Chiral control in Soft Monolayers

The formation of chiral structures in Langmuir monolayers can take place starting from chiral or achiral amphiphilic molecules. In this last case, the influence of chiral forces of different strength and nature, physical or chemical, is essential to select and control the handedness of the monolayer.

It is well known that the presence of chirally active dopants and true chiral forces induces, and in some cases selects, the handedness of supramolecular aggregates. In our group, we have taken the Langmuir monolayers of some azobenzene derivatives as a two-dimensional model to analyze the effect of chiral forces. The key feature of these amphiphiles is the cis-trans photoisomerization. Their monolayers, in absence of any chiral influence and under precise experimental and preparation conditions, display a racemic bi-dimensional emulsion of clockwise and counterclockwise trans-bend-like domains surrounded by an isotropic pool of the cis isomer. The presence of chiral dopants, a chemical chiral influence, and the application of vortical stirring through the subphase, a true physical chiral force, during the formation of these chiral domains breaks their balance promoting an enantiomorphic excess of clockwise or counterclockwise domains. This result can be useful to understand some aspects of the origin of the terrestrial homochirality or to prepare heterogeneous catalysts.