Square networks based on the Br······NO2 supramolecular synthon

·O brid - ges that are promoted by the activation of the C-H groups in these electron-deficient systems. The Br······NO2 interactions act as one -dimensional connec- tors and, in this sense, may be termed as supramole - cular synthons. A networking of these polyfunctional molecules leads to two -dimensional square and hexa- gonal networks. In one favourable case, the overall crystal packing is in a non -centrosymmetric space group and the alignment of chromophores leads to significant SHG properties. Even for small, simple molecules of the type studied here, crystal structure prediction is difficult and no two compounds have exactly the same crystal structure.