| 1. |
- Brede, Jens, et al.
(författare)
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Adsorption and conformation of porphyrins on metallic surfaces
- 2009
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Ingår i: JOURNAL OF VACUUM SCIENCE and TECHNOLOGY B. - : American Vacuum Society. - 1071-1023. ; 27:2, s. 799-804
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Tidskriftsartikel (refereegranskat)abstract
- Tetraphenyl porphyrins (TPP) belong to a highly interesting class of molecules with a large variety of electronic, magnetic, and structural properties. So far, local investigations by scanning probe techniques were primarily focused on larger agglomerates of TPP molecules. Here, experimental results of the observation and manipulation of isolated molecules adsorbed on cold metal substrates by means of low temperature scanning tunneling microscopy are presented. Depending on the surface geometry, i.e., Cu(111) VS, Cu(100) three distinct deformations of the molecular structure are identified reflecting the interaction of the phenyl periphery with the substrate. In a second step, controlled manipulation in terms of deformation of the porphyrin core, ligand dissociation, and lateral displacement of the phenyl periphery are demonstrated.
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| 2. |
- Brede, Jens, et al.
(författare)
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Dynamics of molecular self-ordering in tetraphenyl porphyrin monolayers on metallic substrates
- 2009
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Ingår i: NANOTECHNOLOGY. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 20:27, s. 275602-
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Tidskriftsartikel (refereegranskat)abstract
- A molecular model system of tetraphenyl porphyrins (TPP) adsorbed on metallic substrates is systematically investigated within a joint scanning tunnelling microscopy/molecular modelling approach. The molecular conformation of TPP molecules, their adsorption on a gold surface and the growth of highly ordered TPP islands are modelled with a combination of density functional theory and dynamic force field methods. The results indicate a subtle interplay between different contributions. The molecule-substrate interaction causes a bending of the porphyrin core which also determines the relative orientations of phenyl legs attached to the core. A major consequence of this is a characteristic (and energetically most favourable) arrangement of molecules within self-assembled molecular clusters; the phenyl legs of adjacent molecules are not aligned parallel to each other (often denoted as pi-pi stacking) but perpendicularly in a T-shaped arrangement. The results of the simulations are fully consistent with the scanning tunnelling microscopy observations, in terms of the symmetries of individual molecules, orientation and relative alignment of molecules in the self-assembled clusters.
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| 3. |
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| 4. |
- Hayton, James, et al.
(författare)
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Pi-Conjugated Molecular Nanowire Stacks Investigated by Frequency-Modulation Atomic Force Microscopy in the qPlus Configuration
- 2009
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Ingår i: APPLIED PHYSICS EXPRESS. - 1882-0778. ; 2:9, s. 091501-
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Tidskriftsartikel (refereegranskat)abstract
- Self-assembled pi-conjugated nanowire stacks were imaged using a tuning-fork probe mounted in the qPlus configuration in frequency modulation mode under ultrahigh vacuum. High resolution topographic and dissipation images demonstrate the applicability of such probes to soft conjugated materials, with sub-molecular resolution achieved perpendicular to the stacking axis. A new insight is gained from the damping contrast into the local mechanical properties of edge-on pi-conjugated stacks.
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| 5. |
- Isare, B., et al.
(författare)
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Engineering the cavity of self-assembled dynamic nanotubes
- 2009
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 113:11, s. 3360-3364
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Tidskriftsartikel (refereegranskat)abstract
- By analogy with hydrogen-bonded molecular capsules, self-assembled nanotubes are of interest because they can temporarily isolate guest molecules from the solution. We show here that the stability of a particular bis-urea based dynamic self-assembled nanotube is related to the possibility for solvent molecules to fit inside the tubular cavity. The diameter of the cavity can be finely tuned by introducing a modified monomer in controlled amount.
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| 6. |
- Linares, Mathieu, et al.
(författare)
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Effects of π -stacking interactions on the near carbon K -edge x-ray absorption fine structure : A theoretical study of the ethylene pentamer and the phthalocyanine dimer
- 2009
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Ingår i: Journal of Chemical Physics. - : American Institute of Physics (AIP). - 0021-9606 .- 1089-7690. ; 130:10
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Tidskriftsartikel (refereegranskat)abstract
- X-ray absorption spectra have been determined for ethylene and free base phthalocyanine at the carbon K -edge with use of the complex polarization propagator method combined with Kohn-Sham density functional theory and the Coulomb attenuated method B3LYP exchange-correlation functional. Apart from isolated molecules, the study includes π -stacked systems of the phthalocyanine dimer and the ethylene dimer, trimer, tetramer, and pentamer. For ethylene, π -stacking involves a reduction in transition energy of the valence π* -band by some 70 meV and large spectral changes (regarding also shape and intensity) of the Rydberg bands. For phthalocyanine, there are large spectral changes in the entire valence π * -part of the spectrum. © 2009 American Institute of Physics.
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| 7. |
- Linares, Mathieu, et al.
(författare)
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Expression of chirality in molecular layers at surfaces : Insights from modelling
- 2009
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Ingår i: Chemical Society Reviews. - : Royal Society of Chemistry (RSC). - 0306-0012 .- 1460-4744. ; 38:3, s. 806-816
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Tidskriftsartikel (refereegranskat)abstract
- This tutorial review illustrates how modelling can be used to understand the structure and properties of chiral surfaces formed by adsorption of molecular layers. The two major theoretical approaches for such modelling (Density Functional Theory and classical force-field methods) are briefly described and compared. A few examples of their use are given, focussing on: (i) the expression of chirality at the local and global scale in layers of chiral molecules, (ii) the appearance of chirality in layers of achiral molecules on achiral surfaces, and (iii) the molecular organisation in layers formed from racemic mixtures.
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| 8. |
- Linares, Mathieu, 1979-, et al.
(författare)
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The effect of lattice dynamics on electron localization in poly-(para-phenylene vinylene)
- 2009
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Ingår i: Synthetic metals. - : Elsevier BV. - 0379-6779 .- 1879-3290. ; 159:21-22, s. 2219-2221
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Tidskriftsartikel (refereegranskat)abstract
- The lattice dynamics in poly-(para-phenylenevinylene) and its effect on the electron localization properties have been investigated. The simulations were performed using a hybrid quantum mechanical/molecular dynamics approach including the Pariser–Parr–Pople Hamiltonian. It is found that the dynamic disorder that exists at room temperature induces electron localization. The most important lattice modes in the context of localization are the torsional modes of the phenylene rings, that are shown to couple strongly to the electronic system. Since these modes occur at low frequencies, they will also have a strong impact on intra-chain charge transport.
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| 9. |
- Linares, Mathieu, et al.
(författare)
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The Nature of Resonance in Allyl Ions and Radical
- 2008
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Ingår i: JOURNAL OF PHYSICAL CHEMISTRY A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 112:50, s. 13249-13255
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Tidskriftsartikel (refereegranskat)abstract
- A recent valence bond scheme based on Lewis structures, the valence bond BOND (VBB) method (BOND: Breathing Orbitals Naturally Delocalized) method (Linares, M.; Braida, B.; Humbel, S. J. Phys. Chem. A 2006, 110, 2505-2509), is applied to explore the nature of resonance in allyl systems. Whereas allyl radical is correctly described by the resonance between the two traditional Lewis structures, a third "long-bonded" structure, which apparently creates a pi bond between the two distant carbon atoms, appears to plays an important role in allyl ions description. The similar vertical resonance energy (VRE) for both allyl ions is rather moderate (similar to 37 kcal/mol) in the two-structure description but is significantly enhanced when the long-bonded structure is included into the VBB wave function (by up to 20 kcal/mol). The allyl radical is much less resonant and is correctly described by the traditional two-structure picture. The development of VBB Lewis structures into "pure" valence bond determinants enlightens the role of the third structure in the description of allyl ions. The existence of a long bond between the two distant carbon atoms is clearly ruled out. Charge equilibration effect is shown to be a minor factor. The third structure is finally attributed to one- and three-electron bonding character revealed in the g systems of the cation and anion, respectively. This makes these systems two surprising examples of odd electron bonding within a singlet state. Last, the two-structure description of allyl radical is improved by addition of missing ionic structures.
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| 10. |
- Verlaak, Stijn, et al.
(författare)
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Electronic Structure and Geminate Pair Energetics at Organic-Organic Interfaces: The Case of Pentacene/C-60 Heterojunctions
- 2009
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Ingår i: Advanced Functional Materials. - : Wiley. - 1616-301X .- 1616-3028. ; 19:23, s. 3809-3814
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Tidskriftsartikel (refereegranskat)abstract
- Organic semiconductors are characterized by localized states whose energies are predominantly determined by electrostatic interactions with their immediate molecular environment. As a result, the details of the energy landscape at heterojunctions between different organic semiconductors cannot simply be deduced from those of the individual semiconductors, and they have so far remained largely unexplored. Here, microelectrostatic computations are performed to clarify the nature of the electronic structure and geminate pair energetics at the pentacene/C-60 interface, as archetype for an interface between a donor molecule and a fullerene electron acceptor. The size and orientation of the molecular quadrupole moments, determined by material choice, crystal orientation, and thermodynamic growth parameters of the semiconductors, dominate the interface energetics. Not only do quadrupoles produce direct electrostatic interactions with charge carriers, but, in addition, the discontinuity of the quadrupole field at the interface induces permanent interface dipoles. That discontinuity is particularly striking for an interface with C-60 molecules, which by virtue of their symmetry possess no quadrupole. Consequently, at a pentacene/C-60 interface, both the vacuum-level shift and geminate pair dissociation critically depend on the orientation of the pentacene pi-system relative to the adjacent C-60.
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