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theses

After decades of development, host-guest chemistry has become a highly multidisplinary field, spanning areas of chemistry to material science. The construction of host molecules and materials for housing specific guest species often involves a complicated combination of subtle electronic and structural interactions. The number of interacting sites and type of non-covalent forces that are involved in the host-guest interaction determine the stability of a complex. Thus, a comprehensive understanding of the binding behavior and structure-property relationship is as crucial as the design and synthesis of host materials. Conventional characterization methods of crystalline solid state materials such as single crystal XRD are key but have their limitations. In this case, accurate and structurally sensitive characterization methods are highly demanded. In this dissertation, Raman and X-ray absorption spectroscopy techniques are employed to complement conventional characterization methods, FT-IR, UV and XRD, to help identify the structural change and host-guest interactions on various systems in their solid form, including metal-organic frameworks and macrocycle complexes. The results of these investigations revealed unique information to help understand host-guest interactions in these systems and may help the further improvement of their applications. Chapter 1: An overview of the research background is introduced. The overview will start from a general introduction of host-guest systems, followed by a summary of methods used for probing the structural changes and host-guest interactions. Detailed descriptions of the specific classes of these systems are covered in the remaining part of the chapter, namely metal-organic frameworks and cucurbit[n]uril. Chapter 2: A flexible metal organic framework (MOF) comprised of Co(II) ions and 4,4’-oxybis(benzoic acid) linkers undergoes substantial structure rearrangements upon removal of the metal coordinated water molecules upon activation. Two complementary structurally sensitive methods, X-ray absorption and Raman spectroscopy, are used to build a composite picture of the MOF activation process by revealing the specific local structural changes about both the Co sites and the organic linkers that are not apparent through standard powder X-ray diffraction studies. Chapter 3: The activation and CO2 gas adsorption processes in the rht-type metal-organic framework, [Cu3(TDPAT)(H2O)3]•10H2O•5DMA (TDPAT=2,4,6-tris(3,5-dicarboxylphenylamino)-1,3,5-triazine) were investigated on the molecular level using several spectroscopic characterization methods. The remarkable selectivity of this framework for CO2 is attributed to the high density of coordinatively unsaturated metal nodes and amine linker functionality that both serve as gas binding sites. Spectroscopic evidence for these binding interactions as well as the concomitant electronic and molecular level structural changes of the host framework and CO2 guest is derived from a combination of in situ UV-vis diffuse reflectance, X-ray absorption and Raman spectroscopy studies. Results showed that the activation process produced subtle structural rearrangements of the framework that may be influencing the binding interactions upon subsequent CO2 adsorption. Chapter 4: The activation and CO2 gas adsorption processes of a series of MOFs, M2(dobdc) (M=Mg, Co, Ni, Zn; dobdc= 2,5-dihydroxy-benzene dicarboxylic acid), were studied in-situ by Raman spectroscopy and diffuse reflectance spectroscopy. The high density of open metal sites lining the pore wall internal surfaces gives extraordinary gas storage capacity and selectivity to these frameworks, especially under low pressure. The fact that a considerable capacity gap exists among these iso-structural frameworks remains puzzling. In this chapter, a careful examination of in-situ Raman and diffuse reflectance spectra reveals that other than the well known factor of binding affinity, the unique local and global structure change that each framework experiences upon activation (ie removal of labile metal-bound solvent molecules) may have a significant impact on the gas adsorption properties as well. Chapter 5: A complex formed between cucurbit[7]uril (CB7) and ferrocene (Fc) is investigated in the solid state using Raman spectroscopy. The Raman spectra are compared with those of the CB7 host and Fc guest reference molecules, revealing significant frequency shifts of some vibrational modes upon complexation. These changes expose the structural and electronic interactions between the host and guest components and provide evidence that the ferrocene moiety resides within the cucurbituril cavity in the solid state environment.

ETD_6735

Ph.D.

Includes bibliographical references

by Yuan Chen

doi:10.7282/T3HM5BG8

ETD doctoral

The author owns the copyright to this work.