RESUMO
Smart materials represent an elegant class of (macro)-molecules endowed with the ability to react to chemical/physical changes in the environment. Herein, we prepared new photo responsive azobenzenes possessing halogen bond donor groups. The X-ray structures of two molecules highlight supramolecular organizations governed by unusual noncovalent bonds. In azo dye I-azo-NO2, the nitro group is engaged in orthogonal H···O···I halogen and hydrogen bonding, linking the units in parallel undulating chains. As far as compound I-azo-NH-MMA is concerned, a non-centrosymmetric pattern is formed due to a very rare I···π interaction involving the alkene group supplemented by hydrogen bonds. The Cambridge Structural Database contains only four structures showing the same I···CH2=C contact. For all compounds, an 19F-NMR spectroscopic analysis confirms the formation of halogen bonds in solution through a recognition process with chloride anion, and the reversible photo-responsiveness is demonstrated upon exposing a solution to UV light irradiation. Finally, the intermediate I-azo-NH2 also shows a pronounced color change due to pH variation. These azobenzenes are thereby attractive building blocks to design future multi-stimuli responsive materials for highly functional devices.
RESUMO
Two decades ago, halogen bonding attracted considerable attention as a new type of non-covalent interaction and has now emerged as an important supramolecular tool for crystal engineering, demonstrating its ability to self-assemble low affinity entities. Specific features of halogen bonds have been subsequently harnessed in biological and material sciences to give rise to elegant functional systems with a wide range of applications. In this highlight, we focused on new concepts in polymer science, electrochemistry, electronic and sensing materials. Recent findings on the use of halogenated compounds for transmembrane transport are also discussed, together with a short summary of the current understanding of the nature of the halogen bond, in the light of modern computational tools and energy decomposition.