Stereochemical recognition of helicenes on metal surfaces
The chiral recognition among biomolecules is fundamentally important for many processes of life, including the stereochemistry of evolution. Of special interest is chiral recognition during crystallization of racemates, when either homochiral recognition leads to a conglomerate of homochiral crystals or heterochiral recognition dominates resulting in a racemic compound. The complex nature of molecular recognition at the level of nucleation and crystal growth renders it difficult to understand and calls for manageable model systems. Notably, the approach of studying aggregation of molecules at surfaces under well-defined conditions includes the benefit of the availability of a multitude of highly sensitive investigation methods, of which scanning tunneling microscopy (STM) with its submolecular resolution is tremendously valuable. Heterogeneous nucleation at surfaces is strongly favored over homogeneous nucleation in solution; hence, surfaces are significantly involved in stereochemical recognition during crystallization. Helicenes are a fascinating class of chiral compounds with outstanding optical activity. These π-conjugated, ortho-fused, aromatic hydrocarbons are promising candidates for organic electronic devices such as sensors, circular dichroic photonics, liquid crystal displays or spin filters. But in particular the defined footprint of their terminal benzo rings on a surface makes them interesting for studying stereochemical recognition with different single crystalline surfaces and the impact this has, in turn, on intermolecular recognition.
In this Account, we describe the self-assembly of helicenes on metal surfaces with the focus on stereochemical recognition in two-dimensional structures. Using the isomeric all-carbon helicenes, heptahelicene and dibenzohelicene as examples, different aggregation phenomena on different surfaces of single crystalline copper, silver, and gold are investigated. By means of STM different modes of transmission of molecular handedness from single molecules into extended two-dimensional supramolecular structures are identified. For the problem of racemate versus conglomerate crystallization, the impact of surface and molecular structure and their interplay are analyzed. This leads to detailed conclusions about the importance of the match of molecular and surface binding sites for long-range self-assembly. The absence of polar groups puts emphasis on van der Waals interaction and their maximization by steric overlap of molecular parts in enantiomeric and diastereomeric interactions. With STM as a manipulation tool, dimers are manually separated in order to analyze their chiral composition. And finally, new nonlinear cooperative effects induced by small enantiospecific bias are discovered that lead to single enantiomorphism in two-dimensional racemate crystals as well as in racemic multilayered films. By means of these model studies many details that govern chiral recognition at surfaces are rationalized.