Simulated structure and imaging of NTCDI on Si(1 1 1)-7 × 7 : a combined STM, NC-AFM and DFT study

The adsorption of naphthalene tetracarboxylic diimide (NTCDI) on Si(1 1 1)-7 × 7 is investigated through a combination of scanning tunnelling microscopy (STM), noncontact atomic force microscopy (NC-AFM) and density functional theory (DFT) calculations. We show that NTCDI adopts multiple planar adsorption geometries on the Si(1 1 1)-7 × 7 surface which can be imaged with intramolecular bond resolution using NC-AFM. DFT calculations reveal adsorption is dominated by covalent bond formation between the molecular oxygen atoms and the surface silicon adatoms. The chemisorption of the molecule is found to induce subtle distortions to the molecular structure, which are observed in NC-AFM images.

Role of interaction anisotropy in the formation and stability of molecular templates

Surface templating via self-assembly of hydrogen-bonded molecular networks is a rapidly developing bottom-up approach in nanotechnology. Using the melamine-PTCDI molecular system as an example we show theoretically that the network stability in the parameter space of temperature versus molecular coupling anisotropy is highly restricted. Our kinetic Monte Carlo simulations predict a structural stability diagram that contains domains of stability of an open honeycomb network, a compact phase, and a high-temperature disordered phase. The results are in agreement with recent experiments, and reveal a relationship between the molecular size and the network stability, which may be used to predict an upper limit on pore-cavity sizes.

Bimolecular networks and supramolecular traps on Au(111)

We demonstrate the formation of intermixed phases and self assembled molecular templates on the Au(111) surface. The templates are stabilized by hydrogen bonding between melamine molecules with trigonal symmetry and linear PTCDI (perylene tetra-carboxylic di-imide) molecules. When annealed, these molecules spontaneously form either a chiral intermixed phase or a honeycomb arrangement in which vertexes and edges correspond respectively to melamine and PTCDI molecules. We also observe minority phases with more complex intermolecular junctions. The use of these networks as templates is demonstrated by the controlled capture of fullerenes within the pores of the network to form dimers, hexamers, and heptamers. Our results confirm that bimolecular templates can be realized on a range of substrates.

Dianhydride-Amine Hydrogen Bonded Perylene Tetracarboxylic Dianhydride and Tetraaminobenzene Rows

We have investigated the coadsorption of perylene tetracarboxylic dianhydride (PTCDA) and tetraaminobenzene (TAB) on the Ag/Si(111)-square root(3) x square root(3) R30 degree surface using scanning tunneling microscopy. At room temperature, PTCDA islands with square and herringbone ordering are formed which, on exposure to TAB, are converted into an intermixed phase in which PTCDA and TAB form alternating rows. From our images, we determine the relative placement of TAB and PTCDA molecules and conclude that the row structure is stabilized by hydrogen bonding between dianhydride and diamine groups. We confirm that this hydrogen bonding junction is stable using ab initio calculations and show that the proposed geometry is consistent with calculated intermolecular dimensions.

Hydrogen-Bonded PTCDA−Melamine Networks and Mixed Phases

A stable hydrogen-bonding junction is formed between 3,4,9,10-perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA) and 1,3,5-triazine-2,4,6-triamine (melamine). This bimolecular system was studied on the Ag-Si(111) square root 3 x square root R 30 degrees surface at sub-monolayer coverage, and two distinct phases are observed. A hexagonal lattice is formed that is stabilized by hydrogen bonding between PTCDA and melamine. This phase, in which melamine acts as a 3-fold vertex, is a close analogue to the 3,4,9,10-perylene-3,4,9,10-tetracarboxylic-diimide-melamine network reported recently. To our knowledge this hydrogen-bonding junction has not been previously observed and might not be expected due to lone pair repulsion. However we confirm that this combination is stable using ab initio methods. In the second intermixed phase parallel rows of PTCDA molecules coexist with an array of melamine molecules, and we propose a model for this structure.

Square, Hexagonal, and Row Phases of PTCDA and PTCDI on Ag−Si(111) × R30°

We have investigated the ordered phases of the perylene derivatives perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) and the imide analogue PTCDI on the Ag-Si(111)square root(3) x square root(3)R30 degrees surface using scanning tunneling microscopy. We find that PTCDA forms square, hexagonal, and herringbone phases, which coexist on the surface. The existence of a square phase on a hexagonal surface is of particular interest and is a result of a near commensurability between the molecular dimensions and the surface lattice. Contrast variations across the square islands arise from PTCDA molecules binding to different sites on the surface. PTCDI on Ag-Si(111)square root(3) x square root(3)R30 degrees forms extended rows, as well as two-dimensional islands, both of which are stabilized by hydrogen bonding mediated by the presence of imide groups. We present models for the molecular arrangements in all these phases and highlight the role of hydrogen bonding in controlling this order.

Growth induced reordering of fullerene clusters trapped in a two-dimensional supramolecular network

We have investigated the growth of molecular clusters in confined geometries defined by a bimolecular supramolecular network. This framework provides a regular array of identical nanoscale traps in which further deposited molecules nucleate cluster growth. For the higher fullerene, C84, molecules aggregate into close packed assemblies with an orientation which switches when the cluster size increases by one molecule. This change is controlled by the interactions between the molecules and the confining boundaries of the network pore. We show that, following nucleation of small clusters, further growth requires a reconfiguration of previously captured molecules resulting in a transition between nanoscale phases with different ordering.

Using microscopic techniques to reveal the mechanism of anion exchange in crystalline co-ordination polymers

Co-ordination polymers are currently attracting extensive interest due to their potential applications as supramolecular hosts, vessels, and frameworks for storage and separations. Many applications rely on the ion exchange capabilities of these compounds, and considerable debate surrounds the mechanism by which ion exchange occurs in co-ordination polymers. Here AFM and SEM were applied, for the first time, to investigate this class of materials. In situ AFM studies revealed the mechanism by which anion exchange and the subsequent structural transformations of the crystalline co-ordination polymers [[Ag(4,4'-bipy)]BF(4)](infinity) and [[Ag(4,4'-bipy)]NO(3)](infinity) occur. The process is initiated by the dissolution of the metastable crystalline polymer, followed by the subsequent crystallization of the new stable phase on the surface of the original crystal. The formation of deep clefts in the metastable polymer crystal during the transformation allows the solution to access the successive crystalline layers. Thus, the entire process can be viewed as a self-perpetuating cascade of dissolution and recrystallization throughout the macroscopic crystal. SEM data consolidate the findings of AFM. These techniques collectively illustrate that the anion exchange, and subsequent structural transformation, proceeds via a solvent-mediated mechanism, rather than a purely solid-state one.

Inorganic-organic interpenetrating frameworks: 4,4'-bipyridine N,N'-dioxide as a bridging hydrogen-bond acceptor

4,4'-Bipyridine N,N'-dioxide (L) acts as a hydrogen-bond acceptor in the compounds ([M(NO3)2(H2O)4].L2) (M = Co, Ni) to form doubly-interpenetrated framework materials with sixfold topological connectivity.

Bis(mu-2-cyanopyridine-N:N')bis[(2-cyanopyridine-N)silver(I)] bis(tetrafluoroborate): an anion-linked molecular ladder

In the title compound, [Ag(2)(C(6)H(4)N(2))(4)](BF(4))(2), the Ag(I) cations adopt distorted trigonal-planar coordination geometries. The Ag(I) centres are linked via two bridging 2-cyanopyridine ligands to give a centrosymmetric dinuclear complex in which the Ag(I) coordination environment is completed by monodentate non-bridging 2-cyanopyridine ligands. Bridging Ag...F(BF(2))F...Ag interactions link the dinuclear cations into molecular ladders.