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ABSTRACT: We present the efficient synthesis of a new molecular spoked-wheel structure (MSW-3). Two derivatives with diameters of approximately 4 nm have been prepared. By highlighting the importance of pseudo-high-dilution conditions during cyclization, we were able to access the compounds on a several hundred milligram scale. In addition to the standard characterization (NMR spectroscopy, MS), we describe a detailed investigation of the optical properties of the fluorescent MSWs by comparison with appropriate model chromophores. Furthermore, a comprehensive study of the structure in solution by means of light- and X-ray scattering experiments has been conducted. Scanning tunneling microscopy (STM) revealed the two-dimensional organization of the molecules on highly oriented pyrolytic graphite and emphasized the spoked-wheel structure. The diameter of these molecules measured by small-angle X-ray scattering is in very good agreement with that obtained from STM and matches the results of molecular modeling. This confirms the rigidifying effect of the spokes, which results in highly shape-persistent nanometer-sized oblate organic compounds.
Chemistry 02/2013; · 5.93 Impact Factor
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ABSTRACT: Seeing stars: The two-dimensional patterns of the polycyclic heteroaromatic star molecules 1 on graphite vary with the side chain length. For n=12, frustrated self-assembly leads to hierarchically organized superstructures: up to 10 molecules form triangular aggregates which pack densely into hexagonal patterns with very large (15.5 nm) lattice constants.
Angewandte Chemie International Edition 07/2012; 51(34):8555-9. · 13.45 Impact Factor
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ABSTRACT: The aggregation of shape-persistent macrocycles with an empty cavity, an undecyldiether strand and a tetraethylene glycol strand leads in all cases to a macroscopic gelation of the solvent. However, the gelation temperatures are fine-tuned by the intraannular substituents.
Chemical Communications 05/2012; 48(52):6547-9. · 6.17 Impact Factor
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ABSTRACT: The interactions between two π-conjugated oligomers templated in molecular scaffolds are revealed as a function of separation and orientation, providing models of intermolecular interactions in bulk organic semiconductor materials. For a variety of dimer geometries (acyclic and macrocyclic) of the same model oligomer, no change in fluorescence spectra, fluorescence dynamics, or low-temperature single-molecule emission characteristics is observed. A small red-shift and slowing of fluorescence in the most closely spaced macrocyclic dimer structure is thought to arise both due to an intramolecular solvatochromic shift as well as from weak intramolecular aggregate formation. No corresponding effect is observed in bulk films of the acyclic model oligomer, implying the absence of intermolecular aggregate or excimer formation due to random relative dipole orientations. The largest effect of intramolecular geometry of the model dimer structures is seen in transient fluorescence depolarization, where an open ring geometry leads to rapid depolarization, compared to the corresponding macrocycle, due to the presence of a range of molecular transition dipole moment orientations. Self-assembled monolayers of the molecules on HOPG investigated by scanning-tunneling microscopy further illustrate the conformational variability of the open dimers in contrast to the fixed conformation of the closed dimers.
The Journal of Physical Chemistry B 04/2012; · 3.70 Impact Factor
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ABSTRACT: A route towards phenylene-bithiophene macrocycles via oxidative thiophene coupling under pseudo-high dilution conditions is reported. This method is applied to the synthesis of a shape-persistent thiophene macrocycle with extraannularly attached perylenebisimide moieties that forms supramolecular aggregates at the solid/liquid interface.
Chemical Communications 09/2011; 47(39):11023-5. · 6.17 Impact Factor
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ABSTRACT: Rigid rod oligo(phenylene-ethynylene-butadiynylene)s (oPEBs), "half-rings" of two rigid rods connected via a molecular clamp unit, and shape-persistent macrocycles (cyclic "half-ring dimers") are synthesized and their self-assembled monolayers (SAMs) are investigated by scanning tunneling microscopy (STM) at the interface of 1,2,4-trichlorobenzene (TCB)/highly oriented pyrolytic graphite (HOPG). The results are important for the design of molecular building blocks for two-dimensional nanoscale architectures on solid surfaces.
Chemical Communications 06/2011; 47(31):8838-40. · 6.17 Impact Factor
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ABSTRACT: Shape-persistent rigid phenylene-ethynylene-butadiynylenes form lamellar self-assembled monolayers (SAMs) at the HOPG/TCB interface, which were studied by scanning tunneling microscopy (STM) with submolecular resolution. Substitution of the terminating acetylene functions with polar cyanopropyldimethylsilyl groups leads to 2D phase separation and defined rod-rod interactions, which determine the packing distances between the rigid rods. The results stimulated the connection of rigid rods via septiarylene clamp units. They covalently link two rigid rod units and define the intramolecular rod-rod distance that matches the alkoxy substituent chain lengths. The systems can be described as half-ring structures of two rigid rods connected via a rotatable joint unit. These acetylene-terminated half-ring structures were also oligomerized under Cu and Pd catalysis to yield defined acyclic and cyclic oligomers. Detailed STM studies decoded the molecular origin of the surface patterning of such systems. The dodecyloxy side chains are adsorbed along the HOPG main axes and, together with the alkoxy backbone angle, determine the adsorption direction of the adlayers.
Langmuir 06/2011; 27(13):8205-15. · 4.19 Impact Factor
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Angewandte Chemie International Edition 08/2010; 49(35):6101-5. · 13.45 Impact Factor
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ABSTRACT: A series of dithienophenazines with different lengths of the oligomeric thiophene units (quaterthiophenes and sexithiophenes) was synthesized. The thiophene and phenazine units act as electron donors and acceptors, respectively, resulting in characteristic absorption spectra. The optical spectra were calculated using time-dependent density functional theory at the B3LYP/TZVP level and verify the experimental data. Adsorption of the dithienophenazines on highly ordered pyrolytic graphite (HOPG) was investigated by scanning tunneling microscopy, showing that one of the compounds forms highly organized self-assembled monolayers.
BEILSTEIN JOURNAL OF ORGANIC CHEMISTRY. 01/2010; 6:1180-1187.
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ABSTRACT: The reaction of solid C(58) films with atomic deuterium to yield deuterofullerenes, C(58)D(x), has been investigated by thermal desorption spectroscopy coupled with mass spectrometric detection, ultraviolet photoionization spectroscopy (21.2 eV), and atomic force microscopy (AFM). The average composition of the deuterofullerenes created depends on deuterium dose, beam flux, and surface temperature. Low deuterium exposures at room temperature yield predominantly C(58)D(6-8) cages. Saturation exposures at room temperature yield mass spectra peaked at C(58)D(26). After saturation exposures at elevated surface temperatures (approximately 500 K), the (subsequently) desorbed material reveals a comparatively narrow mass spectral distribution centered at C(58)D(30). Deuteration is associated with cleavage of covalent cage-cage bonds in the starting C(58) oligomer material, as evidenced by a considerable lowering of the sublimation energies of C(58)D(x) compared to desorption of C(58) desorbed from pure oligomer films. Correspondingly, AFM images reveal a D-induced, thermally activated transition from dendritic C(58) oligomer islands into smooth-rimmed islands composed of deuterated cages. Deuterated films exhibit a significantly lower work function than bare C(58) films. Progressing deuteration also gradually raises the surface ionization potential.
The Journal of Chemical Physics 01/2007; 125(22):224705. · 3.33 Impact Factor
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ABSTRACT: Ga+ focused ion beam (FIB) patterning was used to structure highly oriented pyrolytic graphite surfaces with square, periodic arrays of amorphous carbon defects (mesh sizes: 300 nm–2 µm). Controlled oxygen etching of these arrays leads to matrices of uniform, orientationally aligned, nm-sized, hexagonal holes. The properties of the resulting hole assembly (hole depths and lateral hole dimensions) have been investigated by means of atomic force microscopy, scanning electron microscopy and FIB sectioning. The hole dimensions and uniformity both depend on the FIB parameters and etching conditions. Etching temperatures from 500 to 700 °C were applied. Initial etch rates of up to 106 C s−1 per individual hole were observed when using oxygen pressures of 200 mbar. For an etch temperature of 590 °C the rate of etching of individual holes was found to depend measurably on the inter-hole separation. This confirms that the associated reaction kinetics is mediated by the finite diffusion length of reactive oxygen species along the graphite basal plane. Prolonged etching results in hole–hole contact and generation of mesa arrays of controllable size and shape.
Nanotechnology 11/2006; 17(23):5889. · 3.98 Impact Factor
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ABSTRACT: Novel semiconducting materials have been prepared under ultrahigh-vacuum conditions by soft-landing mass-selected Cn+ (50< or =n<60; even n) on highly oriented pyrolytic graphite surfaces at mean kinetic energies of 6 eV. In all cases, Cn films grow according to the Volmer-Weber mechanism: the surface is initially decorated by two-dimensional fractal islands, which in later deposition stages become three-dimensional dendritic mounds. We infer that Cn aggregation is governed by reactive sites comprising adjacent pentagons (or heptagons) on individual cages. The resulting covalent cage-cage bonds are responsible for the unusually high thermal stability of the films compared to solid C60. The apparent activation energies for intact Cn sublimation range from 2.2 eV for C58 to 2.6 eV for C50 as derived from thermal desorption spectra. All Cn films exhibit a common valence-band ultraviolet photoelectron spectroscopy spectral feature located around the center of a broad highest occupied molecular-orbital (HOMO)-derived band (EB approximately 2.5 eV). This feature has been assigned to Cn units covalently linked to each other in polymeric structures. To within experimental accuracy, the same work function (4.8 eV) was determined for thick films of all Cn studied. In contrast, "HOMO" ionization potentials were cage size dependent and significantly lower than that obtained for C60. C58 exhibited the lowest HOMO (6.5 eV). Band gaps of Cn films have been determined by depositing small amounts of Cs atoms onto the topmost film layer. HOMO-lowest unoccupied molecular-orbital-derived band gaps between 0.8 eV (C52) and 1.8 eV (C50) were observed, compared to 1.5 eV for solid C60.
The Journal of Chemical Physics 02/2006; 124(5):054705. · 3.33 Impact Factor
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ABSTRACT: Novel semiconducting materials have been prepared under ultrahigh-vacuum conditions by soft-landing mass-selected C n + 50 n 60; even n on highly oriented pyrolytic graphite surfaces at mean kinetic energies of 6 eV. In all cases, C n films grow according to the Volmer-Weber mechanism: the surface is initially decorated by two-dimensional fractal islands, which in later deposition stages become three-dimensional dendritic mounds. We infer that C n aggregation is governed by reactive sites comprising adjacent pentagons or heptagons on individual cages. The resulting covalent cage-cage bonds are responsible for the unusually high thermal stability of the films compared to solid C 60 . The apparent activation energies for intact C n sublimation range from 2.2 eV for C 58 to 2.6 eV for C 50 as derived from thermal desorption spectra. All C n films exhibit a common valence-band ultraviolet photoelectron spectroscopy spectral feature located around the center of a broad highest occupied molecular-orbital HOMO-derived band E B 2.5 eV. This feature has been assigned to C n units covalently linked to each other in polymeric structures. To within experimental accuracy, the same work function 4.8 eV was determined for thick films of all C n studied. In contrast, "HOMO" ionization potentials were cage size dependent and significantly lower than that obtained for C 60 . C 58 exhibited the lowest HOMO 6.5 eV. Band gaps of C n films have been determined by depositing small amounts of Cs atoms onto the topmost film layer. HOMO-lowest unoccupied molecular-orbital-derived band gaps between 0.8 eV C 52 and 1.8 eV C 50 were observed, compared to 1.5 eV for solid C 60 . © 2006 American Institute of Physics.
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ABSTRACT: Novel semiconducting materials have been prepared under ultrahigh-vacuum conditions by soft-landing mass-selected C n + 50 n 60; even n on highly oriented pyrolytic graphite surfaces at mean kinetic energies of 6 eV. In all cases, C n films grow according to the Volmer-Weber mechanism: the surface is initially decorated by two-dimensional fractal islands, which in later deposition stages become three-dimensional dendritic mounds. We infer that C n aggregation is governed by reactive sites comprising adjacent pentagons or heptagons on individual cages. The resulting covalent cage-cage bonds are responsible for the unusually high thermal stability of the films compared to solid C 60 . The apparent activation energies for intact C n sublimation range from 2.2 eV for C 58 to 2.6 eV for C 50 as derived from thermal desorption spectra. All C n films exhibit a common valence-band ultraviolet photoelectron spectroscopy spectral feature located around the center of a broad highest occupied molecular-orbital HOMO-derived band E B 2.5 eV. This feature has been assigned to C n units covalently linked to each other in polymeric structures. To within experimental accuracy, the same work function 4.8 eV was determined for thick films of all C n studied. In contrast, "HOMO" ionization potentials were cage size dependent and significantly lower than that obtained for C 60 . C 58 exhibited the lowest HOMO 6.5 eV. Band gaps of C n films have been determined by depositing small amounts of Cs atoms onto the topmost film layer. HOMO-lowest unoccupied molecular-orbital-derived band gaps between 0.8 eV C 52 and 1.8 eV C 50 were observed, compared to 1.5 eV for solid C 60 . © 2006 American Institute of Physics.
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ABSTRACT: A new solid material has been created in ultra high vacuum by utilizing the aggregation process of C58 molecules deposited onto highly oriented pyrolytic graphite from a mass selected low-energy ion beam comprising C58+. Cluster fluxes of up to 3x10(11) ions s-1 cm-2 with impinging kinetic energies of 6+/-0.5 eV were typically applied. Growth of the solid C58 phase proceeds according to the cluster-aggregation-based Volmer-Weber scenario where initially ramified 2D islands transform into 3D pyramid-like structures at higher coverages. The C58 films created exhibit much higher thermal stability than the C60 solid phase. Sublimation of C58 sets in at a temperature of 700 K. Ultraviolet photoionization spectra (He I, 21.2 eV) yield a molecular ionization potential in the range between 6.6 and 7 eV. Density functional and Hartree-Fock theories suggest that the formation of C58 dimers and higher multimers upon deposition/aggregation gives rise to the high thermal stability and unique electronic properties of this material.
Physical Chemistry Chemical Physics 08/2005; 7(14):2816-20. · 3.57 Impact Factor
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ABSTRACT: A new solid material has been created in ultra high vacuum by utilizing the aggregation process of C 58 molecules deposited onto highly oriented pyrolytic graphite from a mass selected low-energy ion beam comprising C 58 1 . Cluster fluxes of up to 3 Â 10 11 ions s À1 cm À2 with impinging kinetic energies of 6 AE 0.5 eV were typically applied. Growth of the solid C 58 phase proceeds according to the cluster-aggregation-based Volmer–Weber scenario where initially ramified 2D islands transform into 3D pyramid-like structures at higher coverages. The C 58 films created exhibit much higher thermal stability than the C 60 solid phase. Sublimation of C 58 sets in at a temperature of 700 K. Ultraviolet photoionization spectra (He I, 21.2 eV) yield a molecular ionization potential in the range between 6.6 and 7 eV. Density functional and Hartree–Fock theories suggest that the formation of C 58 dimers and higher multimers upon deposition/aggregation gives rise to the high thermal stability and unique electronic properties of this material.
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ABSTRACT: A new solid material has been created in ultra high vacuum by utilizing the aggregation process of C 58 molecules deposited onto highly oriented pyrolytic graphite from a mass selected low-energy ion beam comprising C 58 1 . Cluster fluxes of up to 3 Â 10 11 ions s À1 cm À2 with impinging kinetic energies of 6 AE 0.5 eV were typically applied. Growth of the solid C 58 phase proceeds according to the cluster-aggregation-based Volmer–Weber scenario where initially ramified 2D islands transform into 3D pyramid-like structures at higher coverages. The C 58 films created exhibit much higher thermal stability than the C 60 solid phase. Sublimation of C 58 sets in at a temperature of 700 K. Ultraviolet photoionization spectra (He I, 21.2 eV) yield a molecular ionization potential in the range between 6.6 and 7 eV. Density functional and Hartree–Fock theories suggest that the formation of C 58 dimers and higher multimers upon deposition/aggregation gives rise to the high thermal stability and unique electronic properties of this material.
