Molecular Alignment and Chiral Bias under Dynamic Nanoconfinement Conditions: The Impact of Molecular Symmetry

Abstract

When molecules assemble in self-assembled molecular networks on highly oriented pyrolytic graphite, typically six orientational domains are formed, which can be grouped into two sets of three, each set characteristic of a packing chirality. Under dynamic nanoconfinement conditions, where the scanning tunneling microscopy (STM)-based lithography process gradually increases the area available for molecule adsorption and self-assembly, the orientational domain equivalence is often disrupted. Additionally, this phenomenon can sometimes be accompanied by the selective formation of one of the two enantiomorphic packings. Until now, the preferential orientation of domains, sometimes accompanied by the favored formation of enantiomorphic packing patterns, has been investigated for anisotropic molecules with a single alkyl chain. We reasoned that a system where different alkyl chains may adsorb along various directions on the substrate, while also adopting different orientations concerning the line-wise movement of the STM tip, can potentially hinder molecular alignment in the nanocorrals and eventually limit the extent of enantiomorph-selective adsorption. To address this hypothesis, in this study, we investigate the self-assembly of an achiral molecule with four alkyl chains, in pairs directed along two different directions, under dynamic nanoconfinement conditions. Despite of this increased molecular structural complexity, dynamic nanoconfinement conditions lead to a clear bias in the formation of rotational domains, as well as partial chiral selection, although in a complex fashion. The factors that contribute to this bias are discussed. This highlights the significance of utilizing dynamic nanoconfinement conditions to control domain orientation and to disturb chirality equivalence, even for intricate systems.

Chiral induction in substrate-supported self-assembled molecular networks under nanoconfinement conditions

“…By carefully controlling the orientation of nanocorrals with respect to the symmetry lattice of the substrate, the area of HOPG exposed by corrals has a precise symmetry and this feature has been demonstrated useful in favoring the formation of structures having a precise chirality. [31][32][33] Note though that the unit cell vector a of the targeted domain is not always perfectly parallel to the lateral side of the corral. This is because the shaving direction is determined by taking the self-assembly contained in previously created corrals as a reference point.…”

“…Using this approach it was possible to preferentially form one out of two possible enantiomorphs starting from prochiral 31 and achiral molecules. 32 Furthermore, the same strategies have been proved effective in discriminating between enantiomers forming a racemic mixture, by adsorbing preferentially one of them inside these nanocontainers. 33 Nanoshaving to produce nanocontainers, also referred to as nanocorrals, is particularly intriguing because it induces chirality by combining achiral elements.…”

Tuning Preferential Molecular Adsorption and Self-Assembled Molecular Network Formation at the Liquid–Solid Interface: The Impact of Annealing and Confinement

“…Nanoconfinement conditions were formed by creating so-called corrals of which the size is approximately 150 × 150 nm 2 . These corrals were formed using a method called “nanoshaving”, a process in which covalently attached molecules, i.e., 3,4,5-trimethoxyphenyl groups, are locally removed from the surface with the STM tip at high tunneling currents. ,,, In situ nanoshaving was used, meaning that the nanoshaving takes place in the liquid phase. As a result, SAMN formation may occur simultaneously with the nanoshaving process as an increasing area of pristine HOPG is exposed to the liquid.…”

“…Self-assembly of organic molecules in two dimensions (2D), i.e., the formation of self-assembled molecular networks (SAMNs), on solid surfaces such as highly oriented pyrolytic graphite (HOPG), gold, silver, and other metals is a promising method for constructing well-defined patterns at the molecular level, with a high level of control. − This has attracted the interest from diverse areas ranging from supramolecular chemistry to nanoscience and nanotechnology. − The potential of the approach to engineer surfaces and interfaces holds promise for applications in fields as diverse as electronics, sensing, and tailored catalysis. − These SAMNs formed on a conductive solid surface are usually investigated using scanning tunneling microscopy (STM), either at ambient or under ultrahigh-vacuum conditions. − This research field has witnessed rapid development in understanding how molecular and interfacial interactions can influence the arrangement of molecules. Diverse nanostructures have been realized ranging from monolayers to bilayers or multilayers, from monocomponent linear architectures to multicomponent host-guest porous networks, and many others. − …”