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Sökning: WFRF:(Ocal C.)

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2.
  • Frederiksen, Thomas, et al. (författare)
  • Exploring the Tilt-Angle Dependence of Electron Tunneling across Molecular Junctions of Self-Assembled Alkanethiols
  • 2009
  • Ingår i: ACS Nano. - : American Chemical Society (ACS). - 1936-0851 .- 1936-086X. ; 3:8, s. 2073-2080
  • Tidskriftsartikel (refereegranskat)abstract
    • Electronic transport mechanisms in molecular junctions are investigated by a combination of first-principles calculations and current-voltage measurements of several well-characterized structures. We study self-assembled layers of alkanethiols grown on Au(111) and form tunnel junctions by contacting the molecular layers with the tip of a conductive force microscope. Measurements done under low-load conditions permit us to obtain reliable tilt-angle and molecular length dependencies of the low-bias conductance through the alkanethiol layers. The observed dependence on tilt-angle is stronger for the longer molecular chains. Our calculations confirm the observed trends and explain them as a result of two mechanisms, namely, a previously proposed intermolecular tunneling enhancement as well as a hitherto overlooked tilt-dependent molecular gate effect.
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3.
  • Iavicoli, P., et al. (författare)
  • Tuning the supramolecular chirality of one-and two-dimensional aggregates with the number of stereogenic centers in the component porphyrins
  • 2010
  • Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 132:27, s. 9350-9362
  • Tidskriftsartikel (refereegranskat)abstract
    • A synthetic strategy was developed for the preparation of porphyrins containing between one and four stereogenic centers, such that their molecular weights vary only as a result of methyl groups which give the chiral forms. The low-dimensional nanoscale aggregates of these compounds Reveal the profound effects of this varying molecular chirality on their supramolecular structure and optical activity. The number of stereogenic centers influences significantly the self-assembly and chiral structure of the aggregates of porphyrin molecules described here. A scanning tunneling microscopy study of monolayers on graphite shows that the degree of structural chirality with respect to the surface increases almost linearly with the number of stereogenic centers, and only one handedness is formed in the monolayers, whereas the achiral compound forms a mixture of mirror-image domains at the surface. In solution, four hydrogen bonds induce the formation of an H-aggregate, and circular dichroism measurements and theoretical studies indicate that the compounds self-assemble into helical structures. Both the chirality and stability of the aggregates depend critically on the number of stereocenters. The chiral porphyrin derivatives gelate methylcyclohexane at concentrations dependent on the number and position of chiral groups at the periphery of the aromatic core, reflecting the different aggregation forces of the molecules in solution. Increasing the number of stereogenic centers requires more material to immobilize the solvent, in all likelihood because of the greater solubility of the porphyrins. The vibrational circular dichroism spectra of the gels show that all compounds have a chiral environment around the amide bonds, confirming the helical model proposed by calculations. The morphologies of the xerogels (studied by scanning electron microscopy and scanning force microscopy) are similar, although more fibrous features are present in the molecules with fewer stereogenic centers. Importantly, the presence of only one stereogenic center, bearing a methyl group as the desymmetrizing ligand, in a molecule of considerable molecular weight is enough to induce singlehanded chirality in both the one-and two-dimensional supramolecular self-assembled structures.
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