Topological Selectivity in a Supramolecular Self-Assembled Host-Guest Network at the Solid-Liquid Interface Christoph Meier, Katharina Landfester, and Ulrich Ziener* Institute of Organic Chemistry III (Macromolecular Chemistry and Organic Materials), Albert-Einstein-Allee 11, 89081 Um, Germany ReceiVed: May 13, 2008; ReVised Manuscript ReceiVed: June 28, 2008 An oligopyridine forms a well-ordered two-dimensional network at the graphite-liquid interface stabilized by weak hydrogen bonds. This network offers two types of geometrically different voids of similar size, oval and circular in shape, respectively. Hierarchical coadsorption of C 60 fullerene onto the oligopyridine monolayer leads to a host-guest network in which the fullerene molecules adsorb exclusively in the circularly shaped voids. This is attributed to the fit of the spherical molecules to the round void according to a geometric lock-and-key principle. 1. Introduction Two dimensional (2D) hydrogen-bonded networks (HBN) have attracted growing interest in the past several years as the self-assembly behavior of the appropriate building blocks can be controlled via slight variations of their chemical structures. 1-4 The feasibility of precise control of the resulting network structures makes them ideal candidates for functionalizing inorganic surfaces for applications in molecular electronics 5-10 and surface-supported sensors. 11-13 From both scientific and technological points of view, the fabrication of highly ordered arrays of fullerenes on solid substrates under ambient conditions is of great importance. Fullerenes are well-known for their outstanding electronic properties, 10,14,15 making them viable building blocks for the realization of molecular electronic devices working under ambient conditions. To investigate the electronic properties of individual fullerene molecules under ambient conditions, they have to be immobilized in a spatially well controlled manner. 16 Within this context, the study of supramolecular binding phenomena at the solid-liquid interface is very important. 4,16-24 On metal surfaces in ultrahigh vacuum, various organic template networks accommodate C 60 through weak van der Waals interactions. 25-29 At the solid-liquid interface, the solubility of fullerene in the supernatant solvent phase plays an important role in adsorption. It was shown that C 60 is incorporated into trimesic acid (TMA) networks from saturated solutions in fatty acids, which do not physically dissolve fullerene molecules. 16,30 As a result of the low solubility, their residence time at the interface is high and their incorporation into the host structure is a rare event. In typical organic solvents such as 1,2,4-trichlorobenzene (TCB), the formation of stable C 60 monolayers on HOPG has not been documented, as a result of the low adsorption energy of C 60 on the bare graphite surface. 31 However, it was shown that stabilizing charge-transfer interactions between C 60 and template building blocks, e.g., thiophenes, are a suitable method for immobilizing C 60 at the solid-liquid interface. 19,20 In this contribution, we report on the lock-and-key principle- like immobilization of C 60 in a 2D supramolecular HBN, composed of the oligopyridine building block 4,3′-BTP (4,3′- bisterpyridine) under ambient conditions at the solid-liquid interface (Scheme 1). We show that fullerene adsorption in the HBN is topologically selective even in the absence of charge- transfer interactions. Therefore, the properties of the self- assembled HBN differ fundamentally from those of the indi- vidual 4,3′-BTP building blocks and are a result of the self- assembly process. 2. Experimental Procedure A drop of a 1,2,4-trichlorobenzene (Aldrich, >99%) solution, containing 4,3′-BTP (0.2 mg/mL), was deposited on a freshly cleaved surface of highly ordered pyrolytic graphite (HOPG) with the tip (80/20 Pt/Ir) in tunnel contact. After successful imaging of the oligopyridine network with the STM (SPM1000, RHK), a drop of a concentrated solution of C 60 in TCB (∼15 mg/mL) was applied and imaged. The MM+ force field calculations were performed with Hyperchem 7. 32 3. Results and Discussion In a previous publication, we reported on the C 2V symmetric oligopyridine 4,3′-BTP which forms a 2D chiral HBN at the solid-liquid interface of highly ordered pyrolytic graphite (HOPG) and TCB, which was investigated with scanning SCHEME 1: Sketch of the Investigated Oligopyridine 4,3′-BTP and the Fullerene C 60 J. Phys. Chem. C 2008, 112, 15236–15240 15236 10.1021/jp804235a CCC: $40.75  2008 American Chemical Society Published on Web 09/10/2008