EPR spectroscopy of paramagnetic centers and adsorption complexes in porous metal organic frameworks: Development and application of dielectric resonators and microresonators for investigations of small single crystals

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Porous metal-organic framework (MOF) compounds are a novel class of materials, which offer a large application potential in various areas such as adsorption, separation, catalysis, and sensing. Many of these materials contain paramagnetic ions as major framework constituents. Therefore, a detailed characterization of paramagnetic ions and their interaction with adsorbates and reactants by EPR spectroscopy is of uttermost importance for the the understanding of the adsorptive and catalytic properties and the envisaged future applications of MOF materials. Unfortunately, EPR investigations of MOF systems are restricted to powder materials so far as single crystals are only available in sub-millimeter sizes. However a proper elucidation of the structure of the formed adsorption complexes requires knowledge about the orientation of the magnetic tensors with respect to the crystallographic axes frame in many cases which can only be deduced from single crystal experiments. Here we will use dielectric resonators with high permittivities (eps = 80) that have been developed in the first funding period to study paramagnetic adsorption complexes in selected small MOF single crystals and to determine the orientation of the magnetic interaction tensors with respect to the crystallographic axes frame. The use of dielectric resonators may improve the EPR sensitivity by one order of magnitude and allows EPR measurements on sub-millimeter MOF single crystals now. In addition we will develop set-ups for pulsed EPR spectroscopy of small MOF single crystals that are based on dielectric resonators and micro resonators. They will allow us to elucidate the orientation of the ligand hyperfine coupling tensors in the adsorption complexes by ESEEM spectroscopy and will provide detailed information about their complex structure.