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.

A light COF: Two‐dimensional covalent organic frameworks of a nickel phthalocyanine have been synthesized. Owing to well‐ordered stacking of the phthalocyanine units, the resulting 2D framework provides enhanced and broad light absorbance and facilitates charge transport. The material becomes highly photoconductive and is exceptionally sensitive to deep‐red visible and near‐infrared light.

Covalent organic frameworks (CoFs) are a class of important porous materials that allow atomically precise integration of building blocks to achieve pre-designable pore size and geometry; however, pore surface engineering in CoFs remains challenging. Here 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 azideappended 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.

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 a belt shape and consists of AA stacking of 2D polymer sheets. Integration of BTDA blocks at the edges of a tetragonal metallophthalocyanine COF causes drastic changes in the carrier-transport mode and a switch from a hole-transporting skeleton to an 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 cm(2) V(-1) s(-1).

Covalent organic frameworks (COFs) are a class of designable crystalline polymers with structural periodicity and inherent porosity. [1][2][3][4][5][6][7][8][9] COFs have emerged as new porous materials for gas adsorption and storage because of their high porosity, robust thermal stability, and low densities. [1,2,7] In addition, the layered structure of 2D COFs provides periodic arrays of p clouds that can greatly facilitate charge-carrier transport; [1f, 3, 9] this structural feature is not seen in conventional 1D and 3D polymers. To date, COFs containing boronate ester, boroxine, imine, triazine, and hydrazone linkages have been synthesized. [1][2][3][4][5][6][7][8][9] The development of new reactions to synthesize such covalent and crystalline frameworks is critical for further progress in this emerging field.We report herein a new reaction based on squaraine that allows for the synthesis of a new type of 2D COF. Squaraines are interesting dyes with a zwitterionic resonance structure and have broad applications in areas such as imaging, nonlinear optics, photovoltaics, photodynamic therapy, and ion sensing. [10] Squaraines are usually prepared through the condensation of squaric acid (SA) with aromatic, heteroaromatic, or olefinic compounds in a simple one-step reaction. [10] For example, the condensation of SA with p-toludine as the donating molecule gives a squaraine (SQ) with a planar yet zigzagged zwitterionic resonance structure (Scheme 1 a and b). By using this linkage, we expect to form a conjugated COF with a zigzagged skeleton. We synthesized copper(II) 5,10,15,20-tetrakis(4-aminophenyl)porphyrin (TAP-CuP) as a building block for the SA condensation and constructed a crystalline 2D conjugated COF (CuP-SQ COF; Scheme 1 c) with a tetragonal mesoporous skeleton. We demonstrated that the SQ-linked COFs have a zigzagged conformation that protects the layered structure from sideslip, are highly stable in solvents, provide an extended p conjugation over the 2D sheets, and have lower band gap energy and greatly enhanced absorbance capability compared to existing COFs. These features extend the structural and functional scope of COFs.The CuP-SQ COF was synthesized under solvothermal conditions through the condensation of SA and TAP-CuP in o-dichlorobenzene/n-butanol (1:1 by vol.) at 85 8C for 7 days. The resulting precipitate was collected by filtration, washed with THF and acetone, and dried at 150 8C under vacuum to provide the CuP-SQ COF as a dark purple powder in 94 % yield. When combinations of n-butanol with other aromatic solvents, such as mesitylene and 1,3,5-tricholorobenzene, were used, the resulting solid had a lower crystallinity. To investiagte the ratio of o-dichlorobenzene to nBuOH, ratios of 1:3, 1:2, 1:1, 2:1, and 3:1 were used; the product with the highest crystallinity was obtained when the 1:1 volume ratio was used. The model compound SQ was synthesized under the same reaction conditions in 99 % yield (Scheme 1 a and the Supporting Information). Thermal gravimetric analysis shows that the...

Light works: Mechanistic insights into the photochemical events and charge dynamics of a donor–acceptor covalent organic framework were given by time‐resolved transient absorption spectroscopy and time‐resolved electron spin resonance spectroscopy (see picture). The organic framework triggers ultrafast electron transfer and enables long‐distance charge delocalization and exceptional long‐term charge separation.

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 through 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.

A azine-linked covalent organic framework, COF-JLU2, was designed and synthesized by condensation of hydrazine hydrate and 1,3,5-triformylphloroglucinol under solvothermal conditions for the first time. The new covalent organic framework material combines permanent micropores, high crystallinity, good thermal and chemical stability, and abundant heteroatom activated sites in the skeleton. COF-JLU2 possesses a moderate BET surface area of over 410 m(2)  g(-1) with a pore volume of 0.56 cm(3)  g(-1) . Specifically, COF-JLU2 displays remarkable carbon dioxide uptake (up to 217 mg g(-1) ) and methane uptake (38 mg g(-1) ) at 273 K and 1 bar, as well as high CO2 /N2 (77) selectivity. Furthermore, we further highlight that it exhibits a higher hydrogen storage capacity (16 mg g(-1) ) than those of reported COFs at 77 K and 1 bar.