- Applications of Advanced Magnetic Resonance Techniques to the Study of Molecule-Based Magnetic Materials.
Greer, Samuel Michael, Hill, S., Shatruk, Mykhailo, Xiong, Peng, Steinbock, Oliver, Florida State University, College of Arts and Sciences, Department of Chemistry and Biochemistry
The highly interdisciplinary study of molecular magnetism spans a wide array of topics, ranging from spintronics and quantum computing to enzyme function and MRI contrast agents. At the core of all these fields is the study of materials whose properties can be controlled through the rational design of molecules. The chemical tailoring of molecular magnetic properties can only be achieved by understanding the relationship between the physical and electronic structures. In this dissertation,...
Show moreThe highly interdisciplinary study of molecular magnetism spans a wide array of topics, ranging from spintronics and quantum computing to enzyme function and MRI contrast agents. At the core of all these fields is the study of materials whose properties can be controlled through the rational design of molecules. The chemical tailoring of molecular magnetic properties can only be achieved by understanding the relationship between the physical and electronic structures. In this dissertation, the interplay between structure and physical properties is probed using a variety of magnetic resonance techniques. In Chapter 1, we give a succinct overview of the various methods utilized in this dissertation. We first describe the experimental methods including electron paramagnetic resonance (EPR), 57Fe nuclear gamma resonance (Mössbauer) spectroscopy, electron double resonance detected nuclear magnetic resonance (ELDOR-NMR), and Fourier transform far-infrared (FTIR) spectroscopy. In addition to the introduction of each technique, we describe how the data is analyzed and what quantities may be extracted from each method. We also introduce the quantum chemical methods used to rationalize the spectroscopic parameters. In Chapter 2, we investigate a recently reported Fe-V triply bonded species, [V(iPrNPPh2)3FeI], using high frequency EPR (HFEPR), field- and temperature-dependent 57Fe Mössbauer spectroscopy, and high-field ELDOR-NMR. From the use of this suite of physical methods, we probe the electron spin distribution as well as the effects of spin-orbit coupling on the electronic structure. This is accomplished by measuring the effective g – factors as well as the Fe/V electro – nuclear hyperfine interaction tensors of the spin S = ½ ground state. We have rationalized these tensors in the context of ligand field theory supported by quantum chemical calculations. This combined theoretical and experimental analysis suggests that the S = ½ ground state originates from a single unpaired electron predominately localized on the Fe site. Chapter 3 describes a combined HFEPR and variable-field Mössbauer spectroscopic investigation of a pair of bimetallic compounds with Fe-Fe bonds, [Fe(iPrNPPh2)3FeR] (R = ≡NtBu and PMe3). Both of these compounds have high spin ground states, where R= PMe3 (S = 7/2) and the R= ≡NtBu displays (S = 5/2). The ligand set employed in this work encapsulates each Fe site in a different coordination environment. This results in polarized bonding orbitals which engender each nuclear site with unique hyperfine tensors as revealed by Mössbauer spectroscopy. Absent the metal-metal bond, the tris-amide bound site in both compounds is expected to be Fe(II). To gain insight into the local site electronic structure, we have concurrently studied a compound containing a single Fe(II) in a tris-amide site. Our spectroscopic studies have allowed us to assess the electronic structure via the determination of the zero field splitting parameters and 57Fe electronuclear-hyperfine tensors for the entire series. Through the insight gained in this study, we propose some strategies for the design of polymetallic single molecule magnets where the metal-metal interactions are mediated by the formation of covalent bonds between metal centers. Recently, a great deal of the work in molecular magnetism has moved away from polymetallic compounds and towards molecules containing only a single magnetic ion. A critical challenge in this endeavor is to ensure the preservation of orbital angular momentum in the groundstate. The stabilization of the ground state orbital moment generates the strong magnetic anisotropy which is often required for the design of magnetic materials. The presence of unquenched orbital angular momentum can be identified by significant shifts in the g-value away from the free ion value. In an initial report of a Ni(I) coordination complex, which was found to exhibit field-induced slow magnetic relaxation, no EPR signal was observed. Given the expectation that orbital angular momentum can shift the g-values beyond the range expected for a typical S= ½ system, we have reexamined this compound using multi-frequency EPR and field-dependent FTIR spectroscopy. Through a combined spectroscopic and theoretical effort, we have characterized the effect of first order spin-orbit coupling on the electronic structure. The final report, Chapter 5, examines an exciting new class of photomagnetic materials based on bisdithiazolyl radicals. These materials, and others with magnetic properties that can be modulated via optical excitation, offer enticing opportunities for the development of next generation technologies. The dimorphic system in this study crystallizes in two phases, one composed of diamagnetic dimers and the other of paramagnetic radicals. Here we report on the use of high-field electron paramagnetic resonance spectroscopy to characterize both the thermally- and light-induced transitions in the dimer phase. During the course of this study we show that signals originating from residual radical defects in the dimer phase can be differentiated from those arising from the radical phase.
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