Commentary

Porous materials are classified according to their structural compositions as inorganic porous materials (zeolites, carbon, etc.), inorganic–organic hybrid porous materials (MOFs, CPs, etc.), and organic porous materials. Organic porous materials, which are emerging as a novel functional platform, are a new research hotspot in the fields of physics, chemistry, and material science. Organic porous materials have high surface areas, tunable architectures, well-defined porosities, and easy machinability as a result of the advantages of both porous solids and polymers. Furthermore, a variety of synthetic techniques make it possible to design and prepare a wide range of organic porous materials that incorporate key physical and chemical properties into the porous skeleton or at the pore surface. Significant progress has been made in the use of these materials in gas storage and separation, catalysis, energy storage, sustained drug release, and a variety of other applications. Despite the rapid progress, the diverse talents involved in this research encourage the development of a universal strategy for synthesising organic porous materials with customised pore structures and specific functionalities.

Porous aromatic frameworks (PAFs), which are well-known for their large surface areas, associated porosity, diverse structures, and excellent stability, have recently piqued the interest of a wide range of researchers. To meet the necessary requirements, customised porous architectures can be prepared exclusively through covalent bonding using widely available building blocks and various coupling strategies. Furthermore, PAFs are made up of phenyl-ring-derived fragments that can be easily modified with desired functional groups using established synthetic chemistry techniques. This review, based on material design and preparative chemistry, focuses on recent advances in the structural and chemical properties of PAFs for potential applications such as molecule storage, gas separation, catalysis, and ion extraction. Because they are primarily composed of 2D/3D periodic aromatic frameworks, PAFs are formed by the effective assembly of organic building blocks via covalent coupling reactions. Unlike conventional COFs, which are formed through reversible organic condensation reactions, PAFs are formed through irreversible cross-coupling reactions (generally, C–C coupling) and, as a result, have high surface areas, open architectures, robust skeletons, and excellent stabilities. The uncorrected orientation of condensed oligomeric fragments and the free rotation of polyhedral monomers result in framework defects and an irregular internal structure. PAF materials with rigid building blocks, topology-oriented construction, short-range ordered structure, superb stability, and intrinsic porosity share unordered structures but differ noticeably from other cross-linked polymers, such as HCPs. The target synthesis of functional PAF materials has harvested the rapid development by conditioning the surface area, pore size, and functionalization sites on the basis of this solid foundation. Typically, the specific features can be controlled by the incorporation of building blocks with precise shape, size, hybridization, or heterocyclic units, followed by simple synthetic methodologies to convert monomers into cross-linked textures.

It is impossible for the field to achieve high yields and great homogeneity, multivariate behaviour, surface area, and hybrid materials without a universal methodology. Concerning the distinct superiority of each strategy, this review focuses on the benefits of post synthetic functionalization for preparing well- defined porous structures and addresses ongoing efforts to establish function-oriented design methods for PAF products. A large surface area is an intriguing property of porous materials because it provides accessible space for guest molecule storage. However, challenging the conventional routine for high porosity construction is difficult. Inspired by the study of PAF-1, our group designed and synthesised two PAF materials, PAF-100 and PAF- 101, using an engineering-specific building unit strategy. PAF-100 and PAF-101 have high BET surface areas that exceed 5000 m2 g–1 and uniform pore size distributions. PAFs combine the significant advantages of inorganic materials and inorganic–organic hybrids, resulting in a tuneable pore environment and physicochemical stability. Their open pores allow various functional groups or molecular assemblies to decorate them further. The high stability and easily modifiable chemistry encourage the incorporation of various desired chemical functionalities in a simple and dense manner. PAF phenyl skeletons can be densely functionalized after post synthetic functionalization, allowing for advanced applications.

PAFs with tailorable compositions, structure, and pore environments can serve as advanced platforms for other far- reaching applications not seen in MOFs/COFs, such as removal of contaminants (especially organic pollutants) and extraction of precious metals from an aquatic environment, in addition to their widely investigated applications such as molecular storage, gas separation, and catalysis. Furthermore, the excellent stability and compatibility of PAFs results in a high level of ease of operation and flexibility for large-scale coatings, films, and membranes in antibacterial devices, gas separation, nuclear material capture, and nuclear waste remediation applications. Furthermore, efforts should be directed toward the scalable preparation of PAFs for industrial mass production using mild and low-cost methods.