Chemistry in complex quantum systems: A study of charge transfer and electron transport at a metal-molecule interface

The complex electronic structure at an organic-metal interface provides a unique testbed for studying the fundamental properties of electron transport at the nanoscale and the effects of charge transfer to chemical reactivity in transforming small molecules. Molecular electronics containing a single molecule, which is sandwiched between two metal electrodes via functionalized anchor groups, have shown potential to be good candidates for atomic-scale transistors, switches, and filters for spintronic devices. Despite this potential, no molecular electronic currently exists on the market owing to the unresolved nature regarding the origin of these systems' conductance distributions. Current literature suggests that the peak width, observed experimentally through various break-junction techniques, is controlled by the interaction between the anchor ground and electrode metal. This work offers a different explanation for conductance fluctuations by utilizing models of single-molecule junctions containing benzene-based organics and gold electrodes in molecular electronic devices. Through a combination of ab initio molecular dynamics and FFT [Fast Fourier Transform] methods, we show that the chosen anchor groups contribute significantly to the construction of the total distribution but have little effect on the observed peak width. Instead, our results demonstrate that motion on the benzene ring, captured in the normal-vibrational modes of the junction, has a more substantial impact on peak width by disrupting the aromaticity (resonance) of the benzene via distortions in the ring's geometry. 2 Additionally, we show that targeting these modes by modifying the benzene ring allows one to tailor conductance distributions. Beyond the transport of electrons in molecular junctions, the metal-ligand interface significantly impacts catalysis. For example, the branching ratio of polyethylene may be tuned by selective control of the charge density of a modified Brookhart-type catalyst. This is demonstrated in Chapter V, whereby the alpha-diimine-ligands' charge-transfer abilities are altered by adding electron-donating and withdrawing groups, affecting the branching ratio of polyethylene polymerization. Furthermore, this work explores the transformation of isopropanol over strontium titanate via the transfer of electrons to the strong Lewis acidic titanium sites at the surface, showcasing two unique mechanisms that have yet to be reported in the literature.