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.

Aspartic acid adsorbed on Cu surfaces is doubly deprotonated. On chiral Cu(643) its enantiomers undergo enantiospecific decomposition via an autocatalytic explosion. Once initiated, the decomposition mechanism proceeds via sequential cleavage of the C3-C4 and C1-C2 bonds each yielding CO, followed by conversion of the remaining species into N[triple bond, length as m-dash]CCH.

Mechanisms for the spontaneous transformation of achiral chemical systems into states of enantiomeric purity have important ramifications in modern pharmacology and potential relevance to the origins of homochirality in life on Earth. Such mechanisms for enantiopurification are needed for production of chiral pharmaceuticals and other bioactive compounds. Previously proposed chemical mechanisms leading from achiral systems to near homochirality are initiated by a symmetry‐breaking step resulting in a minor excess of one enantiomer via statistical fluctuations in enantiomer concentrations. Subsequent irreversible processes then amplify the majority enantiomer concentration while simultaneously suppressing minority enantiomer production. Herein, equilibrium adsorption of amino acid enantiomer mixtures onto chiral and achiral surfaces reveals amplification of surface enantiomeric excess relative to the gas phase; i. e. enantiopurification of chiral adsorbates by adsorption. This adsorption‐induced amplification of enantiomeric excess is shown to be well‐describe by the 2D Ising model. More importantly, the 2D‐Ising model predicts formation of homochiral monolayers from adsorption of racemic mixtures or prochiral molecules on achiral surfaces; i. e. enantiopurification with no apparent chiral driving force.

The study of molecular chirality is essential to understanding the fundamentals of enantiospecific chemical interactions that are ubiquitous in the biochemistry of life on Earth. At a molecular level, there is insufficient understanding of chiral recognition and enantiomer-enantiomer interaction (aggregation) of chiral molecules adsorbed on surfaces. Here, using enantiospecific isotopic labelling and surface sensitive techniques, we show that when the two enantiomers of chiral aspartic acid (Asp) are adsorbed on the naturally chiral Cu(643)R&S surfaces, they decompose enantiospecifically depending on the chirality of the surface. The non-linear kinetics of the surface decomposition mechanism amplifies the difference between the decomposition rate constants of the two adsorbed enantiomers resulting in highly enantiospecific decomposition rates. Further, we also demonstrate that Asp enantiomers aggregate homochirally on several chiral and achiral surfaces, amplifying the enantiomeric excess on the surface with respect to that in the gas phase, |ees |>|eeg. Our results show that it is possible to discern the enantiospecific behavior of a complex adsorbate such as Asp and shed light on molecular level enantiospecific interactions on surfaces. The enantiospecific isotope labelling methods discussed in this paper allow probing of both the qualitative features of the Asp decomposition mechanism on Cu(643)R&S and quantitative aspects of the adsorption equilibria of enantiomer mixtures.

The 2D Ising model is well-formulated to address problems in adsorption thermodynamics. It is particularly well-suited to describing the adsorption isotherms predicting the surface enantiomeric excess, ees, observed during competitive co-adsorption of enantiomers onto achiral surfaces. Herein, we make the direct one-to-one correspondence between the 2D Ising model Hamiltonian and the Hamiltonian used to describe competitive enantiomer adsorption on achiral surfaces. We then demonstrate that adsorption from racemic mixtures of enantiomers and adsorption of prochiral molecules are directly analogous to the Ising model with no applied magnetic field, i.e., the enantiomeric excess on chiral surfaces can be predicted using Onsager’s solution to the 2D Ising model. The implication is that enantiomeric purity on the surface can be achieved during equilibrium exposure of prochiral compounds or racemic mixtures of enantiomers to achiral surfaces.