Covalent organic frameworks in tribology - A perspective

Two-dimensional covalent organic frameworks (2D COFs) are an emerging class of crystalline porous materials formed through covalent bonds between organic building blocks. COFs uniquely combine a large surface area, an excellent stability, numerous abundant active sites, and tunable functionalities, thus making them highly attractive for numerous applications. Especially, their abundant active sites and weak interlayer interaction make these materials promising candidates for tribological research. Recently, notable attention has been paid to COFs as lubricant additives due to their excellent tribological performance. Our review aims at critically summarizing the state-of-art developments of 2D COFs in tribology. We discuss their structural and functional design principles, as well as synthetic strategies with a special focus on tribology. The generation of COF thin films is also assessed in detail, which can alleviate their most challenging drawbacks for this application. Subsequently, we analyze the existing state-of-the-art regarding the usage of COFs as lubricant additives, self-lubrication composite coatings, and solid lubricants at the nanoscale. Finally, critical challenges and future trends of 2D COFs in tribology are outlined to initiate and boost new research activities in this exciting field.

The role of aromaticity in the cyclization and polymerization of alkyne-substituted porphyrins on Au(111)

Aromaticity is an established and widely used concept for the prediction of the reactivity of organic molecules. However, its role remains largely unexplored in on-surface chemistry, where the interaction with the substrate can alter the electronic and geometric structure of the adsorbates. Here we investigate how aromaticity affects the reactivity of alkyne-substituted porphyrin molecules in cyclization and coupling reactions on a Au(111) surface. We examine and quantify the regioselectivity in the reactions by scanning tunnelling microscopy and bond-resolved atomic force microscopy at the single-molecule level. Our experiments show a substantially lower reactivity of carbon atoms that are stabilized by the aromatic diaza[18]annulene pathway of free-base porphyrins. The results are corroborated by density functional theory calculations, which show a direct correlation between aromaticity and thermodynamic stability of the reaction products. These insights are helpful to understand, and in turn design, reactions with aromatic species in on-surface chemistry and heterogeneous catalysis.

Rational design, structure properties, and synthesis strategies of dual-pore covalent organic frameworks (COFs) for potent applications: A review

Dual-pore covalent organic frameworks (COFs) offer a molecular scaffold for introducing building blocks into periodically organized polygonal skeletons to produce fascinating structural features. The rapid development of this material has attracted intensive interest from researchers with diverse expertise. This review selects the leading scientific findings about dual-pore COFs and highlights their functions and perspectives on design, structure properties, and synthesis strategies. Dual-pore COFs, as newly hetero-pore COFs by integrating particular pores into one polygonal skeleton, have been compared to conventional COFs. Dual-pore COFs display hierarchical/heterogeneous porosities and homogeneous porosity, which endow them with exceptional features involving mass diffusion, charge transfer, and large surface area with abundant active sites. Additionally, the strategic dual-pore design by opting for different approaches, such as integration of [D_2h + C₂] symmetries, kagome-type lattices, and other symmetric arrangements of monomers, are inclusively discussed. Identification and construction of dual-pores in COFs via optimal synthetic methods, such as desymmetrization, multiple linking sites, and orthogonal reactions, are highlighted as the primary pore engineering routes to simultaneously regulate the growth and alter the characteristics of COFs for promising applications. Lastly, a focused discussion on various challenges and critical fundamentals of dual-pore engineering is successfully outlined, with potential prospects of introducing dual-pore in COFs.

Constitutional isomerism of the linkages in donor–acceptor covalent organic frameworks and its impact on photocatalysis

When new covalent organic frameworks (COFs) are designed, the main efforts are typically focused on selecting specific building blocks with certain geometries and properties to control the structure and function of the final COFs. The nature of the linkage (imine, boroxine, vinyl, etc.) between these building blocks naturally also defines their properties. However, besides the linkage type, the orientation, i.e., the constitutional isomerism of these linkages, has rarely been considered so far as an essential aspect. In this work, three pairs of constitutionally isomeric imine-linked donor-acceptor (D-A) COFs are synthesized, which are different in the orientation of the imine bonds (D-C=N-A (DCNA) and D-N=C-A (DNCA)). The constitutional isomers show substantial differences in their photophysical properties and consequently in their photocatalytic performance. Indeed, all DCNA COFs show enhanced photocatalytic H₂ evolution performance than the corresponding DNCA COFs. Besides the imine COFs shown here, it can be concluded that the proposed concept of constitutional isomerism of linkages in COFs is quite universal and should be considered when designing and tuning the properties of COFs.

Systematic investigation of isomerism in covalent organic frameworks (COFs) can provide key insights into their properties. Here, the authors reveal that the constitutional isomerism of the linkage i.e., linkage orientations distinctly impact COFs’ structural and photophysical properties.

Recent advances in the tuning of the organic framework materials - The selections of ligands, reaction conditions, and post-synthesis approaches

Organic framework materials, particularly metal-organic frameworks (MOFs), graphene-organic frameworks (GOFs), and covalent organic frameworks (COFs), have led to the revolution across fields including catalysts, sensors, gas capture, and biology mainly owing to their ultra-high surface area-to-volume ratio, on-demand tunable crystal structures, and unique surface properties. While the wet chemistry routes have been the predominant synthesis approach, the crystal phase, morphological parameters, and physicochemical properties of organic framework materials are largely affected by various synthesis parameters and precursors. In this work, we specifically review the influences of synthesis parameters towards crystal structures and chemical compositions of organic framework materials, including selected ligand types and lengths, reaction temperature/solvent/reactant compositions, as well as post-synthesis modification approaches. More importantly, the subsequent impacts on the general electronic, mechanical, surface chemical, and thermal properties as well as the consequent variation in performances towards catalytic, desalination, gas sensing, and gas storage applications are critically discussed. Finally, the current challenges and prospects of organic framework materials are provided.

Covalent Organic Frameworks: Design, Synthesis, and Functions

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure-function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.

Bulk COFs and COF nanosheets for electrochemical energy storage and conversion

The current advances, structure-property relationship and future perspectives in covalent organic frameworks (COFs) and their nanosheets for electrochemical energy storage (EES) and conversion (EEC) are summarized.

Surface-confined single-layer covalent organic frameworks: design, synthesis and application

This review describes the state of the art of surface-confined single-layer covalent organic frameworks, focusing on reticular design, synthesis approaches, and exploring applications in host/guest chemistry.

Covalent Organic Frameworks: Chemical Approaches to Designer Structures and Built-In Functions

A new approach has been developed to design organic polymers using topology diagrams. This strategy enables covalent integration of organic units into ordered topologies and creates a new polymer form, that is, covalent organic frameworks. This is a breakthrough in chemistry because it sets a molecular platform for synthesizing polymers with predesignable primary and high-order structures, which has been a central aim for over a century but unattainable with traditional design principles. This new field has its own features that are distinct from conventional polymers. This Review summarizes the fundamentals as well as major progress by focusing on the chemistry used to design structures, including the principles, synthetic strategies, and control methods. We scrutinize built-in functions that are specific to the structures by revealing various interplays and mechanisms involved in the expression of function. We propose major fundamental issues to be addressed in chemistry as well as future directions from physics, materials, and application perspectives.

Controlling a Chemical Coupling Reaction on a Surface: Tools and Strategies for On-Surface Synthesis

On-surface synthesis is appearing as an extremely promising research field aimed at creating new organic materials. A large number of chemical reactions have been successfully demonstrated to take place directly on surfaces through unusual reaction mechanisms. In some cases the reaction conditions can be properly tuned to steer the formation of the reaction products. It is thus possible to control the initiation step of the reaction and its degree of advancement (the kinetics, the reaction yield); the nature of the reaction products (selectivity control, particularly in the case of competing processes); as well as the structure, position, and orientation of the covalent compounds, or the quality of the as-formed networks in terms of order and extension. The aim of our review is thus to provide an extensive description of all tools and strategies reported to date and to put them into perspective. We specifically define the different approaches available and group them into a few general categories. In the last part, we demonstrate the effective maturation of the on-surface synthesis field by reporting systems that are getting closer to application-relevant levels thanks to the use of advanced control strategies.

Recent advances in covalent organic frameworks (COFs) as a smart sensing material

Recent advances in covalent organic frameworks (COFs) as a smart sensing material are summarized and highlighted.

The impact of grafted surface defects on the on-surface Schiff-base chemistry at the solid-liquid interface

We investigate the effect of covalently modified graphitic surfaces on the formation of single-layer covalent organic frameworks (sCOFs) at the solid-liquid interface. The impact of different densities of grafted aryl species was tested on the on-surface synthesis of three distinct imine-based 2D sCOFs. The grafted aryl species that act as defects provide steric barriers to the progress of the Schiff-base reaction, and can be locally removed to start the 2D polymer healing process. This unique strategy provides a general approach to study in situ this dynamic covalent on-surface chemistry.

Reversibility and intermediate steps as key tools for the growth of extended ordered polymers via on-surface synthesis

Surface-confined polymerization is a bottom-up strategy to create one- and two-dimensional covalent organic nanostructures with a π-conjugated backbone, which are suitable to be employed in real-life electronic devices, due to their high mechanical resistance and electronic charge transport efficiency. This strategy makes it possible to change the properties of the final nanostructures by a careful choice of the monomer architecture (i.e. of its constituent atoms and their spatial arrangement). Several chemical reactions have been proven to form low-dimensional polymers on surfaces, exploiting a variety of precursors in combination with metal (e.g. Cu, Ag, Au) and insulating (e.g. NaCl, CaCO₃) surfaces. One of the main challenges of such an approach is to obtain nanostructures with long-range order, to boost the conductance performances of these materials. Most of the exploited chemical reactions use irreversible coupling between the monomers and, as a consequence, the resulting structures often suffer from poor order and high defect density. This review focuses on the state-of-the-art surface-confined polymerization reactions, with particular attention paid to reversible coupling pathways and irreversible processes including intermediate states, which are key aspects to control to increase the order of the final nanostructure.

The intramolecular H-bonding effect on the growth and stability of Schiff-base surface covalent organic frameworks

The introduction of intramolecular H-bonding by adding -OH functionalities adjacent to the Schiff base centers is considered to be a useful strategy to enhance the stability and crystallinity of bulk covalent organic frameworks (COFs). However, the influence of intramolecular H-bonding on the synthesis of surface COFs (SCOFs) have been barely explored. Herein, SCOFs based on the Schiff-base reaction between 1,3,5-tris(4-aminophenyl)benzene (TAPB) and terephthalaldehydes with symmetry or asymmetrically substituted hydroxyl functional groups are designed. In the absence of a solvent, hydroxyl substituents can be easily oxidized; thus argon protection is required to obtain high-quality SCOFs. Besides, an extended network with uniform pores can be achieved in spite of the symmetry of substituents. Both experimental results and theoretical calculations show that the influence of intramolecular hydrogen bonding on surface synthesis is not as important as that in bulk phase synthesis because the substrate itself can lead to the complanation of adsorbed molecules. The existence of intramolecular H-bonding can enhance the stability of the network in both acid and alkali environments.

Construction of boronate ester based single-layered covalent organic frameworks

Boronate ester based single-layered covalent organic frameworks (sCOFs) with large domain areas and uniform pore sizes have been fabricated on graphite under an ambient atmosphere. The phase separation to generate the boronate ester based sCOFs, boroxine based sCOFs and other nanostructures could be tuned using the molecular ratio of the two precursors, demonstrating a self-sorting process for on-surface dynamic covalent chemistry.

Surface modification and pattern formation by nucleobases and their coordination complexes

This review presents recent progress concerning the organization of nucleobases on highly ordered pyrolytic graphite (HOPG), mica, Cu(110) and Au(111) surfaces, followed by their studies using microscopy methods such as atomic force microscopy (AFM), scanning tunneling microscopy (STM) and transmission electron microscopy (TEM). Interesting research prospects related to surface patterning by nucleobases, nucleobase-functionalized carbon nanotubes (CNTs) and metal–nucleobase coordination polymers are also discussed, which offer a wide array of functional molecules for advanced applications. Nucleobases and their analogs are able to invoke non-covalent interactions such as π–π stacking and hydrogen bonding, and possess the required framework to coordinate metal ions, giving rise to fascinating supramolecular architectures. The latter could be transferred to conductive substrates, such as HOPG and gold, for assessment by high-end tunneling microscopy under various conditions. Clear understanding of the principles governing nucleobase self-assembly and metal ion complexation, and precise control over generation of functional architectures, might lead to custom assemblies for targeted nanotechnological and nanomaterial applications.

This review highlights recent advancements in surface patterning of nucleobases, their analogs including nucleobase-CNT hybrids and metal complexes, using various microscopy techniques for nanotechnological applications.

Covalent Organic Frameworks: From Materials Design to Biomedical Application

Covalent organic frameworks (COFs) are newly emerged crystalline porous polymers with well-defined skeletons and nanopores mainly consisted of light-weight elements (H, B, C, N and O) linked by dynamic covalent bonds. Compared with conventional materials, COFs possess some unique and attractive features, such as large surface area, pre-designable pore geometry, excellent crystallinity, inherent adaptability and high flexibility in structural and functional design, thus exhibiting great potential for various applications. Especially, their large surface area and tunable porosity and π conjugation with unique photoelectric properties will enable COFs to serve as a promising platform for drug delivery, bioimaging, biosensing and theranostic applications. In this review, we trace the evolution of COFs in terms of linkages and highlight the important issues on synthetic method, structural design, morphological control and functionalization. And then we summarize the recent advances of COFs in the biomedical and pharmaceutical sectors and conclude with a discussion of the challenges and opportunities of COFs for biomedical purposes. Although currently still at its infancy stage, COFs as an innovative source have paved a new way to meet future challenges in human healthcare and disease theranostic.

Concentration-Directed Polymorphic Surface Covalent Organic Frameworks: Rhombus, Parallelogram, and Kagome

Polymorphic single-layered covalent organic frameworks (sCOFs) via on-surface synthesis have been investigated by employing the tetradentate monomer 1,3,6,8-tetrakis(p-formylphenyl)pyrene with D_2h symmetry and ditopic linear diamine building blocks. Three kinds of well-ordered sCOFs, including rhombus, parallelogram, and Kagome networks, are observed on the graphite surface by scanning tunnel microscopy. The pore size and periodicity of sCOFs are tunable by employing diamine monomers with different lengths. Statistical analysis reveals that two types of quadrate networks are preferred at high concentration, whereas the occupancy of Kagome networks increases at low concentration. This trend can be understood by the differences in the network density of three kinds of networks. The reversibility and the self-sorting ability of the dynamic covalent reaction make it possible to control the polymorphic distribution similar to the principle demonstrated in supramolecular self-assembly.

Design Principles for Covalent Organic Frameworks in Energy Storage Applications

Covalent organic frameworks (COFs) are an exciting class of porous materials that have been explored as energy-storage materials for more than a decade. This review discusses efforts to develop these materials for applications in gas and electrical power storage. Some of the design strategies for developing the gas sorption properties of COFs and mechanistic studies on their formation are also discussed.

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocatalysis, batteries, supercapacitors, solar cells, photocatalysis, and sensing platforms. Finally, the challenges and outlooks in this promising field are featured on the basis of its current development.

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

The orthogonality between the Schiff base reaction and the boronic acid dehydration reaction is explored during the on-surface synthesis process. By activating the above two reactions in one-step and employing asymmetrical substituted monomers and the 3-fold symmetric monomer 1,3,5-tris(4-aminophenyl)benzene (TAPB), highly ordered imine-boroxine hybrid single-layered covalent organic frameworks (sCOFs) have been successfully constructed on HOPG by a gas-solid interface reaction method and characterized by scanning tunnelling microscopy (STM). In particular, the reaction between the meta-substituted monomer and TAPB generates sCOFB with a windmill structure, which is the first sCOF with surface chirality so far reported. The demonstration of the one-step synthesis of multiple linkages to form sCOFs can further enlarge the sCOF family and expand the design routes for functional 2D organic nanomaterials.

Covalent organic frameworks based on Schiff-base chemistry: synthesis, properties and potential applications

Covalent organic-frameworks (COFs) are an emerging class of porous and ordered materials formed by condensation reactions of organic molecules.

MasterChem: cooking 2D-polymers

This review summarized the current state-of-the-art in the field of synthetic 2D-polymers showing different routes to prepare these polymers and their physical properties.

An azine-linked hexaphenylbenzene based covalent organic framework

We report an azine linked covalent organic framework based on hexaphenylbenzene monomer functionalized with aldehyde groups (“HEX-COF 1”, avg. pore size = 1 nm, surface area >1200 m² g⁻¹, sorption capability at 273 K, 1 atm = 20 wt% for CO² and 2.3 wt% for CH₄).

Tunable porosity of 3D-networks with germanium nodes

Eight hyper cross-linked polymers based on tetrakis(4-ethynylphenyl)germanium and tetrakis(4-ethynylphenyl)methane are presented.

S-Shaped Conformation of the Quaterthiophene Molecular Backbone in Two-Dimensional Bisterpyridine-Derivative Self-Assembled Nanoarchitecture

The conformation and the two-dimensional self-assembly of 4'-(3',4″-dihexyloxy-5,2':5',2″:5″,2‴-quaterthien-2,5‴-diyl)-bis(2,2':6',2″-terpyridine) molecules are theoretically and experimentally investigated. This molecular building block forms a hydrogen-bonded chiral supramolecular nanoarchitecture on graphite at the solid/liquid interface. Scanning tunneling microscopy (STM) shows that the molecule adopts an S-shaped conformation in this structure. DFTB+ calculations reveal that this conformation is not the lowest-energy conformation. The molecular nanoarchitecture appears to be stabilized by hydrogen bonding as well as van der Waals interactions. I-, L-, and D-shaped molecular conformations are, however, locally observed at the domain boundary, but these conformations do not self-assemble into organized 2D structures.