Fused-Ring-Linked Covalent Organic Frameworks

Abstract

The development of linkage chemistry in the research area of covalent organic frameworks (COFs) is fundamentally important for creating robust structures with high crystallinity and diversified functionality. We reach herein a new level of complexity and controllability in linkage chemistry by achieving the first synthesis of fused-ring-linked COFs. A series of bicyclic pyrano [4,3-b]pyridine COFs have been constructed via a cascade protocol involving Schiffbase condensation, intramolecular [4 + 2] cycloaddition, and dehydroaromatization. With a broad scope of Brønsted or Lewis acids as the catalyst, the designed monomers, that is, O-propargylic salicylaldehydes and multitopic anilines, were converted into the fused-ring-linked frameworks in a one-pot fashion. The obtained COFs exhibited excellence in terms of purity, stability, and crystallinity, as comprehensively characterized by solid-state nuclear magnetic resonance (NMR) spectroscopy, powder X-ray diffraction, high-resolution transmission electron microscopy, and so on. Specifically, the highly selective formation (>94%) of pyrano [4,3-b]pyridine linkage was verified by quantitative NMR measurements combined with 13 C-labeling synthesis. Moreover, the fused-ring linkage possesses fully locked conformation, which benefits to the high crystallinity observed for these COFs. Advancing the linkage chemistry from the formation of solo bonds or single rings to that of fused rings, this study has opened up new possibilities for the concise construction of sophisticated COF structures with high controllability.

Metalated covalent organic frameworks: from synthetic strategies to diverse applications

“…Of note, the metallophthalocyanine-COFs with 5th and 6th period metals (e.g., Pt, Rh, Ag, W, Ta, Mo, Os, and Re) have been predicted based on first-principles density functional theory calculations, 223 but their synthesis still remains blank. 3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadecafluorophthalocyanines under alkaline conditions (Fig. 6).…”

“…218 In addition to the pyrazine-linked COFs, the piperazinelinked phthalocyanine-COF has also been reported by Huang and co-workers. By Buchwald-Hartwig cross-coupling reaction of nickel(II) 2,3,9,10,16,17,23,24-octaaminophthalocyanine and cobalt(II) 1,2,3,4,8,9,10,11,15,16,17,18,22,23,24,25-hexadecafluorophthalocyanine under solvothermal conditions (dioxane, triethylamine, 120 1C), the piperazine-linked heterometallic NiPc-NH-CoPcF 8 COF was generated. 219 NiPc-NH-CoPcF 8 crystallized in the tetragonal P4/mmm space group, with AA stacking mode, a pore size of 1.1 nm, and an interlayer spacing of 3.6 Å.…”

“…5 As shown, the boroxinelinked COF-1 features a 2D structure with staggered AB stacking mode, whereas the boronate ester-linked 2D COF-5 adopts an eclipsed AA stacking structure. Since then, COFs with different linkages and topologies have been synthesized one after another by various approaches including solvothermal, [6][7][8][9] ionothermal, 10,11 microwave, 12,13 atmospheric solution, [14][15][16][17][18] mechanochemical, [19][20][21] electrochemical, 22,23 photochemical, 24,25 and other methods. [26][27][28][29] The CURATED-COFs database 30 showed that the query string TITLE-ABS-KEY (''Covalent Organic Framework'') retrieved over 4300 research papers and close to 1000 patents, covering various subject areas such as chemistry, materials science, chemical engineering, and environmental science.…”

“…5 As shown, the boroxine-linked COF-1 features a 2D structure with staggered AB stacking mode, whereas the boronate ester-linked 2D COF-5 adopts an eclipsed AA stacking structure. Since then, COFs with different linkages and topologies have been synthesized one after another by various approaches including solvothermal, 6–9 ionothermal, 10,11 microwave, 12,13 atmospheric solution, 14–18 mechanochemical, 19–21 electrochemical, 22,23 photochemical, 24,25 and other methods. 26–29 The CURATED-COFs database 30 established by Smit et al has included 648 COFs as of June 2021.…”

Tale of COF-on-MOF Composites with Structural Regulation and Stepwise Luminescence Enhancement

“…Since the groundbreaking research on covalent organic frameworks (COFs) reported by Yaghi et al, such type of popular crystalline materials have attracted extensive attention. − COFs are constructed from organic ligands through covalent bonds and possess predesigned structures as well as precise customized functions that facilitate the implementation of chemical and structural control. − Despite enormous attractive research on the design and construction of COFs having been conducted for nearly 20 years, there is still great potential for the development of luminescent COF materials. − Notably, the natural π-conjugated structure of COFs is beneficial to study their luminescence performance, while the π–π stacked structure also causes aggregation-induced quenching (ACQ) and hinders the emission of COFs. , Meanwhile, weakening the stacking degree between layers will increase the intramolecular rotation, which is also a dilemma for luminescence. − At present, fluorescent organic linkers with large conjugated structures or with an aggregation-induced luminescence effect are usually selected for the fabrication of luminescent COFs. − Emissive COFs can also be obtained via postmodification or exfoliation. − To improve the luminescence properties of COFs in an effective way, the emergence of luminescent composites based on COFs stands out from single COFs. − As we know, even though luminescent COFs as emerging functional materials have been dramatically studied over the past few years, ,,− research on directly growing COFs on the core substrate with a limited number of layers to improve the luminous efficiency still remains less understood.…”