Two-Dimensional Crystallization of Enantiopure and Racemic Heptahelicene on Ag(111) and Au(111)

Abstract

Insight into recognition among helical molecules is highly relevant for understanding chiral separation or biomolecular activity. The two-dimensional selfassembly of enantiopure and racemic heptahelicene has been studied on the (111) surfaces of silver and gold by means of scanning tunneling microscopy. As found earlier for racemic heptahelicene on Cu(111), the racemate forms zigzag rows with alternating enantiomers. In contrast to Cu(111), no enantiomorphism in the form of oblique alignment of molecular adlattice vectors with respect to substrate lattice vectors is observed. That is, the zigzag rows run parallel to the high-symmetry directions of the substrate. Because of the larger interatomic distances, stress builds up in the racemic molecular layer on Ag(111), which leads to relaxation into different structures after every third or fourth zigzag row. For enantiopure (M)-heptahelicene, identical structures at monolayer saturation coverage are observed, showing a pronounced transmission of chirality via azimuthal alignment of adjacent molecules.

Synthesis, Properties, and Two‐Dimensional Adsorption Characteristics of 5‐Amino[6]hexahelicene

“…At the liquid–solid interface, the assembly depends drastically on the solvent, most likely due to its role in the determination of the protonation state of the −NH 2 group. In fact, no order emerges in solvents in which the NH 2 group is protonated, whereas the assembly is very similar to that observed for the nonfunctionalized heptahelicene on Au(111) in neutral solvents . For the racemate, incomplete spontaneous resolution was proposed to be a consequence of kinetic trapping of metastable racemic polymorphs due to slow surface diffusion.…”

“…In fact, no order emergesi ns olvents in which the NH 2 group is protonated, whereas the assembly is very similar to that observed for the nonfunctionalized heptahelicene on Au(111)i nn eutrals olvents. [49] For the racemate, incomplete spontaneous resolution was proposed to be ac onsequence of kinetic trapping of metastable racemic polymorphs due to slow surfaced iffusion. The high packing density of the racemate and the absence of enantiopure phases indicate that the driving force for the assembly under UHV conditions is probably relatedt ot he thermodynamic stability of the phase, similar to the 3D crystal in which the racematei sm ore stable, and therefore no resolutiono ccurs.…”

Double layer crystallization of heptahelicene on noble metal surfaces

“…Most relevant here, however, is the previously reported 2D aggregation of [7]H on the three (111) surfaces of Cu, Ag, and Au 22,23 . In all three cases, zigzag rows with alternating enantiomers were observed for the racemate at coverages up to one monolayer.…”

“…On Au(111) [7]H grows into ( M)/(P) zigzag rows, aligned parallel to the three high‐symmetry directions of the surface 23 . Once the coverage on the surface exceeds the amount of molecules that can be accommodated into the first molecular layer, double layer crystallization occurs.…”

“…The rac ‐[7]H first layer zigzag rows on Au(111) and Ag(111) were reported to be aligned parallel to the surface direction 23 . Interestingly, enantiomorphism is established due to the double layer crystallization on Au(111), as the second layer lattice vectors now show an oblique tilt angle with respect to the surface direction.…”