Covalent organic frameworks (COFs) represent an exciting new type of porous organic materials, which are ingeniously constructed with organic building units via strong covalent bonds. The well-defined crystalline porous structures together with tailored functionalities have offered the COF materials superior potential in diverse applications, such as gas storage, adsorption, optoelectricity, and catalysis. Since the seminal work of Yaghi and co-workers in 2005, the rapid development in this research area has attracted intensive interest from researchers with diverse expertise. This critical review describes the state-of-the-art development in the design, synthesis, characterisation, and application of the crystalline porous COF materials. Our own opinions on further development of the COF materials are also presented for discussion (155 references).

Covalent organic frameworks (COFs): from design to applications

Dingcai Wu,*,† Fei Xu,† Bin Sun,† Ruowen Fu,† Hongkun He,‡ and Krzysztof Matyjaszewski*,‡ †Materials Science Institute, Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, People's Republic of China ‡Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States

Design and Preparation of Porous Polymers

Covalent organic frameworks (COFs) are a class of crystalline porous organic polymers with permanent porosity and highly ordered structures. Unlike other polymers, a significant feature of COFs is that they are structurally predesignable, synthetically controllable, and functionally manageable. In principle, the topological design diagram offers geometric guidance for the structural tiling of extended porous polygons, and the polycondensation reactions provide synthetic ways to construct the predesigned primary and high-order structures. Progress over the past decade in the chemistry of these two aspects undoubtedly established the base of the COF field. By virtue of the availability of organic units and the diversity of topologies and linkages, COFs have emerged as a new field of organic materials that offer a powerful molecular platform for complex structural design and tailor-made functional development. Here we target a comprehensive review of the COF field, provide a historic overview of the chemistry of the COF field, survey the advances in the topology design and synthetic reactions, illustrate the structural features and diversities, scrutinize the development and potential of various functions through elucidating structure-function correlations based on interactions with photons, electrons, holes, spins, ions, and molecules, discuss the key fundamental and challenging issues that need to be addressed, and predict the future directions from chemistry, physics, and materials perspectives.

Covalent Organic Frameworks: Design, Synthesis, and Functions.

Conjugated microporous polymers (CMPs) are a class of organic porous polymers that combine π-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.

Conjugated microporous polymers: design, synthesis and application

Two new chemically stable [acid and base] 2D crystalline covalent organic frameworks (COFs) (TpPa-1 and TpPa-2) were synthesized using combined reversible and irreversible organic reactions. Syntheses of these COFs were done by the Schiff base reactions of 1,3,5-triformylphloroglucinol (Tp) with p-phenylenediamine (Pa-1) and 2,5-dimethyl-p-phenylenediamine (Pa-2), respectively, in 1:1 mesitylene/dioxane. The expected enol–imine (OH) form underwent irreversible proton tautomerism, and only the keto–enamine form was observed. Because of the irreversible nature of the total reaction and the absence of an imine bond in the system, TpPa-1 and TpPa-2 showed strong resistance toward acid (9 N HCl) and boiling water. Moreover, TpPa-2 showed exceptional stability in base (9 N NaOH) as well.

Construction of crystalline 2D covalent organic frameworks with remarkable chemical (Acid/Base) stability via a combined reversible and irreversible route

Porous materials have been of intense scientific and technological interest because of their vital importance in many applications such as catalysis, gas separation, and gas storage. Great efforts in the past decade have led to the production of highly porous materials with large surface areas. In particular, the development of metal–organic frameworks (MOFs) has been especially rapid. Indeed, the highest surface area reported to date is claimed for a recently reported MOF material UMCM-2, which has a N2 uptake capacity of 1500 cm g at saturation, from which a Langmuir surface area of 6060 m g (Brunauer–Emmett–Teller (BET) surface area of 5200 m g) can be derived. Unfortunately, the high-surface-area porous MOFs usually suffer from low thermal and hydrothermal stabilities, which severely limit their applications, particularly in industry. These low stability issues could be resolved by replacing coordination bonds with stronger covalent bonds, as observed in covalent organic frameworks (COFs) or porous organic polymers. However, the COFs and porous organic polymers reported to date have lower surface areas compared to MOFs; the highest reported surface area for a COF is 4210 m g (BET) in COF103. Thus, further efforts are required to explore various strategies to achieve higher surface areas in COFs. Herein, we present a strategy that has enabled us to achieve, with the aid of computational design, a structure that possesses by far the highest surface area reported to date, as well as exceptional thermal and hydrothermal stabilities. We report the synthesis and properties of a porous aromatic framework PAF-1, which has a Langmuir surface area of 7100 m g. Besides its exceptional surface area, PAF-1 outperforms highly porous MOFs in thermal and hydrothermal stabilities, and demonstrates high uptake capacities for hydrogen (10.7 wt % at 77 K, 48 bar) and carbon dioxide (1300 mgg 1 at 298 K, 40 bar). Moreover, the super hydrophobicity and high surface area of PAF-1 result in unprecedented uptake capacities of benzene and toluene vapors at room temperature. It is well known that one of the most stable compounds in nature is diamond, in which each carbon atom is tetrahedrally connected to four neighboring atoms by covalent bonds (Figure 1a). Conceptually, replacement of the C C covalent bonds of diamond with rigid phenyl rings should not only retain a diamond-like structural stability but also allow sufficient exposure of the faces and edges of phenyl rings with the expectation of increasing the internal surface areas. By employing a multiscale theoretical method, which

Targeted Synthesis of a Porous Aromatic Framework with High Stability and Exceptionally High Surface Area

A series of porous aromatic frameworks (PAFs) were synthesized via a Yamamoto-type Ullmann reaction containing quadricovalent Si (PAF-3) and Ge (PAF-4). These PAFs are thermally stable up to 465 °C for PAF-3 and 443 °C for PAF-4, corresponding to a 5% weight loss according to the TG pattern. As PAF-1, they exhibit high surface areas (up to 2932 m2 g−1) and excellent adsorption ability to hydrogen, methane and carbon dioxide. Low pressure gas uptake experiments on PAFs show PAF-3 has the highest heat of adsorption (Qst) of hydrogen (6.6 kJ mol−1) and carbon dioxide (19.2 kJ mol−1), while PAF-4 has the highest Qst for methane adsorption (23.2 kJ mol−1) among PAFs. Gas molecule recognition at 273 K was performed and results show only greenhouse gases such as carbon dioxide and methane could be adsorbed onto PAFs.

Gas storage in porous aromatic frameworks (PAFs)

Targeted synthesis of a 3D porous aromatic framework for selective sorption of benzene.

Targeted synthesis of a 2D ordered porous organic framework for drug release

Tetrakis(4-bromophenyl)methane (TBPM) as a tetrahedral unit and a diboronic acid as a linker were selected to couple the phenyl rings into a porous aromatic framework, PAF-11. PAF-11 was polymerized via a Suzuki coupling reaction. A TG analysis showed that PAF-11 is thermally stable up to 400 °C in air. PAF-11 also has a high chemical stability and cannot be dissolved or decomposed in common solvents or concentrated hydrochloric acid. A N2 sorption measurement on activated PAF-11 revealed a surface area of 952 m2 g−1 in the Langmuir model. PAF-11 also shows a considerable adsorption capacity for H2. Interestingly, PAF-11 is a highly hydrophobic material but with a high methanol uptake (654 mg g−1 at saturated vapour pressure and room temperature). PAF-11 also exhibits high adsorption abilities for small aromatic molecules such as benzene and toluene (874 and 780 mg g−1, respectively, at saturated vapour pressure and room temperature) due to its aromatic framework. This ability of PAF-11 could be very useful to eliminate harmful small aromatic molecules produced by industry.

Xiaofei Jing

Papers

Targeted Synthesis of a Porous Aromatic Framework with High Stability and Exceptionally High Surface Area

Gas storage in porous aromatic frameworks (PAFs)

Targeted synthesis of a 3D porous aromatic framework for selective sorption of benzene.

Targeted synthesis of a 2D ordered porous organic framework for drug release

Targeted synthesis of a porous aromatic framework with a high adsorption capacity for organic molecules