2D Spontaneous Resolution DOI: 10.1002/anie.201102627 Self-Assembly and Two-Dimensional Spontaneous Resolution of Cyano-Functionalized [7]Helicenes on Cu(111)** Meike Stçhr,* Serpil Boz, Michael Schär, Manh-Thuong Nguyen, Carlo A. Pignedoli, Daniele Passerone,* W. Bernd Schweizer, Carlo Thilgen, ThomasA. Jung,* and FranÅois Diederich* In memoriam Emanuel Vogel Effective control of chirality in supramolecular systems is an important challenge, for example in the fields of (heteroge- neous) asymmetric catalysis [1] and liquid crystals. [2] The spontaneous resolution of a racemic compound into a conglomerate of enantiomeric crystals is based on a prefer- ence of molecules to make contacts with neighbors of the same chirality sense through supramolecular interactions. [3] Although considerable progress has been made in the prediction of crystal structures, [4] the occurrence of sponta- neous resolution in the course of the formation of crystals in three dimensions (3D) still lacks reliable predictability. Therefore, scanning tunneling microscopy (STM) studies of the formation of 2D conglomerates from surface-supported racemic mixtures of molecules provide valuable insight into the phenomenon of spontaneous resolution [3, 5] and the underlying intermolecular interactions. Helicity is a fundamental element of molecular chirality, [6] and supramolecular interactions between helices are of utmost importance in molecular biology. [7] The carbon- based [n]helicenes, [8] ortho-fused polycyclic aromatic hydro- carbons with n  5 angularly arranged benzene rings, are a prototypical example of cylindrical molecular helices. In particular, the adsorption of [7]helicene on Cu(111) has been at the focus of research attempting to unveil the principles of self-assembly for these chiral hydrocarbons. [9] A racemic mixture of heptahelicene was shown to form zigzag-type rows of alternating P- and M-configured molecules. [9d] These rows assembled into “2D racemate” type chiral domains, the underlying intermolecular interactions being based on non- directional van der Waals forces. Up to now, no spontaneous resolution of enantiomers has been observed for racemic helicenes adsorbed on surfaces. This contrasts with the 3D crystallization behavior of many unsubstituted helicenes which form conglomerates of (micro)crystals, often featuring microtwinning or lamellar twinning. [8a–c] The title compound, 6,13-dicyano[7]helicene (1, Scheme 1 and Figure 1a), on the other hand, crystallized as solvent-free racemate from a solution of ()-1 in CH 2 Cl 2 , and as the solvate (+)-(P)- 1·CH 2 Cl 2 from a solution of pure (+)-(P)-1 (see the Support- ing Information). Here, we present a combined STM and DFT (density functional theory) study for the adsorption of a [7]helicene functionalized with two cyano groups (1) on Cu(111). We demonstrate the formation of enantiopure domains in which homochiral molecules are assembled either in the form of “dimers” or “tetramers”. Through atomistic simulation, we understand the role of supramolecular interactions in this diastereoselective self-assembly process on the copper sur- face. Indeed, our experimental and theoretical findings show that supramolecular synthons based on CN···HC(Ar) hydro- gen bonding and dipolar CN···CN interactions, both of which are well known from 3D crystals [10] and 2D surface architec- tures, [11] play also a role in the conglomerate-type 2D self- assembly (spontaneous resolution) of cyanohelicenes. A versatile method was elaborated for the preparation of pure enantiomers of 6,13-dicyano[7]helicene ((P)-1 and (M)- 1, Scheme 1). It includes the photocyclodehydrogenation of stilbene-type precursors [12] 2 as the key, helicene-forming step as well as a chromatographic resolution of the resulting helicene derivative 3. Distilbene 2 is available in three steps from naphthalene-2,3-dimethanol [13] (see Scheme 1 in the Supporting Information). Taking advantage of the directing effect of the Br substituent (“bromine-auxiliary” strategy), [14] helicene precursor 2 was regioselectively converted into racemic [7]helicene ()-3 by photocyclodehydrogenation [*] Prof. M. Stçhr Zernike Institute for Advanced Materials, University of Groningen Nijenborgh 4, 9747 AG Groningen (The Netherlands) E-mail: m.a.stohr@rug.nl Dr. S. Boz, Prof. T.A. Jung Department of Physics, University of Basel Klingelbergstrasse 82, 4056 Basel (Switzerland) E-mail: thomas.jung@psi.ch Dr. M. Schär, Dr. W. B. Schweizer, Prof. C. Thilgen, Prof. F. Diederich Laboratorium für Organische Chemie, ETH Zürich Wolfgang-Pauli-Strasse 10, 8093 Zürich (Switzerland) E-mail: diederich@org.chem.ethz.ch M.-T. Nguyen, Dr. C.A. Pignedoli, Dr. D. Passerone Empa, Swiss Federal Laboratories for Materials Science and Technology, nanotech@surfaces laboratory Überlandstrasse 129, 8600 Dübendorf (Switzerland) E-mail: Daniele.Passerone@empa.ch [**] This work was supported by the European Union through the Marie Curie Research Training Network PRAIRIES (contract MRTN-CT- 2006-035810), the Swiss National Science Foundation, the NCCR “Nanoscale Science”, and the Wolfermann-Nägeli-Stiftung. The Swiss National Supercomputing Centre (CSCS) is acknowledged for the use of computer time. We thank S. Schnell for his support with building and maintaining the experimental infrastructure. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201102627. Communications 9982  2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. Int. Ed. 2011, 50, 9982 –9986