title: Structure-Property Relationships in Molecular Junctions
Creators: name: Welsh, Jonathan
Publisher: name: University of Liverpool Repository
Year: 2023

description: The growth of the field of molecular electronics from initial theoretical proposals for molecule-based devices to the development of experimental techniques for measuring the electrical conductance of individual molecules indicates the research interest in exploiting the unique capabilities of molecules for electronic device applications. Progress in this field is dependent on gaining knowledge and understanding of how tuning molecular structure influences charge transport behaviour to enable identification of compounds with suitable characteristics for different device functions. This thesis investigates relationships between molecular structure and electrical properties in molecular junctions and aims to gain insight into how structural alterations in closely related molecules can lead to significant differences in charge transport behaviour. Particular emphasis is placed on exploring how molecular conductance can be controlled by quantum interference effects inherent to the small dimensions of molecules, which can lead to large changes in conductance, and anchoring groups, which control the molecule-electrode interface properties. The novel aspects of this work involve investigating the effect of changing structural features on charge transport properties in different series of molecules and explaining the behaviour observed for these compounds. The first series of experiments focuses on the influence of interference effects on the electrical properties of pyridine-terminated molecular wires. This section explores the effect of changing the pyridyl anchoring site positions on molecular conductance. The target molecules have been synthesised via cross-coupling reactions and the experimental results compared with theoretical transmission calculations. The measurements show that changing the pyridine anchoring site positions does not lead to the large variations in conductance predicted by theory, which is attributed to the metal-molecule orbital alignment of the molecular junctions. The second series of experiments focuses on the influence of destructive interference effects on the electrical properties of fluorene-derived molecular wires. This section explores how the addition of cross-conjugated ketone moieties to a linearly conjugated core unit affects the molecular conductance. The results indicate that the presence of cross-conjugated carbonyl groups leads to a decrease in conductance, as the cross-conjugated molecules display lower conductance values, supporting predictions of destructive interference behaviour. The third series of experiments investigates the electrical properties of two molecules with multidentate 1,2,4-triazolyl anchoring groups. These molecules possess a simple core structure, allowing the influence of the novel bidentate anchoring moieties on the conductance properties to be assessed. The experiments indicate that 1,2,4-triazole can function as an anchoring group, as the results show that both structures form stable molecular junctions which display consistent conductance features, albeit with relatively low molecular conductance values.