The halogen bonding and the pi...pi stacking interactions induce the noncovalent self-assembly of modules into photoreactive supramolecular architecture. The pi...pi interaction pre-organizes the template, and the halogen bonding aligns the olefins to conform to the topochemical principle for photoreaction. The UV irradiation of the crystal resulted in a cyclization product with quantitative yield and stereospecificity.
Engineering functional materials endowed with unprecedented properties require the exploitation of new intermolecular interactions, which can determine the characteristics of the bulk materials. The great potential of Halogen Bonding (XB), namely any noncovalent interaction involving halogens as electron acceptors, in the design of new and high-value functional materials is now emerging clearly. This Highlight will give a detailed overview on the energetic and geometric features of XB, showing how some of them are quite constant in most of the formed supramolecular complexes (e.g., the angle formed by the covalent and the noncovalent bonds around the halogen atom), while some others depend strictly on the nature of the interacting partners. Then, several specific examples of halogen-bonded supramolecular architectures, whose structural aspects as well as applications in fields as diverse as enantiomers' separation, crystal engineering, liquid crystals, natural, and synthetic receptors, will be fully described.
The N.Br halogen bonding drives the self-assembly of 1,4-dibromotetrafluorobenzene (1 a) and its 1,3 or 1,2 analogues (1 b,c, respectively) with dipyridyl derivatives 2 a,b. The isomeric supramolecular architectures 3 a-f are obtained as cocrystals that are stable in the air at room temperature. The solid-state features of these 1D infinite chains 3 have been fully characterized by single-crystal X-ray, Raman, and IR analyses. The occurrence of N.Br halogen bonding in solution has been detected with (19)F NMR spectroscopy. The N.Br halogen bonding is highly selective and directional and the geometry of the single strands of noncovalent copolymers 3 is programmed by the geometry of halogen-bonding donor and acceptor sites on the starting modules. The composition and topology of the instructed networks can be predicted with great accuracy. Experiments of competitive cocrystal formation established the strength of the N.Br interaction relative to other halogen bondings and the ability of different modules 1 to be involved in site-selective supramolecular syntheses.
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