Stereochemical Recognition of Helicenes on Metal Surfaces

Abstract

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.

The Critical Role of Tilted Cinchona Surface Species for Enantioselective Hydrogenation

“…[3][4][5] The molecular orientation of adsorbates also plays a crucial role in asymmetric catalysis as the enantiospecific adsorption of the prochiral reactant determines the enantioselectivity. [6][7][8] A practically very simple approach to transfer enantiocontrol to a conventional achiral hydrogenation catalyst is to modify the noble metal with a suitable organic chiral molecule, referred to as chiral modifier. 9,10 Amongst various chiral organic compounds the naturally-occurring pseudo-enantiomers cinchonine (CN) and cinchonidine (CD) stand out as efficient chiral modifiers.…”

Imaging on-surface hierarchical assembly of chiral supramolecular networks

“…Due to the lack of lateral functional groups, such interchain coupling is expected to be mediated by van der Waals forces. 61 All the mechanisms are highly enantioselective and work identically for both molecular chiralities. Concerning the organometallic coordination, the clear correlation between molecular chirality and chain orientation with respect to the Cu(111) crystallographic directions (see Fig.…”

Fivefold Symmetry and 2D Crystallization: Self‐Assembly of the Buckybowl Pentaindenocorannulene on a Cu(100) Surface

“…There is ar easonable expectation that theser ules will hold in general for carbon-rich nonplanar graphenoidss uch as fullerenef ragments or helicenes. [32,33] Molecular tiles with geometrically orthogonal segments offer ac ontrol mechanism in which the segment best suited to optimize the surface coverage will bind flat, and its orthogonal segment will extend normalt othe surfacep lane.T he shaper ecognition of the surface-binding portion should result in tilings with regularly spaced features such as gratings or boxes.…”

“…Hence, a consistent picture of polynuclear aromatic hydrocarbons binding to metal surfaces is developing, in which surface area and benzenoid‐ring disposition play an important role in the nature of binding. There is a reasonable expectation that these rules will hold in general for carbon‐rich nonplanar graphenoids such as fullerene fragments or helicenes . Molecular tiles with geometrically orthogonal segments offer a control mechanism in which the segment best suited to optimize the surface coverage will bind flat, and its orthogonal segment will extend normal to the surface plane.…”