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Strategies Toward the Homogeneous Synthesis and Functionalization of Covalent Organic Frameworks

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Abstract

This dissertation focuses on a range of methods for expanding the complexity of covalent organic frameworks (COFs). COFs are a class of highly porous crystalline materials constructed by stitching organic building blocks together into extended frameworks. Since the first COF was reported in 2005, extensive efforts have been devoted to expanding the scope of this field. In this dissertation, the development of a novel synthetic route and three post-synthetic modifications are covered. The applicability of these strategies is demonstrated by the successful synthesis of monodisperse COF nanoparticles, highly oriented COF thin films, woven COF crystals, and a functionalizable COF platform.

Chapter I encompasses a general introduction of COFs. Unlike amorphous polymers, which are typically formed under kinetic control, COFs represent a thermodynamic product. As such, they possess high structural periodicity that results from reversible bond formation and cleavage, eventually yielding the linkage between the organic building units as the result of dynamic error correction. With directional linkages and rigid molecular building units, COFs can be precisely designed on the atomic level. Efforts have been devoted to developing new synthetic strategies and accessing functionalized imine-linked COFs. The resultant tunability and well-defined morphology of COFs have allowed for application of the materials to numerous fields.

In Chapter II, a newly developed homogeneous synthetic route for imine COFs is presented. Conventional imine-linked COF synthesis is carried out in heterogeneous suspensions, where polyimine formation occurs too rapidly to effectively control crystal nucleation. In order to address this shortcoming, Boc-protected (Boc = tert-butyloxycarbonyl) amine linkers are used as starting materials, which are slowly deprotected in situ and grow into extended structures when reacted with aldehyde counterparts. Under the synthetic conditions reported here, the initial nucleation rate is slowed down such that it is comparable to that of the dynamic error correction for the structure. Additionally, the reaction intermediates are readily soluble in the solution. These phenomena allow for the synthesis of imine COFs to be carried out under homogeneous conditions and enable the growth of imine COFs to be tuned using modulators. The work herein brings COFs into nanometer size regime and paves the way for the integration of COFs into mesoscopic constructs.

Chapter III further discusses how the newly developed homogeneous synthetic route leads to the successful synthesis of high quality COF thin films. By employing the Boc-protected amine building blocks and initiating the reaction under highly acidic conditions, selective secondary nucleation of COF nanoparticles will occur on the polar oxides in the reaction system (e.g., a silicon wafer substrate). The nucleation barrier in the homogeneous COF synthesis enables film growth without precipitation from the solution and eventually yields highly oriented thin films.

Reticular chemistry, the chemistry of linking rigid molecular building units together by strong chemical bonds into extended structures with long-range periodicity, has led to a plethora of COF structures. Chapter IV presents the design and synthesis of a new class of materials, woven COFs with one-dimensional covalently linked molecular chains designed to intersect at regular intervals by means of metal templates. COF-112 is constructed from cobalt bis(diiminopyridine) complexes with a near-perfect tetrahedral geometry and pyridinedicarboxaldehyde, and is an ideal example of molecular weaving. It represents one of the simplest three-dimensional woven structures, as it involves only two sets of threads that are straight and parallel with only one point of registry.

Chapter V continues to focus on expanding the scope of COF chemistry by developing strategies to design and synthesize functionalized COFs in a rapid and efficient manner. In this chapter, three new post-synthetic modification reactions, namely amidation, esterification, and thioesterification, are demonstrated on a novel, highly crystalline, two-dimensional COF, COF-616, bearing pre-installed carboxylic acid groups. The strategy can be used to introduce a large variety of functional groups into COFs and the modifications can be carried out under mild reaction conditions with high yields and an easy work-up protocol. Therefore, COF-616 can serve as a functionalizable platform that can be readily tailored for various applications. As a proof of concept, various chelating functional groups were successfully incorporated into COF-616 to yield a family of adsorbents for efficient removal of heavy metal ions from water.

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This item is under embargo until August 5, 2023.