Unlike bulk JUC-557, its nanosheet counterpart did not present an apparent PXRD pattern (Supplementary Fig. ), indicating that the pristine bulk has been converted to a dissociative nanosheet. Besides, JUC-557-nanosheet was characterized by FT-IR spectroscopy and solid-state C CP-MAS NMR, and these data implied that the exfoliated nanosheet maintained the same chemical composition (Supplementary Figs. and). JUC-557-nanosheet still possessed a good thermal stability (Supplementary Fig. ). Nevertheless, the specific surface area of JUC-557-nanosheet remarkably decreased from 1115 m 2 /g to 43 m 2 /g in comparison with its parent material (Supplementary Figs. and). Due to the structural relaxation arising from thin layers, the Raman spectrum showed a slight shift toward higher wavenumber (Supplementary Fig. ), and the ultraviolet-visible (UV-vis) absorption band was blue-shifted from 374 nm to 345 nm (Fig. ). Furthermore, we conducted the UV-vis diffuse reflectance measurement to understand the optical bandgap changes (E g ) between the bulk powder and nanosheet. As shown in Supplementary Figs. and, the narrow absorption band of JUC-557-nanosheet could be attributed to its uniform dispersion in solution, and the calculated E g was increased to 2.53 eV compared with the bulk COF (2.42 eV).