Formation Of Covalent Organic Frameworks Research Articles

Rapid Synthesis of High Surface Area Imine-Linked 2D Covalent Organic Frameworks by Avoiding Pore Collapse During Isolation.

Imine-linked 2D covalent organic frameworks (COFs) form more rapidly than previously reported under Brønsted acid-catalyzed conditions, showing signs of crystallinity within a few minutes, and maximum crystallinity within hours. These observations contrast with the multiday reaction times typically employed under these conditions. In addition, vacuum activation, which is often used to isolate COF materials significantly erodes the crystallinity and surface area of the several isolated materials, as measured by N2 sorption and X-ray diffraction. This loss of material quality during isolation for many networks has historically obscured otherwise effective polymerization conditions. The influence of the activation procedure is characterized in detail for three COFs, with the commonly used 1,3,5-tris(4-aminophenyl)benzene-terephthaldehyde network (TAPB-PDA COF), the most prone to pore collapse. When the networks are activated carefully, rapid COF formation is general for all five of the imine-linked 2D COFs studied, with all exhibiting excellent crystallinity and surface areas, including the highest surface areas reported to date for three materials. Furthermore, to simplify the workup of COF materials, a simple nitrogen flow method provides high-quality materials without the need for specialized equipment. These insights have important implications for studying and understanding how 2D COFs form.

Dynamic covalent chemistry steers synchronizing nanoparticle self-assembly with interfacial polymerization

Precise organization of matter across multiple length scales is of particular interest because of its great potential with advanced functions and properties. Here we demonstrate a simple yet versatile strategy that enables the organization of hydrophobic nanoparticles within the covalent organic framework (COF) in an emulsion droplet. The interfacial polymerization takes place upon the addition of Lewis acid in the aqueous phase, which allows the formation of COF after a crystallization process. Meanwhile, the interaction between nanoparticles and COF is realized by the use of amine-aldehyde reactions in the nearest loci of the nanoparticles. Importantly, the competition between the nanoparticle self-assembly and interfacial polymerization allows control over the spatial distribution of nanoparticles within COF. As a general strategy, a wide variety of COF-wrapped nanoparticle assemblies can be synthesized and these hybridized nanomaterials could find applications in optoelectronics, heterogeneous catalysis and energy chemistry.

Hierarchical-Coassembly-Enabled 3D-Printing of Homogeneous and Heterogeneous Covalent Organic Frameworks

Covalent organic frameworks (COFs) are crystalline polymers with permanent porosity. They are usually synthesized as micrometer-sized powders or two-dimensional thin films and membranes for applications in molecular storage, separation, and catalysis. In this work, we report a general method to integrate COFs with imine or β-ketoenamine linkages into three-dimensional (3D)-printing materials. A 3D-printing template, Pluronic F127, was introduced to coassemble with imine polymers in an aqueous environment. By limitation of the degree of imine polycondensation during COF formation, the amorphous imine polymer and F127 form coassembled 3D-printable hydrogels with suitable shear thinning and rapid self-healing properties. After the removal of F127 followed by an amorphous-to-crystalline transformation, three β-ketoenamine- and imine-based COFs were fabricated into 3D monoliths possessing high crystallinity, hierarchical pores with high surface areas, good structural integrity, and robust mechanical stability. Moreover, when multiple COF precursor inks were employed for 3D printing, heterogeneous dual-component COF monoliths were fabricated with high spatial precision. This method not only enables the development of COFs with sophisticated 3D macrostructure but also facilitates the heterogeneous integration of COFs into devices with interconnected interfaces at the molecular level.

Porosity Prediction through Hydrogen Bonding in Covalent Organic Frameworks.

Easy and bulk-scale syntheses of two-dimensional (2D) covalent organic frameworks (COFs) represent an enduring challenge in material science. Concomitantly, the most critical aspect is to precisely control the porosity and crystallinity of these robust structures. Disparate complementary approaches such as solvothermal synthesis have emerged recently and are fueled in part by the usage of different modulators and acids that have enriched the COF library. Yet, the fundamental understanding of the integral processes of 2D COF assembly, including their growth from nucleating sites and the origin of periodicity, is an intriguing chemical question that needs to be answered. To address these cardinal questions, a green and easy-to-perform approach of COF formation has been delineated involving acid-diamine salt precursors. The role of hydrogen bonding [ dav(Namine-H···Oacid); dav signifies the average Namine-H···Oacid distances, i.e., the average distance from the H atom of the amine to the O atom of the acid] present in the acid-diamine salts in improving the COFs' crystallinity and porosity has further been decoded by thorough crystallographic analyses of the salt molecules. What is particularly noteworthy is that we have established the hydrogen-bonding distances dav(Namine-H···Oacid) in the acid-diamine salts that are pivotal in maintaining the reversibility of the reaction, which mainly facilitates highly crystalline and porous COF formation. Moreover, this reactant-structure to the product-quality relationship has further been utilized for the synthesis of highly crystalline and porous COFs that are unattainable by other synthetic means.

Colloidal Covalent Organic Frameworks.

Covalent organic frameworks (COFs) are two- or three-dimensional (2D or 3D) polymer networks with designed topology and chemical functionality, permanent porosity, and high surface areas. These features are potentially useful for a broad range of applications, including catalysis, optoelectronics, and energy storage devices. But current COF syntheses offer poor control over the material’s morphology and final form, generally providing insoluble and unprocessable microcrystalline powder aggregates. COF polymerizations are often performed under conditions in which the monomers are only partially soluble in the reaction solvent, and this heterogeneity has hindered understanding of their polymerization or crystallization processes. Here we report homogeneous polymerization conditions for boronate ester-linked, 2D COFs that inhibit crystallite precipitation, resulting in stable colloidal suspensions of 2D COF nanoparticles. The hexagonal, layered structures of the colloids are confirmed by small-angle and wide-angle X-ray scattering, and kinetic characterization provides insight into the growth process. The colloid size is modulated by solvent conditions, and the technique is demonstrated for four 2D boronate ester-linked COFs. The diameter of individual COF nanoparticles in solution is monitored and quantified during COF growth and stabilization at elevated temperature using in situ variable-temperature liquid cell transmission electron microscopy imaging, a new characterization technique that complements conventional bulk scattering techniques. Solution casting of the colloids yields a free-standing transparent COF film with retained crystallinity and porosity, as well as preferential crystallite orientation. Collectively this structural control provides new opportunities for understanding COF formation and designing morphologies for device applications.

Constructing covalent organic frameworks in water via dynamic covalent bonding.

The formation of keto-enamine based crystalline, porous polymers in water is investigated for the first time. Facile access to the Schiff base reaction in water has been exploited to synthesize stable porous structures using the principles of Dynamic Covalent Chemistry (DCC). Most credibly, the water-based Covalent Organic Frameworks (COFs) possess chemical as well as physical properties such as crystallinity, surface area and porosity, which is comparable to their solvothermal counterparts. The formation of COFs in water is further investigated by understanding the nature of the monomers formed using hydroxy and non-hydroxy analogues of the aldehyde. This synthetic route paves a new way to synthesize COFs using a viable, greener route by utilization of the DCC principles in conjunction with the keto-enol tautomerism to synthesize useful, stable and porous COFs in water.

Covalent, Organometallic, and Halogen-Bonded Nanomeshes from Tetrabromo-Terphenyl by Surface-Assisted Synthesis on Cu(111)

The selective temperature-controlled surface-assisted synthesis of covalent, organometallic, and halogen-bonded nanomeshes based on a 3,5,3″,5″-tetrabromo-para-terphenyl (TBrTP) precursor was studied with scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (STM) in ultrahigh vacuum. Vapor deposition of TBrTP onto Cu(111) at 90 K leads to a highly ordered organic monolayer stabilized by Br···Br and Br···H intermolecular bonds between the intact T-type assembled TBrTP molecules, as confirmed by density functional theory (DFT) calculations. Annealing the monolayer to 300 K results in C–Br bond scission and the formation of C–Cu–C bonds, which link adjacent para-terphenyl fragments such that stable organometallic frameworks are formed. Pore sizes correlate with the number of enclosed adatoms (most likely Br atoms), which presumably play a size-determining role during the process of the pore formation. Larger islands of the organometallic framework are obtained by deposition of TBrTP onto the copper surface held at 460 K. A further increase in sample temperature to 570 K during deposition gives rise to the formation of covalent organic frameworks with pores of tetragonal and trigonal symmetry. The covalent nanostructures are not completely planar, but contain phenylene units which are tilted relative to the surface plane, most likely due to steric hindrance between the C–H bonds inside the pores. Comparison of the three different bonding regimes reveals that the degree of long-range order correlates inversely with the strength of the bonds between the building blocks.

Mechanochemical Synthesis of Chemically Stable Isoreticular Covalent Organic Frameworks

Three thermally and chemically stable isoreticular covalent organic frameworks (COFs) were synthesized via room-temperature solvent-free mechanochemical grinding. These COFs were successfully compared with their solvothermally synthesized counterparts in all aspects. These solvent-free mechanochemically synthesized COFs have moderate crystallinity with remarkable stability in boiling water, acid (9 N HCl), and base [TpBD (MC) in 3 N NaOH and TpPa-2 (MC) in 9 N NaOH]. Exfoliation of COF layers was simultaneously observed with COF formation during mechanochemical synthesis. The structures thus obtained seemed to have a graphene-like layered morphology (exfoliated layers), unlike the parent COFs synthesized solvothermally.

A mechanistic study of Lewis acid-catalyzed covalent organic framework formation

Three boronate ester-linked covalent organic frameworks (COFs) were synthesized using a new approach that employs polyfunctional boronic acid and acetonide-protected catechol reactants in the presence of the Lewis acid catalyst BF3·OEt2. This transformation avoids the use of unstable and insoluble polyfunctional catechols. The COF-5 and COF-10 hexagonal lattices were obtained from a triphenylene tris(acetonide) and the appropriate diboronic acid linker, whereas a square Ni phthalocyanine COF was prepared from the appropriate Ni phthalocyanine tetra(acetonide). The powder X-ray diffraction, infrared spectra, and measured surface areas of these materials matched or exceeded previously reported values. A mechanistic study of this transformation revealed that the dehydrative trimerization of boronic acids to boroxines and the formation of a nonproductive aryl boronic acid–BF3 complex strongly affect the rate of boronate ester formation. Crossover experiments employing substituted boronate ester derivatives suggest that esterhydrolysis is the most likely exchange mechanism during COF formation under BF3·OEt2-catalyzed conditions.

On the reticular construction concept of covalent organic frameworks

The concept of reticular chemistry is investigated to explore the applicability of the formation of Covalent Organic Frameworks (COFs) from their defined individual building blocks. Thus, we have designed, optimized and investigated a set of reported and hypothetical 2D COFs using Density Functional Theory (DFT) and the related Density Functional based tight-binding (DFTB) method. Linear, trigonal and hexagonal building blocks have been selected for designing hexagonal COF layers. High-symmetry AA and AB stackings are considered, as well as low-symmetry serrated and inclined stackings of the layers. The latter ones are only slightly modified compared to the high-symmetry forms, but show higher energetic stability. Experimental XRD patterns found in literature also support stackings with highest formation energies. All stacking forms vary in their interlayer separations and band gaps; however, their electronic densities of states (DOS) are similar and not significantly different from that of a monolayer. The band gaps are found to be in the range of 1.7–4.0 eV. COFs built of building blocks with a greater number of aromatic rings have smaller band gaps.