Covalent Organic Frameworks - ACS Publications - American

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Covalent Organic Frameworks: Chemistry beyond the Structure Sharath Kandambeth,†,§ Kaushik Dey,‡,§ and Rahul Banerjee*,‡ †

Physical/Materials Chemistry Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur Campus, Mohanpur 741246, India

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‡

dimensional covalent solids demand in situ crystallization, owing to their insoluble and nonmelting nature. It is noteworthy that during the covalent bond formation at higher dimensions, multiple covalent structures with varying free energies can be generated. This is due to the numerous possible ways of special extension of covalent bonds. As a result, direct crystallization of COFs proved to be an impossible task. It is important to apply the conditions of reversibility in a reaction, to prevent the formation of disordered kinetic products that are amorphous and, subsequently, to crystallize the thermodynamically most stable crystalline covalently networked solid.16,17 The reversibility in bond formation imparts self-healing and error correction during the crystallization. During the reversible covalent bond formation and extension, if any bond formation happens in an undesired direction, the system can repair it through back reaction and bond reformation and, finally, a crystalline thermodynamic product with the lowest free energy is isolated. However, the conditions for reversible covalent bond formation can be achieved only at a very high temperature and pressure owing to the higher covalent bond energies (50− 110 kcal mol−1). At lower temperatures, usually kinetically controlled disordered polymeric products are seen to dominate. Thus, it is difficult to construct the ordered covalent network solids under ambient reaction conditions, as the desired thermodynamic reaction pathways demand very high activation energies. In 2013, Wuest and co-workers reported single crystalline covalent network solids by utilizing low strength covalent bonds under ambient reaction conditions.18 Similarly, single crystals of 2D polymers were constructed independently by the research groups of Schlüter19 and King.20 However, the lack of permanent porosity and low thermal stability limits the suitability of these crystalline polymers for any practical applications. In order to construct thermally and chemically robust porous crystalline COFs, one must focus on chemically nonlabile covalent bonds with higher bond energies. However, thermal reversibility cannot be a solution to construct these COFs, because most of the organic building units fail to survive at high reaction temperatures. This problem was partially solved by employing “chemically induced dynamic covalent chemistry (DCC)”, for the construction of thermally robust extended crystalline covalent organic frameworks (COFs).12,15 In chemically induced DCC, a specific chemical agent was used to maintain the reversibility during the covalent bond formation at two or three dimensions. The “chemical agent” water plays a crucial role

ABSTRACT: Covalent organic frameworks (COFs) represent a new field of rapidly growing chemical research that takes direct inspiration from diverse covalent bonds existing between atoms. The success of linking atoms in two and three dimensions to construct extended framework structures moved the chemistry of COFs beyond the structures to methodologies, highlighting the possibility of prospective applications. Although structure to property relation in COFs has led to fascinating properties, chemical stability, processability and scalability were some of the important challenges that needed to be overcome for their successful implementation. In this Perspective, we take a closer look at the growth of COFs from mere supramolecular structures to potential industrializable materials.

1. INTRODUCTION Covalent bonds are well-known for their strength and diversity as they can be formed by diverse kinds of interactions between a wide range of atoms.1 Covalent bonds connect atoms to form molecules, and then further link these molecules to giant covalent structures.2 The extraordinary strength of a covalent bond is the reason behind the stability of these covalently networked solids, where each atom is connected and extended by covalent bonds to a framework structure. Covalently bonded giant molecular solids with long-range order such as diamond, silicon carbide and boron nitride represent the foremost hardest materials on earth.3 Furthermore, covalent bonds, using the same atom as the basic building unit, can create allotropes with different physical and chemical properties. Hence, it is important to a researcher to understand and master this bond formation chemistry, in order to construct new synthetic covalent structures (Figure 1a).4−8 Organic chemists had mastered the art of controlling covalent bond formation in the zero dimension.9,10 Polymer chemists further extended this covalent chemistry to one dimension. But “in two or three dimensions, it is a synthetic wasteland” as quoted by Nobel laureate Roald Hoffmann.11 Although extended organic framework structures such as covalently networked solids exist in nature, there was no other artificial synthetic analog, until Yaghi and co-workers made a breakthrough in 2005 (Figure 2).12 They could successfully link small symmetric organic building blocks to an extended porous crystalline covalent organic framework (COF) by utilizing the principles of dynamic covalent chemistry (DCC).13−15 However, unlike 0D, 1D organic structures, the higher © XXXX American Chemical Society

Received: September 24, 2018 Published: November 28, 2018 A

DOI: 10.1021/jacs.8b10334 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

Figure 1. (a) Examples for natural and synthetic covalent structures in different dimensions. (b) Schematic representation of crystalline and amorphous covalent organic framework formation by reversible and irreversible reactions. Adapted with permission from ref 12. Copyright 2005 AAAS; ref 6. Copyright 2010 Springer Nature; ref 32. Copyright 2012 RSC.

well-defined pore channels and high structural framework rigidity.34 Further, solid state robustness, together with their insoluble nature, enables COFs to be suitable candidates for heterogeneous catalysis.35 The ordered pi stacked layered structures of 2D COFs were crafted to arrange donor and acceptor organic molecules in ordered stacks for optoelectronic applications.36 The ordered, well aligned pore channels of the COFs were also envisaged as proton conducting37 and Lithium-ion conducting membranes.38 Despite all these initial successes and promising applications, COF research had been restricted within the doors of the laboratory. Commercializing this material and utilizing it for any type of real life application seems to be a far-fetched goal, due to the following serious disadvantages: 1) Chemical instability, 2) Nonscalability and 3) Nonprocessability. In order to promote the COFs for the practical/industrial applications, these three key issues needed to be resolved. In this Perspective, we will mainly discuss the development of COF chemistry in these three research areas.

in maintaining the reversibility under the closed reaction conditions.12 As a result, the ordered COF structure with high crystallinity is isolated at the end of the reaction. The newly synthesized 2D and 3D COFs displayed high thermal stability due to their strong covalent backbone and were tested for gas storage applications despite their modest chemical stability. After this initial success, several other reversible condensation reactions such as the Schiff base reaction, spiro-borane condensation, Knoevenagel condensation and imide condensation were successfully employed for the construction of crystalline COFs (Figure 3b).21−31 The use of rigid symmetric organic linkers minimizes the number of probable covalently extended kinetic structures. Keeping this in perspective, several 2D and 3D COFs were further constructed by using a wide range of organic linkers with a different symmetric combination and by following the principles of reticular chemistry.15 Linking organic molecules to create such extended structures immediately opened a new window for the solid-state organic chemistry research. The predesignable crystalline framework structure of the COFs allows systematic tailor-made arrangement of building blocks and thus permits the construction of the organic framework solid with desired physicochemical properties.32 The porous network structure of the COFs was initially targeted for gas storage applications.33 In addition to gas storage, mesoporous 2D COFs were tested for the storage of guest molecules such as drugs, biomolecules, enzymes and removal of toxic molecules such as dyes, pollutants etc.33 It has been perceived by many researchers, at that time, that if cast in a membrane form, COFs can be an attractive candidate for nanofiltration and gas separation applications owing to their

2. INDUCING CHEMICAL STABILITY IN COFS As discussed earlier, the concept “chemically induced reversibility” became successful for the construction of a wide variety of crystalline COFs. But in turn, it produces a serious disadvantage toward their hydrolytic/chemical stability. Though COFs bear high thermal stability and structural rigidity toward a wide range of chemical environments including organic solvents, they are prone to hydrolytic decomposition. Because water is produced as a byproduct during the reversible COF formation reaction, according to Le Chatelier’s principle, addition of an excess amount of water in B


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Figure 2. Evolution of the covalent organic framework chemistry. Important advances starting from unstable powder COFs to ultrastable processable forms. Adapted with permission from refs 15, 60, 69, 85. Copyright American Chemical Society; ref 84. Copyright Wiley-VCH; ref 88. Copyright AAAS.

industry, gas storage applications are performed under hydrous conditions. The adsorbents are expected to get exposed to a wide variety of reactive species such as water and acidic oxides (SO2, H2S, NO2, etc.).40 Therefore, the porous COFs, despite having high gas storage capacity, could not be implemented for industrial gas storage. Other industrially relevant applications where porous COFs could be suitable candidates are (i) separation of natural gases, (ii) removal of carbon dioxide from

the reaction medium can facilitate the backward reaction, which would lead to the decomposition of COFs (Figure 4a).39 COFs, especially boroxine and boronate ester COFs, in general get structurally degraded upon direct contact with humidity or water. Hence, the chemical stability of COFs became one of the important aspects that needed to be addressed, before considering them for any real-life applications like industrial gas storage or separation. In any chemical C


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Figure 3. (a and b) Different types of reversible organic reactions used for COF construction. (c) Different types of symmetric combinations used for 2D COF construction. Adapted with permission from refs 18, 19, 27. Copyright Springer Nature; refs 15, 21, 24, 26, 31. Copyright American Chemical Society; ref 23. Copyright AAAS; refs 25, 30. Copyright Wiley-VCH; ref 29. Copyright RSC.

flue gases, (iii) CO2 capture from the biofuel stream and (iv) hydrogen storage for the fuel cell. It is noteworthy that natural gas streams always contain water vapor (∼150 ppm) during the excavation process. This water vapor must be removed from natural gas completely before their transport and storage. The presence of water vapor in natural gas can create clogs in the pipelines by the formation of clathrates and gas hydrates. The humidity content within the natural gas also decreases its combustion value. Therefore, the COFs used for natural gas storage/separation must be stable in water. Chemical stability is also important in carbon dioxide sequestration processes. Industrial flue gas contains moisture and acidic oxides such as SO2 (∼0.04%), H2S (∼0.04%) and N2O (∼0.04%) in addition to CO2 (∼15%) and N2 (∼70%).41 Therefore, the intended COFs to be used for the CO2 capture require high chemical stability in acidic and humid conditions. Hydrolytic stability of

the COFs could also play a pivotal feature in hydrogen storage required for the fuel cells.42 During their operation, fuel cells expel water as an end product; and operate in humid conditions (as high as 98% RH). Hence, the hydrolytic stability of the COFs becomes essential for hydrogen storage applications. Even though in dry conditions COFs show high storage capacity and are placed among the benchmark materials regarding the total gas uptake values; the chemical instability of COFs restricted their applications in industry approved operating conditions. Thus, there was an urgent need to improve the chemical stability of the COF materials for potential industrial level applications. A significant effort had already been devoted in literature to solve the chemical stability issues in COFs. Lavigne and coworkers found that unsubstituted boroxine/boronate ester COFs, like COF-5 and COF-18Å are highly susceptible D


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Figure 4. (a) Schematic representation of the hydrolytic decomposition of boronate ester COFs. (b) Synthesis of highly chemically stable COFs by combined reversible and irreversible reactions. Adapted with permission from ref 39. Copyright 2011 American Chemical Society.

toward hydrolytic decomposition (Figure 4a).39 The authors demonstrated that the hydrolytic stability of COFs could be enhanced by backbone functionalization with long chain alkyl groups. The hydrophobic environment created by the alkyl chains protects the boronate ester centers from the attack of water molecules and slows down the hydrolysis of COFs. Later, Calabro and co-workers used pyridine, as a Lewis base, to protect the boronate ester linkages of COF-5 from hydrolysis.43 However, this functionalization of the COF backbone decreased the porosity and the gas storage properties of COFs, even though a slight enhancement in hydrolytic stability was achieved. In 2012, we had successfully developed a novel method to combine a reversible and an irreversible organic synthetic route to synthesize and crystallize highly chemically stable and porous COFs.44 The chemically stable β-ketoenamine-linked COFs were constructed by a one-pot reaction between 1,3,5triformylphloroglucinol (Tp) and aromaticdiamines (Pa-1, Pa2) (Figure 4b). In the first step, reversible Schiff base condensation reaction between Tp and aromatic diamines

(Pa-1, Pa-2) created a crystalline enol-imine COF intermediate. Subsequently, an irreversible proton tautomerization occurred in the second step to result in the formation of the β-ketoenamine linked COF (Figure 4b). Because of the irreversible nature of the second step, the β-ketoenamine form does not revert to the starting materials, while coming in contact with water. As a result, it displayed extraordinary chemical stability. The irreversible nature of the second step, however, did not affect the crystallinity of the final COFs because almost all the atomic positions remained identical during the proton tautomerization and only covalent bonds got rearranged during this process. Because of this locked βketoenamine structure, the newly formed COF displayed unprecedented chemical stability in water, as well as in acidic (9 M HCl) and basic media (9 M NaOH). A series of 2D chemically stable COFs (around 40 in total) of varying porosity and diverse functionalities were further constructed by us and others using this methodology.45−53 Among the others, Yan et al. further extended this β-ketoenamine based chemical stabilization strategy to 3D and synthesized the chemically E


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Figure 5. Attempts to improve chemical stability of 2D imine COFs: (I) decreasing nucleophilicity of imine bonds, (II) postsynthetic modification and (III) strengthening of the inter layer stacking. Adapted with permission from refs 61, 62. Copyright Springer Nature.

stable 3D COF in 2013.54 An analogous reversible-irreversible cascade reaction methodology was recently (2018) introduced by Wang and co-workers to synthesize a chemically stable benzoxazole linked COF (Figure 4b).31 After the work of chemically stable β-ketoenamine COF synthesis, we and others attempted to explore the chemical stability in COFs synthesized using different linkages, especially the imine [−CN] linkage. Because of the high

hydrolytic instability of boronate and boroxine based COFs, the focus shifted toward much more versatile and stable imine based COFs. In comparison to boronate and boroxine linked COFs, imine based COFs are less susceptible to hydrolytic decomposition, as they are generated by the pH-induced reversible reaction.56 A catalytic amount of acid is required to make the imine COF formation reaction completely reversible and to subsequently crystallize the material. However, this in F


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Figure 6. Road map toward scalable solid-state synthesis of COF: (a) conventional sealed tube based COF synthesis, (b) mechanochemical synthesis of β-ketoenamine linked COF, (c) coagent assisted solid state synthesis of β-ketoenamine linked COF. Adapted with permission from ref 69. Copyright 2017 American Chemical Society.

hydrazone-linked COFs for water splitting applications (Figure 5a).59 Jiang and co-workers reported the chemically stable azine-linked 2D COF by the reaction of hydrazine with 1,3,6,8tetrakis(4-formylphenyl)pyrene in 2013 (Figure 5b).60 The azine COF (Py-Azine) retains its crystallinity in aqueous HCl (1 M) and NaOH (1 M) solutions at 25 °C for 1 day. 2.2. Improvement of the Interlayer Stacking. Even though chemically stable imine bonds possess improved hydrolytic stability in comparison to boroxine and boronate ester bonds, still, special care was needed during the isolation and purification of 2D imine linked COFs. Often, dry organic solvents were employed in the imine COF purification process to preserve the crystallinity. The framework sensitivity of 2D imine COFs typically originated from the electrostatic repulsion between polarized CN segments, which significantly weakens the π−π stacking interaction between the individual layers. As a result, 2D imine COF layer stacks are vulnerable to external stimuli. In recent years a significant amount of research has been devoted to improving the crystallinity and chemical stability of the imine linked COFs by strengthening the π−π stacking interaction between the COF layers. As mentioned earlier, we have introduced the concept of intramolecular hydrogen bonding interaction for improving the crystallinity, porosity and chemical stability of the porphyrin based 2D imine COFs (Figure 5e).57 The strong

turn creates a disadvantage, as under acidic conditions, the backward reaction gets accelerated, which leads to the hydrolysis of the imine bonded COFs. In 2013, we, for the first time showcased the possibility of enhancing the stability of imine based COFs using intramolecular[−OH···NC] hydrogen bonding.57 The resulting COFs showcased decent stability in 3 M HCl. The chemical stability improvement of imine COFs were mainly achieved by (a) decreasing the nucleophilicity of the imine bonds, (b) strengthening theinterlayer stacking and by (c) the postsynthetic modifications. However, stability enhancement of imine based COFs is restricted only to 2D COFs. Alternative research approaches to stabilize the imine COFs in 3D is still active. 2.1. Decreasing the Nucleophilicity of the Imine Bonds. Hydrolytic decomposition of the imine bonds is initiated by the protonation of the imine nitrogen. Substitution of the carbon atom attached to the imine nitrogen by an electronegative element such as nitrogen can significantly reduce the nucleophilicity of the imine nitrogen and consequently protect it from proton attack. As a result, nitrogenous hydrazone and azine-linked COFs display higher chemical stability in comparison to simple imine functionalized COFs (Figure 5a,b). Uribe-Romo et al. initiated the synthesis of hydrazone-linked COF series in 2011.58 Later, Lotsch and co-workers successfully utilized the hydrolytic stability of G


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(industrial) application is their limited scalability during synthesis. COF synthesis is mostly restricted to the sealed pyrex tube with low internal pressure (150 mTorr) (Figure 6a) or in a hydrothermal reactor inside a glovebox.12 The desired vacuum inside the tube is generated by laborious freeze pump thaw cycles. A closed reaction condition is essential for maintaining the water equilibrium and the reversibility in the COF formation reaction. The low internal pressure inside the pyrex tube allows the slow diffusion of water out of the reaction medium, thus facilitating the slow nucleation and crystallization of COFs (Figure 6a). Factors that disturb this water equilibrium have a detrimental effect on the COF crystallinity. For example, heating in the open atmosphere can cause the rapid removal of water from the reaction mixture. As a result, COFs synthesized by this method generally show poor structural order. It is difficult to maintain the conditions of water equilibrium and reversibility of COF formation reaction on a large scale. Thus, most of the COF syntheses reported in the literature are only of the order of the milligram scale. Apart from the sealed reaction conditions, COF synthesis also demands the usage of high boiling organic solvents and a very long reaction time (3−7 days). In order to make COF synthesis attractive for industry, these constraints must be eradicated, and an efficient scalable reaction methodology needed that can convert organic linkers to the desired crystalline COFs in a short span of reaction time with the limited use of organic solvents. The best way to achieve this goal is to switch the conventional solution based synthesis to the solid state synthetic methodology. Solid state reactions are highly desirable for industrial applications as they are more economical, scalable and environmentally friendly due to the minimized use of toxic solvents. However, until 2015, COF synthesis was mainly performed via a solvothermal sealed tube reaction methodology developed in 2005.12 In 2009, Cooper and co-workers demonstrated the use of microwave synthesis for the rapid production of boronate ester based 2D and 3D COFs.67 However, apart from the requirement of a microwave reactor, this methodology still demands high boiling organic solvents as the reaction medium and a closed reaction tube for maintaining the reversibility. Wang and co-workers extended further the microwave synthetic methodology for the rapid construction of a β-ketoenamine linked COF (TpPa-1) in 60 min.68 In 2013, we had reported the rapid mechanochemical (MC) solid state synthesis of β-ketoenamine COF, which was a key step in switching solution based COF synthetic methodology to the solid state (Figure 6b).45 The solvent-free mechanochemical synthesis of ketoenamine COF was carried out by simple grinding of 1,3,5-triformylphloroglucinol and aromatic diamines in a mortar and pestle at room temperature. After 40 min of grinding, dark red colored powders of the COFs were isolated in ∼90% yield (Figure 6b). The FT-IR and 13C CPMAS solid-state NMR of the isolated product had shown mechanochemical C−N bond formation and the isolation of the β-ketoenamine-linked structure. However, the mechanochemically formed COF powders had poor crystallinity and porosity in comparison to their solvothermal analog. The key reason for the low crystallinity of the mechanochemically synthesized COFs was the lack of proper control of reversibility in the bond formation during the solid state reaction. Mechanochemical synthesis was performed in the “open”, which causes the rapid removal of water molecules

OH···NC intramolecular hydrogen bonding forces the aromatic rings to align in one plane, which subsequently improves the π−π stacking between the individual COF layers (Figure 5e). Because of this enhanced π−π stacking interaction between COF layers, COF-DhaTph displays improved crystallinity and porosity in comparison to the pristine unfunctionalized COF-366 and methoxy substituted COFDmaTph. Additionally, the strong intramolecular OH···N C hydrogen bonds protect the imine centers from water and acid decomposition. The hydrogen bonded DhaTph-COF preserves its crystallinity even after 7 days of water and 3 M HCl treatment. Along this line, Bein and co-workers have introduced an interesting concept of “molecular docking” to generate structurally robust 2D imine COFs with high crystallinity and porosity (Figure 5d).61 The stable dual-pore 4PE based 2D imine COFs were constructed by the Schiff base reaction between propeller-shaped C4 linker amine (1,1,2,2tetrakis(4-aminophenyl)ethane) and C2 shaped linear aromatic dialdehydes (Figure 5d). The propeller nature of the C4 linker node creates self-repeating docking sites for the effective reinforcement of individual COF layers. As a result, the propeller shaped 2D COFs displayed high crystallinity, porosity and structural stability toward water, as well as with protic and aprotic, organic solvents. Jiang and co-workers later showed that the introduction of methoxy (−OCH 3 ) functionalities in the COF backbone helps to improve the crystallinity and chemical stability of the 2D imine based COFs (Figure 5f).62 Recently, our group demonstrated that incorporation of methoxy (−OCH3) functionalities at the C3 node could also generate ultrastable 2D imine COFs with high porosity.63 The novel, chemically stable imine based COFs were synthesized by the Schiff base condensation between trimethoxy-1,3,5-benzenetricarbaldehyde (C3) and various linear aromatic diamines (C2) (Figure 5g). The synthesized ultrastable 2D imine COFs displayed exceptional chemical resistivity toward strong acids H2SO4 (18 M), HCl (12 M) and base NaOH (9 M). We postulate that this exceptional stability was not due to the mere presence of the −OMe groups but due to the effect of interlayer −CH···N hydrogen bonding64,65 between −OMe functionalities of one layer with the imine nitrogen atom (−CN) in the adjacent COF layer. Detailed theoretical calculations validated our reasoning (Figure 5g). Furthermore, the ultrahigh chemical stability of these COFs was exploited for sulfuric acid recovery applications. 2.3. Postsynthetic Modification. In 2016, Yaghi and coworkers introduced the postsynthetic chemical conversion strategy for transforming labile 2D imine (−CN−) COFs to their chemically stable amide (−CONH−) analogues (Figure 5c).66 In this method, 2D imine linked COFs were chemically converted to stable amide-linkages with excellent yield. Most importantly this post synthetic chemical conversion process does not perturb COFs crystallinity, topology and porosity. The post modified 2D amide COFs display high structural stability toward 12 M HCl and 1 M NaOH treatment for 1 day. The authors proposed that the post modification approach can be useful for bypassing the crystallization problem of COFs with linkages that are difficult to construct by the conventional reversible reaction strategy.

3. SCALABILITY ISSUES Besides chemical instability, the second important factor which drags down the development of COFs toward practical H


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Figure 7. (a) Forty-nine COFs synthesized via salt-mediated crystallization. First, amines and acids have been crystallized, and the crystals have been further reacted with the aldehyde (Tp).70 The highest surface area value of each of the COFs has been highlighted in red (bold font as well). (b) Model representation of the orientation of coagent acids catalyst with respect to the aromatic diamine linker. Adapted with permission from ref 70. Copyright 2018 American Chemical Society.

during the progress of the reaction. Because the “water equilibrium” was not well maintained during the mechanochemical grinding process, the resulting COFs display poor crystallinity and porosity. Synthesis of highly crystalline and porous COFs by solid state synthesis was still far out of reach due to the difficulty in maintaining the conditions of reversibility during the solid state synthesis. In 2017, we have developed a novel “external coagent” assisted solid state synthetic strategy for the rapid and gram scale synthesis of highly crystalline and porous β-ketoenamine linked COFs (Figure 6c).69 Traditionally, acetic acid was employed as an acid catalyst for maintaining the reversibility of the formation and crystallinity of imine and β-ketoenamine linked COFs in solvothermal synthesis. However, the use of acetic acid in mechanochemical solid state COF synthesis did not find success in producing crystalline and porous COFs. The acetic acid alone was not enough to maintain the water equilibrium and the reversibility during the mechanochemical solid state reaction. The introduction of an “external co-agent” that can play multiple roles, such as a catalyst, reactivity and water equilibrium controller could be beneficial in inducing crystallinity and porosity during the solid state COF synthesis. Keeping this in perspective, we had screened the various coagents for enhancing the crystallinity and porosity of COFs prepared via solid state synthesis. After a thorough screening, we have selected, p-toluene sulfonic acid monohydrate (PTSA.H2O) as the most effective “external co-agent” for the rapid, scalable, solid state synthesis of highly crystalline and porous 2D β-ketoenamine linked COFs (Figure 7).70 In this solid state synthetic method, the coagent p-toluene sulfonic acid (PTSA·H2O) was mixed along with the starting materials

(1,3,5-triformylphloroglucinol and aromatic diamines) in the presence of a small amount of water. After thorough mixing, the resulting dough was heated at 170 °C for 60 s (Figure 6c). The resulting COF monoliths, after washing with water, displayed very high surface area (up to 3000 m2 g−1) and excellent crystallinity. COFs synthesized by this method possess superior surface area and crystallinity in comparison to their acetic acid catalyzed solvothermal analogs. The novel solid state synthetic methodology developed is very simple, fast and solvent-free as compared to the laborious sealed tube reaction conditions. The easy-to-perform nature of the synthesis was further implemented for the large-scale synthesis of COFs (∼10 g/h) in a twin-screw extruder (Figure 8a).69 Detailed mechanistic studies revealed that PTSA·H2O plays multiple roles of water reservoir, molecular organizer, catalyst and reactivity controller during the solid phase COF crystallite formation. The masking effect of the coagent (PTSA·H2O) makes the controlled formation of the covalent bond network possible, and thus helps in inducing crystallinity in COFs (Figure 7b).70 Initially, during the solid phase mixing of reactants, PTSA·H2O protonates the aromatic diamine ligands and forms an intermolecular hydrogen bonded network structure. The protonated amine functionality in the hydrogen bonded network structure becomes well protected. Thus, the reaction does not commence after the immediate addition of the Tp. During the heating process the weaker hydrogen bonding interaction disintegrate slowly, and the Tp starts to replace the PTSA by forming the stronger covalent bonds with the amine. The protecting strategy of PTSA on aromatic diamines has a positive influence on the COF crystallinity because the reaction tends to proceed through a controlled I


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Figure 8. (a) Large scale synthesis of TpPa-1 COF by a using twin-screw extruder. (b) Schematic representation of COF-bead and sculpture fabrication using the terracotta technique. (c) Static vapor sorption isotherm of TpPa-1, TpAzo, TpBpy and TpBD. (d) Dynamic vapor sorption of TpPa-1, TpAzo, TpBpy and TpBD. Adapted with permission from ref 69. Copyright 2017 American Chemical Society.

insight into the desired acid-amine interactions required for the successful solid-state crystallization of 25 different COFs. Detailed analyses of the crystal structures of these acid-diamine salts reveal that an optimum intermolecular hydrogen-bonding distance [dav (NamineH···Oacid)] is essential for the reversible proton transfer and subsequent crystallization of the highly porous COFs. Crystallographic studies reveal that acidaromatic diamine salts are having dav (NamineH···Oacid) in between 2.06−2.19 Å produce COFs of the highest percentage of theoretical surface area value. Among trials using various aromatic sulfonic acids, PTSA·H2O was found to be the ideal candidate for generating highly crystalline and porous COFs in solid state synthesis. We could further extend this solid-state synthetic methodology to imine based COFs. By using the PTSA·H2O based solid state synthetic methodology, six novel crystalline and porous 2D imine COFs were constructed in a short reaction time.63

pathway. Additionally, PTSA·H2O plays the role of the water reservoir and maintains the “water equilibrium” required for the COF crystallization (Figure 6c). A detailed investigation revealed that the acid-amine reversible proton transfer plays a crucial role in solid state COF formation. The use of other Brønsted acids such as phosphoric acid (H3PO4), trifluoromethanesulfonic acid (TFMS), hydrochloric acid (HCl) resulted in the formation of COFs with poor crystallinity and porosity during the solid state synthesis. Since strong acids effectively bind with aromatic diamines, reversible acid-amine proton transfer will become difficult to achieve during the progress of the COF formation (Figure 7). The use of a weak acid (e.g., acetic acid) as the catalyst as well as the coagent also leads to the formation of the amorphous framework during the solid state COF synthesis. Weak acids were not able to sufficiently mask the aromatic diamines due to their weak binding. As a result, after the immediate addition of Tp, the reaction proceeds rapidly leading to the formation of the amorphous networks. The optimum pKa (−2.8) value of PTSA was found to be beneficial in creating crystalline and highly porous COFs during the solid phase reaction (Figure 7b). We have crystallized 49 acid-diamine salts to get molecular level

4. PROCESSABILITY Apart from the chemical instability and low scalability, nonprocessability is another downside of the COF research. Low processability of COFs typically originates from their J


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Figure 9. (a) Schematic of COF membrane formation. (b) Schematic representation of interfacial crystallization of COF thin films. (c) Selective molecular separation through COF membranes and thin films. Plot showing selective separation of Rose Bengal (RB) from a mixture of RB and nitroaniline (NA). Adapted with permission from ref 84. Copyright 2017 Wiley-VCH; ref 85. Copyright 2017 American Chemical Society.

Soon after the discovery of COFs,12 researchers were interested in exploring the optoelectronic properties of 2D COFs. But to precisely explore these properties, COFs needed to be fabricated as thin films and had to be integrated into electronic circuits. Therefore, the initial trend in COF fabrication was to develop 2D COF based thin films. Because the conventional solution based drop-casting or spin-coating methodologies failed due to the limited solubility of COF crystallites in any solvent, there was a need to develop in situ COF film fabrication techniques. In 2011, Dichtel and coworkers reported oriented boronate ester based 2D COF thin films (COF-5) on a single layer graphene supported by copper, silicon carbide and transparent fused silica (SiO2) substrates.78 Later, Bein and co-workers had developed an elegant strategy of a vapor assisted conversion reaction for the room temperature synthesis of boronate ester based COF thin films.79 This strategy involves the drop-casting of COF precursors on a clean glass substrate, followed by incubation inside a closed desiccator provided with solvent reservoirs of mesitylene and dioxane in a 1:1 ratio. It is possible to tune the thickness of the COF thin films by controlling the volume of the precursors in the drop casting solution. However, morphological control and large area synthesis of COF films by this method still needed to be achieved.

robust covalent backbone structure, which renders them insoluble and infusible. COFs are usually synthesized as fluffy microcrystalline powders, which need to be densified further to be suitable for industrial applications. Unlike 1D polymers, any type of postfabrication of COF powders is practically impossible due to their stubborn framework structure and limited solubility. These microcrystalline powders need to be further molded into different shapes to suit COFs for any real life applications. For example, membrane separation technology demands the development of defect-free self-standing membranes made from porous crystalline materials (to achieve high selectivity and permeability in separation).71,72 Similarly, to utilize COF as an adsorbent material for industrial applications, microcrystalline powders need to be further molded into molecular sieve beads of different shapes. For optoelectronic applications, the fabrication of defect free COF thin films is necessary.73,74 Even though usage of binders and additives can achieve the desired shapes for the COFs, the possible pore blocking effect from the additives will prevent access to the pores, and to the individual ligands properties, in comparison to the pristine COF material.75−77 Hence, a large scale, additive free COF fabrication methodology needs to be developed, in order to harness the full potential of the COF materials for practical applications. K


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Journal of the American Chemical Society Later, large area thin films of 2D imine COFs were reported by Zhenan Bao’s80 and Wei Zhang’s81 research groups. An air−solvent interface synthetic strategy was employed to synthesize 2D imine COF films. In this method, ligands with long chain alkyl groups were allowed to react at the air/solvent interface under ambient reaction conditions. The self-assembly of the hydrophobic alkyl groups assist in the organization and aggregation of COF crystallites into large area thin films. However, the utility of this method for synthesizing COF thin films from ligands without having alkyl functionalities is yet to be explored. Another important aspect of the mechanical processing was reported by Uribe-Romo and co-workers.55 The authors reported that uniaxial pressure on 2D COF pellets leads to preferred orientation between different crystallographic planes. Such anisotropic ordering of the layers could have enormous implication in device fabrication as exemplified by the authors. In 2017, Dichtel and co-workers developed an elegant colloidal COF based fabrication strategy for the casting of free-standing transparent boronate ester based 2D COF films.82 Despite all these successes in fabrication, optoelectronic properties of the COF thin-films achieved to date are rather low, in comparison to the 1D polymers. The probable reason can be correlated to the inevitable grain boundaries formed in the COF thin films, which significantly reduce the charge migration. Apart from small area thin film fabrication, COFs were never fabricated as large area membranes nor molded into any other geometric shapes. The development of the novel in situ COF processing techniques is highly essential to achieve this task. These techniques, if achieved, should accelerate the development of COFs toward the real-life/industrial level applications. Membrane based separation is one of the research areas that could benefit from the development of large scale COF fabrication techniques. Micro and nanofiltration technology demands robust porous materials with ordered pore channels in subnano meter domains. Commercially used polymer membranes often lack precise pore channels in nano domains, and solvent robustness.71 Development of COF based membranes could be a promising solution for a wide variety of applications such as water purification, recovery of active pharmaceutical ingredients from organic solution, etc. Theoretical studies suggest that 2D COF membranes can outperform commercially used 1D polymers in water desalination applications.83 However, developing large scale COF based defect free membrane is an extremely challenging task due to their poor solubility and processability issues. Initial attempts at the fabrication of COF based membranes were done by the utilization of binders, but as discussed earlier, in order to utilize the full potential of COFs, ordered pore channels and additive free, self-standing COF membranes needed to be developed. In 2017, we have successfully developed a novel molecular baking strategy for the large area fabrication of self-standing, porous and crystalline β-ketoenamine COF membranes (COFMs).84 We were able to fabricate a series of large scale, defect free, stand-alone COF nanofiltration membranes that retained structural integrity and showed long-term durability in water and organic solvents (Figure 9a). These COF membranes, due to their highly porous structure, displayed high permeance toward organic solvents. Among the COFMs series, MTpTD displayed permeance of 278 L m2− h−1 bar−1 to acetonitrile solvent, which is 2.5 times higher than the commercially used NF membranes. Because of the precise pore

channels, COFMs display high selectivity toward molecular separation. The high selectivity and permeability of COFMs were then successfully utilized for water purification applications. From the standpoint of commercialization, we made a rough cost analysis of these membranes. The functional cost of the 5 cm coupon of the membranes were estimated between 2 and 23 USD. This membrane fabrication methodology was further extended in imine linked 2D COFs by us using 2,4,6-trimethoxy-1,3,5-benzenetricarbaldehyde and aromatic diamines as the building blocks.63 Owing to the high chemical resistivity, these imine COF membranes were tested for potential applications in sulfuric acid recovery. Later, in another report, we demonstrated that the solvent permeability and separation performance of the COF membranes can be greatly enhanced by reducing the membrane thickness. Large-scale and defect-free COF thin films were synthesized by using the liquid−liquid interfacial crystallization technique under ambient reaction conditions (Figure 9b).85 The nanoporous COF thin films synthesized by this method displayed high permeability and structural stability toward a wide range of organic solvents in comparison to the commercially used polymeric membranes. Because of the ultrahigh porosity and robust framework structure, COFs can be a promising adsorbent material for the various industrial separation applications such as dehumidification, wastewater purification, cleanup of oils and solvents etc. However, to utilize COFs for such practical applications, polycrystalline COF powders needed to be engineered further. For example, for dehumidification applications, adsorbent powders need to be molded and transformed into molecular sieve beads cartridges, pellets etc. of specific geometric shapes (Figure 8b). This shape transformation is required to improve the mechanical strength, longevity of the adsorbents and gas flow rates in the case of applications such as dehumidification of natural gas and the drying of air steam. Thus, the low processable COF crystallites should be fabricated into different desired geometric shapes. We had observed that the coagent based solid-state synthetic methodology could be advantageous for the fabrication of COFs into the desired geometric shapes.69 The coagent PTSA retard the reactivity of the amine by protonation and subsequently prevent the covalent bond formation (COF formation) at room temperature. This low reactivity is advantageous in the molding of COF precursors into the desired geometric shapes. The novel molding methodology that we developed is analogous to the ancient “terracotta: process. These β-ketoenamine COFs can be fabricated into any desired geometric shapes, including sculptures, using this “organic terracotta process”. In this typical molecular terracotta process the starting materials and the co-agent (PTSA) were mixed thoroughly with the required amount of water. The resultant dough was molded in to different geometric shapes and subjected to baking (60−120 °C) in a programmable oven for 12−24 h. The resulting defect free COF beads and geometrical shapes display high crystallinity, analogous to solvothermally synthesized COF powders. Because of the binder-free nature of this process, the pore blocking effect had been minimized and the resulting COF beads displayed very high surface area. These COF beads outperform commercially used adsorbents such as Basolite A100, zeolite NaX and silica gel in water vapor uptake (Figure 8c,d).69,86 Additionally, the retention of the total water vapor uptake capacity, even after 4 cycles, also portrays that the COF L


Perspective


realization of a single layer COF structure, “COFENE”, could create fascinating electronic properties. By the scalable and processable synthesis of chemically stable COFs, we have made a significant advance in the area of COFs toward real life gas storage applications. Further advancement in this direction could come from the 3D printing of COFs. However, in order to use COFs for industrial storage separation applications, several other physical factors such as (1) specific heat capacity, (2) thermal diffusibility and (3) mechanical strength need to be considered. During the bulk scale gas adsorption process, the heat of adsorption is generated. This thermal energy needs to be dissipated properly from the adsorbent to prevent the structural damage and increase gas storage capacity. Thermal conductivity and other physical properties of the COF adsorbents need to be checked before they can be employed for real life gas storage applications. Making COF hybrids with conducting 2D materials such as graphene could be a possible alternative to enhance the thermal conductivity of the COF samples. Because of the high hydrolytic stability and excellent water uptake capacity, β-ketoenamine COF beads could evolve as promising materials for water vapor storage and removal applications. The high reversible water uptake of 2D βketoenamine COF beads at low relative pressure could be explored for adsorptive water production from low humid desert air. We had shown self-standing 2D COF membranes as promising materials for nanofiltration applications. Usually, COFs with mesoporous pore walls are used for nanofiltration applications. By reducing the pore size to 1 nm, one can improve the applicability of COF membranes for a wide range of applications. One such application is desalination. Conventionally, reverse osmosis membranes are employed for desalination applications. Because of the absence of ordered pore channels and dense structures, RO membranes operate under high pressure and are energy intensive. 2D COF membranes with specific pore channels can be useful for low cost desalination processes, as they demand only low operating pressures. Constructing 3D COF self-standing membranes is another challenging area of research. Because of the narrow microporous pore channels, in the future, 3D COF membranes could be promising materials for gas separation processes.

beads could be desirable candidates for industrial dehumidification applications in the near future.

5. PERSPECTIVE AND CHALLENGES The exploration of the utility of this robust porous covalent backbone structure for various applications should continue. As discussed in the Introduction section, covalent bonds are diverse in nature. They can produce diverse materials with fascinating properties by virtue of the diverse chemical properties of the individual building units and the direction of their linkages. The same principle also applies to COFs, where the entire network is composed of covalent bonds. We have already discussed that a subtle change in the imine bond (−CN−) to enamine bond (−CN−) in the COF backbone can drastically improve the chemical stability of βketoenamine44 and imide24 COFs. Also, the noncovalent and supramolecular interactions can aid in improving the crystallinity, porosity and framework stability in 2D COFs. Therefore, the best way to manipulate the properties of COFs and to overcome the current limitation and challenges is to focus more on basic structural and covalent bonding aspects. Jiang and co-workers showed another recent example of the, covalent bond deciding the properties of the COF materials.23 The authors reported the synthesis of crystalline and porous 2D COFs with the −CC− bond as the basic connecting linkage, by employing the reversible Knoevenagel condensation reaction. The sp2 carbon COF displayed excellent conductivity in comparison to the structurally analogous 2D imine COFs. This example also signifies that a small interchange in atom or bonding can completely alter the electronic properties of the 2D COF frameworks. In literature, several theoretically proposed carbon 2D materials such as graphynes and graphdiynes exist displaying high electrical conductivity.87 However, such type of synthetically crystalline ordered structures has not yet been achieved. In COFs, the reversible reticular framework construction principle can be useful in achieving these hypothesized 2D structures. In order to achieve this goal, one has to investigate the exact reaction conditions to make −CC−, and −CC− reversible. Low electrical conductivity of the 2D COFs is one of the prominent reasons that prevents the use of COFs for practical electrochemical applications. Once the crystalline framework construction from −CC−, −CC− and −CC− covalent bonds are achieved, COFs can advance toward real life electronic applications. Apart from the atoms and bonds, a detailed study of the reaction mechanism of COF formation is also very relevant. We have already discussed in the previous section how the slowing down of the framework formation reaction and the managing of the water equilibrium led to the scalable and processable synthesis of COFs. A recent example for this strategy is the single crystal COF synthesis reported by Yaghi and co-workers.88 By using the imine exchange strategy, the authors could significantly slow down the COF formation rate and were able to synthesize the single crystal COF. Dichtel and co-workers had synthesized micron sized single crystals of 2D boronate ester COFs by the seeded growth mechanism.89 However, the single crystal of 2D imine, sp2 C COFs has not yet been achieved. The study of the optoelectronic properties of these 2D COF single crystals is an important emerging area of research. In 2D materials, the electronic properties and bandgap changes with the layer stacking thickness. The

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Rahul Banerjee: 0000-0002-3547-4746 Author Contributions §

S.K. and K.D. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.K. and K.D. acknowledge CSIR, India for research fellowships. R.B. acknowledges IISER-Kolkata Startup grant and SwarnaJayanti Fellowship grant [DST/SJF/CSA-02/20162017] for funding and Carl Friedrich von Siemens Research Fellowship and the Alexander von Humboldt Foundation. M


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(25) Du, Y.; Yang, H.; Whiteley, J. M.; Wan, S.; Jin, Y.; Lee, S.-H.; Zhang, W. Ionic Covalent Organic Frameworks with Spiroborate Linkage. Angew. Chem., Int. Ed. 2016, 55, 1737−1741. (26) Rao, M. R.; Fang, Y.; De Feyter, S.; Perepichka, D. F. Conjugated Covalent Organic Frameworks via Michael Addition− Elimination. J. Am. Chem. Soc. 2017, 139, 2421−2427. (27) Guo, J.; Xu, Y.; Jin, S.; Chen, L.; Kaji, T.; Honsho, Y.; Addicoat, M. A.; Kim, J.; Saeki, A.; Ihee, H.; Seki, S.; Irle, S.; Hiramoto, M.; Gao, J.; Jiang, D. Conjugated organic framework with threedimensionally ordered stable structure and delocalized π clouds. Nat. Commun. 2013, 4, 2736. (28) Kuhn, P.; Antonietti, M.; Thomas, A. Porous, Covalent Triazine-Based Frameworks Prepared by Ionothermal Synthesis. Angew. Chem., Int. Ed. 2008, 47, 3450−3453. (29) Jackson, K. T.; Reich, T. E.; El-Kaderi, H. M. Targeted synthesis of a porous borazine-linked covalent organic framework. Chem. Commun. 2012, 48, 8823−8825. (30) Nagai, A.; Chen, X.; Feng, X.; Ding, X.; Guo, Z.; Jiang, D. A Squaraine-Linked Mesoporous Covalent Organic Framework. Angew. Chem., Int. Ed. 2013, 52, 3770−3774. (31) Wei, P.-F.; Qi, M.-Z.; Wang, Z.-P.; Ding, S.-Y.; Yu, W.; Liu, Q.; Wang, L.-K.; Wang, H.-Z.; An, W.-K.; Wang, W. Benzoxazole-Linked Ultrastable Covalent Organic Frameworks for Photocatalysis. J. Am. Chem. Soc. 2018, 140, 4623−4631. (32) Ding, S.-Y.; Wang, W. Covalent organic frameworks (COFs): from design to applications. Chem. Soc. Rev. 2013, 42, 548−568. (33) Zhu, L.; Zhang, Y.-B. Crystallization of Covalent Organic Frameworks for Gas Storage Applications. Molecules 2017, 22, 1149. (34) Jimenez-Solomon, M. F.; Song, Q.; Jelfs, K. E.; Munoz-Ibanez, M.; Livingston, A. G. Polymer nanofilms with enhanced microporosity by interfacial polymerization. Nat. Mater. 2016, 15, 760−767. (35) Ding, S.-Y.; Gao, J.; Wang, Q.; Zhang, Y.; Song, W.-G.; Su, C.Y.; Wang, W. Construction of Covalent Organic Framework for Catalysis: Pd/COF-LZU1 in Suzuki−Miyaura Coupling Reaction. J. Am. Chem. Soc. 2011, 133, 19816−19822. (36) Mandal, A. K.; Mahmood, J.; Baek, J.-B. Two-Dimensional Covalent Organic Frameworks for Optoelectronics and Energy Storage. ChemNanoMat 2017, 3, 373−391. (37) Chandra, S.; Kundu, T.; Kandambeth, S.; BabaRao, R.; Marathe, Y.; Kunjir, S. M.; Banerjee, R. Phosphoric Acid Loaded Azo (−NN−) Based Covalent Organic Framework for Proton Conduction. J. Am. Chem. Soc. 2014, 136, 6570−6573. (38) Lei, Z.; Yang, Q.; Xu, Y.; Guo, S.; Sun, W.; Liu, H.; Lv, L.-P.; Zhang, Y.; Wang, Y. Boosting lithium storage in covalent organic framework via activation of 14-electron redox chemistry. Nat. Commun. 2018, 9, 576. (39) Lanni, L. M.; Tilford, R. W.; Bharathy, M.; Lavigne, J. J. Enhanced Hydrolytic Stability of Self-Assembling Alkylated TwoDimensional Covalent Organic Frameworks. J. Am. Chem. Soc. 2011, 133, 13975−13983. (40) Natural gas 1998: Issues and trends; DOE/EIA-0560-98; Energy Information Administration, Office of Oil and Gas: Washington, DC, 1999. (41) Beychok, M. R. Fundamentals of Stack Gas Dispersion; Milton R. Beychok: Irvine, CA, 1994. (42) Peighambardoust, S. J.; Rowshanzamir, S.; Amjadi, M. Review of the proton exchange membranes for fuel cell applications. Int. J. Hydrogen Energy 2010, 35, 9349−9384. (43) Du, Y.; Mao, K.; Kamakoti, P.; Ravikovitch, P.; Paur, C.; Cundy, S.; Li, Q.; Calabro, D. Experimental and computational studies of pyridine-assisted post-synthesis modified air stable covalent−organic frameworks. Chem. Commun. 2012, 48, 4606−4608. (44) Kandambeth, S.; Mallick, A.; Lukose, B.; Mane, M. V.; Heine, T.; Banerjee, R. Construction of Crystalline 2D Covalent Organic Frameworks with Remarkable Chemical (Acid/Base) Stability via a Combined Reversible and Irreversible Route. J. Am. Chem. Soc. 2012, 134, 19524−19527. (45) Biswal, B. P.; Chandra, S.; Kandambeth, S.; Lukose, B.; Heine, T.; Banerjee, R. Mechanochemical Synthesis of Chemically Stable

REFERENCES

(1) Langmuir, I. THE ARRANGEMENT OF ELECTRONS IN ATOMS AND MOLECULES. J. Am. Chem. Soc. 1919, 41, 868−934. (2) Lewis, G. N. THE ATOM AND THE MOLECULE. J. Am. Chem. Soc. 1916, 38, 762−785. (3) Wentorf, R. H.; DeVries, R. C.; Bundy, F. P. Sintered Superhard Materials. Science 1980, 208, 873−880. (4) Nishiyama, Y.; Langan, P.; Chanzy, H. Crystal Structure and Hydrogen-Bonding System in Cellulose Iβ from Synchrotron X-ray and Neutron Fiber Diffraction. J. Am. Chem. Soc. 2002, 124, 9074− 9082. (5) Weller, M.; Overton, T.; Rourke, J.; Armstrong, F. Inorganic Chemistry; Oxford University Press: Oxford, 2018. (6) Grujicic, M.; Bell, W. C.; Glomski, P. S.; Pandurangan, B.; Yen, C. F.; Cheeseman, B. A. Filament-Level Modeling of Aramid-Based High-Performance Structural Materials. J. Mater. Eng. Perform. 2011, 20, 1401−1413. (7) Dawson, R.; Cooper, A. I.; Adams, D. J. Nanoporous organic polymer networks. Prog. Polym. Sci. 2012, 37, 530−563. (8) Corey, E. J.; Czakó, B.; Kürti, L. Molecules and Medicine; John Wiley & Sons: Hoboken, NJ, 2007. (9) Woodward, R. B. THE TOTAL SYNTHESIS OF VITAMIN B12. Pure Appl. Chem. 1973, 33, 145−177. (10) Corey, E. J.; Robinson, R. Retrosynthetic Thinking-Essentials and Examples. Chem. Soc. Rev. 1988, 17, 111−133. (11) Hoffmann, R. How Should Chemists Think? Sci. Am. 1993, 268, 66−73. (12) Côté, A. P.; Benin, A. I.; Ockwig, N. W.; O’Keeffe, M.; Matzger, A. J.; Yaghi, O. M. Porous, Crystalline, Covalent Organic Frameworks. Science 2005, 310, 1166−1170. (13) Rowan, S. J.; Cantrill, S. J.; Cousins, G. R. L.; Sanders, J. K. M.; Stoddart, J. F. Dynamic Covalent Chemistry. Angew. Chem., Int. Ed. 2002, 41, 898−952. (14) Jin, Y.; Wang, Q.; Taynton, P.; Zhang, W. Dynamic Covalent Chemistry Approaches Toward Macrocycles, Molecular Cages, and Polymers. Acc. Chem. Res. 2014, 47, 1575−1586. (15) Waller, P. J.; Gandara, F.; Yaghi, O. M. Chemistry of Covalent Organic Frameworks. Acc. Chem. Res. 2015, 48, 3053−3063. (16) Lehn, J. M. Dynamic Combinatorial Chemistry and Virtual Combinatorial Libraries. Chem. - Eur. J. 1999, 5, 2455−2463. (17) Jin, Y.; Yu, C.; Denman, R. J.; Zhang, W. Recent advances in dynamic covalent chemistry. Chem. Soc. Rev. 2013, 42, 6634−6654. (18) Beaudoin, D.; Maris, T.; Wuest, J. D. Constructing monocrystalline covalent organic networks by polymerization. Nat. Chem. 2013, 5, 830−834. (19) Kory, M. J.; Wörle, M.; Weber, T.; Payamyar, P.; van de Poll, S. W.; Dshemuchadse, J.; Trapp, N.; Schlüter, A. D. Gram-scale synthesis of two-dimensional polymer crystals and their structure analysis by X-ray diffraction. Nat. Chem. 2014, 6, 779−784. (20) Kissel, P.; Murray, D. J.; Wulftange, W. J.; Catalano, V. J.; King, B. T. A nanoporous two-dimensional polymer by single-crystal-tosingle-crystal photopolymerization. Nat. Chem. 2014, 6, 774−778. (21) Han, S. S.; Furukawa, H.; Yaghi, O. M.; Goddard, W. A. Covalent Organic Frameworks as Exceptional Hydrogen Storage Materials. J. Am. Chem. Soc. 2008, 130, 11580−11581. (22) Uribe-Romo, F. J.; Hunt, J. R.; Furukawa, H.; Klöck, C.; O’Keeffe, M.; Yaghi, O. M. A Crystalline Imine-Linked 3-D Porous Covalent Organic Framework. J. Am. Chem. Soc. 2009, 131, 4570− 4571. (23) Jin, E.; Asada, M.; Xu, Q.; Dalapati, S.; Addicoat, M. A.; Brady, M. A.; Xu, H.; Nakamura, T.; Heine, T.; Chen, Q.; Jiang, D. Twodimensional sp2 carbon−conjugated covalent organic frameworks. Science 2017, 357, 673−676. (24) Fang, Q.; Wang, J.; Gu, S.; Kaspar, R. B.; Zhuang, Z.; Zheng, J.; Guo, H.; Qiu, S.; Yan, Y. 3D Porous Crystalline Polyimide Covalent Organic Frameworks for Drug Delivery. J. Am. Chem. Soc. 2015, 137, 8352−8355. N


Perspective

Journal of the American Chemical Society Isoreticular Covalent Organic Frameworks. J. Am. Chem. Soc. 2013, 135, 5328−5331. (46) Chandra, S.; Kandambeth, S.; Biswal, B. P.; Lukose, B.; Kunjir, S. M.; Chaudhary, M.; Babarao, R.; Heine, T.; Banerjee, R. Chemically Stable Multilayered Covalent Organic Nanosheets from Covalent Organic Frameworks via Mechanical Delamination. J. Am. Chem. Soc. 2013, 135, 17853−17861. (47) Shinde, D. B.; Aiyappa, H. B.; Bhadra, M.; Biswal, B. P.; Wadge, P.; Kandambeth, S.; Garai, B.; Kundu, T.; Kurungot, S.; Banerjee, R. A mechanochemically synthesized covalent organic framework as a proton-conducting solid electrolyte. J. Mater. Chem. A 2016, 4, 2682− 2690. (48) DeBlase, C. R.; Silberstein, K. E.; Truong, T.-T.; Abruña, H. D.; Dichtel, W. R. β-Ketoenamine-Linked Covalent Organic Frameworks Capable of Pseudocapacitive Energy Storage. J. Am. Chem. Soc. 2013, 135, 16821−16824. (49) Chandra, S.; Kundu, T.; Dey, K.; Addicoat, M.; Heine, T.; Banerjee, R. Interplaying Intrinsic and Extrinsic Proton Conductivities in Covalent Organic Frameworks. Chem. Mater. 2016, 28, 1489− 1494. (50) Thote, J.; Aiyappa, H. B.; Kumar, R. R.; Kandambeth, S.; Biswal, B. P.; Shinde, D. B.; Roy, N. C.; Banerjee, R. Constructing covalent organic frameworks in water via dynamic covalent bonding. IUCrJ 2016, 3, 402−407. (51) Chandra, S.; Chowdhury, D. R.; Addicoat, M.; Heine, T.; Paul, A.; Banerjee, R. Molecular Level Control of the Capacitance of TwoDimensional Covalent Organic Frameworks: Role of Hydrogen Bonding in Energy Storage Materials. Chem. Mater. 2017, 29, 2074−2080. (52) Khayum, M. A.; Kandambeth, S.; Mitra, S.; Nair, S. B.; Das, A.; Nagane, S. S.; Mukherjee, R.; Banerjee, R. Chemically Delaminated Free-Standing Ultrathin Covalent Organic Nanosheets. Angew. Chem., Int. Ed. 2016, 55, 15604−15608. (53) Mitra, S.; Sasmal, H. S.; Kundu, T.; Kandambeth, S.; Illath, K.; Díaz Díaz, D.; Banerjee, R. Targeted Drug Delivery in Covalent Organic Nanosheets (CONs) via Sequential Postsynthetic Modification. J. Am. Chem. Soc. 2017, 139, 4513−4520. (54) Fang, Q.; Gu, S.; Zheng, J.; Zhuang, Z.; Qiu, S.; Yan, Y. 3D Microporous Base-Functionalized Covalent Organic Frameworks for Size-Selective Catalysis. Angew. Chem., Int. Ed. 2014, 53, 2878−2882. (55) Vazquez-Molina, D. A.; Mohammad-Pour, G. S.; Lee, C.; Logan, M. W.; Duan, X.; Harper, J. K.; Uribe-Romo, F. J. Mechanically Shaped Two-Dimensional Covalent Organic Frameworks Reveal Crystallographic Alignment and Fast Li-Ion Conductivity. J. Am. Chem. Soc. 2016, 138, 9767−9770. (56) Smith, B. J.; Overholts, A. C.; Hwang, N.; Dichtel, W. R. Insight into the crystallization of amorphous imine-linked polymer networks to 2D covalent organic frameworks. Chem. Commun. 2016, 52, 3690− 3693. (57) Kandambeth, S.; Shinde, D. B.; Panda, M. K.; Lukose, B.; Heine, T.; Banerjee, R. Enhancement of Chemical Stability and Crystallinity in Porphyrin-Containing Covalent Organic Frameworks by Intramolecular Hydrogen Bonds. Angew. Chem., Int. Ed. 2013, 52, 13052−13056. (58) Uribe-Romo, F. J.; Doonan, C. J.; Furukawa, H.; Oisaki, K.; Yaghi, O. M. Crystalline Covalent Organic Frameworks with Hydrazone Linkages. J. Am. Chem. Soc. 2011, 133, 11478−11481. (59) Stegbauer, L.; Schwinghammer, K.; Lotsch, B. V. A hydrazonebased covalent organic framework for photocatalytic hydrogen production. Chem. Sci. 2014, 5, 2789−2793. (60) Dalapati, S.; Jin, S.; Gao, J.; Xu, Y.; Nagai, A.; Jiang, D. An Azine-Linked Covalent Organic Framework. J. Am. Chem. Soc. 2013, 135, 17310−17313. (61) Ascherl, L.; Sick, T.; Margraf, J. T.; Lapidus, S. H.; Calik, M.; Hettstedt, C.; Karaghiosoff, K.; Döblinger, M.; Clark, T.; Chapman, K. W.; Auras, F.; Bein, T. Molecular docking sites designed for the generation of highly crystalline covalent organic frameworks. Nat. Chem. 2016, 8, 310−316.

(62) Xu, H.; Gao, J.; Jiang, D. Stable, crystalline, porous, covalent organic frameworks as a platform for chiral organocatalysts. Nat. Chem. 2015, 7, 905−912. (63) Halder, A.; Karak, S.; Addicoat, M.; Bera, S.; Chakraborty, A.; Kunjattu, S. H.; Pachfule, P.; Heine, T.; Banerjee, R. Ultrastable Imine-Based Covalent Organic Frameworks for Sulfuric Acid Recovery: An Effect of Interlayer Hydrogen Bonding. Angew. Chem., Int. Ed. 2018, 57, 5797−5802. (64) Desiraju, G. R. Hydrogen Bridges in Crystal Engineering: Interactions without Borders. Acc. Chem. Res. 2002, 35, 565−573. (65) Desiraju, G. R.; Steiner, T. The weak hydrogen bond in structuralchemistry and biology; Oxford University Press: Oxford, 1999. (66) Waller, P. J.; Lyle, S.; Osborn Popp, T.; Diercks, C. S.; Reimer, J. A.; Yaghi, O. M. Chemical Conversion of Linkages in Covalent Organic Frameworks. J. Am. Chem. Soc. 2016, 138, 15519−15522. (67) Campbell, N. L.; Clowes, R.; Ritchie, L. K.; Cooper, A. I. Rapid Microwave Synthesis and Purification of Porous Covalent Organic Frameworks. Chem. Mater. 2009, 21, 204−206. (68) Wei, H.; Chai, S.; Hu, N.; Yang, Z.; Wei, L.; Wang, L. The microwave-assisted solvothermal synthesis of a crystalline twodimensional covalent organic framework with high CO2 capacity. Chem. Commun. 2015, 51, 12178−12181. (69) Karak, S.; Kandambeth, S.; Biswal, B. P.; Sasmal, H. S.; Kumar, S.; Pachfule, P.; Banerjee, R. Constructing Ultraporous Covalent Organic Frameworks in Seconds via an Organic Terracotta Process. J. Am. Chem. Soc. 2017, 139, 1856−1862. (70) Karak, S.; Kumar, S.; Pachfule, P.; Banerjee, R. Porosity Prediction through Hydrogen Bonding in Covalent Organic Frameworks. J. Am. Chem. Soc. 2018, 140, 5138−5145. (71) Karan, S.; Jiang, Z.; Livingston, A. G. Sub−10 nm polyamide nanofilms with ultrafast solvent transport for molecular separation. Science 2015, 348, 1347−1351. (72) Jue, M. L.; Koh, D.-Y.; McCool, B. A.; Lively, R. P. Enabling Widespread Use of Microporous Materials for Challenging Organic Solvent Separations. Chem. Mater. 2017, 29, 9863−9876. (73) Medina, D. D.; Werner, V.; Auras, F.; Tautz, R.; Dogru, M.; Schuster, J.; Linke, S.; Döblinger, M.; Feldmann, J.; Knochel, P.; Bein, T. Oriented Thin Films of a Benzodithiophene Covalent Organic Framework. ACS Nano 2014, 8, 4042−4052. (74) Medina, D. D.; Petrus, M. L.; Jumabekov, A. N.; Margraf, J. T.; Weinberger, S.; Rotter, J. M.; Clark, T.; Bein, T. Directional ChargeCarrier Transport in Oriented Benzodithiophene Covalent Organic Framework Thin Films. ACS Nano 2017, 11, 2706−2713. (75) Biswal, B. P.; Chaudhari, H. D.; Banerjee, R.; Kharul, U. K. Chemically Stable Covalent Organic Framework (COF)-Polybenzimidazole Hybrid Membranes: Enhanced Gas Separation through Pore Modulation. Chem. - Eur. J. 2016, 22, 4695−4699. (76) Shan, M.; Seoane, B.; Rozhko, E.; Dikhtiarenko, A.; Clet, G.; Kapteijn, F.; Gascon, J. Azine-Linked Covalent Organic Framework (COF)-Based Mixed-Matrix Membranes for CO2/CH4 Separation. Chem. - Eur. J. 2016, 22, 14467−14470. (77) Kang, Z.; Peng, Y.; Qian, Y.; Yuan, D.; Addicoat, M. A.; Heine, T.; Hu, Z.; Tee, L.; Guo, Z.; Zhao, D. Mixed Matrix Membranes (MMMs) Comprising Exfoliated 2D Covalent Organic Frameworks (COFs) for Efficient CO2 Separation. Chem. Mater. 2016, 28, 1277− 1285. (78) Colson, J. W.; Woll, A. R.; Mukherjee, A.; Levendorf, M. P.; Spitler, E. L.; Shields, V. B.; Spencer, M. G.; Park, J.; Dichtel, W. R. Oriented 2D Covalent Organic Framework Thin Films on SingleLayer Graphene. Science 2011, 332, 228−231. (79) Medina, D. D.; Rotter, J. M.; Hu, Y.; Dogru, M.; Werner, V.; Auras, F.; Markiewicz, J. T.; Knochel, P.; Bein, T. Room Temperature Synthesis of Covalent−Organic Framework Films through VaporAssisted Conversion. J. Am. Chem. Soc. 2015, 137, 1016−1019. (80) Feldblyum, J. I.; McCreery, C. H.; Andrews, S. C.; Kurosawa, T.; Santos, E. J. G.; Duong, V.; Fang, L.; Ayzner, A. L.; Bao, Z. Fewlayer, large-area, 2D covalent organic framework semiconductor thin films. Chem. Commun. 2015, 51, 13894−13897. O


Perspective

Journal of the American Chemical Society (81) Dai, W.; Shao, F.; Szczerbiński, J.; McCaffrey, R.; Zenobi, R.; Jin, Y.; Schlüter, A. D.; Zhang, W. Synthesis of a Two-Dimensional Covalent Organic Monolayer through Dynamic Imine Chemistry at the Air/Water Interface. Angew. Chem., Int. Ed. 2016, 55, 213−217. (82) Smith, B. J.; Parent, L. R.; Overholts, A. C.; Beaucage, P. A.; Bisbey, R. P.; Chavez, A. D.; Hwang, N.; Park, C.; Evans, A. M.; Gianneschi, N. C.; Dichtel, W. R. Colloidal Covalent Organic Frameworks. ACS Cent. Sci. 2017, 3, 58−65. (83) Elimelech, M.; Phillip, W. A. The Future of Seawater Desalination: Energy, Technology, and the Environment. Science 2011, 333, 712−717. (84) Kandambeth, S.; Biswal, B. P.; Chaudhari, H. D.; Rout, K. C.; Kunjattu, H. S.; Mitra, S.; Karak, S.; Das, A.; Mukherjee, R.; Kharul, U. K.; Banerjee, R. Selective Molecular Sieving in Self-Standing Porous Covalent-Organic-Framework Membranes. Adv. Mater. 2017, 29, 1603945−1603953. (85) Dey, K.; Pal, M.; Rout, K. C.; Kunjattu, H. S.; Das, A.; Mukherjee, R.; Kharul, U. K.; Banerjee, R. Selective Molecular Separation by Interfacially Crystallized Covalent Organic Framework Thin Films. J. Am. Chem. Soc. 2017, 139, 13083−13091. (86) Seo, Y.-K.; Yoon, J. W.; Lee, J. S.; Hwang, Y. K.; Jun, C.-H.; Chang, J.-S.; Wuttke, S.; Bazin, P.; Vimont, A.; Daturi, M.; Bourrelly, S.; Llewellyn, P. L.; Horcajada, P.; Serre, C.; Ferey, G. EnergyEfficient Dehumidification over Hierachically Porous Metal−Organic Frameworks as Advanced Water Adsorbents. Adv. Mater. 2012, 24, 806−810. (87) Enyashin, A. N.; Ivanovskii, A. L. Graphene allotropes. Phys. Status Solidi B 2011, 248, 1879−1883. (88) Ma, T.; Kapustin, E. A.; Yin, S. X.; Liang, L.; Zhou, Z.; Niu, J.; Li, L.-H.; Wang, Y.; Su, J.; Li, J.; Wang, X.; Wang, W. D.; Wang, W.; Sun, J.; Yaghi, O. M. Single-crystal x-ray diffraction structures of covalent organic frameworks. Science 2018, 361, 48−52. (89) Evans, A. M.; Parent, L. R.; Flanders, N. C.; Bisbey, R. P.; Vitaku, E.; Kirschner, M. S.; Schaller, R. D.; Chen, L. X.; Gianneschi, N. C.; Dichtel, W. R. Seeded growth of single-crystal two-dimensional covalent organic frameworks. Science 2018, 361, 52−57.

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