Angew. Chem. Int. Ed. http://doi.org/fz6jrh (2012)

A tiny and usually undetectable amount of chiral contaminant can induce racemic or achiral mixtures to become optically active as the molecules in solution self-assemble into supramolecular aggregates. Observations of chiral amplifications of this type have been reported by several groups and may help explain the emergence of homochirality in nature. Now, Rint Sijbesma, Anja Palmans, Bert Meijer and colleagues at the Eindhoven Institute of Technology have discovered a unique chiral amplification that takes place in two consecutive steps.

The researchers synthesize a partially fluorinated achiral compound composed of three branching units stemming off a benzene core. One of these units contains an alkyl chain that is completely fluorinated, except for one terminal hydrogen. In solution, as the temperature is decreased, the molecule aggregates to form a racemic mixture of helical structures in a self-assembly process driven by hydrogen-bond interactions. However, on further cooling, larger aggregates form through hierarchical self-assembly of the helices to produce fibre bundles that are about 60 nm in diameter. These nanostructures are found to provide an optically active solution.

The second self-assembly step is driven by the intermolecular dipole–dipole interaction between the electron-poor terminal hydrogen and the fluorine atoms from neighbouring helices. However, the predominant handedness of the bundles seems to be influenced by chiral traces.