesis of polymers on thin insulating films. Additionally, the use of light holds important advantageous as an appropriate choice of photon energy represents a highly selective stimulus. High-resolution scanning probe microscopy will be used to characterize the chemical structures of reactands as well as intermediate and final products of the reaction. This will give, with help of theoretical simulations, detailed understanding of intermediate and final products of the reactions and, due to the use of local microscopy techniques, even rare species will be detected. Thus, this project will give a fundamental insight into such light-driven reactions and the corresponding reaction pathways.

This project studied the coupling of molecules on surfaces to form well-defined molecular nanostructures and to get insight into chemical reactions in a two-dimensional confinement by scanning tunnelling microscopy at the single-molecule level in ultrahigh vacuum. It was found that 2,6-dibromo-anthracene molecules can be polymerized by different stimuli, light (in the UV range) and heat. Both processes lead result in linear oligomers on a Au(111) surface, but a systematic comparison revealed that thermal diffusion plays a crucial role within the photo-approach. A typical problem in molecular coupling reactions on a surface is the influence of the metal substrate, which can alter the intrinsic electronic structure of the molecules. In order to decouple the molecules from the metal, graphene was grown on Pt(111) and halogenated organic molecules were then deposited onto it to perform dehalogenation. It turned out that the molecule-surface interaction is so weak, that even stable imaging is challenging. Also insulating NaCl layers where used on which photo-debromination could be induced very efficiently. While intact and activated molecules were found to be immobile on NaCl at 5K, the formation of oligomers and clusters was observed at higher temperatures. On-surface heterocoupling of a fluorinated carbene (CF2) with an aromatic halide (BTFyl) was studied with the aim of understanding the distinct mechanisms that influence the chemical transformation of reagents into products. Importantly, the CF2 molecule travels far distances along only one atomic row of the Cu(110) surface and coupling was observed only for certain trajectories of the CF2 'projectile'. No reaction was observed when the CF2 path was at an angle of at least 30 to the long-axis of the BTFyl, despite a similar collision position. This approach direction presumably did not produce reaction due to a poor overlap between the dangling bonds of the CF2 and the BTFyl at the point of collision. In another approach even a reversible reaction could be achieved with the CF2 'projectile' molecules. This means that a single CF3 molecule could in a first step be dissociated into its compounds CF2 and F and in a second step the CF2 molecule and the F atom are coupled to a CF3 molecule. Extending this to two individual CF3 molecules (named A and B), the components could even be swapped: After dissociation into CF2 and F, the different components are laterally displaced on the surface so that the coupling experiments lead to a new CF3 molecule that in one case contains the CF2 molecule that origins from A and the F atom that initially comes from B, while the other novel CF3 molecule has the opposite composition.