From Homochiral Clusters to Racemate Crystals: Viable Nuclei in 2D Chiral Crystallization

Abstract

The quest for enantiopure compounds raises the question of which factors favor conglomerate crystallization over racemate crystallization. Studying nucleation and crystal growth at surfaces with submolecular-resolution scanning tunneling microscopy is a suitable approach to better understand intermolecular chiral recognition. Racemic heptahelicene on the Ag(100) surface shows a transition from homochiral nuclei to larger racemic motifs, although the extended homochiral phase exhibits higher density. The homochiral-heterochiral transition is explained by the higher stability of growing nuclei due to a better match of the molecular lattice to the substrate surface. Our observations are direct visual proof of viable nuclei.

Synthesis, Properties, and Two‐Dimensional Adsorption Characteristics of [6]Hexahelicene‐7‐carboxylic acid

“…As has been shown previously, such STM contrast is typical for helicenes and can be used to assign molecular handedness directly . Mapping out the features in a high‐resolution STM image (Figure A) with 1.2 nm circles (helicene dimensions), while maintaining the periodicity and symmetry of the pattern is relatively straightforward and led to a tentative molecular model shown in Figure B.…”

“…The model contains alternating rows of homochiral ( MM and PP ) hydrogen‐bonded helicene dimers, which means that the ordered self‐assembly of 1 is a racemic compound. Such alternating packing of homochiral dimers results in easily recognizable M ‐ P zig‐zag row motif, well‐known stable packing of racemic unsubstituted helicenes . Interestingly, despite the racemic composition, the 2D unit cell is asymmetric and mirror‐image domains are present on the Au(111) surface (Figure C).…”

Double layer crystallization of heptahelicene on noble metal surfaces

“…Interestingly, the same motif was identified for 5‐amino[6]helicene on the same surface, 15 but by alloying with tin racemate formation was induced 16 . A conglomerate‐to‐racemate transition was also observed on Ag(100) for [7]H. Enantiopure quadruplets at small coverage turned into racemic zigzag rows with increasing coverage 17 . Moreover, cyano‐functionalized [7]H forms a 2D conglomerate on Cu(111), 18 while bromo, benzo, and S ‐acetylthiolate functionalized [7]H formed zigzag racemate structures 19 .…”

One‐Dimensionally Disordered Chiral Sorting by Racemic Tiling in a Surface‐Confined Supramolecular Assembly of Achiral Tectons

“…Notably, a strain–relaxation induced conglomerate– racemate transition was previously observed in the ( M )‐ and ( P )‐heptahelicenes on Ag(001) . Such strain‐related mechanisms can be excluded in our case as homochiral, heterochiral, and racemic structures are simultaneously observed for small domains (Figure , Figure a; Supporting Information, Figure S4), and the molecule–molecule and molecule–substrate interactions therein are identical.…”