Highly Flexible Dielectric Films from Solution Processable Covalent Organic Frameworks.

Covalent organic frameworks (COFs) are known to be a promising class of materials for a wide range of applications, yet their poor solution processability limits their utility in many areas. Here we report a pore engineering method using hydrophilic side chains to improve the processability of hydrazone and ß-ketoenamine-linked COFs and the production of flexible, crystalline films. Mechanical measurements of the free-standing COF films of COF-PEO-3 (hydrazone-linked) and TFP-PEO-3 (ß-ketoenamine-linked), revealed a Young's modulus of 391.7 MPa and 1034.7 MPa, respectively. The solubility and excellent mechanical properties enabled the use of these COFs in dielectric devices. Specifically, the TFP-PEO-3 film-based dielectric capacitors display simultaneously high dielectric constant and breakdown strength, resulting in a discharged energy density of 11.22 J cm-3 . This work offers a general approach for producing solution processable COFs and mechanically flexible COF-based films, which hold great potential for use in energy storage and flexible electronics applications.

Supramolecular Reinforcement of a Large-Pore 2D Covalent Organic Framework.

Two-dimensional covalent organic frameworks (2D-COFs) are a class of crystalline porous organic polymers that consist of covalently linked, two-dimensional sheets that can stack together through noncovalent interactions. Here we report 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 the nitrogen adsorption, powder X-ray 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 noncovalent structural reinforcement in 2D-COFs to produce larger and persistent pore sizes than previously possible.

Supramolecular design in 2D covalent organic frameworks.

2D covalent organic frameworks (COFs) are a class of porous polymers with highly crystalline structures and tunable function. The structure of a 2D-COF consists of two dimensional sheets held together through covalent bonds which are then stacked together through non-covalent forces. Since their first report, the synthesis of new COFs has relied mostly on imparting functionality to the monomer structures through covalent modification, or through the use of new thermodynamically controlled covalent bond forming methods. This tutorial review will discuss recent efforts to use supramolecular design to leverage the non-covalent forces between COF monomers and sheets to improve their properties and function. The importance of supramolecular interactions in COFs to their mechanisms of formation and overall structure will also be covered.

2D-Covalent Organic Frameworks with Interlayer Hydrogen Bonding Oriented through Designed Nonplanarity.

We report the synthesis and characterization of a new class of 2D-covalent organic frameworks, called COFamides, whose layers are held together by amide hydrogen bonds. To accomplish this, we have designed monomers with a nonplanar structure that arises from steric crowding, forcing the amide side groups out of plane with the COF sheets orienting the hydrogen bonds between the layers. The presence of these hydrogen bonds provides significant structural stabilization as demonstrated by comparison to control structures that lack hydrogen bonding capability, resulting in lower surface area and crystallinity. We have characterized both azine and imine-linked versions of these COFs, named COFamide-1 and -2, respectively, for their surface areas, pore sizes, and crystallinity. In addition to these more conventional characterization methods, we also used variable temperature infrared spectroscopy methods and van der Waals density functional calculations to directly observe the presence of hydrogen bonding.

Hierarchical Co-Assembly Enhanced Direct Ink Writing.

Integrating intelligent molecular systems into 3D printing materials and transforming their molecular functions to the macroscale with controlled superstructures will unleash great potential for the development of smart materials. Compared to macromolecular 3D printing materials, self-assembled small-molecule-based 3D printing materials are very rare owing to the difficulties of facilitating 3D printability as well as preserving their molecular functions macroscopically. Herein, we report a general approach for the integration of functional small molecules into 3D printing materials for direct ink writing through the introduction of a supramolecular template. A variety of inorganic and organic small-molecule-based inks were 3D-printed, and their superstructures were refined by post-printing hierarchical co-assembly. Through spatial and temporal control of individual molecular events from the nano- to the macroscale, fine-tuned macroscale features were successfully installed in the monoliths.

Computational and Experimental Studies on the Effects of Monomer Planarity on Covalent Organic Framework Formation.

We report the synthesis of one new boronate ester-based covalent organic framework (COF) and two new covalent organic polymers (COPs) made with fluoranthene-containing monomers and hexahydroxytriphenylene. The structure of the monomer heavily influences whether this material forms a highly ordered mesoporous material (COF) or an amorphous, microporous material (COP). The synthesis of the fluoranthene monomers was carried out using a divergent strategy that allows for systematic structural variation and the ability to conduct a careful structure-function study. We found that small structural variations in the monomers dramatically affected the crystallinity, surface area, pore structure, and luminescence properties of the polymers. While each of the monomers contains the same fluoranthene core, the resultant pore sizes range from microporous (10 Å) to mesoporous (37 Å), with surface areas ranging from ∼500 to 1200 m2/g. To help explain how these small structural differences can have such a large effect, we carried out a series of molecular dynamics simulations on the polymers to obtain information with atomic-scale resolution on how the monomer structure affects non-covalent COF layer stacking.

An Elastic Hydrogen-Bonded Cross-Linked Organic Framework for Effective Iodine Capture in Water.

A crystalline microporous hydrogen-bonded cross-linked organic framework has been developed through covalent photo-cross-linking of molecular monomers that are assembled in a crystalline state. The elastic framework expands its void space to adsorb iodine rapidly with a high uptake capacity in an aqueous environment as well as recovering its crystalline form after the release of iodine.

Enhanced Structural Organization in Covalent Organic Frameworks Through Fluorination.

Here, we report a structure-function study of imine covalent organic frameworks (COFs) comparing a series of novel fluorine-containing monomers to their non-fluorinated analogues. We found that the fluorine-containing monomers produced 2D-COFs with not only greatly improved surface areas (over 2000 m2 g-1 compared to 760 m2 g-1 for the non-fluorinated analogue), but also with improved crystallinity and larger, more defined pore diameters. We then studied the formation of these COFs under varying reaction times and temperatures to obtain a greater insight into their mechanism of formation.

Template-Directed Synthesis of Porous and Protective Core-Shell Bionanoparticles.

Metal-organic frameworks (MOFs) are promising high surface area coordination polymers with tunable pore structures and functionality; however, a lack of good size and morphological control over the as-prepared MOFs has persisted as an issue in their application. Herein, we show how a robust protein template, tobacco mosaic virus (TMV), can be used to regulate the size and shape of as-fabricated MOF materials. We were able to obtain discrete rod-shaped TMV@MOF core-shell hybrids with good uniformity, and their diameters could be tuned by adjusting the synthetic conditions, which can also significantly impact the stability of the core-shell composite. More interestingly, the virus particle underneath the MOF shell can be chemically modified using a standard bioconjugation reaction, showing mass transportation within the MOF shell.

Rapid, high-capacity adsorption of iodine from aqueous environments with amide functionalized covalent organic frameworks.

The uses and production of radionuclides in nuclear energy production and medical therapy are becoming more significant in today's world. While these applications have many benefits, they can produce harmful pollutants, such as radioactive iodine, that need to be sequestered. Effective capture and storage of radioactive iodine waste remains a major challenge for nuclear energy generation and nuclear medicine. Here we report the highly efficient capture of iodine in a series of mesoporous, two-dimensional (2D) covalent organic frameworks, called COFamides, which contain amide sidechains in their pores. COFamides are capable of rapidly removing iodine from aqueous solution at concentrations as low as 50 ppm, with total capacities greater than 650 wt%. In order to explain the high affinity of the COFamide series for iodine and iodide species in water, we performed a computational analysis of the interactions between the COFamide framework and iodine guests. These studies suggest that the origin of the large iodine capacity in these materials can be explained by the presence of multiple, cooperative, non-covalent interactions between the framework and both iodine, and iodide species.

Hierarchically Structured Metal-Organic Framework Polymer Composites for Chemical Warfare Agent Degradation.

Metal-organic frameworks (MOFs) have captured the imagination of researchers for their highly tunable properties and many potential applications, including as catalysts for a variety of transformations. Even though MOFs possess significant potential, the challenges associated with processing of these crystalline powders into usable form factors while retaining their functional properties limit their end use applications. Herein, we introduce a new approach to construct MOF-polymer composites via 3D photoprinting to overcome these limitations. We designed photoresin composite formulations that use polymerization-induced phase separation to cause the MOF catalysts to migrate to the surface of the printed material, where they are accessible to substrates such as chemical warfare agents. Using our approach, MOF-polymer composites can be fabricated into nearly any shape or architecture while retaining both the excellent catalytic activity at 10 wt % loading of the MOF components and the flexible, elastomeric mechanical properties of a polymer.

A scalable synthesis of adjuvanting antigen depots based on metal-organic frameworks.

Vaccines have saved countless lives by preventing and even irradicating infectious diseases. Commonly used subunit vaccines comprising one or multiple recombinant proteins isolated from a pathogen demonstrate a better safety profile than live or attenuated vaccines. However, the immunogenicity of these vaccines is weak, and therefore, subunit vaccines require a series of doses to achieve sufficient immunity against the pathogen. Here, we show that the biomimetic mineralization of the inert model antigen, ovalbumin (OVA), in zeolitic imidazolate framework-8 (ZIF-8) significantly improves the humoral immune response over three bolus doses of OVA (OVA 3×). Encapsulation of OVA in ZIF-8 (OVA@ZIF) demonstrated higher serum antibody titers against OVA than OVA 3×. OVA@ZIF vaccinated mice displayed higher populations of germinal center (GC) B cells and IgG1+ GC B cells as opposed to OVA 3×, indicative of class-switching recombination. We show that the mechanism of this phenomenon is at least partly owed to the metalloimmunological effects of the zinc metal as well as the sustained release of OVA from the ZIF-8 composite. The system acts as an antigen reservoir for antigen-presenting cells to traffic into the draining lymph node, enhancing the humoral response. Lastly, our model system OVA@ZIF is produced quickly at the gram scale in a laboratory setting, sufficient for up to 20 000 vaccine doses.

Quantitative emergence of hetero[4]rotaxanes by template-directed click chemistry.

In one fell swoop, polyrotaxanes comprising up to 64 rings can be synthesized as a result of cucurbit[6]uril-templated 1,3-dipolar azide-alkyne cycloadditions accelerated in the presence of cyclodextrins as a consequence of self-sorting and positive cooperativity, brought about by hydrogen bonding. Mixing six components in one pot affords a hetero[4]rotaxane in one minute in quantitative yield.

Positive cooperativity in the template-directed synthesis of monodisperse macromolecules.

Two series of oligorotaxanes R and R' that contain -CH(2)NH(2)(+)CH(2)- recognition sites in their dumbbell components have been synthesized employing template-directed protocols. [24]Crown-8 rings self-assemble by a clipping strategy around each and every recognition site using equimolar amounts of 2,6-pyridinedicarboxaldehyde and tetraethyleneglycol bis(2-aminophenyl) ether to efficiently provide up to a [20]rotaxane. In the R series, the -NH(2)(+)- recognition sites are separated by trismethylene bridges, whereas in the R' series the spacers are p-phenylene linkers. The underpinning idea here is that in the former series, the recognition sites are strategically positioned 3.5 Å apart from one another so as to facilitate efficient [π···π] stacking between the aromatic residues in contiguous rings in the rotaxanes and consequently, a discrete rigid and rod-like conformation is realized; these noncovalent interactions are absent in the latter series rendering them conformationally flexible/nondiscrete. Although in the R' series, the [3]-, [4]-, [8]-, and [12]rotaxanes were isolated after reaction times of <5-30 min in yields of 72-85%, in the R series, the [3]-, [4]-, [5]-, [8]-, [12]-, [16]-, and [20]rotaxanes were isolated in <5 min to 14 h in 88-98% yields. It follows that while in the R' series the higher order oligorotaxanes are formed in lower yields more rapidly, in the R series, the higher order oligorotaxanes are formed in higher yields more slowly. In the R series, the high percentage yields are sustained throughout, despite the fact that up to 39 components are participating in the template-directed self-assembly process. Simple arithmetic reveals that the conversion efficiency for each imine bond formation peaks at 99.9% in the R series and 99.3% in the R' series. This maintenance of reaction efficiency in the R series can be ascribed to positive cooperativity, that is, when one ring is formed it aids and abets the formation of subsequent rings presumably because of stabilizing extended [π···π] stacking interactions between the arene units. Experiments have been performed wherein the dumbbell is starved of the macrocyclic components, and up to five times more of the fully saturated rotaxane is formed than is predicted based on a purely statistical outcome, providing a clear indication that positive cooperativity is operative. Moreover, it would appear that as the R series is traversed from the [3]- to the [4]- to the [5]rotaxane, the cooperativity becomes increasingly positive. This kind of cooperative behavior is not observed for the analogous oligorotaxanes in the R' series. The conventional bevy of analytical techniques (e.g., HR-MS (ESI) and both (1)H and (13)C NMR spectroscopy) help establish the fact that all the oligorotaxanes are pure and monodisperse. Evidence of efficient [π···π] stacking between contiguous arene units in the rings in the R series is revealed by (1)H NMR spectroscopy. Ion-mobility mass spectrometry performed on the R and R' series yielded the collisional cross sections (CCSs), confirming the rigidity of the R oligorotaxanes and the flexibility of the R' ones. The extended [π···π] stacking interactions are found to be present in the solid-state structures of the [3]- and [4]rotaxanes in the R series and also on the basis of molecular mechanics calculations performed on the entire series of oligomers. The collective data presented herein supports our original design in that the extended [π···π] stacking between contiguous arene units in the rings of the R series of oligorotaxanes facilitate an essentially rigid rod-like conformation with evidence that positive cooperativity improves the efficiency of their formation. This situation stands in sharp contrast to the conformationally flexible R' series where the oligorotaxanes form with no cooperativity.

Nanoporous carbohydrate metal-organic frameworks.

The binding of alkali and alkaline earth metal cations by macrocyclic and diazamacrobicyclic polyethers, composed of ordered arrays of hard oxygen (and nitrogen) donor atoms, underpinned the development of host-guest supramolecular chemistry in the 1970s and 1980s. The arrangement of -OCCO- and -OCCN- chelating units in these preorganized receptors, including, but not limited to, crown ethers and cryptands, is responsible for the very high binding constants observed for their complexes with Group IA and IIA cations. The cyclodextrins (CDs), cyclic oligosaccharides derived microbiologically from starch, also display this -OCCO- bidentate motif on both their primary and secondary faces. The self-assembly, in aqueous alcohol, of infinite networks of extended structures, which have been termed CD-MOFs, wherein γ-cyclodextrin (γ-CD) is linked by coordination to Group IA and IIA metal cations to form metal-organic frameworks (MOFs), is reported. CD-MOF-1 and CD-MOF-2, prepared on the gram-scale from KOH and RbOH, respectively, form body-centered cubic arrangements of (γ-CD)(6) cubes linked by eight-coordinate alkali metal cations. These cubic CD-MOFs are (i) stable to the removal of solvents, (ii) permanently porous, with surface areas of ~1200 m(2) g(-1), and (iii) capable of storing gases and small molecules within their pores. The fact that the -OCCO- moieties of γ-CD are not prearranged in a manner conducive to encapsulating single metal cations has led to our isolating other infinite frameworks, with different topologies, from salts of Na(+), Cs(+), and Sr(2+). This lack of preorganization is expressed emphatically in the case of Cs(+), where two polymorphs assemble under identical conditions. CD-MOF-3 has the cubic topology observed for CD-MOFs 1 and 2, while CD-MOF-4 displays a channel structure wherein γ-CD tori are perfectly stacked in one dimension in a manner reminiscent of the structures of some γ-CD solvates, but with added crystal stability imparted by metal-ion coordination. These new MOFs demonstrate that the CDs can indeed function as ligands for alkali and alkaline earth metal cations in a manner similar to that found with crown ethers. These inexpensive, green, nanoporous materials exhibit absorption properties which make them realistic candidates for commercial development, not least of all because edible derivatives, fit for human consumption, can be prepared entirely from food-grade ingredients.

Stereochemistry of molecular figures-of-eight.

A trans isomer of a figure-of-eight (Fo8) compound was prepared from an electron-withdrawing cyclobis(paraquat-p-phenylene) derivative carrying trans-disposed azide functions between its two phenylene rings. Copper(I)-catalyzed azide-alkyne cycloadditions with a bispropargyl derivative of a polyether chain, interrupted in its midriff by an electron-donating 1,5-dioxynaphthalene unit acting as the template to organize the reactants prior to the onset of two click reactions, afforded the Fo8 compound with C(i) symmetry. Exactly the same chemistry is performed on the cis-bisazide of the tetracationic cyclophane to give a Fo8 compound with C(2) symmetry. Both of these Fo8 compounds exist as major and very minor conformational isomers in solution. The major conformation in the trans series, which has been characterized by X-ray crystallography, adopts a geometry which maximizes its C-H···O interactions, while maintaining its π···π stacking and C-H···π interactions. Ab initio calculations at the M06L level support the conformational assignments to the major and minor isomers in the trans series. Dynamic (1)H NMR spectroscopy, supported by 2D (1)H NMR experiments, indicates that the major and minor isomers in both the cis and trans series equilibrate in solution on the (1)H NMR timescale rapidly above and slowly below room temperature.

Carrier gas triggered controlled biolistic delivery of DNA and protein therapeutics from metal-organic frameworks.

The efficacy and specificity of protein, DNA, and RNA-based drugs make them popular in the clinic; however, these drugs are often delivered via injection, requiring skilled medical personnel, and producing biohazardous waste. Here, we report an approach that allows for their controlled delivery, affording either a burst or slow release without altering the formulation. We show that when encapsulated within zeolitic-imidazolate framework eight (ZIF-8), the biomolecules are stable in powder formulations and can be inoculated with a low-cost, gas-powered "MOF-Jet" into living animal and plant tissues. Additionally, their release profiles can be modulated through judicious selection of the carrier gas used in the MOF-Jet. Our in vitro and in vivo studies reveal that when CO2 is used, it creates a transient and weakly acidic local environment that causes a near-instantaneous release of the biomolecules through an immediate dissolution of ZIF-8. Conversely, when air is used, ZIF-8 biodegrades slowly, releasing the biomolecules over a week. This is the first example of controlled-biolistic delivery of biomolecules using ZIF-8, which provides a powerful tool for fundamental and applied science research.

Strong and reversible binding of carbon dioxide in a green metal-organic framework.

The efficient capture and storage of gaseous CO(2) is a pressing environmental problem. Although porous metal-organic frameworks (MOFs) have been shown to be very effective at adsorbing CO(2) selectively by dint of dipole-quadruple interactions and/or ligation to open metal sites, the gas is not usually trapped covalently. Furthermore, the vast majority of these MOFs are fabricated from nonrenewable materials, often in the presence of harmful solvents, most of which are derived from petrochemical sources. Herein we report the highly selective adsorption of CO(2) by CD-MOF-2, a recently described green MOF consisting of the renewable cyclic oligosaccharide γ-cyclodextrin and RbOH, by what is believed to be reversible carbon fixation involving carbonate formation and decomposition at room temperature. The process was monitored by solid-state (13)C NMR spectroscopy as well as colorimetrically after a pH indicator was incorporated into CD-MOF-2 to signal the formation of carbonic acid functions within the nanoporous extended framework.

Balancing Self-Healing and Shape Stability in Dynamic Covalent Photoresins for Stereolithography 3D Printing.

Dynamic covalent bonds impart new properties to 3D printable materials that help to establish 3D printing as an accessible and efficient manufacturing technique. Here, we studied the effect of a thermally reversible Diels-Alder cross-linker on the shape stability of photoprintable resins and their self-healing properties. Resins containing different concentrations of dynamic covalent cross-links in a polyacrylate network showed that the content of dynamic cross-links plays a key role in balancing shape stability with self-healing ability. The shape stability of the printed objects was evaluated by measuring the dimensional changes after thermal treatment. The self-healing efficiency of the 3D printed resins was characterized with a scratch test and tensile testing. A dynamic covalent cross-link concentration of 1.8 mol % was enough to provide 99% self-healing efficiency without disrupting the shape stability of the printed objects. Our work shows the potential of dynamic covalent bonds in broadening the availability of 3D printable materials that are compatible with vat photopolymerization.

Stabilization of supramolecular membrane protein-lipid bilayer assemblies through immobilization in a crystalline exoskeleton.

Artificial native-like lipid bilayer systems constructed from phospholipids assembling into unilamellar liposomes allow the reconstitution of detergent-solubilized transmembrane proteins into supramolecular lipid-protein assemblies called proteoliposomes, which mimic cellular membranes. Stabilization of these complexes remains challenging because of their chemical composition, the hydrophobicity and structural instability of membrane proteins, and the lability of interactions between protein, detergent, and lipids within micelles and lipid bilayers. In this work we demonstrate that metastable lipid, protein-detergent, and protein-lipid supramolecular complexes can be successfully generated and immobilized within zeolitic-imidazole framework (ZIF) to enhance their stability against chemical and physical stressors. Upon immobilization in ZIF bio-composites, blank liposomes, and model transmembrane metal transporters in detergent micelles or embedded in proteoliposomes resist elevated temperatures, exposure to chemical denaturants, aging, and mechanical stresses. Extensive morphological and functional characterization of the assemblies upon exfoliation reveal that all these complexes encapsulated within the framework maintain their native morphology, structure, and activity, which is otherwise lost rapidly without immobilization.