Urea-Linked\nCovalent Organic Frameworks

2D\ncovalent organic frameworks (COFs) with flexible urea linkages\nhave been synthesized by condensation of 1,3,5-triformylphloroglucinol\n(TFP) with 1,4-phenylenediurea (BDU) or 1,1′-(3,3′-dimethyl-[1,1′-biphenyl]-4,4′-diyl)­diurea\n(DMBDU). The resulting COF-117 and COF-118 undergo reversible structural\ndynamics within their layers, in response to inclusion and removal\nof guest molecules, emanating from urea CN bond rotation and\ninterlayer hydrogen-bonding interactions. These compounds are the\nfirst urea-linked COFs, serving to expand the scope of reticular chemistry.