A surface coordination network based on copper adatom trimers.

Surface coordination networks formed by co-adsorption of metal atoms and organic ligands have interesting properties, for example regarding catalysis and data storage. Surface coordination networks studied to date have typically been based on single metal atom centers. The formation of a novel surface coordination network is now demonstrated that is based on network nodes in the form of clusters consisting of three Cu adatoms. The network forms by deposition of tetrahydroxybenzene (THB) on Cu(111) under UHV conditions. As shown from a combination of scanning tunneling microscopy, X-ray photoelectron spectroscopy, and density functional theory calculations, all four hydroxy groups of THB dehydrogenate upon thermal activation at 440 K. This highly reactive ligand binds to Cu adatom trimers, which are resolved by high-resolution STM. The network creates an ordered array of mono-dispersed metal clusters constituting a two-dimensional analogue of metal-organic frameworks.

Chiral induction with chiral conformational switches in the limit of low "sergeants to soldiers" ratio.

Molecular-level insights into chiral adsorption phenomena are highly relevant within the fields of asymmetric heterogeneous catalysis or chiral separation and may contribute to understand the origins of homochirality in nature. Here, we investigate chiral induction by the "sergeants and soldiers" mechanism for an oligo(phenylene ethynylene) based chiral conformational switch by coadsorbing it with an intrinsically chiral seed on Au(111). Through statistical analysis of scanning tunneling microscopy (STM) data, we demonstrate successful chiral induction with a very low concentration of seeding molecules down to 3%. The microscopic mechanism for the observed chiral induction is suggested to involve nucleation of the intrinsically chiral seeds, allowing for effective transfer and amplification of chirality to large numbers of soldier target molecules.

Adsorption and dehydrogenation of tetrahydroxybenzene on Cu(111).

Adsorption of tetrahydroxybenzene (THB) on Cu(111) and Au(111) surfaces is studied using a combination of STM, XPS, and DFT. THB is deposited intact, but on Cu(111) it undergoes gradual dehydrogenation of the hydroxyl groups as a function of substrate temperature, yielding a pure dihydroxy-benzoquinone phase at 370 K. Subtle changes to the adsorption structure upon dehydrogenation are explained from differences in molecule-surface bonding.

On-surface azide-alkyne cycloaddition on Cu(111): does it "click" in ultrahigh vacuum?

Using scanning tunneling microscopy, we demonstrate that the 1,3-dipolar cycloaddition between a terminal alkyne and an azide can be performed under solvent-free ultrahigh vacuum conditions with reactants adsorbed on a Cu(111) surface. XPS shows significant degradation of the azide upon adsorption, which is found to be the limiting factor for the reaction.

Steering organizational and conformational surface chirality by controlling molecular chemical functionality.

Molecular chirality on surfaces has been widely explored, both for intrinsically chiral molecules and for prochiral molecules that become chiral upon adsorption due to the reduced symmetry which follows from surface confinement. However, little attention has been devoted to chiral effects that originate from conformational degrees of freedom for adsorbed molecules. Here we have used scanning tunneling microscopy to investigate the self-assembled structures formed when a class of six linear, organic molecules (oligo-phenylene-ethynylenes) are adsorbed on a Au(111) surface under ultrahigh vacuum conditions. All of the investigated compounds are intrinsically achiral, but most display conformational chirality in the sense that the molecules can adsorb on the surface in different conformations giving rise to either one of two chiral surface enantiomers or a mirror-symmetric achiral meso form. A total of eleven observed adsorption structures are systematically investigated with respect to conformational chirality as well as point chirality (arising where molecular adsorption locally breaks the substrate symmetry) and organizational chirality (arising from the tiling pattern of the molecular backbones). A number of interesting correlations are identified between these different levels of chirality. Most importantly, we demonstrate that it is possible through control of the terminal group functionalization to steer the oligo(phenylene-ethynylene) molecular backbones into surface assemblies that either display pronounced organizational chirality or have mirror symmetric tiling patterns, and that it is furthermore possible to control the conformational surface chirality so the compounds preferentially assume either chiral or achiral surface conformers.

Hydrogen-bonded molecular networks of melamine and cyanuric acid on thin films of NaCl on Au(111).

Self-assembly of organized molecular structures on insulators is technologically very relevant, but in general rather challenging to achieve due to the comparatively weak molecule-substrate interactions. Here the self-assembly of a bimolecular hydrogen-bonded network formed by melamine (M) and cyanuric acid (CA) on ultrathin NaCl films grown on a Au(111) surface is reported. Using scanning tunneling microscopy under ultrahigh-vacuum conditions it is demonstrated that it is possible to exploit strong intermolecular forces in the M-CA system, resulting from complementary triple hydrogen bonds, to grow 2D bimolecular networks on an ultrathin NaCl film that are stable at a relatively high temperature of approximately 160 K and at a coverage below saturation of the first molecular monolayer. These hydrogen-bonded structures on NaCl are identical to the self-assembled structures observed for the M-CA system on Au(111), which indicates that the molecular self-assembly is not significantly affected by the isolating NaCl substrate.

Formation of trioctylamine from octylamine on Au(111).

The adsorption of octylamine on Au(111) under ultrahigh vacuum conditions is investigated. The molecules surprisingly undergo a thermally activated chemical reaction, resulting in formation of trioctylamine as confirmed both by X-ray photoelectron spectroscopy (XPS) and by comparison to the scanning tunneling microscopy (STM) signature of trioctylamine deposited directly onto the surface.

Molecular self-assembly from building blocks synthesized on a surface in ultrahigh vacuum: kinetic control and topo-chemical reactions.

Self-assembly of organic molecules on solid surfaces under ultrahigh vacuum conditions has been the focus of intense study, in particular utilizing the technique of scanning tunneling microscopy. The size and complexity of the organic compounds used in such studies are in general limited by thermal decomposition in the necessary vacuum sublimation step. An interesting alternative approach is to deposit smaller molecular precursors, which react with each other on the surface and form the building blocks for the subsequent self-assembly. This has however hitherto not been explored to any significant extent. Here, we perform a condensation reaction between aldehyde and amine precursors codeposited on a Au(111) surface. The reaction product consists of a three-spoke oligo-phenylene-ethynylene backbone with alkyl chains attached through imine coupling. We characterize the self-assembled structures and molecular conformations of the complex reaction product and find that the combined reaction and self-assembly process exhibits pronounced kinetic effects leading to formation of qualitatively different molecular structures depending on the reaction/assembly conditions. At high amine flux/low substrate temperature, compact triimine structures of high conformational order are formed, which inherit organizational motifs from structures formed from one of the reactants. This suggests a topochemical reaction. At low amine flux/high substrate temperature, open porous networks with a high degree of conformational disorder are formed. Both structures are entirely different from that obtained when the triimine product synthesized ex-situ is deposited onto the surface. This demonstrates that the approach of combined self-assembly and on-surface synthesis may allow formation of unique structures that are not obtainable through self-assembly from conventionally deposited building blocks.