Halogen bonding interaction: investigation through charge density studies and applications in the design of new functional materials

Halogen bonding, any noncovalent interaction involving halogens as electrophilic sites, is a relatively new item in the supramolecular toolbox and shares numerous properties with the better known hydrogen bonding. The topological analysis of the X-ray multipole refined charge density (CD) proved to be an effective tool to elucidate the nature of such interaction. This presentation will be mainly focused on the Br...N interaction in two different complexes of 1,4-dibromotetrafluorobenzene with dipyridyl derivatives, that is, 1,2-bis(4-pyridyl)ethylene and 4,4'-dipyridyl. Both experimental and theoretical CD distributions for the two cocrystals have been determined. The experimental CDs have been derived from X-ray data collected at 90 K, using the aspherical atom formalism of Stewart. The topological features of the Br...N interaction will be discussed and compared with those of the previously investigated I...N halogen bonding. The potentialities of halogen bonding in the rational design of new electrooptic materials will be illustrated by showing the results of an experimental and theoretical study on the NLO properties of a push-pull stilbene derivative, N,N-dimethyl-4-[(E)-2-(2,3,5,6-tetrafluoro-4-iodophenyl) vinyl]aniline, which self-organizes in the crystal giving rise to head-to-tail halogen bonded polymers. The second-order NLO response of such system in solution has been proven to be strongly influenced, in both magnitude and sign, by the halogen bond interaction.