Lord Kelvin's definition (1904): …I call any geometrical figure, or group of points, chiral, and say it has chirality, if its image in a plane mirror, ideally realized, cannot be brought to coincide by itself...

Chirality abounds in natural and synthetic systems, and also it is therefore not surprising that it is manifested in self-assembled monolayers at the liquid-solid interface, although observing it directly has been virtually impossible until relatively recently with modern tools such as atomic force microscopy (AFM) and scanning tunneling microscopy (STM). When molecules, chiral or achiral, adsorb on surfaces, two dimensional patterns (monolayers) can be formed which are (locally) chiral. On the macroscopic scale though, such surfaces are (most often) achiral for achiral molecules, though chiral for enantiomers.

Understanding how a stereogenic center influences conformations at the molecular scale and organization at the supramolecular level is an elusive and intriguing challenge in a number of scientific disciplines. We focus on surface-confined systems and study how the chiral nature of molecules affects the ordering at the liquid-solid interface. What drives molecules to form two-dimensional chiral patterns? What happens when equimolar mixtures of enantiomers are crystallized or are physisorbed at a surface? And what about the role of the solvent at the liquid-solid interface?

Example1

2D self-assembly of DBAs at the liquid/solid interface. a, Chemical structures of achiral and chiral DBAs. b, An STM image of the honeycomb structure of DBA-OC12 at the 1-phenyloctane/graphite interface (Iscl = 0.32 nA, Vbias = –0.17 V, 3.0 × 10–6 M). Molecular models are superimposed on the image. The white line indicates a domain boundary between the CW and CCW honeycomb structures. The black rhombi and six white arrows indicate the unit cell of the honeycomb structure and the main symmetry axes of graphite (1210 directions), respectively. c, Molecular models forming the + (upper) and – (lower) interdigitation motifs. d, The CW or CCW rotation of a nanowell is defined by the vectors pointing from the DBA core to the end of alkyl chain at the rim of a nanowell.

Example 2

STM image of an achiral oligo-p-phenylenevinylene derivative at the liquid-sold interface. Despite being achiral, this molecule self-assembles into chiral 'windmill' structures. Surprisingly, the supramolecular 'windmill' rotation depends on the chirality of the solvent.

Example 3

The separation of enantiomers - an important step in the production of optically active chemicals used in a variety of applications - is frequently performed using "classical" diastereomeric resolution, or Pasteurian resolution. We demonstrated diastereoselective adsorption on an achiral surface via surface-mediated complex formation and its in situ visualisation at the molecular level by employing STM at the liquid-solid interface. This discovery bodes well for the development of studies of diastereoselective phenomena at interfaces to deepen understanding of interfacial diastereomer complexes and their possible exploitation. The observation of this diastereoselectivity at an interface also opens the opportunity for the study of more complex resolution phenomena under similar conditions in dynamic systems. The results presented show the potential for STM to probe stereochemical processes usually associated with much larger scales, giving sub-molecular level information.

A cartoon depicting diastereoselective crystallisation at the liquid-solid interface. The hammer-like features symbolize the enantiopure resolving agents. The black and grey ovals represent the enantiomers of the racemate to be resolved.

Chemical structures of the chiral resorcinol derivative 1 and 1,2-diaminocyclohexane 2 (a). * indicates the location of a stereogenic centre. STM images of monolayers at the 1-phenyloctane - HOPG interface formed b) by (R)-1; c) & d) upon premixing 1 and 2 at a 2:1 ratio. c) (R)-1: (R,R)-2 (2:1), d) (S)-1 : (S,S)-2 (2:1). The solution composition is shown in a black text box with white text whereas the domain composition is given in black letters in a white text box. White solid lines indicate graphite main symmetry axes. Models of resorcinol and diamine molecules are superimposed on the STM image. e) Molecular model of the z-shaped (R)-1/(R,R)-2 self-assembled structure on graphite obtained via molecular mechanics simulations. The model at the right is the s-shaped (S)-1/(S,S)-2 structure which is obtained by reflection. The yellow dashed line is the graphite reference axis. f) Snapshot of representative structure (side view) of the core of the (R)-1/(R,R)-2 assembly (nitrogen in blue; oxygen in red).