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In Search of Flexible Molecular Wires with Near Conformer-Independent Conjugation and Conductance: A Computational Study
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - BMC, Physical Organic Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - BMC, Physical Organic Chemistry. State Key Laboratory of Physical Chemistry of Solid Surfaces, Fujian Provincial Key Laboratory of Theoretical and Computational Chemistry, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University.
Uppsala University, Disciplinary Domain of Science and Technology, Physics, Department of Physics and Astronomy, Materials Theory.
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2014 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, no 11, 5637-5649 p.Article in journal (Refereed) Published
Abstract [en]

Oligomers of 1,4-disila/germa/stannacyclohexa-2,5-dienes as well as all-carbon 1,4-cyclohexadienes connected via E—E single bonds (E = C, Si, Ge, or Sn) were studied through quantum chemical calculations in an effort to identify conformationally flexible molecular wires that act as molecular “electrical cords” having conformer-independent conjugative and conductive properties. Our oligomers display neutral hyperconjugative interactions (σ/π-conjugation) between adjacent σ(E—E) and π(C═C) bond orbitals, and these interactions do not change with conformation. The energies and spatial distributions of the highest occupied molecular orbitals of methyl-, silyl-, and trimethylsilyl (TMS)-substituted 1,4-disilacyclohexa-2,5-diene dimers, and stable conformers of trimers and tetramers, remain rather constant upon Si–Si bond rotation. Yet, steric congestion may be a concern in some of the oligomer types. The calculated conductances for the Si-containing tetramers are similar to that of a σ-conjugated linear all-anti oligosilane (a hexadecasilane) with equally many bonds in the conjugated paths. Moreover, the Me-substituted 1,4-disilacyclohexadiene tetramer has modest conductance fluctuations with Si–Si bond rotations when the electrode–electrode distance is locked (variation by factor 30), while the fluctuations under similar conditions are larger for the analogous TMS-substituted tetramer. When the electrode–electrode distance is changed several oligomers display small conductance variations within certain distance intervals, e.g., the mean conductance of TMS-substituted 1,4-disilacyclohexa-2,5-diene tetramer is almost unchanged over 9 Å of electrode–electrode distances.

Place, publisher, year, edition, pages
American Chemical Society (ACS), 2014. Vol. 118, no 11, 5637-5649 p.
National Category
Physical Chemistry
Research subject
Chemistry with specialization in Organic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-209260DOI: 10.1021/jp409767rISI: 000333381300003OAI: oai:DiVA.org:uu-209260DiVA: diva2:656506
Available from: 2013-10-16 Created: 2013-10-16 Last updated: 2017-12-06Bibliographically approved
In thesis
1. Computational Studies of Electron Transport in Nanoscale Devices
Open this publication in new window or tab >>Computational Studies of Electron Transport in Nanoscale Devices
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis, a combination of density functional theory (DFT) based calculations and nonequilibrium Green’s functions are employed to investigate electron transport in molecular switches, molecular cords and nanoscale devices.

  Molecular electronic devices have been proposed as an approach to complement today’s silicon based electronic devices. However, engineering of such miniature devices and design of functional molecular components still present significant challenges.

  First, the way to connect a molecule to conductive electrodes has to be controlled. We study, in a nanoelectrode-nanoparticle platform, how structural changes affect the measured conductance and how current fluctuations due to these structural changes can be decreased. We find that, for reproducible measurements, it is important to have the molecules chemically bonded to the surfaces of adjacent nanoparticles. Furthermore, we show by a combination of DFT and theoretical modeling that we can identify signals from single-molecules in inelastic electron spectroscopy measurements on these devices.

  Second, active elements based on molecules, some examples being switches, rectifiers or memory devices, have to be designed. We study molecular conductance switches that can be operated by light and/or temperature. By tuning the substituents on the molecules, we can optimize the shift of the most conducting molecular orbital and increase the effective coupling between the molecule and the electrodes when going from the OFF to the ON-state of the switches, giving high switching ratio (up to three orders of magnitude). We also study so called mechanoswitches that are activated by a mechanical force elongating the molecules, which means that these switches could operate as sensors.

  Furthermore, we have studied two different classes of compounds that may function either as rigid molecular spacers with a well-defined conductance or as molecular cords. In both cases, we find that it is of great importance to match the conjugation of the anchoring groups with the molecular backbone for high conductance.

  The last part of the thesis is devoted to another interesting semiconductor material, diamond. We have accurately calculated the band structure and effective masses for this material. Furthermore, these results have been used to calculate the Hall coefficient, the resistivity and the Seebeck coefficient.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. i-x, 89 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1090
Keyword
Density functional theory, Molecular electronics, Organosilicon chemistry, Diamond, Molecular switches, Nanoelectrode bridge platform, Molecular cords
National Category
Condensed Matter Physics Physical Chemistry
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-209261 (URN)978-91-554-8781-2 (ISBN)
Public defence
2013-11-29, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2013-11-08 Created: 2013-10-16 Last updated: 2014-01-23
2. Conjugation in Organic Group 14 Element Compounds: Design, Synthesis and Experimental Evaluation
Open this publication in new window or tab >>Conjugation in Organic Group 14 Element Compounds: Design, Synthesis and Experimental Evaluation
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

This thesis focuses on the chemical concept of conjugation, i.e., electron delocalization, and the effect it has on electronic and optical properties of molecules. The emphasis is on electron delocalization across a saturated σ-bonded segment, and in our studies these segments are either inserted between π-conjugated moieties or joined together to form longer chains. The electronic and optical properties of these compounds are probed and compared to those of traditionally π-conjugated compounds. The investigations utilize a combination of qualitative chemical bonding theories, quantum chemical calculations, chemical syntheses and different spectroscopic methods.

Herein, it is revealed that a saturated σ-bonded segment inserted between two π-systems can have optical and electronic properties similar to a cross-conjugated compound when substituents with heavy Group 14 elements (Si, Ge or Sn) are attached to the central atom. We coined the terminology cross-hyperconjugation for this interaction, and have shown it by both computational and spectroscopic means. This similarity is also found in cyclic compounds, for example in the 1,4-disilacyclohexa-2,5-dienes, as we reveal that there is a cyclic aspect of cross-hyperconjugation. Cross-hyperconjugation can further also be found in smaller rings such as siloles and cyclopentadienes, and we show on the similarities between these and their cross-π-conjugated analogues, the fulvenes. Here, this concept is combined with that of excited state aromaticity and the electronic properties of these systems are rationalized in terms of “aromatic chameleon” effects. We show that the optical properties of these systems can be rationally tuned and predicted through the choice of substituents and knowledge about the aromaticity rules in both ground and excited states.

We computationally examine the relation between conjugation and conductance and reveal that oligomers of 1,4-disilacyclohexa-2,5-dienes and related analogues can display molecular cord properties. The conductance through several σ-conjugated silicon compounds were also examined and show that mixed silicon and carbon bicyclo[2.2.2]octane compounds do not provide significant benefits over the open-chain oligosilanes. However, cyclohexasilanes, a synthetic precursor to the bicyclic compounds, displayed conformer-dependent electronic structure variations that were not seen for cyclohexanes. This allowed for computational design of a mechanically activated conductance switch.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 70 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1140
Keyword
conjugation, conductance, electronic structure, Group 14 elements, hyperconjugation, molecular electronics, organosilicon chemistry
National Category
Organic Chemistry Physical Chemistry
Research subject
Chemistry with specialization in Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-221683 (URN)978-91-554-8929-8 (ISBN)
Public defence
2014-05-27, B42, BMC, Husargatan 3, Uppsala, 13:15 (English)
Opponent
Supervisors
Available from: 2014-05-06 Created: 2014-04-03 Last updated: 2014-06-30

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Publisher's full texthttp://pubs.acs.org/doi/full/10.1021/jp409767r

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Emanuelsson, RikardLöfås, HenrikAhuja, RajeevGrigoriev, AntonOttosson, Henrik

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