169 research outputs found
From Au-Thiolate Chains to Thioether Sierpiński Triangles: The Versatile Surface Chemistry of 1,3,5-Tris(4-Mercaptophenyl)Benzene on Au(111)
Self-assembly of 1,3,5-tris(4-mercaptophenyl)benzene (TMB) – a three-fold symmetric, thiol functionalized aromatic molecule – was studied on Au(111) with the aim to realize extended Au-thiolate linked molecular architectures. The focus lay on resolving thermally activated structural and chemical changes by a combination of microscopy and spectroscopy. Thereby Scanning Tunneling Microscopy provided submolecularly resolved structural information, while the chemical state of sulfur was assessed by X-ray Photoelectron Spectroscopy. Directly after room temperature deposition only less well ordered structures were observed. Mild annealing promoted the first structural transition into ordered molecular chains, partly organized in homochiral molecular braids. Further annealing led to self-similar Sierpiński triangles, while annealing at even higher temperatures again resulted in mostly disordered structures. Both the irregular aggregates observed at room temperature and the chains were identified as metal-organic assemblies, whereby two out of the three intermolecular binding motifs are energetically equivalent according to Density Functional Theory simulations. The emergence of Sierpiński triangles is driven by a chemical transformation, i.e. the conversion of coordinative Au-thiolate to covalent thioether linkages, and can be further understood by Monte Carlo simulations. The great structural variance of TMB on Au(111) can on one hand be explained by the energetic equivalence of two binding motifs. On the other hand, the unexpected chemical transition even enhances the structural variance and results in thiol-derived covalent molecular architectures
Broken symmetry and the variation of critical properties in the phase behaviour of supramolecular rhombus tilings
The degree of randomness, or partial order, present in two-dimensional
supramolecular arrays of isophthalate tetracarboxylic acids is shown to vary
due to subtle chemical changes such as the choice of solvent or small
differences in molecular dimensions. This variation may be quantified using an
order parameter and reveals a novel phase behaviour including random tiling
with varying critical properties as well as ordered phases dominated by either
parallel or non-parallel alignment of neighbouring molecules, consistent with
long-standing theoretical studies. The balance between order and randomness is
driven by small differences in the intermolecular interaction energies, which
we show, using numerical simulations, can be related to the measured order
parameter. Significant variations occur even when the energy difference is much
less than the thermal energy highlighting the delicate balance between entropic
and energetic effects in complex self-assembly processes
Two-Dimensional 1,3,5-Tris(4-carboxyphenyl)benzene Self-Assembly at the 1-Phenyloctane/Graphite Interface Revisited
International audienceTwo-dimensional (2D) self-assembly of star-shaped 1,3,5-tris(4-carboxyphenyl)benzene molecules is investigated. Scanning tunneling microscopy reveals that this molecule can form three hydrogen-bonded networks at the 1-phenyloctane/graphite interface. One of these structures is close-packed and the two other ones are porous structures, with hexagonal and rectangular cavities. The network with rectangular cavities appears to be the most stable structure
Bio-inspired nanocatalysts for the oxygen reduction reaction
Electrochemical conversions at fuel cell electrodes are complex processes. In particular, the oxygen reduction reaction has substantial overpotential limiting the electrical power output efficiency. Effective and inexpensive catalytic interfaces are therefore essential for increased performance. Taking inspiration from enzymes, earth-abundant metal centres embedded in organic environments present remarkable catalytic active sites. Here we show that these enzyme-inspired centres can be effectively mimicked in two-dimensional metal-organic coordination networks self-assembled on electrode surfaces. Networks consisting of trimesic acid and bis-pyridyl-bispyrimidine coordinating to single iron and manganese atoms on Au(111) effectively catalyse the reduction and reveal distinctive catalytic activity in alkaline media. These results demonstrate the potential of surface-engineered metal-organic networks for electrocatalytic conversions. Specifically designed coordination complexes at surfaces inspired by enzyme cofactors represent a new class of nanocatalysts with promising applications in electrocatalysis.Fil: Grumelli, Doris Elda. Consejo Nacional de Investigaciones CientĂficas y TĂ©cnicas. Centro CientĂfico TecnolĂłgico la Plata. Instituto de Investigaciones FisicoquĂmicas TeĂłricas y Aplicadas; Argentina. Max Planck Institute for Solid State Research; AlemaniaFil: Wurtser, Benjamin. Max Planck Institute for Solid State Research; AlemaniaFil: Stepanow, Sabastian. Max Planck Institute for Solid State Research; AlemaniaFil: Kern, Klaus. Max Planck Institute for Solid State Research; Alemania. Ecole Polytechnique Federale de Lausanne; Suiz
Supramolecular networks stabilise and functionalise black phosphorus
The limited stability of the surface of black phosphorus (BP) under atmospheric conditions is a significant constraint on the exploitation of this layered material and its few layer analogue, phosphorene, as an optoelectronic material. Here we show that supramolecular networks stabilised by hydrogen bonding can be formed on BP, and that these monolayer-thick films can passivate the BP surface and inhibit oxidation under ambient conditions. The supramolecular layers are formed by solution deposition and we use atomic force microscopy to obtain images of the BP surface and hexagonal supramolecular networks of trimesic acid and melamine cyanurate (CA.M) under ambient conditions. The CA.M network is aligned with rows of phosphorus atoms and forms large domains which passivate the BP surface for more than a month, and also provides a stable supramolecular platform for the sequential deposition of 1,2,4,5-tetrakis(4-carboxyphenyl)benzene to form supramolecular heterostructures
On surface Ullmann polymerization via intermediate organometallic networks on Ag 111
The role of organometallic intermediates during on-surface polymerization via Ullmann coupling was studied on Ag(111).</p
The Role of Kinetics versus Thermodynamics in Surface Assisted Ullmann Coupling on Gold and Silver Surfaces
Surface-assisted Ullmann coupling
is the workhorse of on-surface
synthesis. Despite its obvious relevance, many fundamental and mechanistic
aspects remain elusive. To shed light on individual reaction steps
and their progression with temperature, temperature-programmed X-ray
photoelectron spectroscopy (TP-XPS) experiments are performed for
a prototypical model system. The activation of the coupling by initial
dehalogenation is tracked by monitoring Br 3d core levels, whereas the C 1s
signature is used to follow the emergence of metastable organometallic
intermediates and their conversion to the final covalent products
upon heating in real time. The employed 1,3,5-trisÂ(4-bromophenyl)Âbenzene
precursor is comparatively studied on Ag(111) versus Au(111), whereby
intermolecular bonds and network topologies are additionally characterized
by scanning tunneling microscopy (STM). Besides the well-comprehended
differences in activation temperatures for debromination, the thermal
progression shows marked differences between the two surfaces. Debromination
proceeds rapidly on Ag(111), but is relatively gradual on Au(111).
While on Ag(111) debromination is well explained by first-order reaction
kinetics, thermodynamics prevail on Au(111), underpinned by a close
agreement between experimentally deduced and density functional theory
(DFT) calculated reaction enthalpies. Thermodynamically controlled
debromination on Au(111) over a large temperature range implies an
unexpectedly long lifetime of surface-stabilized radicals prior to
covalent coupling, as corroborated by TP-XPS of C 1s core levels.
These insights are anticipated to play an important role regarding
our ability to rationally synthesize atomically precise low-dimensional
covalent nanostructures on surfaces
Steering Self Assembly of Three Dimensional Iptycenes on Au 111 by Tuning Molecule Surface Interactions
Self assembly of three dimensional molecules is scarcely studied on surfaces. Their modes of adsorption can exhibit far greater variability compared to nearly planar molecules that adsorb mostly flat on surfaces. This additional degree of freedom can have decisive consequences for the expression of intermolecular binding motifs, hence the formation of supramolecular structures. The determining molecule surface interactions can be widely tuned, thereby providing a new powerful lever for crystal engineering in two dimensions. Here, we study the self assembly of triptycene derivatives with anthracene blades on Au 111 by Scanning Tunneling Microscopy, Near Edge X ray Absorption Fine Structure and Density Functional Theory. The impact of molecule surface interactions was experimentally tested by comparing pristine with iodine passivated Au 111 surfaces. Thereby, we observed a fundamental change of the adsorption mode that triggered self assembly of an entirely different structur
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