Achieving chiral resolution in self-assembled supramolecular structures through kinetic pathways

Tailoring long-range superlattice chirality in molecular self-assemblies via weak fluorine-mediated interactions

Controllable fabrication of enantiospecific molecular superlattices is a matter of imminent scientific and technological interest. Herein, we demonstrate that long-range superlattice chirality in molecular self-assemblies can be tailored by tuning the interplay of weak intermolecular non-covalent interactions between hexaphenylbenzene-based enantiomers. By means of high-resolution scanning tunneling microscopy measurements, we demonstrate that the functionalization of a hexaphenylbenzene-based molecule with fluorine (F) atoms leads to the formation of molecular self-assemblies with distinct long-range chiral recognition patterns. We employed density functional theory calculations to quantify F-mediated lone pair F⋯π, C-H⋯F, and F⋯F interactions attributed to the distinct enantiospecific molecular self-organizations. Our findings underpin a viable route to fabricate long-range chiral recognition patterns in supramolecular assemblies by engineering the weak non-covalent intermolecular interactions.

Thermally Induced Chiral Aggregation of Dihydrobenzopyrenone on Au(111)

The realization of chiral supramolecular architectures on solid surfaces has triggered much interest due to its potential enantiospecific applications. An in-depth study of chiral aggregation on surfaces is significant for developing functional chiral surfaces. Herein, we report thermally induced chiral aggregation of dihydrobenzopyrenone on Au(111). By high-resolution low-temperature scanning tunneling microscopy, a racemate monolayer consisting of levorotatory and dextrorotatory dihydrobenzopyrenones was found to aggregate into conglomerate domains after moderate annealing treatment. Combined with first-principles calculations, we suggest that the intermolecular dipole-dipole interaction plays an important role in chiral aggregation, which takes place via molecular in-plane diffusion rather than molecular out-of-plane flipping. This work unveils one underlying mechanism of thermally induced chiral aggregation, thus enabling potential applications such as fabricating supramolecular architectures for functional chiral surfaces.

Observations of Gradual Chiral Self-Recognition of Adsorbed Aromatic Compound

The self-assembly of two-dimensional chiral 1 H,5 H-benzo(1,2- d:4,5- d')bistriazole (H₂bbta) on a Ag(110) surface was investigated by ultra-high-vacuum scanning tunneling microscopy. The gradual formation of ordered structures by H₂bbta molecules with the same chirality recognizing each other was observed as the annealing temperature was increased from 300 to 333 K. When the sample was annealed at 355 K, the homochiral structures were converted to coexisting structures containing λ-H₂bbta and δ-H₂bbta in a ratio of 6:1. Density functional theory (DFT) calculations revealed that thermally driven and intermolecular interactions induced chiral self-recognition to form enantiomorphous H₂bbta structures in which N-H···N hydrogen bonds and C-H···N hydrogen bonds are the main attractive forces.

Enantiomer surface chemistry: conglomerate versus racemate formation on surfaces

Research on surface chirality is motivated by the need to develop functional chiral surfaces for enantiospecific applications. While molecular chirality in 3D has been the subject of study for almost two centuries, many aspects of 2D chiral surface chemistry have yet to be addressed. In 3D, racemic mixtures of chiral molecules tend to aggregate into racemate (molecularly heterochiral) crystals much more frequently than conglomerate (molecularly homochiral) crystals. Whether chiral adsorbates on surfaces preferentially aggregate into heterochiral rather than homochiral domains (2D crystals or clusters) is not known. In this review, we have made the first attempt to answer the following question based on available data: in 2D racemic mixtures adsorbed on surfaces, is there a clear preference for homochiral or heterochiral aggregation? The current hypothesis is that homochiral packing is preferred on surfaces; in contrast to 3D where heterochiral packing is more common. In this review, we present a simple hierarchical scheme to categorize the chirality of adsorbate-surface systems. We then review the body of work using scanning tunneling microscopy predominantly to study aggregation of racemic adsorbates. Our analysis of the existing literature suggests that there is no clear evidence of any preference for either homochiral or heterochiral aggregation at the molecular level by chiral and prochiral adsorbates on surfaces.

Spontaneous symmetry breaking on ordered, racemic monolayers of achiral theophylline: formation of unichiral stripes on Au(111)

We report the observation of spontaneous chiral symmetry breaking within ordered, racemic monolayers of theophylline, manifesting itself as extended, nanoscale unichiral stripes at the interface between molecular domains. Theophylline is a xanthine derivative playing an important role in several biochemical processes. Molecular chirality is induced by adsorption on the Au(111) surface, resulting in extended domains with two different racemic, ordered structures, coexisting with a disordered phase. By combining low-temperature scanning tunneling microscopy (LT-STM) and ab initio density functional theory calculations, we first provide a detailed picture of the interactions within the ordered assemblies, and we uncover the origin of the distinct contrast features in STM images. Secondly, experiments reveal the existence of nanoscale stripes of unichiral molecules separating racemic domains of one of the two ordered phases, giving rise to a local enantiomeric imbalance. Systematic theoretical investigation of their structure and chiral composition confirm their unichirality, with the specific handedness related to the registry between the two ordered domains facing the stripes. These findings can open the way to new insights into the elusive mechanisms leading to local chiral imbalances in racemic systems, possibly at the origin of biomolecular homochirality, as well as suggest novel approaches for stereoselective heterogeneous catalysis.

Self-assembly of pyrene derivatives on Au(111): substituent effects on intermolecular interactions

The adsorption behaviour as well as the influence of bromine substituents on the formation of highly-ordered two-dimensional structures of pyrene derivatives on Au(111) are studied by a combination of scanning tunnelling microscopy (STM) and density functional theory (DFT) calculations.

Supramolecular Cl⋅⋅⋅H and O⋅⋅⋅H interactions in self-assembled 1,5-dichloroanthraquinone layers on Au(111)

The role of halogen bonds in self-assembled networks for systems with Br and I ligands has recently been studied with scanning tunneling microscopy (STM), which provides physical insight at the atomic scale. Here, we study the supramolecular interactions of 1,5-dichloroanthraquinone molecules on Au(111), including Cl ligands, by using STM. Two different molecular structures of chevron and square networks are observed, and their molecular models are proposed. Both molecular structures are stabilized by intermolecular Cl⋅⋅⋅H and O⋅⋅⋅H hydrogen bonds with marginal contributions from Cl-related halogen bonds, as revealed by density functional theory calculations. Our study shows that, in contrast to Br- and I-related halogen bonds, Cl-related halogen bonds weakly contribute to the molecular structure due to a modest positive potential (σ hole) of the Cl ligands.

Polymorphic porous supramolecular networks mediated by halogen bonds on Ag(111)

Intermolecular structures of porous two-dimensional supramolecular networks are studied using scanning tunnelling microscopy combined with density functional theory calculations. The local configurations of halogen bonds in polymorphic porous supramolecular networks are directly visualized in support of previous bulk crystal studies.