Alexander Langner

Charge-transfer-induced structural rearrangements at both sides of organic/metal interfaces

Organic/metal interfaces control the performance of many optoelectronic organic devices, including organic light-emitting diodes or field-effect transistors. Using scanning tunnelling microscopy, low-energy electron diffraction, X-ray photoemission spectroscopy, near-edge X-ray absorption fine structure spectroscopy and density functional theory calculations, we show that electron transfer at the interface between a metal surface and the organic electron acceptor tetracyano-p-quinodimethane leads to substantial structural rearrangements on both the organic and metallic sides of the interface. These structural modifications mediate new intermolecular interactions through the creation of stress fields that could not have been predicted on the basis of gas-phase neutral tetracyano-p-quinodimethane conformation.

Self-recognition and self-selection in multicomponent supramolecular coordination networks on surfaces

Self-recognition, self-selection, and dynamic self-organization are of fundamental importance for the assembly of all supramolecular systems, but molecular-level information is not generally accessible. We present direct examples of these critical steps by using scanning tunneling microscopy to study mixtures of complementary organic ligands on a copper substrate. The ligands coordinate cooperatively with iron atoms to form well ordered arrays of rectangular multicomponent compartments whose size and shape can be deliberately tuned by selecting ligands of desired length from complementary ligand families. We demonstrate explicitly that highly ordered supramolecular arrays can be produced from redundant ligand mixtures by molecular self-recognition and -selection, enabled by efficient error correction and cooperativity, and show an example of failed self-selection due to error tolerance in the ligand mixture, leading to a disordered structure.

Ordering and Stabilization of Metal-Organic Coordination Chains by Hierarchical Assembly through Hydrogen Bonding at a Surface

Keywords: copper ; scanning probe microscopy ; self-assembly ; supramolecular chemistry ; surface chemistry ; Cu(100) ; Networks ; Chemistry ; Template ; Polymers ; Acid ; Architectures ; Confinement ; Interface ; Molecules Reference EPFL-ARTICLE-160497doi:10.1002/anie.200803124View record in Web of Science Record created on 2010-11-30, modified on 2017-05-12

Assembling Isostructural Metal‐Organic Coordination Architectures on Cu(100), Ag(100) and Ag(111) Substrates

Keywords: bipyrimidine ligands ; nanostructures ; scanning probe microscopy ; self-assembly ; supramolecular chemistry ; Scanning-Tunneling-Microscopy ; Oriented Pyrolytic-Graphite ; Weak Hydrogen-Bonds ; Terephthalic Acid ; Surface ; Networks ; Coadsorption ; Organization ; Monolayers ; Interface Reference EPFL-ARTICLE-160376doi:10.1002/cphc.200800575View record in Web of Science Record created on 2010-11-30, modified on 2017-05-12

Selective Coordination Bonding in Metallo‐Supramolecular Systems on Surfaces

The confinement of molecular species in nanoscale environments strongly modifies the interaction pathways compared to homogenous, three-dimensional (bulk) conditions. A new field of chemistry featuring weak interactions, coordination bonding, and covalent chemistry at solid surfaces has recently emerged. In particular, the combination of surface-confined chemistry and scanning probe techniques with subnanometer resolution allows immediate insights into molecular self-organization processes on the nanometer level. Extended monolayers of open, two-dimensional (2D) coordination networks with high organizational periodicity, controlled symmetries, and modular dimensionality have been achieved by using designed, self-instructedmolecular building blocks. The deposition of mixtures of precursor molecules has led to more sophisticated architectures, mainly built on weak intermolecular interactions or weak interactions in combination with coordination bonding, that is, hierarchical motifs. The cooperative assembly of instructed mixtures of molecular bricks enables a high degree of structural control and functionality, for example, the stability and ordering of primary structures can be increased, or the dimensionality and geometry of supramolecular structures can be steered. Observations of molecular-level self-recognition and error correction have demonstrated collective dynamics in surface-confined supramolecular systems. A grand challenge in materials chemistry is the capability to design adaptive materials, that is, to develop systemic methods for tailored structure and function. To exploit the opportunities of systemic chemistry, a detailed understanding of the selectivity in the interaction mechanisms of molecular mixtures, if possible by direct studies at the single-molecule level, is of pivotal interest. Herein, we report on the observation of supramolecular selectivity in the simultaneous coordinative interaction of two different molecular ligands, aromatic bipyrimidines and dicarboxylic acids, with Cu and Fe atoms resulting in a selfsegregation into two distinct, surface-confined coordination network domains. The random mixture of ligands and metals separates into subdomains of pure bipyrimidine–Cu and carboxylate–Fe networks, while heteroleptic ligand combinations, though feasible, are not observed. Each 2D coordination network exhibits a tetragonal geometry with metal atom coordination nodes, but expresses unique molecular composition and spatial organization. The molecular components PBP (5,5’-bis(4-pyridyl)(2,2’bipyrimidine)) and BDA (1,4’-biphenyl-dicarboxylic acid, see Scheme 1) are co-evaporated in a 1:1 number ratio onto a Cu(100) substrate at room temperature under ultra-high vacuum (UHV) conditions. At this temperature, a diffusing copper adatom gas is present at the Cu(100) surface, which has been shown to be available for the formation of extended

Two- to one-dimensional transition of self-assembled coordination networks at surfaces by organic ligand addition

Two-dimensional metal-organic coordination networks at a Cu(100) surface are transformed to a new supramolecular structure with one-dimensional coordination character by the addition of a second organic ligand.

Modeling Ferro- and Antiferromagnetic Interactions in Metal− Organic Coordination Networks

Magnetization curves of two rectangular metal–organic coordination networks formed by the organic ligand TCNQ (7,7,8,8-tetracyanoquinodimethane) and two different (Mn and Ni) 3d transition metal atoms [M(3d)] show marked differences that are explained using first-principles density functional theory and model calculations. We find that the existence of a weakly dispersive hybrid band with M(3d) and TCNQ character crossing the Fermi level is determinant for the appearance of ferromagnetic coupling between metal centers, as it is the case of the metallic system Ni-TCNQ but not of the insulating system Mn-TCNQ. The spin magnetic moment localized at the Ni atoms induces a significant spin polarization in the organic molecule; the corresponding spin density being delocalized along the whole system. The exchange interaction between localized spins at Ni centers and the itinerant spin density is ferromagnetic. On the basis of two different model Hamiltonians, we estimate the strength of exchange couplings betwee...