Per- and polyfluorinated alkyl substances (PFAS), such as perfluorooctanoic acid (PFOA), perfluorooctanesulfonate (PFOS), and ammonium perfluoro-2-propoxypropionate (GenX), contaminate ground and ...surface waters throughout the world. The cost and performance limitations of current PFAS removal technologies motivate efforts to develop selective and high-affinity adsorbents. Covalent organic frameworks (COFs) are unexplored yet promising adsorbents because of their high surface area and tunable pore sizes. Here we show that imine-linked two-dimensional (2D) COFs bearing primary amines adsorb GenX rapidly at environmentally relevant concentrations. COFs with partial amine incorporation showed the highest capacity and fastest removal, suggesting that the synergistic combination of the polar group and hydrophobic surfaces are responsible for GenX binding. A COF with 28% amine loading also removed more than 90% of 12 out of 13 PFAS. These results demonstrate the promise of COFs for PFAS removal and suggest design criteria for maximizing adsorbent performance.
Synthetic chemists have developed robust methods to synthesize discrete molecules, linear and branched polymers, and disordered cross-linked networks. However, two-dimensional polymers (2DPs) ...prepared from designed monomers have been long missing from these capabilities, both as objects of chemical synthesis and in nature. Recently, new polymerization strategies and characterization methods have enabled the unambiguous realization of covalently linked macromolecular sheets. Here we review 2DPs and 2D polymerization methods. Three predominant 2D polymerization strategies have emerged to date, which produce 2DPs either as monolayers or multilayer assemblies. We discuss the fundamental understanding and scope of each of these approaches, including: the bond-forming reactions used, the synthetic diversity of 2DPs prepared, their multilayer stacking behaviors, nanoscale and mesoscale structures, and macroscale morphologies. Additionally, we describe the analytical tools currently available to characterize 2DPs in their various isolated forms. Finally, we review emergent 2DP properties and the potential applications of planar macromolecules. Throughout, we highlight achievements in 2D polymerization and identify opportunities for continued study.
Imine-linked two-dimensional covalent organic frameworks (2D COFs) are crystalline polymer networks with enhanced stability compared to boronate ester-linked systems and with broad monomer scope. ...They are traditionally prepared by condensing polyfunctional aldehydes and amines at elevated temperature in a mixture of organic solvents and aqueous CH3CO2H, which catalyzes imine formation and exchange. Here we employ metal triflates, which are water-tolerant Lewis acids, to accelerate 2D imine-linked COF synthesis and improve their materials quality. Low catalyst loadings provide crystalline polymer networks in nearly quantitative yields. These conditions are demonstrated for several COFs, including heteroatom-containing systems of interest for optoelectronic applications.
Covalent organic frameworks (COFs) consist of monomers arranged in predictable structures with emergent properties. However, improved crystallinity, porosity, and solution processability remain major ...challenges. To this end, colloidal COF nanoparticles are useful for mechanistic studies of nucleation and growth and enable advanced spectroscopy and solution processing of thin films. Here we present a general approach to synthesize imine-linked 2D COF nanoparticles and control their size by favoring imine polymerization while preventing the nucleation of new particles. The method yields uniform, crystalline, and high-surface-area particles and is applicable to several imine-linked COFs.
X-ray scattering experiments reveal the nucleation of amorphous polymers, which crystallize
imine exchange processes during and after particle growth, consistent with previous mechanistic studies of imine-linked COF powders. The separation of particle formation and growth processes offers control of particle size and may enable further improvements in crystallinity in the future.
Two-dimensional (2D) covalent organic frameworks (COFs) are composed of structurally precise, permanently porous, layered macromolecular sheets, which are traditionally synthesized as polycrystalline ...solids with crystalline domain lengths smaller than 100 nm. Here, we polymerize imine-linked 2D COFs as suspensions of faceted single crystals in as little as 5 min at moderate temperature and ambient pressure. Single crystals of two imine-linked 2D COFs were prepared, consisting of a rhombic 2D COF (TAPPy-PDA) and a hexagonal 2D COF (TAPB-DMPDA). The sizes of TAPPy-PDA and TAPB-DMPDA crystals were tuned from 720 nm to 4 μm and 450 nm to 20 μm in width, respectively. High-resolution transmission electron microscopy revealed that the COF crystals consist of layered, 2D polymers comprising single-crystalline domains. Continuous rotation electron diffraction resolved the unit cell and crystal structure of both COFs, which are single-crystalline in the a–b plane but disordered in the stacking c dimension. Single crystals of both COFs were incorporated into gas chromatography separation columns and exhibited unusual selective retention of cyclohexane over benzene, with single-crystalline TAPPy-PDA significantly outperforming single-crystalline TAPB-DMPDA. Polycrystalline TAPPy-PDA exhibited no separation, while polycrystalline TAPB-DMPDA exhibited poor separation and the opposite order of elution, retaining benzene more than cyclohexane, indicating the importance of improved material quality for COFs to exhibit properties that derive from their precise, crystalline structures. This work represents the first example of synthesizing imine-linked 2D COF single crystals at ambient pressure and short reaction times and demonstrates the promise of high-quality COFs for molecular separations.
The transformative applications of quantum information science (QIS) require precise design and integration of networks of qubits, the fundamental units of QIS systems. Chemical synthesis is a ...powerful approach, offering routes to modular, atomically precise arrangements of identical qubits. Herein, we employed the versatility of framework chemistry to investigate spin and lattice dynamics of the expanded copper(II) porphyrinic framework Zr–Cu–NU-1102 (2) possessing Cu–Cu distances of 18.0 Å. Pulse electron paramagnetic resonance spectroscopy revealed a significant reduction in relaxation processes mediated by qubit–qubit interactions compared with the more spin-dense Cu–PCN-224 (1) framework. With the reduction in the spin–spin relaxation process, phonon-mediated processes emerged as the primary driver of spin–lattice relaxation. We synthesized the isoreticular Hf–Cu–NU-1102 (3) to elucidate the impact of the nodes versus the ligands on the phonon-mediated relaxation process. Measurement of 3 revealed identical spin–lattice relaxation dynamics to 2, thereby excluding involvement of node-centered or bulk framework acoustic modes. Supported by theoretical calculations of the ligand vibrational modes, these results implicated linker-based motions as dominant contributors to phonon-mediated spin–lattice relaxation. These findings provide clear guidelines for synthetic design to control spin and phonon interactions in modular arrays of molecular qubits.
Two-dimensional (2D) covalent organic frameworks (COFs) are a class of crystalline polymer networks that polymerize and crystallize into layered structures, characterized by their atomically precise ...structure, permanent porosity and high modularity. Imine-linked COFs are one of the most important and promising classes of macromolecular sheets, which form by the condensation of polyfunctional amines with aldehydes. These materials exhibit one of the broadest structural diversity of 2D networks and are attractive partly because of their enhanced hydrolytic and oxidative stability compared to their boron-linked counterparts. This stability coupled with their immense structural versatility has led to their development with desirable properties for various applications. As such, the field has currently explored these stable frameworks by probing many testable properties of the polycrystalline COF powders. Major developments, however, are still needed. A key, currently unmet need that will provide morphological control as well as improved materials quality is better mechanistic understanding of the COF formation and growth. In this dissertation, I will discuss the design and synthesis, formation mechanisms, different morphologies, as well as molecular adsorption and separation applications of 2D imine-linked COFs. In chapter 1 I will discuss a broad overview of imine linkage chemistries and the 2D COFs generated by these strategies. This chapter will start with the outline of contemporary strategies used to successfully polymerize imine-linked macromolecular sheets and the exploration of the chemical and structural COF variants produced through each of these methods. Next, the chapter discusses the nano- and macroscale morphologies of COFs and molecular adsorption and separation applications using imine COFs. Finally, Chapter 1 will end by highlighting additional insight needed to address some of the major challenges in the field and identify opportunities for continued study. Chapter 2 focuses on the design and synthesis of the amine-functionalized, 2D imine-linked COFs for perfluorinated alkyl substances (PFAS) removal from water. Chapter 3 is about thermal depolymerization and recrystallization of 2D imine-linked COFs, a novel phenomenon which can be used to improve and assess COF materials quality. Finally, Chapter 4 discusses a novel synthetic strategy for preparing imine-linked 2D COF single crystals of four different architectures.
Mechanistic understanding into the formation and growth of imine-linked two-dimensional (2D) covalent organic frameworks (COFs) is needed to improve their materials quality and access larger ...crystallite sizes, both of which limit the promise of 2D COFs and 2D polymerization techniques. Here we report a previously unknown temperature-dependent depolymerization of colloidal 2D imine-linked COFs, which offers a new means to improve their crystallinity. 2D COF colloids form at room temperature but then depolymerize when their reaction mixtures are heated to 90 °C. As the solutions are cooled back to room temperature, the 2D COFs repolymerize and crystallize with improved crystallinity and porosity, as characterized by X-ray diffraction, infrared spectroscopy and N
2
porosimetry. The evolution of COF crystallinity during the solvothermal depolymerization and repolymerization processes was characterized by
in situ
wide angle X-ray scattering, and the concentrations of free COF monomers as a function of temperature were quantified by variable temperature
1
H NMR spectroscopy. The ability of a 2D COF to depolymerize under these conditions depends on both the identity of the COF and its initial materials quality. For one network formed at room temperature (
TAPB-PDA COF
), a first depolymerization process is nearly complete, and the repolymerization yields materials with dramatically enhanced crystallinity and surface area. Already recrystallized materials partially depolymerize upon heating their reaction mixtures a second time. A related 2D COF (
TAPB-DMTA COF
) forms initially with improved crystallinity compared to
TAPB-PDA COF
and then partially depolymerizes upon heating. These results suggest that both high materials quality and network-dependent properties, such as interlayer interaction strength, influence the extent to which 2D COFs resist depolymerization. These findings offer a new means to recrystallize or solvent anneal 2D COFs and may ultimately inform crystallization conditions for obtaining large-area imine-linked two-dimensional polymers from solution.
Conditions for which imine-linked 2D COF polymerizations are temperature-sensitive are identified that enable a dissolution/repolymerization process akin to molecular recrystallization.
Colloidal COF-300 synthesis with substituted benzoic acid catalysts demonstrates that acidity plays a role in reaction kinetics and the resulting particle's colloidal stability.
Colloidal covalent organic framework (COF) synthesis enables morphological control of crystallite size and shape. Despite numerous examples of 2D COF colloids with various linkage chemistries, 3D ...imine-linked COF colloids are more challenging synthetic targets. Here we report a rapid (15 min-5 day) synthesis of hydrated COF-300 colloids ranging in length (251 nm-4.6 μm) with high crystallinity and moderate surface areas (150 m
2
g
−1
). These materials are characterized by pair distribution function analysis, which is consistent with the known average structure for this material alongside different degrees of atomic disorder at different length scales. Additionally, we investigate a series of
para
-substituted benzoic acid catalysts, finding that 4-cyano and 4-fluoro substituted benzoic acids produce the largest COF-300 crystallites with lengths of 1-2 μm.
In situ
dynamic light scattering experiments are used to assess time to nucleation in conjunction with
1
H NMR model compound studies to probe the impact of catalyst acidity on the imine condensation equilibrium. We observe cationically stabilized colloids with a zeta potential of up to +14.35 mV in benzonitrile as a result of the carboxylic acid catalyst protonating surface amine groups. We leverage these surface chemistry insights to synthesize small COF-300 colloids using sterically hindered diortho-substituted carboxylic acid catalysts. This fundamental study of COF-300 colloid synthesis and surface chemistry will provide new insights into the role of acid catalysts both as imine condensation catalysts and as colloid stabilizing agents.
Colloidal COF-300 synthesis with substituted benzoic acid catalysts demonstrates that acidity plays a role in reaction kinetics and the resulting particle's colloidal stability.
PDF
