Graphene-like single-layered covalent organic frameworks: synthesis strategies and application prospects.

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Graphene-Like Single-Layered Covalent Organic Frameworks: Synthesis Strategies and Application Prospects Xuan-He Liu, Cui-Zhong Guan, Dong Wang,* and Li-Jun Wan* chalocogenides, silicene, and so on, which also display many interesting dimensionrelated materials properties.[5] The exotic properties of these 2D nanomaterials promise a great potential regarding their application in molecular electronics, sensors, and optoelectronic devices. The instinctive curiosity of chemists to create and modify matter brings out the question as to whether we can synthesize the organic counterpart of 2D nanomaterials. To be more specific, the target is to covalently interconnect the functional building blocks into 2D frameworks while keeping the film to a single-molecule thickness. In fact, even before the discovery of graphene, people had tried to synthesize 2D polymers, which, unlike conventional 1D polymers, feature periodic covalent bonding of monomers in two or more orthogonal directions.[6] Theoretical calculations have predicted that the electronic properties of 2D polymers with a conjugated backbone can be greatly tuned by the periodicity and topology of the materials.[7,8] Therefore, great interest has been paid to 2D polymers with well-defined in-plane crystalline structures, or single-layered covalent organic frameworks (sCOFs). sCOFs are structurally reminiscent of graphene, except the C atoms are replaced with organic building blocks. The merit of the unprecedented diverse structures by tailoring abundant building blocks and linking functional groups endows them with designable and tunable structures and promising electronic/optoelectronic properties. The straightforward way to get sCOFs is to exfoliate bulk COFs.[9–11] However, although a great variety of bulk COFs have been reported,[12–23] the synthesis of large-size COF crystals remains a big challenge. Besides, the tendency of multilayer formation and mechanical damage after exfoliation in practice makes the top-down exfoliation method to obtain sCOFs less appealing. Therefore, much more effort has been made to achieve sCOFs with few defects via the “bottom-up” synthesis strategy.[24–26] In 2007, Hecht and Grill reported well-defined 2D covalently bound molecular nanostructures obtained by the surface catalyzed Ullmann reaction of tetrabromine porphyrin.[27] Since then, a great variety of surface reactions have been exploited to obtain sCOFs.[28–43] However, the key challenge urgent to address for the synthesis of high-quality sCOFs is how to obtain desired topological structures with perfect structural uniformity, which has impeded the fundamental understanding of the intrinsic properties of sCOFs and potential applications thereafter.

Two-dimensional (2D) nanomaterials, such as graphene and transition metal chalcogenides, show many interesting dimension-related materials properties. Inspired by the development of 2D inorganic nanomaterials, singlelayered covalent organic frameworks (sCOFs), featuring atom-thick sheets and crystalline extended organic structures with covalently bonded building blocks, have attracted great attention in recent years. With their unique graphene-like topological structure and the merit of structural diversity, sCOFs promise to possess novel and designable properties. However, the synthesis of sCOFs with well-defined structures remains a great challenge. Herein, the recent development of the bottom-up synthesis methods of 2D sCOFs, such as thermodynamic equilibrium control methods, growth-kinetics control methods, and surface-assisted covalent polymerization methods, are reviewed. Finally, some of the critical properties and application prospects of these materials are outlined.

1. Introduction The blossoming development of nanomaterials in recent years has demonstrated the basic principle that dimensionality is one of the most important factors in determining material properties.[1] This principle has been perfectly validated once again by the fascinating properties associated with 2D nanomaterials.[2] Graphene, as the most-representative 2D nanomaterial, featuring atomic thickness and well-defined crystalline in-plane structure, possesses distinct electronic properties as compared with its parent graphite, and has been subjected to intensive investigation in the past decade.[3,4] More importantly, the discovery of graphene has inspired great interest in other single-layered 2D nanomaterials, covering transition metal dichalcogenides, metal

X.-H. Liu, Dr. C.-Z. Guan, Prof. D. Wang, Prof. L.-J. Wan Key Laboratory of Molecular Nanostructure and Nanotechnology and Beijing National Laboratory for Molecular Sciences Institute of Chemistry Chinese Academy of Sciences (CAS) Beijing 100190, P. R. China E-mail: [email protected]; [email protected] X.-H. Liu University of CAS Beijing 100049, P. R. China

DOI: 10.1002/adma.201305317

Adv. Mater. 2014, DOI: 10.1002/adma.201305317

© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Herein, recent developments in 2D sCOFs synthesis strategies are reviewed. First, the general retro-synthesis design for sCOFs is introduced. Then, synthetic strategies to obtain 2D sCOFs via on-surface synthesis approaches are summarized. We will show several examples of obtaining high-quality sCOFs with the employment of suitable reaction conditions to tailor the thermodynamics, growth kinetics, and catalytic process. In addition, some interesting examples of the synthesis of sCOFs based on the liquid–air interface reaction and the solutionphase reaction are surveyed to indicate other alternatives to 2D sCOFs.[44,45] Finally, a brief outlook for the application prospects of 2D sCOFs concludes this Research News.

2. Structural Design and Diversity of sCOFs Covalent assemblies from repetitive modules with periodic structure in a low dimension should meet the requirements of having conformationally rigid building blocks and appropriate

reactive groups on the building blocks. The rigid conformation of the building blocks, especially those with planar, highsymmetry structures, enables the topological design of the sCOFs. The building blocks should have substantially free mobility and be able to connect with each other to form thermally robust periodic structures at the designated reaction conditions. The precursors bearing multiple reaction moieties with different symmetries are designed to target a variety of topological structures ranging from hexagonal, trigonal networks to square grids. Topologically, a sCOF network is composed of nodes and spacers. From the viewpoint of a retro-synthesis design, a fully cross-linked 2D network can be obtained by “node” construction or “spacer” construction. For the first approach, multicomponent (>3) reactions that can form symmetric connectors are employed. As schematic presented in Scheme 1A, the self-condensation of ditopic building blocks bearing two functional groups can produce 2D networks. For example, hexagonal nodes can be generated by the cyclocondensation of three

Scheme 1. A) The design of 2D sCOFs by the “node” approach. The reaction scheme of cyclocondensation of three boronic acids molecules and tetramerization of four TCNB molecules with one Fe atom. B) The design of 2D sCOFs by the spacer approach and a schematic representation of the linkage reactions for the construction of 2D sCOFs: the reaction scheme for the condensation reaction between boronic acids and diols, the Schiff-based reaction, polyamide formation, polyimide formation, polyester condensation, Ullmann coupling, and Glaser coupling.

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3. On-Surface Synthesis of sCOFs Although almost all bulk COFs are synthesized in the solution phase, on-surface polymerization has been demonstrated to be effective for sCOF synthesis. Well-defined single-crystal surfaces can offer an ideal atomically flat interface to template polymerization reactions in two dimensions and support the resulting film. Appreciable substrate-molecule interactions can help the dispersion of precursors into a monolayer, which is beneficial for avoiding the growth of multilayer COFs, and even orientate the precursors, which is helpful for growth of highquality sCOFs. In addition, the employment of metal-substratecatalyzed coupling reaction for construction of sCOFs makes the reaction only occur at the interface and guarantees the formation of sCOFs. Scanning tunneling microscopy (STM) is a powerful technique to study adsorbates on conductor surfaces, and thus can be utilized to image and characterize sCOFs on conductive surfaces with high resolution. Hecht and Grill reported the first 2D covalently bonded molecular nanostructures via an on-surface synthesis approach and confirmed the structures by ultrahigh vacuum STM.[27] Molecular nanostructures of dimers, linear chains, and networks precisely corresponding to the molecular design of


porphyrins with varied numbers of Br substituents were obtained. Henceforth, experimental efforts aimed at creating sCOFs based on Ullmann radical coupling, polyimide formation, imine coupling, boronic anhydridation reaction, etc. have received increasing attention.[28–43] Nevertheless, some of these attempts were not further reproduced due to poorly controlled polymerization processes, and the prepared 2D sCOFs suffer from small domain sizes and non-uniform frameworks. It is well-established that organic molecules can form wellordered 2D self-assembled molecular nanostructures on solid surfaces via non-covalent intermolecular interactions, such as van der Waals forces, hydrogen bonding, and metal-organic interactions. Such weak and reversible interactions allow effective reorganization and diffusion of molecules on the solid surfaces, to form thermodynamically well-ordered self-assemblies. However, covalent bonds generally possess high bond strength and poor reversibility, which makes the remedy of the structural defects difficult. Learning from non-covalent-bond-driven supramolecular assembly processes, successful preparative strategies to tailor the reaction conditions are beginning to emerge for the growth of high-quality 2D sCOFs.[41,49–51,54–57]

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monomers into one ring, covering cyclocondensation of three boronic acid building blocks into a six-membered boroxine ring (B3O3) and trimerization reactions of nitriles to triazine.[32,46] Similarly, tetramerization of 1,2,4,5-tetracyanobenzene (TCNB) with the assistance of Fe atoms as a template can produce covalent iron–phthalocyanine networks with four-fold symmetry.[47] On the other hand, in the “spacer” construction approach, coupling reactions forming linear connectors are used in the construction of extended regular 2D sCOFs, as shown in Scheme 1B. For this type of reaction, the construction of 2D continuous reticular structures requires the use of precursors with more than 2 reactive positions on each precursor. Molecular building blocks utilized can be divided into different categories according to the directional symmetry of the reactive groups on the molecular structures. Configurations of C2, C3 and C4, which possesses two, three and four reactive sites respectively, capable of attaching rigid molecular termini at regular angles in the plane of 2D sheets, are displayed in Scheme 1B. For example, honeycomb hexagonal networks of 2D sCOFs have been constructed by integrating C3 building blocks with C2 or C3 blocks. Similarly, tetragonal grid structures of 2D sCOFs have been constructed by reactions of C4 building blocks and C2 blocks, as well as the condensation of C4 and C4 building blocks. Exploited bicomponent coupling reactions include: the coupling of boronic acid and diol groups to form a dioxaborole heterocycle;[48] the Schiff-base reaction of aldehydes and amines to form an imine;[30,31] and acylation reactions to form amides, imides, and esters.[29,35,36] The formation of a carbon–carbon covalent bond is a very attractive type of coupling reaction, since the resulting networks are expected to be conjugated if proper precursors are used. The Ullmann reaction between haloarene and Glaser coupling between alkyne moieties have been demonstrated for the construction of 2D sCOFs, typically under the catalytic mediation of metal substrates.[27,49]

3.1. Thermodynamic Equilibrium Control The key difference between covalent and non-covalent bonds is their different degrees of reversibility. Therefore, control over thermodynamic equilibrium is considered to be most important for the synthesis of larger-scale high-quality sCOFs, just similar to those molecular networks formed through weak intermolecular interactions. The cyclo-condensation of boronic acid to a boroxine ring was employed to construct the first bulk COF materials.[12] Zwaneveld et al. carried out a sCOF synthesis based on the dehydration of 1,4-benzenediboronic acid on Ag(111) surface in ultrahigh vacuum (UHV).[32] Disordered nanoporous networks with mixed pentagon, hexagon, and heptagon structures are obtained. Recently, a thermodynamic-equilibrium control strategy was applied to construct large-scale high-quality sCOFs based on the dehydration reaction of boronic acids using water as the chemical-equilibrium-manipulating agent.[50] In a typical reaction protocol, ditopic diboronic acid was deposited onto highly oriented pyrolytic graphite (HOPG) surfaces from solution, and the dried precursor film was sealed in the autoclaves and subjected to the designated temperature to induce the formation of the sCOFs. Critically, a small amount of CuSO4·5H2O powder was placed in the closed system to act as a water “reservoir’’ to regulate the chemical equilibrium of the dehydration reaction. The water molecules released from the CuSO4·5H2O during the heating process can promote a defect remedy process by dissociating and reconnecting mislinked units. These water molecules would be reabsorbed by the CuSO4 during the cooling process, preventing the decomposition of the sCOFs. The synthesis route and STM images of the formed flawless sCOF are shown in Figure 1. Over 98% of the surface is covered by interconnected molecular hexagons, by statistical analysis, in the presence of CuSO4·5H2O. In the control experiment, where the reaction is carried out without the presence of equilibrium regulator, the coverage



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Figure 1. The synthesis routes to sCOFs via the dehydration reaction of biphenyldiboronic acid and the pertaining STM images of the formed sCOFs.

of six-member rings on the HOPG surface was only approximately 7%, which is comparable to the result formed on metal surfaces in vacuum conditions. The important role of water in the formation process of an ordered sCOF is understandable by le Chatelier’s principle. This presence of H2O during the reaction process changes the Gibbs energy of the reaction and pushes the equilibrium backward to favor a reverse reaction to a certain extent. The improved reversibility can improve the self-adjustment or self-healing abilities of the sCOFs and result in the formation of highly ordered porous molecular nanostructures. Similar results are obtained when carrying out the experiment in an open environment but under a H2O atmosphere generated by adding 50 µL of H2O to the bottom of the reactor.[51,52] By employing the chemical-equilibrium strategy, a series of 2D sCOFs with various lattice parameters and pore sizes were obtained based on self-condensation of boronic acids with varied organic backbones Similarly, Kunitake and co-workers used H+ as a thermodynamic-equilibrium regulator in the Schiff base reaction between an aldehyde and an amine.[31,53] It is well known that the Schiff base reaction can be catalyzed by an acid. They carefully set a slightly acidic value relative to the pKa of the molecules bearing primary amino units to manipulate the reaction equilibrium. Lowering the pH of the reaction media makes the bond formation much slower. The real-space observation of bond formation and cleavage by STM clearly demonstrates the reversibility of this process. It is worth mentioning that chemically inert and hydrophobic iodine-modified Au(111) (I/Au(111)) was selected as the substrate in order to weaken physisorption and enhance molecular mobility and/or the dynamic adsorption equilibrium. By utilizing thermodynamic

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control of the equilibrium polymerization reactions, robust, extended, supramacromolecular networks and arrays based on Schiff base coupling have been achieved.

3.2. Growth Process Control The formation of 2D sCOFs on surfaces involves several kinetic processes including nucleation, diffusion, and growth. To grow large-scale highly ordered 2D sCOFs, it is desired to have a low density of nuclei to ensure a large domain area, a high diffusion rate of the precursors to increase the reaction efficiency, and a slow growth rate to facilitate the formation of thermodynamically most-stable structures. Recently, wereported the method of self-limiting solid−vapor interface reaction for growth of 2D sCOFs nanostructures with extended order.[54] The synthesis route for producing sCOF is schematically shown in Figure 2A and 2B. Briefly, the precursor A was deposited on HOPG surfaces, which were then placed in a closed system containing the other precursor B. During thermo-activated polymerization process, precursor B was vaporized and then landed on the surface covered with precursor A. By this means, two precursors are tailored to form covalent bonds at the solid−vapor interface. During the reaction process, the gas-phase dosing of precursor B at the solid-vapor interface effectively promotes the attachment of precursor B to the available sCOF nuclei, leading to the growth of high-quality sCOFs. Large-scale high-quality sCOFs based on the Schiff base reaction were successfully designed and synthesized through self-limiting solid−vapor interface reaction methods. The sCOFs constructed by this strategy can be steered relatively




RESEARCH NEWS Figure 2. A, B) The synthesis route to the sCOF via the Schiff base reaction of tris(4-aminophenyl)benzene and gaseous terephthaldicarboxaldehyde. C,D) STM images of the sCOF on HOPG showing the hexagonal structure. The inset depicts the corresponding fast Fourier transform (FFT) of the image. E) Chemical structure of the trans-Br2I2TPP molecules. F) Scheme of the sequential activation mechanism. G,H) STM images of trans-Br2I2TPP molecules on Au(111): after deposition (at 80 K), after heating to 120 °C (Step 1), and after further heating to 250 °C (Step 2). Reproduced with permission.[55] Copyright 2012, Nature Publishing Group.




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easily to be single-layered. The large-scale and high-resolution STM images of the formed nearly flawless sCOFs are displayed in Figure 2C and 2D respectively. A control experiment to construct sCOFs by heating precursors with mixing uniformity on surfaces shows lots of defects and small domains, due to the instantaneous formation of small nuclei (oligomers) on surfaces. A mechanism of nucleation and orientated growth for the self-limiting solid−vapor interface reaction method is proposed based on ex situ observation of the growth process of the sCOFs at different reaction times. The STM results show that sCOF flakes tend to expand after nuclei formation for the selflimiting solid−vapor interface reaction method. The formation of fully developed 2D sCOFs requires precise control over the conformation of each building units, while it is much easier to construct 1D linear polymers due to their uncomplicated conformation. Grill and co-workers developed a sequential growth strategy to obtain 2D sCOFs.[55] The building block they designed bears a porphyrin core equipped with bromine and iodine reactive groups at orthogonal positions (Figure 2E). As shown in Figure 2F–I, they first created linear chains of porphyrin molecules by initial thermal activation of the iodine sites after heating to 120 °C (Step 1), and then 2D conjugated networks were synthesized by subsequent activation of the bromine sites after further heating to 250 °C (Step 2). The sequential strategy can maximize the possibility of each building block to adapt optimal conformations and avoid defect formation. Control of the growth-kinetics during carboncarbon formation leads to improved network quality and enables the fabrication of sophisticated multicomponent molecular architectures.

3.3. Surface-Catalyzed Polymerization Use of carbon–carbon coupling reactions such as Ullmann coupling and Glaser coupling is popular for forming sCOFs materials. The resulted sCOFs are expected to have great inplane π-conjugation and promise interesting electronic properties. However, the carbon–carbon bond formation is highly irreversible, which makes the formation of highly ordered sCOFs difficult. Coinage metals, such as Ag, Cu, and Au, are usually utilized as substrates to induce carbon–carbon covalent bond formation. It is generally recognized that the metal substrates serve two purposes: as static templates to confine polymerization reactions in two dimensions and as catalysts to be actively involved in reaction steps of the carbon–carbon covalent bond formation. The catalytic reactivity, diffusion and surface mobility of the metal substrates are major factors regulating the growth of sCOFs based on Ullmann coupling. Bieri et al. thoroughly exploited the impact of the nature of such metal surfaces on the carbon–carbon bond formation during the annealing process of a prototypical multidentate molecular precursor, hexaiodo-substituted macrocycle cyclohexa-m-phenylene (CHP), at different metal substrates, namely, Cu(111), Au(111) and Ag(111).[41] Based on STM measurements combined with density-functional theory (DFT) calculations, they found that higher reactivity but lower diffusion and mobility are shown on Cu(111) surface, compared with Au(111) and Ag(111) surfaces.

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Ag(111) surface manifests itself as a more-favorable substrate for the formation of extended regular 2D networks synergistically influenced by these major factors in the research system. Theoretical studies by Stafstrom and co-workers indicate the Cu(111), Au(111) and Ag(111) surfaces show different catalytic reactivities, diffusion barriers, and the impact of halogen byproducts, which is crucial in dehalogen-reaction-based covalent self-assembly on metal surfaces.[56] The elucidation of the reaction mechanism for surfacecatalyzed Ullmann coupling is important for the development of efficient protocols for growth of sCOFs. Two intermediate states (i.e., radicals and organometallics) have been proposed based on previous studies. Bieri et al. highlighted the prominent domains of uncoupled radicals surrounded by iodine atoms and domains of polymerized radicals in an STM image obtained by homo-coupling of CHP.[41] The observed iodine-iodine distance adjacent to single CHP radicals (CHPRs) varies with the calculated distance of the related Cu adatoms binding to the CHPRs, suggesting a free-radical intermediate. On the other hand, Lin and co-workers proposed an organometallic intermediate, a Cu atom linked by two terphenyl radicals via the C–Cu–C bridge in the debromination reactions of 4, 4’’-dibromo-p-terphenyl (Br-(ph)3-Br) on a Cu(111) surface, based on a joint computational and experimental study.[57] The Cu atoms are released and the organometallic intermediate is converted into the carbon–carbon coupling bond at a higher temperature. Zhang et al. investigated the potential of Glaser coupling between terminal alkynes to construct conjugated graphdiyne.[49] Thermoannealing of a densely packed phase of 1,3,5triethynyl-benzene (TEB) molecules formed at 170 K results in the deposition of the monomer before the coupling reaction occurs. Exposure of the substrate, kept at 330 K, to a high molecular flux induces the formation of a TEB dimer. Further annealing of the monolayer results in the hierarchical growth of disordered covalent networks. The result indicates that the formation of the sCOFs is strongly affected by the balance of the catalytic reactivity of the metal substrate and the diffusion/ surface mobility of the precursor. Although the detailed mechanism of the reaction still remains controversial,[58,59] manipulation of metal-catalyzed carbon–carbon coupling reactions to varied structures covering molecular wires, nanoribbons, and nanotroughs with high 2D order has been reported.[27,41,43,58,60] Right now, extending the knowledge from these results is promising for steering the synthesis of high-quality sCOFs by further optimization of the experimental conditions, such as substrate materials and precursor design, to control molecular diffusion and the catalytic activity of the covalent bond-forming process.

4. Other Synthetic Methods for sCOFs Similar to solid-surface-supported sCOF synthesis, the liquidair interface can be utilized for the synthesis of sCOFs. Michl and Magnera[44] designed a pedestal-mounted star connector, the sandwich anion lanthanum(III) bis[5,10,15,20-tetrakis(4-pyridyl) porphyrinate]. The lower deck is the pedestal and its pyridine moieties act as the tentacles to orient and confine the molecules on the surface of mercury, which provides




5. Application Prospects 5.1. Electronic and Optoelectronic Properties One of the promising applications of sCOFs is in organic electronics, as they can organize the functional building blocks in the materials in a well-defined manner. 2D sCOF materials with backbones containing an extended π-conjugation may exhibit interesting in-plane carrier-transportation behavior (Figure 3A), which can be a promising characteristic for applications in optoelectronic devices and electron transportation. Theoretical calculations have predicted the properties of 2D conjugated polymers for more than two decades. Semiconducting behavior and metallic conductivity might be obtained by tailoring the topology of the networks and the chemical properties of the building blocks.[62] Recent theoretical work indicated that the bandgap of 2D sCOFs can be significantly tuned by the crystalline size, and is much narrower than their 1D polymer

counterpart.[63] In addition, a free choice of prespecified locations, orientation, and the nature of the active groups, which could be light absorbers or emitters, electron donors or acceptors, charged or magnetic groups, non-linear optical chromophores, complexing or reactive groups, etc., endows sCOFs with related functions by general synthetic procedures.[15–23] It is worth mentioning that the crystalline size and orientated domains of sCOFs are crucial for the above-mentioned development to really take off. Therefore, it is imperative for researchers to pay great attention to the development of synthetic routes that allow 2D conjugated sCOFs to be generated with extended order and high uniformity, and the relationships between the structures and properties of the sCOFs to be exploited.

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firm adhesion combined with 2D translational and rotational freedom. The upper deck is the star connector, and its pyridine rings are the arms for the desired grid formation by linear coupling when treated with p-xylylene dibromide in absolute ethanol. Under the proper reaction conditions, it is slowly converted to 2D sCOFs with a grid structure. Recently, it has been shown that formation of 2D sCOFs by a solution-phase reaction is possible with elaborate precursor design. EI-Kaderi et al. showed that interlayer interactions between the COF monolayers could be weakened by integrating triptycene unit into the backbone of COFs.[45] Zhao and coworkers introduced two methyl groups into the 9 and 10-positions of triptycene tricatechol (TPTC) to further increase the distance between the monolayers, and therefore substantially suppress the interlayer interaction between the as-formed polymeric monolayers.[61] By this strategy, free-standing, monolayer 2D sCOFs in solution were obtained.

5.2. sCOF-Scaffold-Based Hybrid Materials Intriguingly, 2D sCOFs sheets can turn into scaffolds with position-defined anchor groups. As schematically shown in Figure 3B, certain anchor groups, either covalent or supramolecular bonding sites, modified in the z-direction of the sCOFs sheets can act as anchors to attach chemical entities of various kinds.[64] The defined anchor groups could be used for sensors, catalysis and molecular recognition, after capturing functional target chemical entities. For example, some monodispersed metal nanoparticles can be decorated on amino-functionalized sCOFs, and these serve as highly specific sensing platforms for the detection of heavy metal ions.[65] Realization of applications in photonic devices, optical devices, information storage, and sensor arrays could profit from such platforms as soon as knowledge is gained on how to decorate 2D polymers with useful ordered anchor groups or metal nanoparticles. In addition, 2D sCOF sheets can turn into a scaffold for the construction of structurally well-defined 3D materials by wrapping, rolling, folding, or face to face stacking. By placing one sheet on top of another in a layer-by-layer fashion, implementation of fabricating 3D bulk materials with a certain number of layers in a controllable way becomes feasible, which make it possible for systematic research on the relation of the number of layers and the intrinsic properties. Ultimately, the structural diversity enables sCOF monolayers to have many advanced functions and properties, such as semiconducting behavior or metallic conductivity, chemical stability, topological tunability, which would be advantageous for applications in sensors, and molecular electronics, nanofiltration, and catalysis. Furthermore, other dramatic and unpredictable performance is still on the way to be being discovered and exploited.

6. Conclusion and Outlook

Figure 3. Schematic illustration of the possible application of 2D sCOFs in electronic properties (A) and post-synthesis for their functionalization (B).


The crystallization problem is one of the most-important issues for the synthesis of graphene-like sCOFs and it is desirable that it is settled, as defects in sCOFs are detrimental to their functionality. We have outlined here several representative synthesis strategies effective for producing 2D sCOF nanostructures with extended order, including thermodynamic equilibrium



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manipulation methods, methods of controlling the growth kinetics, and surface-assisted covalent polymerization methods. It is apparent that bottom-up synthesis methods for 2D sCOFs enable both extended order and structural diversity, thus indeed opening up a new phase for the rational tailoring of sCOFs with high performance. Although the properties of sCOFs are only now being explored, the diversity of available building blocks and linking functional groups facilitates various sCOF functionalities to be utilized in applications ranging from electronic and optoelectronic devices, catalysis, and molecular recognition, to sensors. Thus, the design of building blocks and linkage reactions and the development of new synthetic methods are necessary to expand the sCOF family. The design of pores and skeletons or post-synthetic modification of prepared sCOFs provide more chances to construct sCOFs of various functions with free choice of pre-specified locations, orientation, and the nature of the active groups. Nevertheless, it is worth noting that the way to technological applications of sCOFs still faces the following challenges. First, only a small part of the huge pool of linkage chemistries has been used in high-quality sCOF synthesis, which is far from enough for fundamental research and technological applications. Second, the methods applicable in the synthesis of highquality 2D sCOFs are still not enough. The use of sCOFs as materials requires new methods to construct large-scale SCOFs with few defects and subsequently manipulate them. Most important of all, there are only a few examples of freestanding 2D sCOF sheets being obtained. The development of a transfer technique for 2D sCOFs onto an arbitrary substrate is urgent for both the fundamental understanding of the intrinsic properties of sCOFs and for further applications.

Acknowledgements This work was supported by the National Key Project on Basic Research (Grants2011CB808700, 2011CB932300), the National Natural Science Foundation of China (91023013, 21121063, 21127901), and the Chinese Academy of Sciences.

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Colloidal Covalent Organic Frameworks.

Designed synthesis of double-stage two-dimensional covalent organic frameworks.

Chemistry of Covalent Organic Frameworks.

Covalent organic frameworks: Crossing the channel.

Chemically stable multilayered covalent organic nanosheets from covalent organic frameworks via mechanical delamination.

Synthesis of Surface Covalent Organic Frameworks via Dimerization and Cyclotrimerization of Acetyls.

Bottom-up synthesis of chiral covalent organic frameworks and their bound capillaries for chiral separation.

Flatbands in 2D boroxine-linked covalent organic frameworks.

Isomeric routes to Schiff-base single-layered covalent organic frameworks.

Catalytic covalent organic frameworks via pore surface engineering.

Two-dimensional tetrathiafulvalene covalent organic frameworks: towards latticed conductive organic salts.

The preparation of metal-organic frameworks and their biomedical application.

Homochiral metal-organic frameworks and their application in chromatography enantioseparations.

Simultaneous construction of two linkages for the on-surface synthesis of imine-boroxine hybrid covalent organic frameworks.

Zeolite-like metal-organic frameworks (ZMOFs): design, synthesis, and properties.

Synthesis and characterization of functionalized metal-organic frameworks.

Colloidal-sized metal-organic frameworks: synthesis and applications.

Growth rates and water stability of 2D boronate ester covalent organic frameworks.

Towards covalent organic frameworks with predesignable and aligned open docking sites.

Electronic, structural, and substrate effect properties of single-layer covalent organic frameworks.

Synthesis of ordered carbonaceous frameworks from organic crystals.

From Highly Crystalline to Outer Surface-Functionalized Covalent Organic Frameworks--A Modulation Approach.

Photoelectric covalent organic frameworks: converting open lattices into ordered donor-acceptor heterojunctions.

3D microporous base-functionalized covalent organic frameworks for size-selective catalysis.