Structural geometries of halogen∙∙∙halogen synthons

The halogen bond continues to draw the attention on crystal engineering because of the scope it offers for design and application (Fig. 7). Non linear optics, optoelectronic transducers, ferro-, piezo- and pyro-electric materials are required for applications in chemistry, physics, material sciences and for important technological and industrial applications. Materials used for these purposes require polar molecules in polar crystals for necessary chemical and physical properties. The XB bonding are extremely important in crystal engineering to create sites beyond classic ones behaving as nucleophiles or electrophiles towards acids or basis. For example CHI3 is able to bond three bases creating a non centrosymmetric structure.