Competing Interactions in the Self-Assembly of NC-Ph₃-CN Molecules on Cu(111)

Abstract

We report on low-temperature scanning tunneling microscopy and spectroscopy measurements on NC-Ph 3 -CN molecules adsorbed at 300 K on a Cu(111) surface. Upon cooling, the molecules form chain and honeycomb structures, incorporating Cu adatoms supplied by the substrate as metal linkers. In these assemblies, the molecules align along two main directions, with a relative abundance that depends on the coordination number and on the bond length. We show spectroscopic data about the unoccupied molecular orbitals and investigate the patterns obtained by depositing different amounts of molecules. Comparison of these results with the ones obtained for NC-Ph 5 -CN molecules on the same substrate enables us to establish a hierarchy of the different interaction forces at work in the system. ■ INTRODUCTIONSupramolecular chemistry at surfaces is a topic of widespread interest because low-dimensional architectures with specific functionalities can be created. 1−4 A detailed understanding of the mechanism underlying the self-assembly of the various organic and metal−organic networks is important to realize the desired morphology, chemical composition, and eventually functionality. In this respect, regular porous networks are especially promising: They act as templates for the positioning of molecules or metal atoms and clusters onto well-defined sites within the cavities 5−8 as well as on the ligand molecules. 9 Moreover, depending on the metal atoms used for the coordination nodes, regular arrays of transition-metal 10,11 or rare earth atoms 12 can be created.Molecular adsorption and assembly are controlled by several competing interactions. 13−16 For the case of metal−organic porous networks, there are the van der Waals forces between the organic molecules and the surface and the corrugation of this potential energy surface upon translation and rotation of the molecules. A second ingredient is the potential energy surface felt by the metal coordination atoms, that is, their preferred adsorption sites and their diffusion barriers. Finally, there are the bond angle, distance, and coordination number of the metal−organic coordination bond.To shed light on the hierarchy of these energies, we report here on the self-assembly of NC-Ph 3 -CN molecules on Cu(111). When deposited on the substrate kept at room temperature, upon cooling NC-Ph 3 -CN molecules form chain and honeycomb structures and the molecules align with orientations that deviate from the crystallographic high symmetry directions of the surface. The comparison with the motifs formed by NC-Ph 5 -CN molecules on the same substrate 17 enables us to estimate which interaction dominates in each case, depending on the number of phenyl rings in the system. To complement the characterization of the observed metal−organic structures, we report spectroscopic data for the molecular orbitals. Moreover, the dependence of the obtained structures on the molecular coverage is discussed.

Two-Orbital Kondo Screening in a Self-Assembled Metal–Organic Complex

“…The higher intensity observed on one side of the molecules is ascribed to the tilt of the phenyl rings with respect to the surface and to each other. 25,28,29 Notice that very similar results have been obtained for Fe atoms buried by NC-Ph 5 -CN molecules (see Supporting Information).…”

“…NC-Ph 3 -CN molecules were evaporated from a molecular effusion cell at 418 K onto the Cu(111) substrate at 300 K. The obtained (NC-Ph 3 -CN) 3 Cu 2 honeycomb network has a period of 3.51 nm. 25 Fe and Co were evaporated from an e-beam evaporator, while the sample was placed in the STM. One monolayer of Fe or Co is defined as one atom per Cu(111) unit cell.…”

On‐Surface Metalation and 2D Self‐Assembly of Pyrphyrin Molecules Into Metal‐Coordinated Networks on Cu(111)

“…In solution, CoPyr derived complexes have been shown to catalyze the reduction of water under illumination with light . Cyano‐functionalized molecules in general may also act as ligands for surface adatoms . Such coordination at radial or terminal functional groups leads often to the formation of highly ordered 2D metal‐organic coordination networks …”

Patterns of Organics on Substrates with Metallic Surface States: Why?, So??

“…Some examples on Cu(111) in- clude NC-Ph5-CN, 3deh-DPDI, and dehydro-DPDI [26][27][28][29]; Br-DNT on Ag(111) is another [30]. In all these cases, there is a single molecule on edge of the hexagonal pores.…”

“…Brune and coworkers [29,30] used different-length linear polyphenyl molecules, specifically chains of 3 and of 5 phenyl rings (coupled with Cu atoms from the substrate). They discuss the differences between the irregular honeycombs in the two cases in terms of a competition between admolecule-substrate and admolecule-admolecule interactions.…”