A Small Molecule Walks Along a Surface Between Porphyrin Fences That Are Assembled In Situ

An on‐surface bimolecular system is described, comprising a simple divalent bis(imidazolyl) molecule that is shown to “walk” at room temperature via an inchworm mechanism along a specific pathway terminated at each end by oligomeric “fences” constructed on a monocrystalline copper surface. Scanning tunneling microscopy shows that the motion of the walker occurs along the [1$\bar 1$ 0] direction of the Cu surface with remarkably high selectivity and is effectively confined by the orthogonal construction of covalent porphyrin oligomers along the [001] surface direction, which serve as barriers. Density functional theory shows that the mobile molecule walks by attaching and detaching the nitrogen atoms in its imidazolyl “legs” to and from the protruding close‐packed rows of the metal surface and that it can transit between two energetically equivalent extended and contracted conformations by overcoming a small energy barrier.     

Clean coupling of unfunctionalized porphyrins at surfaces to give highly oriented organometallic oligomers

The direct coupling of complex, functional organic molecules at a surface is one of the outstanding challenges in the road map to future molecular devices. Equally demanding is to meet this challenge without recourse to additional functionalization of the molecular building blocks and via clean surface reactions that leave no surface contamination. Here, we demonstrate the directional coupling of unfunctionalized porphyrin molecules--large aromatic multifunctional building blocks--on a single crystal copper surface, which generates highly oriented one-dimensional organometallic macromolecular nanostructures (wires) in a reaction which generates gaseous hydrogen as the only byproduct. In situ scanning tunneling microscopy and temperature programmed desorption, supported by theoretical modeling, reveal that the process is driven by C-H bond scission and the incorporation of copper atoms in between the organic components to form a very stable organocopper oligomer comprising organometallic edge-to-edge porphyrin-Cu-porphyrin connections on the surface that are unprecedented in solution chemistry. The hydrogen generated during the reaction leaves the surface and, therefore, produces no surface contamination. A remarkable feature of the wires is their stability at high temperatures (up to 670 K) and their preference for 1D growth along a prescribed crystallographic direction of the surface. The on-surface formation of directional organometallic wires that link highly functional porphyrin cores via direct C-Cu-C bonds in a single-step synthesis is a new development in surface-based molecular systems and provides a versatile approach to create functional organic nanostructures at surfaces.     

NO and dichloroethene reactivity on single crystal and supported Cu nanoparticles: just how big is the materials gap?

Infrared and molecular beam experiments are used to compare and contrast the adsorption and reaction of NO and trans-1,2-dichloroethene on Cu(110) and on Cu nanoclusters deposited on a 5 A thick Al(2)O(3) film. The overall reaction of NO, leading to decomposition, is almost identical in the two systems, with both types of Cu surfaces promoting the formation of NO dimers, which are precursors to the dissociation products N(2)O, N(2) and O. Although the overall reaction is independent of surface structure, the IR spectra clearly show differences in the adsorption sites occupied on the single crystal and the clusters, a disparity that is also shown by CO adsorption experiments. In contrast, the reaction pathway of dichloroethene does show differences on the two types of Cu surfaces. On both surfaces, the initial reaction step is insensitive to structure and efficient dechlorination leads to the production of adsorbed acetylene. However, the fate of this intermediate depends critically on the underlying surface. On Cu(110), the acetylene trimerises readily into benzene at 350 K. However, this reaction shows a significant size dependent behaviour on the supported nanocluster systems, with the probability for trimerisation diminishing with decreasing cluster size.     

Hydrogen and coordination bonding supramolecular structures of trimesic acid on Cu(110)

The adsorption of trimesic acid (TMA) on Cu(110) has been studied in the temperature range between 130 and 550 K and for coverages up to one monolayer. We combine scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), reflection absorption infrared spectroscopy (RAIRS), X-ray photoemission spectroscopy (XPS), and density functional theory (DFT) calculations to produce a detailed adsorption phase diagram for the TMA/Cu(110) system as a function of the molecular coverage and the substrate temperature. We identify a quite complex set of adsorption phases, which are determined by the interplay between the extent of deprotonation, the intermolecular bonding, and the overall energy minimization. For temperatures up to 280 K, TMA molecules are only partly deprotonated and form hydrogen-bonded structures, which locally exhibit organizational chirality. Above this threshold, the molecules deprotonate completely and form supramolecular metal-organic structures with Cu substrate adatoms. These structures exist in the form of single and double coordination chains, with the molecular coverage driving distinct phase transitions.     

Direct visualization of chirality in two dimensions

Surface science techniques have been used to investigate 2D chirality induced by molecular adsorption at the Cu(1 1 0) surface. Particular emphasis has been devoted to presenting molecular resolution scanning tunnelling microscope images which provide direct, real-space access to chiral behaviour at the nanoscale. The systems chosen demonstrate the gamut of chiral organization: conglomerates, racemates and solid solutions. They show the creation of chirality from achiral adsorbates, and the enantiospecific hosting of an intrinsic chiral guest. In short, chirality at surfaces manifests its full range of behaviours, and STM provides the means by which that behaviour can be captured.     

Challenges and prospects of metal sulfide materials for supercapacitors

Worldwide, the research on advanced materials for energy storage devices has drawn greater attention. Numerous works on different energy storage materials has been reported and still continuing. Among the energy storage devices, electrochemical supercapacitors (ESs) are one of the most investigated topics. The globalization and increasing demand of smart and flexible devices has forced the current research to develop low-cost, high-energy density and stable ESs. In this regard, metal sulfides (MSs)–based materials have been envisioned for ESs applications owing to their unique and promising properties. Recently, several research articles have been published on MSs-based electrodes for ESs with enhanced performances. This review presents a brief survey on such recent developments towards synthesis of MSs and their use as an efficient electrode material in ESs. The challenges and future aspect involved with MSs to develop and establish it as a promising energy storage material are also discussed.     

Racemic versus enantiopure alanine on Cu(110): an experimental study

The adsorption of racemic alanine on the Cu(110) surface has been compared to that of enantiopure alanine using low-energy electron diffraction (LEED), reflection absorption infrared spectroscopy (RAIRS), and scanning tunneling microscopy (STM). No evidence of chiral resolution at the surface was observed for the racemic system, indicating that the formation of separate enantiopure areas is not preferred. Also, in contrast to the enantiopure system, no chirally organized phase was observed for the racemic system. LEED shows that both systems display a common (3 x 2) phase at high coverage. However, the pathway and kinetic barriers to this phase differ markedly for the racemic and the enantiopure systems, with the racemic (3 x 2) appearing at a temperature that is more than 100 K below that required for the enantiopure system. In addition, we report intriguing complexities for the (3 x 2) LEED structure that is ubiquitous in amino acid/Cu(110) systems. First, a common (3 x 2) pattern with a zigzag distortion can be associated with both the racemic and enantiopure systems. For the racemic system, the coverage can be increased further to give a "true" (3 x 2) LEED pattern, which is a transformation that is impossible to enact for the enantiopure system. Most importantly, STM images of the "distorted" and "true" (3 x 2) structures created in the racemic system show subtle differences with neither arrangement being fully periodic over distances greater than a few molecules. Thus, the (3 x 2) phase appears to be more complicated than at first indicated and will require more complex models for a full interpretation.     

Tailoring homochirality at surfaces: going beyond molecular handedness

Chirality can be bestowed upon a surface by the adsorption of molecules and is usually discussed in terms of the molecular handedness. However, the adsorption process often leads to a new manifestation of chirality in the form of the adsorption footprint, which can also be chiral and generate mirror-images in 2-D. Therefore, in describing the chirality of the interface, one must consider both the handedness and the adsorption 'footedness' of the system. Specifically, the creation of a truly homochiral surface must ensure that a single chirality is expressed for each aspect, and requires not only the control of molecule handedness but also direct control over footedness. Here, we demonstrate the ability to engineer homochiral footedness by a structural modification of enantiopure (S)-proline, which normally creates a (4 × 2) organization on a Cu(110) surface with heterochiral footedness. This modification of proline via the addition of a double bond within the pyrrolidine ring, yielding 3-pyrroline-2-carboxylic acid (PCA), is sufficient to drive the footprints of the entire (4 × 2) assembly from heterochiral to homochiral, leading to the creation of a truly homochiral interface The effects of modifications upon the footprint arrangements were characterized at the single-molecule level by scanning tunnelling microscopy, reflection absorption infrared spectroscopy and periodic density functional theory calculations. The control of adsorption footprints is not only pivotal to tailoring chirality at surfaces but also plays a key role in dictating the organization, the outward facing functionalities and the response of the organic-inorganic interface.