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Jürgen P Rabe

Publications and source records attributed to Jürgen P Rabe.

At least 19 recordsLinked to original sources
PubMed2006

Weak charge transfer between an acceptor molecule and metal surfaces enabling organic/metal energy level tuning.

Evidence for charge transfer (CT) between the electron acceptor molecule octafluoroanthraquinone (FAQ) and the metal surfaces Ag(111) and polycrystalline Au is provided by ultraviolet photoelectron spectroscopy. The energy level alignment of subsequently deposited sexithienyl (6T) on FAQ-precovered metal substrates was investigated. Due to the metal work function change induced by the FAQ-metal CT, the hole injection barrier of 6T on FAQ-precovered metals could be reduced by up to 0.60 eV compared to that of 6T on pristine metal surfaces.

Journal Article↗
PubMed2006

Covalent connection of individualized, neutral, dendronized polymers on a solid substrate using a scanning force microscope.

The synthesis of a neutral, high-molar-mass, acrylamide-based, third-generation dendronized polymer (denpol) with a defined number of azide groups at its periphery is reported. An attach-to route is used in which a first-generation denpol is reacted with second-generation (G2) dendrons. The degree of structure perfection of the resulting denpol is quantified as 99.8 %. This value was obtained after the introduction of a fluorescence label at the sites that remained unaffected by the dendronization. The high coverage was independently confirmed for the dendronization of another first-generation polymer and a closely related G2 dendron. The third-generation denpol resulting from the first dendronization experiment was spin-coated as a sub-monolayer onto highly oriented graphite precoated with an ultrathin layer of C12H25NH2, which was introduced to provide a well-defined substrate for denpol adsorption and manipulation. Scanning force microscopy revealed single denpols, which could be moved across the surface and "welded" by covalent cross-linking induced by photochemical decomposition of the azides into highly reactive nitrenes. The successful formation of covalent bonds between two denpols was confirmed by mechanically challenging the link with the scanning force microscope (SFM) tip. This is the second reported case of a move-connect-prove sequence using polymers and the SFM, which for the first time employs noncharged denpols, thus widening the applicability of this method significantly.

Journal Article↗
PubMed2006

Synthesis of an anionically chargeable, high-molar-mass, second-generation dendronized polymer and the observation of branching by scanning force microscopy.

An efficient synthesis of a methacrylate-based, second-generation (G2) dendronized macromonomer and its free radical polymerization to the corresponding high-molar-mass G2 dendronized polymer are described. The molar mass is determined by gel permeation chromatography (GPC), light-scattering, and analytical ultracentrifugation and compared with values estimated from a scanning force microscopy (SFM) contour lengths analysis of individualized polymer strands on mica. The polymer carries terminal tert-butyl-protected carboxyl groups, the degree of deprotection of which with trifluoroacetic acid is quantified by NMR spectroscopy using the highest molar mass sample. SFM imaging of both protected (noncharged) and unprotected (charged) dendronized polymers on solid substrates reveals mostly linear chains but also some with main-chain branches. The nature of these branches is investigated and the degree roughly estimated to which they are formed. Finally, a synthetic model experiment is described which sheds some light on the aspect of whether chain transfer, a process that could lead to covalent branching, is of importance in the synthesis of the present dendronized polymers.

Journal Article↗
PubMed2006

A simple tagging system for protein encapsulation.

Molecular containers that encapsulate specific cargo can be useful for many natural and non-natural processes. We report a simple system, based on charge complementarity, for the encapsulation of appropriately tagged proteins within an engineered, proteinaceous capsid. Four negative charges per monomer were added to the lumazine synthase from Aquifex aeolicus (AaLS). The capsids formed by the engineered AaLS associate with green fluorescent protein bearing a positively charged deca-arginine tag upon coproduction in Escherichia coli. Analytical ultracentrifugation and scanning force microscopy studies indicated that the engineered AaLS retains the ability to form capsids, but that their average size was substantially increased. The success of this strategy demonstrates that both the container and guest components of protein-based encapsulation systems can be convergently designed in a straightforward manner, which may help to extend their versatility.

Amino Acid Substitution↗
PubMed2006

Adsorption of polyelectrolyte molecules to a nanostructured monolayer of amphiphiles.

We investigate the conformation of atactic poly(sodium 4-styrene sulfonate) (PSS) molecules both in water-ethanol solutions and adsorbed to a template provided by a nanostructured monolayer of flat-lying octadecylamines on graphite. The conformations of the PSS molecules in these solutions undergo two conformational transitions, a re-entry transition at 40 vol % and a coil-globule conformational transition at 80 vol % ethanol. On the surface the conformations are characterized by two different length scales: on a short length scale they are defined by the template, whereas the large-scale structures reflect the conformations in solution. We conclude that the conformational transitions are discontinuous on the level of single molecules and appear continuous in the ensemble.

Adsorption↗
PubMed2005

Optimized hole injection with strong electron acceptors at organic-metal interfaces.

The energy-level alignment at interfaces between three electroactive conjugated organic materials and Au was systematically varied by adjusting the precoverage of the metal substrate with the electron acceptor tetrafluoro-tetracyanoquinodimethane (F4-TCNQ). Photoelectron spectroscopy revealed that electron transfer from Au to adsorbed F4-TCNQ was responsible for lowering the hole-injection barrier by as much as 1.2 eV. This novel interface modification scheme is independent of the charge transfer complex formation ability of the organic materials with the electron acceptor.

Journal Article↗
PubMed2005

Solvent molecules in an epitaxially grown scaffold of star-shaped nanographenes.

A scanning tunneling microscopy (STM) study of a star-shaped hexa-peri-hexabenzocoronene (HBC-star) derivative at solid-solution interfaces is presented. The star-shape of the molecules provides voids at their periphery which can be filled by smaller molecules. The use of solvents with different affinities to fill the voids allows for the fine-tuning of the structure of self-assembled architectures of HBC-stars. This concept is demonstrated by the use of solvents of different polarity and size, which leads to the formation of complex, epitaxial architectures at the interface. For small polar solvent molecules, a surprising decrease of the tunneling barrier is observed. The self-assembled architecture may serve as a useful model system for studying the dependence of electron tunneling on order, mobility, and polarity of adsorbates.

Molecular Structure↗
PubMed2005

Double-helical ultrastructure of polycationic dendronized polymers determined by single-particle Cryo-TEM.

The ultrastructure of cationic dendronized polymers (denpols) of third and fourth generations (PG3 and PG4) in water was determined by using single-particle cryo-transmission electron microscopy (cryo-TEM). At concentrations in the region of 50 mg L(-1), networks of double-stranded fibers were revealed that exhibit well-defined diameters of 5.9 nm+/-0.4 nm for PG3 and 7.4 nm+/-0.4 nm for PG4. The structure varies with progression along the fibers, and includes a double helix with a pitch of 7.0+/-0.4 nm for PG3 and 9.0+/-0.4 nm for PG4. The formation of the double strands is attributed to the hydrophobic effect and limited crowding in the dendron shell of the third and fourth generation denpols investigated. From solutions of lower concentrations (around 10 mg L(-1)), isolated molecular fibers were adsorbed onto high-energy surfaces and examined by performing scanning force microscopy (SFM) on mica, and after staining, TEM on glow-discharged carbon films. In both cases, characteristic undulations of single strands were observed, which are attributed largely to the adsorption process.

Journal Article↗
PubMed2004

Synthesis and self-assembly of a rigid exotopic bisphenanthroline macrocycle: surface patterning and a supramolecular nanobasket.

The synthesis and characterisation of a rigid nanoscale macrocycle with two exotopic phenanthroline binding sites is reported. Scanning tunnelling microscopy (STM) at the solid-liquid interface reveals the formation of highly ordered monolayers of macrocycles with dimensions that are in good agreement with the calculated structure. Using the HETPHEN concept several bisheteroleptic coordination complexes with other phenanthrolines and a nanoscale basket assembly were prepared in presence of copper(I) ions. NMR spectroscopy, mass spectrometric data and elemental analysis point to three distinct isomers of the basket assembly in solution. A silver basket was prepared and readily converted to its copper analogue. Electrospray ionisation (ESI)-MS and spectrophotometric investigations provided additional mechanistic insight into the assembly process. Hence, the exotopic bisphenanthroline macrocycle in combination with HETPHEN concept proves to be very effective in controlling the compositional aspects of multicomponent self-assembly.

Journal Article↗
PubMed2004

Tuning intermolecular interactions in a rodlike polymer assembled at surfaces and in solution.

The isolation of single polyelectrolyte chains of water-soluble poly(isocyanodipeptide)s (PICs) bearing carboxylic acid terminated side chains occurring both at surfaces and in solution was accomplished by reducing the intermolecular interactions through complexation with cations or positively charged surfactants. Scanning force microscopy and viscosity analyses revealed that this method allows to tune the conformation of the macromolecule, which is of importance for tailoring the physicochemical properties of the material. This is particularly significant for the use of these polymer chains as seed for biomineralization processes.

Carboxylic Acids↗
PubMed2004

Scanning force microscopy of DNA translocation by the Type III restriction enzyme EcoP15I.

Type III restriction enzymes are multifunctional heterooligomeric enzymes that cleave DNA at a fixed position downstream of a non-symmetric recognition site. For effective DNA cleavage these restriction enzymes need the presence of two unmethylated, inversely oriented recognition sites in the DNA molecule. DNA cleavage was proposed to result from ATP-dependent DNA translocation, which is expected to induce DNA loop formation, and collision of two enzyme-DNA complexes. We used scanning force microscopy to visualise the protein interaction with linear DNA molecules containing two EcoP15I recognition sites in inverse orientation. In the presence of the cofactors ATP and Mg(2+), EcoP15I molecules were shown to bind specifically to the recognition sites and to form DNA loop structures. One of the origins of the protein-clipped DNA loops was shown to be located at an EcoP15I recognition site, the other origin had an unspecific position in between the two EcoP15I recognition sites. The data demonstrate for the first time DNA translocation by the Type III restriction enzyme EcoP15I using scanning force microscopy. Moreover, our study revealed differences in the DNA-translocation processes mediated by Type I and Type III restriction enzymes.

Adenosine Triphosphatases↗
PubMed2004

Self-assembly of electron donor-acceptor dyads into ordered architectures in two and three dimensions: surface patterning and columnar "double cables".

We report the synthesis and characterization of covalent dyads and multiads of electron acceptors (A) and donors (D), with the purpose of exploiting their nanophase separation behavior toward (a) two-dimensional (2D) surface patterning with well-defined integrated arrays of dissimilar molecular electronic features and (b) bulk self-assembly to noncovalent columnar versions of the so-called "double cable" systems, the likes of which could eventually provide side-by-side percolation pathways for electrons and holes in solar cells. Soluble, alkylated hexa-peri-hexabenzocoronenes (HBCs) bearing tethered anthraquinones (AQs) are shown by scanning tunneling microscopy (STM) to self-assemble at the solution-graphite interface into either defect-rich polycrystalline monolayers or extended 2D crystalline domains, depending on the number of tethered AQs. In the bulk, the thermal stability of the room-temperature HBC columnar phase is increased, which is attributed to the desired nanotriphase separation of HBC columns, insulating alkyl sheaths, and AQ units. Homeotropic alignment (columns normal to surfaces), predicted to be ideal for potential exploitation of such "double cables" in photovoltaic devices, is demonstrated.

Journal Article↗
PubMed2004

A hexa-peri-hexabenzocoronene cyclophane: an addition to the toolbox for molecular electronics.

Cyclophanes with the largest-to-date polycyclic aromatic hydrocarbon (hexa-peri-hexabenzocoronene, HBC) to be entrained in such a structural motif are reported. The two disks are covalently captured by intermolecular ring-closing olefin metathesis of dienes in good yield. DSC, optical microscopy, and WAXD show the new cyclophanes to self-assemble to thermotropic columnar liquid crystal mesophases similar to monomeric analogues. Solution spectroscopic studies reveal that the two disks within a single unit lie face-to-face, with a small average lateral offset. Self-assembly into two-dimensional crystals at a solid-liquid interface was visualized by STM, and the electrical properties of single molecules were assessed by scanning tunneling spectroscopy revealing a diode-like behavior which is similar to that previously reported for single HBC disks, laying the groundwork for future electrical interrogations of dynamic molecular complexes.

Journal Article↗
PubMed2003

Supramolecular helices via self-assembly of 8-oxoguanosines.

The cooperative effect of solvophobic interactions and hydrogen bonding has been exploited to self-assemble supramolecular helical architectures of 8-oxoguanosines in different environments. This self-assembly into helical structures is completely different from that of the parent guanosines which, in the same experimental conditions, form flat, ribbonlike structures. While optical microscopy and X-ray diffraction suggest a chiral columnar aggregate in the LC phase, NMR and Circular Dichroism reveal the presence of a helical structures in solution. Scanning Tunneling Microscopy made it possible to visualize hexagonally arranged G-quartets on graphite, which are sections of the helices packed with their long axis perpendicular to the basal plane of the substrate. Due to their rectifying electrical properties, such helices are interesting for fabricating (opto)electronic biodevices.

Circular Dichroism↗