Amplification of chirality in surface-confined supramolecular bilayers

Hai Cao,Steven De Feyter

doi:10.1038/s41467-018-05962-3

Abstract

One of the most dramatic effects of supramolecular assembly is the generation of homochirality in near-racemic systems. It is normally infeasible though to flip the absolute chirality of a molecule. Here we rationalize this seemingly contradictory chiral amplification mechanism with a combined scanning tunneling microscopy (STM) and modeling study of surface-grown enantiomerically unbalanced supramolecular bilayers. We identify a chemical equilibrium between opposite but not mirror-image-related twisting molecular geometries of the pure enantiomer, and accordingly two competing aggregation pathways. The nonlinear chiral amplification effect in bilayers of near-racemic mixtures involves the biased adsorption and organization of the majority enantiomer, and the compliance of the minority enantiomer to adopt an energetically less favorable twisting molecular conformation and handed organization. By establishing a direct link between molecular building block architectures and chiral amplification effect, this study provides a general approach to gain insight into cooperative supramolecular assembly in mixed enantiomer systems.

Highlights

One of the most dramatic effects of supramolecular assembly is the generation of homochirality in near-racemic systems
One particular promising avenue for the amplification of chirality is via cooperative supramolecular assembly[7,8,9,10], whereby a small enantiomeric excess at the molecular level is able to steer the aggregation of a near-racemic system toward homochiral assemblies
While high-resolution local probe microscopies such as scanning tunneling microscopy (STM) can be applied to examine twodimensional (2D) chiral networks at the molecular scale on surfaces[17,18,19,20,21,22], so far only few particular cases on chiral amplification in mixed enantiomer systems have been reported[23,24,25], all of them focussing on monolayer systems

Summary

Introduction

One of the most dramatic effects of supramolecular assembly is the generation of homochirality in near-racemic systems. Despite the variability in molecular conformation and 1D organization, as indicated by the molecular modeling approach, experimentally only parallel-arranged oblique 1D arrays were observed for enantiopure (L)-FAC18 and (D)-FAC18 upon depositing their 1phenyloctane solutions (c = 0.2 mM) onto the surface of highly ordered pyrolytic graphite (HOPG), as revealed by STM (Fig. 4a–e).

Results

Conclusion