| 1. |
- Crispin, Annica, et al.
(author)
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Influence of dopant on the electronic structure of spiro-oligophenyl-based disordered organic semiconductors
- 2002
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In: Journal of Chemical Physics. - : AIP Publishing. - 0021-9606 .- 1089-7690. ; 116:18, s. 8159-8167
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Journal article (peer-reviewed)abstract
- The influence of the dopant on the electronic structure of spiro-oligophenyl-based disordered organic semiconductors was studied by means of photoelectron spectroscopy. With lithium atoms as dopants, two charges were stored on the same spiro branch in the form of bipolarons, for spiro-quarterphenyl and spiro-sexiphenyl. For doping with the sodium atoms, the size of the counter ions made it less energetically desirable to store two charges onto a single branch, and the charged species were polarons independent of the level of doping which was confirmed by optical absorption data.
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| 2. |
- Linares, Mathieu, et al.
(author)
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On the interface dipole at the pentacene-fullerene heterojunction : A theoretical study
- 2010
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In: The Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 114:7, s. 3215-3224
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Journal article (peer-reviewed)abstract
- The electronic structure at organic/organic interfaces plays a key role, among others, in defining the quantum efficiency of organics-based photovoltaic cells. Here, we perform quantum-chemical and microelectrostatic calculations on molecular aggregates of various sizes and shapes to characterize the interfacial dipole moment at pentacene/C60 heterojunctions. The results show that the interfacial dipole mostly originates in polarization effects due to the asymmetry in the multipolar expansion of the electronic density distribution between the interacting molecules, rather than in a charge transfer from donor to acceptor. The local dipole is found to fluctuate in sign and magnitude over the interface and appears as a sensitive probe of the relative arrangements of the pentacene and C60 molecules (and of the resulting local electrical fields sensed by the molecular units).
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| 3. |
- Bröms, P., et al.
(author)
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Optical absorption studies of sodium doped poly(cyanoterephthalylidene)
- 1994
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In: Synthetic metals. - : Elsevier. - 0379-6779 .- 1879-3290. ; 67:1-3, s. 93-96
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Journal article (peer-reviewed)abstract
- The effects of doping poly(cyanoterephthalylidene) with sodium in ultrahighvacuum been studied by optical absorption spectroscopy. Upon doping, new optical transitions are observed within the bandgap; the characteristics of these transitions are consistent with the formation of bipolarons. The optical absorption results are confirmed by direct measurements of the doping-induced gap states using ultraviolet photoelectron spectroscopy.
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| 4. |
- Crispin, Xavier, et al.
(author)
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Characterization of the interface dipole at organic/metal interfaces
- 2002
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 124:27, s. 8131-8141
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Journal article (peer-reviewed)abstract
- In organics-based (opto)electronic devices, the interface dipoles formed at the organic/metal interfaces play a key role in determining the barrier for charge (hole or electron) injection between the metal electrodes and the active organic layers. The origin of this dipole is rationalized here from the results of a joint experimental and theoretical study based on the interaction between acrylonitrile, a p-conjugated molecule, and transition metal surfaces (Cu, Ni, and Fe). The adsorption of acrylonitrile on these surfaces is investigated experimentally by photoelectron spectroscopies, while quantum mechanical methods based on density functional theory are used to study the systems theoretically. It appears that the interface dipole formed at an organic/metal interface can be divided into two contributions: (i) the first corresponds to the "chemical" dipole induced by a partial charge transfer between the organic layers and the metal upon chemisorption of the organic molecules on the metal surface, and (ii) the second relates to the change in metal surface dipole because of the modification of the metal electron density tail that is induced by the presence of the adsorbed organic molecules. Our analysis shows that the charge injection barrier in devices can be tuned by modulating various parameters: the chemical potential of the bare metal (given by its work function), the metal surface dipole, and the ionization potential and electron affinity of the organic layer.
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| 5. |
- Crispin, Xavier, 1972-, et al.
(author)
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Electronic delocalization in discotic liquid crystals : A joint experimental and theoretical study
- 2004
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In: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 126:38, s. 11889-11899
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Journal article (peer-reviewed)abstract
- Discotic liquid crystals emerge as very attractive materials for organic-based (opto)electronics as they allow efficient charge and energy transport along self-organized molecular columns. Here, angle-resolved photoelectron spectroscopy (ARUPS) is used to investigate the electronic structure and supramolecular organization of the discotic molecule, hexakis(hexylthio)diquinoxalino[2,3-a:2′,3′-c]phenazine, deposited on graphite. The ARUPS data reveal significant changes in the electronic properties when going from disordered to columnar phases, the main feature being a decrease in ionization potential by 1.8 eV following the appearance of new electronic states at low binding energy. This evolution is rationalized by quantum-chemical calculations performed on model stacks containing from two to six molecules, which illustrate the formation of a quasi-band structure with Bloch-like orbitals delocalized over several molecules in the column. The ARUPS data also point to an energy dispersion of the upper π-bands in the columns by some 1.1 eV, therefore highlighting the strongly delocalized nature of the π-electrons along the discotic stacks.
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| 6. |
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| 7. |
- Crivillers, N, et al.
(author)
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Photoinduced work function changes by isomerization of a densely packed azobenzene-based SAM on Au: a joint experimental and theoretical study
- 2011
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In: Physical Chemistry, Chemical Physics - PCCP. - : Royal Society of Chemistry. - 1463-9076 .- 1463-9084. ; 13:32, s. 14302-14310
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Journal article (peer-reviewed)abstract
- Responsive monolayers are key building blocks for future applications in organic and molecular electronics in particular because they hold potential for tuning the physico-chemical properties of interfaces, including their energetics. Here we study a photochromic SAM based on a conjugated azobenzene derivative and its influence on the gold work function (Phi(Au)) when chemisorbed on its surface. In particular we show that the Phi(Au) can be modulated with external stimuli by controlling the azobenzene trans/cis isomerization process. This phenomenon is characterized experimentally by four different techniques, kelvin probe, kelvin probe force microscopy, electroabsorption spectroscopy and ultraviolet photoelectron spectroscopy. The use of different techniques implies exposing the SAM to different measurement conditions and different preparation methods, which, remarkably, do not alter the observed work function change (Phi(trans)-Phi(cis)). Theoretical calculations provided a complementary insight crucial to attain a deeper knowledge on the origin of the work function photo-modulation.
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| 8. |
- Karazazi, Y., et al.
(author)
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Influence of contact geometry and molecular derivatization on the interfacial interactions between gold and conjugated wires
- 2004
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In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 387:4-6, s. 502-508
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Journal article (peer-reviewed)abstract
- Self-assembled monolayers made of thiolated conjugated wires attached on gold surfaces currently attract a considerable interest in the field of nanoelectronics. The interactions taking place at the metal/molecule interface govern the electronic structure of the complex, and hence the barriers for charge injection from the electrodes to the molecules. Considering benzenethiol as a prototype molecule, we investigate here the way the electronic structure is affected by the nature of the anchoring site of the sulfur atom on the gold surface and by the relative orientation of the molecule with respect to the surface. We also assess whether the changes in the molecular electronic properties upon substitution are similar for the isolated molecule and for the molecule attached on the gold surface. Our results provide strong evidences that, in order to introduce functionalities and/or improve charge injection in molecular devices, the electronic properties of conjugated molecular wires can be tailored by derivatization independently of the metal electrodes. copy,
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| 9. |
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| 10. |
- Cornil, J, et al.
(author)
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Influence of interchain interactions on the absorption and luminescence of conjugated oligomers and polymers: A quantum-chemical characterization
- 1998
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In: Journal of the American Chemical Society. - : American Chemical Society. - 0002-7863 .- 1520-5126. ; 120:6, s. 1289-1299
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Journal article (peer-reviewed)abstract
- Correlated quantum-chemical calculations are used to investigate the influence of interchain interactions on the absorption and emission of pi-conjugated chains. The results are discussed in relation to the utilization of conjugated materials as active elements in electro-optic devices; they provide guidelines on how to prevent a substantial decrease in luminescence yield in solid films. In high-symmetry cofacial configurations, interchain interactions lead to a blue shift of the lowest optical transition compared to that calculated for an isolated chain; the appearance of an additional red-shifted component is expected when positional disorder is considered. The absence of any significant oscillator strength in the transition between the ground state and the lowest excited state in highly symmetric complexes implies that the luminescence emission will be strongly quenched. This picture is. however, modified when one takes account of the relaxation processes which occur in the lowest excited state. The nature of the most stable photogenerated species and the role played by chemical impurities are also addressed.
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| 11. |
- Geskin, V., et al.
(author)
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Bianthrone at a Metal Surface: Conductance Switching with a Bistable Molecule Made Feasible by Image Charge Effects
- 2015
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In: AIP Conference Proceedings. - : AIP Publishing LLC. - 1551-7616 .- 0094-243X. ; 1642, s. 469-472
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Conference paper (peer-reviewed)abstract
- Bianthrone is a sterically hindered compound that exists in the form of two non-planar isomers. Our experimental study of single-molecule junctions with bianthrone reveals persistent switching of electric conductance at low temperatures, which can be reasonably associated to molecular isomerization events. Temperature dependence of the switching rate allows for an estimate of the activation energy of the process, on the order of 35-90 meV. Quantum-chemical calculations of the potential surface of neutral bianthrone and its anion, including identification of transition states, yields the isolated molecule isomerization barriers too high vs. the previous estimate, though in perfect agreement with previous experimental studies in solution. Nevertheless, we show that the attraction of the anion in the vicinity of the metal surface by its image charge can significantly alter the energetic landscape, in particular, by reducing the barrier to the values compatible with the observed switching behavior.
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| 12. |
- Lara Avila, Samuel, 1983, et al.
(author)
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Bianthrone in a Single-Molecule Junction: Conductance Switching with a Bistable Molecule Facilitated by Image Charge Effects
- 2010
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In: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 114:48, s. 20686-20695
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Journal article (peer-reviewed)abstract
- Bianthrone is a sterically hindered compound that exists in the form of two nonplanar isomers. Our experimental study of single-molecule junctions with bianthrone reveals persistent switching of electric conductance at low temperatures, which can be reasonably associated with molecular isomerization events. Temperature dependence of the switching rate allows for an estimate of the activation energy of the process, on the order of 120 +/- 50 meV. Quantum-chemical calculations of the potential energy relief of neutral bianthrone and its anion, including identification of transition states, yields the isolated molecule isomerization barriers too high vs the previous estimate, though compatible with previous experimental studies in solution. Nevertheless, we show that the attraction of the anion in the vicinity of the metal surface by its image charge can change the energetic landscape, in particular, by significantly reducing the barrier to values compatible with the observed switching behavior.
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| 13. |
- Lindell, Linda, et al.
(author)
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Transparent, plastic, low-work-function poly(3,4-ethylenedioxythiophene) electrodes
- 2006
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In: Chemistry of Materials. - : American Chemical Society (ACS). - 0897-4756 .- 1520-5002. ; 18:18, s. 4246-4252
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Journal article (peer-reviewed)abstract
- Novel applications for flexible electronics, e.g., displays and solar cells, require fully flexible, transparent, stable, and low-work-function electrodes that can be manufactured via a low-cost process. Here, we demonstrate that surface chemistry constitutes a route to producing transparent low-work-function plastic electrodes. The work function of the conducting polymer poly(3,4-ethylenedioxythiophene)-tosylate, or PEDOT-Tos, is decreased by submonolayer surface redox reaction with a strong electron donor, tetrakis-(dimethylamino)ethylene (TDAE), allowing it to reach a work function of 3.8 eV. The interface formed between TDAE and PEDOT is investigated in a joint experimental and theoretical study using photoelectron spectroscopy and quantum chemical calculations. © 2006 American Chemical Society.
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| 14. |
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| 15. |
- Sancho-Garcia, JC, et al.
(author)
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Joint theoretical and experimental characterization of the structural and electronic properties of poly(dioctylfluorene-alt-N-butylphenyl diphenylamine)
- 2004
-
In: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1520-6106 .- 1520-5207. ; 108:18, s. 5594-5599
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Journal article (peer-reviewed)abstract
- Fluorene-based copolymers are currently attracting considerable interest for use in a wide range of optoelectronic devices. Here, we present the results of a joint quantum-chemical and experimental characterization of the structural, electronic, and optical properties of an alternating fluorene-triphenylamine copolymer. We compare the results from this study with those from similar studies of polyfluorene. Although calculations are performed for the gas phase and experiments are performed on the solid state, the results from the two methodologies are in good agreement: the relevant electronic levels, HOMO and LUMO, of polyfluorene are found to be destabilized by incorporation of triphenylamine units in the conjugated backbone, whereas the optical properties of polyfluorene chains are largely unperturbed by the presence of triphenylamine.
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| 16. |
- Tassignon, Benjamin, et al.
(author)
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Site Selectivity of Peptoids as Azobenzene Scaffold for Molecular Solar Thermal Energy Storage
- 2023
-
In: Chemistry - A European Journal. - 1521-3765 .- 0947-6539. ; 29:70
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Journal article (peer-reviewed)abstract
- Storing solar energy is a key challenge in modern science. MOlecular Solar Thermal (MOST) systems, in particular those based on azobenzene switches, have received great interest in the last decades. The energy storage properties of azobenzene (t1/2<4 days; ΔH~270 kJ/kg) must be improved for future applications. Herein, we introduce peptoids as programmable supramolecular scaffolds to improve the energy storage properties of azobenzene-based MOST systems. We demonstrate with 3-unit peptoids bearing a single azobenzene chromophore that dynamics of the MOST systems can be tuned depending on the anchoring position of the photochromic unit on the macromolecular backbone. We measured a remarkable increase of the half-life of the metastable form up to 14 days at 20 °C for a specific anchoring site, significantly higher than the isolated azobenzene moiety, thus opening new perspectives for MOST development. We also highlight that liquid chromatography coupled to mass spectrometry does not only enable to monitor the different stereoisomers during the photoisomerization process as traditionally done, but also allows to determine the thermal back-isomerization kinetics.
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| 17. |
- Tzamalis, G., et al.
(author)
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Fluorescence light emission at 1 eV from a conjugated polymer
- 2010
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In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614. ; 489:1-3, s. 92-95
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Journal article (peer-reviewed)abstract
- While polymer light-emitting diodes are currently finding commercial applications in displays and lighting, the development of low bandgap polymers emitting in the infrared has received much less attention in spite of potential applications for instance in the field of communication technologies. We report here a light emission at 1 eV from a low bandgap polymer made of an alternation of dialkoxy-phenylene units and a low bandgap monomer composed of an electron accepting 2-thia-1,3,5,8-tetraaza-cyclopenta[b]naphthalene group fenced with electron donating thiophene units. The electronic structure of the polymer chains has been characterized at a quantum-chemical level to shed light into the experimental results. (C) 2010 Published by Elsevier B.V.
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| 18. |
- Van Regemorter, Tanguy, et al.
(author)
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Electronic Structure of Self-Assembled Monolayers on ZnO Surfaces
- 2011
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Conference paper (other academic/artistic)abstract
- Controlling the electronic properties at the interface between organic and inorganic materials is an issue of high interest since it is known to strongly affect the charge injection and the overall efficiency of opto-electronic devices. Self-assembled monolayers (SAMs) can be used to change the electronic properties of the underlying inorganic material and optimize the charge injection occurring at the interface. In this context, we aim here at a fundamental understanding of the electronic processes taking place when SAMs are deposited on ZnO. In particular, our theoretical investigations focus on the origin of the shift in the conduction band upon addition of SAMs on the ZnO surface. Calculations were performed within the framework of density functional theory (DFT) using periodic boundary conditions. The SAMs investigated here are 4-tertbutylpyridine (SAM1) and benzoic acid (SAM2), this latter with three different terminations. We show how the conduction band varies in the presence of SAMs on both polar and non-polar ZnO surfaces, in comparison to the bare surface. We find a shift in opposite direction for SAM1 and SAM2 for both surface orientations. A charge density analysis has been performed to identify the role of the surface orientation, bond dipole and dipole moment of the individual molecules in the observed effect. These theoretical observations have been confronted to preliminary experimental data for ZnO nano-particles with and without SAMs.
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| 19. |
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