Showing papers by "Peter H. Beton published in 2011"

Vernier templating and synthesis of a 12-porphyrin nano-ring

Abstract: Chemists use template-directed synthesis to position molecular components so that they can be covalently linked into complex molecules that are not readily accessible by classical synthetic methods. But as larger structures are targeted, the synthesis of the templates themselves becomes challenging. O'Sullivan et al. now show that 'molecular Verniers', based on the principle of moire pattern formation, can solve this problem. Using a Vernier template with six binding sites and molecular building blocks with four porphyrins acting as binding sites, the authors create a 12-porphyrin nano-ring with a diameter of 4.7 nanometres. The ease and efficiency of this synthesis establishes Vernier templating as a powerful new strategy for producing large monodisperse macromolecules. Templates are widely used to arrange molecular components so they can be covalently linked into complex molecules that are not readily accessible by classical synthetic methods. But, as larger structures are targeted, the synthesis of the templates themselves becomes challenging. It is now shown that 'molecular Verniers' can solve this problem: using a template with six binding sites and molecular building blocks with four porphyrins acting as binding sites, a 12-porphyrin nano-ring with a diameter of 4.7 nm is created. The ease and efficiency of this synthesis establishes Vernier templating as a powerful new strategy for producing large monodisperse macromolecules. Templates are widely used to arrange molecular components so they can be covalently linked into complex molecules that are not readily accessible by classical synthetic methods1,2,3,4,5,6,7. Nature uses sophisticated templates such as the ribosome, whereas chemists use simple ions or small molecules. But as we tackle the synthesis of larger targets, we require larger templates—which themselves become synthetically challenging. Here we show that Vernier complexes can solve this problem: if the number of binding sites on the template, nT, is not a multiple of the number of binding sites on the molecular building blocks, nB, then small templates can direct the assembly of relatively large Vernier complexes where the number of binding sites in the product, nP, is the lowest common multiple of nB and nT (refs 8, 9). We illustrate the value of this concept for the covalent synthesis of challenging targets by using a simple six-site template to direct the synthesis of a 12-porphyrin nano-ring with a diameter of 4.7 nm, thus establishing Vernier templating as a powerful new strategy for the synthesis of large monodisperse macromolecules.

Guest-induced growth of a surface-based supramolecular bilayer

Abstract: Self-assembly of planar molecules on a surface can result in the formation of a wide variety of close-packed or porous structures. Two-dimensional porous arrays provide host sites for trapping guest species of suitable size. Here we show that a non-planar guest species (C(60)) can play a more complex role by promoting the growth of a second layer of host molecules (p-terphenyl-3,5,3″,5″-tetracarboxylic acid) above and parallel to the surface so that self-assembly is extended into the third dimension. The addition of guest molecules and the formation of the second layer are co-dependent. Adding a planar guest (coronene) can displace the C(60) and cause reversion to a monolayer arrangement. The system provides an example of a reversible transformation between a planar and a non-planar supramolecular network, an important step towards the controlled self-assembly of functional, three-dimensional, surface-based supramolecular architectures.

Two-dimensional supramolecular chemistry on surfaces

Abstract: Self-assembly of two-dimensional supramolecular arrays on surfaces represents a significant challenge to chemists, materials scientists and physicists. This article highlights advances in using supramolecular interactions, particularly hydrogen bonding, to self-assemble such two-dimensional arrays on surfaces. Scanning-probe microscopies, particularly scanning tunnelling microscopy (STM), can be used to determine the precise molecular arrangement of the self-assembled structures allowing insight into the self-assembly process at the molecular level. The use of such supramolecular assemblies to trap guest species, mimicking host–guest chemistry in the solution phase, will also be discussed. Such images provide great insight into the advantages and restrictions of working in two dimensions in comparison to the solution phase or the solid state.

Dimerization of Tri(4-bromophenyl)benzene by Aryl―Aryl Coupling from Solution on a Gold Surface

Abstract: Monolayers of monomer 1,3,5-tri(4-bromophenyl)benzene (TBPB) and the dimerized product 3,3′′′,5,5′′′-tetra(4-bromophenyl)quaterphenyl (TBPQ) on a gold surface have been studied using ambient scanning tunneling microscopy and time-of-flight secondary ion mass spectrometry (ToF-SIMS). Molecular layers are prepared by allowing sessile drops of solution to dry on a gold substrate. For room-temperature deposition we observe ordered arrays of TBPB in three distinct packing arrangements. Deposition on a heated substrate leads to the formation of the dimerized product, TBPQ, through a surface-induced aryl−aryl coupling. Regions of TBPQ coexist with regions of disordered multiply linked molecules. The conversion of monomer TPBP to dimers is confirmed using ToF-SIMS. Our results demonstrate an alternative, solution-phase approach to the formation of large molecules and nanostructures by coupling reactions.

Graphene Formation by Decomposition of C60

Abstract: Graphene is formed through the thermally induced decomposition of C60 in combination with a Ni thin film. After transfer to a SiO2 substrate, optical microscopy and Raman spectroscopy confirm the presence of graphene for films of C60 buried under nickel or alternatively for C60 adsorbed on a nickel surface. For buried films the graphene thickness is shown to depend on the fullerene dosage, with evidence of domain growth from nucleation sites separated by tens of micrometers. The adsorption of C60 and the changes arising from annealing are also monitored using scanning tunneling microscopy. We relate our results to previous studies of the decomposition of C60 on nickel and other transition metals.

Single molecule magnets on a gold surface: in situ electrospray deposition, x-ray absorption and photoemission.

Abstract: Single molecule magnets based on the dodecamanganese (III, IV) cluster with two different types of ligand (acetate and benzoate) have been studied on the Au(111) surface. Due to the non-volatile and fragile nature of the molecules, we have used ultra-high vacuum electrospray deposition to produce a series of surface coverages from a fraction of a monolayer to multilayer films in both cases. Synchrotron radiation based electron spectroscopy has been used to study the adsorption of the molecules on the Au(111) surface and the effect that this has on the oxidation states of the manganese atoms in the core. In both cases, reduction of the core is observed due to the interaction with the surface, to essentially the same extent despite substantial differences in the size and nature of the ligand shell.