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Sebastian Stepanow

Publications and source records attributed to Sebastian Stepanow.

10 recordsLinked to original sources
PubMed2006

Surface-template assembly of two-dimensional metal-organic coordination networks.

The self-assembly of iron-coordinated two-dimensional metal-organic networks on a Cu(100) surface has been investigated by scanning tunneling microscopy under ultra-high-vacuum conditions. We applied three rodlike polybenzene dicarboxylic acid molecules with different backbone lengths as organic linkers. The three linker molecules form topologically identical rectangular networks with Fe, all comprising iron pairs as the network nodes. Whereas the length of the linker molecules defines the dimension of the networks, the substrate also significantly influences the structural details, e.g., network orientation with respect to the substrate, geometric shape of the network cavities, Fe-carboxylate coordination configuration, and iron-iron distance.

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PubMed2006

Asymmetry induction by cooperative intermolecular hydrogen bonds in surface-anchored layers of achiral molecules.

The mesoscale induction of two-dimensional supramolecular chirality (formation of 2D organic domains with a single handedness) was achieved by self-assembly of 1,2,4-benzenetricarboxylic (trimellitic) acid on a Cu(100) surface at elevated temperatures. The combination of spectroscopic [X-ray photoelectron spectroscopy (XPS) and near-edge X-ray absorption fine structure (NEXAFS)], real-space-probe [scanning tunneling microscopy (STM)], and computational [density functional theory (DFT)] methods allows a comprehensive characterization of the obtained organic adlayers, where details of molecular adsorption geometry, intermolecular coupling, and surface chemical bonding are elucidated. The trimellitic acid species, comprising three functional carboxylic groups, form distinct stable mirror-symmetric hydrogen-bonded domains. The chiral ordering is associated with conformational restriction in the domains: molecules anchor to the substrate with an ortho carboxylate group, providing two para carboxylic acid moieties for collective lateral interweaving through H bonding, which induces a specific tilt of the molecular plane. The ease of molecular symmetry switching in domain formation makes homochiral-signature propagation solely limited by the terrace width. The molecular layer modifies the morphology of the underlying copper substrate and induces mum-sized strictly homochiral terraces.

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PubMed2006

Surface-assisted coordination chemistry and self-assembly.

This article discusses different approaches to build up supramolecular nanoarchitectures on surfaces, which were simultaneously investigated by scanning tunneling microscopy (STM) on the single-molecule level. Following this general road map, first, the hydrogen-bonding guided self-assembly of two different, structural-equivalent molecular building blocks, azobenzene dicarboxylic acid and stilbene dicarboxylic acid, was studied. Secondly, the coordination chemistry of the same building blocks, now acting as ligands in metal coordination reactions, towards co-sublimed Fe atoms was studied under near surface-conditions. Extended two-dimensional tetragonal network formation with unusual Fe2L(4/2)-dimers at the crossing points was observed on copper surfaces. Complementary to the first two experiments, a two-step approach based on the solution-based self-assembly of square-like tetranuclear complexes of the M4L4-type with subsequent deposition on graphite surfaces was investigated. One- and two-dimensional arrangements as well as single molecules of the M4L4-complexes could be observed. Moreover, the local electronic properties of a single M4L4-complexes could be probed with submolecular resolution by means of scanning tunnelling spectroscopy (STS).

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PubMed2006

Density functional theory analysis of carboxylate-bridged diiron units in two-dimensional metal-organic grids.

Carboxylate-bridged diiron units have been realized recently in two dimensions via metal-directed self-assembly of simple organic linkers. They represent notably a coupling motif stabilizing a series of nanoporous metal-organic coordination networks and are reminiscent of catalytically active sites in metalloproteins. To rationalize their atomic structure, chemical bonding, and electronic properties, we performed density functional theory calculations for exemplary Fe-terephthalate grids assembled on the Cu(100) surface. The obtained atomistic description and understanding is decisive for the considerate use of two-dimensional metal-organic coordination networks as templates, guest systems, or catalysts.

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PubMed2005

Chiral phase transition in two-dimensional supramolecular assemblies of prochiral molecules.

The self-assembly of the rodlike two-dimensional chiral molecule 4-[trans-2-(pyrid-4-yl-vinyl)] benzoic acid on the Cu(100) surface has been investigated by scanning tunneling microscopy. Upon adsorption at T>or=300 K, the molecules are deprotonated and assemble in parquet patterns when the coverage remains below a critical value. Corresponding high-resolution data reveal that the ordering implies mesoscopic chiral resolution as a result of chiroselective interactions (i.e., two domains comprise exclusively one enantiomer). When the critical coverage is exceeded, an abrupt transition to a single racemic phase is observed with a different lateral molecular coupling scheme. The shifting of the subtle balance between the weak lateral coupling, substrate bonding, and the packing requirements encountered with the increased molecular coverage is suggested to be the driving force for this homochiral-to-heterochiral phase transition.

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PubMed2005

Manipulating 2D metal-organic networks via ligand control.

High-resolution scanning tunneling microscopy has revealed how ligand control can be successfully employed to eliminate isomeric phases and defects in 2D coordination networks that are self-assembled at a surface support by replacing symmetric dicarboxylato linker ligands to dissymmetric carboxylpyridyl linker ligands.

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PubMed2005

Programming supramolecular assembly and chirality in two-dimensional dicarboxylate networks on a Cu(100) surface.

We report a comparative study on the 2D self-assembly of two related ditopic benzoic acid species, which have similar shape and endgroups but different backbone symmetry. High-resolution scanning tunneling microscopy data reveal how the symmetry information of molecular building blocks is readily expressed in the resulting chiral or nonchiral supramolecular networks. The underlying square Cu(100) surface steers network orientation and accounts for carboxylate formation, resulting in an unusual intermolecular hydrogen bond motif. Our results demonstrate that symmetry and chiral resolution in 2D supramolecular assembly can be controlled via the design of functional molecules and choice of substrate.

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PubMed2004

Towards surface-supported supramolecular architectures: tailored coordination assembly of 1,4-benzenedicarboxylate and Fe on Cu(100).

We present a comprehensive investigation of the modular assembly of surface-supported metal-organic coordination systems with specific topologies and high structural stability formed by vapor deposition of 1,4-benzenedicarboxylic acid molecules and iron atoms on a Cu(100) surface under ultra-high vacuum conditions. By making use of the two carboxylate moieties available for lateral linkage to Fe atoms, we succeeded in the fabrication of distinct Fe-carboxylate coordination architectures at the surface by carefully adjusting the ligand and metal concentration ratio and the temperature of the post-deposition annealing treatment. The mononuclear, 1D-polymeric and fully 2D-reticulated metallosupramolecular arrangements obtained were characterized in situ at the single-molecule level by scanning tunneling microscopy.

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PubMed2004

Steering molecular organization and host-guest interactions using two-dimensional nanoporous coordination systems.

Metal-organic coordination networks (MOCNs) have attracted wide interest because they provide a novel route towards porous materials that may find applications in molecular recognition, catalysis, gas storage and separation. The so-called rational design principle-synthesis of materials with predictable structures and properties-has been explored using appropriate organic molecular linkers connecting to metal nodes to control pore size and functionality of open coordination networks. Here we demonstrate the fabrication of surface-supported MOCNs comprising tailored pore sizes and chemical functionality by the modular assembly of polytopic organic carboxylate linker molecules and iron atoms on a Cu(100) surface under ultra-high-vacuum conditions. These arrays provide versatile templates for the handling and organization of functional species at the nanoscale, as is demonstrated by their use to accommodate C(60) guest molecules. Temperature-controlled studies reveal, at the single-molecule level, how pore size and chemical functionality determine the host-guest interactions.

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