title: On-Surface Genesis of Complex Molecular Architectures Based on Polycyclic Aromatic Compounds
Creators: name: Wit, B
Publisher: name: University of Liverpool Repository
Year: 2016

description: This work describes three systems in the emerging field of on-surface chemistry. The aim of on-surface chemistry is to employ covalent bond forming reactions to create complex well-defined molecular architectures directly on surfaces from simple precursors. However, the number of viable on-surface reactions is still very limited and the resulting oligomers, polymers and networks are often highly defective due to the irreversible nature of the reactions. The present work aims to increase the knowledge of different aspects of on-surface chemistry and its potential applications by examining three distinct systems. The first system employs the established on-surface reaction of porphyrins on Cu(110) to form a barrier to the diffusion of a second mobile component, 1,3-bis(imidazol-1-yl methyl)benzene. The diffusion of this molecule is highly anisotropic; it diffuses readily in the &lt;110&gt; direction but hardly in the &lt;001&gt; direction. Moreover, it was found through calculations that the diffusion of this molecule is of a walking type due to the specifics of the molecule-surface interaction. The ‘walker’ binds with two identical ‘feet’ to the surface and it can only detach one ‘foot’ at a time. The barriers are linear one dimensional organometallic chains grown exclusively in the &lt;001&gt; direction, perpendicular to the diffusion direction of 1,3-bis(imidazol-1-yl methyl)benzene, and as such these porphyrin chains can act as ‘fences’ to the diffusion of the walker. Additionally, there is some attractive interaction between the porphyrin fences and the walker. Most notably, the walker can be trapped temporarily by the fences, creating something resembling stations for the walker. The second system investigated is that of two porphyrins, either porphine or zinc diphenyl porphyrin, on Au(111). In this case, the porphyrin derivatives are connected on the relatively inert Au(111) surface even though these porphyrins lack marked functional groups. All available peripheral C-H bonds were sufficiently activated such that the hydrogen could be cleaved off and C-C bonds could be formed upon annealing. Unfortunately, the selectivity that was found on Cu(110) among the different C-H bonds of diphenyl porphyrins was not observed on Au(111). The reaction initiates at step edges, but propagates on the terraces at slightly higher temperatures. This results in large scale irregular networks. In the third system the effect of chirality in on-surface reactions between helicenes on Cu(110) is investigated. Two different hexahelicenes, one with a single methyl group and one with two ethynyl groups, were used. It was found that both helicenes can undergo an indiscriminate reaction of the peripheral C-H bonds, forming random oligomers. However, the helicenes bearing the ethynyl functionalities can also undergo on-surface Glaser coupling at room temperature. For this reaction, the two types of dimers could be distinguished in scanning tunnelling microscopy images which revealed a significant difference between the observed and expected ratio of the products: the formation of heterochiral dimers is favoured over the formation of homochiral dimers.