Amplification of chirality in two-dimensional enantiomorphous lattices

Abstract

The concept of chirality dates back to 1848, when Pasteur manually separated left-handed from right-handed sodium ammonium tartrate crystals. Crystallization is still an important means for separating chiral molecules into their two different mirror-image isomers (enantiomers), yet remains poorly understood. For example, there are no firm rules to predict whether a particular pair of chiral partners will follow the behaviour of the vast majority of chiral molecules and crystallize together as racemic crystals, or as separate enantiomers. A somewhat simpler and more tractable version of this phenomenon is crystallization in two dimensions, such as the formation of surface structures by adsorbed molecules. The relatively simple spatial molecular arrangement of these systems makes it easier to study the effects of specific chiral interactions; moreover, chiral assembly and recognition processes can be observed directly and with molecular resolution using scanning tunnelling microscopy. The enantioseparation of chiral molecules in two dimensions is expected to occur more readily because planar confinement excludes some bulk crystal symmetry elements and enhances chiral interactions; however, many surface structures have been found to be racemic. Here we show that the chiral hydrocarbon heptahelicene on a Cu111 surface does not undergo two-dimensional spontaneous resolution into enantiomers, but still shows enantiomorphism on a mesoscopic length scale that is readily amplified. That is, we observe formation of racemic heptahelicene domains with non-superimposable mirror-like lattice structures, with a small excess of one of the heptahelicene enantiomers suppressing the formation of one domain type. Similar to the induction of homochirality in achiral enantiomorphous monolayers by a chiral modifier, a small enantiomeric excess suffices to ensure that the entire molecular monolayer consists of domains having only one of two possible, non-superimposable, mirror-like lattice structures.

Stirring competes with chemical induction in chiral selection of soft matter aggregates

“…In particular, one anticipates that achiral substances can be transformed, at most, into a racemic mixture of equally represented enantiomers, or into an enantiopure system with equal probability for the two enantiomers. Apart from the handedness imparted by proto-chiral organizing surfaces 3 , oriented reacting interfaces 4 , and aligning 5 or confining 6 boundaries, the balance above may be broken under the intervention of one within a few physical forces of signed helicity 7,8 , or through the effect of a chiral chemical modifier. In general terms, there is still the pending issue of whether a chiral physical force may be opposed in an effective way to a chiral chemical intervention.…”

Coexisting Chiral Two-Dimensional Self-Assembled Structures of 1,2,3,4-Tetrahydronaphthalene Molecules: Porous Pinwheel Nanoarchitecture and Close-Packed Herringbone Arrangement

“…49 Chiral nanoarchitectures can emerge from molecular self-assembly. 50,51 2D chirality can result from the self-assembly of chiral building blocks 52,53 and prochiral building blocks. 54−56 In that case, the chirality of the molecule is transferred to the molecular assembly.…”

Expression and Amplification of Chirality in Two-Dimensional Molecular Crystals

“…2). [13] Nevertheless, mirror domains are observed via LEED and STM. [6b,13] Overall, three pairs of enantiomorphous structures, denoted as ε/δ, λ ' /ρ' .…”

“…The chiral bias from a small ee is sufficient in order to suppress formation of one unfavorable mirror domain and induces homochiral lattice order. [13] Because of strong steric constraints, any ee is expelled from the racemic enantiomorphous domains during crystallization. From the domain edges, however, the excess molecules have an influence on the relative alignment of the heterochiral pairs at the domain edge.…”