Resolving transformations of molecules and their chirality in the submolecular scale

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Scientists from the Institute of Physics and the Institute of Organic Chemistry and Biochemistry of the Czech Academy of Sciences observed a chemical transformation of individual molecules on the silver surface and demonstrated chirality transfer during the reaction. They employed the latest advances of scanning probe microscopy, which allows scientists to determine the chemical bond between individual atoms within molecules and thus determine their molecular structure and the chirality. The results were published in the prestigious journal Nature Chemistry.

Chirality is a geometrical property an object (usually a molecule or ion) that cannot be matched with its mirror image. Chirality plays a key role in nature and can be demonstrated for example on the relationship of right and left hand, which are not identical in terms of symmetry. The phenomenon of chirality is important in many areas e.g. stereoselective reactions, self-assembly of molecules, biological processes (where proteins, nucleic acids or polysaccharides are involved), the polarization of light or electron spin. Control of chirality in chemical reactions in solutions represents one of the greatest achievements of organic chemistry in the last fifty years.

Molecules that are not chiral in solution or gas phase (i.e. prochiral), may become chiral under certain conditions, specifically after adsorption to a solid surface, when a so-called chiral adsorbate is formed. The breakthrough method, that allows one to create an extensive two-dimensional molecular layer of chosen chirality, was developed through the combined efforts of researchers from the Institute of Physics and the Institute of Organic Chemistry and Biochemistry (IOCB Prague) of Czech Academy of Sciences, led by Dr Pavel Jelinek and Dr Ivo Starý. This achievement represents the first practical demonstration of the possibility that prochiral molecules at solid surfaces occupy either right or left-handed orientation on purpose.

Namely, the Czech scientists achieved that the adsorbed molecules adopted a single chirality in their whole monolayer, by using a thermally controlled chemical transformation of chiral helical molecules, so-called Helicenes to planar polyaromatic molecules (see Fig. 1). The transformation was made on the surface of a silver crystal. Moreover, the scientists could accurately determine a sequence of multi-step chemical transformations of the molecules using the atomic force microscope that operates at temperatures close to the absolute zero and under conditions of ultra-high vacuum. They managed to identify both intermediates and final products of the cascade of chemical reactions, employing the unique submolecular resolution that allowed the direct deduction of the molecular structures of the relevant molecules (see Fig. 2).

Fig.1. Schematic view of the chirality-preserving transformation of molecules deposited on a solid surface (A) using a temperature-controlled chemical reaction (T1). This reaction allows to get the same chirality of adsorbed molecules across the whole monolayer (green frame). Importantly, this chiral arrangement cannot be reached by simple deposition of the prochiral molecules onto the surface (B and C).

This work represents a revolutionary approach enabling in-depth studies of chemical reactions on solid state surfaces and a precise determination of the chemical structure of molecules, with the possibility to track conversions of their chirality. The original method opens up new possibilities for the preparation of well-defined chiral surfaces that have significant potential for use in the field of heterogeneous catalysis, sensors of chirality, molecular electronics, spintronics, photonics and biochemistry.

Fig.2. Molecular structures of polyaromatic molecules obtained during different stages of the chemical transformation on a crystalline silver surface (a-c). The accurate determination of the chemical structure of the individual products was carried out using scanning probe microscopes with high spatial resolution (d-k,l-o). The cascade of chemical reactions transforming the initial helicene molecules (DBH) into various intermediates and final products is schematically illustrated in (p).

From helical to planar chirality by on-surface chemistry
O. Stetsovych, M. Švec, J. Vacek, J. Vacek Chocholoušová, A. Jančařík, J. Rybáček, K. Kosmider, I. G. Stará, P. Jelínek, I. Starý
Nature Chemistry
DOI: 10.1038/nchem.2662