Self-Assembly of Heterogeneous Supramolecular Structures with Uniaxial Anisotropy M. Ruiz-Ose ´ s, ² N. Gonza ´ lez-Lakunza, ‡ I. Silanes, § A. Gourdon, ⊥ A. Arnau, ‡,# and J. E. Ortega* ,²,# Departamento de Fı ´sica Aplicada I, UniVersidad del Paı ´s Vasco, Plaza de On ˜ ate 2, E-20018 Donostia-San Sebastia ´ n, Spain, Departamento de Fı ´sica de Materiales UPV/EHU, Facultad de Quı ´mica, Apartado 1072, San Sebastian E-20080, Spain, Kimika Fakultatea, Euskal Herriko Unibertsitatea, P.K. 1072, 20080 Donostia, Euskadi, Spain, Groupe NanoSciences, CEMES-CNRS (UPR 8011), BP 94347, 29 Rue J. MarVig, 31055 Toulouse Cedex 4, France, and Unidad de Fı ´sica de Materiales, Centro Mixto CSIC-UPV/EHU, Manuel Lardizabal 3, E-20018 San Sebastia ´ n, Spain ReceiVed: October 3, 2006; In Final Form: NoVember 21, 2006 Uniaxial anisotropy in two-dimensional self-assembled supramolecular structures is achieved by the coadsorption of two different linear molecules with complementary amine and imide functionalization. The two-dimensional monolayer is defined by a one-dimensional stack of binary chains, which can be forced to line up along steps in vicinal surfaces. The competing driving forces in the self-organization process are discussed in light of the structures observed during single molecule adsorption and coadsorption on flat and vicinal surfaces and the corresponding theoretical calculations. Molecular recognition, that is, selective molecular interaction, is at the basis of fundamental biological processes in living systems, ranging from DNA replication or virus attachment to cells, to a number of nanotechnology applications that mimic these phenomena, such as nanoparticle surface functionalization for molecule or cell targets. Molecular recognition is also the driving force in the supramolecular self-assembly of organic molecular multilayers used in nanoelectronic devices. 1 Both supramolecular chemistry and molecular recognition strategies can be jointly exploited in the fabrication of nanostructured templates for molecule adsorption on surfaces. For such purposes, planar ring systems (polyarenes) are very suitable, particularly with noble metals as substrates, since they adsorb flat and diffuse quickly on the surface plane. In particular, stable supramolecular structures can be obtained by coadsorbing two molecules with complementary end-groups, such as pairs of DNA bases. 2 Synthetic chemistry allows one to tune the lattice parameter, by varying the size of the polyarenes, and the symmetry, by inserting functional groups in the appropriate geometry. One good example is the coadsorption of diamino- triazine (melamine) and perylene tetracarboxylic diimide (PTC- DI), 3,4 which leads to a supramolecular (H-bonded) honeycomb network driven by both the strong PTCDI-melamine (imide/ amine) affinity and the threefold symmetry of the melamine. Here we show that, by selecting a pair of chemically comple- mentary polyarenes with linear shape and imide/amine end- groups, one obtains the one-dimensional (1D) heterogeneous molecular network analogue, that is, a 1D binary chain with strong intermolecular H-bonding. Functional structures made of molecular recognition patterns require well-defined arrays at higher hierarchical levels. Here, the substrate plays the key role. Homogeneous patterns spanning large surface areas are effectively limited by terrace morphol- ogies and substrate reconstruction domains. 5 Mesoscopic order- ing can be achieved by self-assembled prepatterned surfaces, such as vicinal surfaces, that is, 1D arrays of monatomic steps. 6 The latter not only introduce the uniaxial anisotropy in the system, but also can induce single domains in the mesoscopic scale. 7 As we show in this work, one can combine vicinal substrates and a pair of linearly shaped molecules with complementary (imide/amine) end-groups to achieve self- organized supramolecular assemblies with a well-defined uni- axial anisotropy with respect to a surface direction, namely, the step direction. In order to test this prepatterning-plus-self-assembly ap- proach of binary structures, we use benzodiguanamine (BDG) (C 12 H 12 N 10 ), which is a linear version of melanine, and naphtalene tetracarboxylic diimide (NTCDI) (C 14 H 8 N 2 O 4 ), which is similar to PTCDI, as well as a Au(111) vicinal surface with periodic faceting. The atomic structures of these two molecules are schematically shown in Figure 1. The comple- mentary functionalization is expected to favor the formation of dimers with strong triple H-bonds, as depicted in the figure. BDG is synthesized by the reaction of dicyandiamide with terephthalonitrile in EGME/potassium hydroxide under micro- wave irradiation. The product of the reaction that we call BDG is 1,4-di(4,6-diamino-1,3,5-triazin-2-yl)benzene. On flat sur- faces, a two-dimensional (2D), binary supramolecular structure is readily achieved, which is formed by packing 1D heteroge- neous NTCDI/BDG rows. Theoretical calculations fit structural observations and demonstrate that the triple H-bond in the frontal * Corresponding author. ² Universidad del Paı ´s Vasco. ‡ Facultad de Quı ´mica. § Euskal Herriko Unibertsitatea. ⊥ CEMES-CNRS (UPR 8011). # Centro Mixto CSIC-UPV/EHU. 25573 2006, 110, 25573-25577 Published on Web 12/07/2006 10.1021/jp066493x CCC: $33.50 © 2006 American Chemical Society