Thomas A. Jung

Controlling spins in adsorbed molecules by a chemical switch

The development of chemical systems with switchable molecular spins could lead to the architecture of materials with controllable magnetic or spintronic properties. Here, we present conclusive evidence that the spin of an organometallic molecule coupled to a ferromagnetic substrate can be switched between magnetic off and on states by a chemical stimulus. This is achieved by nitric oxide (NO) functioning as an axial ligand of cobalt(II)tetraphenylporphyrin (CoTPP) ferromagnetically coupled to nickel thin-film (Ni(001)). On NO addition, the coordination sphere of Co2+ is modified and a NO–CoTPP nitrosyl complex is formed, which corresponds to an off state of the Co spin. Thermal dissociation of NO from the nitrosyl complex restores the on state of the Co spin. The NO-induced reversible off–on switching of surface-adsorbed molecular spins observed here is attributed to a spin trans effect.

Aggregation and Contingent Metal/Surface Reactivity of 1,3,8,10-Tetraazaperopyrene (TAPP) on Cu(111)

The structural chemistry and reactivity of 1,3,8,10-tetraazaperopyrene (TAPP) on Cu(111) under ultra-high-vacuum (UHV) conditions has been studied by a combination of experimental techniques (scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy, XPS) and DFT calculations. Depending on the deposition conditions, TAPP forms three main assemblies, which result from initial submonolayer coverages based on different intermolecular interactions: a close-packed assembly similar to a projection of the bulk structure of TAPP, in which the molecules interact mainly through van der Waals (vDW) forces and weak hydrogen bonds; a porous copper surface coordination network; and covalently linked molecular chains. The Cu substrate is of crucial importance in determining the structures of the aggregates and available reaction channels on the surface, both in the formation of the porous network for which it provides the Cu atoms for surface metal coordination and in the covalent coupling of the TAPP molecules at elevated temperature. Apart from their role in the kinetics of surface transformations, the available metal adatoms may also profoundly influence the thermodynamics of transformations by coordination to the reaction product, as shown in this work for the case of the Cu-decorated covalent poly(TAPP-Cu) chains.

Band Formation from Coupled Quantum Dots Formed by a Nanoporous Network on a Copper Surface

Coupled Copper Surface States Periodic arrays of quantum dots can create new electronic states that arise from coupling of the states created by confinement. Lobo-Checa et al. (p. 300) show that the electronic surface-state of copper can be converted into a regular array of quantum dots. An organic overlayer that is created on the copper surface has pores 1.6 nanometers in diameter that trap the surface states. The coupling of these trapped states is revealed in photoemission experiments, in which a shallow dispersive electronic band is formed. Trapped electronic states induced by a nanoporous overlayer create an artificial electronic band structure. The properties of crystalline solids can to a large extent be derived from the scale and dimensionality of periodic arrays of coupled quantum systems such as atoms and molecules. Periodic quantum confinement in two dimensions has been elusive on surfaces, mainly because of the challenge to produce regular nanopatterned structures that can trap electronic states. We report that the two-dimensional free electron gas of the Cu(111) surface state can be trapped within the pores of an organic nanoporous network, which can be regarded as a regular array of quantum dots. Moreover, a shallow dispersive electronic band structure is formed, which is indicative of electronic coupling between neighboring pore states.

Trimodular Engineering of Linear Supramolecular Miniatures on Ag(111) Surfaces Controlled by Complementary Triple Hydrogen Bonds

Thespecificity, directionality, dynamics, and complementarity ofsuchinteractionscanallowforthedesignofalargelibraryoforganic modules bearing H-bond donor (D) and/or acceptor(A) moieties with specific programmed functions and struc-tures that could ultimately lead to the construction of manydesired functional assemblies. So far, this method has beensuccessfullyemployedonsolidsurfacesforthepreparationofextendedone-

Supramolecular synthons on surfaces: controlling dimensionality and periodicity of tetraarylporphyrin assemblies by the interplay of cyano and alkoxy substituents.

The self-assembly of three porphyrin derivatives was studied in detail on a Cu(111) substrate by means of scanning tunneling microscopy (STM). All derivatives have two 4-cyanophenyl substituents in diagonally opposed meso-positions of the porphyrin core, but differ in the nature of the other two meso-alkoxyphenyl substituents. At coverages below 0.8 monolayers, two derivatives form molecular chains, which evolve into nanoporous networks at higher coverages. The third derivative self-assembles directly into a nanoporous network without showing a one-dimensional phase. The pore-to-pore distances for the three networks depend on the size and shape of the alkoxy substituents. All observed effects are explained by 1) different steric demands of the alkoxy residues, 2) apolar (mainly dispersion) interactions between the alkoxy chains, 3) polar bonding involving both cyanophenyl and alkoxyphenyl substituents, and 4) the entropy/enthalpy balance of the network formation.

Self‐Assembly and Two‐Dimensional Spontaneous Resolution of Cyano‐Functionalized [7]Helicenes on Cu(111)

Birds of a feather flock together: STM and DFT studies provide the first example of spontaneous chiral resolution of a helicene on a surface. Racemic 6,13-dicyano[7]helicene forms fully segregated domains of pure enantiomers (2D conglomerate) on Cu(111). The propensity of the system to optimize intermolecular CN⋅⋅⋅HC(Ar) hydrogen bonding and CN⋅⋅⋅CN dipolar interactions translates into chiral recognition with preferential assembly of homochiral molecules.

STM fingerprint of molecule-adatom interactions in a self-assembled metal-organic surface coordination network on Cu(111)

A novel approach of identifying metal atoms within a metal-organic surface coordination network using scanning tunnelling microscopy (STM) is presented. The Cu adatoms coordinated in the porous surface network of 1,3,8,10-tetraazaperopyrene (TAPP) molecules on a Cu(111) surface give rise to a characteristic electronic resonance in STM experiments. Using density functional theory calculations, we provide strong evidence that this resonance is a fingerprint of the interaction between the molecules and the Cu adatoms. We also show that the bonding of the Cu adatoms to the organic exodentate ligands is characterised by both the mixing of the nitrogen lone-pair orbitals of TAPP with states on the Cu adatoms and the partial filling of the lowest unoccupied molecular orbital (LUMO) of the TAPP molecule. Furthermore, the key interactions determining the surface unit cell of the network are discussed.

Reduction of the contact resistance by doping in pentacene few monolayers thin film transistors and self-assembled nanocrystals

The authors study the contact resistance of gold-pentacene interface by applying the transmission-line method to a few monolayers thick pentacene films in thin film transistor geometry. It was found that tetrafluorotetracyanoquinodimethane (F4TCNQ) doping reduces the contact resistance by more than a factor of 20. In addition, a significant improvement of the conductance of pentacene nanocrystals self-assembled on 10nm gap Au nanojunction devices by F4TCNQ doping is observed. The result demonstrates the importance of doping on the performance of organic electronic devices from 10nm scale up to 100μm scale.

Chirality Transfer in 1D Self-Assemblies : Influence of H-Bonding vs Metal Coordination between Dicyano[7]helicene Enantiomers

Chiral recognition as well as chirality transfer in supramolecular self-assembly and on-surface coordination is studied for the enantiopure 6,13-dicyano[7]helicene building block. It is remarkable that, with this helical molecule, both H-bonded chains and metal-coordinated chains can be formed on the same substrate, thereby allowing for a direct comparison of the chain bonding motifs and their effects on the self-assembly in experiment and theory. Conformational flexure and both adsorbate/adsorbent and intermolecular interactions can be identified as factors influencing the chiral recognition at the binding site. The observed H-bonded chains are chiral, however, the overall appearance of Cu-coordinated chains is no longer chiral. The study was performed via scanning tunneling microscopy, X-ray-photoelectron spectroscopy and density functional theory calculations. We show a significant influence of the molecular flexibility and the type of bonding motif on the chirality transfer in the 1D self-assembly.

On-surface coordination chemistry of planar molecular spin systems: novel magnetochemical effects induced by axial ligands

Paramagnetic transition-metal complexes assembled on surfaces are of great interest for potential applications in organic spintronics. The magnetochemical interactions of the spin of the metal centers with both ferromagnetic surfaces and optional axial ligands are yet to be understood. We use a combination of X-ray magnetic circular dichroism (XMCD) and quantum-chemical simulations based on density functional theory (DFT + U) to investigate these metal–organic interfaces with chemically tunable magnetization. The interplay between an optional axial ligand (NO, spin S = 1/2 or NH3, S = 0) and Ni and Co ferromagnetic surfaces affecting the spin of Co(II) tetraphenylporphyrin (d7, S = 1/2), Fe(II) tetraphenylporphyrin (d6, S = 1), Mn(II) tetraphenylporphyrin (d5, S = 5/2) and Mn(II) phthalocyanine (d5, S = 3/2) is studied. We find that the structural trans effect on the surface rules the molecular spin state, as well as the sign and strength of the exchange interaction with the substrate. We refer to this observation as the surface spin-trans effect.