Porous Organic Materials: Strategic Design and StructureâFunction

Review pubs.acs.org/CR

Porous Organic Materials: Strategic Design and Structure−Function Correlation Saikat Das,† Patrick Heasman,‡ Teng Ben,*,† and Shilun Qiu† †

Department of Chemistry, Jilin University, Changchun 130012, People’s Republic of China Department of Chemistry, Lancaster University, Lancaster LA1 4YB, United Kingdom

‡

ABSTRACT: Porous organic materials have garnered colossal interest with the scientific fraternity due to their excellent gas sorption performances, catalytic abilities, energy storage capacities, and other intriguing applications. This review encompasses the recent significant breakthroughs and the conventional functions and practices in the field of porous organic materials to find useful applications and imparts a comprehensive understanding of the strategic evolution of the design and synthetic approaches of porous organic materials with tunable characteristics. We present an exhaustive analysis of the design strategies with special emphasis on the topologies of crystalline and amorphous porous organic materials. In addition to elucidating the structure−function correlation and state-of-the-art applications of porous organic materials, we address the challenges and restrictions that prevent us from realizing porous organic materials with tailored structures and properties for useful applications.

CONTENTS 1. Introduction 2. Family of Porous Organic Materials 2.1. Hyper-Cross-Linked Polymers (HCPs) 2.2. Polymers of Intrinsic Microporosity (PIMs) 2.3. Covalent Organic Frameworks (COFs) 2.4. Conjugated Microporous Polymers (CMPs) 2.5. Covalent Triazine Frameworks (CTFs) 2.6. Porous Aromatic Frameworks (PAFs) 2.7. Extrinsic Porous Molecules 2.8. Porous Organic Cages 2.9. Genesis of Porous Liquids 3. Design of Porous Organic Materials 3.1. Topology Design 3.1.1. Topology of Crystalline Porous Organic Materials 3.1.2. Topology of Amorphous Porous Organic Materials 3.2. Theoretical Design 4. Chemical Synthesis of Porous Organic Materials 4.1. Thermodynamic and Kinetic Control 4.2. Reactions Involved in the Chemical Synthesis of Porous Organic Materials 4.3. Approaches To Minimize Structural Defects 5. Functionalization of Porous Organic Materials 5.1. Presynthetic and Postsynthetic Functionalization 5.2. Theoretical Approach to Property Determination 6. Applications of Porous Organic Materials 6.1. Gas Adsorption and Storage 6.2. Separation of Gases 6.2.1. Breakthrough Experiment © XXXX American Chemical Society

6.2.2. Membrane-Based Gas Separation 6.3. Catalysis 6.4. Energy Storage 6.5. Proton Conduction 6.6. Semiconduction 6.7. Photocatalysis and Photoluminescence 6.8. Photoenergy Conversion in Solar Cells 6.9. Enantioseparation 6.10. Drug Delivery and Release 6.11. Iodine Adsorption 6.12. Detection and Removal of Pollutants/ Contaminants from Water and Other Liquids 7. Outlook and Future Perspectives 7.1. Rational Design of Novel Porous Organic Materials 7.2. Technique for Structural Characterization of Amorphous Porous Organic Materials 7.3. Synthetic Challenges 7.4. Conclusion Author Information Corresponding Author Notes Biographies Acknowledgments Abbreviations References

A D D E E F G H I I J L L P S V Y Y Y Y Z Z AB AC AC AF AF

AF AI AI AI AI AJ AL AM AM AM

AM AN AN AN AN AN AO AO AO AO AO AO AO

Received: July 8, 2016

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DOI: 10.1021/acs.chemrev.6b00439 Chem. Rev. XXXX, XXX, XXX−XXX

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Figure 1. Interpreting porosity based on the concept of ineffective molecular packing. (a) Exemplary concave shape prototype. (b) Interconnected grid formed by concave shape prototype. (c) Typical porous organic material, namely, COF-5 formed from molecular building blocks. Carbon, boron, oxygen, and hydrogen atoms are colored gray, pink, red, and white, respectively. Hydrogen atoms are excluded for clarity in the COF-5 structure.

Figure 2. (a) Cross-section of a porous medium illustrating open and closed pores (1 and 2 closed pores; 3 and 4 open pores). Adapted with permission from ref 71. Copyright 1994 IUPAC. (b) Structural representation of 3D-Py-COF exemplifying open pores. Carbon and nitrogen atoms are colored gray/green and blue, respectively. Hydrogen atoms are excluded for clarity.

1. INTRODUCTION Science is the organized knowledge of the behavior of nature and its application to the welfare of mankind. Porous structures are also inherent to natural processes. The honeycomb, which is the “magnum opus” of honeybees, is a porous structure. It stockpiles honey, royal jelly, 1 and pollen2 as well as accommodates the brood.2 Sponges are aquatic animals that have pores on the surfaces of their bodies. The pores manage a steady water flux through their bodies to filter food, absorb oxygen, and eliminate waste products. The natural porous structures and their uses provided materials scientists with the idea of mimicking them in artificial structures. The porous organic materials fabricated accordingly manage to procure their porosity via inefficient molecular packing, which occurs when the component molecules have concavities.3−5 Figure 1a

illustrates an exemplary concave shape prototype. This concave shape can form an interconnected grid (Figure 1b) such as COF-5 (COF = covalent organic framework),6 which is formed from molecular building blocks (Figure 1c). The porous organic materials are effective in the adsorption of greenhouse gases such as carbon dioxide and methane.7−13 The upsurge in greenhouse gas emissions has a damaging impact on the environment. Porous organic materials can also be productively applied for gas separation,14−20 catalysis,21−26 energy storage,27−35 proton conduction,36,37 semiconduction,38−44 photocatalysis and photoluminescence,45−58 photoenergy conversion in solar cells,59 enantioseparation,60,61 drug delivery and release,62 iodine adsorption,63,64 detection and removal of pollutants/contaminants from water/other liquids,65−70 along with other essential applications. B


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As the title of the present review suggests, porous organic materials refer to hydrocarbons that include pores (voids). Conforming to the IUPAC recommendations,71 pores that have continuous connection pathways with the outer surfaces of the porous structure are called open pores. On the other hand, pores that are detached from other pores are called closed pores. Figure 2a illustrates the cross-section of a porous medium. As defined, pores 1 and 2 are closed pores, while pores 3 and 4 are open pores. The open pores play a pivotal role in fluid dynamics and gas adsorption.71 From applicationbased perspectives, materials chemists are interested with open pores. The IUPAC also categorizes porous materials according to their pore size as microporous (pore widths less than 2 nm), mesoporous (pore widths between 2 and 50 nm), and macroporous (pore widths greater than 50 nm).71,72 Figure 2b illustrates the structure of a porous organic material, namely, 3D-Py-COF (three-dimensional pyrene-based covalent organic framework),73 exemplifying open pores. Porous organic materials have some attributes that may be imperative to describe the materials. The IUPAC71 defines the attributes of a porous solid as follows: By “pore volume” (Vp), we mean the “volume of the pores, as measured by a given method which must be stated”. The “porosity” is given by the “ratio of the total pore volume Vp to the apparent volume V of the particle or powder (excluding interparticle voids)”. Oftentimes, the surface area of a porous solid is represented as the “specific surface area”, which is given by the “accessible (or detectable) area of solid surface per unit mass of material”. The “pore size” is described as “the distance between two opposite walls of the pore (diameter of cylindrical pores, width of slit-shaped pores)”. The “pore size distribution” can be “represented by the derivatives dAp/drp or dVp/drp as a f unction of rp, where Ap, Vp, and rp are the wall area, volume, and radius of the pores. The size in question is here the radius, which implies that the pores are known to be, or assumed to be, cylindrical. In other cases rp should be replaced by the width.” Given the porous structures/media discussed thus far, it is time we analyze the nature of a single “pore”. The pore space74 of a porous medium is usually taken into account in relation to single pores, the definition of which can be elucidated subjectively as follows:74 The pore space can be thought of as a compilation of capillaries that allows fluids to pass through them. The comparatively constricted parts of such capillaries are called “pore openings”, while the comparatively broader/ more spacious parts of the capillaries are called “pore bodies”. Figure 3a illustrates the mechanisms of unloading and loading of fluid in a capillary. The pore pressure P is governed by the pore openings throughout the unloading process and the pore bodies throughout the loading process. During the course of unloading of fluid in a capillary, the pore pressure decreases (P1 > P2 > P3 > P4) and the radius of curvature of the meniscus formed at the air−liquid interface increases steadily. This continues up to a point (shown as position 4 in Figure 3a75) when the interface at the most constricted portion of the pore opening cannot accept the steady increment of the radius of curvature of the meniscus. Instead, the liquid immediately departs for other narrower capillaries. A similar circumstance arises in the most spacious portion of the pore body in the course of loading of fluid in a capillary. In lieu of a steady decrement of the radius of curvature of the meniscus, the liquid instantly fills up the whole capillary (shown as position 8 in Figure 3a75). The space that is filled and unfilled by such means is referred to as a single “pore”. Figure 3b illustrates COF-30076 with fluid occupying the pore exemplifying a capillary. The

Figure 3. (a) Illustration of the loading and unloading of fluid in a capillary. Adapted with permission from ref 75. Copyright 1956 American Institute of Physics. (b) COF-300 with fluid occupying the pore exemplifying a capillary. Carbon and nitrogen atoms are colored gray and blue, respectively. Hydrogen atoms are excluded for clarity. (c) Nitrogen sorption isotherms of COF-300 powder (degassed) and the pore size distribution of COF-300 (shown in the inset figure) evaluated from nitrogen sorption isotherms implemented using density functional theory (DFT). In the isotherms, the solid symbols represent adsorption and the open symbols represent desorption. Reprinted with permission from ref 17. Copyright 2016 American Chemical Society.

sorption of gases by porous organic materials is applied to analyze the specific surface area of materials utilizing Langmuir theory when considering the adsorption of a monomolecular layer77−79 and Brunauer−Emmett−Teller theory when considering the adsorption of multimolecular layers.80 The pore size and pore size distribution of the materials can be assessed from the gas sorption isotherms by employing density functional theory (DFT).81−83 Figure 3c17 illustrates the nitrogen sorption isotherms of degassed COF-300 powder and the pore size distribution of COF-300 evaluated from the nitrogen sorption isotherms using DFT. The present review will focus on the design, structure− function correlation, and applications of porous organic materials. On the basis of the states of matter, the porous organic materials can be classified as porous solids and porous liquids.84 Porous solids include hyper-cross-linked polymers C


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Figure 4. (Left) Simulated model of poly(p-dichloroxylene). The dimension of the simulation box was taken as 3.3175 nm. Adapted with permission from ref 110. Copyright 2007 American Chemical Society. (Right) Visuals of 2-D slices through the simulated pore structures of (a) benzene-based HCP, (b) biphenyl-based HCP, and (c) 1,3,5-triphenylbenzene-based HCP. Red and blue portions correspond to the occupied and unoccupied volume, respectively. The dimension of the simulation cell was taken as approximately 3.3 nm. Adapted with permission from ref 111. Copyright 2011 American Chemical Society.

easy scale-up. HCPs find prospective applications in gas adsorption and storage, catalysis, adsorption of aromatic molecules from water, etc. There are three techniques practiced for the synthesis of HCPs, namely, post-cross-linking of polymers, direct one-step polycondensation, and using external cross-linkers.92 The advent of the third synthesis technique has helped to improve the diversifiability of HCPs. The post-cross-linking technique commences with the dissolving of the polymer precursors in solvent. Upon swelling, the polymer chains untangle, and the space between them becomes occupied by the solvent, which helps to introduce space between the chains. The polymer chains are thereafter subjected to cross-linking. Upon removal of the solvent, the polymer chains are held apart by the cross-links, eventually resulting in an interlinked porous polymer.97 Hyper-crosslinked polystyrene came to be known first in the late 1960s by Davankov et al.85 Since then, Davankov et al. have made substantial contributions in this discipline to enrich it immensely. They continued to synthesize HCPs adopting linear polystyrene as a precursor while taking advantage of different cross-linkers such as p-xylylenedichloride,98 chloromethyl methyl ether,99,100 etc. The maximum BET surface area obtained was 1106 m2·g−1.100 As polymer precursors, polystyrene-divinylbenzene101,102 and vinylbenzyl chloridedivinylbenzene103−105 showed their merits, the latter yielding HCPs with a BET surface area as high as 2090 m2·g−1.105 Tan et al. ascertained that the pore size of HCPs can be customized accordingly by varying the divinylbenzene concentration in poly(divinylbenzene-covinylbenzyl chloride) precursors followed by hyper-cross-linking the precursors.106 This study presented the ease with which the porous character of the HCPs can be manipulated. In 2012, Tan and co-workers107 reported the synthesis of HCPs with highly dispersed Pt nanoparticles that resulted in “hydrogen spillover”,108 thereby improving the H2 storage capacity of the HCPs. Recently, Dai et al.109 reported the synthesis of ordered mesoporous phenolic resin polymers via interfacial hyper-cross-linking. The strategy helped to increase the robustness of the framework as well as enhance the CO2 adsorption capacity of the material. Unlike the post-cross-linking technique which makes use of polymer precursors followed by cross-linking, the direct one-step polycondensation technique involves the synthesis of HCPs directly from small monomers, for instance, dichloroxylene, 4,4′-bis(chloromethyl)-1,1′-biphenyl, etc.110 The Cooper group adopted this technique to successfully synthesize HCPs as monolithic blocks that helped to subside the volumetric methane storage concerns of HCPs pertaining to the packing of porous particulate materials.96 The simulated model of HCP,

(HCPs),85 polymers of intrinsic microporosity (PIMs),86 covalent organic frameworks (COFs),6 conjugated microporous polymers (CMPs),87 covalent triazine frameworks (CTFs),88 porous aromatic frameworks (PAFs),89 extrinsic porous molecules,90 and porous organic cages.91 The present review covers the family of microporous and mesoporous organic materials and excludes macroporous organic materials. In addition, natural porous organic materials and hybrid organic−inorganic porous materials are also beyond the scope of this review. HCPs antecede all of the other porous organic materials here.85 These amorphous polymers can be developed via post-cross-linking of polymers and direct onestep polycondensation and using external cross-linkers.92 They present unique features, such as swelling, and are cheaper (suitable for scale-up synthesis) but chemically less diversifiable. PIMs are one-of-a-kind amorphous porous polymers, some of which are soluble in organic solvents. Unlike HCPs and PIMs, COFs are crystalline frameworks. CMPs are amorphous polymers with high stability, high surface area, and adjustable pore size and are made of C−C and C−H bonds. They derive from conjugated starting materials which offer the opportunity to tune the luminescence band gap.93 CTFs are formed by the trimerization of rigid nitrile compounds.88 The first PAF with phenomenal surface area (SBET = 5600 m2·g−1) and outstanding stability, viz. PAF-1, was formed via Yamamoto-type Ullmann coupling of tetrakis(4-bromophenyl)methane.89 PAF-1 owes its exceptionally high surface area to the particular monomer structure as well as the chemistry and conditions selected. The extrinsic porosity in the porous molecules results from the inefficient packing of the molecules, thereby leading to intermolecular voids.94 Porous organic cages and porous liquids are molecules with intrinsic porosity (intramolecular voids) taken into account.91,95

2. FAMILY OF POROUS ORGANIC MATERIALS In this section, we intend to review the different genres of porous organic materials discussing the characteristic features of each family. 2.1. Hyper-Cross-Linked Polymers (HCPs)

HCPs are amorphous polymers endued with impressive surface areas, microporosities, and low densities. They are synthesized using considerably different chemistry (generally, Friedel− Crafts alkylation chemistry) from other porous organic materials. Properties such as conspicuous swelling and superb stability may be attributed to the high degree of cross-linking of HCPs. From the synthetic point of view, HCPs render certain advantages, such as mild reaction conditions, inexpensive reagents, monolithic products,96,92 etc., which reinforce their D


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solution-processability and thereby film-forming property of PIMs. In the articles that followed, the membranes fabricated from PIM-1123,18 and PIM-718 revealed impressive gas permeabilities and selectivities. Figure 5120 illustrates the

namely, poly(p-dichloroxylene), is illustrated in Figure 4(left).110 On the other hand, the synthesis of HCPs using external cross-linkers was first reported by Tan’s group in 2011.111 This strategy adopts the knitting of aromatic units with the help of an external cross-linker, for instance, formaldehyde dimethyl acetal to yield HCPs, especially with special functionality and controlled micromorphology. This is a major advantage since it allows the use of a very broad range of aromatic monomers without any custom synthesis. In 2012, the Cooper group reported the synthesis of knitted HCPs that exhibit superb CO2 adsorption capabilities in dry conditions; however, they have poor performance in wet conditions, which are considered more realistic conditions for CO2 capture.112 Nevertheless, the potency of knitted HCPs for precombustion as well as postcombustion CO2 capture were reported in the course of time by the Cooper group113 and the Jiang group,114 respectively. Tan’s group reported the synthesis of a Pd catalyst immobilized in knitted aryl network polymers for catalysis in the Suzuki−Miyaura cross-coupling reaction.115 Their group recently used 1,4-dimethoxybenzene as an external cross-linker to yield novel microporous knitted HCPs with conjugated structures.116 The conjugated structures impart the polymers with high conductivities, thereby making them useful for electronic applications. The knitting strategy was also adopted by their group to synthesize soluble HCPs by using formaldehyde dimethyl acetal as an external cross-linker.117

Figure 5. Structural model pertaining to a fragment of PIM-1. Reprinted with permission from ref 120. Copyright 2006 The Royal Society of Chemistry.

structural model pertaining to a fragment of PIM-1. In 2007, triptycene-based PIMs were reported with surface areas (SBET = 1065 m2·g−1) improved upon the previously reported surface areas and with moderate hydrogen adsorption capacities.124 Upon functionalization, the triptycene-based PIMs revealed BET surface areas as high as 1760 m2·g−1.125 More recent work has focused on developing applications for these porous polymers, as well as enhancing their apparent properties. One example is the restructuring of PIM-1 via electrospinning, offering a greater surface area as polymer fibers rather than as powder.126 Another example is the fabrication of porous membranes with the postmodification of PIM-1.127,128 Several polymers that exhibit intrinsic microporosity and extended conjugation throughout their chains have been synthesized.129 These resulting polymers form a bridge between PIMs and CMPs, opening up the field of applications for PIMs into those that can make use of polymeric conjugation in membranes, such as organic supercapacitors. The recent breakthrough in the strategic synthesis of PIMs (especially PIMs with high molecular weight) is the mechanochemical synthesis approach that offers advantages such as speedy synthesis and solvent-free reaction.130,131 To sum things up, PIMs differ greatly from other porous polymers. They possess microporosity but lack designed frameworks. Their porosity results from the flexibility of the polymer chains themselves rather than interconnected bonds within the material itself. PIMs demonstrate not only high surface areas but also individual properties that allow these porous polymers to be soluble, leading to the development of porous membranes and extending their reach in industry.

2.2. Polymers of Intrinsic Microporosity (PIMs)

PIMs are amorphous microporous polymers with rigid polymer chains. They resemble a series of interclasped aromatic rings put together like a chain with contorted/disfigured sites, thereby dispossessing them of the ability to pack efficiently. The solubility in organic solvents of some PIMs and the filmforming properties of PIMs have enabled their adoption to fabricate membranes for gas separation. PIMs, which were introduced in 2002, are the brainchild of McKeown et al. The design evolved 4 years before this when they reported the design of a porous polymer from phthalocyanines conjoined with spirocyclic groups.118,119 In 2002, they eventually came up with the synthesis technique of phthalocyanine-based network polymers adopting the phthalocyanine-forming reaction of a bis(phthalonitrile) monomer.86a A metal ion template aids the reaction. The monomer ensues from the dioxane-forming reaction between 4,5-dichlorophthalonitrile and 5,5′,6,6′-tetrahydroxy-3,3,3′,3′tetramethyl-1,1′spirobisindane.120 The phthalocyanine-based network polymers revealed BET surface areas of 450−950 m2·g−1.86a At the same time, they published another article in which they reported the synthesis of porphyrin-based network polymers (SBET = 900− 1000 m2·g−1) adopting the dibenzodioxane-forming reaction between meso-tetrakis(pentafluorophenyl)porphyrin and bis(catechol) monomer.86b In 2003, the group reported the synthesis of a hexaazatrinaphthylene-based PIM, which is in the same league as the aforementioned PIMs in terms of surface area, with competence as a catalyst and adsobent.121 In late 2003, they came up with a series of PIMs (PIM 1−6)122 with specific surface area as good as the previously reported PIMs. PIMs 1−3 are a good example of how altering a comonomer can have a significant effect on the porosity of porous polymers. The increased size between the three fluorinated aromatic monomers122 reduces the surface area from 850 (exhibited in PIM-1) to 600 (PIM-2) and 560 m2·g−1 (PIM-3). Nevertheless, the study was noteworthy on the grounds that it reported the

2.3. Covalent Organic Frameworks (COFs)

Endowed with crystallinity as well as porosity, COFs comprise molecular building blocks linked via covalent bonds. The crystallinity of the COFs is backed by the reversible character of the polymerization reactions adopted to synthesize COFs, which favors the formation of thermodynamically controlled polymers. The variety of molecular building blocks (see section 3.1.1) and custom-made attributes (attained by functionalization) have enabled the adoption of COFs for a wide range of E


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Figure 6. Structural models of (left) COF-1 and (right) COF-5. Carbon, boron, and oxygen atoms are colored gray, pink, and red, respectively. Hydrogen atoms are excluded for clarity.

g−1), by the imine condensation reaction of 1,3,6,8-tetrakis(4formylphenyl)pyrene and tetra(p-aminophenyl)methane was reported recently.73 The imine-based COFs are extremely promising for exploration from a synthetic point of view owing to their impressive stabilities, easy approach to postsynthetic functionalization by coordinating the N atoms with metal ions, etc.136 Imide condensation reactions were also adopted to successfully synthesize polyimide COFs, viz. PI-COF-4 (SBET = 2403 m2·g−1) and PI-COF-5 (SBET = 1876 m2·g−1), that were utilized for drug delivery and release.62 However, the reaction is especially slow and takes approximately 5 days to complete. Dichtel et al. thoroughly analyzed the nucleation and growth mechanism of COF-5 under homogeneous conditions that enabled the assessment of the kinetics of COF formation.137 Xu et al. recently reported a synthesis approach of chiral COFs straight from chiral building units (sans postsynthetic functionalization). The gradual design of chiral building units (starting from a backbone followed by a scaffold design and finally a chiral building unit) is interesting.138 In addition to the solvothermal conditions adopted routinely for COF synthesis, the microwave heating method for the synthesis of 2-D and 3-D COFs139,140 was also reported. The microwave heating method disproved the idea that slow condensation is imperative to the synthesis of crystalline COFs. This approach has been used to rapidly synthesize COF-5 and COF-102 in less than 20 min.139,140

applications including gas adsorption and storage, catalysis, energy storage, and others. The pioneering first work on COFs was reported in 2005 by the Yaghi group. The earliest documented COFs, viz. COF-1 and COF-5, were synthesized by the self-condensation reaction of 1,4-benzenediboronic acid (boroxine anhydride formation reaction) and the co-condensation reaction of 1,4-benzenediboronic acid with 2,3,6,7,10,11-hexahydroxytriphenylene (boronate ester formation reaction), respectively.6 The solvent for the reaction was chosen so that 1,4-benzenediboronic acid was sparingly soluble in it, which in turn enables the slow condensation of 1,4-benzenediboronic acid, thereby facilitating the nucleation and growth of crystalline COFs.6 As revealed, the structures of COF-1 (SBET = 711 m2·g−1) and COF-5 (SBET = 1590 m2·g−1) conform to staggered and eclipsed networks, respectively.6 The structural models of COF-1 and COF-5 are illustrated in Figure 6. In 2007, the group reported the synthesis of 3-D COFs, viz. COF-102, COF-103, COF-105, and COF108. 132 The self-condensation reactions of tetra(4dihydroxyborylphenyl)methane and tetra(4dihydroxyborylphenyl)silane (boroxine anhydride formation reactions) yielded COF-102 and COF-103, respectively.132 Likewise, the co-condensation reactions of tetra(4dihydroxyborylphenyl)methane and tetra(4dihydroxyborylphenyl)silane with 2,3,6,7,10,11-hexahydroxytriphenylene (boronate ester formation reactions) yielded COF105 and COF-108, respectively.132 The family of 2-D COFs started to burgeon when the syntheses of COF-6, COF-8, and COF-10 were reported by the co-condensation reactions (boronate ester formation reactions) of 2,3,6,7,10,11-hexahydroxytriphenylene with 1,3,5-benzenetriboronic acid, 1,3,5benzenetris(4-phenylboronic acid), and 4,4′-biphenyldiboronic acid, respectively.133 Dichtel et al. reported for the first time the fabrication of COF thin films on single-layered graphene, which enabled the porous polymers processable to be adopted for membrane and optoelectronic applications in the future.134 Apart from the aforementioned boron-comprising COFs, other COFs, such as imine-based COFs (COF-300,76 3D-Py-COF,73 COF-505,135 etc.) and polyimide COFs (PI-COF-4, PI-COF562), were also reported in the past few years. The imine condensation reaction (Schiff base reaction) of tetra(4anilyl)methane and terephthalaldehyde yielded COF-300 (SBET = 1360 m2·g−1).76 The synthesis of first 3-D COF featuring fluorescence, namely, 3D-Py-COF (SBET = 1290 m2·

2.4. Conjugated Microporous Polymers (CMPs)

A conjugated microporous polymer refers to a macromolecule that possesses a microporous network that has building blocks within the system that give rise to π-conjugation.141−143 This conjugation is a result of alternating single and multiple bonds which arise from the overlap of two p orbitals (or d orbitals) crosswise with an intermediary σ-bond.144 CMPs possess this conjugation to a certain degree. The alternation of π and σ bonds throughout the network readily allows their structure to be utilized electronically. In tandem with inefficient molecular packing, CMPs offer properties for not only electronic but also porous applications. These materials are amorphous due to the freedom of rotation about the s bonds formed between building blocks. CMPs began their scientific journey in 2007 when Cooper’s lab reported the synthesis of a series of conjugated microporous poly(aryleneethynylene) networks, namely, CMP-1, CMP-2, CMP-3 and CMP-4, by Sonogashira−Hagihara cross-coupling F


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Figure 7. Generated structural fragments of CMP-1 (left) and CMP-3 (right). Highlighted (gray) are the 1,3,5-connected nodes and the aromatic struts. Adapted with permission from ref 87. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Figure 8. Structural models of (left) CTF-1 and (right) CTF-0. Carbon and nitrogen atoms are colored gray and blue, respectively. Hydrogen atoms are excluded for clarity.

of alkyne and halogen monomers.87 The study establishes that the length of branched arm varies inversely with the specific surface area of the polymers. Bearing two ethynes and one benzene in the branched arm, CMP-1 surpasses the other three CMPs in BET surface area. The structural fragments of CMP-1 and CMP-3 are illustrated in Figure 7.87 The following year, Cooper’s lab145 came up with the synthesis of two other CMPs, namely, CMP-0 and CMP-5. CMP-0 (SBET = 1018 m2·g−1), with one ethyne and one benzene in its branched arm, outdoes CMP-1 in BET surface area. In 2010, Cooper’s lab146 reported the synthesis of a series of CMPs bearing high surface areas from tetrahedral monomers. The CMPs formed by Yamamoto coupling of tetrakis(4-iodophenyl)methane and tetrakis(4bromophenyl)-1,3,5,7-adamantane monomers with BET surface areas of 3160 and 3180 m2·g−1, respectively, top the list of CMPs in terms of surface area. The study asservated the fact that the monomer structure and reaction conditions play a crucial role in determining the surface area of the polymers. Despite the fact that Yamamoto coupling tops the reaction list for the synthesis of CMPs with high surface areas, other coupling reactions also provided CMPs with impressive surface areas. For instance, Sonogashira−Hagihara cross-coupling, Suzuki−Miyaura cross-coupling, and phenazine ring fusion reactions yielded PSN-2147 (SBET = 1042 m2·g−1), HPOP-1148 (SBET = 1148 m2·g−1), and Aza-CMP@50028 (SBET = 1227 m2· g−1), respectively. Lithiation imparts CMPs with competence for H2 storage (6.1 wt % at 1 bar and 77 K), thereby making them useful for clean fuel applications.149 The competence of CMPs (and perhaps other porous polymers) in CO2 capture at

standard ambient temperature and pressure depends on its composition in the first place. Despite a lower surface area than its analogues functionalized with dihydroxy and dimethyl groups, CMP-1 nonetheless outperforms the functionalized analogues in CO2 sorption capacity at standard ambient temperature and pressure.150 Developing research on CMPs is ever increasing, with porous properties such as surface area exceeding 1000 m2·g−1. In addition to applications such as molecular adsorption, catalysis, and separation, extended conjugation opens up opportunities in alternative applications. CMPs show high capability for electronic applications, with such materials being used in photocatalysis or optoelectronics.93 2.5. Covalent Triazine Frameworks (CTFs)

CTFs were first reported in 2008 by Thomas et al. This exclusive genre of COFs emanates from the cyclotrimerization of the functional nitrile group of nitrile compounds, developing s-triazine rings.151,152 The earliest CTF, namely, CTF-1, was synthesized by the cyclotrimerization of 1,4-dicyanobenzene using a ZnCl2 catalyst at 400 °C under ionothermal conditions.88 The monomer structure, monomer:ZnCl2 ratio, reaction temperature, and reaction time play pivotal roles in the structure and properties of CTFs. Albeit bearing a moderate BET surface area of 791 m2·g−1, CTF-1 is significant in its own way, being the first documented crystalline and microporous CTF. The use of alternate monomers or changes in the monomer:ZnCl2 ratio may offer higher surface areas; however, the resulting triazine-based polymers produced were found to be amorphous.88 Following from this, the effects that varying G


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another case of band gap engineering with the use of additional functionalized comonomers, as previously seen with pyrenebased CMPs.161

reaction temperatures and reaction times can have on the structures and properties of CTFs were explored.153 In consideration of 1,4-dicyanobenzene, the BET surface area of the polymer increased remarkably from 920 to 3270 m2·g−1 when using a combination of temperatures (400 °C for 20 h, followed by 600 °C for 96 h). As the temperature of the cyclotrimerization reaction for the synthesis of CTFs is increased to 600 °C, the mesoporous character of the resulting CTFs also increases. In 2010, Thomas et al. reported the synthesis of the crystalline porous organic material, CTF-2, by the cyclotrimerization of 2,6-napthalenedinitrile using ionothermal conditions (ZnCl2 catalyst).154 However, the polymer structure revealed a poor BET surface area of only 90 m2·g−1. Altering the monomer:ZnCl2 ratio to 1:5 and the temperature to 450 °C for 40 h resulted in a considerable increase in the surface area of the polymer (SBET = 2255 m2·g−1), although the resulting triazine-based polymer lacked the long-range order previously exhibited, instead proving to be amorphous with micromesoporous properties.154 Thomas et al. also reported the synthesis of CTF-0 polymers (SBET = 232−687 m2·g−1) via the cyclotrimerization of 1,3,5-tricyanobenzene under ionothermal conditions (ZnCl2 catalyst).152 Upon adoption of dual reaction temperature (400 °C followed by 600 °C), the resulting CTF-0 polymer showed an appreciable increase in surface area as well as CO2 sorption capacity.152 The structural models of CTF-1 and CTF-0 are illustrated in Figure 8. The ionothermal synthesis of CTFs described here suffers from some serious drawbacks including a prolonged process, elevated temperature, and problems separating ZnCl2 completely at the end of the reaction.155 In the interest of resolving these issues, Cooper et al. reported the synthesis of a series of CTFs from different aromatic nitriles (monomers) in the presence of trifluoromethanesulfonic acid (catalyst) at room temperature and under microwave heating conditions.155 The materials were found to be principally amorphous; however, some of them synthesized under microwave heating conditions revealed limited order. This is an adaptation of the study performed on molecular triazine compounds by Meyer et al., in which they produced symmetrical triazine-based compounds with the use of metal-free acid catalysts.156 At the same time as Cooper, Dai et al.157 published a study on the synthesis of porous triazine framework membranes by superacid-catalyzed sol−gel-like polymerization of 4,4′-biphenyldicarbonitrile. This was the first report to synthesize highly cross-linked polymer membranes. They achieved a surface area similar to that of CTF-1 (TFM-1, 738 m2·g−1; TFMT-550,158 421 m2·g−1) as well as exceptional CO2 permeability. Martinez-Alvarez et al. further reported on the use of trifluoromethanesulfonic anhydride for the formation of triazine compounds.159 The use of such strong Lewis and Brønsted acids allows for significantly reduced reaction conditions for CTF synthesis. This method of superacid catalysis has not only been performed for CTF synthesis, but Dai et al. also proposed the use of trifluoromethanesulfonic acid for the condensation of acetyl compounds for porous membrane synthesis,160 successfully synthesizing porous carbonaceous membranes, as well as N-doped porous carbonaceous membranes, which exhibit surface areas of 651 and 382 m2·g−1, respectively. Cooper et al. further investigated the implementation of triazine rings in alternative porous polymers, studying their electronic properties and their structural integrities.161 Introducing triazine rings into CMP complexes resulted in high surface areas (TNCMP2; 995 m2·g−1) and extended conjugation. This also illustrates

2.6. Porous Aromatic Frameworks (PAFs)

PAFs are open framework porous polymers bearing outstanding surface area and top-notch stability. They owe their outstanding surface area to the monomer structure and reaction conditions. The superb stability of PAFs can be ascribed to its covalent bonds and rigid phenyl framework.162 PAFs can be readily functionalized to extend their scope of application. PAFs, with PAF-3 in the lead, present commendable performance in the selective adsorption of greenhouse gases, such as carbon dioxide, methane, etc. In addition, PAFs are very effective in other pursuits. PAFs came into the scientific limelight in 2009 when PAF-1, which has a phenomenal BET surface area of 5600 m2·g−1, outstanding H2 and CO2 storage capacities, and superb stability, was synthesized by Yamamoto coupling.89 This reaction provides a direct method for the homocoupling of monomers, offering a simple synthetic technique for large networks like PAFs, CMPs, etc., to form. The differing factor between PAFs and CMPs is that PAFs lose their framework conjugation inasmuch as the conjugation is broken by the central atom of the tetrahedral monomers, whereas CMPs maintain the conjugation that originates from the monomers. The crystalline structure of diamond acted as the cue for the design of PAF-1 wherein it was reasonably speculated that substituting phenyl groups for C-C covalent bonds of diamond would sustain the stability (comparable to diamond) in the structure of PAF-1 besides indulging in ample exposure of the phenyl groups, which might help expand the internal surface area of PAF-1.89 Characterization of PAF-1 via theoretical simulations resulted in an unprecedented BET surface area of 5640 m2·g−1 and a density as low as 0.315 g·cm−3. On that account, tetrakis(4bromophenyl)methane was thoughtfully chosen as the building block for PAF-1 synthesis. In addition to the contemplative design of PAF-1, which shows a phenomenally high surface area, the coupling reaction adopted to realize PAF-1, viz. Yamamoto coupling, also supported the theoretical calculations. This particular coupling reaction is more efficient for the removal of terminal halo groups, which are responsible for reductions in surface area, than other coupling reactions.162 The successful synthesis of PAF-1 also incited the adoption of Yamamoto coupling for the synthesis of PAF-3 and PAF-4 using monomers of tetrakis(4-bromophenyl)silane and tetrakis(4-bromophenyl)germane, respectively. Both of these new materials have demonstrated impressive surfaces areas (PAF-3, 2932 m2·g−1; PAF-4, 2246 m2·g−1) and exceptional selectivity for the adsorption of gases (PAF-3 CO2/N2 selectivity, 87; CH4/N2 selectivity, 30).13 Prior to these, Cooper et al. demonstrated Yamamoto coupling for PAF-3 with a BET surface area of 1102 m2·g−1.146 Zhou’s lab followed up with the synthesis of PAF-3 (by the name PPN-4), which is endowed with an incredible surface area of 6461 m2·g−1 and unparalleled gas storage capabilities.163 The Yamamoto coupling reaction proves to be the most effective procedure for the synthesis of PAFs with outstanding surface areas and gas sorption capabilities. PAF-5 is another example, demonstrating a BET surface area of 1503 m2·g−1 with 1,3,5-tris(4-bromophenyl)benzene building blocks.164 Alternative coupling reactions were also explored, yielding more moderate/mediocre surface areas. For instance, PAF-2 was synthesized via the trimerization of H


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criteria specified in the paper to help search for unidentified crystal structures with microporosities surpassing those of the prevalent microporous crystals.185 In the following year, they published an article documenting the synthesis of phthalocyanine unsolvated nanoporous crystals (by implementation of single-crystal-to-single-crystal transformation techniques) that exhibit BET surface areas of 850−1000 m2·g−1.186 In 2012, Mastalerz et al. reported the synthesis of an extrinsic porous crystal by the self-assembly of triptycene trisbenzimidazolone via H bonds. The BET surface area of the porous crystal was assessed as high as 2796 m2·g−1.187 This paper invalidated the belief that a high surface area is attainable exclusively with intrinsic porous materials.188,189

tetrakis(4-cyanophenyl)methane, with a BET surface area of 981 m2·g−1.165 PAF-11 synthesized by the Suzuki−Miyaura cross-coupling of tetrakis-(4-bromophenyl)methane and 4,4′biphenyldiboronic acid presented a BET surface area of 704 m2· g−1.166 JUC-Zs, a class of PAFs, took the first step in 2011 when JUC-Z1 (“JUC” stands for Jilin University, China and “Z” for Zeolites), a porous framework that comprises p-iodiooctaphenylsilsesquioxane building blocks linked together by covalent bonds and may bear zeolitic LTA topology,167 was reported.168 The syntheses of JUC-Z2, a microporous electroactive polymer,169 and JUC-Z12, a microporous polymer featuring impressive catalytic competence in the Knoevenagel reaction,170 were reported in 2011 and 2014, respectively. JUCZ4 and JUC-Z5 were synthesized by implementing redox reactions from JUC-Z4-Cl.171 The synthesis of JUC-Z13-JUCZ19 by Yamamoto coupling reaction of tetrakis (4bromophenyl)methane and poly-4-bromostyrene has been reported recently.172 Postsynthesis functionalization of PAFs bestows it with competence of highly selective amine uptake173 and high-caliber CO2 capture.174,175 In 2014, Sozzani et al.176 published a paper where “mechanics” teamed up with “chemistry” in consideration of PAFs. The insertion of pphenylene groups in PAF-3 renders dynamism in the material, which can thereby come up with enthralling applications. Zhu’s lab177 reported a series of PAFs, namely, F-PAF-50, Cl-PAF-50, Br-PAF-50, and 2I-PAF-50, with tailor-made pore sizes (realized by the adoption of an ion exchange technique) functioning as sieves for the selective separation of gases besides connecting the Cl-PAF-50 and 2I-PAF-50 columns for full separation of a quinary gas mixture. Zhu et al.178 recently reported the synthesis of fullerene-based PAFs, which are interesting from a structural point of view. Albeit the research on PAFs has made great headway, there are many facets to explore.

2.8. Porous Organic Cages

The development of porous molecular systems is increasingly gaining momentum. Molecular solids demonstrate porous properties that are not only porous but can possess solubility, making them suitable as membranes.190 These materials attain their porosity via one of two ways: intrinsically, with the porosity based inside cage-like structures or extrinsically, having voids form via the inefficient packing of the solid.191 Some materials can possess both properties, having inefficiently packed cages, offering potential for exceptional molecular storage, as well as separation. Similar to crystalline porous polymers, cages can be manipulated into rigid structures. However, their structure can deviate from the ideal upon removal of the occupying solvent, packing into dense solids, where only intrinsic porosity is available.192 Figures 9193 and

2.7. Extrinsic Porous Molecules

In the context of porous molecules, intrinsic porosity refers to the porosity that arises out of intramolecular voids and that is ingrained within the molecules. In contrast, extrinsic porosity refers to the porosity arising out of voids intervening the molecules (outside of molecules) owing to inefficient packing of molecules.90,179 In 1964, Allcock and colleagues reported the synthesis of a molecular crystal, viz. tris(o-phenylenedioxy)phosphonitrile, by the reactions of phosphonitrilic chlorides with catechol and triethylamine.180,181 In the paper181 the authors did not speak of the porosity of the crystal; however, they conjectured the possibility of inefficient packing of the crystal and that its behavior/activity “in contact with organic liquids or vapors may be described as that of a crystalline molecular sponge”.181 The porosity and selective gas sorption capacity of the crystal were substantiated by the study of Sozzani and coworkers.90,182 In 2006, Hulliger and colleagues determined the Langmuir surface area of tris(o-phenylenedioxy)phosphonitrile in a pseudohexagonal guest-free state as only 240 m2·g−1.183 Shimizu et al. analyzed the CO2 sorption capacity of a crystalline porous host arising out of the self-assembly of a bisurea macrocycle to confirm the microporosity of the host and that the host bears a BET surface area of 316 m2·g−1 (evaluated at 0 °C).184 In 2009, McKeown et al. reported the synthesis of a microporous organic crystal (SBET = 278 m2·g−1) constituting 3,3′,4,4′-tetra(trimethylsilylethynyl)biphenyl and featuring impressive stability.185 In addition to the reported microporous organic crystal, this paper is particularly significant owing to the

Figure 9. (a) Strategic design of the porphyrin box with square and triangular building units. (b) Synthesis of the same. Reprinted with permission from ref 193. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

10189 depict the rationalization behind the design of crystalline porous organic cages, which is also exemplified by the corresponding building blocks and the topology of the cages. The attainment of crystalline porous organic cages, with a specific topology, lies in the thoughtful choice of dissimilar monomers, which aggregate to develop Archimedean solids.193 Figure 9193 shows the design of a porous organic cage (PB-1; porphyrin box) with rhombicuboctahedron topology. The combination of the 2-connected angular triptycene tetaol with the 3-connected triangular triboronic acid yields a porous organic cage with cuboctahedral topology, illustrated in Figure 10.189 The topology of porous organic cages emerges as more complex with interlocking cages. Cooper et al. reported the designed synthesis of tetrahedral cages, triply interlocked to constitute a dimer (Figure 11194). The topology of each tetrahedral cage is analogous to that of a triple torus.194 The interlocking of these organic cages may provide an ideal I


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Figure 11. Topology of triply interlocked cages: (a) 3-D schematic representation of interlocking dimers, (b) top-down 2-D representation of interlocking 3-torus dimer, (c) individually highlighted (yellow) interlocking crossing cycles (i−iii) and the outer, nonpenetrating cycle (iv) of the interlocking cage molecules. Reprinted with permission from ref 194. Copyright 2010 Macmillan Publishers Limited.

Figure 10. (a) One-step condensation reaction of triptycene tetraol with triboronic acid. Resulting structure of a cuboctahedral cage. (b) Pore structure of cuboctahedral cage, illustrating selected dimensions of the molecular structure. Edges of the cage structure are represented in red. Reprinted with permission from ref 189. Copyright 2014 WileyVCH Verlag GmbH & Co. KGaA, Weinheim.

method for the development of complex porous organic materials, along with other supramolecular constructs. 2.9. Genesis of Porous Liquids

Among porous organic materials, the “hot off press” porous liquids deserve special mention. The title of the recently published paper84 is very significant because the ephemerality of the pores was one of the problems that materials chemists had been struggling with. In effect, the study of porous liquids began in 2007. In an article published that year, Giri and coworkers95 shared their perception of microporous liquids. They clearly set the target to attain liquids bearing intramolecular voids, which means that the porosity is ingrained within the liquid molecules (intrinsic porosity), and urged for three types of liquids (depicted in Figure 1295) as (i) type 1 “neat liquids”, (ii) type 2 “empty hosts dissolved in sterically hindered solvents”, and (iii) type 3 “f ramework materials dispersed in hindered solvents”. The observations and findings195 by the same research group reported in 2012 brought them a step closer to the target of realizing type 1 microporous liquids. They observed that organic imine cages could be easily liqueified upon alkylation by n-C5, n-C6, iso-C6, and n-C8 functional groups. However, the alkyl tails can get in the hollow cavities to undermine the microporosity. In 2014, they published a paper in which they continued their pursuit to explore three more alkylated organic

Figure 12. Schematic representation of porous liquids with intrinsic porosity compared to ordinary liquids with extrinsic porosity. Adapted with permission from ref 95. Copyright 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

cages, viz. n-C12, iso-C13, and neo-C14 (as shown in Figure 13196).196 Despite the fact that the melting point of n-C5 organic cage is higher than that of n-C12 cage, scientists determined the former to be a plausible type 1 microporous liquid because the small-length n-C5 chains cannot fill or take over the whole space of the hollow cavities, thus leaving behind an unfilled segment (indicated by theoretical simulation as illustrated in Figure 13196) in comparison with the n-C12 chains, which almost fill the entire space of the hollow cavities. Attributed to the micropores in the n-C5 cage, the solubility of methane gas is augmented (indicated by theoretical simulation as shown in Figure 13196). This was the first report on experimentally synthesized microporous liquids; however, no direct experimental proof on the accessibility of the pores to an J


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Figure 13. (Top, left) Synthesis route for the organic cages. (Top, right) Graphical representation showing the fraction of cages occupied by the tails of the chains (⟨XOC⟩) with respect to the temperature (T) corresponding to n-C5, n-C12, and n-C20 (vertical bars show the equilibrium root-meansquare fluctuations of XOC). (Bottom, left) CH4 absorption isotherms obtained from Grand Canonical Monte Carlo simulations corresponding to nC12, n-C5, and neo-C14. Average mole ratio ⟨NCH4/NC⟩ describes the amount of CH4 absorbed by the sample. (Bottom, right) Fraction of CH4 molecules (guest), ⟨xinCH4⟩, inside the host cages for n-C12, n-C5, and neo-C14. Adapted with permission from ref 196. Copyright 2014 the Owner Societies.

external gas was furnished. In 2014, the other group led by Dai197 reported the synthesis of an inorganic porous liquid. Despite these significant studies, the “quantum leap” was yet to come. In 2015, Giri and co-workers84 hit a masterstroke to come up with type 2 microporous liquids by developing cage molecules (functionalized with six crown ether groups to enhance solubility) endowed with empty space and impressive solubility in 15-crown-5 solvent, the molecules of which are overly voluminous to be able to get in the cage cavities (Figure 14a84). The solubility of methane gas is octupled, which is attributed to the cage micropores. Considering practical feasibility, scale-up synthesis, etc., another type 2 microporous liquid was prepared by employing a mixture of diamines to yield a mixture of scrambled cages (Figure 14b84), which were dissolved in hexachloropropene solvent. The cages included empty space, but the molecules of the solvent were too large to pass into the cage cavities. No functional group that could get into the cage cavities was used in the synthesis of the scrambled cages, and the microporous liquid (developed from the scrambled cages) was endowed with outstanding solubility for

methane and other gases. Upon inclusion of chloroform in a xenon-saturated microporous liquid, the xenon gas was promptly released, making room for the smaller chloroform molecules to get into the cage cavities. However, there is no such outcome when 1-tert-butyl-3,5-dimethylbenzene (oversized molecules) was introduced. The method introduced in this study made many important contributions. First, in lieu of making the cage play the role of a liquid, the cage was dissolved in a solvent whose molecules cannot access the cage cavity. Second, the functional groups that could disturb the system by getting inside the cage cavity were judiciously excluded from this synthesis. Third, crown ethers were used to functionalize the cage molecules and as the solvent. Crown ethers, which are amphiphilic by nature, help in the lowering of the melting point. On the basis of the way that this porous liquid responded to the guest size, it is clear that the porous liquid can be used for gas separation. Porous liquids at this initial stage cannot match other porous organic materials, such as COFs, PAFs, etc., in terms of surface area, gas storage capacity, etc. In K


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Figure 14. (a) Realization of microporous liquid by developing cage molecules functionalized with six crown ether groups to improve solubility in 15-crown-5 solvent, the molecules of which are larger than the cage cavities. Carbon, oxygen, nitrogen, and hydrogen atoms are colored gray, red, blue, and white, respectively. (b) Mixture of scrambled cages was developed upon reaction of trialdehyde with two diamines (designated here as 3 and 13). Mixture of scrambled cages was subsequently dissolved in hexachloropropene solvent to yield microporous liquid. Adapted with permission from ref 84. Copyright 2015 Macmillan Publishers Limited.

contrast to these limitations, the ability of the porous liquids to flow can be exploited for useful applications.

3. DESIGN OF POROUS ORGANIC MATERIALS The design of porous organic materials considers the topology, reticular chemistry involved with the generation of extended structures, and other structural aspects (that have become accessible with the advent of theoretical techniques) of the materials. In this section, we intend to review the topology of porous organic materials followed by the reticular chemistry and eventually the design approaches of porous organic materials.

Figure 15. Illustration of a pore (marked in yellow) in a hollow cube.

3.1. Topology Design

1 is called the Euler number198 or Euler characteristic (χ) and is represented as

The topological consideration of porous organic materials is fundamental to the design of the materials because it describes the connectedness between the structural building units constituting the materials. The tactical/strategic design of porous structures seeks to understand the topology of porous media. The concept of topology germinated from the polyhedron formula198 by Euler is given as V−E+F=2

χ=V−E+F

(2)

The Euler characteristic is an intrinsic attribute (i.e., invariant198) of each topological space. Examples of such invariants of topological space include connectedness, compactness, etc. The study of topology of porous organic materials helps to understand the connectedness between the structural building units forming the materials and thereby to realize the pore structure of the materials. The topology of crystalline porous organic materials (such as COFs) can be determined conveniently; however, the topology of amorphous porous organic materials (such as PAFs) is rather difficult to ascertain. Crystalline porous organic materials are endowed with both long-range and short-range structural order. On the other hand, amorphous porous organic materials do not

(1)

where V, E, and F correspond to the number of vertices, edges, and faces, respectively, of a polyhedron. For instance, Figure 15 shows a hollow cube (pore/void inside the cube represented by the yellow sphere) that has V = 8, E = 12, and F = 6 and thereby complies with the above formula. Nevertheless, Euler’s polyhedron formula does not hold for all topological surfaces (topological spaces). The expression on the left-hand side of eq L


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Table 1. Topological Synopsis of Typical Crystalline (Ordered) Porous Organic Materialsa

M


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Table 1. continued

N


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Table 1. continued

O


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Table 1. continued

a

Carbon, boron, oxygen, nitrogen, and silicon atoms are colored gray/green, pink, red, blue, and yellow respectively. Hydrogen atoms are excluded for clarity. The pore size values in parentheses were determined from crystal structures. bThe topological symbols srs, nbo, bnn, gra, ctn, bor, dia, and pts have come from the Si net of SrSi2, NbO net, boron nitride net, graphite structure, a hypothetical carbon nitride (C3N4) structure, boracite net, diamond net, and cooperite (PtS) net, respectively. cThe topological symbol corresponding to orthorhombic crystalline material bearing space group Cmm2 was not available in the Reticular Chemistry Structure Resource (RCSR) database. dPore size calculated by implementing nonlocal DFT to gas sorption isotherms. ePore size calculated by implementing quenched solid DFT to gas sorption isotherms. fPore size calculated by implementing Barrett−Joyner−Halenda (BJH) model to gas sorption isotherms.

“quasiregular nets”200 as those with symmetrically equivalent edges but lack symmetrically equivalent angles. Hence, these nets are reassuring to realize. The coordination figures201 in consideration of regular nets are equilateral triangle (srs topology), square (nbo topology), regular tetrahedron (dia topology), etc. The aforementioned topological codes for crystal nets were laid down by Yaghi et al. in the Reticular Chemistry Structure Resource (RCSR) database.203,205−208 Table 1 illustrates the topologies of the crystal nets reported thus far based on some typical crystalline porous organic materials. For CTF-1 and COF-1, the oligomers (triangular) during polymerization have been shown in the table and not the monomers/starting materials. The aforementioned crystalline porous organic materials designed from especially symmetric building blocks are endowed with homogeneous pore sizes. The designed synthesis of crystalline porous organic materials with heterogeneous pore sizes is a challenging exercise. Nevertheless, such materials present the opportunity to find atypical topologies and unusual attributes. Structural heterogeneity in crystalline porous organic materials depends on the “fit” of molecular symmetry between judiciously selected connectors (building units yielding

have long-range structural order but may have short-range order. We intend to elucidate the topology of crystalline and amorphous porous organic materials under two different subsections for a thorough analysis of the topic. 3.1.1. Topology of Crystalline Porous Organic Materials. The study of the topology of crystal structures comes to light with the representation of crystal structures as nets.199−202 O’Keeffe and colleagues described the net as “a periodic connected simple graph”199 wherein a graph has been described as “A graph consists of vertices labeled i, j,... and edges corresponding to pairs of vertices (i, j)”.199 The crystal structures are constituted of atoms acting as the vertices and chemical bonds connecting the atoms as edges.203 In relation to crystalline porous organic materials, especially COFs, the secondary building units204 (monomers) are represented as polygons that are connected via covalent bonds yielding vertices and edges. As stated categorically by O’Keeffe and colleagues, the nets that bear “simple high-symmetry topologies”200 are fundamentally important and favorable for designed synthesis.200,201 Taking this idea forward, they interpreted “regular nets”200 as those characterized with symmetrically equivalent vertices, edges, and angles and P


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and the other functional group undergoes self-condensation (yielding two constituent COF) or alternatively a reversible reaction with a third monomer (yielding three constituent COF). The entire approach is schematically illustrated in Figures 19214 and 20.214 Jiang et al.215 adopted the double-stage synthesis technique that renders the advantages of controlling the arrangement of molecular building blocks and adjusting the pore size to yield state-of-the art COFs involving structural intricacy (Figures 21, 22, 23, and 24215). The scientific fraternity has lately strived for unique approaches216,217 to attain multiple constituent COFs in the interest of reinforcing its structural heterogeneity and conceivably its sphere of application. A systematic approach toward the design and synthesis of crystalline porous organic materials from suitable connectors and linkers was first laid down by the breakthrough concept of “reticular chemistry”.218−220 The word “reticular” literally means entangled like a net. The concept of reticular chemistry goes as follows:220 First, an ordered porous organic material structure can be thought of in advance (anticipating the topology of the structure). Thereafter, we can associate pertinent building blocks with strong bonds to obtain extended structures (chains in 1-D, sheets in 2-D, networks in 3-D) of porous polymers that resemble a net. The joints and links of the extended structures of porous polymers are identical to the vertices and edges of a net, respectively. During reticular modeling of porous polymers, the building units are joined by robust bonds (thereby retaining the structural characteristics of the blocks) to develop extended porous organic material structures. Hence, this approach applies in the design of ordered crystalline porous organic materials, such as COFs. The science behind the successful reticular modeling of COFs from building blocks has been explained in detail by Heine et al.221 and is recounted as follows: Figure 25221 illustrates the building blocks, namely, four connectors “I−IV” and five linkers “a−e”, pertaining to the development of 2-D COFs. Characterized by geometry, the connectors can be classified as trigonal “T” (connectors I, II, and III) and hexagonal “H” (connector IV). The linkers can likewise be classified as linear “l” (linkers a, b, and e) and trigonal “t” (linkers c and d). Considering that one connector combines with one linker to yield every particular COF structure and that the topology of the COF structure is regulated by the geometries of the combining connector and linker, the resulting COFs can be classified as “Tl” (I-a, I-b, I-e, II-a, II-b, II-e, III-a, III-b, III-e), “Tt” (I-c, I-d, II-c, II-d, III-c, III-d), “Hl” (IV-a, IV-b, IV-e), and “Ht” (IV-c, IV-d). For instance, the extended structures of I-a, II-a, II-b, II-c, and II-d give COF-1,6 COF-5,6 COF-10,133 COF-8,133 and COF-6,133 respectively. Considering the four types of stacking, namely, AA (hexagonal), AB (hexagonal), serrated (orthorhombic), and inclined (monoclinic), that are formed after crystallization of the 2-D COF sheets, the COF-5 structures formed with the four types of stacking are shown in Figure 25.221 After assessment, the structural characteristics (bond lengths, bond angles, etc.) of the molecular building blocks were found to be retained in the respective COF structures.221 This is the basic foundation on which the concept of reticular modeling of crystalline porous organic materials stands. As discussed previously, a common method of codification for the topology of crystal nets has been laid down by Yaghi and co-workers in the Reticular Chemistry Structure Resource (RCSR) database.203,205 Computer pro-

vertices) and linkers (building units yielding edges). Zhao et al.211 reported the designed synthesis of dual-pore 2-D COF, endowed with triangular micropores (7.1 Å) as well as hexagonal mesopores (26.9 Å), by the combination of D2hsymmetric 4,4′,4″,4‴-(ethene-1,1,2,2-tetrayl)-tetraaniline connectors and C2-symmetric terephthalaldehyde linkers. Figure 16211 shows the graphical representation of differential pore

Figure 16. Differential pore volume plotted against pore width for COF bearing triangular micropores and hexagonal mesopores. Adapted with permission from ref 211. Copyright 2014 American Chemical Society.

volume plotted against pore width for the dual-pore COF. Zhang et al.212 recently brought in the idea of using molecularly desymmetrized building blocks to attain 2-D crystalline COFs endowed with dual pore sizes. The COFs, namely, HP-COF-1 and HP-COF-2, were synthesized using C2ϑ-symmetric 5-(4formylphenyl)isophthalaldehyde and 5-((4-formylphenyl)ethylene)isophthalaldehyde connectors and C2-symmetric hydrazine linker. Figure 17212 depicts the design approach and structural models of HP-COF-1 and HP-COF-2. Very recently, Zhao et al.213 reported the designed synthesis of ternary-pore COFs endowed with three different pore sizes via a heterostructural mixed linker strategy, a very hot topic sought after in supramolecular chemistry. The design involves the combination of a D2h-symmetric tetraamine connector and two C2-symmetric dialdehyde linkers of varied lengths (illustrated in Figure 18213). This is the first report on the adoption of heterostructural mixed linker strategy to successfully synthesize COFs with unique topology. The novel topologies of crystalline porous organic materials, thus reported of late, can bring forth promising applications. The designed synthesis of COFs with more than two building blocks has added to their structural intricacy and heterogeneity. Zeng and co-workers214 adopted an approach where monomers undergo orthogonal reactions conceiving two covalent bonds between differing monomers to bring about COFs. The development of these multiple constituent COFs, namely, NTU-COF-1 (two constituents) and NTU-COF-2 (three constituents), requires a monomer to accommodate a minimum of two functional groups (for instance, 4formylphenylboronic acid accommodates aldehyde group as well as boronic acid). In regard to the two functional groups, one undergoes a reversible reaction with a secondary monomer Q


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Figure 17. (a−c) Adoption of molecularly desymmetrized building blocks to realize dual-pore COFs. Structural models (pores shown as yellow balls) of (d and e) isomers of dual-pore HP-COF-1 and (f and g) those of dual-pore HP-COF-2. Adapted with permission from ref 212. Copyright 2015 American Chemical Society.

Figure 18. Strategic design conceiving dual- and triple-pore COFs. Reprinted with permission from ref 213. Copyright 2016 American Chemical Society.

grams such as TOPOS222 and Systre223,224are also very useful to ascertain the topology of crystal nets. The most recent turning point in the design of porous organic materials came when Yaghi and co-workers reported the synthesis of woven COF135 (namely, COF-505). Despite not being topologically novel (COF-505 bears dia topology),

this COF is a unique example of scientific artistry and craftsmanship that is structurally flexible/adaptable compared to routine COFs. The interesting aspect of this study is the consideration of the Cu(I) complex as a monomer, which provides two look-alike horseshoe-shaped molecules that form the two weaving threads and copper ions that act as templates R


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atoms of glass arrange themselves as three-dimensional extended “networks” analogous to those of crystals. However, the X-ray diffraction results illustrate that the networks in glass lack periodicity and symmetry unlike those in crystals. This indicates that the networks in glass are “random” or disordered. The networks are “continuous” in the sense that they are devoid of any break or discontinuity in the bonds forming the network. In addition, Zachariasen226 also illustrated that the crystalline analogues of glass oxides constitute oxygen polyhedra surrounding some other atom. Assuming the interatomic forces in glasses and the corresponding crystals are alike, he inferred that the glass networks also consist of oxygen polyhedra surrounding some other atom (as illustrated in Figure 27226). For instance, the vitreous silica network consists of oxygen tetrahedra surrounding silicon atoms. The different tetrahedra share corners, confirming that an oxygen atom is bonded to two adjacent silicon atoms in the threedimensional network. Figure 28227 describes the three-dimensional network of vitreous silica comprised of corner-shared SiO4 tetrahedra with illustration of bond and torsion angles. The bond angle β formed by an oxygen atom between two adjacent silicon atoms varies randomly with each oxygen atom. The distribution of the torsion angles is also random.227,228 Nevertheless, the networks in glass are not purely random inasmuch as the internuclear distances between the atoms cannot drop beyond the least possible value.226 The distribution of the bond angle β ranges from 120° to 180° with a maximum at β = 144°.228 PAF-1 was introduced in 200989 where the dia topology (illustrated in Figure 29a229 and referred to as structure P2 in the original article89) was proposed for it. There were several reasons backing this move. First, the basic structure based on which the monomer was designed was that of diamond. Second, some powder X-ray diffraction (PXRD) peaks (although not sharp) were observed at the beginning of the PAF-1 synthesis. The PXRD pattern of PAF-1 was revealed to be analogous in basic character to the theoretically simulated pattern of structure P2. Third, the experimental N2 sorption isotherms of PAF-1 (yielding a BET surface area of 5600 m2· g−1) matched the simulated N2 sorption isotherms of the model P2 (yielding a BET surface area of 5640 m2·g−1).89 In the next year, the Cooper group developed the amorphous model229 of PAF-1 (illustrated in Figure 29c229) based on the CRN model of amorphous silica (illustrated in Figure 29b229). They suggested that the absence of sharp peaks in the diffraction pattern of PAF-1 reveals its amorphous structure. They also maintained that the theoretically calculated density and elemental composition of PAF-1 obtained from its amorphous model are comparable to those obtained from its diamondoid model. The CRN model of PAF-1 conceived by Goodwin et al.225 (illustrated in Figure 30225) to explain the topology of tetrahedral amorphous porous organic materials is in keeping with the amorphous structure model of PAF-1 by the Cooper group. The diffraction pattern of amorphous porous organic materials is a diffuse scattering pattern with nearly no Bragg peaks. This fact prevents it from being interpreted using common crystallographic methods.225,230 The common crystallographic methods characterize the single atomic positions of a material. On the other hand, the diffuse scattering characterizes the pairs of atomic positions of a material.231 The diffuse scattering patterns of amorphous porous organic materials upon Fourier transform can yield a pair distribution

Figure 19. (a) Binary and (b) ternary COFs pertaining to the formation of two kinds of covalent bonds attained by adopting the orthogonal reactions approach. Reprinted with permission from ref 214. Copyright 2015 American Chemical Society.

Figure 20. Pathways of orthogonal reactions to successfully synthesize NTU-COF-1 and NTU-COF-2. Other reaction pathways (bearing cross-marks) to synthesize NTU-COF-2 proved futile. Please note that the abbreviation TPBA in the figure is an error in the original article.214 The correct abbreviation is TAPB corresponding to 1,3,5tris(4-aminophenyl)-benzene. Reprinted with permission from ref 214. Copyright 2015 American Chemical Society.

aiding the two threads to team up, creating an extended woven structure. In the event that the copper ions are withdrawn from the woven COF structure, the polymer threads become unfettered, and thus, the elasticity of the material is augmented incredibly albeit with a decrease in crystallinity. The material returns to its initial elasticity and crystallinity when the copper ions are redeemed. Hence, this woven COF exhibits more elasticity than common COFs, which consist of molecular building blocks linked via robust covalent bonds. Figure 26135 depicts the design and demetalation−remetalation technique of the material. 3.1.2. Topology of Amorphous Porous Organic Materials. 3.1.2.1. Continuous Random Network (CRN) Model. The topology of amorphous porous organic materials has been explained by Goodwin et al.,225 making use of the “continuous random network” (CRN) model. To comprehend the concept of a continuous random network, we revert back to the year 1932 when Zachariasen published an article “The Atomic Arrangement in Glass”226 explaining the concept in the context of glass structure. As described by Zachariasen,226 the S


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Figure 21. Design of COFs by (a) an ordinary/routine approach and (b) a special double-stage approach (involving structural intricacy). (c) Linkages pertaining to COFs formed from the ordinary approach compared to those from the double-stage approach. FPBA is as a bifunctional linker aiding in the double-stage approach to realize COFs. Reprinted with permission from ref 215. Copyright 2015 Macmillan Publishers Limited.

significant to the topology of the internal structure, as solvent effects are apparent for pore development. On the basis of comparisons of the PXRD, N2 isotherms, and pore size distributions from the experimental data with those from simulated structures, the amorphous structure bears more resemblance to the experimentally produced data. This agrees with the amorphous silica model represented by Cooper et al.,229 where the amorphous properties alongside the surface area and pore size distribution agree with PAF-1 having a topology that is similar to that of amorphous silica. The structural properties of PAF-1 are dependent on the bonding formation of the building blocks during synthesis. Trewin et al.234 describe how not only the rigidity of the PAF-1 building blocks is key to interpenetration reduction but also solvent templating during synthesis helps to mold the pores formed. Simulating the formation of clusters for PAF-1 growth has demonstrated how the rigidity of building blocks leads to a “snap-out” effect when bonding to additional building blocks or clusters. Each building block bonded to a cluster protrudes directly outward from the structure, reaching close to their thermodynamic minima, limiting the potential for overlap or flexibility. Solvent templating, on the other hand, inhibits building blocks from flexing and folding in on each other. Dimethylformamide molecules potentially congregate together, forming a solvent cluster for the PAF-1 framework to mold around. These two factors of PAF synthesis conclude the lack of interpenetration in PAF-1 structures and aid in the design

function (PDF). PDF is an alternative practice to express the diffraction information imparting structural information on a material on the basis of atom-pair distributions in real space.232 The PDF realized by the Fourier transform of the scattering pattern can be expressed as230,232 G (r ) =

2 π

∫Q

Q max

Q [S(Q ) − 1]sin(Qr )dQ

min

(3)

where Q refers to the magnitude of the scattering vector, S(Q) the total scattering structure function, and r the radial distance. The scattering structure function can be expressed as230,232 S (Q ) =

I coh(Q ) − ∑ ci |fi (Q )|2 |∑ cifi (Q )|2

+1 (4)

where Icoh(Q) refers to the experimentally observed scattering intensity subjected to normalization, background correction, and experimental correction, ci the atomic concentrations, and f i the X-ray atomic form factors, corresponding to atoms i. Qindependent neutron scattering lengths will be substituted for f i in neutron diffraction. Hereafter, the PDF approach is able to competently interpret the total scattering, i.e., Bragg scattering as well as diffuse scattering.225,230 The models simulated by Trewin et al. have demonstrated how topological simulations of PAF-1 differ from the experimental data produced from the synthesis.233 Simulating the reaction conditions during the formation of PAF-1 proves T


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Figure 22. Adoption of a double-stage approach to realize rhombic COFs. Adapted with permission from ref 215. Copyright 2015 Macmillan Publishers Limited.

Figure 23. Strategic double-stage synthesis approach to attain tetragonal COFs. Adapted with permission from ref 215. Copyright 2015 Macmillan Publishers Limited.

and rationalization of porous aromatic frameworks as well as other high surface area porous organic materials. 3.1.2.2. Ideal Amorphous Solid (IAS) Model. Prior to reviewing the “ideal amorphous solid” (IAS) model it may be

pertinent to mention that this model has not been explored yet for the topological analysis of amorphous porous organic materials; however, this model may be advantageously adapted to study the topology of amorphous porous organic materials. U


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Figure 24. Implementing the double-stage approach to realize hexagonal COFs. Adapted with permission from ref 215. Copyright 2015 Macmillan Publishers Limited.

The IAS model was conceived by Stachurski et al.235 with the intent that it would act as a standard reference or prototype in the topological analysis of amorphous solids. They followed a more stringent rendition of an ideal amorphous solid as characterized with impeccably random packing and the absence of short-, medium-, and long-range order. The development of the ideal amorphous solid configuration commences with the formation of an atomic aggregate constituting a middle atom surrounded by other nearest atoms. The formation of the atomic aggregate initiates with a sphere encompassing equidistant surface points followed by assigning some points (in a random approach) as points of contingence for coordinating spheres. The points assigned ought to be in the angular range (ϕ) of π/3 ≤ ϕ ≤ π. The atomic aggregate is obtained after inclusion of coordinating spheres in contact with the assigned points. The evolving of the atomic aggregate is illustrated in Figure 31.235 After this, the subsequent concentric shell of spheres is annexed. This is repeated as a 3-D structure encompassing randomly packed spheres, specified as a round cell (if the structure is finite) or ideal amorphous solid (if the structure is infinite), is obtained. Figure 32235 (left) illustrates the cross-section of a round cell of monatomic spheres and (right) an ideal amorphous solid corresponding to Zr-based metallic alloy. The above-mentioned ideal amorphous solid configuration presumes the atoms as impervious to one another and that the atomic aggregates involve the coordination shell atoms conceiving irregular triangles, thereby indicating irregular positioning of the atoms. As explained, exact structural models for amorphous porous organic materials are not readily available, as experimental data and simulated data cannot be directly compared. Furthering the study of interpenetration, the technique demonstrated by Trewin et al.234 illustrates how an entirely amorphous system cannot be justified but can possess many features and characteristics that coincide with experimental analysis. This technique involves simulating models of PAF-1 via a random arrangement of clusters developed, further building a larger system using these differing building blocks. They were able to attain an entirely amorphous structural representation of PAF1. However, this structural example only shows several chemical and physical features comparable to the experimental data of PAF-1, one example being a deviation from the low density

observed, although determining the key characteristic toward the lack of interpenetration. This technique allows for potential structural development of entirely amorphous representations of porous organic materials. However, these systems cannot be used as a direct representation of the physical material and will always possess dissimilarities. This is one of many studies showing the complexity of illustrating a representative model for an ideal amorphous porous organic material. Figure 33 shows a comparison of the PAF-1 system, generated using molecular dynamics simulations, against the isomorphic substitution structures of dia and amorphous silica. In the short range, there are visible similarities between all three structures, in particular with the well-defined building unit of PAF-1. However, PAF-1 lacks long-range order, which is typical for an amorphous topology. There is much speculation about the topology of PAF-1, with its structure initially being suggested as a dia topology and later suggested as a completely amorphous system. Analysis of the pair distribution function for differing PAF-1 topologies has indicated structural similarities to each of the differing topologies. Short-range order is present throughout each of the structures; however, the simulated system of PAF-1 lacks in long-range order, indicating some degree of dia topology in the short range. This leads us to suggest that the monomer of PAF-1 is highly unique, with structural properties that allow for a highly porous system with a complete lack of interpenetration, thereby maximizing the surface area. Despite the fact that several tetrahedral monomers had been explored, the monomer adopted for the synthesis of PAF-1 imparted the material with the highest surface area. However, more data are required to identify the exact structure and topology of PAF-1. At this point, it may be pertinent to discuss the importance of tetrahedral monomers in enhancing the surface areas of amorphous porous organic materials as in amorphous microporous poly(aryleneethynylene) networks.236 The tetrahedral carbon- and silicon-centered monomers impart the materials with BET surface areas of up to 1213 m2·g−1, which are higher than those for similar networks made from 2-D monomers. 3.2. Theoretical Design

Structural simulation and characterization of porous organic materials is an important exercise and rather exacting for V


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Figure 25. (A) Molecular building blocks yielding vertices (I−IV) and edges (a−e) of crystalline porous organic materials. Carbon, boron, oxygen, and hydrogen atoms are colored green, magenta, red, and white, respectively. (B) COFs bearing different topologies designed from red building blocks yielding vertices and blue building blocks yielding edges. (C) Structural representations of stacked (AA, AB, serrated, and inclined) COF-5. Adapted with permission from ref 221. Copyright 2010 Lukose et al; licensee Beilstein-Institut.

amorphous porous organic materials. Rationalization of porous organic materials can aid in the realization of structural properties and potential functionalization of these materials. Many techniques have been applied to analyze porous organic materials, with the bulk analysis being presented for crystalline materials. However, due to the random nature of amorphous porous organic materials, it is difficult to produce accurate representations or structural models of these materials. Rationalization of new materials involves analysis of reported amorphous porous organic materials, targeting monomers to provide specifically low densities, or challenging experimentally realized densities. This involves a combination of compression and relaxation phases to obtain models at energy minima that are still able to retain their porosities. There are several techniques that research groups have adopted or developed for this challenge. Many research groups

take the molecular dynamics (MD) approach for the extensive research of porous organic materials. The overall structure of a porous organic material is dependent on the movement of atoms within the system. In addition, liquid and gas flow are important to study based on how effective generated models are for gas adsorption. Cooper et al.237 reported a theoretical design technique (illustrated in Figure 34237) for structure formation of porous organic cages and further analyzed the experimentally attained selective gas uptake of the cages by MD simulations. These simulations in particular undergo a series of steps repeated multiple times to achieve the generation of larger systems or small clusters. Figure 35 illustrates a generic simulation route adapted from the published work of Trewin et al.233,234,237 Similar processes have been reported, such as that from Colina et al.238 in which HCPs are developed. Figure 36238 illustrates the cross-linking steps performed during W


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Figure 27. Two-dimensional glass network bearing composition A2O3. Circles filled with black color represent A atoms, and circles filled with white color represent oxygen atoms. Please note that the two oxygen atoms in the right of the figure are missing (a misprint) in the original article.226 Reprinted with permission from ref 226. Copyright 1932 American Chemical Society.

Figure 26. (A) Representative synthetic route for the formation of woven COF with a Cu(I) complex as a monomer along with two other linear monomers. (B) Demetalation−remetalation technique of the woven COF. Reprinted with permission from ref 135. Copyright 2016 American Association for the Advancement of Science.

synthesis, starting with the recognition of the closest leaving groups of the binding molecular. Together with the introduction of Grand Canonical Monte Carlo (GCMC) simulations, it is possible to simulate the gas uptake into these simulated systems. At this point, it may be pertinent to mention that despite the lack of academic work to date, where one can unmitigatedly control the structures, properties, and functions of porous organic materials by only adjusting the building blocks without any experimental contribution, we are optimistic that this approach can be viable regularly and effortlessly in the future.

Figure 28. Three-dimensional network of vitreous silica comprised of SiO4 tetrahedra with illustration of bond and torsion angles. Reprinted with permission from ref 227. Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We anticipate that this is the impending future of porous organic materials. X


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Gibbs energy) as239 “...the greatest amount of mechanical work which can be obtained f rom a given quantity of a certain substance in a given initial state, without increasing its total volume or allowing heat to pass to or f rom external bodies, except such as at the close of the processes are lef t in their initial condition.” In consideration of the energy landscape illustrated in Figure 37240 (bottom), product C is the thermodynamically controlled product since it is at lower energy (thereby more stable) than product D. The difference in Gibbs energies between the initial and the final states (“driving force”240) corresponding to product C, i.e., ΔG0 is greater than that corresponding to product D, i.e., ΔG0′. Product D is the kinetically controlled product since it encounters a lower “energy barrier”240 than product C. In light of the system at thermodynamic equilibrium, we can write241 ΔG° = ΔH ° − T ΔS ° = −RT ln K

(6)

where ΔH° and ΔS° are the enthalpy and entropy changes between the initial and the final states of the system, T is the absolute temperature, R is the universal gas constant, and K is the equilibrium constant. Figure 38209,210 illustrates the thermodynamically controlled COF-42 synthesized via the imine condensation reaction. The syntheses of kinetically controlled homocoupled CMPs, namely, HCMP-1 and HCMP-2, by oxidative coupling reaction are depicted in Figure 39.242 Cyclization is a very useful technique to realize pores in a porous organic material. A low concentration of reagent(s) facilitates cyclization, whereas a high concentration of reagent(s) facilitates linear polymerization.243−252 If all cyclizations are accomplished (reacted) in the same way, uniform pores are obtained. There is always a competition between cyclization and linear polymerization/hyperbranching. A high concentration of the reagent(s) facilitates linear polymerization/hyperbranching, and it follows that chain packing (random packing) is attained but not uniform pores. If a narrow pore size distribution is desired, a low concentration of reagent(s) must be used.

Figure 29. (a) Structural model for PAF-1 with dia topology.89 Adapted with permission from ref 89. Copyright 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. (b) CRN model for amorphous silica. Silicon and oxygen atoms are colored yellow and red, respectively. (c) Amorphous model of PAF-1 realized from CRN model for amorphous silica after substituting the tetrahedral carbon nodes for silicon atoms and biphenyl struts for oxygen atoms. (b and c) Reprinted with permission from ref 229. Copyright 2010 WileyVCH Verlag GmbH & Co. KGaA, Weinheim.

4.2. Reactions Involved in the Chemical Synthesis of Porous Organic Materials

Table 2 elucidates the significant reactions employed in the chemical synthesis of porous organic materials. 4.3. Approaches To Minimize Structural Defects

The perpetuation of crystallinity (long-range order) in COFs is challenging due to the defects that originate during stacking. To counter this issue, a hydrogen bond was included in the COFs as reported by Banerjee et al.253 The inclusion of a hydrogen bond increases the rigidity of the framework and thereby its crystallinity. Figure 40253 depicts the synthesis of COFs, namely, DmaTph and DhaTph, with the latter including hydrogen bonds. Bein et al.254 adopted a different approach of using propeller-shaped monomer (building block), namely, 1,1,2,2-tetrakis(4-aminophenyl)ethene, to counter the same issue. This building block is very interesting because its propeller shape facilitates dense packing. It allows a limited space where there are the most possible docking sites. Hence, an ordered structure can be formed precisely in this space (here, just underneath the green energy surface shown in Figure 41,254 left). Therefore, there is a choice of a specific location that is of the lowest energy and is the most suitable for crystallization.

Figure 30. Understanding the topology of PAF-1: (top) CRN model of amorphous silica; (bottom) CRN model of PAF-1 proceeding from the CRN model of amorphous silica. Adapted with permission from ref 225. Copyright 2013 The Royal Society of Chemistry.

4. CHEMICAL SYNTHESIS OF POROUS ORGANIC MATERIALS 4.1. Thermodynamic and Kinetic Control

The crystalline or amorphous nature of a porous organic material depends on whether the material is thermodynamically or kinetically controlled in the course of its chemical synthesis. About 140 years ago, Gibbs published an article,239 which has become a landmark article by now, in which he first explained the concept of “available energy”239 (later came to be called Y


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Figure 31. Evolving of the atomic aggregate sets about with (a) a sphere encompassing equidistant surface points followed by (b) designating some points in a random fashion as points of contingence for coordinating spheres and eventually (c) the inclusion of coordinating spheres in contact with the designated points. Reprinted with permission from ref 235. Copyright 2010 The Minerals, Metals & Materials Society and ASM International.

Figure 32. (Left) Cross-sectional view of a round cell encompassing randomly packed monatomic spheres (about 105 spheres). (Right) Exterior view of IAS corresponding to Zr-based metallic alloy (approximately 7000 spheres representing five types of atoms). Reprinted with permission from ref 235. Copyright 2010 The Minerals, Metals & Materials Society and ASM International.

can be customized by accommodating the building blocks with precise shape, size, and/or precisely included functional moieties. Scientists follow two methodologies to pursue the functionalization of porous organic materials, namely, presynthetic functionalization and postsynthetic functionalization. The presynthetic functionalization (bottom-up development) of porous organic materials is concerned with functionalizing the building blocks (monomers) followed by polymerization reaction transforming the monomers to develop porous organic materials. On the other hand, the postsynthetic functionalization (top-down development) of porous organic materials involves modification of initially synthesized porous organic

The successful synthesis of single-crystal COFs, albeit challenging, was reported for the first time by Wuest et al.255 The single-crystal COFs formed by reversible self-addition polymerization of tetranitroso monomers exhibit characteristics different from other porous organic materials.

5. FUNCTIONALIZATION OF POROUS ORGANIC MATERIALS 5.1. Presynthetic and Postsynthetic Functionalization

Considering specific applications of porous organic materials, the specific surface area, density, and pore size of the materials Z


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Figure 33. Pair distribution function (PDF) comparison of a PAF-1 simulation against PAF-1 dia and amorphous silica models. PAF-1 structure was generated via molecular dynamics simulations, while isomorphic substitution was used to replace the dia and amorphous silica models for PAF-1 building blocks. Visual Molecular Dynamics (VMD) was used to perform the PDF analysis of these structures.

Figure 34. Theoretical design technique for structure formation of amorphous porous organic cages. NVE represents an ensemble with constant volume and constant energy, while NPT represents an ensemble at constant pressure and constant temperature. Reprinted with permission from ref 237. Copyright 2013 American Chemical Society.

materials. The postsynthetic functionalization methodology is especially effective for the incorporation of several functional groups in one porous organic material, thereby making the materials more worthy for advanced applications. Some functional groups, if added to the monomers during presynthetic functionalization, may hamper the polymerization, whereupon lies the necessity of postsynthetic functionalization. The following study of presynthetic functionalization recently reported by Gu’s lab256 illustrates the structure− function interrelationship (agreement) of porous organic materials: CTFs, namely, PCTF-1, PCTF-2, PCTF-3, and PCTF-4, each bear a triphenylamine core with different branched arms. In light of the research, the extension of branched arms in porous organic materials results in better packing of the materials, contributing to an increase in density, reduction in pore volume, and thereby a reduction in specific surface area. PCTF-1 (SBET = 853 m2·g−1) surpasses PCTF-2 (SBET = 811 m2·g−1) and PCTF-3 (SBET = 391 m2·g−1) in specific surface area. The structural difference between PCTF-2 and PCTF-4 is that the latter consists of benzothiadiazole taking the place of an intermediate benzene ring in the branched arms of the former. Benzothiadiazole in PCTF-4 imparts to the CTF an impressive BET specific surface area

Figure 35. Adaptation of the theoretical technique proposed by Trewin et al.234 for the formation of amorphous porous organic materials. The figure is representative of a simulation cell with periodic boundaries, and the completion is dependent on the possible formation of bonds and/or if the number of atoms exceeds the size of the simulation cell. AA


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adsorption/separation performance. This study categorically describes how precisely adjusted branch lengths of monomers and precisely included functional moieties to the monomers influence the surface areas and porosities of porous organic materials. The functionalization of the monomers prior to the synthesis of each of the CTFs is illustrated schematically in Figure 42.256 Presynthetic functionalization of porous organic materials is exemplified by the aforementioned study reported of late; it is time we consider postsynthetic functionalization. The amine cages lack perpetuity of shape and tend to collapse (consequent to desolvation) resulting in a loss of porosity. Cooper’s lab257 reported the postsynthetic functionalization of amine cages by tying aptly chosen molecules (acetone and formaldehyde) to the cage vertices to sustain the shape and solid-state porosity as well as to enhance the chemical stability of the cage. Figure 43257 (top) schematically illustrates the development of acetone- and formaldehyde-tied cages, namely, AT-RCC3 and FT-RCC3, respectively, from the amine cage RCC3. The formaldehyde-tied cage reveals a tremendous upsurge in porosity compared to the acetone tied and amine cages (Figure 43,257 bottom, left). The formaldehyde-tied cage features outstanding chemical stability as well (Figure 43,257 bottom, right). With respect to CMPs, Cooper’s lab258 reported the postsynthetic functionalization of amine-functionalized CMP (CMP-1-NH2) to develop amide-functionalized CMPs (CMP1-AMDs with varying alkyl chain lengths) that imparted control over surface area, pore volume, gas adsorption capacity, etc., of the material. The results indicated that the BET surface area and pore volume of CMP-1-NH2 are 656 and 0.41 cm3·g−1, respectively, whereas the corresponding values for the CMP-1AMDs can be tuned over the range 316−68 and 0.21−0.09 cm3·g−1, respectively. Postsynthetic functionalization of aminefunctionalized CMP revealing the CO2 adsorption capacity of 1.65 mmol·g−1 at 1 bar/273 K helps to tune the CO2 adsorption capacity of the amide-functionalized CMPs over the range 1.51−0.87 mmol·g−1 at 1 bar/273 K.

Figure 36. Cross-linking exercise of dichloroxylene starting from (a) recognition of closest reactive pair of atoms (here a Cl atom and a H atom) followed by (b) development of a bond linking the cross-linking sites corresponding to the closest pair of reactive atoms whereupon the reactive pair of atoms is removed and eventually ending with (c) equilibration. Carbon, chlorine, and hydrogen atoms are colored gray, green, and white, respectively. Reprinted with permission from ref 238. Copyright 2011 American Chemical Society.

5.2. Theoretical Approach to Property Determination

The properties of porous organic materials can be determined using theoretical methods such as GCMC simulations and MD simulations. These methods are very useful to handle large extended structures of porous organic materials. Several research groups have adopted GCMC simulation to determine the gas sorption and gas storage capacities of porous organic materials. Garberoglio259 utilized GCMC simulation to determine the storage capacities of Ar, H2, and CH4 in 3-D COFs. The same set of COFs was explored by Froudakis et al.260 using a compilation of several methods to determine the H2 storage capacity of the COFs as well as the binding strength of H2 found in the adsorption sites of the COFs. Using the GCMC method, Zhong et al.261 interpreted that the stepped behaviors pertaining to gas adsorption in 2-D COFs may be attributed to the stacking of COFs. The Yaghi group262 determined the methane storage capacities of several COFs using GCMC simulations. The theoretically simulated results of COF-5 and COF-8 were comparable to the experimental results, whereas the theoretically simulated result for COF-1 was much higher than its experimental result. This is because the real structure of COF-1 is staggered and the pore space is hardly accessible. However, this is not the case in the theoretical method. Figures 44262 and 45262 illustrate the ensemble average obtained from the GCMC simulations

Figure 37. (Top) Driving force acting on a body moving from its initial state to its final state is related to thermodynamics. On the other hand, the kinetic barrier experienced by the body along a particular route has implications for kinetics. (Bottom) Energy landscape depicting the thermodynamic product opposed to the kinetic product. Adapted with permission from ref 240. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

(1404 m2·g−1), high CO2 adsorption capability (20.5 wt % at 1 bar/273 K), and excellent gas separation selectivities (CO2/N2, 56; CO2/CH4, 20). PCTF-4 outdoes PCTF-2 (CO2 adsorption capacity, 12.5 wt % at 1 bar/273 K; CO2/N2 selectivity, 17; CO2/CH4 selectivity, 19) in specific surface area and gas AB


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Figure 38. Chemical reaction involved in the synthesis of COF-42. Adapted with permission from ref 209. Copyright 2011 American Chemical Society.

adsorption, and the removal of pollutants/contaminants from water and other liquids, among others. 6.1. Gas Adsorption and Storage

H2, CO2, and CH4 are the three gases that are predominantly explored for gas adsorption and storage applications of porous organic materials. Carbon dioxide is a greenhouse gas that causes global warming, precariously affecting the environment. Hence, effective sorbents of CO2 are necessary. The storage of hydrogen and methane are mainly carried out with the intent for their use as clean fuels. For low-pressure gas adsorption by porous organic materials, it is imperative to attain porous organic materials with high heats of adsorption and large binding sites. Considering high-pressure gas storage by porous organic materials, the porous organic materials need to have high surface areas. However, if the binding sites are increased, the surface area always decreases. Therefore, a disagreement presumably persists between porous organic materials with outstanding surface areas and materials with large binding sites, and it is indeed challenging to combine the two in a single porous organic material. Our group reported the synthesis of carbonized PAFs264 that are suitable for low-pressure gas sorption as well as high-pressure gas storage. This rare character of the carbonized PAFs can be imputed to the unique dual-pore system that arises after carbonization. The kinetic diameters of H2, CO2, and CH4 molecules are 2.89, 3.30, and 3.80 Å, respectively. The most favorable approach for gas sorption/ storage by porous organic materials is to match the pore size with the kinetic diameter of the gas and the requisite binding energy between the material and the gas molecules. There are three different practices of CO2 capture:265 precombustion,113 postcombustion, and oxy-fuel combustion. Precombustion capture pertains to the water−gas shift reaction266 of syngas265 (CO and H2) and steam to form CO2 and H2. CO2 is an acid gas, and an acid gas removal setup may be utilized for CO2 capture. The H2 can find use as a clean fuel.267 The postcombustion capture addresses the capture of CO2 from

Figure 39. Chemical reactions involved in the synthesis of HCMP-1 and HCMP-2. Adapted with permission from ref 242. Copyright 2008 The Royal Society of Chemistry.

concerning the methane adsorption by the COFs. Noticeably, the figures show the pore and how it adsorbs gas. The gas sorption and gas storage capacities of porous organic materials help to understand the porosity of the materials. At the same time, the strain of a porous organic material gives knowledge about the rigidity of the framework. Schmid et al.263 proposed a theoretical method to study the strain energy of COF-102.

6. APPLICATIONS OF POROUS ORGANIC MATERIALS In this section, we review the applications of porous organic materials in gas adsorption and storage, the separation of gases, catalysis, energy storage, proton conduction, semiconduction, photocatalysis, photoluminescence, photoenergy conversion in solar cells, enantioseparation, drug delivery and release, iodine AC


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Table 2. Significant Reactions Employed in the Chemical Synthesis of Porous Organic Materials

AD


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Table 2. continued

a

R, functional group/H; X, halogens. bBrunauer−Emmett−Teller surface area (m2·g−1).

flue gas that is formed from the combustion of fossil fuels.267 The oxy-fuel combustion pertains to the capture of CO2 after water vapor (one of the components of flue gas that is formed from the combustion of fuel in oxygen) is condensed to water.268 With respect to the CO2 adsorption capability of porous organic materials at low pressures, the CTF viz. FCTF1-60014 leads with a gravimetric capacity of 24.34 wt % at 1 bar/273 K followed by JUC-Z2-900174 with 22.20 wt % at 1 bar/273 K and FCTF-114 with 20.55 wt % at 1 bar/273 K. The CTFs owe their outstanding CO2 adsorption capacities at low pressures to their high micropore volumes relative to total pore volumes. The CO2 adsorption capacities of porous organic materials at low pressures are summarized in Table 3. The U.S. Department of Energy (DOE) has laid down a target of 5.5 wt % (gravimetric capacity) and 0.04 kg·L−1 (volumetric capacity) in the temperature range 233−333 K for H2 storage by 2020.269 Among the porous organic materials, PPN-4163 reveals the highest gravimetric hydrogen storage capacity (9.10 wt %, 55 bar/77 K) followed by COF-102132 (7.24 wt %, saturation pressure/77 K), COF-103132 (7.05 wt %, saturation pressure/ 77 K), and PAF-189 (7.0 wt %, 48 bar/77 K). Table 4 summarizes the H2 storage capacities of porous organic materials. Under the aegis of the “Methane Opportunities for Vehicular Energy” (MOVE) program, the DOE has laid down a target of 0.5 g(CH4)/g(sorbent) (gravimetric capacity) and 263 cc(STP)/cc (volumetric capacity) for CH4 storage. With a 25% packing loss taken into account, the targeted volumetric capacity equals 330 cc(STP)/cc.270,271 Again, PPN-4163 leads the family of porous organic materials in methane storage with a gravimetric capacity of 0.27 g(CH4)/g(sorbent) at 55 bar/295 K followed by COF-102132 (0.24 g(CH4)/g(sorbent) at 85 bar/298 K) and COF-103132 (0.23 g(CH4)/g(sorbent) at 85 bar/298 K). However, they are far from the DOE target. The CH4 storage capacity of porous organic materials is summarized

Figure 40. Inclusion of hydrogen bonds in DhaTph helps to augment its crystallinity. Reprinted with permission from ref 253. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

AE


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Figure 41. (Left) 1,1,2,2-Tetrakis(4-aminophenyl)ethene monomer has a specific space with the lowest energy (indicated by the red arrow) just underneath the green energy surface for the formation of an ordered structure. This monomer includes carbon (dark gray for four core rings and light gray for others), nitrogen (blue), and hydrogen (white). The layer that follows after the annexation includes carbon (orange), nitrogen (blue), and hydrogen (white). (Right) TEM image of the COF presenting COF crystallites inclusive of mesopores arranged in a hexagonal fashion. Insert figure shows a magnified view of a single COF crystallite. Adapted with permission from ref 254. Copyright 2016 Macmillan Publishers Limited.

mixture). The gas permeability and selectivity of such membranes are likely to be opposing each other. A porous organic material-derived membrane, with large pore size and requisite binding energy between the membrane and gas molecules, facilitates speedy diffusion of gas molecules through its pores, which in turn imparts the membrane with high gas permeability. On the other hand, it is imperative for the porous organic material-derived membrane to have pores that are of commensurate size as the gas molecules in order to present impressive selectivity. It is indeed a demanding task to develop a porous organic material-derived membrane with both high gas permeability and superb selectivity. Very recently, we developed the first COF membrane and composite membrane (from two porous polymers) that presented high gas permeability and first-rate selectivity for H2/CO2 mixture and transcended the Robeson upper bound as well.17 The composite membrane is illustrated schematically in Figure 46.17 The membranes owed their superb selectivities and gas permeabilities to the chemical character of the different porous materials used and their synergistic interplay. The large surface area in conjunction with the microporosity backs PIMs as highly competent for membrane-based gas separation applications. PIM-1 and PIM7 are two typical PIMs that reveal high gas permeability together with outstanding selectivity in terms of membranebased gas separation.18 PTMSP18 exhibits notable gas permeability but moderate selectivity owing to large pore sizes. The microporosity of the PIM membranes has a strong influence on increasing the apparent solubility of gases in the membranes, which helps escalate the permeability while preserving the selectivity. The binding energy between the membrane and the gas molecules is also critical in determining the competency of the membrane. The inclusion of polar functional groups (such as nitrile groups in PIM-1) helps to improve this binding energy, which in turn elevates the gas sorption capability of the PIM-1 membrane.18 Dai et al. reported the in situ cross-linking of nonporous polystyrene membranes (templates) to yield free-standing porous polymer membranes functioning as molecular sieves that present superb gas separation performance.19 Despite challenges, the Cooper

in Table 5. It is unfortunate that no porous organic material surpassing (or equalizing) the DOE target for methane storage has been reported thus far. 6.2. Separation of Gases

6.2.1. Breakthrough Experiment. Breakthrough experiments are accomplished for the separation of gases from gas mixtures by porous organic materials that are in powdered form (unprocessable). Compared to membrane-based gas separation, this method requires a considerable amount of sample to be adopted for the separation of gases and thereby has scale-up concerns. Nevertheless, this method is useful to determine the gas separation performances of porous organic materials considering that most of the porous organic materials are synthesized as powders (unprocessable). The gas separation selectivity of a porous organic material in the breakthrough experiment can be expressed as14 α = (q1/y1)/(q2 /y2 )

where qi refers to the absolute amount of gas i adsorbed and yi refers to the molar fraction of gas i in the gas mixture. Han and co-workers14 reported a perfluorinated CTF that is useful for selective CO2 sorption and features a selectivity of 77 corresponding to the CO2/N2 gas pair in breakthrough experiments. Ma’s lab15 reported a functionalized PAF that can separate ethylene and ethane from a mixture of the two components, as evinced by the breakthrough experiments. The porous organic cage reported by the Cooper group, namely, CC3α, presented a selectivity of 76.5 for the N2/SF6 gas pair in breakthrough experiments.16 6.2.2. Membrane-Based Gas Separation. A porous membrane is produced from the processing of porous organic materials, for which the shape can be readily manipulated. There has been increasing enthusiasm for the development of membranes from porous organic materials over the past few years. The competency of a porous organic material-derived gas-separation membrane is assessed using two parameters, viz. the permeability coefficient (in consideration of a single gas) and selectivity (of one gas opposed to other gas from a gas AF


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Figure 42. Functionalization of monomers preceding the synthesis of CTFs thereby exemplifying the presynthetic functionalization methodology of porous organic materials. Reprinted with permission from ref 256. Copyright 2015 The Royal Society of Chemistry. AG


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Figure 43. (Top) Functionalization of cages by tying acetone and formaldehyde to cage vertices. RCC3, AT-RCC3, and FT-RCC3 denote the original cage, acetone-tied cage, and formaldehyde-tied cage, respectively. (Bottom, left) Formaldehyde-tied cage reveals a conspicuous escalation in porosity compared to the original and acetone-tied cages, as evident from the N2 sorption isotherms of the cages. (Bottom, right) (a) PXRD patterns corresponding to the formaldehyde-tied cage after immersion in 0.02 M HCl (pH 1.7) for 12 days. (b) N2 sorption isotherms of the formaldehydetied cage: as synthesized (black circles), post-treatment with basic solution (blue squares), and post-treatment with acidic solution (red triangles). Solid symbols indicate adsorption and open symbols desorption in the N2 isotherms. Adapted with permission from ref 257. Copyright 2014 American Chemical Society.

Figure 44. GCMC simulation of methane adsorption in 2D-COFs: (a) COF-10, pore diameter = 35 Å; (b) COF-5, pore diameter = 27 Å; (c) COF8, pore diameter = 16 Å; (d) COF-6, pore diameter = 11 Å. Methane is represented as blue, and the accessible surface, calculated using the van der Waals radii of each atom within the framework and the methane kinetic radii, is represented as purple. Carbon, oxygen, boron, silicon, and hydrogen atoms are colored black, red, pink, yellow, and blue, respectively. Reprinted with permission from ref 262. Copyright 2010 American Chemical Society.

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Figure 45. Simulated methane adsorption at varying pressures of (a) COF-103, (b) COF-105, and (c) COF-108 (C, gray; O, red; Si, yellow; B, pink). Methane is represented as blue. Accessible surface was calculated in a similar fashion to Figure 44. Reprinted with permission from ref 262. Copyright 2010 American Chemical Society.

The modified COF film shows superb potential for energy storage.32 The cooperative potentials of COF and carbon nanotubes for energy storage were explored for the first time by Jiang et al.33 PAFs can be used as hosts for sulfur in Li−S batteries by exploiting entrapment in their pores. The [JUCZ2]-[S] composite material can be aptly employed as a cathode in Li−S batteries.34 The capacitance of carbonized PAF-1 can be tuned by tuning its pore size. K-PAF-1, which shows superb capacitance, may bring forth potential applications as a supercapacitor in future.35

group successfully synthesized porous organic cage-based membranes20 for exploration in gas separation. 6.3. Catalysis

Porous organic materials have been considered for catalytic applications owing to their large surface areas and porosities. Functionalized COFs with precisely included functional moieties allow us to analyze their catalytic performances. Wang et al. reported the synthesis of a Pd-incorporated COF that exhibited catalytic activity in the Suzuki−Miyaura crosscoupling reaction.21 Fang et al. successfully synthesized 3-D base-functionalized COFs, which when studied for their catalytic performance in the Knoevenagel condensation reaction exhibited excellent conversion besides giving superb size selectivity and good recyclability.22 Jiang’s lab23 recently reported postsynthetically functionalized COFs that present a rare combination of high porosity, crystallinity, and stability. In addition, they function as chiral organocatalysts. Kamiya and co-workers explored the electrocatalytic performance of platinum-functionalized CTFs hybridized with carbon nanoparticles24 and copper-functionalized CTFs hybridized with carbon nanoparticles.25 Soft-oxometalatePAF dispersion revealed its catalytic activity in the oxidation of benzaldehyde.26

It is important to look out for alternative sources of energy because energy consumption across the globe is increasing. In this context, fuel cells can be a potential substitute. Proton conduction is indispensably related to the operation of fuel cells. Very recently, Jiang et al.36 reported a mesoporous and crystalline COF that is useful for proton conduction across its channels, which invalidated the notion that large-pore materials enable the displacement of proton carriers in place of the conduction of protons. Zhu’s lab37 brilliantly combined cationic COF with polyoxometalates which markedly increased the proton conductivity of the COF.

6.4. Energy Storage

6.6. Semiconduction

The human population is increasing by leaps and bounds and so is the demand for future energy storage and usage. Porous organic materials have proved their usefulness for energy storage. Jiang et al.27 reported a hexaazatrinaphthalene-based CMP that owes its energy storage ability to its π-conjugation, redox-active units, large surface area, and excellent microporosity. Their lab also reported the synthesis of aza-fused CMPs that exhibit impressive supercapacitive performance.28 The functionalization of CMPs enabled their usage for supercapacitive energy storage, as reported by a recent study by Cooper et al.29 Jiang et al.30 reported an approach to transform an electrochemically inert COF bearing ethynyl groups into COFs that show high potential for energy storage and high capacitance values throughout a large range of current densities. The ethynyl groups present in the pristine COF react with 4-azido-2,2,6,6-tetramethyl-1-piperidinyloxy to give functionalized COFs bearing redox-active TEMPO radicals that facilitate energy storage. The energy storage capacity of βketoenamine-linked COFs was reported by Dichtel et al.31 Recently, Dichtel’s lab reported the modification of a COF film by electropolymerizing 3,4-ethylenedioxythiophene in its pores.

Semiconducting COFs owe their semiconducting properties to their highly ordered structures composed of π-conjugated building blocks. The first reported semiconducting crystalline COF, namely, TP-COF, consisting of triphenylene units linked by pyrene units exhibits photoluminescence and allows energy transfer between the building units. The eclipsed stacking arrangement of π-conjugated components endows the COF with photocurrent generation and hole mobility.38 Similar to TP-COF, PPy-COF made from pyrene units reveals p-type semiconducting behavior with electrical conductivity of the same order as that of TP-COF.39 Following a study reporting the semiconducting behavior of nickel phthalocyanine-based COF, namely, NiPc COF,40 with a carrier mobility of 1.3 cm2 V−1 s−1, the photoconductivities of several other metallophthalocyanine COFs41 were assessed. Unlike the aforementioned p-type semiconducting COFs, an n-type semiconducting COF was developed by accommodating a nickel phthalocyanine-based COF with electron-deficient benzothiadiazole units included as linkers. The resulting COF, namely, 2D-NiPc-BTDA COF, presented electron mobilities as high as 0.6 cm2 V−1 s−1.42 Porphyrin-based COFs bearing eclipsed

6.5. Proton Conduction

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Table 3. Summary of Carbon Dioxide Adsorption Capacity of Porous Organic Materials porous organic materials HCPs BINOL network 4 1,3,5-triphenylbenzene-based HCP meso-PR-36 SHCP-5 HCP 1 HCP 2 HCP 3 HCP 4 PIMs PIM-1 amidoxime-PIM-1 COFs COF-1 COF-5 COF-6 COF-8 COF-10 COF-102 COF-103 ACOF-1 [HO2C]100%-H2P-COF CMPs CMP-0 CMP-1 CMP-1-(OH)2 CMP-1-(CH3)2 CMP-1-COOH CMP-3 CMP-5 TCMP-0 TCMP-3 TCMP-5 TNCMP-2 NCMP-2

P (bar)

T (K)

wt %

porous organic materials

ref

P (bar)

T (K)

wt %

1

298

5.06

161

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

273 273 273 273 273 273 298 273 298 273 298 273 298 273 298 273 273 273 273 273 273 273 273 273 273 273

15.70 10.87 20.55 16.81 24.34 15.36 11.24 12.63 9.65 10.55 7.29 9.33 5.74 11.87 7.54 18.70 14.38 13.91 18.84 18.18 13.63 14.81 18.36 14.34 10.14 11.42

152 14 14 14 14 12 12 12 12 12 12 12 12 12 12 277 277 278 279 279 155 155 155 280 280 280

1 1 1 1 1 1 1 1 1 1

273 273 273 273 273 273 273 273 273 273

9.10 19.80 15.30 10.70 13.75 22.20 11.39 14.73 15.13 16.70

13 175 13 13 281 174 281 281 281 281

1 1 1

275 275 275

5.59 13.20 10.87

91 91 91

ref

CMPs 1 1 1 1.13 1 1 1 1

273 273 273 273 298 298 298 298

17.43 15.90 9.50 8.11 7.48 7.48 7.04 7.04

112 111 109 117 272 272 272 272

1 1 1 1

273 298 273 298

11.14 6.20 12.05 7.24

273 273 273 273

1 1 1 1 1 1 1 1 1

273 273 273 273 273 273 273 273 273

10.21 5.90 16.90 6.29 5.33 6.87 7.48 17.70 17.40

274, 274, 274, 274, 274, 274, 274, 7 10

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

273 298 298 298 298 273 273 298 273 298 273 298 273 298 273 298 273 298 273

9.24 5.33 5.19 4.71 4.14 7.04 9.59 3.56 4.89 2.77 10.47 5.90 9.90 5.55 5.37 2.99 11.53 6.38 9.24

161 161 150, 276 150, 276 150, 276 150 161 161 161 161 161 161 161 161 161 161 161 161 161

CTFs CTF-0 CTF-1 FCTF-1 CTF-1-600 FCTF-1-600 CTF-FUM-350 CTF-FUM-400 CTF-FUM-500 CTF-DCN-400 275 275 275 275 275 275 275

CTF-DCN-500 CTF-TPC CTF-FL MCTF@500 f l-CTF350 f l-CTF400 CTF-P4 CTF-P6 CTF-P6M PCTF-1 PCTF-4 PCTF-5 PAFs PAF-1 PAF-1-450 PAF-3 PAF-4 JUC-Z2 JUC-Z2-900 JUC-Z7 JUC-Z8 JUC-Z9 JUC-Z10 porous organic cages cage 1 cage 2 cage 3

stacking structures, viz. COF-66 and COF-366, exhibit p-type semiconducting characteristics with excellent hole mobilities of 3.0 and 8.1 cm2 V−1 s−1.43 In 2012, Jiang’s lab reported the three different modes of conduction (i.e., hole, electron, and ambipolar) in metalloporphyrin COFs with the same framework but different central metal atoms in the porphyrin rings. The results indicated that the free base porphyrin-based COF, viz. H2P-COF, is hole conducting, whereas the copper porphyrin-based COF, viz. CuP-COF, is electron conducting and the zinc porphyrin-based COF, viz. ZnP-COF, is ambipolar by nature.44

the synthesis of hydrogen-generating CMP photocatalysts in which the optical band gap can be tuned by adjusting the monomer content which impacts the photocatalytic performance of the CMPs. The photochemical splitting of water by COFs derived from triphenylarene aldehydes and hydrazine was reported by Lotsch et al.46 The hydrogen thereby generated provides a solution to our overdependency on fossil fuels and can be used as a clean fuel. Liras et al.47 reported a dye-based CMP that shows photocatalytic action to oxidize thioanisole into methylphenyl sulfoxide. Ir/Ru-included CMPs, 4 8 Rose Bengal-based CMPs, 4 9 and tris(2phenylpyridine)iridium(III) complexes-derived CMPs (polycarbazoles)50 have been reported to function as photocatalysts in aza-Henry reactions.

6.7. Photocatalysis and Photoluminescence

Porous organic materials, viz. CMPs, COFs, etc., have recently been reported to impart photocatalytic performance in the reduction of water to yield hydrogen. Sprick et al.45 reported AJ


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Table 4. Summary of Hydrogen Storage Capacity of Porous Organic Materials porous organic materials HCPs hyper-cross-linked polystyrene p-DCX network BCMBP/p-DCX network hyper-cross-linked polyaniline 6 diiodomethane-hyper-crosslinked polypyrrole iodoform-hyper-cross-linked polypyrrole boron triiodide-hyper-crosslinked polypyrrole p-diaminobenzene/ tribromobenzene hypercross-linked network PIMs PIM-1 HATN network CTC network Trip-PIM OFP-3 COFs COF-1 COF-5 COF-6 COF-8 COF-10 COF-102 COF-103 CMPs CMP-0 CMP-1 CMP-2 CMP-5 NCMP-0 Li-CMP CTFs f l-CTF400 CTF1-600 lut-CTF600 bipy-CTF600 PAFs PAF-1 PAF-3 PAF-4 JUC-Z2 JUC-Z7 JUC-Z8 JUC-Z9 JUC-Z10 PPN-4 porous organic cages cage 2 cage 3

P (bar)

T (K)

wt %

Table 5. Summary of Methane Storage Capacity of Porous Organic Materials

ref

10

77

2.75

282

15 15 15 30

77 77.3 77.3 77

3.04 3.18 3.68 2.20

282 110 110 97

4

77

1.60

283

4

77

1.30

283

1.2

77

0.63

283

1.2

77

0.97

284

porous organic materials HCPs 1 (BCMBP/DCX) 2 (DCX) 3 (DCX) 4 (BCMBP) COFs COF-1 COF-5 COF-6 COF-8

10 10 10 10 10

77 77 77 77 77

1.44 1.56 1.70 2.71 3.94

285 285 285 124 286

COF-10

saturation saturation saturation saturation saturation saturation saturation

77 77 77 77 77 77 77

1.48 3.58 2.26 3.50 3.92 7.24 7.05

275 275 275 275 275 275 275

1.13 1.13 1.13 1.13 1.13 1

77.3 77.3 77.3 77.3 77.3 77

1.4 (approx) 1.14 0.92 0.6 (approx) 1.5 (approx) 6.10

145 145 145 145 287 149

PAFs PAF-1 JUC-Z2 JUC-Z7 JUC-Z8 JUC-Z9 JUC-Z10 PPN-4 porous organic cages cage 1 cage 2 cage 3

20 25 25 25

77 77 77 77

4.36 4.34 4.18 4.00

279 288 288 288

48 60 60 48 48 48 48 48 55

77 77 77 77 77 77 77 77 77

7.00 5.50 4.20 3.89 6.89 6.65 6.34 5.08 9.10

89 13 13 281 281 281 281 281 163

7 7

77.3 77.3

1.78 1.50

91 91

COF-102 COF-103

P (bar)

T (K)

g(CH4)/g(sorbent)

ref

20 20 20 36 20 36

298 298 298 298 298 298

0.08 0.07 0.06 0.07 0.07 0.09

96 96 96 96 96 96

35 85 35 85 35 85 35 85 35 85 35 85 35 85

298 298 298 298 298 298 298 298 298 298 298 298 298 298

0.04 0.04 0.09 0.13 0.07 0.07 0.09 0.11 0.08 0.12 0.19 0.24 0.18 0.23

275 275 275 275 275 275 275 275 275 275 275 275 275 275

35 35 35 35 35 35 55

298 298 298 298 298 298 295

0.19 0.13 0.19 0.18 0.18 0.15 0.27

289 281 281 281 281 281 163

12 12 12

289 289 289

0.02 0.05 0.05

91 91 91

framework, which reveals a minimum carrier mobility of 0.04 cm2 V−1 s−1. The CMP (electron donor) can readily sheathe coumarin 6 (electron acceptor) in the pores, giving green emission. The antennas comprising the porous CMPs and the dye molecules encased in the pores exhibit artificial light harvesting. The light-harvesting performance of pyrene-based CMPs including 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran and Nile red dyes in the pores was recently reported by Rao et al.52 Jiang et al.53 recently reported the designed synthesis of a dual-pore COF (Figure 4753) with strong luminescence inasmuch as the nonemissive tetraphenylethene cores (yielding vertices of the COF) was induced to emit light via aggregation.54,55 A carbazole-based CMP synthesized by Jiang et al. revealed photoluminescence as well as sensing properties for arenes.56 Their lab also reported the synthesis of photoluminescent COFs with rhombic topology (comprised of pyrene connectors and azine linkers) that exhibited high sensitivity and high selectivity for the detection of the explosive 2,4,6-trinitrophenol via fluorescent quenching.57 Porous and photoluminescent covalent organic polymers, namely, COPs, were successfully synthesized by Cao’s lab. COP-3 and COP-4 revealed sensing properties for trace detection of nitroaromatic explosives. In addition, they showed

Porous organic materials have also been reported to exhibit photoluminescence. Jiang et al.51 reported the designed synthesis of polyphenylene-based CMPs that give blue emission. The excitation energy can migrate over the CMP AK


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Figure 46. Diagrammatic representation of the composite membrane with two porous polymers. Reprinted with permission from ref 17. Copyright 2016 American Chemical Society.

Figure 47. (Left) Luminescent COF designed from tetraphenylethene as the core of the connector. (Right) Fluorescence microscopy images of the luminescent COF at different reaction times. Adapted with permission from ref 53. Copyright 2016 American Chemical Society.

(illustrated in Figure 48,59 left, top). The CMP:C60 film performs as the photoactive layer in this construct. The J−V

high selectivity for picric acid and 2,4,6-trinitrotoluene at low concentrations (less than 1 ppm).58 6.8. Photoenergy Conversion in Solar Cells

Solar cells have been of key interest for porous organic materials. The first solar cell was reported in 1954 by Chapin et al., consisting of silicon p−n junctions.290 Considering the economic advantages, the scientific fraternity strived to further investigate organic solar cells,291 reporting a conducting single crystal of anthracene.292 However, the initial report of organic solar cells yielded particularly low power conversion efficiencies due to the generation of excitons in organic solar cells compared to the free electron−hole pairs in conventional (inorganic) solar cells.291 The issue of lower power conversion efficiencies was resolved with the adoption of heterojunctions (the use of an electron-donor layer and an electron-acceptor layer simultaneously side by side), which was first reported for organic solar cells by Tang in 1986.293 The concept of using bulk heterojunction organic solar cells in which electron-donor and -acceptor materials are allowed to intermix was introduced later.291 Jiang et al.59 recently reported the fabrication of CMP:fullerene bulk heterojunction solar cells with the ITO/ PEDOT:PSS/CMP:C60 film/LiF/Al structural arrangement

Figure 48. (Left, top) Layer architecture of solar cells wherein the CMP:C60 film performs as the photoactive layer. (Left, bottom) CMP structure together with the fullerene dopants. (Right) Current density versus voltage curves of the solar cells. Red and blue curves correspond to the solar cells comprising BTT-CMP:C60 and TTB-CMP:C60 films as photoactive layers. Abbreviations used in the figure have been defined in the abbreviations section. Adapted with permission from ref 59. Copyright 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. AL


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Figure 49. Structure of the CC9 porous cage showing the triangular opening of the pore (a), solid-state packing viewed along the c axis (b), and further packing viewed down the a axis (c). Hydrogen atoms are excluded for clarity. Reprinted with permission from ref 61. Copyright 2015 Elsevier B.V.

characteristics of the solar cells are illustrated in Figure 48,59 (right). While the power conversion efficiencies of 5.02% and 2.55% corresponding to the BTT-CMP:C60 and TTBCMP:C60 solar cells, respectively, are average and there is still extensive research to be performed before porous organic materials can be considered for adoption in solar cells, CMPs have demonstrated their capabilities.

porous polymers for drug delivery is new. Fang et al.62 reported the synthesis of 3-D polyimide COFs, namely, PI-COF-4 (bearing noninterpenetrating dia topology) and PI-COF-5 (bearing 4-fold interpenetrating dia topology), that can conveniently confine drug (ibuprofen, captopril, and caffeine) molecules in their pores to give superb loadings. The COFs provide impressive release control of the drugs as well.

6.9. Enantioseparation

6.11. Iodine Adsorption

The resolution of racemates (i.e., the separation of racemates into their constituent enantiomers) in a chiral setting has become an imperative requisite in the pharmaceutical and other industries. In 2015, Yuan’s lab reported the adoption of a chiral porous organic cage, namely, CC3-R, for gas chromatographic resolution of racemates.60 Very recently, they reported the development of a capillary column coated with another chiral porous organic cage, namely, CC9, that finds applications in the gas chromatographic separation of positional isomers and enantiomers and may complement the CC3-R-coated capillary column.61 The chiral nature of CC9 favors the separation of racemates into their constituent enantiomers. Figure 4961 illustrates the cage structure and cage−cage packing of CC9. The structure of CC9 has tetrahedral geometry. CC9 molecule packs with another CC9 molecule next to it in window-to-face mode.

Iodine-129 and iodine-131 are two radioisotopes of iodine that can be discharged into the environment from nuclear reactors in small amounts and nuclear accidents in large amounts. PAF1 and JUC-Z2 exhibit outstanding iodine adsorption performance (PAF-1, 1.86 g g−1; JUC-Z2, .44 g g−1).63 The especially high iodine adsorption capacities of PAF-21 and PAF-22 (PAF21, 1.52 g g−1; PAF-22, .96 g g−1) can be attributed to their charged frameworks. In addition, PAF-21 and PAF-22 have the potential to reversibly release iodine molecules in ethanol solution.64 6.12. Detection and Removal of Pollutants/Contaminants from Water and Other Liquids

Porous organic materials have proved their competence in ascertaining the presence of and removing pollutants/ contaminants from water and liquids; therefore, they are useful for applications such as wastewater treatment. Li et al.65 reported the synthesis of β-cyclodextrin-based HCPs that can not only competently adsorb aromatic molecules from water but also act as potent catalysts for the conversion of the

6.10. Drug Delivery and Release

Although research on the use of polymers in drug delivery systems has been going on for the last few decades, the topic of AM


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explain the structure of the material, and more research is imperative to comprehensively understand the structure of the material. An ideal amorphous solid (IAS) model for amorphous porous organic materials may act as the standard paradigm to analyze the topology of amorphous porous organic materials and needs to be explored. One vital challenge is to come up with a suitable imaging technique that can accurately analyze the structural details of amorphous porous organic materials.

adsorbed molecules when the HCPs are functionalized with gold nanoparticles. A 2-D polycationic COF was reported for the first time by Yu and co-workers66 in which the bipyridinium cations show affinity for anionic organic dyes, thereby justifying the competency of the COF for the adsorption of anionic dyes from water. The thioether functional groups in COF-LZU8 reported by Wang et al.67 assist in the removal of Hg2+ from water. PFCMP-0, which was synthesized by Deng et al.68 and possesses remarkable adsorption capability for the removal of a wide range of pollutants from water, owes its commendable performance to the presence of fluorine atoms and the large specific surface area of the porous polymer. PAF-45, which was synthesized by Zhao and co-workers69 and has a positively charged surface, has an affinity for negatively charged perfluorooctanesulfonate molecules and can suitably act as an adsorbent for the removal of pollutants from water. The PAF/ ionic liquid-coated solid-phase microextraction fiber, which was used in a gas chromatography-electron capture detector as reported by Wu et al.,70 can determine the presence of organochlorine pesticides in milk and juice specimen samples.

7.3. Synthetic Challenges

There are certain synthetic challenges as well. With the exception of linear PIMs, some CMPs (such as SCMP1 reported by Cooper et al.297) and some HCPs (such as SHCP 3−5 recently reported by Tan et al.117), other porous organic materials, such as COFs, CTFs, PAFs, etc., are not soluble in common organic solvents. The goal is to attain solutionprocessable porous organic materials. In addition, one-step yield reactions (such as that adopted by Yaghi et al.6 to attain COF-1 and that by Ben et al.89 to attain PAF-1) that give the highest efficiency are sought after. With respect to the synthesis of COFs, the challenge is to come up with novel reversible reactions yielding novel COFs. The subject of porous liquids, still at its germinal stage, entails across-the-board research in the pursuit of facile synthetic approaches, functionalization to attain customized attributes, etc. Future efforts may be directed to successfully synthesize type 3 microporous liquids. With respect to the fiscal aspects, inexpensive reagents and catalysts can be considered for the scale up of porous organic material synthesis. For instance, Li et al.298 recently reported the synthesis of HCPs from inexpensive pitch. Similarly, Al, Fe, Zn (or their compounds), etc., can be feasible catalysts in polymerization reactions. Unfortunately, most of the porous organic materials are expensive and have scale-up concerns that can be reduced by increasing the synthetic efficiency. Future research may be aimed at bringing forth breakthrough applications for the recently reported porous liquids, woven COF, etc. Porous liquids, endowed with permanent microporosity as well as fluidity, and woven COF may find applications that are common in other microporous organic materials. In addition, the fluid dynamics of porous liquids and the flexibility/pliability of woven COF can be put to use to find several intriguing applications for these materials.

7. OUTLOOK AND FUTURE PERSPECTIVES The latter-day studies reveal that the discipline of porous organic materials has enriched considerably. However, there are certain challenges that come to the fore. The scientific fraternity has always remained motivated to rise above and surpass challenges. 7.1. Rational Design of Novel Porous Organic Materials

In regard to the design of porous organic materials, the goal lies in the attainment of full control over the structures, properties, and functions (the three must relate mutually) of the materials by adjusting the building blocks/monomers. The challenge to strategic design of porous organic materials with customized functionalities by accommodating the building blocks with supreme accuracy, precision, and exactitude needs to be addressed. Despite the fact that many topologies, for instance, pcu (coordination figure octahedron, regular net),294 bcu (coordination figure cube, regular net),295 fcu (coordination figure cuboctahedron, quasiregular net),296 etc., have been reported for metal−organic frameworks, they are undiscovered thus far in COFs and CTFs. The prime challenge in realizing these topologies in COFs and CTFs is to identify the requisite and appropriate monomers. In addition, suitable reaction conditions and reaction methods (for instance, template method) are essential to attain these topologies in COFs and CTFs. For example, an appropriate eight-node monomer and another appropriate two-node monomer may combine to yield a COF with bcu topology. In regard to the pcu topology, the right combination of a suitable six-node monomer and another two-node monomer may help achieve the same. Likewise, an appropriate six-node monomer and another befitting threenode monomer may combine correctly to present a COF bearing fcu topology.

7.4. Conclusion

Endowed with large surface areas, impressive stabilities, and tailored attributes, porous organic materials have been extensively considered for diverse applications. This review offers an in-depth view of porous organic materials, noting their topological structures, how they differ, and how their structures link to their functionalities and properties. With each class of porous organic material, we see how the reaction conditions and the types of monomer used result in their topologies. HCPs as the first porous organic material observed, demonstrated high surface areas from interlinking polymer chains (hyper-cross-linked polystyrene; SA approximately 1000 m2·g−1). We observe unique structures from PIMs (PIM-1; SA 850 m2·g−1), allowing them potential solubility in conventional solvents, and they are readily synthesized. In addition, we see a high degree of crystallinity in the form of COFs (COF-1; SA 711 m2·g−1) and CTFs (CTF-1; SA 791 m2·g−1). COFs differ from CTFs in the consistency of the long-range order; the reaction conditions used for CTF synthesis show the formation of amorphous networks. However, the use of metal catalysts and high temperatures yield the thermodynamically stable

7.2. Technique for Structural Characterization of Amorphous Porous Organic Materials

Coming to structural analysis, the common crystallographic methods do not apply for structural analysis of amorphous porous organic materials (such as PAFs). The pair distribution function (PDF) analysis for structural information (topology, disorder, etc.) of amorphous porous organic materials entails contemplative research. Considering the structure of PAF-1, the continuous random network (CRN) model can partially AN


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ABBREVIATIONS BTT benzotrithiophene FPBA 4-formylphenylboronic acid ITO indium tin oxide PEDOT:PSS poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) SA surface area TTB 1,3,5-tri(2-thienyl)-benzene

structures of each. CMPs are another example of amorphous porous organic materials. Their extended conjugations throughout their networks offer the potential for not only porous applications but also electronic applications (CMP-1; SA 834 m2·g−1). Similarly, the focal point of porous organic materials is PAFs. PAF-1 offers a much greater surface area combined with exceptional physicochemical stability (PAF-1; SA 5600 m2·g−1), making it an ideal research candidate for the absorption of gas molecules. PAF-1 shows a unique porous structure. It reveals short-range order (some degree of dia topology in short range) but no long-range order (amorphous character in long range). The short-range order comes from the rigidity of the monomer, which is unique and has interesting properties. Not only have porous solids shown exceptional properties in applications such as gas storage and separation but also there is an increase in interest in the material, with considerable focus on the solubility and functionalization of porous organic materials as well as research into porous liquids. With the many differing structures and topologies and the vast array of building blocks already researched, the materials have a cornucopia of potential to explore.

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AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Saikat Das is currently pursuing his Ph.D. (2015−present) in macromolecular chemistry and physics under the supervision of Prof. Teng Ben at Jilin University, China. His research interests include the structural exploration of porous organic frameworks and related compounds. Patrick Heasman was born in 1992 in Leicester, U.K. He received his M.Chem. degree in 2014 from the University of Hull, U.K. He joined Dr. Abbie Trewin’s group at Lancaster University, U.K., as a Ph.D. student in 2014. His research interests include the design and synthesis of porous organic polymers with a focus on amorphous structures. Teng Ben is Professor of Chemistry at Jilin University, China where he has been in this position since 2010. He received his Ph.D. in 2002 from Jilin University in Polymer Science. He was a postdoctoral fellow with Prof. Eiji Yashima at Nagoya University, Japan from 2005−2008. His research interests include the fundamental understanding of host− guest interactions in nanoporous materials, gas storage, and separation using porous organic frameworks. Shilun Qiu is Professor of Chemistry at Jilin University, China. He received his Ph.D. in Chemistry from Jilin University in 1988. He became a professor in the Department of Chemistry, Jilin University in 1994. He was the recipient of the Second Grade Award of the State Natural Science Award of China in 2008 and the Fellowship of the Royal Society of Chemistry (FRSC). His research group is actively engaged in the synthesis of novel micro- and mesoporous materials for diverse applications.

ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grant no. 21390394, 21261130584, 21471065) and the “111” project (B07016) is gratefully acknowledged. The authors thank Abbie Trewin for the discussion on amorphous porous organic materials. AO


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