Chirality is ubiquitous in the molecular world. Chiral recognition has an important impact on various processes such as enantioselective catalysis, crystallization-induced separation of chiral molecules into their enantiomers and the design of non-linear optics and spin filters. Unfortunately, our current understanding of chiral intermolecular interactions is still poor and calls for the study of adequate model systems. The use of scanning tunneling microscopy (STM) on well-defined metal single crystals allows to gather information with sub-molecular resolution. We present an elucidation of chiral recognition of different helicenes (a type of helical non-planar aromatic hydrocarbons) in monolayered and multilayered films on a Cu (111) surface. Starting at low coverages, racemic pentahelicene ([5]H, C22H14), form non-covalently-bonded homochiral dimers, i.e. both molecules have the same sense of helicity. Close to monolayer coverage, a transition from a 2D racemate, which means that it is composed of homochiral pairs with opposite handedness, to a conglomerate of homochiral mirror domains is observed in the first layer. This phenomenon is induced by second layer nucleation and indicates a long-range chiral communication between second layer islands and other areas on the surface. A similar effect was also observed for heptahelicene ([7]H, C30H18), which, in addition may show single enantiomorphism in multilayers via the so-called ‘Sergeants-and-Soldiers’ mechanism induced by small enantiomeric excess. Both systems show a drastic transformation of the self-assembly due to a small enantiomeric bias.

Chiral recognition among helical non-planar aromatic hydrocarbons on metal surfaces

255th National Meeting (2018)

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