Beyond Molecules: Mesoporous Supramolecular Frameworks Self‐Assembled from Coordination Cages and Inorganic Anions

Biological function arises by the assembly of individual biomolecular modules into large aggregations or highly complex architectures. A similar strategy is adopted in supramolecular chemistry to assemble complex and highly ordered structures with advanced functions from simple components. Here we report a series of diamond‐like supramolecular frameworks featuring mesoporous cavities, which are assembled from metal‐imidazolate coordination cages and various anions. Small components (metal ions, amines, aldehydes, and anions) are assembled into the hierarchical complex structures through multiple interactions including covalent bonds, dative bonds, and weak C? H???X (X=O, F, and π) hydrogen bonds. The mesoporous cavities are large enough to trap organic dye molecules, coordination cages, and vitamin B₁₂. The study is expected to inspire new types of crystalline supramolecular framework materials based on coordination motifs and inorganic ions.     

Recent advances of hexaazatriphenylene (HAT) derivatives: Their applications in self-assembly and porous organic materials

As a rigid and planar aza-based heteroaromatic scaffold, hexaazatriphenylene (HAT) exhibits excellent electron-deficient property and high π-π stacking tendency, which makes it an ideal building block in the construction of supramolecular architectures and functional materials. In addition, HATs have also been picked out as building blocks for the construction of novel porous organic polymers, one of the most attractive fields of porous materials in the past decade, which includes intrinsic microporosity (PIMs), π-conjugated microporous polymers (CMPs), and covalent organic frameworks (COFs). In this digest paper, the synthetic methods of HAT derivatives have been briefly introduced and some recent advances of HATs in the applications of supramolecular self-assembly and porous organic materials have been highlighted.     

Viologen derivatives with extended π-conjugation structures: From supra-/molecular building blocks to organic porous materials

Recently, increasing attention has been paid on extending the π-conjugation structures of viologens (1,1′-disubstituted-4,4′-bipyridylium salts) by incorporating planar aromatic units into the bipyridinium backbones. Various viologen derivatives with extended π-conjugation structures have been synthesized, including the N-termini aromatic substituted viologens, the extended π-conjugated viologens (denoted as ECVs) as well as the π-conjugated oligomeric viologens (denoted as COVs). These compounds typically exhibit interesting properties distinguished from those of an isolated viologen unit, which make them as new class of electron deficient supra-/molecular building blocks in supramolecular chemistry and materials science. In this review, we would like to highlight the recent advances of viologen derivatives with extended π-conjugation structures in versatile applications ranging from electrochromic and energy storage materials, the ECV/COV-based supramolecular self-assembly systems including the linear supramolecular polymers and 2D/3D supramolecular organic frameworks (SOFs), to the viologen-based covalent organic frameworks (COFs)/networks. We hope this review will serve as an in-time summary worthy of referring, more importantly, to provide inspiration in the rational design of novel molecules with unexplored properties and functions.     

Metal–Covalent Organic Frameworks (MCOFs): A Bridge Between Metal–Organic Frameworks and Covalent Organic Frameworks

Many sophisticated chemical and physical properties of porous materials strongly rely on the presence of the metal ions within the structures. Whereas homogeneous distribution of metals is conveniently realized in metal–organic frameworks (MOFs), the limited stability potentially restricts their practical implementation. From that perspective, the development of metal–covalent organic frameworks (MCOFs) may address these shortcomings by incorporating active metal species atop highly stable COF backbones. This Minireview highlights examples of MCOFs that tackle important issues from their design, synthesis, characterization to cutting-edge applications.     

Metal–Covalent Organic Frameworks (MCOFs): A Bridge Between Metal–Organic Frameworks and Covalent Organic Frameworks

Many sophisticated chemical and physical properties of porous materials strongly rely on the presence of the metal ions within the structures. Whereas homogeneous distribution of metals is conveniently realized in metal–organic frameworks (MOFs), the limited stability potentially restricts their practical implementation. From that perspective, the development of metal–covalent organic frameworks (MCOFs) may address these shortcomings by incorporating active metal species atop highly stable COF backbones. This Minireview highlights examples of MCOFs that tackle important issues from their design, synthesis, characterization to cutting‐edge applications.     

Water‐Soluble Three‐Dimensional Polymers: Non‐Covalent and Covalent Synthesis and Functions†

Water‐soluble three‐dimensional (3D) polymers are structurally ideal for the construction of ordered porous materials for in‐situ and tunable loading and release of guests. For many years, studies on ordered porous materials have been confined to crystalline solids. Since 2014, self‐assembly has been developed as a robust strategy for the preparation of water‐soluble 3D polymers that possess defined and intrinsic porosity. Through the encapsulation of cucurbit[8]uril for aromatic dimers, ordered diamondoid supramolecular organic frameworks can be assembled from tetrahedral monomers. With [Ru(bipy)₃]²⁺‐derived octahedral complexes as precursors, cubic supramolecular metal‐organic frameworks have been assembled. One supramolecular organic framework has also been utilized to prepare the first homogeneous covalent organic framework through the [2+2] alkene cycloaddition, whereas the quantitative formation of the hydrazone bonds can be utilized to synthesize flexible porous organic frameworks. The new water‐soluble ordered and flexible polymeric frameworks are able to include drugs and biomacromolecules to accomplish in situ loading and intracellular delivery and to enrich photosensitizers and catalysts to enhance discrete visible light‐induced reactions. This review highlights the advances.     

Porous Solids Arising from Synergistic and Competing Modes of Assembly: Combining Coordination Chemistry and Covalent Bond Formation

Design and synthesis of porous solids employing both reversible coordination chemistry and reversible covalent bond formation is described. The combination of two different linkage modes in a single material presents a link between two distinct classes of porous materials as exemplified by metal–organic frameworks (MOFs) and covalent organic frameworks (COFs). This strategy, in addition to being a compelling material‐discovery method, also offers a platform for developing a fundamental understanding of the factors influencing the competing modes of assembly. We also demonstrate that even temporary formation of reversible connections between components may be leveraged to make new phases thus offering design routes to polymorphic frameworks. Moreover, this approach has the striking potential of providing a rich landscape of structurally complex materials from commercially available or readily accessible feedstocks.     

Research Progress in Covalent Organic Frameworks for Photoluminescent Materials

Covalent organic frameworks (COFs) are an emerging kind of crystalline porous polymers that present the precise integration of organic building blocks into extensible structures with regular pores and periodic skeletons. The diversity of organic units and covalent linkages makes COFs a rising materials platform for the design of structure and functionality. Herein, recent research progress in developing COFs for photoluminescent materials is summarised. Structural and functional design strategies are highlighted and fundamental problems that need to be solved are identified, in conjunction with potential applications from perspectives of photoluminescent materials.     

Covalent Organic Frameworks and Cage Compounds: Design and Applications of Polymeric and Discrete Organic Scaffolds

Porous organic materials are an emerging class of functional nanostructures with unprecedented properties. Dynamic covalent assembly of small organic building blocks under thermodynamic control is utilized for the intriguingly simple formation of complex molecular architectures in one‐pot procedures. In this Review, we aim to analyze the basic design principles that govern the formation of either covalent organic frameworks as crystalline porous polymers or covalent organic cage compounds as shape‐persistent molecular objects. Common synthetic procedures and characterization techniques will be discussed as well as more advanced strategies such as postsynthetic modification or self‐sorting. When appropriate, comparisons are drawn between polymeric frameworks and discrete organic cages in terms of their underlying properties. Furthermore, we highlight the potential of these materials for applications ranging from gas storage to catalysis and organic electronics.     

共价有机骨架材料在样品前处理中的研究进展

共价有机骨架（COFs）是由有机单体通过共价键连接形成的二维或三维晶体多孔结构。作为一种新兴的晶体多孔材料,COFs已经在气体储存、催化、传感、药物输送等各个领域广泛应用。近年来,COFs材料由于密度低、表面积大、结构可控等优点,在分析化学方面显示出巨大的潜力。该文综述了多孔COFs及其复合材料在样品前处理中的研究应用,包括分散固相萃取、固相微萃取和磁性固相萃取等。     

Recent advances of application of porous molecular cages for enantioselective recognition and separation

Porous materials with well‐defined pore structures have received considerable attention in the past decades due to their unique structures and wide applications. Most porous materials such as zeolites, metal‐organic frameworks, covalent organic frameworks, and porous organic polymers are extended to infinite frameworks or networks by robust covalent or coordination bonds. Porous molecular cages composed of discrete molecules with permanent cavities are an emerging class of porous material and the discrete molecules assemble into solids by weak intermolecular interaction. In comparison to porous extended solids such as metal‐organic frameworks and covalent organic frameworks, porous molecular cage solids are generally soluble in organic solvents thus allowing solution processing, making them more convenient to apply in many fields. This review mainly focuses on the recent advances of application of porous molecular cages (porous organic cages and metal‐organic cages) for enantioselective recognition and separation from 2010 to present, including gas chromatography, capillary electrochromatography, chiral fluorescent recognition, chiral potentiometric sensing, and enantioselective adsorption. Furthermore, the two important family members of porous molecular cages, porous organic cages and metal‐organic cages, are also discussed.     

Cucurbit[n]uril-calix[n]arene-based supramolecular frameworks assembled using the outer surface interactions of cucurbit[n]urils

Based on the crystal structures of two cucurbit[6]uril/calix[n]arene-based supramolecular frameworks reported by Long and co-workers,we further investigated the interactions of cucurbit[6]uril with 4-sulfocalix[4]arene and 4-sulfocalix[6]arene using ¹H NMR spectroscopy and isothermal titration calorimetry(ITC),respectively.Moreover,solid fluorescent materials were prepared via the adsorption of fluorescent dyes by these porous supramolecular frameworks,which exhibit a selective response to certain volatile organic compounds.     

Synthesis of Robust MOFs@COFs Porous Hybrid Materials via an Aza-Diels–Alder Reaction: Towards High-Performance Supercapacitor Materials

Metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) have attracted enormous attention in recent years. Recently, MOF@COF are emerging as hybrid architectures combining the unique features of the individual components to enable the generation of materials displaying novel physicochemical properties. Herein we report an unprecedented use of aza-Diels–Alder cycloaddition reaction as post-synthetic modification of MOF@COF-LZU1, to generate aza-MOFs@COFs hybrid porous materials with extended π-delocalization. A a proof-of-concept, the obtained aza-MOFs@COFs is used as electrode in supercapacitors displaying specific capacitance of 20.35 μF cm⁻² and high volumetric energy density of 1.16 F cm⁻³. Our approach of post-synthetic modification of MOFs@COFs hybrids implement rational design for the synthesis of functional porous materials and expands the plethora of promising application of MOFs@COFs hybrid porous materials in energy storage applications.     

Hydrogen‐Bonded Organic Frameworks (HOFs): A New Class of Porous Crystalline Proton‐Conducting Materials

Two porous hydrogen‐bonded organic frameworks (HOFs) based on arene sulfonates and guanidinium ions are reported. As a result of the presence of ionic backbones appended with protonic source, the compounds exhibit ultra‐high proton conduction values (σ) 0.75× 10^?2 S cm^?1 and 1.8×10^?2 S cm^?1 under humidified conditions. Also, they have very low activation energy values and the highest proton conductivity at ambient conditions (low humidity and at moderate temperature) among porous crystalline materials, such as metal–organic frameworks (MOFs) and covalent organic frameworks (COFs). These values are not only comparable to the conventionally used proton exchange membranes, such as Nafion used in fuel cell technologies, but is also the highest value reported in organic‐based porous architectures. Notably, this report inaugurates the usage of crystalline hydrogen‐bonded porous organic frameworks as solid‐state proton conducting materials.     

Applications of covalent organic frameworks (COFs):From gas storage and separation to drug delivery

Covalent organic frameworks (COFs) are an emerging class of porous covalent organic structures whose backbones were composed of light elements (B, C, N, O, Si) and linked by robust covalent bonds to endow such material with desirable properties, i.e., inherent porosity, well-defined pore aperture, ordered channel structure, large surface area, high stability, and multi-dimension. As expected, the above-mentioned properties of COFs broaden the applications of this class of materials in various fields such as gas storage and separation, catalysis, optoelectronics, sensing, small molecules adsorption, and drug delivery. In this review, we outlined the synthesis of COFs and highlighted their applications ranging from the initial gas storage and separation to drug delivery.     

共价有机骨架材料设计及其在分离领域的应用

共价有机骨架(COFs)材料是由有机小分子单体通过共价键连接形成的结晶多孔聚合物。与传统的线性聚合物不同的是,COFs可以在二维和三维空间上对其骨架结构进行控制,从而合成具有高度有序的刚性多孔结构,并且能够调节骨架的化学和物理性质。这种由COF形成的纳米级孔道和空间为分子存储、释放和分离提供了理想的环境。因此它在能量储存、分离、催化等领域有着广泛的应用前景。本文综述了近年来COFs材料的研究进展,主要包括材料的合成策略及其在分离领域的应用,并对COFs材料未来的发展方向进行了展望。     

Recent advances in metal‐organic frameworks and covalent organic frameworks for sample preparation and chromatographic analysis

In the field of analytical chemistry, sample preparation and chromatographic separation are two core procedures. The means by which to improve the sensitivity, selectivity and detection limit of a method have become a topic of great interest. Recently, porous organic frameworks, such as metal‐organic frameworks (MOFs) and covalent organic frameworks (COFs), have been widely used in this research area because of their special features, and different methods have been developed. This review summarizes the applications of MOFs and COFs in sample preparation and chromatographic stationary phases. The MOF‐ or COF‐based solid‐phase extraction (SPE), solid‐phase microextraction (SPME), gas chromatography (GC), high‐performance liquid chromatography (HPLC) and capillary electrochromatography (CEC) methods are described. The excellent properties of MOFs and COFs have resulted in intense interest in exploring their performance and mechanisms for sample preparation and chromatographic separation.     

Porous Organic Molecular Frameworks with Extrinsic Porosity: A Platform for Carbon Storage and Separation

The assembly of porous organic molecular frameworks (POMFs), which typically evidence common feasibility and compatibility, purification, and regeneration at practical conditions, remains a strategic challenge in modern materials science and is crucial for their favorable applications in biological, medical, and environmental realms. However, instructive knowledge of well‐organized POMF assembly by supramolecular interactions is, in general, ambiguous to date. Nevertheless, a significant advance in controlled POMF assembly has been recently achieved. This Minireview highlights these approaches, with a particular focus on the design of molecular constituents and assembly strategies. We also look beyond the field of solid‐state POMF materials into the assembly and recognition in solution, thus covering recent advances in POMFs based on material design and applications in carbon storage and separation.     

Multienzyme Biocatalytic Cascade Systems in Porous Organic Frameworks for Biosensing

Multienzyme biocatalytic cascade systems (MBCS) have attracted widespread research in the field of biosensing due to selective substrate transformations and signal amplification function. However, the poor stability of enzymes significantly restricts their effectiveness in practical applications. The spatial organization of MBCS within porous organic frameworks (POFs), such as metal–organic frameworks, covalent organic frameworks, and hydrogen-bonded organic frameworks, is regarded as a promising strategy to overcome these challenges. This advanced biotechnology sets up a POFs microenvironment for enzymes immobilization, and thus make it possible to shield the enzyme from the external stimulus by POFs-guided structural confinement. Simultaneously, the tailorable porous structure of POFs shell allows for the selective transport of substrates into interior enzymes, thereby accelerating the sensing process. Herein, we present the concept of this POFs-confined MBCS, wherein enzymes were completely encapsulated into, rather than adsorption onto, the POFs. We highlight the new strategies for MBCS spatial organization through rational POFs support, and describe how this new bio-nanosystem that integrates framework and enzymes functions can be designed as a versatile biosensing platform. In addition, the challenges and outlooks are also discussed.     

Metal–Organic Frameworks and Other Crystalline Materials for Ultrahigh Superprotonic Conductivities of 10−2 S cm−1 or Higher

Proton-conducting materials in the solid state have received immense attention for their role as electrolytes in proton-exchange membrane fuel cells. Recently, crystalline materials—metal–organic frameworks (MOFs), hydrogen-bonded organic frameworks (HOFs), covalent organic frameworks (COFs), polyoxometalates (POMs), and porous organic crystals—have become an exciting research topic in the field of proton-conducting materials. For a better electrolyte, a high proton conductivity on the order of 10⁻² S cm⁻¹ or higher is preferred as efficient proton transport between the electrodes is ultimately necessary. With an emphasis on design principles, this Concept will focus on MOFs and other crystalline solid-based proton-conducting platforms that exhibit “ultrahigh superprotonic” conductivities with values in excess of 10⁻² S cm⁻¹. While only a handful of MOFs exhibit such an ultrahigh conductivity, this quality in other systems is even rarer. In addition to interpreting the structural–functional correlation by taking advantage of their crystalline nature, we address the challenges and promising directions for future research.