UC San Diego
Mixed valency and electronic structure in self-assembled monolayers, self-exchange, and hydrogen bonded assemblies
- Author(s): Goeltz, John Christopher
- et al.
Mixed valency and electron transfer are explored in self- assembled monolayers, in intermolecular electron self- exchange reactions in solution, and in hydrogen bonded assemblies. Tetrathiafulvalene is derivatized for binding to gold in self-assembled monolayers, but the trinuclear ruthenium cluster Ru₃O(OAc)₆L₃ (where L is an ancillary ligand) is used as a building block for the majority of the work. While oxo-centered trinuclear hexaacetate clusters of many transition metals are known, the triruthenium cluster is particularly versatile because of the kinetically stable binding of a wide variety of ligands. The electronic structure can be depicted by molecular orbitals diagrams or more recently by computer generated combinations of atomic orbitals, and remains relatively unchanged for variously substituted clusters. The important differences with respect to getting an electron in or out of a cluster lie in electron delocalization onto ligand based orbitals. In combination with reorganization energies calculated from accumulated structural and vibrational data, the molecular orbital diagrams offer a great deal of explanatory power. When allowed by symmetry and energy matching, electrons in reduced clusters are delocalized onto pyridine [pi]* orbitals, greatly easing the transfer to an oxidized cluster in the face of a large reorganization energy. When electron delocalization is not allowed, electron self- exchange can be fast only if the reorganization energy is small. In hydrogen bonded assemblies of these ruthenium clusters, the electronic structure is still dominant in electron transfer behavior. In these cases the increase in delocalization upon dimerization appears to induce large changes in the orbital energies. This is consistent with the electronic absorptions and the thermal electron transfer behavior observed. The take-home message of this dissertation is that one must understand to electronic structure of a complex in order to understand its behavior in electron transfer reactions. This may seem obvious, but is often overlooked. With the knowledge of the electronic structure of reactants and products, one has a much greater chance of understanding the path between them. Molecular orbital diagrams seem cumbersome and outdated in this age of calculated chemistry, but many cases drawing them out is worth the investment in time. Who knows, you may even learn something