Integrating Levels of Hierarchical Organization in Porous Organic Molecular Materials

Porous organic molecular materials(POMMs)are an emergent class of molecular-based materials characterized by the formation of extended porous frameworks,mainly held by non-covalent interactions.POMMs represent a variety of chemical families,such as hydrogen-bonded organic frameworks,porous organic salts,porous organic cages,C-H···πmicroporous crystals,supramolecular organic frameworks,π-organic frameworks,halogen-bonded organic framework,and intrinsically porous molecular materials.In some porous materials such as zeolites and metal organic frameworks,the integration of multiscale has been adopted to build materials with multifunctionality and optimized properties.Therefore,considering the significant role of hierarchy in porous materials and the growing importance of POMMs in the realm of synthetic porous materials,we consider it appropriate to dedicate for the first time a critical review covering both topics.Herein,we will provide a summary of literature examples showcasing hierarchical POMMs,with a focus on their main synthetic approaches,applications,and the advantages brought forth by introducing hierarchy.

Covalent organic frameworks as heterogeneous catalysts

Covalent organic frameworks （COFs）, established as an emerging class of crystalline porous polymers with high surface area, structural diversity, and esignability, attract much interest and exhibit potential applications in catalysis. In this review, we summarize the use of COFs as a versatile platform to develop heterogeneous catalysts for a variety of chemical reactions. Catalytic COFs are categorized in accordance with the types of active sites, involving single functional active sites, bifunctional active sites, and metal nanoparticles （NPs） embedded in pores. Special emphasis is placed on the deliberate or incidental synthesis strategies, the stability, the heterogeneity, and the shape/size selectivity for COF catalysis. Moreover, a description of the application of COFs as photocatalysts and electrocatalysts is presented. Finally, the prospects of COFs in catalysis and remaining issues in this field are indicated.

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

Recently,increasing atte ntion has been paid on extending theπ-conjugation structures ofviologens(1,1’-disubstituted-4,4’-bipyridylium salts)by incorporating planar aromatic units into the bipyridinium backbones.Various viologen derivative s with extendedπ-conjugation structures have been synthesized,including the N-termini aromatic substituted viologens,the extendedπ-conjugated viologens(denoted as ECVs)as well as theπ-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 extendedπ-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.

Template synthesis of imine-based covalent organic framework core-shell structure and hollow sphere:a case of COFTTA-DHTA

Controllably synthesizing well-dispersed covalent organic frameworks （COFs） with uniform both morphology and size is still a challenge. Herein, we report the template-directed synthesis of COFTTA-D.rA-based core-shell hybrids under solvothermal conditions by using amino-functionalized SiO2 microspheres as templates coupled with stepwise addition of initial monomer molecules. The modified amino groups on the surfaces of SiO2 templates play an important role in the formation of well-defined NH2-f-SiO2@COFTTA-DHXA core-shell hybrids. COFTTA-DHTA hollow spheres can be obtained by etching SiO2 cores of NH2-f-SiO2@COFTTA-DHTA. Both the NH2-f-SiO2@COFTTA-I）HTA and COFxTA-DHVA hollow spheres possess the well-defined morphology, high crystallinity and porosity, excellent dispersion property and high chemical stability. The template synthesis method demonstrated in this work provides a general method for the shape-controlled synthesis of COF-based materials, which is important for the further applications in the fields such as energy storage, drug delivery and catalysis.

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 abovementioned 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.

Assemblies of covalent organic framework microcrystals:multiple-dimensional manipulation for enhanced applications

Covalent organic frameworks (COFs) are well known as the next generation of shape-persistent zeolite analogues, which have brought new impetus to the development of porous organic materials as well as two-dimensional polymers. Since the advent of COFs in 2005, many striking findings have definitely proven their great potentials expanding applications across energy,environment and healthcare fields. With thorough exploration over a decade, research interest has been drawn on the scientific challenges on chemistry, while making full play of COF values has remained far from satisfactory yet. Thus opening an avenue to modulating COF assemblies on the multi-scale is no longer just an option, but a necessity for matching the application requirements with enhanced performances. In this mini-review, we summarize the recent progress on design of nanoscale COFs with varying forms. Detailed description is concentrated on the synthetic strategies of COF assemblies such as spheres, fibers,tubes, coatings and films, thereby shedding light on the flexible manipulation over dimensions, compositions and morphologies.Meanwhile, the advanced applications of nanoscale COFs have been discussed here with comparison of their bulky counterparts.

Spiro-fused bicyclo[3,2,2]octatriene-cored triptycene:synthesis,molecular packing,and functional aggregates

Aggregate science has led to the existence of functional properties at the aggregate level that significantly differs from those of corresponding single-molecule species,leading to a number of novel conceptual materials with state-dependent functionality.For developing such materials,new molecular scaffolds that allow modulating the structural and functional properties of aggregates in a controlled manner could be extremely helpful.In this study,we report a class of spiro-fused bicyclo[3,2,2]octatriene triptycene derivatives with a unique 3 D molecular scaffolding structure that displays aggregation-induced emission(AIE)activity with bright solid-state blue luminescence.The core scaffold features a Y-shaped bicyclo[3,2,2]octatriene backbone and a vertical crossover arrangement of adjacent spiro-conjugated rings.Unlike classic bicyclo[2,2,2]octatrienes with high rigidity,this stereoscopic bicyclo[3,2,2]octatriene containing a seven-membered ring allows retaining certain flexibility in the molecular scaffold,which results in a weak fluorescence in dilute solutions(Φ_(F)<10%).However,the corresponding molecular aggregates emit intense fluorescence(Φ_(F)=50%-75%)and show prominent AIE activity because of the restriction of intramolecular scaffold motions.Moreover,in principle,these stereoscopic scaffold constructions with partially restricted flexibility facilitate the assembly of multiple different crystalline packing structures in both porous and nonporous forms depending on the growth conditions of crystalline aggregates.The obtained crystals with regular voids(12.85×6.89Aeach)show reversible luminescence in response to the adsorption/desorption of iodine vapor.These materials based on an unusual 3 D scaffold exhibit a high degree of structural modulation at the aggregate level and demonstrate potential applications in chemical sensing and flexible optoelectronics,which may open up the exploration of highly stereoscopic molecular building blocks for developing functional aggregates.