Metal-Ligand Bond Dynamics in Metal-Organic Frameworks Confirmed by Variable Temperature Vibrational Spectroscopy
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Date
2020
Authors
Andreeva, Stacey
Journal Title
Journal ISSN
Volume Title
Publisher
University of Oregon
Abstract
Dynamic chemical bonds reversibly break and reform with minimal heat, light, or pressure. This type of bonding is responsible for the basic mechanism of crystallization for many material systems because erroneous bond formation can be corrected through facile reversal until the material settles into the most favorable crystalline phase. A particularly important class of crystalline materials that emerge from this dynamic process are metal-organic frameworks (MOFs). MOF architecture is dependent on two building blocks: the metal ions or metal clusters and the organic ligands that bridge the metals. For the past two decades, MOFs have been viewed as rigid structures, but we propose that even after formation, MOFs contain metal-ligand bonds that remain dynamic such that the crystalline structure contains mixtures of partially bound and unbound arrangements. We hypothesize that metal-carboxylate bonds – which constitute the majority of MOFs – are especially dynamic, with large fraction of these bonds existing in unbound states. To understand this metal-ligand interaction, our research focuses on monitoring the changes in the vibrational frequencies as a function of temperature. Variable temperature vibrational spectroscopy shows a lowering in energy of the stretches associated with these dynamic bonds at increased temperatures, indicative of bond weakening. By understanding this relationship, more general insights can be made regarding important material behavior such as crystallization and self-healing responsiveness. Insight into their labile nature would provide a predictive model of their growth mechanism and inspire important applications such as the use of MOF for self-healing membranes.
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Keywords
Metal-Organic Frameworks, Dynamics, Vibrational Spectroscopy