Large-Scale Synthesis of Covalent Organic Frameworks: Challenges and Opportunities

Connecting organic building blocks by covalent bonds to design porous crystalline networks has led to covalent organic frameworks (COFs), consequently transferring the flexibility of dynamic linkages from discrete architectures to extended structures. By virtue of the library of organic building blocks and the diversity of dynamic linkages and topologies, COFs have emerged as a novel field of organic materials that propose a platform for tailor-made complex structural design. Progress over the past two decades in the design, synthesis, and functional exploration of COFs in diverse applications successively established these frameworks in materials chemistry. The large-scale synthesis of COFs with uniform structures and properties is of profound importance for commercialization and industrial applications; however, this is in its infancy at present. An innovative designing and synthetic approaches have paved novel ways to address future hurdles. This review article highlights the fundamental of COFs, including designing principles, coupling reactions, topologies, structural diversity, synthetic strategies, characterization, growth mechanism, and activation aspects of COFs. Finally, the major challenges and future trends for large-scale COF fabrication are outlined.


Introduction
The covalent chemistry of organic molecules has matured throughout the 20th century and at the core of many important advances in science.The synthesis of chemicals, polymers, and pharmaceuticals via the modification of organic molecules through stable covalent bonds have fundamentally changed our way of life [1,2].The fabrication of well-defined macromolecules with precise primary and high-order structures, as found in biological polymers, such as proteins and enzymes, is challenging [3,4].Biological polymers with a well-defined structure amalgamate the chemistry of covalent bonds to understand primary-order chain structure and intermolecular force of attraction to shape up the high-order morphology [5,6].Inspired by biological polymer systems, the role of covalent bonds and non-covalent interactions in achieving synthetic primary and highorder structure with predesigned functionalities is significant to address fundamental challenges, thus expanding chemistry of hierarchical structures.Covalent organic frameworks (COFs) are porous and crystalline polymers discovered nearly two-decades ago by stitching molecular building blocks together through the covalent bonds and noncovalent interactions in the polymerization systems [7][8][9][10].As stated by Nobel laureate Roald Hoffmann: "Organic chemists are masterful at exercising control in the zero dimensions.One subculture of organic chemists has learned to exercise control in one dimension.These are polymer chemists, the chain builders. ... But in two or three dimensions, it's a synthetic wasteland" [11].In 2005, Yaghi and co-workers made a breakthrough in successfully connecting boronic acid-and catechol-based building blocks to an extended porous crystalline boroxine-and boronic-ester-linked COFs using the principle of dynamic covalent chemistry [10].Since then, the exponential rate of frameworks grown via the emergence of novel dynamic linkages, such as imine, hydrazone, imide, azine, and β-ketoenamine, is extraordinary (Figure 1) [12][13][14][15].27], energy storage [28][29][30], biomedical applications [31][32][33], and other app 36].In recent times, a number of notable reviews have summarized diffe spectives, especially in the area of reticular chemistry [37][38][39], covalent chem pore surface engineering [16][17][18]42], and diverse applications [43][44][45][46] (Table fundamental aspects of the large-scale synthesis of COFs have not been rev review, we cover recent advances in the fundamental concepts and summa chemistry, design principles, topologies, growth mechanism, synthetic a strategies, and scale-up fabrication and predict the future directions from spectives to boost commercialization.A crystalline framework with exclusive conformation and morphology generates a confined molecular space (pores) with the accessibility of atoms facilitating host-guest interactions.Furthermore, the porosity and structure stability are of profound importance to expand functional development via post-synthetic modification [16][17][18].Pore surface engineering permits the steric and electronic tunability of the pore environment by using the principles of organic and organometallic chemistry.Based on these properties, COFs are at the forefront in heterogeneous catalysis [19][20][21], environmental remediation [22][23][24][25][26][27], energy storage [28][29][30], biomedical applications [31][32][33], and other applications [34][35][36].In recent times, a number of notable reviews have summarized different COF perspectives, especially in the area of reticular chemistry [37][38][39], covalent chemistry [9,40,41], pore surface engineering [16][17][18]42], and diverse applications [43][44][45][46] (Table 1), but to date, fundamental aspects of the large-scale synthesis of COFs have not been reviewed.In this review, we cover recent advances in the fundamental concepts and summarizes coupling chemistry, design principles, topologies, growth mechanism, synthetic and activation strategies, and scale-up fabrication and predict the future directions from multiple perspectives to boost commercialization.

Coupling Chemistry
Coupling chemistry refers to a variety of organic reactions where two fragments are stitched together.COFs are built from organic building blocks via reversible condensation reactions.This micro-reversibility prevents the formation of disordered amorphous kinetic products and bolsters thermodynamically stable crystalline covalent networks.Furthermore, the reversibility in bond formation imparts self-healing and error-correction during crystallization.Throughout the reversible covalent bond formation and extension, if any bond formation happens in an undesired direction, the system can repair it through a back reaction and bond reformation, thereby supporting crystalline thermodynamic products with the lowest free energy.However, the condition for reversible covalent bond formation can be achieved only at a very-high temperature and pressure owing to the higher covalent bond energies (50-100 kcal mol −1 ).At lower temperatures, kinetically controlled disordered polymeric products are seen to dominate; thus, it is difficult to construct the ordered covalent network solids under ambient reaction conditions.Moreover, the desired thermodynamic reaction pathways demand a very-high activation energy.

Design Principles and Topologies
Linking organic building blocks toward topology-directed framework growth occurs in concurrence with the geometry of building blocks.A high-order crystalline structure with unique conformation and morphology relies on the directional nature of covalent bonds.A covalent bond is a chemical bond made by sharing electrons between atoms.Organic molecule synthesis exhibits the full benefits of the directionality of covalent bonds, as exemplified by the synthesis of vitamin B12 [121,122].Organic building blocks with rigid backbone and distribution of reactive sites in a distinct geometry ensure directional bonding.Furthermore, building blocks of particular symmetricity and point group guide spatial orientation and determine the relative position of the repeated units (chain growth direction) that firmly follow the predesigned topology.The principle of directional bonding is established in the construction of transition metal-based discrete architectures, such as metalla-rectangle and metalla-prism [123,124].The stitching of symmetrical organic building blocks via dynamic linkages affords 2D atomic layers with specific topology.The periodic growth of a 2D layer under the influence of noncovalent interactions inherently generates discrete nanopores.This bestows a well-defined hierarchical system with spatial lattice orientation and crystallinity through controlled interlayer interactions.The high-order structure with an accessible one-dimension open channels are primarily dependent on the geometry of building blocks often referred to as bottom-up approach.The topology of COFs with varying pore shapes and sizes can be understood to have different permutations and combinations of building blocks.For instance, the combination of [C2 + C2 + C2], [C3 + C2], and [C3 + C3] symmetric organic building blocks form hexagonal COFs with different pore sizes and stacking patterns [125,126].However, [C4 + C4] and [C4 + C2] linkers afford a tetragonal framework of varying pore size (Figure 4).

B-O Boronate Ester
Excellent crystallinity and thermal stability (600 • C), but sensitive to water, acid, and base.

Boroxine
Excellent crystallinity and thermal stability (500 • C), but sensitive to water, acid, and base.

Imine
Good crystallinity and excellent thermal (500 • C) and chemical stability.

Hydrazone Imide
Good crystallinity and thermal stability (300 • C), but better chemical stability as compared to imine linkages.Good crystallinity and chemical stability, but excellent thermal stability (500 • C).

Design Principles and Topologies
Linking organic building blocks toward topology-directed framework growth occurs in concurrence with the geometry of building blocks.A high-order crystalline structure with unique conformation and morphology relies on the directional nature of covalent bonds.A covalent bond is a chemical bond made by sharing electrons between atoms.Organic molecule synthesis exhibits the full benefits of the directionality of covalent bonds, as exemplified by the synthesis of vitamin B 12 [121,122].Organic building blocks with rigid backbone and distribution of reactive sites in a distinct geometry ensure directional bonding.Furthermore, building blocks of particular symmetricity and point group guide spatial orientation and determine the relative position of the repeated units (chain growth direction) that firmly follow the predesigned topology.The principle of directional bonding is established in the construction of transition metal-based discrete architectures, such as metalla-rectangle and metalla-prism [123,124].The stitching of symmetrical organic building blocks via dynamic linkages affords 2D atomic layers with specific topology.The periodic growth of a 2D layer under the influence of noncovalent interactions inherently generates discrete nanopores.This bestows a well-defined hierarchical system with spatial lattice orientation and crystallinity through controlled interlayer interactions.The highorder structure with an accessible one-dimension open channels are primarily dependent on the geometry of building blocks often referred to as bottom-up approach.The topology of COFs with varying pore shapes and sizes can be understood to have different permutations and combinations of building blocks.For instance, the combination of , and [C 3 + C 3 ] symmetric organic building blocks form hexagonal COFs with different pore sizes and stacking patterns [125,126].However, [C 4 + C 4 ] and [C 4 + C 2 ] linkers afford a tetragonal framework of varying pore size (Figure 4).Notably, either the , and [C 4 + C 2 ] yield mesoporous COFs (2-50 nm).The symmetricity, point group, and structural features of organic building blocks bolster interlayer (π-π) interactions and facilitates COF topology.
Apart from 2D COFs, a 3D COF design entails at least one tetrahedral (T d ) or orthogonal geometry of building blocks to enable the extension of the backbone into a 3D network.For instance, the stitching of tetrahedral or orthogonal nodes with C 1 , C 2 , C 3 , C 4 , and tetragonal organic building blocks produce 3D COFs (Figure 4) [45,95] The dia net among ctn, bor, srs, rra, pts, she, scu, bcu, fjh, and other networks constitute the largest family of 3D COFs due to the diversity and library of organic building blocks, which in turn emphasize the pivotal role of novel linkers and dynamic linkages.Notably, the mechanistic study and crystallization complications due to imbalance between strong covalent bonds and reversible linkages are still unexplored.Furthermore, 3D COFs suffer from lower porosity due to multifold interpenetration, which is one of the critical tasks in terms of design and construction perspectives.These fundamentals restrict the library of frameworks with unique dynamic linkages and an electronic and steric environment.
tetragonal organic building blocks produce 3D COFs (Figure 4) [45,95].The ctn network can be obtained by linking [Td + C3] and [Td + Td] symmetric building blocks, whereas a bor network can be obtained by [Td + C3].The pts network can be formed by either [Td + C2] or [Td + C4], in which the C2 or C4 symmetric unit exhibits four reactive sites.In addition, the dia and srs networks can be formed by [Td + C2] and [Td + C3], respectively.The dia net among ctn, bor, srs, rra, pts, she, scu, bcu, fjh, and other networks constitute the largest family of 3D COFs due to the diversity and library of organic building blocks, which in turn emphasize the pivotal role of novel linkers and dynamic linkages.Notably, the mechanistic study and crystallization complications due to imbalance between strong covalent bonds and reversible linkages are still unexplored.Furthermore, 3D COFs suffer from lower porosity due to multifold interpenetration, which is one of the critical tasks in terms of design and construction perspectives.These fundamentals restrict the library of frameworks with unique dynamic linkages and an electronic and steric environment.

Pore Shapes and Structures
Numerous building blocks with unique geometrical and structural features enable the design of COFs with varying pore shapes and sizes.This permits the frameworks' topological diversity via the bottom-up approach.Furthermore, building blocks of varying sizes and functionalities have been extensively explored to tune the electronic and steric environment of 2D frameworks (Figures 5 and 6).On the other hand, 3D COFs exhibit a limited structural diversity due to the restricted number of monomers with Td or

Pore Shapes and Structures
Numerous building blocks with unique geometrical and structural features enable the design of COFs with varying pore shapes and sizes.This permits the frameworks' topological diversity via the bottom-up approach.Furthermore, building blocks of varying sizes and functionalities have been extensively explored to tune the electronic and steric environment of 2D frameworks (Figures 5 and 6).On the other hand, 3D COFs exhibit a limited structural diversity due to the restricted number of monomers with T d or orthogonal nodes and inadequate dynamic linkages.Nonetheless, the expansion of 3D COFs with conventional coupling reactions is still a major challenge.
orthogonal nodes and inadequate dynamic linkages.Nonetheless, the expansion of 3D COFs with conventional coupling reactions is still a major challenge.
In addition to olefin-linked and imine-linked COFs, Dalapati et al. reported azinelinked rhombic Py-Azine COF by the reversible condensation of hydrazine and 1,3,6,8tetrakis(4-formylphenyl)pyrene at 120 • C for 7 days [106].The pyrene units were located at the vertices, whereas diazabutadiene linkers occupied the edges of the rhombic-shaped layer structure (Figure 6a).The framework was characterized by physiochemical analysis and displayed a BET surface area and pore size of 1210 m 2 /g and 1.76 nm, respectively.

Hexagonal COFs
Hexagonal COFs were prepared by a reversible condensation reaction between organic building blocks with the precise positioning of binding units in a closed system.Dynamic imine-linked hexagonal pore frameworks with varying pore sizes and a tunable electronic and steric environment benefitted from the innumerable advantages of the bottom-up approach.The hexagonal COF-LZU1 was synthesized by the Schiff-base reaction of C 3 -symmetric 1,3,5-triformylbenzene and C 2 -symmetric 1,4-diaminobenzene under solvothermal conditions.The stable and porous nature (410 m 2 /g) of COF-LZU1 provoked post-synthetic modification by reacting with Pd(OAc) 2 .Pd/COF-LZU1 served as a heterogeneous catalyst for a Suzuki-Miyaura coupling reaction between phenylboronic acid and electron-donating or electron-withdrawing aryl halide [146].In addition, Banerjee's research group emphasized the significance of building block planarity to construct highly crystalline and porous hexagonal COFs.Hexagonal-pore 2,3-DhaTta was synthesized by the Schiff-base reaction of C 3 -symmetric 1,3,5-tris(4-aminophenyl)triazine with a planar core (triazine), and C 2 -symmetric 2,3-dihydroxyterephthalaldehyde exhibited a high crystallinity and porosity (1700 m 2 /g).In contrary, a hexagonal 2,3-DhaTab COF was synthesized by the condensation reaction of 1,3,5-tris(4-aminophenyl)benzene with a nonplanar core (benzene), and 2,3-dihydroxyterephthalaldehyde displayed a low crystallinity and porosity (413 m 2 /g) [147].This is attributed to the strong π-π stacking interaction between the hexagonal layers in the 2,3-DhaTta COF due to the three phenyl rings' connection to the central triazine core (the torsion angles were 174.7 • , 176.3 • , and 179.9 • ) versus the non-planar core (the torsion angles were 145.5 • , 149.6 • , and 154.5 • ) in the 2,3-DhaTab framework.To demonstrate the fundamental role of interlayer interactions in COF stabilization, Jiang and co-workers reported the fabrication of a hexagonal TPB-DMTP COF by connecting C 3 -symmetric 1,3,5-tris(4-aminophenyl)benzene and C 2 -symmetric 2,5-dimethoxyterephthalaldehyde.The introduction of two electron-donating methoxy groups to the phenyl edge delocalized the lone pairs from oxygen over the phenyl ring and strengthened the interlayer interaction to stabilize the framework (the stacking energy of TPB-DMTP COF = 106.862kcal/mol) [148].However, a TPB-TP COF, constructed by using terephthalaldehyde instead of 2,5-dimethoxyterephthalaldehyde, possessed a low stacking energy of 94.084 kcal/mol.Furthermore, the BET surface area of the TPB-DMTP COF (2105 m 2 /g) was significantly higher than that of TPB-TP COF (610 m 2 /g).

Trigonal COFs
Triangular frameworks constructed by the incorporation of C6-symmetric building blocks are rare in the field.For instance, Jiang and co-workers reported triangular HPB-COF (Figure 6e) and HBC-COF by the Schiff-base solvothermal reaction of terephthalaldehyde with C6-symmetric hexaphenylbenzene and hexabenzocoronene, respectively [162].C6-symmetric vertices play a critical role in controlling interlayer interactions.HPB-

Trigonal COFs
Triangular frameworks constructed by the incorporation of C 6 -symmetric building blocks are rare in the field.For instance, Jiang and co-workers reported triangular HPB-COF (Figure 6e) and HBC-COF by the Schiff-base solvothermal reaction of terephthalaldehyde with C 6 -symmetric hexaphenylbenzene and hexabenzocoronene, respectively [162].C 6symmetric vertices play a critical role in controlling interlayer interactions.HPB-COF and HBC-COF displayed excellent thermal stability (500 • C) and chemical stability in HCl, NaOH, and protic and aprotic solvents.The BET surface areas and pore sizes of HPB-COF and HBC-COF were 965 m 2 /g and 1.2 nm and 469 m 2 /g and 1.8 nm, respectively.In addition, the dual-pore triangular HAT-COF and HFPTP-BPDA COFs with pore sizes of 1.13 nm and 1.52 nm and 1.27 nm and 1.55 nm, respectively, were constructed via solvothermal reactions between C 3 -symmetric hexaazatriphenylene and 2,3,6,7,10,11-hexakis(4formylphenyl)triphenylene and C 2 -symmetric terephthalaldehyde and 1,1 -biphenyl-4,4diamine, respectively [163,164].Remarkably, the BET surface area of HFPTP-BPDA COF (1024 m 2 /g) is higher compared to that of HAT-COF (486 m 2 /g), possibly due to the incorporation of an elongated linker.In addition to imine, azine-linked HEX-COF 1 and sp 2 carbon-linked CCP-HATN frameworks were designed by a reversible Schiff-base reaction and Knoevenagel condensation reaction, respectively [165].
The photocatalytic efficacy of 3D COFs for oxidative amine coupling and cycloaddition reactions were examined by a novel scu topology NKCOF-25-X (X = H or Ni) [198].The [8 + 4] construction approach using octatopic 4 ,5 -bis ( The NKCOF 21-23 displayed uniformly distributed block crystal and square pores with extremely high thermal stability.The BET surface areas and pore volumes were 1397 m 2 /g and 0.23 m 3 g −1 , 1580 m 2 /g and 0.34 m 3 g −1 , and 1900 m 2 /g and 1.05 m 3 g −1 for NKCOF-21, NKCOF-22, and NKCOF-23, respectively.Due to hydrogen-bond interactions (C-H••••N), NKCOFs displayed high ethane and ethylene adsorption at 273K, 298K, and 308K.For instance, NKCOF-21, -22, and -23 for ethene at 298K and 1 bar were 74.3 cm 3 /g, 40.7 cm 3 /g, and 51.0 cm 3 /g, respectively, while the adsorption capacity for ethane reached 97.9 cm 3 /g, 65.9 cm 3 /g, and 60.5 cm 3 /g, respectively.More importantly, bcu-network COFs displayed ethylene/ethane separation for 10 cycles with a complete retention of their structural robustness and crystallinity [199].One of the fundamental challenges in the field of 3D COFs is the fabrication of frameworks with a specific topology and the rejection of other possibilities.The COF research community has developed synthetic strategies to construct organic building blocks with sufficient structural, symmetrical, and reactive information to target specific topologies.This was illustrated by Nguyen et al. by the selective construction of COF-790 with an fjh topology using triangle and square building blocks.The solvothermal reaction of triangular 1,3,5-trimethyl-2,4,6-tris(4-formylphenyl)benzene with square 1,1,2,2,-tetrakis(4-aminophenyl)ethene in nitrobenzene and mesitylene at 85 • C for 3 days afforded a crystalline 3D COF-790.Furthermore, the dihedral angle between organic monomer units was in the range of 75 • -90 • .For instance, the dihedral angle of aldehyde monomer due to the presence of methyl groups was at 74 • , 83 • , and 90 • , thereby bolstering a framework with the desired topology.Notably, the stitching of aldehyde monomers without methyl group and with amine afforded an amorphous solid that has a paramount role in conformation processes.However, the same monomer under solvother-mal conditions in an alternative reaction condition (1,2-dichlorobenzne, 1-butanol, and acetic acid, at 120 • C, for 3 days) afforded the tth topology COF-340.To further show the fundamental role of building block conformation, isoreticular COF-791 and COF-792 were constructed by connecting 1,3,5-trimethyl-2,4,6-tris(4-formylphenyl)benzene with 1,2,4,5tetrakis(4-aminophenyl)benzene and 1,2,4,5-tetrakis(4-aminophenyl)-3 ,6 -dimethylbenzne, respectively [200].This bolstered the role of building blocks' properties, reaction conditions, catalyst, and other parameters to construct COFs with numerous topologies.For instance, Cooper and co-workers reported nbo topology spiroborate-linked SPB-COF-DBA by the control alignment of building blocks.The solvothermal reaction of square-planar cobalt (II)phthalocyanine and trimethylborate in N,N-dibutylformamide at 120 • C for 3 days afforded a crystalline ionic SPB-COF-DBA.The cubic pores of the moderately stable SPB-COF-DBA displayed a BET surface area of 1726 m 2 /g [201].

Synthetic Methods
The judicious choice of building blocks and the directional nature of dynamic linkages are essential to construct crystalline and porous frameworks.A de novo approach creates the formation of robust, reversible covalent linkage, thereby bolstering the thermodynamic control of condensation reactions.However, a kinetically controlled reaction enables the irreversible bond.This self-healing procedure encompasses the error checking and proof reading of structures to afford stable networks without defects.In addition to the fundamental role of reversibility, irreversible coupling reactions, such as dioxin, olefin, and nucleophilic aromatic substitution, have been reported in the design of porous networks and expansion of the library of COFs via different synthetic routes [100-103].

Solvothermal Synthesis
Solvothermal synthesis is one of the most widely used techniques for the synthesis of porous frameworks.In a typical protocol, organic monomers, solvents, and catalysts are placed in a Pyrex tube followed by sonication, degassed through freeze-pump-thaw cycles (liquid N 2 bath), sealed using a burner, and set aside for the required amount of time at a suitable temperature.Then, the precipitate is collected, washed, and dried to produce fluffy, solid-powdered COFs (Figure 7).This method was the first and is the most commonly used procedure to synthesize two 2D boroxine-linked and boronate-ester-linked COFs.Moreover, this methodology was used to synthesize imine-linked COFs, hydrazone-linked COFs, and azine-linked COFs, as highlighted above.The degree of crystallization and porosity depend highly on the reaction time, solvent, catalyst, symmetricity/reactivity, and solubility of the building blocks.The thermodynamics of COF synthesis using the solvothermal method can provide enough energy to overcome the Gibbs free energy of crystallization.A prolonged reaction time and tedious stepwise synthetic route hinder the large-scale synthesis of frameworks; however, the growth of alternative methods is still in its infancy.For instance, TPT-COF-1 was prepared using a gram scale by the solvothermal reaction of 2,4,6-tris(4-aminophenoxy)-1,3,5-triazine and 2,4,6-tris(4-formylphenoxy)-1,3,5triazine (TPT-CHO) with a BET surface area of 1589 m 2 /g [202].

Microwave Synthesis
The extended reaction time and synthetic conditions during solvothermal synthesis deter the large-scale synthesis of frameworks.An early approach to overcome this challenge was reported by Campbell et al. using microwave irradiation for COF fabrication.The rapid preparation (20 min) of COF-5 followed by purification and activation yielded a surface area over 2000 m 2 /g, which is higher than the surface value projected by the solvothermal synthesis [203].The general method of preparing COFs via microwave synthesis initiated by mixing building blocks in a suitable solvent system under inert atmosphere in a sealed microwave tube.It is important to activate COFs with low-surface-tension solvents by the Soxhlet extraction to remove building blocks and monomers adsorbed in the pores.In addition to COF-5, azine-linked, β-ketoenamine-linked, and imide-linked COFs were synthesized using microwave irradiation [204][205][206].For instance, Wei and co-workers reported the synthesis of β-ketoenamine TpPa-1 and TpPa-2 COFs under microwave irradiation within one hour.Notably, both COFs exhibited better crystallinity and porosity in comparison to the preparation via the solvothermal route.In another work, Lee et al. [206] conducted microwave-assisted PI-COF synthesis using pyromellitic dianhydride and tris(4aminophenyl)amine.

Microwave Synthesis
The extended reaction time and synthetic conditions during solvothermal synthesis deter the large-scale synthesis of frameworks.An early approach to overcome this challenge was reported by Campbell et al. using microwave irradiation for COF fabrication.The rapid preparation (20 min) of COF-5 followed by purification and activation yielded a surface area over 2000 m 2 /g, which is higher than the surface value projected by the solvothermal synthesis [203].The general method of preparing COFs via microwave synthesis initiated by mixing building blocks in a suitable solvent system under inert atmosphere in a sealed microwave tube.It is important to activate COFs with low-surface-tension solvents by the Soxhlet extraction to remove building blocks and monomers adsorbed in the pores.In addition to COF-5, azine-linked, β-ketoenamine-linked, and imide-linked COFs were synthesized using microwave irradiation [204][205][206].For instance, Wei and coworkers reported the synthesis of β-ketoenamine TpPa-1 and TpPa-2 COFs under microwave irradiation within one hour.Notably, both COFs exhibited better crystallinity and porosity in comparison to the preparation via the solvothermal route.In another work, Lee et al. [206] conducted microwave-assisted PI-COF synthesis using pyromellitic dianhydride and tris(4-aminophenyl)amine.

Mechanochemical Synthesis
To take COF chemistry to an interdisciplinary level, it is essential to explore simple synthetic methods, thus avoiding any complicated conditions, such as a reaction in a sealed Pyrex tube, inert atmosphere, and elevated temperatures.A mechanochemical synthesis prompted the construction of COFs via stable covalent bonds through a simple, economical, and environment friendly route [207].Despite the advantage of this synthetic route, the poor crystallization and amorphous nature of the as-synthesized COFs is a prime challenge; however, by the addition of either solvent or molecule organizer, such as p-toluenesulfonic acid frameworks, the crystallinity and porosity can be considerably improved.In the mechanochemical synthesis of β-ketoenamine TpPa-1, TpPa-2, TpPa-NO2, TpPa-F4, TpBD-(NO2)2, TpBD-(OMe)2, TpBpy, and other COFs, the building blocks were mechanically grinded using a mortar and pestle followed by heating at 170 °C for 1 min [153,208,209].The yielded COFs exhibited less crystallinity and porosity compared to the frameworks synthesized using the solvothermal route.Moreover, sulfonic-acid-decorated NUS-9 and NUS-10 were constructed mechanochemically by the condensation reaction of 1,3,5-triformylphloroglucinol with 2,5-diaminobenzenesulfonic acid and 2,5-diaminobenzene-1,4-disulfonic acid, respectively, exhibiting low porosity and crystallinity [149].

Mechanochemical Synthesis
To take COF chemistry to an interdisciplinary level, it is essential to explore simple synthetic methods, thus avoiding any complicated conditions, such as a reaction in a sealed Pyrex tube, inert atmosphere, and elevated temperatures.A mechanochemical synthesis prompted the construction of COFs via stable covalent bonds through a simple, economical, and environment friendly route [207].Despite the advantage of this synthetic route, the poor crystallization and amorphous nature of the as-synthesized COFs is a prime challenge; however, by the addition of either solvent or molecule organizer, such as p-toluenesulfonic acid frameworks, the crystallinity and porosity can be considerably improved.In the mechanochemical synthesis of β-ketoenamine TpPa-1, TpPa-2, TpPa-NO 2 , TpPa-F 4 , TpBD-(NO 2 ) 2 , TpBD-(OMe) 2 , TpBpy, and other COFs, the building blocks were mechanically grinded using a mortar and pestle followed by heating at 170 • C for 1 min [153,208,209].The yielded COFs exhibited less crystallinity and porosity compared to the frameworks synthesized using the solvothermal route.Moreover, sulfonic-acid-decorated NUS-9 and NUS-10 were constructed mechanochemically by the condensation reaction of 1,3,5-triformylphloroglucinol with 2,5-diaminobenzenesulfonic acid and 2,5-diaminobenzene-1,4-disulfonic acid, respectively, exhibiting low porosity and crystallinity [149].

Sonochemical Synthesis
The sonochemical method provides an alternative approach to the fabrication of frameworks with uncomplicated reaction conditions.The sonochemical method uses the process of cavitation, in which the reaction conditions, temperature, and pressure can be raised to initiate and accelerate the extent of polymerization.Yang and co-workers reported the use of the sonochemical method for COF-1 and COF-5 syntheses in a short reaction time (30 min-2 h) with high crystallinity and porosity (BET surface area = 2122 m 2 /g) [210].Although this method demonstrates the boundless potential of constructing framework with high crystallinity and stability under mild conditions, it is not apt to study different dynamic linkages to expand the library of COFs.

Ionothermal Synthesis
Covalent triazine frameworks (CTFs) synthesized using the ionothermal route are crystalline in nature; however, most CTFs are amorphous materials with the absence of a long-range order.In a usual method, organic building blocks of zinc chloride are placed in an ampule, which is sealed and heated to an elevated temperature (350-450 • C) for a long time (2-3 days).The solid product is washed with water and stirred in diluted hydrochloric acid to remove the unreacted monomers and zinc chloride [109].Using this protocol, crystalline CTF-1 and CTF-2 were synthesized using cyanuric chloride via the trimerization reaction [211].Recently, a milder and greener synthetic route was reported using ionic liquid as the solvent.For instance, 3D ionic-liquid-containing COFs (3D-IL-COF-1, 3D-IL-COF-2, and 3D-COF-3) were synthesized using 1-butyl-3-methylimidazolium bis-((trifluoromethyl)sulfonyl) imide as a green solvent to facilitate the rate of reaction between tetrakis(4-formylphenyl)methane with p-phenylenediamine, 4,4 -diaminobuiphenyl, and 4,4 -diamino-p-phenylenediamine, respectively [212].

Room-Temperature Synthesis
Most widely used COF synthetic methods require elevated temperatures and prolonged reaction times.The innumerable building blocks are unstable at high temperatures, and synthetic approaches can be hazardous and dangerous.In comparison to the solvothermal route, the room-temperature method offers simplicity and ease of operation.It is of profound importance to explore novel synthetic conditions, catalysts, and linkages to enable the large-scale synthesis of COFs.Within this context, the solutionsuspension approach is employed to prepare COFs at room temperature.In this approach, 1,4-phenylenediamine and 1,3,5-triformylbenzene were dissolved in dioxane at room temperature, followed by the addition of acetic acid as the catalyst.The mixture was kept for 3 days at ambient temperature conditions to construct COFs with good crystallinity and porosity (410-1537 m 2 /g) [213].This method is fast, effective, and can be used for the large-scale commercial production of frameworks for practical applications.These stable building blocks offer strong π interactions and high solubility to form COFs. The continuous flow of COF-LZU1 synthesis demonstrated a production rate of 41 mgh −1 at an extremely high space-time yield of 703 Kgm −3 day −1 .In another work, Bein and coworkers reported the synthesis of boroxine-based COF films from (4,6-diethoxybenzo[1,2b:4,5-b ]dithiophene-2,6-diyl)diboronic acid and hexahydroxy triphenylene through the vapor-assisted conversion method.This method is based on the conversion of precursors in a cast solution layer into a continuous crystalline and porous film by exposure to a vapor of specific composition at moderate temperatures [214].The role of time and vapor mixture is crucial for the formation of a highly regular structure.
COF membrane synthesis was viable a decade after the first ever COF fabrication performed under a solvothermal condition.To date, the development of COF membranes is still in its early stages.The prominent drawbacks come from three aspects, including low crystallinity, inferior processability, and irregular pore channel size.As such, the interfacial polymerization of β-ketoenamine-linked COF membranes at room temperature has advanced as a prominent method to prepare COF membranes.Banerjee and co-workers fabricated Tp-Bpy, Tp-Azo, Tp-Ttba, and Tp-Tta COF membranes by the liquidliquid interfacial polymerization of 1,3,5-triformylphloroglucinol with 2,2 -bipyridine-5,5diamine, 4,4 -azodianiline, 4,4 ,4 -(1,3,5-triazine-2,4,6-triyl)tris(1,1 -biphenyl)trianiline, and 4,4 ,4 -(1,3,5-triazine-2,4,6-triyl)trianiline, respectively, at ambient temperature [218].In this method, 1,3,5-triformylphloroglucinol suspended in dichloromethane and amine monomer dissolved in water was employed as the bottom layer.Since dichloromethane and water are immiscible with each other, a liquid-liquid interface was generated at the interface to afford membranes on the top of the aqueous layer after 72 h (Figure 8).Tp-Bpy, Tp-Azo, Tp-Ttba, and Tp-Tta COFs displayed BET surface areas of 1151 m 2 /g, 647 m 2 /g, 626 m 2 /g, and 333 m 2 /g, respectively.Notably, porous and crystalline Tp-Bpy thin films exhibited an unprecedented acetonitrile permeance of 339 Lm −2 h −1 bar −1 .The interfacial polymerization of a COF membrane is usually conducted in ambient conditions.In general, high temperature can increase the solubility of monomer units, but interrupts the interface morphology, which can lead to defects in the framework membrane.In addition to interfacial polymerizations at the liquid/liquid interface, interfacial polymerization at the liquid/air interface was established for the fabrication of COF membranes.Lai and co-workers reported the fabrication of a TFP-DHF 2D COF membrane from 1,3,5-triformylphloroglucinol and 9,9-dihexylfluorene-2,7-diamine through the Langmuir-Blodgett method [219].A monomer layer was formed on the surface of water by spreading a toluene solution of amine and aldehyde monomer on the water, followed by the evaporation of toluene.The polymerization at the interface was catalyzed by trifluoroacetic acid into the water to form 3 nm of a TFP-DHF membrane at room temperature.
triformylphloroglucinol and 9,9-dihexylfluorene-2,7-diamine through the Langmuir-Blodgett method [219].A monomer layer was formed on the surface of water by spreading a toluene solution of amine and aldehyde monomer on the water, followed by the evaporation of toluene.The polymerization at the interface was catalyzed by trifluoroacetic acid into the water to form 3 nm of a TFP-DHF membrane at room temperature.The challenges associated with the liquid-liquid interface encouraged the development of the present synthetic method.The in situ growth of COF membranes on a porous substrate or free-standing COF membrane offers a high separation performance.Jiang and co-workers reported a polydopamine-modulated in situ crystallization route to prepare sulfonated imine-linked TFP-DABA COF membranes on a polyacrylonitrile (PAN) substrate [220].The polydopamine layer (10 nm) deposited on PAN contained numerous functional groups serving as linking functionalities that adsorbed and bound framework units to promote nucleation.The sequential addition of 1,3,5-triformylphloroglucinol and 2,5-diaminobenzenesulfonic acids in 1,4-dioxane and a water solution to the modified PAN produced a β-ketoenamine membrane after 72 h at room temperature.Notably, the SCOF/PDA/PAN membrane exhibited water permeance of up to 1346 Lm −2 h −1 MPa −1 with desirable dye rejection.Furthermore, a heterostructure COF bilayer membrane was fabricated by a mixed-assembly strategy under an ambient condition.The Jiang research group illustrated the assembly of a TpDHBD nanosheet on a polydopamine-modified PAN substrate by vacuum filtration.Subsequently, the TpHZ COF layer displayed an in situ growth on the framework nanosheet layer via the vapor-liquid interfacial synthesis method.This exclusive promotion of heterogeneous nucleation assisted the creation of a second TpHZ COF layer.Remarkably, the crystallinity of the bilayer membrane was greater as compared to the membrane fabricated by liquid-liquid interfacial polymerization due to the epitaxial growth of a COF nanosheet and slow diffusion of monomers.Furthermore, the heterostructural membrane displayed a separation factor of 4464 for water/butanol separation [221].COF membranes fabricated at room temperature via the interfacial polymerization strategy suffered from low crystallinity.The low crystallinity led to truncated selectivity and permeability; thus, the advancement of synthetic strategies is essential.The crystallinity of the COF membrane influenced by the reversibility and robustness of COF linkage (bond strength), reaction rate, and the synthetic condition (high temperature, catalyst, concentration) initiated reversible condensation reactions.For instance, Banerjee and co-workers reported the in-situ growth of crystalline COF-based The challenges associated with the liquid-liquid interface encouraged the development of the present synthetic method.The in situ growth of COF membranes on a porous substrate or free-standing COF membrane offers a high separation performance.Jiang and co-workers reported a polydopamine-modulated in situ crystallization route to prepare sulfonated imine-linked TFP-DABA COF membranes on a polyacrylonitrile (PAN) substrate [220].The polydopamine layer (10 nm) deposited on PAN contained numerous functional groups serving as linking functionalities that adsorbed and bound framework units to promote nucleation.The sequential addition of 1,3,5-triformylphloroglucinol and 2,5-diaminobenzenesulfonic acids in 1,4-dioxane and a water solution to the modified PAN produced a β-ketoenamine membrane after 72 h at room temperature.Notably, the SCOF/PDA/PAN membrane exhibited water permeance of up to 1346 Lm −2 h −1 MPa −1 with desirable dye rejection.Furthermore, a heterostructure COF bilayer membrane was fabricated by a mixed-assembly strategy under an ambient condition.The Jiang research group illustrated the assembly of a TpDHBD nanosheet on a polydopamine-modified PAN substrate by vacuum filtration.Subsequently, the TpHZ COF layer displayed an in situ growth on the framework nanosheet layer via the vapor-liquid interfacial synthesis method.This exclusive promotion of heterogeneous nucleation assisted the creation of a second TpHZ COF layer.Remarkably, the crystallinity of the bilayer membrane was greater as compared to the membrane fabricated by liquid-liquid interfacial polymerization due to the epitaxial growth of a COF nanosheet and slow diffusion of monomers.Furthermore, the heterostructural membrane displayed a separation factor of 4464 for water/butanol separation [221].COF membranes fabricated at room temperature via the interfacial polymerization strategy suffered from low crystallinity.The low crystallinity led to truncated selectivity and permeability; thus, the advancement of synthetic strategies is essential.The crystallinity of the COF membrane influenced by the reversibility and robustness of COF linkage (bond strength), reaction rate, and the synthetic condition (high temperature, catalyst, concentration) initiated reversible condensation reactions.For instance, Banerjee and co-workers reported the in-situ growth of crystalline COF-based membranes [222].The aromatic diamine co-reagent (PTSA• H 2 O) was mixed with water to form salt.The resultant organic salt and 1,3,5-triformylphloroglucinol were shaken to produce a dough followed by a knife-cast on a plate to fabricate film.Lastly, baking the film at elevated temperatures (60-120 • C) for 12-72 h led to the in situ growth of the COF membrane.This strategy extended to synthesize TpBD(Me) 2 -, TpAzo-, TpBpy-, TpOMe-Pa1-, TpOMe-BD(NO 2 ) 2 -, TpOMe-Azo-, and TpOMe-Bpy-based COF membranes [223,224].Notably, the M-TpTD COF-based membrane displayed an acetonitrile flux of 260 Lm −2 h −1 bar −1 .In addition to the liquid-liquid interface, solid-vapor or liquid-vapor interfaces for COF membrane fabrications required high temperatures [26,225,226].The interfacial polymerization of 1,3,5-triformylphloroglucinol and 1,4-phenylenediamine (vapor phase) in the presence of noctanoic acid afforded crystalline and porous TFP-PDA COF membranes with a thickness of 120 nm.Notably, the TFP-PDA membrane exhibited an ultrahigh permeance towards water (411 Lm −2 h −1 bar −1 ) and acetonitrile (583 Lm −2 h −1 bar −1 ).These varying synthetic strategies were used to compare the advantages and disadvantages of constructing frameworks (Table 3).Notably, COF synthesis in ambient conditions elevated the interdisciplinary research to address other challenges.For instance, room-temperature synthetic methodology creates biomolecule building blocks that are unstable at higher temperatures.Despite the simplicity, the prepared frameworks in ambient conditions exhibited stability in harsh mediums and displayed a high degree of porosity and crystallinity.

Synthetic Method Advantages Disadvantages
Solvothermal Widely-used method for range of monomers and some COFs can be synthesized on a large scale.
Ionothermal Promotes green chemistry.Molten salts are used as solvents and catalysts.
Lacks long-range order in framework (amorphous) and requires high temperatures.

Microwave
Lower reaction time and uses an alternative source of energy.Fast and cleaner products.Requires high temperatures in some cases.
Requires high-temperature conditions and hinders large-scale synthesis.

Mechanochemical
Solvent-free, room temperature, economical, and environmentally friendly.Simple manual grinding is required.
Suffers from diversity of building blocks and dynamic linkages.
Room temperature Simple, easy, and promotes green chemistry.
Restricted to a few building blocks.

Structural Analysis and Characterization
COFs synthesized by stitching organic building blocks using a range of coupling reactions are typically characterized by various physiochemical techniques to establish structural and physical properties.In this section, we describe the analytical techniques to comprehend the structural, physical, and chemical features of porous frameworks.

Powder X-ray Diffraction and Crystallography
Powder X-ray diffraction (PXRD) patterns with distinct and clear diffraction signals helped us to evaluate the structure and crystallinity of the framework.Structural simulations together with calculated PXRD comparisons with experimental PXRD patterns aided the correct prediction of COF structures.To further evaluate the stacking layer patterns (AA or AB stackings) of 2D COFs, density functional tight binding (DFTB) calculations along with optimization of conformation was pivotal [227,228].In addition to eclipsed and staggered modes, the slipped AA-stacking mode was also observed, which, in turn, underlined the roles of topology, planarity, bulkiness, and dynamic linkages.It is worth understanding that stacking energy differs drastically as the stacking pattern changes.Generally, the AB-staggered mode has reduced π-π interlayer interactions resulting in low stabilization energy as compared to the AA-staggered mode, thereby influencing PXRD profiles.In addition to valuable comparisons between calculated and experimental PXRD patterns, the Pawley refinement offers a measure to justify the space groups of COFs, which is reflected in the parameters R wp and R p .In addition to PXRD, small-angle and wide-angle scattering values (SAXS/WAWS) reflect the extent of the polymerization and crystallinity of COF suspensions [229,230].
The influence of temperature and pressure on COF structural changes are usually investigated via in situ XRD.To reveal the thermal and pressure stability values, the diffraction intensity against pressure or heat underlined the structural information.High temperature favored a change in the crystalline structure of the framework, whereas high pressure was unfavorable to crystalline COFs.Three-dimensional imine-based COF-300, COF-303, LZU-79, and LZU-111 were synthesized as single crystals.These COF crystals were resolved using single-crystal X-ray diffraction (SXRD) to generate detailed crystal parameters, including atomic positions, bond lengths, bond angles, and unit cell parameters [231].This comprehensive crystal structure analysis is still in its infancy; however, it offers a method to understand host-guest interactions and mechanistic studies.

Porosity
Both 2D and 3D COFs should be appropriately activated before understanding the textural parameters.In general, COFs are usually activated by solvent exchange (lowsurface-tension solvents), super critical drying, Soxhlet extraction, and vacuum drying, followed by N 2 adsorption isotherm at 77 K to obtain pore volume, pore size, and surface area.The physisorption isotherms of the framework are divided into six types of micropores, mesopores, and macropores.More importantly, the number of COFs is of significant importance; in general, the weight of the tested COF multiplied by specific surface area (m 2 /g) should be equal to 100 m 2 or greater.

Thermal Stability and Chemical Stability
Thermogravimetric analysis (TGA) measures the mass loss versus temperature in a controlled atmosphere.This helps us to understand the thermal stability of frameworks.Moreover, the frameworks first undergo a loss of solvent molecules within a temperature range of 60-120 • C, followed by the structural modification or collapse of frameworks (350-450 • C).Furthermore, TGA measurements as mass loss are not inevitably associated with structural change; therefore, the measurement should be complementary to variable temperature powder X-ray diffraction (VT-PXRD) and in situ PXRD.Notably, TGA measurements can be used to roughly estimate the pore volume.The mass loss ascribed to the release of the trapped solvent in the pores near the boiling point can reflect the pore volume of the COFs.In addition to thermal stability, the chemical stability of COFs is fundamentally important to tune the pore environment via established principles of organic and organometallic chemistry.To evaluate the frameworks' chemical stability, COFs were immersed in acid (diluted and concentrated hydrochloric acid), base (sodium hydroxide), protic (water, ethanol), and aprotic solvents (tetrahydrofuran, dichloromethane, acetone) for multiple days to weeks.The physiochemical analysis comparison of pristine and immersed frameworks underlined structural integrity, porosity, crystallinity, and morphology, which is fundamental to develop scaffolds such as novel sensors and catalytic systems.For instance, Py-Azine COFs exhibit excellent chemical stability in 1M HCl and 1M NaOH [106].In addition, TpOMe-BD(NO 2 ) 2 , TpOMe-Azo, and TpOMe-Bpy with a 2,4,6-trimethoxy-1,3,5-benzenetricarbaldehyde group displayed high chemical stability outcomes in H 2 SO 4 (18M), HCl (12M), NaOH (9M), boiling water, and common organic solvents [224].
6.4. 13C NMR and Fourier-Transform Infrared Spectroscopy 13 C CP-MAS NMR and FT-IR spectroscopy bolstered the successful synthesis, purity, and local chemical environment of the framework.This reinforced the chemical state of multiple functionalities and identities, for instance, dynamic imine-linked COFs showed the appearance of -C=N functionalities at 159 ppm in 13 C CP-MAS NMR and at 1620 cm −1 in FT-IR spectroscopy with a disappearance of -C=O and -NH 2 moieties.In addition, FT-IR spectra showed absorption values at 1774 cm −1 and 1720 cm −1 corresponding to the asymmetric and symmetric vibrations, respectively, of the -C=O groups of five-membered imide rings.Moreover, 13 C CP-MAS NMR exhibited the carbonyl carbon of an imide ring at 163.0-165.0ppm.

Morphology
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are useful to investigate the morphology, elemental composition, and particle size of frameworks.To obtain high-quality SEM images, sputter coating can be performed by applying an ultra-thin coating of electrically conductive metal, such as gold.These microscopies can be combined with energy dispersive spectroscopy (EDS) or energy dispersive X-ray analysis (EDX) to assist elemental quantitative and qualitative composition determinations in COFs.This technique is particularly important to study the presence of immobilized transition metals in COFs via pore surface engineering, which is fundamental to characterize heterogeneous catalysts.In addition, high-resolution TEM allows the visualization of structural features using the low-dose TEM technique based on the use of a directdetection electron-counting camera.For instance, Peng et al. reported the honeycomb-like porous structure of a TPA-COF nanosheet using high-resolution TEM.The high-resolution TEM image displayed hexagonally arranged white contrasts surrounded by six black dots, which corresponded to a 1D pore channel and building blocks [232].Furthermore, Li et al. exhibited the honeycomb-like porous structure of COF-1 along with the [111] direction with a pore opening of 3.0 ± 0.2 nm using a low-dose TEM technique.Notably, the easy structural damage of COFs under electron beams created novel challenges to produce magnified-resolution images [233].

Growth Mechanisms of COFs
Considerable progress has been achieved in understanding the growth mechanisms of COFs from organic building blocks.The amorphous-to-crystalline transformation mechanism was highlighted by Dichtel and co-workers.The robust and rigid monomer units in solution after the addition of acid catalyst rapidly produced a solid product, which, in most cases, was an amorphous polymer.The solid amorphous product at elevated temperatures under a sealed condition converted kinetically generated amorphous product into thermodynamically stable crystalline networks.For instance, the growth mechanism process of the TAPD-PDA COF was studied under two isolated conditions.First, the initial polymer was characterized by PXRD, underlining the amorphous nature.Thereafter, the amorphous product was re-subjected to the optimized COF synthetic condition (70 • C) without any addition of solvent, and monomers exhibited intense diffraction peaks, underlining the crystalline nature of frameworks.Second, the solid amorphous product without any addition of catalysts of specific concentrations displayed an absence of intense diffraction peaks, thereby confirming an amorphous-to-COF transformation.Notably, the transformation process did not show any major change in morphology.Furthermore, the initial TAPB-PDA amorphous product exhibited a surface area of 18 m 2 /g and experience exponential increase in the crystalline TAPB-PDA framework (287 m 2 /g) after being subjected to two days of optimized reaction conditions [234].The research report by Gao et al. and the Guo research group for the synthesis of imine-linked COF-1 and β-ketoenamine TpBD COF also demonstrated a reformation mechanism [235,236].In addition to the amorphous-to-crystalline transformation, Liu and co-workers illustrated a dissolutionrecrystallization mechanism using 2,6-dihydroxynaphthalene-1,5-dicarbaldehyde, and 2,4,6-tris(4-aminophenyl)pyridine building blocks, which was similar to the amorphous-tocrystallization transformation mechanism with one observed difference in the change in morphology during the growth process.First, the initial solid precipitate showed irregular nanoparticle features followed by an increment in the reaction time creating a microsphereto-uniform-microfiber transformation.Finally, the process of amorphous-to-crystalline transformation showed intense peaks in the PXRD profile [237].
The reaction-induced mechanism was entirely different from amorphous-to-crystalline and dissolution-recrystallization mechanisms due to no amorphous polymer formation in the early stage of the reaction.Dichtel and co-workers first investigated this mechanism using boron-containing COF-5 [238] (Figure 9).In the initial stage, building blocks condensed into soluble oligomers, which, upon nucleation, afforded crystallites aggregates.Notably, it was observed that precipitates collected after five minutes displayed identical PXRD profiles to COF-5, with no evidence of a co-crystallized monomer units or any impurities.This ordered crystalline network of initially formed solid products was the differentiating point of two other mechanisms, which usually formed amorphous solid products.Furthermore, Bredas et al. illustrated two pathways of COF-5 nucleation and growth processes, i.e., the lateral growth of stacked structures and stacking between large oligomers using the kinetic Monte Carlo simulation [239].In addition, attempts were made to investigate the organic linker exchange of stable dynamic-linked COFs.For instance, Zhao et al. [240] reported the introduction of benzidine at elevated temperatures to an imine precursor synthesized by a reaction of 1,3,5triformylphloroglucino and mono-functional amine.The PXRD profile displayed crystallinity and higher porosity in comparison to the framework synthesized by the solvothermal process.A similar strategy was employed by Miao et al. for the construction of crystalline frameworks from an amorphous covalent organic polymer [241].Covalent organic polymer synthesized from 1,4-phthaldehyde and 1,3,5-tris(4-amidophenyl)triazine underwent ligand exchange using different building blocks; 2,5-dihydroxyterephthaldehyde, 2,5-dimethoxyterephthaldehyde, 2,3-dihydroxyterephthaldehyde, and 2,3-dimethoxyterephthaldehyde afforded COF 1-4.This one-step direct synthesis method developed our understanding of the mechanistic viewpoint and expanded the scope of crystalline COFs.Yaghi and co-workers highlighted the COFto-COF conversion protocol for linker substitution, cyclization, and oxidation.Imine-linked ILCOF-1 based on 1,3,6,8-tetrakis(4-formylphenyl)pyrene and1,4-phenylenediamine converted to organic linker (2,5-diaminobenzene-1,4-dithioldihydrochloride and 2,5-diaminobenzene-1,4dihydroxydihydrochloride) substitution and oxidative cyclization afforded two isostructural COF-921 and LZU-192.Both COF-921 and LZU-192 displayed good thermal stability (400 • C) and a higher degree of crystallinity.Furthermore, the frameworks exhibited a surface area and pore size larger than 1500 m 2 /g and 20 Å, respectively [143].
ating a microsphere-to-uniform-microfiber transformation.Finally, the process of amorphous-to-crystalline transformation showed intense peaks in the PXRD profile [237].
The reaction-induced mechanism was entirely different from amorphous-to-crystalline and dissolution-recrystallization mechanisms due to no amorphous polymer formation in the early stage of the reaction.Dichtel and co-workers first investigated this mechanism using boron-containing COF-5 [238] (Figure 9).In the initial stage, building blocks condensed into soluble oligomers, which, upon nucleation, afforded crystallites aggregates.Notably, it was observed that precipitates collected after five minutes displayed identical PXRD profiles to COF-5, with no evidence of a co-crystallized monomer units or any impurities.This ordered crystalline network of initially formed solid products was the differentiating point of two other mechanisms, which usually formed amorphous solid products.Furthermore, Bredas et al. illustrated two pathways of COF-5 nucleation and growth processes, i.e., the lateral growth of stacked structures and stacking between large oligomers using the kinetic Monte Carlo simulation [239].In addition, attempts were made to investigate the organic linker exchange of stable dynamic-linked COFs.For instance, Zhao et al. [240] reported the introduction of benzidine at elevated temperatures to an imine precursor synthesized by a reaction of 1,3,5-triformylphloroglucino and mono-functional amine.The PXRD profile displayed crystallinity and higher porosity in comparison to the framework synthesized by the solvothermal process.A similar strategy was employed by Miao et al. for the construction of crystalline frameworks from an amorphous covalent organic polymer [241].Covalent organic polymer synthesized from 1,4phthaldehyde and 1,3,5-tris(4-amidophenyl)triazine underwent ligand exchange using different building blocks; 2,5-dihydroxyterephthaldehyde, 2,5-dimethoxyterephthaldehyde, 2,3-dihydroxyterephthaldehyde, and 2,3-dimethoxyterephthaldehyde afforded COF 1-4.This one-step direct synthesis method developed our understanding of the mechanistic viewpoint and expanded the scope of crystalline COFs.Yaghi and co-workers highlighted the COF-to-COF conversion protocol for linker substitution, cyclization, and oxidation.Imine-linked ILCOF-1 based on 1,3,6,8-tetrakis(4-formylphenyl)pyrene and1,4phenylenediamine converted to organic linker (2,5-diaminobenzene-1,4-dithioldihydrochloride and 2,5-diaminobenzene-1,4-dihydroxydihydrochloride) substitution and oxidative cyclization afforded two isostructural COF-921 and LZU-192.Both COF-921 and LZU-192 displayed good thermal stability (400 °C) and a higher degree of crystallinity.Furthermore, the frameworks exhibited a surface area and pore size larger than 1500 m 2 /g and 20 Å, respectively [143].

Scalability Challenge
The limited scalability and fragility components are critical challenges in the advancement of COFs towards commercialization and industrial applications.The stable dynamic linkage is essential to connect building blocks in a network structure, which usually require closed conditions to maintain the reversibility of condensation reactions.This is usually achieved by sealed Pyrex tubes with low internal pressures or a hydrothermal reactor.The low internal pressure inside the Pyrex tube permits the slow diffusion of water to enable nucleation and crystallization.In contrast, heated building blocks at elevated temperatures in an open atmosphere can disturb reversibility via a loss of water molecules, thereby exhibiting a poor structural order.Consequently, it is difficult to maintain an ideal reaction condition to synthesize COFs on a larger scale, thereby the percentage yield of most COFs is in the order of milligrams.In addition, a prolonged reaction time, high-temperature boiling solvents, and activation prohibit COFs' development in the interdisciplinary research.

Synthetic and Crystallization Aspects
COFs' high degree of crystallinity differentiates the porous organic material from conventional amorphous polymers.The entire organic character of COFs presents their fundamental role and the utilization of robust dynamic coupling reactions to impart chemical/thermal stability.For example, β-ketoenamine and benzoxazole COFs showed tolerance towards acidic and basic conditions [242].The high network order in COFs arises due to reversible bond formations, which permits self-corrections to minimize defects and circumvents the formation of amorphous products.This reversibility induces the error-correction mechanism, where building blocks oriented in a specific direction within the growing network balance the kinetics to obtain an ordered structure.For instance, the amorphous-to-crystalline transition in imine-linked 2D COFs and building block exchange confirms the potent role of the error-correction mechanism [234,243].Furthermore, the slow growth of 3D imine-linked COF single crystals in the presence of competitive reagent.These mechanistic studies foreshadowed the advancements in synthetic strategies of COFs; however, a large gap still remains regarding large-scale COF production.One of the prime issues highlight the time required for reversibility-induced error corrections.Additionally, the stability of dynamic linkages significantly extends the crystallization timeline.For instance, imine-linked, β-ketoenamine-linked, and other COFs required two to seven days to synthesize crystalline frameworks [102].As a result, it is inevitable to invent novel synthetic strategies to minimize lower degrees of long-range orders and reduce errorcorrection durations.The novel synthetic methods for COFs' large-scale syntheses address the challenges and limitations directly associated with the reversibility-induced error corrections.Two-dimensional crystalline COFs obtained by the slow addition of building blocks and reagents tuned the reaction equilibrium and facilitated error corrections in a closed system (Pyrex tube, low internal pressure = 150 mTorr) as per the Le Châtelier's principle.Furthermore, this one-pot multistep process may lead to the involvement of side products.This is a common practice at the lab scale; however, an imitation of this approach on a large scale is dubious.
The addition of catalysts is highly relevant to hasten COFs reaction rates.However, an aspect of green chemistry is equally relevant.The stitching of building blocks via ultrastable dynamic linkages is scarcely practicable without a catalytic mediation.Acetic acid is one of the most widely used catalysts for COF synthesis.Brønsted acids, such as H 3 PO 4 , HCl, and CF 3 SO 3 H, displayed poor crystallinity and porosity properties due to the strong binding with aromatic diamine, which in turn negatively influenced the nucleophilicity and acid-amine proton transfer reaction required for COF synthesis.Moving beyond the acid catalyst, Lewis acid catalysis for a transamination reaction drastically reduced the reaction time and temperature, however, it introduced activation challenges.In addition, the nucleophilic catalyst and ionic liquids positively influenced the reaction rate.The generalization and widespread usage of ionic liquid in COF synthesis is a significant challenge to expand the library of frameworks in ambient conditions.The sustainability of the catalysts has yet to be considered to a large extent.Nonetheless, under the principle of green chemistry, it is conceivable that toxicity, efficacy, aquatic damage, and safe handling will emerge as relevant factors during synthetic expansion.Apart from the mechanisms and green chemistry, stable linkages are indispensable to avert the initiation of the hydrolytic decomposition of COFs.This primarily depends on the choice of building blocks and dynamic linkage.In general, an electron-withdrawing group directly attached to imine nitrogen exponentially decreases the nucleophilicity of imine nitrogen makes proton attack unfavorable.Consequently, β-ketoenamine, hydrazone, and azine linkages exhibited extrachemical and thermal stability; however, limited building blocks do not help in the design of stable COFs.It is important to discover novel stable linkages to synthesize COFs at ambient temperature conditions.Furthermore, the robust interlayer stacking interactions bolster addition stability.The electrostatic repulsion among polarized bonds and π-π stacking interactions between an individual layers improves stability.Notably, the presence of intramolecular hydrogen bonding interaction via the uniform distribution of hydroxy group, methoxy groups significantly improve crystallinity, porosity, and chemical stability of COFs [138,140].Therefore, an effective, economical, scalable, and environment friendly synthetic methodology is desirable to fabricate stable COFs.
Apart from synthetic challenges, the crystallization of extended structures is relatively difficult as compared to discrete structures.The crystallization process requires reversible bond formation to allow the self-correction of defects and avoid the design of an amorphous network.This involves the judicious design of building blocks and the specific positioning of functionalities to add covalent linkages between organic molecules.In addition to the building blocks' symmetricity, the appropriate stoichiometric quantities of building blocks are important to control the equilibrium in a closed system.The combination of hydrophilic and hydrophobic solvents (o-dichlorobenzene/n-butanol, mesitylene/1,4-dioxane) further facilitates water partition between a reaction mixture, which is effective in reversibly controlled reaction rates.COFs constructed through an irreversible coupling reaction are pivotal to address crystallization.The solvent trapped in the pores adds activation issues and differentiates frameworks as ultra-stable and fragile COFs.

Activation Aspect
During COF synthesis, solvent molecules, solvated counterions, and building blocks are trapped in the pores of the frameworks.The necessity to access the permanent porosity for an ingress of foreign functionalities permits various chemical transformations.In order to tune the chemical and steric environment of COFs, the trapped solvent or monomer molecules must be removed-a process that is called activation.This task is more stimulating due to the insolubility of COFs.The most common activation techniques include solvent exchange, vacuum drying, and supercritical CO 2 drying.

Solvent Exchange and Vacuum Drying
In most instances, the as-synthesized COFs, heated (60 • C-80 • C) under vacuum, may be sufficient to remove the trapped solvent.This is observed in an ultra-robust iminelinked, β-ketoenamine-linked, hydrazone-linked, azine-linked, imide-linked, and other COFs.On the contrary, fragile COFs collapse under vacuum due to high surface tension and capillary forces imposed on the structure by the liquid-to-gas phase transformation of trapped solvent molecules (Figure 10).One of the plausible methods is to exchange the solvent with a lower boiling-point and lower-surface-tension solvent prior to heating the sample under vacuum.For instance, the N 2 adsorption desorption isotherms for TAPB-PDA COF activated directly from perfluorohexane and hexane exhibited BET surface areas of 2121 m 2 /g and 2060 m 2 /g, respectively, compared to TAPB-PDA COF activated from methanol (173 m 2 /g), acetone (91 m 2 /g), tetrahydrofuran (28 m 2 /g), dioxane (113 m 2 /g), and dimethylformamide (172 m 2 /g) [244].To appropriately accomplish solvent exchange, COFs should be washed carefully with the reaction solvent to exclude either impurities or unreacted building blocks followed by COFs soaked in low-boiling-point and low-surfacetension solvent to ensure complete infiltration inside the pores.This process requires a prolonged amount of time to make sure the fresh solvent penetrates into the pores of COFs.Once the solvent exchange is complete, heating under vacuum can be applied to ensure complete the activation of the sample.Notably, the heating temperature should be above the solvent boiling point, but below the decomposition temperature of the framework, as obtained by TGA analysis.
higher as compared to vacuum activation.In contrast to TAPB-PDA COF, the influence of vacuum or scCO2 activation on TAPB-OMePDA and TAPPy-PDA COFs is less noticeable, thereby approving the stabilizing effect due to docking behavior.

Mobile Robotics
The synthetic development of COFs can benefit greatly by establishing manufacturing equipment.A mobile robot is widely used in the industry to exponentially increase the productivity.The robot has human-like dimensions and can operate to dispense insoluble and soluble solutions with high accuracy and repeatability.Furthermore, the robot arm and the mobile base comply with safety standards.To date, no approaches have integrated COFs with mobile robotics to shorten the timeline for either the synthesis or optimization of reaction conditions.Cooper's research group recently reported a mobile robot for improved photocatalysts for hydrogen production from water [249].Notably, the robot performed 688 experiments within a ten-variable experimental space over eight days.This bolstered and shortened the screening timeline to discover an ideal reaction condition for COF synthesis with a high degree of crystallinity and porosity.For instance, Ma's research group reported the screening of multiple reaction conditions for TPAPC-COF synthesis.The nine reaction conditions were composed of different ratios of solvents, building blocks' stoichiometry, and activation strategies.The solvothermal reaction of 5,10,15-tris(p-aminophenyl)corrole and terephthaldehyde in mesitylene/n-butanol at 120 °C exhibited good crystallinity and a BET surface of 745 m 2 /g.However, the similar stoichiometric ratio of building blocks in either ethanol/mesitylene or butanol/o-dichlorobenzene displayed lower BET surface areas [250].This laborious work can be curtailed by assimilating COFs with robotics.In another work, the ideal hydrophobic/hydrophilic solvent combination and catalyst concentration for successful Schiff-base condensation reaction to synthesizing TAT-COF-1 and TAT-COF-2 were reported by Zheng and co-workers.Seventeen reaction conditions comprised varying precursor ratios, catalyst concentrations, and solvents correlated with PXRD profile [251].The ideal synthetic condition is central to obtain high degree of crystallinity and porosity.Notably, TAT COFs displayed moderate-to-good hydrogen and carbon dioxide adsorption.
Recently, Beuerle, Würthner and co-workers reported the influence of varying reaction conditions to fabricate crystalline COFs from amorphous polymers.The solvothermal reaction of ruthenium(2,2′-bipyridine-6,6′-dicarboxylate)dialdehyde and tetra-(4-anilyl)methane at 60 °C in N,N-dimethylacetamide/mesitylene (1:1) displayed high crystallinity; however, nine solvothermal reactions in one solvent (CH3OH, THF, DMAc, DMF, 1,4-dioxane, and dimethylsulfoxide) or a mixture of solvents (DMAc/methanol, DMAc/1,4-dioxane, and DMAc/Mes) at varying temperatures (60 °C and 120 °C) and times (72 h and 96 h) showed poor crystallinity or no solid product [252].This substantial challenge of optimization for prolonged reactions can be curtailed by designing a suitable robotic system for COF chemistry.This inevitably generates a library of COFs using novel dynamic linkages and bolsters gram-scale synthesis.To summarize, it is important to uncover the optimal conditions for the gram-scale synthesis of ultra-stable and fragile COFs In another work, Verduzco and co-workers elaborate an insight into the relationship between structural integrity and pore size, pore functionality, and pore architecture [245].Rhombic Py-1P COF with a small pore size (1.5 nm) displayed high crystallinity and porosity after activation with solvents of varying surface tensions, methanol, tetrahydrofuran, and perfluorohexane.Moreover, rhombic Py-2P COF with a larger pore size (2.6 nm) exhibited excellent crystallinity and surface area after activation with methanol or perfluorohexane (3121 m 2 /g and 3312 m 2 /g, respectively), whereas activation with tetrahydrofuran disrupted crystallinity and porosity (91 m 2 /g).Notably, hydroxyl-functionalized rhombic Py-OH1P COF displayed structural robustness, crystallinity, and porosity after activation with methanol, tetrahydrofuran, and perfluorohexane.This underlined the profound role of hydrogen bonding in maintaining the structural integrity of frameworks.A similar activation effect was unambiguously observed for imine-linked COFs of varying pore sizes, TAPB-BTCA (0.9 nm), TAPB-TFPA (1.7 nm), TAPB-TFPB (2.0 nm), and TAPB-PDA (3.3 nm), and pore functionalities, TAPB-FPDA, TAPB-OH PDA, and TAPB-Br PDA.In addition, Soxhlet extraction is another important means for solvent exchange, particularly in the case of linkers, which are difficult to separate due to poor solubility and the solvent being difficult to remove from the pores.For instance, COF-ETTA-EDDA, COF-ETTA-DMDA, TPAPC-COF, and others activated by Soxhlet extraction using anhydrous tetrahydrofuran and acetone displayed significant improvement in crystallinity and porosity [175].

Supercritical Drying
Supercritical carbon dioxide (scCO 2 , surface tension = 0.6 dynes/cm) is a milder activation technique especially used for fragile framework in which conventional solvent exchange is unsuccessful and causes structure collapse.Using scCO 2 , the activation process avoids the liquid-to-gas phase transformation of the guest solvent and instead experiences a supercritical phase, thereby eliminating capillary forces and the surface tension effect.From an experimental perspective, the solvated COFs can be placed in a scCO 2 dryer and cooled to 2-10 • C, followed by exchange with CO 2 .The sample chamber temperature should not drop below 0 • C and fresh CO 2 can be purged every hour.After multiple exchanges (4-5 times), the sample can be heated to 31 • C and 73 atm, the supercritical temperature and pressure of CO 2 to release gaseous CO 2 , respectively, often called as bleeding.The activation challenge associated with a fragile framework can be resolute to access porosity.For instance, Medina and co-workers underlined scCO 2 activation to access the porosity of TA TAPB-COF, TT TAPB-COF, and BDT TAPB-COF fragile COFs with BET surface areas of over 1000 m 2 /g [246].On the contrary, the extent of crystallinity and porosity decreases using 1,4-dioxane, toluene, and acetone.Additionally, scCO 2 activated hydrazone-linked TFPB-DHz COF, TFPT-DHz COF, and Py-DHz COF exhibited excellent crystallinity and porosity [247].The BET surface areas of TFPT-DHz COF, TFPB-DHz COF, and Py-DHz COF were found to be 1199 m 2 /g, 790 m 2 /g, and 932 m 2 /g respectively, while the conventional solvent-activated COFs appeared to be amorphous.Recently, Feriante et al. highlighted the minimization of capillary strain to avoid pore collapse using scCO 2 and to bolster the rapid synthesis of ultra-robust imine-linked TAPB-PDA, TAPB-OHPDA, TAPB-OMePDA, TAPPy-PDA, and TAPPy-NDI-DA COFs in four hours [248].The surface area of TAPB-PDA COF after scCO 2 activation is approximately ten times higher as compared to vacuum activation.In contrast to TAPB-PDA COF, the influence of vacuum or scCO 2 activation on TAPB-OMePDA and TAPPy-PDA COFs is less noticeable, thereby approving the stabilizing effect due to docking behavior.

Mobile Robotics
The synthetic development of COFs can benefit greatly by establishing manufacturing equipment.A mobile robot is widely used in the industry to exponentially increase the productivity.The robot has human-like dimensions and can operate to dispense insoluble and soluble solutions with high accuracy and repeatability.Furthermore, the robot arm and the mobile base comply with safety standards.To date, no approaches have integrated COFs with mobile robotics to shorten the timeline for either the synthesis or optimization of reaction conditions.Cooper's research group recently reported a mobile robot for improved photocatalysts for hydrogen production from water [249].Notably, the robot performed 688 experiments within a ten-variable experimental space over eight days.This bolstered and shortened the screening timeline to discover an ideal reaction condition for COF synthesis with a high degree of crystallinity and porosity.For instance, Ma's research group reported the screening of multiple reaction conditions for TPAPC-COF synthesis.The nine reaction conditions were composed of different ratios of solvents, building blocks' stoichiometry, and activation strategies.The solvothermal reaction of 5,10,15-tris(paminophenyl)corrole and terephthaldehyde in mesitylene/n-butanol at 120 • C exhibited good crystallinity and a BET surface of 745 m 2 /g.However, the similar stoichiometric ratio of building blocks in either ethanol/mesitylene or butanol/o-dichlorobenzene displayed lower BET surface areas [250].This laborious work can be curtailed by assimilating COFs with robotics.In another work, the ideal hydrophobic/hydrophilic solvent combination and catalyst concentration for successful Schiff-base condensation reaction to synthesizing TAT-COF-1 and TAT-COF-2 were reported by Zheng and co-workers.Seventeen reaction conditions comprised varying precursor ratios, catalyst concentrations, and solvents correlated with PXRD profile [251].The ideal synthetic condition is central to obtain high degree of crystallinity and porosity.Notably, TAT COFs displayed moderate-to-good hydrogen and carbon dioxide adsorption.
Recently, Beuerle, Würthner and co-workers reported the influence of varying reaction conditions to fabricate crystalline COFs from amorphous polymers.The solvothermal reaction of ruthenium(2,2 -bipyridine-6,6 -dicarboxylate)dialdehyde and tetra-(4-anilyl)methane at 60 • C in N,N-dimethylacetamide/mesitylene (1:1) displayed high crystallinity; however, nine solvothermal reactions in one solvent (CH 3 OH, THF, DMAc, DMF, 1,4-dioxane, and dimethylsulfoxide) or a mixture of solvents (DMAc/methanol, DMAc/1,4-dioxane, and DMAc/Mes) at varying temperatures (60 • C and 120 • C) and times (72 h and 96 h) showed poor crystallinity or no solid product [252].This substantial challenge of optimization for prolonged reactions can be curtailed by designing a suitable robotic system for COF chemistry.This inevitably generates a library of COFs using novel dynamic linkages and bolsters gram-scale synthesis.To summarize, it is important to uncover the optimal conditions for the gram-scale synthesis of ultra-stable and fragile COFs and design innovative linkages in structures.Still, it will be stimulating and offer great intellectual challenges to link synthetic techniques and activation instruments.

Conclusions, Outlook, and Summary
Covalent organic frameworks, a novel porous organic material, has been broadly examined in various fields and displays an imperative role in nanotechnology.COFs' porosity and crystallinity confirms the unambiguous validation of their structures constructed by dynamic covalent linkages using the principle of reticular chemistry.Dynamic covalent chemistry proposes distinct properties, such as thermal stability, chemical stability, and high porosity, of the designed frameworks, which is important in industrial applications.Furthermore, the library of symmetrical building blocks bearing multiple binding sites at a specific position shows the structural and topological diversity of COFs.This expansion of organic chemistry to an extended network was comprehensively discussed in this review.The design principle can be useful in building either reported or hypothesized structures with varying pore shapes and sizes.This variable pore geometry offers ample opportunities to create an ideal electronic and steric environment either via pore surface engineering or de novo approaches.With these accomplishments, COFs have a unique position in the fields of material sciences and chemistry.However, despite their many favorable characteristics and great advances, the fields require considerable research to urgently address the level of synthetic control, from molecular geometry to network geometry.This requires manipulation at the fundamental level, including synthetic, crystallization, activation, controlled morphology, and coupling chemistry, which hinder the large-scale synthesis of COFs.The review underlined these issues, which deserve attention in future research.
1.The building blocks for constructing COFs are either unavailable commercially or expensive.The most common starting materials involve multistep tedious reaction under severe conditions, and the complicated purification step hampers gram-scale synthesis.In addition, the role of the catalyst in COF synthesis remains a nascent field from a mechanistic perspective, which requires in-depth investigation, especially when moving from protic acid to metal salt.The stitching of building blocks in the presence of a novel catalyst may provide a plausible solution to poor-crystalline COFs and extends the reaction time, but limits safety and presents sustainability concerns.
2. The uniformity and reproducibility in the large-scale synthesis of crystalline and porous COFs are central to commercialization.However, the complexity in structural, activation, and crystallization aspects at the milligram scale demands sustainable tactics for scalable production and batch-to-batch reproducibility.The solvent-free mechanosynthesis might be a plausible method, but suffer from a limited scope.It is imperative to focus on irreversible coupling reactions to develop a simple and effective method for well-defined, uniform, and reproducible COFs, which is still in its infancy.
3. The scalable and sustainable preparation of COFs will significantly be assisted by using manufacturing equipment.The use of automation and robotics can not only bolster the optimization of reaction conditions to fabricate established or novel linkages in a short time, but also positively influence large-scale synthesis at the industrial level for wider applications.Thus, this is only the beginning of this exciting field.

Figure 1 .
Figure 1.Number of published papers on covalent organic frameworks in the last obtained from Web of Science using the keyword "covalent organic frameworks".

Figure 1 .
Figure 1.Number of published papers on covalent organic frameworks in the last ten years.Data obtained from Web of Science using the keyword "covalent organic frameworks".
good thermal (350 °C) and chemical stability.C-O Ester Good crystallinity.No precise description on thermal and chemical stability.1,4-dioxin Good crystallinity and thermal stability (400 °C).Good chemical stability in both acid (HCl) and base (NaOH).

Figure 2 .
Figure 2. Widely accepted reactions for the formation of COFs.

Figure 2 .
Figure 2. Widely accepted reactions for the formation of COFs.

Figure 3 .
Figure 3. Timeline of various dynamic linkages for COF formation.

Figure 3 .
Figure 3. Timeline of various dynamic linkages for COF formation.
. The ctn network can be obtained by linking [T d + C 3 ] and [T d + T d ] symmetric building blocks, whereas a bor network can be obtained by [T d + C 3 ].The pts network can be formed by either [T d + C 2 ] or [T d + C 4 ], in which the C 2 or C 4 symmetric unit exhibits four reactive sites.In addition, the dia and srs networks can be formed by [T d + C 2 ] and [T d + C 3 ], respectively.

Figure 5 .
Figure 5. Organic building blocks with different geometries and reactive groups.

Figure 8 .
Figure 8. Synthesis of Tp-Bpy COF film via interfacial crystallization at room temperature.Reprinted with permission from ref. [218] (Copyright © American Chemical Society).

Figure 8 .
Figure 8. Synthesis of Tp-Bpy COF film via interfacial crystallization at room temperature.Reprinted with permission from ref. [218] (Copyright © American Chemical Society).

Figure 10 .
Figure 10.Surface tension of the most common organic solvents for COF activation.

Figure 10 .
Figure 10.Surface tension of the most common organic solvents for COF activation.

Table 2 .
Characteristics of the most widely used dynamic linkages for COFs.

Table 3 .
Comparison between different synthetic methods.