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DOI: 10.1002/ijch.201800066

Postsynthetic Covalent Modification in Covalent OrganicFrameworksYusran Yusran,[a] Qianrong Fang,[a] and Shilun Qiu*[a]

Abstract: The modification of covalent organic frameworks(COFs) based on postsynthetic covalent linkages is dis-cussed in this review. In this strategy, the COF is preparedasscaffold and then assembled with functional groups withpreservation of structural skeleton. Recent studies indicatethat a number of COFs, such as COF-5 and 3D-OH-COOF,are controllable to postsynthetic modification. Furthermore,

covalentmodifications including triazole, ester, amide,sulfide, o-carbamate, ether, and oxime are suitable forpostsynthetic functionalization. The rapid development ofpostsynthetic modification demonstrates that this approachwill provide a general platform to create COFs as robustfunctional porous materials for wide applications.

Keywords: covalent organic framework · postsynthetic modification · covalent linkage

1. Introduction

Crystalline porous materials combined the merit of long-orderstructural rigidity and the high surface area with richofporosity.[1,2] Covalent organic frameworks (COFs) are theexotic example of crystalline porous material composed ofsolely light-organic elements (B, C, O, N, and S) containing-building blocks that made them as the lowest density amongother porous materials.[3,4] Contrast with metal organic frame-works (MOFs) built by metal nodes and organic linkersthrough coordination bonds,[5] COFs are tunable in term oftopology, structure, and functionality that can be designedthrough reticular chemistry principle linked by strong covalentbonds.[6] In term of topology and structure, COF is obedient tothe geometrical and connective point of the building blocksresulting in an extended framework with periodic skeleton andordered pores within 2D or 3D structures. As the wider rangeof bulding blocks available (commercially or attempted syn-thesis) with varied geometrical and connective points, theextensive library of 2D/3D COFs has been increased rapidlysince their first discovery in 2005.[7] One distinguished featurein COFs is the free-adjustable of porosity (size and dimension)within the framework compared to their counterparts (MOFsand inorganic zeolites). The linker geometry and lengthdetermine the pore architectures (size and dimension). Theselead COFs useful in gas storage/separation,[8] energy storage,[9]

catalyst,[10] adsorbent,[11] drug delivery[12] and so on. Moreover,the strong covalent bonds in COFs strengthen chemical andthermal stabilities which are crucial issues in MOFs andzeolites. Since structural-property relationship in COFs is pre-determined, design and construction of COFs are such an artin chemistry. Owing to their high stability and adjustableporosity, structural modifications in COFs are welcome andinteresting, render the desired and/or additional properties inthe final framework could be achieved. Similarly found inMOFs, modification in COFs could tune their physical and

chemical properties which in turn greatly affected the overallperformances such as chemical and thermal stabilities, optical-electrical performances, gaseous or adsorbates uptake-releaseand host-guest interactions.Since then, it plays an importantrole in improving the structural quality and performance ofCOFs.

Structural modification can be carried out through pre-synthetic approach (bottom-up) by means the availablebuilding blocks contain or undergo certain functionality[13] andpostsynthetic modification (PSM) approach on the establishedframework.[11,14] Both approaches however, need the stabilityand compatibility consideration of the pre-/established COFframework. In bottom-up approach, the building blockprimarily undergoes modification or guest anchoring proce-dure which is stable during COF synthesis. However, thisapproach may hold some limitations such as the anchoredfunctionalities could potentially react with other active sitesduring COF synthesis and it may be leached out (in case formetal-anchored) during synthesis and activation processes.Meanwhile, in the PSM approach, the established COFframework faced further modification through covalent linkingbetween the existing pendant group (e.g.�OH, �NH2, =CH2

and�CN3) in the COF skeleton and the incoming constituent.More importantly, under postsynthetic covalent modification,the types and the quantities of functionalities are controllablestitching by strong covalent linkage.

Several reported reviews have been discussed the discov-ery, synthesis and recent progress of COFs including structuralmodification.[3,4,6,15,16] However, comprehensive review in term

[a] Y. Yusran, Q. Fang, S. QiuState Key Laboratory of Inorganic synthesis and preparativeChemistryDepartment of Chemistry, Jilin UniversityChangchun, 130012, P.R. ChinaE-mail: sqiu@jlu.edu.cn

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of postsynthetic covalent modification has not been elaboratedyet. In this review, we specifically summarized the recentprogress of structural modification in COFs via postsyntheticcovalent modification of the pre-designed framework. Further-more, the effect of modification over the structural quality andperformance of the COFs to certain applications were well-discussed. Since this new class of crystalline porous materialsare under hotspot researches, their structural modifications arecertainly interesting and paramount important.

2. Design Strategy in COFs

The synthesis of COFs follows the reticular chemistryprinciple which allows the precise integration of buildingblocks into a periodic skeleton and long-order crystallinestructure connected by strong covalent bonds. Compared toother porous crystalline materials (MOFs and inorganiczeolites), the synthesis of COFs is flexible in terms of porosity,topology and functionality. All these features are predictablethrough the geometry and functionalities of the availablebuilding blocks.To design COFs, crystallinity and porosity arethe main aspects that should be considered which made themdiffer from amorphous polymers. The covalent bonds stitchingthe building blocks are the starting point to create a rigidframework and their shape and configuration determined thepore-dimensions and structural topology. Thus, designedstrategy in COFs can be categorized as bottom-up approach toobtain a preferred framework.

COFs are crystalline organic polymer governed by strongcovalent bond/linkage between the building blocks.Generally

speaking, all covalent bonds applied in synthesizing organicpolymers are, essentially, possible to construct COFs. How-ever, not all the covalent bonds could successfully link thebuilding blocks to generate crystalline materials as generallyfound in amorphous polymers. It has been widely discussedthat only the covalent linkage involving the reversiblereactions could manifest crystalline product, since it providedefect-healing mechanism during synthesis progress.[16] Thisreversible reaction generates insoluble crystalline powder.

Figure 1 depicts typical covalent linkages which have beenemployed for synthesizing COFs.Linkage among B and O(B�O) under dehydration reaction resulted boroxine,[7] boro-nate-ester,[17] borosilicate[18] and spiroborate[19] based COFs.Meanwhile, linking C and N (C=N) generated other covalentlinkages of imine,[20] hydrazone[21] and squaraine[22] basedCOFs. Imine-based COFs have been widely synthesized fortheir easy crystallization and they are more stable thanboroxine based COFs. Similar to the linkage found in organicamino-acid, b-ketoenamine,[9] and amide[23] covalent linkagesbased COFs have also enriched the COFs compound library.Generally, these COFs were obtained under relatively highertemperature (up to 200 8C) with pronounce thermal andchemical stabilities. Another C and Naromatic (C�Naromatic) link-age was also involved in synthesizing COFs resulting indimerization (phenazine, imidazole)[24] and cyclo-trimerization(triazine)[25] based COFs. The sp2-carbon (C=C) linkage hasalso been realized in COF synthesis which adopted thestructural backbone of 2D graphene.[26] Due to the high interesttowards this new class of porous material, new linkage thatcan generate crystalline framework is still under exploration.

Yusran Yusran received his B.Sc. degree inChemistry with honors from State University ofMakassar, Indonesia in 2012 and M.Sc. degreein chemistry from Gadjah Mada University(UGM), Yogjakarta Indonesia in 2014. He thenattended state key laboratory of inorganic syn-thesis and preparative chemistry (SKLIPC), JilinUniversity China as research student workingon metal organic frameworks (MOFs) underguidance of Prof. Shilun Qiu. From 2016 tillpresent he has been a Ph.D student in thesame lab under supervision of Prof. QianrongFang and Prof. Shilun Qiu. His current researchis focus on porous materials includingMOFsand COFs and their potential application incatalyst and energy storage.

Qianrong Fang obtained his B.S. (2001) andPh.D (2007) degrees in Chemistry from JilinUniversity in China. From 2007 to 2014, hecompleted his postdoctoctoral study in Univer-sity of California at Los Angeles, Texas A&MUniversity, University of California at Riversideas well as University of Delaware. In 2014, hebecame a senior research fellow in CatalyticScience and Technology Center at University of

Delaware. In 2015, he received 1000 YoungTalent Plan of China and went back to the StateKey Laboratory of Inorganic Synthesis & Prepa-rative Chemistry at Jilin University, as a fullprofessor. His current research focuses on thedesign and synthesis of covalent organic frame-works (COFs) for applications in adsorption,se-paration, catalysis, and a number of others.

Shilun Qiu received his Ph.D in chemistry fromJilin University, China, in 1988. He joined theUniversity de Haute-Alsace for postdoctoralresearch. In 1994, he was promoted to be a fullprofessor in Jilin University. He won the SecondGrade Award for the State Natural ScienceAward of China in 2008. He received the GuestProfessorship in Tohoku University, DrexelUniversity, Stockholm University, and is nowthe Honorary Professor of University ofQueensland. His recent research interestsfocus on the studies of molecular engineering,synthesis, structure and applications of porousmaterials and membranes, involving zeolites,mesostructured materials, MOFs and COFs.

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On the other hand, the length and geometry of the organicbuilding blocks determine the porosity of the resulting COFframework. Thus, porosity adjustment (size and dimension) inCOFs is more flexible compared to MOFs and inorganiczeolites. Porosity in inorganic zeolite is limited by thegeometry of tetrahedral silica and alumina secondary buildingunits (SBUs) which is similar to those in MOFs where theirporosity depend on the fixed metal ions and organic linkersgeometry.[27] Meanwhile, the wider availability of organicbuilding block possible to employ in COFs synthesis lead themicro- to mesoporous COFs with 2D and 3D dimensions havebeen designed so far.[13,28,29] Moreover, heteropores-type COFshave also been realized recently demonstrating the greatpotentials in size selective separation and conversion.[30,31] Inaddition, the design structure in COFs should satisfy therigidity issue to establish long-order and defined pore structure(e. g. 2D or 3D tetragonal or hexagonal shapes) as finalframework. Thus, only certain shape and geometry of buildingblocks could be possible to employ in designing COFs.

Figure 2 displays the typical organic building blocksemployed for construction of COFs framework. Linearmonomers (1–3) are the most widely used in synthesizingCOFs since their length is adjustable and can be coupled withany kinds of building block geometries to build 2D or 3DCOFs structures. Triangular geometry building blocks (4–6)are used to construct 2D hexagonal-pores containing COFsand several 3D microporous COFs. Coupled it with linearbuilding blocks, we reported large-pore 2D COFs (5.3 nm, PI-COF-3) by choosing larger triangular building block TABPB(6).[28] Square-like pores in COFs can be designed by stitchingsquare-shape monomers such as porphyrin-based buildingblock (8) and linear building block.[32] Similar in inorganiczeolites, ultra-microporous COF can be obtained with con-densation of hexaphenyl-amine containing propeller-likebuilding blocks (9) with shorter di-aldehyde monomer.[33] For

constructing 3D COFs, it is simply just coupling tetrahedral-type building blocks (e. g. TAM (10) or TFPM (11)) and otherlinear or triangular building blocks. Hence, COF-300 and 3D-IL-COFs possessing 3D pore structure have been reported.[8,20]

One of the great example of pore design flexibility in COFs ispre-designed ofheteropore structure in one framework. Oneexample of this strategy was reported by employing D2h-symmetrical building block.[30] Indeed, the abundant possiblecreation in designing pore environment in COFs promise thatthe structural modification in COFs shall be richer and widerpossibility compared to their counterparts (MOFs and inor-ganic zeolite). Since then, structural modification by introduc-ing certain functional moiety to the pre-designed COFstructure could be versatile.

3. Covalent Linkage Modification

Structural modifications of pre-designed COFs are performedto upgrade their structural quality and performance related tocertain applications. COFs could provide more versatilestructural modification since their porosity and stability can betuned towards chosen building blocks. Generally, modificationeffort can be performed in bottom-up and postsyntheticmodification (PSM) approaches as illustrated in Scheme 1.The bottom-up strategy elaborated the modification of COFsby employing functionalized-building blocks with desiredmoieties. Thus, this strategy could be straightforward byutilizing commercially available functionalized-buildingblocks[9,34–38] or two-steps approach by previously anchoringthe desired moieties on one or both building blocks thenfurther performing COF synthesis.[39,40] However, this strategymay not versatile since the synthetic parameters of COFs(solvent and catalyst with elevated temperature up to 200 8C)may be unable to preserve the functional moieties and induce

Figure 1. Typical covalent linkages employed in construction of COFs.

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possible further and uncontrollable side reaction betweenactive sites within the pre-established framework duringsynthesis. As a consequence, the type of functional groups thatcan be incorporated into framework is limited and control overfunctional density is challenging. Furthermore, bulky constitu-ents (e. g. macromolecules, dyes and fluorescent molecules)are tedious to be pre-functionalized on building block since

they may disturb the structural rigidity when COF frameworkis being constructed during synthesis.

Contrast to the bottom-up approach, PSM approach ismore flexible in term of control over the types and density ofdesired functional moieties anchored onto the established COFframework without altering the crystallinity of the formerframework. In general, COF with certain pendant groups isdesigned as scaffold. Then, the desired functional moiety iscovalently linked with the available pendant group undercertain reaction to generate functionalized-COF. Since themodification involves strong covalent bond between thereactive pendant groups within the COF framework and theactive group of incoming constituent, the resulted anchoredmoiety is generally stable under harsh environment and avoidpossible leaching while at the same time provide additionalproperty to the former framework. This postsynthetic covalentmodification will afford strong attachment of functionalmoiety on the COF framework. Postsynthetic covalent mod-ifications in COFs have been demonstrated so far, since thewider pendant group containing building blocks (e. g. R�OH,R�CN3, R�C�CH, Figure 2) can be incorporated into COFsframework through pre-designed strategy as a scaffold.Similarly in postsynthetic efforts found in MOFs, several typesof covalent linkages have been employed in structuralmodification of COFs including triazole, ester, amide, sulfide,ether and oxime linkages. Through these covalent linkages,several functional moieties have been anchored on the pore-wall of COF to generate multifunctional COFs those are

Figure 2. Typical building blocks used in construction of COFs.

Scheme 1. Structural modification strategies in COFs (bottom-upand postsynthetic modification (PSM)).

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potential for several applications such as high performanceadsorbent, catalyst, optoelectronic materials, energy storageand so on.2D COFs have been widely functionalized sincetheir pore geometry and size (one-dimensional alignedpore)are adjustable. Meanwhile, the high surface area and richof miroporosity found in 3D COFs render the postsyntheticcovalent modification may provide undoubted properties.

3.1 Triazole Linkage

Covalent triazole linkage can be constructed through commonclick reaction by reacting the azide (R�N3) and ethynyl (R�CH) functional groups under milder temperature in the presentof CuI as catalyst as depicted in Figure 3.The PSM of COF viacovalent triazole linkage has been widely demonstrated due totheir facile and its chemical stability during COF synthesis.

Mesoporous hexagonal-pore 2D COF-5 has been designedwith anchored azide pendant group generating azide-COF-5([N3]x-COF-5).[14] Under three-components approach, the con-tent (x) of azide groups on pore-wall were controlled (Fig-ure 4a). The present quantitative azide group on the pore-wall,render the N3-COF-5 as potential scaffold to perform furtherfunctional modification to obtain the desired functionalized-COF. The quantitative CuI assisted-click reaction betweenazide pendant group and ethynyl group of the incomingconstituent afforded covalently linked triazole-functionalizedCOF-5 (RTrz-COF-5). Interestingly, this protocol was compat-ible for wider functional groups attached on the ethynylgroups (R=Ac, Bu, Ph, Es or Py) while preserved thecrystalline nature of the former scaffold.The control over theazide group content lead the density of functional group at thefinal product were tuned. The diversity of this protocol wasdemonstrated by performing another covalent click reaction onazide-appended square-like mesoporous Ni-phtalocyanineCOF (N3�NiPc-COF) with similar incoming ethynyl func-tional moiety resulted triazole-decorated Ni-PhtalocyanineCOF (RTrz-NiPc-COF). This work however demonstrated thatpostsynthetic covalent modification could provide pore surfaceengineering in COFs with pre-designed composition, compo-nents and functions.

On one hand, under the similar protocol, organocatalystwas successfully incorporated into a square-like 2D mesopo-rous porphyrin COF via postsynthetic covalent modificationthrough click reaction of ethynyl containing COF as a scaffold([CH�C]x�H2P-COF) and azide containing organocatalystpyrrolydine as incoming constituent.[41] Three-components

strategy allowed for control ethynyl pendant group density (x)and location within the pore of the established H2P-COF(Figure 4b). The pendant ethynyl groups were postsyntheti-cally modified with azide containing pyrrolidine moiety undercommon CuI-asissted click reaction to afford catalyticallyactive pyrrolidine-COF ([Pyr]x�H2P-COF). Covalent modifica-tion however did not affect the crystalline nature of thepristine framework. As predicted [Pyr]x�H2P-COF couldperform 100 % Michael addition reaction in only 1 h relativelyfaster compared to non-modified structure (3.3 h). Since thedensity of the organocatalyst moiety can be controlled, thecatalytic performance can be optimized. Moreover, theanchored moiety was stable even when the catalytic test wasperformed under flow system. On the other hand, bulkyconstituents such as macromolecules or buckyball are consid-ered tedious to incorporate into the porous materials due totheir large size and possible aggregation within the hostframework.However, as the pore size in 2D COFs is adjust-able, the incorporation of such large molecules is possible.The electron-acceptor buckyball molecule has been anchoredonto the open-lattice mesoporous 2D phthalocyanine COFthrough postsynthetic covalent modification approach toobtain donor-acceptor heterojuction based COF. To realizethis, an azide pendant group containing mesoporous phthalo-cyanine COF ([N3]x�ZnPc-COF) was constructed under three-components strategy allowing for controlled quantity (x) ofthe azide pendant group (Figure 4c).[42] Under common clickreaction, the ethynyl containing buckyball molecule wascovalently linked with the azide pendant group appended onthe COF scaffold to afford [C60]x�ZnPc-COF which exhibitedphoto-induced electron transfer and allowed charge separationwith radical species delocalized in p-arrays, whereas thecharge separation efficiency was dependent on the buckyballcontent which was adjustable relative to the azide pendantgroup content. In addition, the covalent triazole linkageemployed in this structural modification guaranteed theincorporation of buckyball molecules and also may avoidphysical elution from pore channel. This work represents theenhancement of optoelectronic property of COFs throughpostsynthetic covalent modification while preserved thecrystallinity of the former framework.

Chiral molecule has also been covalently linked to theporechannel of highly porous and stable 2D mesoporous TBP-DMTP-COF via triazole linkage to generate chiral organo-catalyst based COF. In this context, a COF scaffold wasdesigned by appending both of methoxy (�OCH3) and alkynyl(�C�CH) functional groups into the pore wall of the COFtowards three-components condensation system which are notonly couldincrease the stability and crystallinity of the hostbut also provide a pendant group for further modification(Figure 4c).[43] Controlled covalent linkage attachment ofchiral molecule was then successfully performed via clickreaction between the alkynyl pendant group and azide groupof chiral pyrazine molecules to afford chiral [Pyr]x-TBP-DMTP-COF. The proposed protocol allowed for controlledstability and chiral functionality, thus highly stable multifunc-

Figure 3. Triazole linkage formation through common click reaction.

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Figure 4. Postsynthetic covalent modification of COFs via covalent triazole linkage.

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tional based COF was obtained. Interestingly, the crystallinityand porosity of the modified COF were retained lead thecombination of catalytic activity, enantioselectivity and recy-clability properties were shown by [Pyr]x-TBP-DMTP-COFwhich are attractive in heterogeneous organocatalyst. Thischiral organocatalyst based COF promoted asymmetric C�Cbond formation in water under ambient temperature. Thisreport showed the transformation of highly stable COF intohigh stable chiral-organocatalyst COF under covalent linkagemodification with control property and performance. Anothereffort in modifying the structural backbone of COF viapostsynthetic covalent triazole linkage was the incorporationof polyradical moiety ([TEMPO]) into a mesoporous square-like porphyrin COF for high performance capacitive energystorage. The ethynyl (HC�C�) pendant group decorated 2Dmesoporous NiP-COF ([HC�C]x�NiP-COF) was preparedunder three-components strategy allowing for control over thedensity of [TEMPO] radical moiety anchored into the COFbackbone after modification.[44] The resulted [TEMPO]x�NiP-COF was electrochemically active since the TEMPO radicalmoiety could undergo one-electron reversible redox reaction.More importantly, the stable of imine-linked COF coupledwith the covalently linked the radical [TEMPO] moiety on thepore wall render the modified COF exhibited sufficientpseudo-capacitive performance and good cycelability. Thereported protocol demonstrated the transformation of electro-chemically inactive COF to electrochemically active COFthrough covalent linkage modification without sacrificing thecrystalline nature of the former framework even 100% of theavailable pendant groups were modified.

Organizing diverse functional groups from both hydro-phobic to hydrophilic and from acid to basic moieties on porewall of COFs were also demonstrated through postsyntheticcovalent triazole modification under click reaction.[45] Undercontrollable load of pendant groups, the tailor-made COFsthrough pore surface engineering with wider functionalitiescan be constructed while retaining the crystalline nature ofCOF framework. The systematic integration of functionalgroups in combination with a manageable pore size made theresulting modified COFs attractive for CO2 adsorption. So far,postsynthetic covalent modification of COFs via covalenttriazole linkage demonstrated general platform to enhance thestructural quality and performance of COFs.

3.2 Ester Linkage

Ring opening reaction between succinic anhydride andhydroxyl group (�OH) affords extended carboxylic moiety viacovalent ester linkage as depicted in Figure 5. This reactioncould be performed under metal-free reaction with mildertemperature which has been employed for modifying MOF-containing hydroxyl pendant group.[46] Since hydroxyl pendantgroup decorated COF can be designed by chosen buildingblock, PSM of COFs via covalent ester linkage has also beenrealized.

Postsynthetic covalent modification of COF via covalentester linkage has been reported. The hydroxyl group decoratedsquare-like porphyrin based COF ([HO]x�H2P-COF) wasdesigned as scaffold to generate carboxyl-containing porphyrinCOF (HOOC]x�H2P-COF) via covalent esterlinkage.[47] Underthree-components strategy, [HO]x�H2P-COFs were synthe-sized by controlling hydroxyl pendant group content (x=25%, 50%, 75 % and 100%). Later, the control postsyntheticcovalent modification was performed by metal-free ringopening reaction of hydroxyl pendant group with succinicanhydride manifested carboxylic-functionalized COF(HOOC]x�H2P-COF) via ester linkage (Figure 6a). Since thecarboxylic group could trigger a dipolar interaction with CO2,HOOC�H2P-COF was predicted to enhance CO2 absorptioncompared to the pristine framework. The CO2 absorptionanalysis at 1 atm and 273 K showed that [HO]x�H2P-COFexhibited low capacities (46–63 mg/g) while [HOOC]x�H2P-COF demonstrated dramatically CO2 capacities up to 180 mg/g. More importantly, the CO2 capacities are improved in linewith COOH content in the framework. Moreover, they alsoshowed good performance in terms of reusability, selectivityand separation productivity. The modification effort clearlyshowed the improvement of CO2 storage compared to thepristine COF. In addition, this covalent modification wasrelatively green approach without involving an additionalmetal catalyst.

Inspired by the protocol mentioned above, we recentlydesigned a hydroxyl-appended 3D COF (3D-OH-COF) withdia net topology as scaffold to obtain carboxyl-functionalized3D COF which is remained unexplored via postsyntheticcovalent ester modification.[11] We designed the 3D-OH-COFfor the first time by condensing tetrahedral knot (TFPM) andlinear link (DHBD) afforded 3D COF structure with 1.35 nmrectangular micropores (Figure 6b). The present of hydroxylpendant groups on the pore channel were converted tocarboxyl group by reacting them with succinic anhydride inanhydrous acetone affording a new 3D-carboxyl-COF (3D-COOH-COF). Importantly, the carboxyl functionalized-COFshowed retained crystallinity and significant porosity intake.The present carboxyl group enhanced the performance of theresulted 3D-COOH-COF toward selective adsorption oflanthanide ions with good absorption selectivity compared tothe former structure. In all, the both reported protocolsdemonstrated that postsynthetic covalent modification via esterlinkage is versatile for structural modification of 2D/3D COFs

Figure 5. The ester linkage formation afforded under metal-freeopening reaction of hydroxyl group (-OH) and succinic anhydride togenerate carboxyl-appended group.

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providing strong linkage over functionality, thus it may avoidactive sites dissolution during absorption test and could be ageneral platform to create another multifunctional COFs forwider application.

3.3 Amide Linkage

Inspired by the strong covalent linkage found in protein andamino acid, covalent amide bond has been utilized as linkagein structural modification of MOFs.[48] The amide bond can beconstructed by the condensation reaction of primary aminegroup (�NH2) and acetic anhydride as illustratedin Figure 7.

The PSM through covalent amide linkage in COFs can bedesigned by anchoring primary amine pendant group on thepore-wall of COF scaffold. However, since the primary aminegroup may react during COF synthesis, the amine pendantgroup in the final COF framework could not be preserved.Since then, it needs a strategy to append this functional groupin COF backbone. Meanwhile, the rigid and stable backboneof 2D COFs framework allow for sequential pore modificationunder postsynthetic strategy to obtain certain functionality onthe final product. In this context,sequential modification ofnitro-anchored 2D mesoporous keto-enamine COF (TpBd(NO2)2-COF) was reported to obtain primary amine function-alized COF which then possible to be converted to secondaryamine and carboxyl containingCOF via amide linkage.[49] Theprotocol was set as follow, the nitro-functionalized COF waspreviously designed as scaffold by reacting linear nitro-functionalized DNB and TPG monomers afforded TpBd(NO2)2-COF (Figure 8). The COF scaffold was then reducedby SnCl2 to obtain primary amine pendant group on the porewall (TpBd(NH2)2-COF) while preserved the porosity andcrystallinity of the skeleton.The present of these pendantgroups allowed for covalent modification by reacting themwith acetic anhydride manifested secondary amine and

Figure 6. Postsynthetic covalent modification of hydroxyl-containing COFs via ester linkage afforded carboxyl-functionalized COFs.Postsynthetic covalent modification of mesoporous 2D [OH]�H2P-COF to [HOOC]�H2P-COF (a) and 3D-OH-COF to 3D-COOH-COF (b) viaester linkage.

Figure 7. The amide linkage formation afforded by condensationreaction of primary amine group and acetic anhydride.

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carboxyl containing COF (TpBd(NHCOCH3)2-COF) linked bystrong amide linkage as final product. Interestingly, thecrystallinity of the obtained functionalized COF was retainedunder sequential (reduction and covalent modification) treat-ments. As of the covalent amide linkage is relatively stabletoward acid environment, this amide-COF was assessed inlactic acid adsorption. The acidic absorption performance ofthe obtained product was accessed through batch systemwhich showed that the absorption was varied for all samplesdepending on H-bonding strength. This study clearly elabo-rated that the stability of the COF as porous materials couldprovide possible abundant framework decoration while pre-served the crystallinity and porosity.

Very recently, the amide linkage has also been adopted inincorporation of bulk-molecule based enzyme on the pore-wallof the predesigned COF through PSM strategy allowing fordevelopment of bio-catalyst based COF composite. Themesoporous 2D COF was synthesized as perfect scaffold toaccommodate the incoming lipase molecules onto its pore-wall via covalent amide linkage between the active site ofCOF scaffold and functional group of lipase molecules(proven by FT-IR analysis).[50] Due to the strong bond among

the host and incoming guest, lipase@COF composite exhibithigh enzyme uptake and high catalytic activity compared toother prepared lipase@porous materials composites, thusalerting high compatibility. Remarkably, the prepared lipa-se@COF-OMe sample exhibited superior catalytic perform-ance in terms of conversion rate and shorter reaction timerelative to the prepared lipase@amorphous polymer, lipase@-MOF and lipase@zeolite samples. This report exemplifying avaluable impact ofpostsynthetic covalent modification of theCOF backbone to realize biocomposite based COF catalyst.

3.4 Sulfide Linkage

Coupling reaction between thiol (R-SH) and alkene com-pounds (R�CH=CH) generates alkane-thiol compounds linkedvia covalent sulfide linkage (R�CH2-S�R) as depicted inFigure 9. The flexible pre-functionalization efforts in COFssynthesis render the possibility to design alkene or thiol-appended COF as a scaffold for further PSM via covalentsulfide linkage.

Figure 8. Sequential transformation of nitro-COF to secondary amine-COF via highly stable covalent amide linkage.

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While PSM via covalent linkages in 2D COFs have beenwidely investigated due to their flexible porosity and highstability, postsynthetic covalent modification in 3D COFs waslargely unexplored due to their relatively limited pore sizeadjustable compared to 2D COFs. A 3D COF-102 containingallyl pendant group was designed allowing for further attach-ment functional moiety via covalent sulfide linkage.[51] Thetruncated mixed linker (TML) strategy of tetrakis-benzenediboronic acid and allyl-functionalized tris-benzene diboronicacid afforded an allyl-functionalized 3D COF (COF-102-allyl)under hydrothermal treatment. The PSM under covalentsulfide linkage was then performed via thiol-ene couplingreaction between allyl pendant group and propanethiol assistedby photo-initiator in THF generated an alkane-thiol modifiedCOF-102 (COF-102-SPr) (Figure 10). Interestingly, the COF-102-SPr still possessed high surface area even the modifiedagent has long aliphatic chain which forecasted that this COFhold promise for potential application.

The PSM via covalent sulfide linkage approach has alsobeen demonstrated on newly design vinyl (-CH=CH2)containing mesoporous 2D COF with the incoming sulfurfunctional group transforming the inactive adsorbent basedCOF to highly active adsorbent based COF for efficient andeffective mercury removal. To realize this approach, vinyl-functionalized mesoporous 2D COF was designed underimine-based condensation reaction of Tab and Dva monomersgenerated a high crystalline and high surface area mesoporous(2.8 nm) 2D COF (COF-V).[52] The precisely bottom-upintegration of vinyl group onto pore wall allowing for furthercontrolled sulfur derivative attachment toward postsyntheticcovalent sulfide linkage by treating the COF scaffold with1,2ethanedithiol manifesteda new thiol-functionalized 2DCOF (COF-S-SH). Notably, the established functionalized-

COF exhibited sufficient crystallinity and porosity intake thusalerting structural integrity. Remarkably, COF-S-SH demon-strated high efficiency mercury (Hg2+ and Hg0) contaminantsremoval from aqueous solution and air, exceeding all those ofthiol and thioether functionalized materials so far. This exoticexample of structural modification of COFs representing thevital role of postsynthetic covalent linkage modification inenhancing the structural quality and performance of thepristine COFs for high performance adsorbent.

Very recently, another excellent example of the structuralmodification of predesigned COF under postsynthetic covalentsulfide linkage was performed exemplifying the robustnessemployment of the functionalized 2D COF as water-repellentcoating agent, an application provided by hydrophobicmaterial. To made this comes to reality, the superhydrophobicbased COF was created by modifying thepore-wall of the COFscaffold with hydrophobic perfluoroalkyl group thus impartingthe superwettability issuein COFs.[53] Systematically, theprevious vinyl pendant group decorating 2D COF (COF-V)was utilized as perfect scaffold. Then, then hydrophobicincoming constituent of perfluorodecanthiol trifluorotoluenemolecule was covalently linked with the available vinylpendant groupthrough covalent sulfide linkage in the presentof azobisisobutyronitrile (AIBN) as catalyst affording thesuperhydrophobic based COF (COF-VF). Interestingly, undercontrollable structural modification, the resultant functional-ized COF retained their porosity ad crystallinity relative to thepristine framework. The present of bulky superhyhydrophobicmoiety on the pore-wall, made the COF-VF to possess water-repellent properties of superhydrophobic surfaces, virtually allaqueous liquids (including inorganic acidic and basic solution)were prevented from permeating the functionalized COF, thusgreatly improved its tolerance against variable pH environ-ments. Notably, the superhydrophobic property of COF-VFwas applicable when used in coating several substratesincluding melamine foam, paper and magnetic liquid. Again,this inspired report witnessed the performance-aided in COFsby postsynthetic covalent modification.

3.5. o-Carbamate Linkage

Another covalent linkage that has been utilized in functional-ization of material such as polymer was carbamate-typelinkage that can be obtained from coupling reaction ofhydroxyl (�OH) and thiocarbamate groups generating o-carbamate linkage as illustrated in Figure 11.[54The covalentcarbamate linkage has been adopted to obtain fluorescent-based COF through PSM approach on the COF scaffold.Utilizing the hydroxyl group appended COF as scaffold, largefluorescent molecule has been incorporated into mesoporousboronate ester COF via postsynthetic covalent o-carbamatelinkage.[55] The mesoporous 2D boronate-ester COF containinghydroxyl pendant group (T-COF-OH) with 4.1 nm accessiblepore channel was prepared through co-condensation ofDHTBA and HHTP monomers (Figure 12). Owing to its large

Figure 9. The covalent sulfide linkage formation by coupling reactionof thiol (R-SH) and alkenyl group (R=CH2).

Figure 10. Postsynthetic covalent modification of 3D COF-102-allylvia covalent sulfide linkage.

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pore-channel, T-COF-OH is attractive scaffold for incorpora-tion large molecules. In this context, fluorescent active ofcarbamate-containing FITC molecule was covalently incorpo-rated into the pore channel of T-COF-OH to afford photoactiveT-COF-OFITC via covalent o-carbamate linkage. Remarkably,the obtained functionalized COF showed retained crystallinityand exhibited strong fluorescent signal indicating the success-ful transformation of photo-inactive COF to photoactive COF.Moreover, the versatility of this protocol was evaluated byanchoring another fluorescent molecule of n-octhylisocyanatemanifested a new photoactive COF. This protocol establisheda general method to achieve multifunctional COF forfluorescence labelling application with strong attachment offluorescent molecule.

3.6 Ether Linkage

Another covalent linkage used for postsynthetically modifyingthe pre-designed COF scaffold was covalent ether linkage thatis generated from coupling reaction of hydroxyl group (R-OH)and alkyl-bromide as illustrated in Figure 13.

To realize this protocol on PSM in COF, mesoporousimine-linked pyrene COF containing hydroxyl pendant groupwas constructed by condensation reaction of three-components

system of PyTTA, PA and DHPA monomers allowing forcontrolling over hydroxyl content on pore wall of COFscaffold ([HO]x�Py-COF) (Figure 14).[56]Subsequently, ionicliquid (Et4NBr) was immobilized onto the pore wall viacovalent ether linkage under Williamson ether reactionafforded [Et4NBr]x�Py-COF with controlled over ionic liquidcontent. This protocol allowing for strong anchoring of ionicliquid moiety, thus catalytically active COF was achievedprovided by ionic liquid. As predicted, the crystalline natureof COF scaffold was retained after immobilization of ionicliquid on the pore wall. Importantly, the [Et4NBr]x�Py-COFsexhibited elevated CO2 uptake capacities and high isostericheat absorption (Qst) than those [HO]x�Py-COFs samplesindicating the effectiveness of pore-wall modification for CO2

capture. This report however showed the quality and perform-ance improvement of COFs via postsynthetic covalent mod-ification.

3.7 Oxime Linkage

Another prospective covalent linkage that may amenable to beemployed in structural modification of COF is oxime linkage.In general, oxime based compounds have general formula ofR1R2C=NOH with vary R chain forming oxime type com-pounds of aldoxime (aldehyde-type oxime), ketoxime (ketone-type oxime) and amidoxime (amide-type oxime).[57]The laterone can be obtained under condensation reaction of nitrile

Figure 11. Typical covalent o-carbamate linkage afforded by couplingreaction of hydroxyl and carbamate groups.

Figure 12. Fluorescent molecule (FITC) incorporation into T-COF-OH pore channel via covalent o-carbamate linkage afforded photoactive T-COF-OFITC.

Figure 13. The covalent ether linkage formation by couplingreactionofhydroxyl group (R-OH) andalkyl-bromide.

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(�CN) compound and hydroxylamine (NH2OH) in the presentof anhydrous methanol as depicted in Figure 15.

Covalent amidoxime-type linkage has been adopted instructural modification of predesigned COF backbone underPSM approach to obtain the desired functionalized-COF. Thereported approach provide efficient step to anchor amidoximegroup on the pore-wall of COF which is an effective chelatingsite for radionuclide sequestration.[58] Specifically, the nitrilependant group containing 2D COF (COF-TpDb) was designedunder solvothermal condensation reaction of 2,5-diaminoben-zonitrile (Db) and triformylphloroglucinol (Tp) as scaffold(Figure 16). Amidoximated treatment was then performedover the COF scaffoldby reacting it with hydroxylamine in thepresent of anhydrous methanol generated amidoxime-function-alized 2D COF (COF-TpAb-AO). Having established amidox-ime groups anchored on the pore- wall of the functionalizedCOF,COF-TpAb-AO is an attractive solid sorbent for uraniumcontaminant sequestration. Remarkably, the COF-TpAB-AOfar outperformed their corresponding amorphous sorbentanalogs in terms of adsorption capacities, kinetics andaffinities towards uranium waste sequestration. This report

clearly demonstrated the performance enhancement in COFstoward postsynthetic covalent modification.

Although this type of covalent linkage modification is stillrarely be reported in COFs modification, this protocol iscompatible to be widely applied since nitrile and/or aminegroup are tunable to be appended in COFs pore-wall via pre-designed building blocks or sequential pore-wall engineering.

4. Summary and Outlook

Owing to the ability to pre-design the structural andfunctionality of their framework, structural modifications inCOFs are versatile which in turn could enhance their structuralquality (chemical and physical properties) and performancerelated to certain application. Postsynthetic covalent modifica-tion allow for strong attachment of desired moieties on thefinal framework with wider organic components that can beadded resulting in a framework with great complexity. Moreimportantly, the PSM efforts allow for control over the typesand contents of functional moieties anchored on the pristineframework leading to manageable performance of the resultedfunctionalized COF which is difficult to achieve by bottom-upapproach.

As of the structural modifications in COF are still in theirearly stage, the high concerned of researches to discover newstructures and topologies with varied functionality containingCOFs could greatly guarantee that the postsynthetic covalentmodification may expand from attachment of simple organicmoieties to large organic threads. Moreover, the types ofcovalent linkage employed in PSM approach could potentiallybe varied since the high flexibility in pre-designed structural

Figure 14. Covalent immobilization of ionic liquid into mesoporous hydroxyl containing pyrene COF via covalent ether linkage.

Figure 15. Typical covalent oxime linkage (amidoxime) afforded bycondensation reaction of nitrile compound and hydroxylamine.

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functionality offered by high stable crystalline framework ofCOFs. In addition, sequential functional modification has alsoprovided new route to achieve the desired functionalized-COFwhich in turn could expand the PSM strategy in COFs. Finally,we forecasted that the great potential in utilizing COF asmultifunctional porous materials, lead the PSM efforts viacovalent linkage to play crucial role in enhancing theirperformance towards relevant applications.

Acknowledgements

This work was supported by National Natural ScienceFoundation of China (21571079,21621001,21390394,21571076,and21571078), “111” project (B07016 andB17020), Guangdong and Zhuhai Science and TechnologyDepartment Project (2012D0501990028), and the program forJLU Science and Technology Innovative Research Team.Q.F.acknowledge the Thousand Talents program (China).

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Received: June 9, 2018Accepted: July 10, 2018

Published online on &&&, &&&&

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REVIEW

Y. Yusran, Q. Fang, S. Qiu*

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Postsynthetic Covalent Modificationin Covalent Organic Frameworks