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Enantioselectivity at surfaces: covalent grafting as new tool for chiral surface modifications

Periodic Reporting for period 1 - EnSurf (Enantioselectivity at surfaces: covalent grafting as new tool for chiral surface modifications)

Reporting period: 2018-03-01 to 2020-02-29

The term chirality is derived from the Greek word for hand and describes objects which are non-superimposable with their mirror image. Two object related by this mirror symmetry are called enantiomers. Chiral objects can be found everywhere, ranging from artificial objects like screws to the shell of snails down to the molecular level, including all molecules of life, e.g. sugars, DNA and amino acids, which naturally appear only as one of the two possible mirror structures. When chiral molecules interact, their handedness can significantly affect the interaction and as a result opposite enantiomers of the same molecule can, for example, have different smells or in the case of pharmaceuticals different effects.
Therefore, the production of enantiopure molecules is particularly important in the pharmaceutical industry, which can be achieved for example by the separation of a racemic mixture via crystallization or separation using high-performance liquid chromatography (HPLC). In HPLC, the creation of homochiral surfaces and the interaction of chiral molecules with these surfaces play are major role. One approach to design and fabricate chiral surfaces is the adsorption and self-assembly of chiral or prochiral molecules on surfaces. Homochiral surfaces can be fabricated either by the adsorption of enantiopure chiral molecules, or by creating a bias in the adsorption of prochiral molecule. Characterization and visualization of these surfaces is possible with sub-molecular resolution by scanning probe microscopy, in particular scanning tunneling microscopy (STM). However, in STM studies dealing with chiral induction, the chiral dopants used are generally enantiopure chiral molecules, i.e. molecular point chirality is required for the fabrication of homochiral surfaces.
The overall objectives of this project were the exploration of new methods for chiral induction at surfaces and interfaces. We showed that chiral symmetry breaking based on lateral geometric nanoconfinement can be used to create enantioselectivity in molecular self-assembly under nanoconfinement. Thus, we introduced a new method for chiral symmetry breaking, in which the enantiomorph can be chosen freely during the nanoconfinement creation without the need for any molecular point chirality. In addition to enantioselectivity, the selective formation of concomitant polymorphs could be achieved in lateral nanoconfinement conditions.
The main part of the work has been performed on nanoconfinement at the solid/liquid interface and the impact of the nanoconfinement on two-dimensional (2D) molecular self-assembly. The nanoconfinement is created by a scanning probe lithography method. Therefore, highly-oriented pyrolytic graphite in covalently modified with aryl species, which prevent the physisorption and self-assembly of molecules from a supernatant solution. Subsequent removal of the covalently bound aryl species by scanning probe lithography allows self-assembly in a well-defined area.
In the first part, the 2D self-assembly of so-called prochiral molecules, i.e. molecules that can adsorb in two different mirror structures on a surface, under nanoconfined conditions has been investigated. By adjusting the relative orientation between the nanoconfinement and the substrate symmetry, a clear bias towards one of the two mirror domains could be created.
The self-assembly of a molecular system forming two concomitant polymorphs, i.e. two different structures forming under the same conditions, on pristine HOPG has been investigated. Thereby, the formation of one of the two polymorphs could be selectively induced by adjusting the orientation of the nanoconfinement.
The last part of the project is about enantioselective adsorption in nanoconfinement from a racemic mixture, i.e. the separation of enantiomers based on nanoconfinement. The results obtained so far indicate the selective adsorption of one enantiomer in the nanoconfinement.
The results have been published, or are about to be published, in prestigious international peer-reviewed scientific journals such as the Journal of the American Chemical Society and Angewandte Chemie – International Edition. Further, the results were presented at a number of international conferences, including the International Conference on Nanoscience and Technology in Brno, Czechia, Advanced Materials and Nanotechnology in Wellington, New Zealand, ChinaNano in Beijing, China, and Chirality at the Nanoscale in Ascona, Switzerland.
The potential of lateral geometric confinement to control molecular self-assembly at surfaces and interfaces has only been shown very recently. In this project, the confinement based control was extended to chiral systems as well as concomitant polymorphs. While the studies performed are of fundamental interest, they demonstrate that the creation of chirality is possible based on controlled confinement based chiral symmetry breaking.
Schematic for summary