Spontaneous discrimination of polycyclic aromatic hydrocarbon (PAH) enantiomers on a metal surface.

Stereochemistry is an issue of great relevance in both technological and fundamental disciplines. For instance, many biological molecules present a characteristic chiral structure, which make them suitable for targeting singleenantiomer drugs. Also of basic importance in the framework of the origin of life, it is accepted that a first enantiomeric selection process took place at some stage on the early Earth and resulted in exclusively l-amino acids being incorporated into the proteins of living beings. In these two examples, the enantiomeric separation is performed either between individual molecules or between a molecule and a surface without the need for clustering enantiomorphic molecular crystals. Recently, the role played by the surface in the adsorption of molecular chiral species has been emphasized. For example, it has been shown that amino acids can be enantiospecifically adsorbed at chiral surface sites, and that a surface is capable of forming chiral domains from the adsorption of achiral molecules or amplifying the separation of enantiomorphous molecular networks. These examples show the capability of surfaces to provide an effective template for chiral molecular discrimination, with double fundamental and technological significance. However, in most of the previous studies the molecule–molecule interactions are the cause of chiral recognition. Herein we report on a confined surface-mediated recognition process, in which the local surface–molecule interaction exclusively results in enantiomeric discrimination, and we present large-scale DFT calculations to explain the atomistic mechanism behind the enantiomeric recognition Thus, we show that thermal under-vacuum deposition on a surface of a three-fold symmetrical polycyclic aromatic hydrocarbon (PAH) leads to adsorption-induced chirality. This chirality is produced because the molecular landing on the surface could be on one side or the other, which induces a prochiral symmetry break. We conclusively demonstrate that the surface distinguishes the way in which an individual molecule lands: rightor left-hand adsorbed molecules clearly show different orientations with respect to the underlying substrate. Moreover, we present an atomistic view of the enantiomeric recognition mechanism. We rationalized the interaction of large PAHs on single crystal surfaces and concluded that the final configuration maximizes the favorable adsorption of its constituent benzene-like subunits. For this study we chose the C60H30 molecule. [6a] This PAH has acquired a particular relevance after the very recent publication by our group that showed that a surface-catalyzed cyclodehydrogenation process of this planar precursor can transform it into C60 fullerenes. [6b] A key point for understanding the cyclization mechanism is the interaction of those precursors with the catalytic surface. Figure 1, left, shows a sketch of the molecular geometry of the C60H30 PAH as determined from ab initio calculations. Although the 2D projected molecular form is achiral (Figure 1, left), the free molecule adopts a helical (propeller-shaped) conformation and is, therefore, chiral. However, upon adsorption, the molecule planarizes but is still chiral because of the surface and shows different handedness depending on its landing side. Figure 1, center, shows an STM image at the same scale of an individual deposited PAH on Pt ACHTUNGTRENNUNG(111). This image shows an intensity enhancement in the central part [a] G. Otero, Dr. B. G mez-Lor, Dr. J. M ndez, Dr. J. A. Mart n-Gago Structure of Nanoscopic Systems Group Instituto de Ciencia de Materiales de Madrid Sor Juana In s de la Cruz, 3, Cantoblanco, 28049, Madrid (Spain) Fax: (+34)913729623 E-mail : [email protected] [b] G. Biddau, Dr. R. P rez Departamento de F sica Te rica de la Materia Condensada Universidad Aut noma de Madrid, 28049-Madrid (Spain) Fax: (+34)914974950 E-mail : [email protected] [c] Dr. T. Ozaki Research Center for Integrated Science (Japan) Advanced Institute of Science and Technology (JAIST) 1–1 Asahidai, Nomi, Ishikawa 923-1292 (Japan) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201002079.