Supramolecular Design, Synthesis, and Characterization of 2D Covalent Organic Frameworks

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2022-05-01T05:00:00.000Z

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Abstract

Covalent Organic Frameworks (COFs) are a class of crystalline, porous, polymers that grow in either 2D- or 3D- dimensionally which became an emerging class of material from its first finding. These polymers consist of lightweight elements (C, H, N, O, B etc.) and are formed through different dynamic covalent bonds. The most common types of covalent linkages that can be found in 2D-COFs are boronate esters, imines, hydrazones, and azines. The promising properties of COFs such as permanent porosity, crystallinity, and low density make them a good candidate for different applications including gas storage and separation, sensing, electronic and conductive materials, and catalysis. Moreover, the 2D sheets of these materials are held together by different non-covalent interlayer interactions including dipole-dipole, aromatic stacking, and hydrogen bonding interactions. The structure of 2D-COFs is controlled by the interplay between noncovalent interlayer interactions and dynamic covalent bond formation. The overall goal of this research effort is to design and synthesize 2D-COFs containing interlayer hydrogen bonding through supramolecular design to expand the scope of the materials for different applications and processability. The first chapter provides a literature review of recent efforts to use supramolecular design to leverage the non-covalent interactions between COF monomers and sheets to improve their properties and function. The importance of supramolecular interactions in 2D-COFs to their mechanisms of formation and overall structure is discussed. In the second chapter, the synthesis and characterization of a new class of 2D-covalent organic frameworks, called COFamides, whose layers are held together by amide hydrogen bonds is discussed. The designed monomer with a nonplanar structure that arises from steric crowding, forcing the amide side groups out of the plane with the COF sheets orienting the hydrogen bonds between the layers was used in this regard. This study was extended to the synthesis of a novel COF, called PyCOFamide, which has an experimentally observed pore size that is greater than 6 nm in diameter. This is among the largest pore size reported to date for a 2D-COF. PyCOFamide exhibits permanent porosity and high crystallinity as evidenced by nitrogen adsorption, powder Xray diffraction, and high-resolution transmission electron microscopy. We show that the pore size of PyCOFamide is large enough to accommodate fluorescent proteins such as Superfolder green fluorescent protein and mNeonGreen. This work demonstrates the utility of non-covalent structural reinforcement in 2D-COFs to produce larger, persistent pore sizes than previously possible. The third chapter describes the synthesis of 2D hydrazone-linked COFs by incorporating a new side chain free hydrazide monomer (terephthalohydrazide). The crystallinity and porosity of synthesized COFs were lost under conventional solvent activation. Mechanistically, the loss of long-range order of the framework during COF formation and isolation can be associated with two possible hypotheses, random displacement of the COF layers or pore collapse during the activation process. In this finding, the problem was mitigated by applying mild activation conditions especially using supercritical carbon dioxide to regain the crystallinity and porosity of the synthesized 2D hydrazone-linked COFs. This is the first report on the synthesis of 2D hydrazonelinked COFs using side chain free hydrazide monomers under mild activation conditions. The fourth chapter discusses the preparation of thin films from COF powders using solution processing methods. The mechanical properties of the synthesized films were studied by nanoindentation aiming to develop high mechanical strength materials like Kevlar. In the fifth chapter, the synthesis and characterization of 2D imine-linked COFs to study their fluorescence will be discussed.

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Chemistry, Organic

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