Research

Covalent Organic Frameworks: A Materials Platform for Structural and Functional Designs
Nat. Rev. Mat. 2016, 1, 16068.
Covalent organic frameworks (COFs) are a class of crystalline porous polymer that allows the atomically precise integration of organic units into extended structures with periodic skeletons and ordered nanopores. One important feature of COFs is that they are designable; that is, the geometry and dimensions of the building blocks can be controlled to direct the topological evolution of structural periodicity. The diversity of building blocks and covalent linkage topology schemes make COFs an emerging materials platform for structural control and functional design. Indeed, COF architectures offer confined molecular spaces for the interplay of photons, excitons, electrons, holes, ions and guest molecules, thereby exhibiting unique properties and functions. In this Review, we summarize the major progress in the field of COFs and recent achievements in developing new design principles and synthetic strategies. We highlight cutting-edge functional designs and identify fundamental issues that need to be addressed in conjunction with future research directions from chemistry, physics and materials perspectives.
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Covalent Organic Frameworks
Chem. Soc. Rev. 2012, 41, 6010-6022.
Covalent organic frameworks (COFs) are a class of crystalline porous polymers that allow the atomically precise integration of organic units to create predesigned skeletons and nanopores. They have recently emerged as a new molecular platform for designing promising organic materials for gas storage, catalysis, and optoelectronic applications. The reversibility of dynamic covalent reactions, diversity of building blocks, and geometry retention are three key factors involved in the reticular design and synthesis of COFs. This tutorial review describes the basic design concepts, the recent synthetic advancements and structural studies, and the frontiers of functional exploration.

Conjugated Microporous Polymers: Design, Synthesis and Application
Chem. Soc. Rev. 2013, 42, 8012-8031. (Front Cover)

Conjugated microporous polymers (CMPs) are a class of organic porous polymers that combine p-conjugated skeletons with permanent nanopores, in sharp contrast to other porous materials that are not π-conjugated and with conventional conjugated polymers that are nonporous. As an emerging material platform, CMPs offer a high flexibility for the molecular design of conjugated skeletons and nanopores. Various chemical reactions, building blocks and synthetic methods have been developed and a broad variety of CMPs with different structures and specific properties have been synthesized, driving the rapid growth of the field. CMPs are unique in that they allow the complementary utilization of π-conjugated skeletons and nanopores for functional exploration; they have shown great potential for challenging energy and environmental issues, as exemplified by their excellent performance in gas adsorption, heterogeneous catalysis, light emitting, light harvesting and electrical energy storage. This review describes the molecular design principles of CMPs, advancements in synthetic and structural studies and the frontiers of functional exploration and potential applications.

Angew. Chem., Int. Ed. 2008, 47, 6628-6630 (VIP & Frontispieces; Highlighted by C&EN 2008, 86 (49), 19.)
Condensation polymerization of pyrene (blue) and triphenylene (green) monomers leads to the construction of a hexagonal mesoporous COF that adopts belt shape, emits blue luminescence and is semiconductive.

This work exemplified for the first time that covalent organic framework allows energy transfer, facilitates exciton migration and emits blue luminescence. Covalent organic framework is semiconducting and capable of repetitive on-off current switching.

Angew. Chem., Int. Ed. 2009, 48, 5439-5442.
One-component pyrene-based covalent organic framework facilitates exciton migration and carrier transportation, harvests visible photons and triggers prominent photocurrent generation with quick response to light irradiation, and is capable of repetitive photocurrent switching with large on-off ratio.

Chem. Commun. 2009, 3119-3121.
A benzene-bridged metallosalphen dimer froms a noncovalent tape that is semiconducting with large carrier mobility, shows anisotropic transportation character, and forms p- and n- channel semiconductors tunable upon doping. The tape is photoconductive and allows repetitive switching with large on/off ratios.

J. Am. Chem. Soc. 2009, 131, 7287-7292.
Noncovalently netted, photoconductive sheets from triphenylene-fused metal trigon conjugates collect a wide wavelength range of photons from ultraviolet to visible regions. The noncovalent sheets allow exciton migration and are semiconducting with an extremely large intrinsic carrier mobility of 3.3 cm2 V–1 s–1. They are highly photoconductive, produce photocurrent with a quick response to light irradiation, and are capable of repetitive on–off switching. Moreover, these sheets facilitate a conduction path perpendicular to the sheet plane, thus exhibiting a spatially distinctive anisotropy in conduction.

J. Am. Chem. Soc. 2010, 132, 9138-9143.
This article describes the synthesis and functions of a porous catalytic framework based on conjugated micro- and mesoporous polymers with metalloporphyrin building blocks (FeP-CMP). FeP-CMP was newly synthesized via a Suzuki polycondensation reaction and was developed as a heterogeneous catalyst for the activation of molecular oxygen to convert sulfide to sulfoxide under ambient temperature and pressure. FeP-CMP is intriguing because the polymer skeleton itself is built from catalytic moieties and serves as built-in catalysts, bears inherent open nanometer-scale pores that are accessible for substrates, and possesses large surface areas (1270 m2 g-1) that facilitate the transformation reaction. It is highly efficient with high conversion (up to 99%) and a large turnover number (TON ) 97,320), is widely applicable to various sulfides covering from aromatic to alkyl and cyclic substrates, displays high selectivity (up to 99%) to form corresponding sulfoxides, and is highly chemoselective for the oxidation of a sulfide group even in the coexistence of other oxidative functionalities. Owing to the covalent linkages between catalytic sites in the frameworks, FeP-CMP can be recycled with good retention of its porous structure and allows for large-scale transformation. These unique characteristics clearly originate from the covalent porous catalytic framework structure and demonstrate the usefulness of CMPs in the exploration of built-in heterogeneous catalysts, a new potential of these materials that have thus far been reported to exhibit noteworthy gas adsorption functions.

Angew. Chem., Int. Ed. 2011, 50, 1289-1293.
A protocol for the synthesis of 2D covalent organic frameworks of metallophthalocyanine is described. Owing to well-ordered stacking of the phthalocyanine units, the resulting 2D framework provides enhanced and broad light absorbance and facilitated charge transport. The material becomes highly photoconductive to show panchromatic response to light irradiation and is exceptionally sensitive to deep-red visible and near-infrared photons.

Chem. Commun. 2011, 47, 1979-1981.
A two-dimensional porphyrin covalent organic framework is solvothermally synthesized and the macroscopic structure and pore parameters can be synthetically controlled. A remarkable and positive COF size effect on the pore parameters enables such structural control with a synchronized feature.

J. Am. Chem. Soc. 2010, 132, 6742-6748 (Cover Page).
This article describes the synthesis and functions of a polyphenylene-based conjugated microporous polymer (PP-CMP). PP-CMP was recently designed and synthesized by Suzuki polycondensation reaction and used as an antenna for the noncovalent construction of a light-harvesting system. In contrast to linear polyphenylene, PP-CMP consists of conjugated three-dimensional polyphenylene scaffolds and holds inherent porous structure with uniform pore size (1.56 nm) and large surface area. It emits blue photoluminescence, is capable of excitation energy migration over the framework, and enables rapid transportation of charge carrier with intrinsic mobility as high as 0.04 cm2 V-1 s-1. The microporous structure of PP-CMP allows for the spatial confinement of energy-accepting coumarin 6 molecules in the pores and makes the high-throughput synthesis of light-harvesting systems with designable donor-acceptor compositions possible. Excitation of the PP-CMP skeleton leads to brilliant green emission from coumarin 6, with an intensity 21-fold as high as that upon direct excitation of coumarin 6 itself, while the fluorescence from PP-CMP itself is wholly quenched as a result of energy transfer from the light-harvesting PP-CMP framework to coumarin 6. The PPCMP skeleton is highly cooperative, with an average of 176 phenylene units working together to channel the excitation energy to one coumarin 6 molecule, and features the energy-transfer process with quick, efficient, and vectorial character. These unique characteristics clearly originate from the conjugated porous structure and demonstrate the usefulness of CMPs in the exploration of π-electronic functions, in addition to their gas adsorption properties thus far reported

Adv. Mater. 2011, 23, 3149-3154.
We have developed a nanoporous metalloporphyrin framework for the activation of molecular oxygen and demonstrated its utility in highly effi cient aerobic epoxidation of olefi ns under ambient conditions. FeP-CMP achieves high activity, excellent selectivity, broad substrate applicability, good reusability, and enzyme-like large TON and TOF values. The porous framework outperforms the monomeric metalloporphyrin and nonporous metalloporphyrin polymer analogues. Our strategy also highlights a new approach for the design of heterogeneous catalysts with built-in catalytic skeletons and inherent nanopores; these structural features together with the excellent catalytic performance were not accomplished by other heterogeneous and homogeneous catalysts reported so far.

Angew. Chem., Int. Ed. 2011, 50, 8753-8757 (VIP; HIghlighted by Nature Nanotechnology)
A conjugated microporous polymer with aza-fused framework is described. Owing to aza-fused structure features, the resulting porous framework is conductive and allows electrolyte ions to move into the pores. The materials becomes highly cooperative in the formation of electrostatic charge-separation layers to show exceptional capacitance in supercapacitive energy storage, provide high energy density, and offer excellent cycling life.

J. Am. Chem. Soc. 2011, 133, 17622-17625. (Highlighted by ACS Noteworthy Chemistry)
We report a strategy for the design of highly luminescent conjugated polymers by restricting rotation of the polymer building blocks through a microporous network architecture. We demonstrate this concept using tetraphenylethene (TPE) as a building block to construct a light-emitting conjugated microporous polymer. The interlocked network successfully restricted the rotation of the phenyl units, which are the major cause of fluorescence deactivation in TPE, thus providing intrinsic luminescence activity for the polymers. We show positive “CMP effects” that the network promotes π-conjugation, facilitates exciton migration, and improves luminescence activity. Although the monomer and linear polymer analogue in solvents are nonemissive, the network polymers are highly luminescent in various solvents and the solid state. Because emission losses due to rotation are ubiquitous among small chromophores, this strategy can be generalized for the de novo design of light-emitting materials by integrating the chromophores into an interlocked network architecture.

J. Am. Chem. Soc. 2011, 133, 14510-14512. (Highlighted by C&EN, September 12, 2011)
Co-condensation of metallophthalocyanine with an electron deficient benzothiadiazole (BTDA) block leads to the formation of a two-dimensional covalent organic framework (2D-NiPc-BTDA COF) that assumes belt shape and consists of AA stacking of 2D polymer sheets. The integration of BTDA blocks to the edges of tetragonal metallophthalocyanine COF causes drastic change in the carrier-transporting mode and switches from the hole-transporting skeleton to electron-transporting framework. 2D-NiPc-BTDA COF exhibits broad and enhanced absorbance up to 1000 nm, shows panchromatic photoconductivity, is highly sensitive to near infrared photons, and has excellent electron mobility as high as 0.6 cm2 V–1 s–1.

Nature Communications 2011, 2:536, doi: 10.1038/ncomms1542.
We introduce pore surface engineering to COF chemistry, which allows the controlled functionalization of COF pore walls with organic groups. This functionalization is made possible by the use of azide-appended building blocks for the synthesis of COFs with walls to which a designable content of azide units is anchored. The azide units can then undergo a quantitative click reaction with alkynes to produce pore surfaces with desired groups and preferred densities. The diversity of click reactions performed shows that the protocol is compatible with the development of various specific surfaces in COFs. Therefore, this methodology constitutes a step in the pore surface engineering of COFs to realize pre-designed compositions, components and functions.

Angew. Chem., Int. Ed. 2012, 51, 2618-2622.
A two-dimensional porphyrin covalent organic framework is described. Owing to the eclipsed stacking alignment, the resulting framework is conductive and allows high-rate carrier transport through the porphyrin columns. The central metal in porphyrin rings drastically changes the conducting nature of the materials, switches the framework from hole to electron, and to ambipolar conductions, and drives the high on-off ratio photoconductivity of the frameworks.

I. Covalent Organic Frameworks
Focusing on the discovery of new reactions, new structures, and new functions of Covalent organic frameworks

III. Conjugated Microporous Polymers
Focusing on the discovery of new reactions, new structures, and new functions of conjugated porous polymers

IV. Noncovalent Multinuclear Metallo-conjugates
Focusing on the discovery of novel assembly and
functions of multinuclear metallo-conjugates

Adv. Mater. 2012, 23, 3026-3031.
A donor-acceptor built-in covalent organic framework with segregated, periodic, and bicontinuous electron donor-acceptor ordering is reported. The framework consists of pre-organized periodic independent pathways for ambipolar electron and hole conduction and constitutes vertically ordered p-n heterojunctions with a broad D-A interface for enhanced photoconductivity.

J. Am. Chem. Soc. 2012, 134, 8738-8742.
Through the construction of a conjugated microporous network, we developed a highly luminescent CMP that can be used to sense chemicals. TCB-CMP is unique in that it discriminates between electron-rich and electron-deficient arenes and provides opposite output with fluorescence-on and fluorescence-off characteristics. Remarkably, the structural features of CMPs work cooperatively in the process of sensing. Specifically, the conjugated network facilitates exciton migration over the skeletons, the large surface area of the skeleton broadens the interface between the CMP and arenes, and the micropores confine arene molecules. These characteristics are inherent and endow CMPs with rapid response times and high sensitivity. The CMP architecture provides a unique platform for the design of de novo chemosensing systems, which can be difficult to achieve with conventional conjugated polymers. Considering the structure versatility and design flexibility of the conjugated skeletons and nanopores of CMPs, we anticipate that a fertile and exciting field will emerge focused on the development of highly sensitive sensors and sensor arrays based on luminescent CMPs.

Chem. Commun. 2012, 48, 8952-8954.
Phthalocyanine covalent organic frameworks with different central metals are synthesized and the AA-stacking structure of the 2D polymer sheets results in periodic phthalocyanine π-columns. The central metals control the π-electronic functions including the improvement of light absorbance, the ease of carrier transport, and the gain of photocurrent.

Chem. Commun. 2013, 49, 1591-1593 (Inside Cover).
A core–shell strategy is demonstrated for designing conjugated microporous polymer that allows the tune of light emission over a wide-wavelength range in a controlled manner. The polymers not only emit efficiently with an eight-fold enhanced luminescence but also sustain light emissions, irrespective of solvent and state.

J. Am. Chem. Soc. 2013, 135, 546-549.
Fluoro-substituted and nonsubstituted aromatic units at different molar ratios were integrated into the edge units that stack to trigger self-complementary π-electronic interactions in the COFs. The interactions improve the crystallinity and enhance the porosity by maximizing the total crystal stacking energy and minimizing the unit cell size. Consequently, the COF consisting of equimolar amounts of fluoro-substituted and nonsubstituted units showed the largest effect. These results suggest a new approach to the design of COFs by managing the interlayer interactions.

Angew. Chem., Int. Ed. 2013, 52, 2017-2021 (Inside Cover).
The mechanistic insights into the photochemical events and charge dynamics of a donor–acceptor covalent organic framework were revealed by time-resolved transient absorption spectroscopy in conjunction with time-resolved electron-spin resonance spectroscopy. The organic framework triggers ultrafast electron transfer and enables long-distance charge delocalization and exceptional long-term charge separation in both solvated and solid states as a result of its periodically ordered bicontinuous heterojunction structure.

CrystEngComm. 2013, 15, 1508-1511. (Themed Issue on COFs)
A C3-symmetric molecule based on phenanthrene cyclotrimer was developed as a new building block to condense with three C2-symmetric units and create a class of star-shaped 2D covalent organic frameworks with an extended hexagonal topology, large surface area, and tunable pore size.

Angew. Chem., Int. Ed. 2013, 52, 3770-3774 (Hot Paper).
A squaraine-linked, conjugated two-dimensional porphyrin covalent organic framework (COF; see scheme; C: gray, H: white, N: blue, Cu: red) was synthesized. Owing to the pconjugated linkage together with the eclipsed stacking of the units, this COF exhibits enhanced chemical and thermal stabilities. It absorbs a broad range of light, from the ultraviolet to the visible, and near-infrared regions, and shows potential as a photocatalyst.

Chem. Commun. 2013, 49, 3233-3235.
Conjugated microporous polymers exhibit a synergistic structural effect on the exceptional uptake of amines, whereas the dense porphyrin units facilitate uptake, the high porosity offers a large interface and the swellability boosts capacity. They are efficient in the uptake of both vapor and liquid amines, are applicable to various types of amines, and are excellent for cycle use.

Chem. Sci. 2013, 4, 4505-4511.
We report the synthesis and structural characterization of large pore covalent organic frameworks (COFs) integrated with donor and acceptor building blocks. The donor and acceptor based on triphenylene and diimide are topologically linked to form COFs with stacked donor and acceptor columns and 5.3-nm width channels, which show high crystallinity and large surface area. By varying donor–acceptor structure in conjunction with time-resolved spectroscopy, these large-pore COFs constitute benchmark frameworks to elucidate not only the importance of donor–acceptor paring but also the role of lattice structure in charge transfer and separation, thereby casting a general principle for the structural design of optoelectronic and photovoltaic COFs.

J. Am. Chem. Soc. 2013, 135, 17310-17313.
Condensation of hydrazine with 1,3,6,8-tetrakis(4-formylphenyl)pyrene under solvothermal conditions yields highly crystalline two-dimensional covalent organic frameworks. The pyrene units occupy the vertices and the diazabutadiene (-C=N-N=C-) linkers locate the edges of rohmbic-shaped polygon sheets, which further stack in an AA-stacking mode to constitute periodically ordered pyrene columns and one-dimensional microporous channels. The azine-linked frameworks feature permanent porosity with high surface area and exhibit outstanding chemical stability. By virtue of the pyrene columnar ordering, the azine-linked frameworks are highly luminescence, whereas the azine units serve as open docking sites for hydrogen-bonding interactions. These synergestic functions of the vertices and edges units endow the azine-linked pyrene frameworks with extremely high sensitivity and selectivity in chemosensing, for example, the selective detection of 2,4,6-trinitrophenol explosive. We anticipate that the extension of the present azine-linked strategy would not only increase the structural diversity but also expand the scope of functions based on this highly stable class of covalent organic frameworks.

II. Two-Dimensional Conjugated Macromolecules
Focusing on the discovery of new reactions, new structures,
and new functions of 2D conjugated polymers

Nature Communications, 2013, 4:2736.
DOI: 10.1038/ncomms3736 (2013)
We report a chemically stable, electronically conjugated organic framework with topologically designed wire frameworks and open nanochannels, whereas the π conjugation spans to the two-dimensional sheets. Our framework permits inborn periodic ordering of conjugated chains in all three dimensions and exhibits a striking combination of remarkable properties: high chemical stability, extended π-delocalization, ability to host guest molecules, and superb hole mobility. We show that the π-conjugated organic framework is useful for high on-off ratio photoswitches and photovoltaic cells. Therefore, this strategy constitutes a step to realize ordered semiconducting porous materials for innovations based on two-dimensionally extended π-systems.

Chem. Commun. 2014, 50, 1292-1294 (Back Cover)
We report a synthetic strategy for the pore surface engineering of imine-linked covalent organic frameworks to predesign pore functions by using click chemistry. This method enables the precise tune of pore wall surfaces with desired functional groups and controlled densities. The integration of organocatalytic sites onto the pore walls creates robust organocatalytic frameworks with significantly enhanced activity and retained stereoselectivity in aqueous solution. The COF catalyst combines a number of striking features, including broad applicability, good recyclability, and high capability to perform quantitative transformation under continuous columnar flow. These results open a way towards precisely organized and synthetically controlled nanoreactors – a highly desired and long-pursed structure for heterogeneous catalytic systems.

Chem. Commun. 2014, 50, 2781-2783.
We report structural insights into the function origin of conjugated microporous polymers (CMPs), by elucidating the vital role of linkage geometry in controlling the porosity, gas adsorption, conjugation, exciton transport and luminescence. This unprecedented yet crucial role of geometry constitutes a general principle for the rational design of CMPs.

Chem. Commun. 2014, 50, 4788-4790.
Conjugated microporous polymers are developed as a new platform for lithium-battery energy storage, which features near-unity coulombic efficiency, high capacity and cycle stability. The polymers exhibit synergistic structural effects on facilitating charge dynamics by virtue of their built-in redox skeletons, open nanopores and large surface areas.

Chem. Commun. 2014, 50, 6161-6163.
A strategy for the synthesis of covalent organic frameworks with open docking sites is developed. The docking sites are ordered on the channel walls and structurally predesignable for meeting various types of noncovalent interactions, thus opening a way towards designing supramolecular materials based on crystalline porous organic frameworks.

Chem. Eur. J. 2014, 20, 14608-14612.
Here we report the construction of a new class of covalent TTF lattice by integrating TTF units into two-dimensional covalent organic frameworks (2D COFs). We explored a general strategy based on the C2 + C2 topological diagram and applied to the synthesis of microporous and mesoporous TTF COFs. Structural resolutions reveal that both COFs consist of layered lattices with periodic TTF columns and tetragonal open nanochannels. The TTF columns offer predesigned pathways for high-rate hole transport, predominate the HOMO and LUMO levels of the COFs, and are redox active to form organic salts that exhibit enhanced electric conductivity by several orders of magnitude. On the other hand, the linkers between the TTF units play a vital role in determining the carrier mobility and conductivity via the perturbation of 2D sheet conformation and interlayer distance. These results open a way towards designing a novel type of TTF materials with stable and predesignable lattice structures for functional exploration.

Angew. Chem. Int. Ed. 2014, 53, 4850-4855.
We developed a CMP thin film with controllable thickness on substrates or as freestanding films. The CMP films combine a number of striking physical properties, including high porosity, extended π conjugation, facilitated exciton delocalization, and high-rate electron transfer. We explored the CMP films as versatile platforms for highly sensitive and label-free chemo- and bio-sensings of electron-rich and -poor arenes, metal ions, dopamine, and hypochloroic acid, featuring rapid response, excellent selectivity, and robust reusability.

J. Am. Chem. Soc. 2014, 136, 9806-9809.
Ordered one-dimensional open channels represent the typical porous structure of two-dimensional covalent organic frameworks (COFs). Here we report a general synthetic strategy for converting these open lattice structure structures into ordered donor-acceptor heterojunctions. A three-component topological design scheme was explored to prepare electron-donating intermediate COFs, which upon click reaction were transformed to photoelectric COFs with segregated donor-acceptor alignments, whereas electronaccepting buckyballs were spatially confined within the nanochannels via covalent anchoring on the channel walls. The donor-acceptor heterojunctions trigger photoinduced electron transfer and allow charge separation with radical species delocalized in the π-arrays, whereas the charge separation efficiency was dependent on the buckyball content. This new donor-acceptor strategy explores both skeletons and pores of COFs for charge separation and photoenergy conversion.

Scientific Reports 2014, 4, 7278.
Protection of metal nanoparticles from agglomeration is critical forto their functions and applications. The cConventional method for enhancing their stability is to cover them nanoparticles with passivation layers to prevent their direct contact. However, the presence of a protectiveon shell blocks the exposure of the metal species to reactants, thereby significantly impedinginterfering with the nanoparticles’ir utilityapplication as catalysts. Here, we report that metal nanoparticles can be prepared and used in a surface-exposed state that renders them inherently catalytically activeity. This strategy is realisedmade possible by spatial confinement and electronic stabilizisation with a dual-module mesoporous- and microporous three-dimensional ππ-network, in which surface-exposed nanoparticles are crystallizised upon in -situ reduction. The uncovered palladium nanoparticles of palladium serve as heterogeneous catalysts that are exceptionalsuperlatively active in water, catalysze unreactive aryl chlorides for straightforward carbon–carbon bond formation and are stable for repeated usecycle in various types of cross couplings. Therefore, our results open new perspectives in developing practical heterogeneous catalysiscatalysts.

Scientific Reports 2015, 5, 8225.
Organic batteries free of toxic metal species could lead to a new generation of consumer energy-storage devices that are safe and environmentally benign. However, the conventional organic electrodes remain problematic because of their structural instability, slow ion-diffusion dynamics, and poor electrical conductivity. Here, we report on the development of a redox-active, crystalline, mesoporous covalent organic framework (COF) on carbon nanotubes for use as electrodes; the electrode stability is enhanced by the covalent network, the ion transport is facilitated by the open meso-channels, and the electron conductivity is boosted by the carbon nanotube wires. These effects work synergistically for the storage of energy and provide lithium-ion batteries with high efficiency, robust cycle stability, and high rate capability. Our results suggest that redox-active COFs on conducting carbons could serve as a unique platform for energy storage and may facilitate the design of new organic electrodes for high-performance and environmentally benign battery devices.

Angew. Chem. Int. Ed 2015, 54, 2986-2990.
Ordered open channels found in two-dimensional covalent organic frameworks (2D COFs) could render them able to adsorb carbon dioxide. However, the frameworks’ dense layer architecture results in low porosity that has thus far restricted their potential for carbon dioxide adsorption. Here we report a strategy for converting a conventional 2D COF into an outstanding platform for carbon dioxide capture via channel-wall functionalization. The dense layer structure enables the dense integration of functional groups on the channel walls, creating a new version of COFs with high capacity, reusability, selectivity, and separation productivity for flue gas. These results suggest that channel-wall functional engineering could be a facile and powerful strategy to develop 2D COFs for high-performance gas storage and separation.

J. Am. Chem. Soc. 2015, 137, 3241-3247.
A series of two-dimensional covalent organic frameworks (2D COFs) locked with intralayer hydrogen-bonding (H-bonding) interactions were synthesized. The H-bonding interaction sites were located on the edge units of the imine-linked tetragonal porphyrin COFs, and the contents of the H-bonding sites in the COFs were synthetically tuned using a three-component condensation system. The intralayer H-bonding interactions suppress the torsion of the edge units and lock the tetragonal sheets in a planar conformation. This planarization enhances the interlayer interactions and triggers extended π-cloud delocalization over the 2D sheets. Upon AA stacking, the resulting COFs with layered 2D sheets amplify these effects and strongly affect the physical properties of the material, including improving their crystallinity, enhancing their porosity, increasing their light-harvesting capability, reducing their band gap, and enhancing their photocatalytic activity toward the generation of singlet oxygen. These remarkable effects on the structure and properties of the material were observed for both freebase and metalloporphyin COFs. These results imply that exploration of supramolecular ensembles would open a new approach to the structural and functional design of COFs.

Scientific Reports 2015, 5, 8867.
Light-harvesting antennae are the machinery for exciton pumping in natural photosynthesis, whereas cascade energy transfer through chlorophyll is key to long-distance, efficient energy transduction. Numerous artificial antennae have been developed. However, they are limited in their cascade energy-transfer abilities because of a lack of control over complex chromophore aggregation processes, which has impeded their advancement. Here we report a viable approach for addressing this issue by using a light-harvesting porous polymer film in which a three-dimensional π-network serves as the antenna and micropores segregate multiple dyes to prevent aggregation. Cascade energy-transfer engines are integrated into the films; the rate and efficiency of the energy-funneling engines are precisely manipulated by tailoring the dye components and contents. The nanofilms allow accurate and versatile luminescence engineering, resulting in the production of thirty emission hues, including blue, green, red and white. This advance may open new pathways for realising photosynthesis and photoenergy conversion.

Angew. Chem., Int. Ed. 2015, 54, 6814-6818.
Ordered π-columns and open nanochannels found in covalent organic frameworks (COFs) could render them able to store electric energy. However, the synthetic difficulty in achieving redox-active skeletons has thus far restricted their potential for energy storage. Here we report a general strategy for converting a conventional COF into an outstanding platform for energy storage through post-synthetic functionalization with organic radicals. The radical frameworks with open-accessible polyradicals immobilized on the pore walls undergo rapid and reversible redox reactions, leading to capacitive energy storage with high capacitance, high-rate kinetics, and robust cycle stability. The results suggest that channel-wall functional engineering with redox-active species will be a facile and versatile strategy to explore COFs for energy storage.

Chem. Commun. 2015, 51, 10096-10098. (Front Cover)
We report a strategy for developing π-electronic covalent organic frameworks as heterogeneous catalysts that enable the use of columnar π-walls as catalytic beds to facilitate organic transformations in their one-dimensional open channels. The π-frameworks exhibit outstanding catalytic activity, promote Diels-Alder reactions under ambient conditions and are robust for cycle use.

Angew. Chem., Int. Ed. 2015, 54, 8704-8707. (VIP)
Ordered π-columnar structures found in covalent organic frameworks (COFs) render them attractive as smart materials. However, external-stimuli-responsive COFs have not been explored. Here we report the design and synthesis of a photoresponsive COF with anthracene units as the photoresponsive π-building blocks. The COF is switchable upon photoirradiation to yield a concavo-convex polygon skeleton via the interlayer [4π + 4π] cycloaddition of anthracene units stacked in the π-columns. This cycloaddition reaction is thermally reversible; heating resets the anthracene layers and regenerates the COF. These external-stimuli-induced structural transformations are accompanied by profound changes in properties, including gas adsorption, π-electronic function, and luminescence. The results suggest that COFs are useful for designing smart porous materials with properties that are controllable by external stimuli.

$J. Am. Chem. Soc. 2015, 137, 7817-7827. By developing metallophthalocyanines and diimides as electron-donating and accepting building blocks, herein, we report the construction of new electron donor-acceptor covalent organic frameworks (COFs) with periodically ordered electron donor and acceptor π-columnar arrays via direct polycondensation reactions. X-ray diffraction measurements in conjunction with structural simulations resolved that the resulting frameworks consist of metallophthalocyanine and diimide columns, which are ordered in a segregated yet bicontinuous manner to form built-in periodic π-arrays. In the frameworks, each metallophthalocyanine donor and diimide acceptor units are exactly linked and interfaced, leading to the generation of superheterojunctions – a new type of heterojunction machinery, for photoinduced electron transfer and charge separation. We show that this polycondensation method is widely applicable to various metallophthalocyanines and diimides as demonstrated by the combination of copper, nickel, and zinc phthalocyanine donors with pyrommellitic diimide, naphthalene diimide, and perylene diimide acceptors. By using time-resolved transient absorption spectroscopy and electron spin resonance, we demonstrated that the COFs enable long-lived charge separation, whereas the metal species, the class of acceptors, and the local geometry between donor and acceptor units play roles in determining the photochemical dynamics. The results provide insights into photoelectric COFs and demonstrate their enormous potential for charge separation and photoenergy conversions.$

J. Am. Chem. Soc. 2015, 137, 7079-7082.
Imine-linked covalent organic frameworks (COFs) were synthesized to bear content-tunable, accessible, and reactive ethynyl groups on the walls of one-dimensional pores. These COFs offer an ideal platform for pore-wall surface engineering aimed at anchoring diverse functional groups ranging from hydrophobic to hydrophilic units and from basic to acidic moieties with controllable loading contents. This approach enables the development of various tailor-made COFs with systematically tuned porosities and functionalities while retaining the crystallinity. We demonstrate that this strategy can be used to efficiently screen for suitable pore structures for use as CO2 adsorbents. The pore-surface-engineered walls exhibit an enhanced affinity for CO2, resulting in COFs that can capture and separate CO2 with high performance.

Nature Communications 2015, 6:7786.
Covalent organic frameworks (COFs) are an emerging　class of highly ordered porous polymers with many potential applications. They are currently designed and synthesised through hexagonal and tetragonal topologies, limiting the access to and exploration of new structures and properties. Here, we report that a triangular topology can be developed for the rational design and synthesis of a new class of COFs. The triangular topology features small pore sizes down to 12 Å, which is among the smallest pores for COFs reported to date, and high π-column densities of up to 0.25 nm–2, which exceeds those of supramolecular columnar π-arrays and other COF materials. These crystalline COFs facilitate π-cloud delocalisation and are highly conductive, with a hole mobility that is among the highest reported for COFs and polygraphitic ensembles.

Angew. Chem. Int. Ed. 2015, 54, 11540.
Porous organic polymers allow the integration of various π-units into robust porous π-networks, but they are usually synthesized as unprocessable solids with poor light-emitting performance as a result of aggregation-related excitation dissipation. Herein, we report a general strategy for the synthesis of highly emissive photofunctional porous polymer films on the basis of a complementary scheme for the structural design of aggregation-induced-emissive π-systems. We developed a high-throughput and facile method for the direct synthesis of large-area porous thin films at the liquid–electrode interface. The approach enables the preparation of microporous films within only a few seconds or minutes and allows precise control over their thickness with sub-nanometer precision. By virtue of rapid photoinduced electron transfer, the thin films can detect explosives with enhanced sensitivity to low parts-per-million levels in a selective manner.

Nature Chemistry 2015, 7, 905-912.
The periodic layers and ordered nanochannels of covalent organic frameworks (COFs) make these materials viable open catalytic nanoreactors, but their low stability has precluded their practical implementation. Here we report the synthesis of a crystalline, porous COF that is stable against water and strong acid/base, and we demonstrate its utility as a material platform for structural design and functional development. A crystalline, porous, imine-based COF was endowed with stability by incorporating methoxy groups to its pore walls to reinforce interlayer interactions. The resulting achiral material was subsequently converted into a series of chiral organocatalysts - while retaining its high crystallinity and porosity - by appending chiral centres and catalytically active sites on its channel walls. The COFs thus prepared combine catalytic activity, enantioselectivity and recyclability, which are attractive in heterogeneous organocatalysis, and were shown to promote the asymmetric C-C bond formation in water under ambient conditions.

Angew. Chem. Int. Ed. 2015, 54, 13594-13598.
Conjugated microporous polymers are a unique class of polymers that combine extended π-conjugation with inherent porosity. However, these polymers are synthesized through solution-phase reactions to yield insoluble and unprocessable solids, which preclude not only the evaluation of their conducting properties but also the fabrication of thin films for device implementation. Here, we report a strategy for the synthesis of thin films of π-conjugated microporous polymers by designing thiophene-based electropolymerization at the solution--electrode interface. High-quality films are prepared on a large area of various electrodes, the film thickness is controllable, and the films are used for device fabrication. These films are outstanding hole conductors and, upon incorporation of fullerenes into the pores, function as highly efficient photoactive layers for energy conversions. Our film strategy may boost the applications in photocatalysis, energy storage, and optoelectronics.

Sci. Rep. 2015, 5, 14650.
Covalent organic frameworks (COFs) are an emerging class of crystalline porous polymers in which organic building blocks are covalently and topologically linked to form extended crystalline polygon structures, constituting a new platform for designing π-electronic porous materials. However, COFs are currently synthesised by a few chemical reactions, limiting the access to and exploration of new structures and properties. The development of new reaction systems that avoid such limitations to expand structural diversity is highly desired. Here we report that COFs can be synthesised via a double-stage connection that polymerises various different building blocks into crystalline polygon architectures, leading to the development of a new type of COFs with enhanced structural complexity and diversity. We show that the double-stage approach not only controls the sequence of building blocks but also allows fine engineering of pore size and shape. This strategy is widely applicable to different polymerisation systems to yield hexagonal, tetragonal and rhombus COFs with predesigned pores and π-arrays.

$Chem. Commun. 2016, 52, 1498-1500. We demonstrate profound effects of spatially confined guest molecules in one-dimensional nanochannels on X-ray diffraction behaviors of covalent organic frameworks. Our results give insights into the abnormal X-ray diffraction patterns and suggests a novel molecular dynamic strategy for resolving crystalline structures.$

Angew. Chem. Int. Ed. 2016, 55, 3049-3053.
Organic optoelectronics offer promising technologies for energy conversions. However, electrode interlayer – a key material between active layers and conducting electrodes, which controls the transport of charge carriers in and out of devices, is still a chemical challenge. Here, we report a new class of porous organic polymers with tunable work function as hole and electron-selective electrode interlayers. The network with organoborane and carbazole units exhibits extremely low work function selective electron flow; while upon ionic ligation and electro-oxidation, the network significantly increases the work function and turn into hole conduction. We demonstrate their outstanding functions as anode and cathode interlayers in energy-converting solar cells and light-emitting diodes.

Nature Materials 2016, 15, 722-727.
Progress over the past decades in proton-conductingmaterials has generated a variety of polyelectrolytes1–5 and microporous polymers. However, most studies are still based on a preconception that large pores eventually cause neat flow of proton carrier themselves other than ecient conduction of proton ions, which precludes the exploration of large-pore polymers for proton transport. Here, we demonstrate proton conduction across mesoporous channels in a crystalline covalent organic framework. The frameworks are designed to constitute hexagonally aligned, dense, mesoporous channels that allow for loading of N-heterocyclic proton carriers. The frameworks achieve proton conductivities that are 2–4 orders of magnitude higher than those ofmicroporous and non-porous polymers. Temperature-dependent and isotopic experiments revealed that the proton transport in these channels is controlled by a low-energy-barrier hopping mechanism. Our results reveal a novel platform based on porous covalent organic frameworks for proton conduction.

J. Am. Chem. Soc. 2016, 138, 5797-5780.
Highly luminescent covalent organic frameworks (COFs) are rarely achieved because of the aggregation-caused quenching (ACQ) of π-π stacked layers. Here, we report a general strategy to design highly emissive COFs by introducing an aggregation-induced emission (AIE) mechanism. The integration of AIE-active units into the polygon vertices yields crystalline porous COFs with periodic π-stacked columnar AIE arrays. These columnar AIE π-arrays dominate the luminescence of the COFs, achieve exceptional quantum yield via a synergistic structural locking effect of intralayer covalent bonding and interlayer noncovalent π-π interactions, and serve as a highly sensitive sensor to report ammonia down to sub ppm level. Our strategy breaks through the ACQ-based mechanistic limitations of COFs and opens a way to explore highly emissive COF materials.

Nat. Rev. Mater. 2016, 1, 16068.
Covalent organic frameworks are crystalline porous polymers with precisely ordered polygon architectures; this Review summarizes recent advances in the design principles and synthetic reactions, highlights the current status in structural construction and functionality design, and predicts future directions.

Nat. Commun. 2016, 1, 16068.
Covalent organic frameworks are a class of crystalline porous polymers that integrate molecular building blocks into periodic structures and are usually synthesized using two-component [1+1] condensation systems comprised of one knot and one linker. Here we report a general strategy based on multiple-component [1+2] and [1+3] condensation systems that enable the use of one knot and two or three linker units for the synthesis of hexagonal and tetragonal multiple-component covalent organic frameworks. Unlike two-component systems, multiple-component covalent organic frameworks feature asymmetric tiling of organic units into anisotropic skeletons and unusually shaped pores. This strategy not only expands the structural complexity of skeletons and pores but also greatly enhances their structural diversity. This synthetic platform is also widely applicable to multiple-component electron donor–acceptor systems, which lead to electronic properties that are not simply linear summations of those of the conventional [1+1] counterparts.

J. Am. Chem. Soc. 2017, 139, 2428-2434.
The predesignable porous structures found in covalent organic frameworks (COFs) render them attractive as a molecular platform for addressing environmental issues such as removal of toxic heavy metal ions from water. However, a rational structural design of COFs in this aspect has not been explored. Here we report the rational design of stable COFs for Hg(II) removal through elaborate structural design and control over skeletons, pore size, and pore walls. The resulting framework is stable under strong acid and base conditions, possesses high surface area, has large mesopores, and contains dense sulfide functional termini on the pore walls. These structural features work together in removing Hg(II) from water and achieve a benchmark system that combines capacity, efficiency, effectivity, applicability, selectivity, and reusability. These results suggest that COFs offer a powerful platform for tailor-made structural design to cope with various types of pollutions.

Small 2016, 12, 6513-6527.
Enormous research efforts are focusing on the design and synthesis of advanced luminescent systems, owing to their diverse capability in scientific studies and technological developments. In particular, fluorescence systems based on aggregation-induced emission (AIE) have emerged great potential for sensing, bio-imaging and optoelectronic applications. Among them, integrating AIE mechanism to design porous polymers is unique because it enables the combination of porosity and luminescence activity in one molecular skeleton for functional design. In recent years rapid progress on exploring AIE-based porous polymers has developed a new class of luminescent materials that exhibit broad structural diversity, outstanding property and functions and promising applications. By classifying structural nature of the skeleton, herein we elucidate the design principle, synthetic development and structural features of different porous luminescent materials, including crystalline covalent organic frameworks (COFs), metal-organic frameworks (MOFs) and amorphous porous organic polymers (POPs). We highlight the functional exploration of these luminescent porous polymers by emphasizing electronic interplay within the confined nanospace, disclose fundamental issues to be addressed and propose future directions from chemistry, physics and materials science perspectives.

Angew. Chem. Int. Ed. 2017, 56, 4982-4986.
Covalent organic frameworks (COFs) have emerged as a tailor-made platform for designing layered two-dimensional polymers. However, most of them are obtained as neutral porous materials. Here, we report the construction of ionic crystalline porous COFs with positively charged walls that enable the creation of well aligned yet spatially confined ionic interface. The unconventional reversed AA-stacking mode alternately orientates the cationic centers to both sides of the walls; the ionic interface endows COFs with unusual electrostatic functions. Because all of the walls are decorated with electric dipoles, the uptake of CO2 is enhanced by three fold compared to the neutral analogue. By virtue of sufficient open space between cations, the ionic interface exhibits exceptional accessibility, efficiency and selectivity in ion exchange to trap anionic pollutants. These findings suggest that construction of ionic interface of COFs offer a new way to the structural and functional designs.

Chem. Commun. 2017, 53, 4242-4245.
Covalent organic frameworks are designed to have backbones with different yet discrete contents of triarylamine units that inetract weakly with CO2. Adsorption experiments indicate that the triarylamine units dominate the CO2 adsorption process and the CO2 uptake increases monotonically with the triarylamine content. These profound collective effects reveal a principle for designing backbones targeting for CO2 capture and separation.

Science 2017, 357, 673-676.
We synthesized a two-dimensional (2D) crystalline covalent organic framework (sp2c-COF) that was designed to be fully π-conjugated and constructed from all sp2- carbons by C=C condensation reactions of tetrakis(4-formylphenyl)pyrene and 1,4-phenylenediacetonitrile. The C=C linkages topologically connect pyrene knots at regular intervals into a 2D lattice with π- conjugations extended along both x and y directions, and develop an eclipsed layer framework rather than the more conventionally obtained disordered structures. The sp2c-COF is a semiconductor with a discrete band gap of 1.9 electron volts and can be chemically oxidized to enhance conductivity by 12 orders of magnitude. The generated radicals are confined on the pyrene knots, enabling the formation of a paramagnetic carbon structure with high spin density. The sp2- carbon framework induces ferromagnetic phase transition to develop spin-spin coherence and align spins unidirectionally across the material.

Chem. Commun 2017, 53, 11334-11337.
A highly redox-active building block, bicarbazole, is developed as a monomer for designing crystalline porous covalent organic frameworks and is successfully integrated to the vertices of microporous tetragonal frameworks, leading to densely aligned redox-active arrays. The frameworks with large porosity and high accessiblity of the redox-active sites exhibit synergistic structural effects and achieve ultrahigh-performance energy storage.

Chem. Commun 2017, 53, 11690-11693.
Here we describe a general strategy based on template pyrolysis for converting conventional covalent organic frameworks into high-performance carbons, which combine conductivity, microporosity and heteroatom density, thus casting a distinct contrast to these obtained upon direct pyrolysis. The carbons serve as electrode and exhibit exceptional performance in energy storage.