Substrate effect on supramolecular self-assembly: from semiconductors to metals.

Terephthalic acid (TPA) deposited on Si(111)-7 x 7, Si(111)-square root 3 x square root 3-Ag and Ag(111) has been studied as a model system to understand how much passivated semiconductor surfaces differ from semiconductor and metal surfaces in respect of supramolecular self assembly. By scanning tunneling microscopy it is found that TPA molecules do not form any ordered supramolecular structure on the pristine semiconductor surface, due to a strong molecule-substrate interaction. On the contrary, TPA has a weaker interaction with Si(111)-square root 3 x square root 3-Ag, leading to the formation of an ordered supramolecular layer stabilized by carboxyl hydrogen bonds. These structures are very similar to the supramolecular layer of TPA formed on Ag(111), indicating that the two substrates behave similarly for what concerns the adsorption of functional organic molecules. However, the deposition of Fe on the TPA layers on Si(111)-square root 3 x square root 3-Ag does not induce the formation of two-dimensional metal-organic frameworks which, on the contrary, readily develop on Ag(111). Possible origins of this difference are discussed.

Toward mechanical switching of surface-adsorbed [2]catenane by in situ copper complexation.

Using scanning tunneling microscopy (STM), electrospray ionization mass spectrometry (ESI-MS), and X-ray photoelectron spectroscopy (XPS), we demonstrate that a free [2]catenane consisting of two interlocking 30-membered rings (cat-30) can be deposited on a Ag(111) surface by vacuum sublimation without decomposition. The deposited cat-30 molecules self-organize as ordered dimer chain structures at the surface, presumably via intermolecular pi-pi stacking. An in situ addition of Cu atoms to the surface-adsorbed catenanes induces a drastic change in the molecular organization, i.e., from the dimer chain structure to isolated species. The nitrogen core level spectra suggest that the cat-30 phenanthroline units coordinate with Cu, indicating that the free catenane has been transformed into a Cu-complexed [2]catenane. Since it is known that the two interlocked macrocyclic rings of the free ligand cat-30 completely rearrange, i.e., circumrotate, upon complexation to copper, our results reveal that when adsorbed on the silver surface, the two macrocyclic rings of the free [2]catenane can glide within one another so as to generate the corresponding copper complex by in situ Cu complexation.

Ionic hydrogen bonds controlling two-dimensional supramolecular systems at a metal surface.

Hydrogen-bond formation between ionic adsorbates on an Ag(111) surface under ultrahigh vacuum was studied by scanning tunneling microscopy/spectroscopy (STM/STS), X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), and molecular dynamics calculations. The adsorbate, 1,3,5-benzenetricarboxylic acid (trimesic acid, TMA), self-assembles at low temperatures (250-300 K) into the known open honeycomb motif through neutral hydrogen bonds formed between carboxyl groups, whereas annealing at 420 K leads to a densely packed quartet structure consisting of flat-lying molecules with one deprotonated carboxyl group per molecule. The resulting charged carboxylate groups form intermolecular ionic hydrogen bonds with enhanced strength compared to the neutral hydrogen bonds; this represents an alternative supramolecular bonding motif in 2D supramolecular organization.

2D supramolecular assemblies of benzene-1,3,5-triyl-tribenzoic acid: temperature-induced phase transformations and hierarchical organization with macrocyclic molecules.

Two-dimensional supramolecular honeycomb networks with cavities of an internal diameter of 2.95 nm were formed by the self-assembly of 4,4',4' '-benzene-1,3,5-triyl-tribenzoic acid (BTA) on a Ag(111) surface at room temperature. Annealing to higher temperatures resulted in two sequential phase transformations into closer-packed supramolecular arrangements. The phase transformations are associated with stepwise deprotonation of the carboxylic acid groups. The voids of the honeycomb network of BTA have a suitable size for the construction of hierarchical structures with guest molecules. Single molecules of the macrocyclic compound mt-33 were successfully confined inside 2D nanocavities of the honeycomb networks and released when the phase was transformed to the close-packed structure.