Steering On-Surface Self-Assembly of High-Quality Hydrocarbon Networks with Terminal Alkynes Nenad Kepc ̌ ija, † Yi-Qi Zhang, † Martin Kleinschrodt, † Jonas Bjö rk, ‡ Svetlana Klyatskaya, § Florian Klappenberger,* ,† Mario Ruben, §,∥ and Johannes V. Barth † † Physik Department E20, Technische Universitä t Mü nchen, James-Franck-Straße, 85748 Garching, Germany ‡ Department of Physics, Chemistry and Biology (IFM), Linkö ping University, 58183 Linkö ping, Sweden § Institute of Nanotechnology, Karlsruhe Institute of Technology, 76344 Eggenstein-Leopoldshafen, Germany ∥ IPCMS-CNRS, Universite ́ de Strasbourg, 23 Rue de Loess, 67034 Strasbourg, France * S Supporting Information ABSTRACT: The two-dimensional (2D) self-assembly of 1,3,5-triethynyl-benzene (TEB) and de novo synthesized 1,3,5- tris-(4-ethynylphenyl)benzene (Ext-TEB) on Ag(111) was investigated by means of scanning tunneling microscopy (STM) under ultrahigh vacuum (UHV) conditions. Both 3- fold symmetric molecules form long-range ordered nano- porous networks featuring organizational chirality, mediated by novel, planar 6-fold cyclic binding motifs. The key interaction for the expression of the motifs is identified as C−H···π bonding. For Ext-TEB, an additional open-porous phase exists with the 3-fold motif. The nature of the underlying noncovalent bonding schemes is thoroughly analyzed by density functional theory (DFT) calculations including van der Waals corrections. The comparison of calculations focusing on isolated 2D molecular sheets and those including the substrate reveals the delicate balance between the attractive molecule−molecule interaction, mediated by both the terminal alkyne and the phenyl groups, and the molecule−substrate interaction responsible for the commensurability and the regularity of the networks. Comparison with bulk structures of similar molecules suggests that these strictly planar cyclic binding motifs appear only in 2D environments. ■ INTRODUCTION Supramolecular chemistry has evolved into one of the most important approaches for the engineering of novel functional materials. 1−3 Among the possible noncovalent interactions hydrogen bonding is most suited because it provides selectivity and directionality combined with a reversible formation process. 4,5 For the rational design of crystal properties by supramolecular synthons, i.e., structural units that assemble by conceivable intermolecular interactions, a profound under- standing of the driving forces behind the attraction between the numerous functional groups is mandatory. 6,7 Aside from the classical, strong hydrogen bridges, their C−H···π analogs, where an acidic CH moiety acts as proton donor and a weakly electron rich π-system plays the role of the acceptor, have been identified as versatile ingredients. 8,9 In this context, terminal alkynes are especially interesting because they unite comparatively strong proton donor capabilities, which result from the high acidity of the alkynyl atom, with great versatility originating from the fact that their π system can simultaneously act as proton acceptor. 8,10 Initially, they have been recognized as secondary structural force in crystals of molecules featuring at the same time classical hydrogen bonding functionalities. 10−13 Later it was demon- strated that in the absence of such functionalities, the terminal alkyne interactions are the dominant ones. 14−16 It is worth mentioning that the competing interaction with the π-system of an aromatic ring is not established in any of these crystal structures. 14−16 Often infinite zigzag patterns of nearly T- shaped arrangements of the ethyne groups are estab- lished, 10,12,14,15,17 but also three- and six-membered synthons have been reported. 15,17,18 A somewhat controversially discussed topic is the manifestation of the cooperative effect, i.e., an increase of the binding energy per bond with increasing number of connected bonds, 10,19 in synthons built-up by terminal alkynes. On the one hand, cooperative stabilization is suggested by experimental results 10,12,15,20,21 as well as early quantum chemical calculations; 12 on the other hand, such effects are not found with the same methods in cases appearing quite similar. 13 A more recent theoretical investigation focusing on ethyne concluded that no cooperative effect appear in between C 2 H 2 molecules and only small additional stabilization (10%) results in the presence of water. 22 It could be tentatively concluded from this discussion that the backbone to which terminal alkynes are attached might have a pronounced influence on the manifestation of cooperative gain. Received: October 26, 2012 Revised: January 24, 2013 Published: January 24, 2013 Article pubs.acs.org/JPCC © 2013 American Chemical Society 3987 dx.doi.org/10.1021/jp310606r | J. Phys. Chem. C 2013, 117, 3987−3995