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Woven Crystalline Covalent Organic Frameworks


Described in this dissertation is the designed synthesis and characterization of a new class of materials – woven crystalline covalent organic frameworks (COFs). Molecular building units are stitched together through strong bonds in a process termed reticular synthesis. In conjunction with metal-templated synthesis woven frameworks are obtained. The products of this reticulation are extended covalently linked one-dimensional (1D) organic molecules that mutually interweave at regular intervals to construct crystalline two- and three-dimensional (2D and 3D) architectures. The mechanical entanglement between the constituents (i.e. 1D chains) allows for large spatial deviations to take place without the breaking of covalent bonds, leading to unusual mechanical properties and ultimately to materials with exceptional resilience and dynamics.

In Chapter I, a general introduction of covalent organic frameworks (COFs) and their designed synthesis based on the fundamental concepts of reticular synthesis is given. Since the discovery of COFs in 2005, a plethora of COF structures with various structure types have been synthesized by reticulating judiciously selected building blocks of desirable connectivity and geometry. The structural tunability and crystallinity of COFs make them the ideal candidates for the synthetic realization of woven extended structures. The metal templation strategy, widely explored in the field of supramolecular chemistry, employs metal ions as templates to pre-organize organic linkers into desirable orientations, which can then be reticulated into woven extended frameworks. These metal ions also serve as the crossing points (points of registry) of the resultant woven threads, and upon demetaltion, the threads have high degrees of freedom to move about these points of registry without undoing the weaving.

Chapter II describes the implementation of the design strategy of COFs toward the successful synthesis of the first woven material, COF-505. A copper(I)-bisphenanthroline core, a well-established template in the field of supramolecular chemistry, was functionalized with aldehyde groups and used as a building block for COF synthesis. Specifically, the tetratopic and tetrahedrally-shaped complex was reacted with a linear ditopic benzidine (BZ) linker through reversible imine condensation to form a 3D crystalline framework, COF-505, of dia topology. The structure was determined with atomic precision, by a combined 3D electron diffraction tomography (3D-EDT) and powder X-ray diffraction (PXRD) approach. On a fundamental level, COF-505 is constructed from organic helices that are covalently linked within the thread, while neighboring helices are held woven with copper(I) ions serving as the points of registry. Upon removal of these copper(I) templates, the threads are endowed with large degrees of freedom to move about the crossing points, which results in a ten-fold increase of elasticity, as observed by nanoindentation using atomic force microscopy (AFM). Owing to the entanglement of the threads, the underlying topology of the structure remains intact upon demetalation. Copper(I) ions can be added back to the demetalated material to recover the structure of the as-synthesized COF; the demelation and remetalation process can be performed in a reversible manner.

In Chapter III, the design principle of woven COFs expanded to structures with large guest accessible internal void space within the structure. It was speculated that this parameter determines the magnitude of the thread movement upon demetalation. The original COF-505 was found to have a two-fold interpenetrated and consequently dense structure. Since it was believed that this might restrict the dynamics of the threads, two strategies were devised to prevent the formation of interpenetration in derivative woven structures: (i) A shorter ditopic linker, p-phenylenediamine (PDA), was employed instead of BZ to yield COF-504. The decreased pore size of the structure has limited internal space thus preventing framework interpenetration. (ii) Alternatively a non-interpenetrated framework can be formed using BZ as the linker when a bulky anion (diphenylphosphinate) was employed during COF synthesis. Post-synthetic anion exchange with smaller BF4- counterions yielded COF-506, a non-interpenetrated analogue of COF-505. The porosity of the two frameworks was investigated by vapor and dye adsorption, which confirmed the guest accessibility of their interior. In COF-506, the member of the series with the largest guest accessible void space, demetalation was found to affect structural dynamics within the structure. Spatial dislocations of the threads within the demetalated COF-506 structure allow for uptake of dye molecules that exceed the pore size of its metalated counterpart.

As the crystallinity of COFs heavily relies on the full reversibility of bond formation during the solvothermal synthesis, COF formation conditions can be highly specific for one type of linkage, or even one particular framework. The development of solid-state anthracene photodimerization as a new linkage for COF synthesis is detailed in Chapter IV. The key feature of this strategy is to pre-organize photoreactive anthracene monomers in a face-to-face manner in the solid state. This arrangement allows for exclusively lateral [4 + 4] cycloaddition reactions in the solid state upon UV exposure to target single-crystal-to-single-crystal transformation through C-C bond formation. To employ this approach to the synthesis of woven COFs the copper(I)-bisphenanthroline core was functionalized with anthracene moieties that serve as points of extension. Various packing modes resulted from different crystallization conditions, however only the undesirable face-to-edge anthracene stacking was observed. Fluorine atoms were subsequently installed on the 1, 2, 3, and 4 positions of the anthracene units which effectively enhanced the cofacial interaction of neighboring anthracenes, but no reactivity towards dimerization was observed upon UV treatment. Further studies of the molecular analogue, 1,2,3,4-tetralfuoro-9-phenylanthracene, revealed that the dimerization was only observed in solution, but not in the solid state, which could be attributed to the large energy cost of phenyl rotation and movement upon formation of the C9-C10 bond. Future work should thus focus on the functionalization of the 2 position of anthracene which is remote from the reaction site for dimerization.

Chapter V focuses on the discovery of higher modes of entanglement beyond woven structures. By reacting the tetrahedral aldehyde-functionalized copper(I) bisphenanthroline complex with square planar 4',4''',4''''',4'''''''-(ethene-1,1,2,2-tetrayl)tetrakis(([1,1'-biphenyl]-4-amine)) linkers (TBPA), the first interlocking COF-500 was synthesized. COF-500 is comprised of entangled 1D corner-sharing ladders and crystallized in a pts topology. Interlocking structures are fundamentally different from woven structures as their entanglement is based on closed rings which ensures that the entities remain entangled. Upon demetalation, the structure became less open, indicated by the decreased nitrogen sorption capacity. Elucidation of structural change accompanying this process is provided by solid-state photoluminescence measurement. Fluorescence of the material was turned on after demelataion and this can be attributed to restricted rotation of the phenyl rings of TBPA when the interlocked ladders move towards each other within the rigid ring systems. THF vapor adsorption studies highlight the dynamic behavior of these interlocked ladders as the demetalated material has a similar THF uptake with COF-500, in a manner that is analogues to swelling of polymers. A significant decrease of fluorescence response further confirms opening-up of demetalated structure in the presence of THF.

Finally, in Chapter VI, a library of woven topologies were identified. By translating the geometries into linear line segments (sticks) joined at corners in their optimal embeddings, 2D and 3D woven and polycatenated structures can be viewed as an extended family of molecular knots and links. Based on mathematical calculations, the most plausible topologies that can be woven were systematically enumerated. Ripe synthetic targets of 2D and 3D weavings and polycatenanes were also provided.

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