Fused-Ring-Linked Covalent Organic Frameworks

The development of linkage chemistry in the researcharea of covalent organic frameworks (COFs) is fundamentallyimportant for creating robust structures with high crystallinity anddiversified functionality. We reach herein a new level of complexityand controllability in linkage chemistry by achieving thefirst synthesisof fused-ring-linked COFs. A series of bicyclic pyrano[4,3-b]pyridineCOFs have been constructed via a cascade protocol involving Schiff-base condensation, intramolecular [4 + 2] cycloaddition, anddehydroaromatization. With a broad scope of Bronsted or Lewisacids as the catalyst, the designed monomers, that is,O-propargylicsalicylaldehydes and multitopic anilines, were converted into thefused-ring-linked frameworks in a one-pot fashion. The obtainedCOFs exhibited excellence in terms of purity, stability, and crystallinity, as comprehensively characterized by solid-state nuclearmagnetic 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 measurementscombined with13C-labeling synthesis. Moreover, the fused-ring linkage possesses fully locked conformation, which benefits to thehigh crystallinity observed for these COFs. Advancing the linkage chemistry from the formation of solo bonds or single rings to thatof fused rings, this study has opened up new possibilities for the concise construction of sophisticated COF structures with highcontrollability