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RESEARCH DIRECTIONS
Metal-Organic Frameworks (MOFs)
The NC Lab is developing chemical science and applications using the precise assembly of materials by the molecular building block approach. For this, we use supramolecular chemistry, where molecular building blocks (organic molecules, inorganic clusters, complexes, proteins, peptides, and dendrimers) are linked into extended frameworks with strong bonds. This chemistry allows us to translate the high functionality of molecules into solids without losing the robustness needed for making useful materials and the dynamics and molecular flexibility required for highly functional materials. Thus, in our group, new materials are created by 'stitching' metal ions and metal complexes with organic linkers to make extended porous frameworks called metal-organic frameworks (MOFs). These are all new classes of porous crystals studied for their applications to clean energy storage and generation, clean water generation and delivery, supercapacitors, thermal batteries, ion-conductivity, and electronic conductivity.
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Confined Dynamic Bonding
The NC lab is interested in the dynamic molecule interactions in metal-organic frameworks. In nano-space, the molecular motion is highly restricted, and molecules have specific directionality which facilitates us to (observe/discover/explore) the weak interactions between them. For example, we published a paper about the weak bonding between the metal ion and alkyl halide. In addition, we also discovered the metal-water molecule interactions and secondary water molecules bonding which shows the bonding is consistently interacting with each other. In this dynamic bonding situation, the weak bond can substitute the strong bond, which is an abnormal phenomenon in macroscopic chemistry. We published this amazing discovery and suggested to other researchers that the solvent molecule bonding characters must be considered seriously during the catalytic reaction in MOF.
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Conductivity Through nanochannels
An ultimate goal in proton-exchange membrane fuel cells, lithium-ion batteries, and supercapacitors is to find better ionic conducting systems. MOFs can be considered ideal systems for these applications because of their potentially high ion transport abilities. Meanwhile, porous carbon materials have been used for commercial lithium-ion batteries and supercapacitors that operate by storing charge on electrochemical double layers. By contrast, pseudo capacitors typically made from metal oxides can store charge by redox reactions. These classes of supercapacitors have both merits and demerits: carbon-based materials operate at very high charge/discharge rates with a long lifecycle but have low capacitance, while metal oxide materials have high capacitance but their redox reactions lead to low lifecycles. We are working on ways of bridging the gap by separating these two classes using MOFs.
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Catalytic Chemistry in MOF
The bonding between the solvent molecules and the metal nodes can be the reaction site of a catalytic reaction. This interaction intensity is different when the different solvent molecules are coordinated. Furthermore, this bond strength can be measured by the Raman spectroscopy. We published that different bond strength is shown from different solvent molecules in HKUST-1. This bond strength is important because it affects the catalytic reactivity. When the catalytic reactions are conducted, the solvent molecules must be detached so that the reactants can approach open metal sites for reaction. Therefore, the strong bond strength from solvent molecules can distract the chemical reaction in MOF. We published a paper about the relation between the catalytic reaction and bond strength and gave a message to other researchers that they must be careful to select the solvent for chemical reactions. In addition, we discovered that ligand-to-metal transfer can be conducted in MOF when we give the thermal treatment to MOF. By using this, we can control the oxidation state of metal in MOF, which gives us enhanced hydrolytic stability. Our lab is still interested in the coordinative reduction reactions in MOF.
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