A Photoconductive Covalent Organic Framework: Self-Condensed Arene Cubes Composed of Eclipsed 2D Polypyrene Sheets for Photocurrent Generation

Covalent organic frameworks (COFs): perspectives of industrialization

In this highlight, we review the state-of-the-art development of COFs from an industrial point of view in five aspects, including their types, growth mechanisms, synthetic methods, processability and applications.

Covalent Organic Frameworks: From Materials Design to Biomedical Application

Covalent organic frameworks (COFs) are newly emerged crystalline porous polymers with well-defined skeletons and nanopores mainly consisted of light-weight elements (H, B, C, N and O) linked by dynamic covalent bonds. Compared with conventional materials, COFs possess some unique and attractive features, such as large surface area, pre-designable pore geometry, excellent crystallinity, inherent adaptability and high flexibility in structural and functional design, thus exhibiting great potential for various applications. Especially, their large surface area and tunable porosity and π conjugation with unique photoelectric properties will enable COFs to serve as a promising platform for drug delivery, bioimaging, biosensing and theranostic applications. In this review, we trace the evolution of COFs in terms of linkages and highlight the important issues on synthetic method, structural design, morphological control and functionalization. And then we summarize the recent advances of COFs in the biomedical and pharmaceutical sectors and conclude with a discussion of the challenges and opportunities of COFs for biomedical purposes. Although currently still at its infancy stage, COFs as an innovative source have paved a new way to meet future challenges in human healthcare and disease theranostic.

Pyrene-cored covalent organic polymers by thiophene-based isomers, their gas adsorption, and photophysical properties

Two new pyrene‐cored covalent organic polymers (COPs), CK‐COP‐1 and CK‐COP‐2, were synthesized via the one‐step polymerization of two thiophene‐based isomers, 1,3,6,8‐tetra(thiophene‐2‐yl) pyrene (L₁) and 1,3,6,8‐tetra(thiophene‐3‐yl) pyrene (L₂). The resulting pyrene‐cored COPs exhibit rather different surface areas of 54 m² g⁻¹ and 615 m²g⁻¹ for CK‐COP‐1 and CK‐COP‐2, respectively. The CO₂ uptake capacities of CK‐COP‐1 and CK‐COP‐2 also show different values of 2.85 and 9.73 wt % at 273 K, respectively. Furthermore, CK‐COP‐2 offers not only a larger CO₂ adsorption capacity but also a better CO₂/CH₄ selectivity at 273 K compared with CK‐COP‐1. CK‐COP‐1 and CK‐COP‐2 also exhibit considerable differences in their photophysical property. The different structure and properties of CK‐COPs could be attributed to the isomer effect of their corresponding thiophene‐based monomers. © 2017 Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 2383–2389

Crystallization of Covalent Organic Frameworks for Gas Storage Applications

Covalent organic frameworks (COFs) have emerged as a new class of crystalline porous materials prepared by integrating organic molecular building blocks into predetermined network structures entirely through strong covalent bonds. The consequently encountered "crystallization problem" has been conquered by dynamic covalent chemistry in syntheses and reticular chemistry in materials design. In this contribution, we have reviewed the progress in the crystallization of COF materials and their hydrogen, methane and carbon dioxide gas storage properties for clean energy applications.

Covalent Organic Frameworks as a Platform for Multidimensional Polymerization

The simultaneous polymerization and crystallization of monomers featuring directional bonding designs provides covalent organic frameworks (COFs), which are periodic polymer networks with robust covalent bonds arranged in two- or three-dimensional topologies. The range of properties characterized in COFs has rapidly expanded to include those of interest for heterogeneous catalysis, energy storage and photovoltaic devices, and proton-conducting membranes. Yet many of these applications will require materials quality, morphological control, and synthetic efficiency exceeding the capabilities of contemporary synthetic methods. This level of control will emerge from an improved fundamental understanding of COF nucleation and growth processes. More powerful characterization of structure and defects, improved syntheses guided by mechanistic understanding, and accessing diverse isolated forms, ranging from single crystals to thin films to colloidal suspensions, remain important frontier problems.

Design Principles for Covalent Organic Frameworks in Energy Storage Applications

Covalent organic frameworks (COFs) are an exciting class of porous materials that have been explored as energy-storage materials for more than a decade. This review discusses efforts to develop these materials for applications in gas and electrical power storage. Some of the design strategies for developing the gas sorption properties of COFs and mechanistic studies on their formation are also discussed.

Imparting Catalytic Activity to a Covalent Organic Framework Material by Nanoparticle Encapsulation

Integrating covalent organic frameworks (COFs) with other functional materials is a useful route to enhancing their performances and extending their applications. We report herein a simple encapsulation method for incorporating catalytically active Au nanoparticles with different sizes, shapes, and contents in a two-dimensional (2D) COF material constructed by condensing 1,3,5-tris(4-aminophenyl)benzene (TAPB) with 2,5-dimethoxyterephthaldehyde (DMTP). The encapsulation is assisted by the surface functionalization of Au nanoparticles with polyvinylpyrrolidone (PVP) and follows a mechanism based on the adsorption of nanoparticles onto surfaces of the initially formed polymeric precursor of COF. The incorporation of nanoparticles does not alter obviously the crystallinity, thermal stability, and pore structures of the framework matrices. The obtained COF composites with embedded but accessible Au nanoparticles possess large surface areas and highly open mesopores and display recyclable catalytic performance for reduction of 4-nitrophenol, which cannot be catalyzed by the pure COF material, with activities relevant to contents and geometric structures of the incorporated nanoparticles.

A quaternary-ammonium-functionalized covalent organic framework for anion conduction

A new anion conducting covalent organic framework (COF) was prepared by covalently tethering quaternary ammonium (QA) ions onto the pore walls of COF 1,3,5-triformylphloroglucinol-o-tolidine (TpBD-Me) through bromination and quaternization.

CO2adsorption and separation in covalent organic frameworks with interlayer slipping

In layered COFs, slipping results in non-monotonous variation in CO₂adsorption and higher uptakes were found near a slipping distance of 10 Å.

Controllable preparation of core–shell magnetic covalent-organic framework nanospheres for efficient adsorption and removal of bisphenols in aqueous solution

A monomer-mediated in situ growth strategy has been developed for the controllable fabrication of magnetic COF core–shell nanostructures with great potential for wide applications.

Pore wall fluorescence labeling of covalent organic frameworks

A novel covalent organic framework (COF) based on terphenyldiboronic acid exhibiting open pores of about 4.1 nm is presented. The pore walls of the COF could be functionalized with a fluorescent dye.

Spiers Memorial Lecture: : Progress and prospects of reticular chemistry

Reticular chemistry, the linking of molecular building units by strong bonds to make crystalline, extended structures such as metal–organic frameworks (MOFs), zeolitic imidazolate frameworks (ZIFs), and covalent organic frameworks (COFs), is currently one of the most rapidly expanding fields of science. In this contribution, we outline the origins of the field; the key intellectual and practical contributions, which have led to this expansion; and the new directions reticular chemistry is taking that are changing the way we think about making new materials and the manner with which we incorporate chemical information within structures to reach additional levels of functionality. This progress is described in the larger context of chemistry and unexplored, yet important, aspects of this field are presented.

Isoreticular covalent organic frameworks for hydrocarbon uptake and separation: the important role of monomer planarity

We report three isoreticular covalent organic frameworks (COFs) sharing identical two-dimensional layered structures but with different planarity of the building monomers, leading to various crystallinities, porosities and hydrocarbon uptake and separation performances.

Rationally Designed 2D Covalent Organic Framework with a Brick-Wall Topology

We report the design and synthesis of an imine-based two-dimensional covalent organic framework (2D COF) with a novel brick-wall topology by judiciously choosing a tritopic T-shaped building block and a ditopic linear linker. Unlike the main body of COF frameworks reported to-date, which consists of higher-symmetry 2D topologies, the unconventional layered brick-wall topology have only been proposed but never been realized experimentally. The brick-wall structure was characterized by powder X-ray diffraction analysis, FT-IR, solid state ¹³C NMR spectroscopy, nitrogen, and carbon oxide adsorption-desorption measurements as well as theoretical simulations. Our present work opens the door to the design of novel 2D COFs and will broaden the scope of emerging COF materials.

Luminescent Porous Polymers Based on Aggregation-Induced Mechanism: Design, Synthesis and Functions

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 to show great potential for sensing, bio-imaging, and optoelectronic applications. Among them, integrating AIE mechanisms 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 in exploring AIE-based porous polymers has developed a new class of luminescent materials that exhibit broad structural diversity, outstanding properties and functions and promising applications. By classifying the structural nature of the skeleton, herein the design principle, synthetic development and structural features of different porous luminescent materials are elucidated, including crystalline covalent organic frameworks (COFs), metal-organic frameworks (MOFs), and amorphous porous organic polymers (POPs). The functional exploration of these luminescent porous polymers are highlighted by emphasizing electronic interplay within the confined nanospace, fundamental issues to be addressed are disclosed, and future directions from chemistry, physics and materials science perspectives are proposed.

Advanced Photoemission Spectroscopy Investigations Correlated with DFT Calculations on the Self-Assembly of 2D Metal Organic Frameworks Nano Thin Films

Metal-organic frameworks (MOFs) deposited from solution have the potential to form 2-dimensional supramolecular thin films suitable for molecular electronic applications. However, the main challenges lie in achieving selective attachment to the substrate surface, and the integration of organic conductive ligands into the MOF structure to achieve conductivity. The presented results demonstrate that photoemission spectroscopy combined with preparation in a system-attached glovebox can be used to characterize the electronic structure of such systems. The presented results demonstrate that porphyrin-based 2D MOF structures can be produced and that they exhibit similar electronic structure to that of corresponding conventional porphyrin thin films. Porphyrin MOF multilayer thin films were grown on Au substrates prefunctionalized with 4-mercaptopyridine (MP) via incubation in a glovebox, which was connected to an ultrahigh vacuum system outfitted with photoelectron spectroscopy. The thin film growth process was carried out in several sequential steps. In between individual steps the surface was characterized by photoemission spectroscopy to determine the valence bands and evaluate the growth mode of the film. A comprehensive evaluation of X-ray photoemission spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), and inverse photoemission spectroscopy (IPES) data was performed and correlated with density functional theory (DFT) calculations of the density of states (DOS) of the films involved to yield the molecular-level insights into the growth and the electronic properties of MOF-based 2D thin films.

Synthesis of chemically stable covalent organic frameworks in water

The development of environmentally benign and scalable synthetic routes to chemically stable covalent organic frameworks (COFs) is key to their real world application in areas such as gas storage and proton conduction. Banerjee et al. [IUCrJ (2016), 3, 402-407] have exploited the high chemical stability of the keto-enamine linkage to develop a 'green' water-mediated procedure, presenting a scalable route to chemically robust COFs.

Room-Temperature Synthesis of Covalent Organic Frameworks with a Boronic Ester Linkage at the Liquid/Solid Interface

With various prospected applications in the field of nanoelectronics and catalysis, on-surface synthesis of single-layer covalent organic frameworks (surface COFs) with designable structures and properties have attracted enormous interest. Herein, we report on a scanning tunneling microscopic investigation of the surface-confined synthesis of a covalently bonded boronic ester network directly at the octanoic acid/ highly oriented pyrolytic graphite(HOPG) interface under room temperature. The dynamic reaction process was investigated in detail. STM results indicate that the surface networks undergo structural evolution from a hybrid covalent/noncovalent multiwall porous network to single-wall hexagonal COF with the decrease of monomer concentration. Further experimental observation disclosed that the boronic ester-linked system is sensitive to instantaneous voltage pulses and the stimulation of the STM tip. In addition, the ¹ H NMR spectra has further confirmed that the surface and octanoic acid may play important roles in promoting the reaction between 4,4'-phenylazobenzoyl diboronic acid (ABBA) and 2,3,6,7,10,11-hexahydroxytriphenylene (HHTP) building units.

Two-dimensional artificial light-harvesting antennae with predesigned high-order structure and robust photosensitising activity

Highly ordered discrete assemblies of chlorophylls that are found in natural light-harvesting antennae are key to photosynthesis, which converts light energy to chemical energy and is the principal producer of organic matter on Earth. Porphyrins and phthalocyanines, which are analogues of chlorophylls, exhibit a strong absorbance of visible and near-infrared light, respectively. A highly ordered porphyrin-co-phthalocyanine antennae would harvest photons over the entire solar spectrum for chemical transformation. However, such a robust antennae has not yet been synthesised. Herein, we report a strategy that merges covalent bonds and noncovalent forces to produce highly ordered two-dimensional porphyrin-co-phthalocyanine antennae. This methodology enables control over the stoichiometry and order of the porphyrin and phthalocyanine units; more importantly, this approach is compatible with various metalloporphyrin and metallophthalocyanine derivatives and thus may lead to the generation of a broad structural diversity of two-dimensional artificial antennae. These ordered porphyrin-co-phthalocyanine two-dimensional antennae exhibit unique optical properties and catalytic functions that are not available with single-component or non-structured materials. These 2D artificial antennae exhibit exceptional light-harvesting capacity over the entire solar spectrum as a result of a synergistic light-absorption effect. In addition, they exhibit outstanding photosensitising activities in using both visible and near-infrared photons for producing singlet oxygen.

A novel Schiff base network-1 nanocomposite coated fiber for solid-phase microextraction of phenols from honey samples

A novel covalent organic framework, Schiff base network-1 (SNW-1), was synthesized and used as a solid-phase microextraction (SPME) fiber coating material. The SNW-1 coated SPME fiber was fabricated by a covalent chemical cross-linking between the SNW-1 nanocomposite and a silanol-functionalized stainless steel wire substrate. Scanning electron microscopy and nitrogen isothermal adsorption results indicate that the new fiber coating exhibited a porous, homogenous surface with the Brunauer-Emmett-Teller surface of 668m²g^-1. The prepared fiber was explored for the SPME of phenols from honey samples prior to their determination by gas chromatography-mass spectrometry. The developed method had large enrichment factors (136-816), low limits of detection (0.06-0.2ngg^-1), good linearity (0.1-100.0ngg^-1) and repeatability (<9.7%) for phenols. The recoveries for spiked phenols (1.0ngg^-1 and 10.0ngg^-1) in Wolfberry, Robinia and Codonopsis honey samples were in the range of 84.2-107.2% with the relative standard deviations ranging from 3.8% to 12.7%. The developed method was suitable for the determination of phenols from honey samples.

Sophisticated Design of Covalent Organic Frameworks with Controllable Bimetallic Docking for a Cascade Reaction

Precise control of the number and position of the catalytic metal ions in heterogeneous catalysts remains a big challenge. Here we synthesized a series of two-dimensional (2D) covalent organic frameworks (COFs) containing two different types of nitrogen ligands, namely imine and bipyridine, with controllable contents. For the first time, the selective coordination of the two nitrogen ligands of the 2D COFs to two different metal complexes, chloro(1,5-cyclooctadiene)rhodium(I) (Rh(COD)Cl) and palladium(II) acetate (Pd(OAc)2 ), has been realized using a programmed synthetic procedure. The bimetallically docked COFs showed excellent catalytic activity in a one-pot addition-oxidation cascade reaction. The high surface area, controllable metal-loading content, and predesigned active sites make them ideal candidates for their use as heterogeneous catalysts in a wide range of chemical reactions.

Recent Advances in Boron-Containing Conjugated Porous Polymers

Porous polymers, integrating the advantages of porous materials and conventional polymers, have been well developed and exhibited tremendous attention in the fields of material, chemistry and biology. Of these, boron-containing conjugated porous polymers, featuring tunable geometric structures, unique Lewis acid boron centers and very rich physical properties, such as high specific surface, chargeable scaffold, strong photoluminescence and intramolecular charge transfer, have emerged as one of the most promising functional materials for optoelectronics, catalysis and sensing, etc. Furthermore, upon thermal treatment, some of them can be effectively converted to boron-doped porous carbon materials with good electrochemical performance in energy storage and conversion, extensively enlarging the applicable scope of such kinds of polymers. In this review, the synthetic approaches, structure analyses and various applications of the boron-containing conjugated porous polymers reported very recently are summarized.

Highly Emissive Covalent Organic Frameworks

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.

High Conductive Two-Dimensional Covalent Organic Framework for Lithium Storage with Large Capacity

A high conductive 2D COF polyporphyrin (TThPP) linked by 4-thiophenephenyl groups was synthesized through an in situ chemical oxidative polymerization on the surface of copper foil. The TThPP films were used as the anode of lithium-ion battery, which exhibited high specific capacities, excellent rate performances, and long cycle lives due to the alignment of 2D polyporphyrin nanosheets, and they (i) can highly efficiently adsorb Li atoms, (ii) have short-ended paths for the fast lithium ion diffusion, and (iii) open nanopores holding electrolyte. The reversible capacity is up to 666 mAh/g. This is the first example of an organic 2D COF for an anode of lithium-ion battery and represents an important step toward the use of COFs in the next-generation high-performance lithium-ion battery.

Crystal structure of (E)-4-nitro-2-(((3-(tetrahydro-8λ4-[1,3,2]oxazaborolo[2,3-b][1,3,2]oxaborol-8-yl)phenyl)imino)methyl)phenol – water (1/2), C17H18BN3O5·2H2O

The title compound has been synthesized and characterized by IR, 1H NMR, elemental analysis as well as single crystal X-ray diffraction techniques. Crystal structure analysis reveals that the boron atom is tetrahedrally bonding with one carbon atom, one nitrogen atom, and two oxygen atoms. The bond between nitrogen atom and boron atom is a coordination bond.

Self-Exfoliated Guanidinium-Based Ionic Covalent Organic Nanosheets (iCONs)

Covalent organic nanosheets (CONs) have emerged as functional two-dimensional materials for versatile applications. Although π-π stacking between layers, hydrolytic instability, possible restacking prevents their exfoliation on to few thin layered CONs from crystalline porous polymers. We anticipated rational designing of a structure by intrinsic ionic linker could be the solution to produce self-exfoliated CONs without external stimuli. In an attempt to address this issue, we have synthesized three self-exfoliated guanidinium halide based ionic covalent organic nanosheets (iCONs) with antimicrobial property. Self-exfoliation phenomenon has been supported by molecular dynamics (MD) simulation as well. Intrinsic ionic guanidinium unit plays the pivotal role for both self-exfoliation and antibacterial property against both Gram-positive and Gram-negative bacteria. Using such iCONs, we have devised a mixed matrix membrane which could be useful for antimicrobial coatings with plausible medical benefits.

Chemically Stable Covalent Organic Framework (COF)-Polybenzimidazole Hybrid Membranes: Enhanced Gas Separation through Pore Modulation

Highly flexible, TpPa-1@PBI-BuI and TpBD@PBI-BuI hybrid membranes based on chemically stable covalent organic frameworks (COFs) could be obtained with the polymer. The loading obtained was substantially higher (50 %) than generally observed with MOFs. These hybrid membranes show an exciting enhancement in permeability (about sevenfold) with appreciable separation factors for CO2/N2 and CO2/CH4. Further, we found that with COF pore modulation, the gas permeability can be systematically enhanced.

Tubes or sheets: divergent aggregation pathways of an amphiphilic 2,7-substituted pyrene trimer

The self-assembly of an amphiphilic 2,7-linked pyrene trimer in an aqueous environment into two morphologically related forms is described. Supramolecular polymerization leads to the simultaneous formation of nanosheets and nanotubes.

A supramolecular strategy based on molecular dipole moments for high-quality covalent organic frameworks

Attractive intermolecular interactions of building blocks with strong dipole moments result in a COF with high crystallinity and large surface area.

Immobilization of ionic liquids to covalent organic frameworks for catalyzing the formylation of amines with CO2 and phenylsilane

We presented the immobilization of ionic liquids on the channel walls of COFs using a post-synthetic strategy.

Microporous polyurethane material for size selective heterogeneous catalysis of the Knoevenagel reaction

For the first time, a microporous polyurethane (MPU) is prepared – it acts as an organocatalyst for aldol-type C–C bond forming reactions with high yields and under mild conditions.

Band-structure engineering in conjugated 2D polymers

The band structures of several conjugated 2D polymers are calculated through DFT and the influence of the polymer's repeat unit on its electronic structure is discussed.

Ionic Covalent Organic Frameworks with Spiroborate Linkage

A novel type of ionic covalent organic framework (ICOF), which contains sp(3)  hybridized boron anionic centers and tunable countercations, was constructed by formation of spiroborate linkages. These ICOFs exhibit high BET surface areas up to 1259 m(2)  g(-1) and adsorb a significant amount of H2 (up to 3.11 wt %, 77 K, 1 bar) and CH4 (up to 4.62 wt %, 273 K, 1 bar). Importantly, the materials show good thermal stabilities and excellent resistance to hydrolysis, remaining nearly intact when immersed in water or basic solution for two days. The presence of permanently immobilized ion centers in ICOFs enables the transportation of lithium ions with room-temperature lithium-ion conductivity of 3.05×10(-5)  S cm(-1) and an average Li(+) transference number value of 0.80±0.02. Our approach thus provides a convenient route to highly stable COFs with ionic linkages, which can potentially serve as absorbents for alternative energy sources such as H2, CH4, and also as solid lithium electrolytes/separators for the next-generation lithium batteries.