Halogen-bonding-based supramolecular

Fig. 1 Most frequently used halogen-bonding-based supramolecular synthons. Eleetron-pair donor sites were reported in blaek, while the aeeeptor sites are in grey. (View this art in color at www.dekker.com)...

In the past 2 years or so, halogen-bond-based supramolecular materials have emerged in the context of aU the aforementioned phenomena. No matter whether dealing with photoactive polymers, liquid crystals, or crystals, the directionality of the halogen bond is the key to its success. Its tunability, on the other hand, provides unprecedented possibilities to carry out fundamental studies on light-induced motions unrelated to other types of material systems. The road is wide open, and the first steps taken are presented in the following sections. 

As described most elegantly elsewhere in this volume, the halogen bond is an intermolecular, charge-transfer interaction between a Lewis base and an electron-deficient halogen. Other chapters that accompany this chart its use in, for example, supramolecular chemistry, molecular conductors and coordination chemistry. In this chapter, a much more recent application of halogen bonding is described, namely in the realisation of liquid-crystalline materials. 

Highly fluorinated molecules are also of significant interest for construction of supramolecular LC based on halogen bonding between perfluoroalkyl- or... 

Supramolecular liquid crystals have also been reported based on more unusual supramolecular interactions such as halogen bonding (Section 1.8.9).12 Neither of the components in 13.17 are mesomorphic by themselves but the presence of the highly activated iodine atom in pentafluoroiodo-benzene results in a strong N---I interaction (N---I distance 2.81 A by X-ray crystallography). These... 

With this background, it is not surprising that supramolecular architectures based on halogen bonding appear in the... 

Figure 11 Halogen bonding leading to the formation of a honeycomb array, (a) Chemical structures of the constituents 1,3,5-tritrifluoro-2,4,6-triiodobenzene and tetraethylammonium iodide in the cation the nitrogen atom is partiaUy substituted by phosphorus (34%) and (b) supramolecular 2-D architecture based on a honeycomb mediated by charge-assisted I - -I halogen bonds [3.48-3.58 A], shown as blue dashed lines the iodide anions are shown as olive green spheres.

Another example of photoresponsive liquid crystalline material based on halogen bonding appeared recently [61]. The idea here was to extend the concept of photoinduced order-disorder phase transition (Fig. Ic) from conventional photoresponsive LCs to halogen-bonded supramolecular complexes. To do this, the authors complexed azopyridines (AzPy) and molecular dihalogens (Fig. 11) in a... 

Supramolecular self-organization, with different amounts of complexity based upon dative bonds (see Section 3.2.3.1) and secondary bonds, is observed for organoindium halides. Long indium-halogen interatomic distances (although shorter than the expected van der Waals distance (ca. 3.8 A for In- -Cl)) are observed for most structurally characterized organoindium halides. 

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