Introduction
Alkynyl COFs linkers here refer to the compounds containing alkyne moieties (such as acetylene, diacetylene) for synthesis of COFs. In the synthesis of COFs, the synthesis of acetylene, diacetylene-bridged COFs seems quite challenging. Because these organic linkers should on the one hand yield large pore sizes but on the other hand have very weak van der Waals interactions, and both factors do not assist for interlayer stacking of the 2D polymers, which is the rate determining step in COFs synthesis. Despite that, such structures could be of high interest especially due to their highly conjugated structures, high charge career mobility, ability to provide active sites for facile migration of photogenerated excitons to the surface and others. Indeed, alkynyl linkers have been gradually used in the synthesis of various COFs in recent years, although they are not as widely used as aldehyde, amine and boric acid linkers in the construction of COFs.
Applications
Some of typical COFs synthesis application examples of alkynyl COFs linkers are listed below.
- Synthesis of COFs for photocatalytic hydrogen generation: Pachfule et al. synthesized two novel highly porous and chemically stable β-ketoenamine COFs bearing acetylene and diacetylene moieties, named TP-EDDA and TP-BDDA, by the acid-catalyzed solvothermal reaction of 1,3,5-triformylphloroglucinol (TP) with two alkynyl linkers of 4,4′-(ethyne-1,2-diyl)dianiline (EDDA) or 4,4′-(buta-1,3-diyne-1,4-diyl)dianiline (BDDA), respectively. The results showed that the diacetylene moieties have a profound effect in terms of photocatalytic activity. In addition, as a combined effect of high porosity, easily accessible diacetylene functionalities and considerable chemical stability, an efficient and recyclable heterogeneous photocatalytic hydrogen generation is achieved by the diacetylene-based COF [1].
Fig. 1. The schematic diagram of synthesis of TP-EDDA and TP-BDDA COFs and their photocatalytic hydrogen generation application.
- Synthesis of COFs for high-rate EDLCs: The electrochemical double-layer capacitors (EDLCs) are highly demanded electrical energy storage devices due to their high power density with thousands of cycle life compared with pseudocapacitors and batteries. COF is a potential EDLCs material, however, COF-based EDLCs have rarely been constructed. Fortunately, the material has been gradually synthesized using alkynyl COFs linkers in recent years. For example, Yusran et al. designed mesoporous 2D COF series (termed as JUC-510, JUC-511, and JUC-512) by reaction of a linear alkynyl building block of hydroxyl containing 4,4-(1,2-ethynediyl)bis-2-hydroxybenzaldehyde as the basic backbone with the square-shape 5,10,15,20-tetrakis[(4-aminophenyl)porphyrin, or 5,10,15,20-tetrakis[(4-aminophenyl)porphyrinato] nickel (II), or 5,10,15,20-tetrakis[(4-aminophenyl)porphyrinato]copper(II). This COF-based capacitor cells achieve high areal capacitance (5.46 mF cm−2 at 1,000 mV s−1), high gravimetric power (55 kW kg−1), and relatively low τ0 value (121 ms) [2].
Fig. 2. Synthesis of JUC-510, JUC-511, and JUC-512.
- Synthesis of COFs with tailor-made pore environments: By reaction of alkynyl linker of 2,5-bis(2-propynyloxy)terephthalaldehyde (BPTA) with 2,5-dihydroxy terephthalaldehyde (DHTA) or 2,5-dimethoxyterephthalaldehyde (DMTA) at various molar ratios can synthesize various COFs with different alkyne contents on their edges. The alkyne units on these COFs that can further react with various azides to develop a pore surface engineering strategy for functionalization of pore walls to create tailor-made pore environments and interfaces of new COFs with different functions. For example, Xu et al. used a mixture of BPTA and DHTA at varying molar ratios (X = [BPTA]/([BPTA] + [DHTA]) ╳ 100 = 0, 25, 50, 75, 100) as the edge units, and 5,10,15,20-tetrakis(40 -tetraphenylamino) porphyrin (H2P) as the vertices to synthesize the COFs (HC≡C]X-H2P-COFs). Then the click reaction of the alkyne units with the azide compounds takes place quantitatively and anchors the desired groups to the pore walls at the desirable contents to synthesize new [Pyr]X-H2P-COFs [3].
![Fig. 3. (A) Synthesis of HC≡C]X-H2P-COFs and [Pyr]X-H2P-COFs. (B) A graphical representation of [Pyr]X-H2P-COFs with different densities of catalytic sites on the pore walls.](/upload/image/alkynyl-cofs-linkers-3.jpg)
Fig. 3. (A) Synthesis of HC≡C]X-H2P-COFs and [Pyr]X-H2P-COFs. (B) A graphical representation of [Pyr]X-H2P-COFs with different densities of catalytic sites on the pore walls.
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References:
- Pachfule P., et al. Diacetylene functionalized covalent organic framework (COF) for photocatalytic hydrogen generation[J]. Journal of the American Chemical Society, 2018, 140(4): 1423-1427.
- Yusran Y., et al. Exfoliated mesoporous 2D covalent organic frameworks for high‐rate electrochemical double‐layer capacitors[J]. Advanced Materials, 2020, 32(8): 1907289.
- Xu H., et al. Catalytic covalent organic frameworks via pore surface engineering[J]. Chemical Communications, 2014, 50(11): 1292-1294.
