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.

New, halogen-bonded fluorinated mesogens are reported; the expected microphase separation associated with perfluoroalkyl chains is surprisingly absent in the mesophase.

The strong halogen bonding occurring between naked iodide ions and iodoperfluoroalkanes can overcome the low affinity that hydrocarbon derivatives and inorganic salts have for perfluorinated derivatives, resulting in the compatibilization of otherwise immiscible compounds. Small amounts of iodoperfluoroalkanes boost the solubility of inorganic salts in fluorinated solvents. In the solid phase, the halogen bonding between the iodide ion of cryptated salts and the differently sized diiodoperfluoroalkanes allows the design and obtainment of three-component mixed supramolecular architectures. Segregation phenomena and halogen bonding features drive the formation of new crystalline materials containing fluorous layerlike and interpenetrated networks.

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The beautiful examples of helical structures provided by nature have inspired huge efforts in the generation of synthetic helical assemblies.[1] Many of the homochiral supramolecular helical structures that have been described were created through the self-assembly of chiral molecules and polymers.[2] A few reports on the preparation of artificial homochiral helices from achiral components also exist.[3] The generation of double-helical architectures retains a unique fascination, as life itself is encoded within a double-helical structure. Both metal-ion coordination [4] and hydrogen bonding [5] are well-established agents for the self-assembly of double helices.Herein, we demonstrate how halogen bonding (XB) [6] directs the self-assembly of naked halide anions and longchain diiodoperfluoroalkanes (diiodo-PFAs), leading to the formation of a new type of artificial molecular double helix. In the homochiral supramolecular complex 4 (Figure 1), the perfluorinated module 3 exclusively adopts a right-handed (P) helical conformation (molecular chirality), which translates into chiral and enantiopure fluorous [7] double helices (supramolecular chirality), owing to strong and directional I À ···IÀC halogen bonding.Naked halide anions function as particularly effective halogen-bond acceptors.[8] Ternary mixtures of calixcrownarenes, KI, and diiodo-PFAs give supramolecular complexes in which the calixarene modules complex the K + ions, and the naked I À ions assemble the diiodo-PFAs into infinite fluorous halogen-bonded chains, which segregate the calixarene domains.[8e]We chose 4-tert-butylcalix[4]arenetetra-N,Ndiethylacetamide (1) for its ability to coordinate alkali and alkaline-earth metal cations, and to form separated ion pairs. Interestingly, calixarene 1 forms a supramolecular complex with KI that is characterized by a columnar stacking of 1&K + supercations intercalated by naked I À ions; this arrangement leaves voids parallel to the stacking direction.[9a] When 1 is combined with a divalent cation in 1&Sr(Pic) 2 (Pic = picrate), Figure 1. Structural formulas of the individual components 1-3, and a section of the crystal structure of the supramolecular complex 4. Hydrogen atoms and water molecules are omitted for clarity.

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