Supramolecular synthons, tectons, and crystal structures of noncovalent organic frameworks

In reticular chemistry, framework structures formed from organic molecules by noncovalent intermolecular interactions [i.e., noncovalent organic frameworks (nCOFs)] have been intensively investigated over the last two decades as another type of candidate for porous materials. Noncovalent interactions, such as hydrogen bonds (H-bonds), are weaker than covalent bonds or coordination bonds; therefore, various structural factors should be considered when a certain nCOF is designed. In this review, we examine nCOFs possessing three types of classic molecular scaffolds: 1,3,5-trisubstituted benzene, 1,2,3,4,5,6-hexasubstituted benzene, and tetraphenylethene, from viewpoints of the strength and directionality of intermolecular interactions (supramolecular synthons) and the geometry and rigidity of building block molecules (tectons). Combination of rigid tectons and robust supramolecular synthons is useful for preorganization of the framework structures, while flexible tectons and synthons can provide various photochemical functionality such as stimuli-responsiveness and chemosensing. Relationship between the structure of tectons, type of supramolecular synthons, and property of nCOFs is overviewed.