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Conjugation in Organic Group 14 Element Compounds: Design, Synthesis and Experimental Evaluation
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - BMC, Physical Organic Chemistry. (Henrik Ottosson)
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 [en]
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: urn:nbn:se:uu:diva-221683ISBN: 978-91-554-8929-8 (print)OAI: oai:DiVA.org:uu-221683DiVA: diva2:709833
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
List of papers
1. Cross-hyperconjugation: An unexplored orbital interaction between pi-conjugated and saturated molecular segments
Open this publication in new window or tab >>Cross-hyperconjugation: An unexplored orbital interaction between pi-conjugated and saturated molecular segments
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2013 (English)In: Angewandte Chemie International Edition, ISSN 1433-7851, E-ISSN 1521-3773, Vol. 52, no 3, 983-987 p.Article in journal (Refereed) Published
Abstract [en]

Crossing a barrier: Molecules with saturated ER2 units (E=C or Si, R=electron-releasing group) inserted between two π-conjugated segments have electronic and optical properties that resemble those of cross-conjugated molecules (see figure). This cross-hyperconjugation provides a deeper understanding of the conjugation phenomenon, and is an alternative to cross-conjugation in the design of molecules for nano and materials applications.

Keyword
conjugation, cross-conjugation, Group 14 elements, hyperconjugation, optical tuning
National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-194990 (URN)10.1002/anie.201206030 (DOI)000313688100036 ()
Available from: 2013-02-21 Created: 2013-02-20 Last updated: 2017-12-06Bibliographically approved
2. Charge transfer through cross-hyperconjugated versus cross-pi-conjugated bridges: an intervalence charge transfer study
Open this publication in new window or tab >>Charge transfer through cross-hyperconjugated versus cross-pi-conjugated bridges: an intervalence charge transfer study
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2013 (English)In: Chemical Science, ISSN 2041-6520, Vol. 4, no 9, 3522-3532 p.Article in journal (Refereed) Published
Abstract [en]

Recently there has been much interest in electron transfer and transport through cross-conjugated molecules as interesting test cases for the interplay between molecular and electronic structure as well as potential motifs in the design of new compounds for molecular electronics. Herein we expand on this concept and present the synthesis and characterization of a series of four organic mixed-valence dyads to probe the effect of the bridge structure on the electronic coupling. The electronic coupling between two triarylamine units could be mediated either by cross-hyperconjugation through a saturated ER2 bridge (E = C or Si, R = alkyl or silyl group), or via a cross-conjugated pi-system. The aim of the study is to compare the electron transfer through the various saturated bridges to that of a cross-pi-conjugated bridge. The electronic coupling in these mixed-valence compounds was determined by analysis of intervalence charge transfer bands, and was found to be in the range of 100-400 cm(-1). A complementary DFT and TD-DFT study indicated that the electronic coupling in the dyads with saturated ER2 segments is highly conformer dependant. Furthermore, the calculations showed that two types of interactions contribute to the electronic coupling; a through-bond cross-(hyper)conjugation mechanism and a through-space mechanism. Taken together, these findings suggest the possibility for new architectures for molecular electronics applications utilizing cross-hyperconjugation through properly selected saturated segments which have comparable electron transfer characteristics as regular cross-pi-conjugated molecules.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-208680 (URN)10.1039/c3sc50844g (DOI)000322391800021 ()
Available from: 2013-10-07 Created: 2013-10-07 Last updated: 2014-06-30Bibliographically approved
3. 1,4-Disilacyclohexa-2,5-diene: a molecular building block that allows for remarkably strong neutral cyclic cross-hyperconjugation
Open this publication in new window or tab >>1,4-Disilacyclohexa-2,5-diene: a molecular building block that allows for remarkably strong neutral cyclic cross-hyperconjugation
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2014 (English)In: Chemical Science, ISSN 2041-6520, Vol. 5, no 1, 360-371 p.Article in journal (Refereed) Published
Abstract [en]

2,3,5,6-Tetraethyl-1,4-disilacyclohexa-2,5-dienes with either four chloro (1a), methyl (1b), or trimethylsilyl (TMS) (1c) substituents at the two silicon atoms were examined in an effort to design rigid compounds with strong neutral cross-hyperconjugation between pi- and sigma-bonded molecular segments arranged into a cycle. Remarkable variations in the lowest electronic excitation energies, lowest ionization energies, and the first oxidation potentials were observed upon change of substituents, as determined by gas phase ultraviolet (UV) absorption spectroscopy, ultraviolet photoelectron spectroscopy (UPS), and cyclic voltammetry. A particularly strong neutral cyclic cross-hyperconjugation was observed in 1c. Its lowest electron binding energy (7.1 eV) is distinctly different from that of 1b (8.5 eV). Molecular orbital analysis reveals a stronger interaction between filled pi(C=C) and pi(SiR2) group orbitals in 1c than in 1a and 1b. The energy shift in the highest occupied molecular orbital is also reflected in the first oxidation potentials as observed in the cyclic voltammograms of the respective compounds (1.47, 0.88, and 0.46 V for 1a, 1b and 1c, respectively). Furthermore, 1,4-disilacyclohexadiene 1c absorbs strongly at 273 nm (4.55 eV), whereas 1a and 1b have no symmetry allowed excitations above 215 nm (below 5.77 eV). Thus, suitably substituted 1,4-disilacyclohexa-2,5-dienes could represent novel building blocks for the design of larger cross-hyperconjugated molecules as alternatives to traditional purely cross-p-conjugated analogues, and could allow for design of molecules with properties that are not accessible to those that are exclusively pi-conjugated.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-213891 (URN)10.1039/c3sc52389f (DOI)000327601600045 ()
Available from: 2014-01-06 Created: 2014-01-05 Last updated: 2016-03-08Bibliographically approved
4. Optimization of the Cyclic Cross-Hyperconjugation in 1,4-Ditetrelcyclohexa-2,5-dienes
Open this publication in new window or tab >>Optimization of the Cyclic Cross-Hyperconjugation in 1,4-Ditetrelcyclohexa-2,5-dienes
2014 (English)In: Organometallics, ISSN 0276-7333, E-ISSN 1520-6041, Vol. 33, no 12, 2997-3004 p.Article in journal (Refereed) Published
Abstract [en]

Cyclic cross-hyperconjugation can exist to variable extents in 1,4-ditetrelcyclohexa-2,5-dienes, i.e., all-carbon cyclohexa-1,4-dienes and 1,4-disila/digerma/distanna/diplumbacyclohexa-2,5-dienes. In this study we first use density functional theory (DFT) computations to optimize the conjugation strength by seeking the optimal atom E and substituent group E'Me-3 in the two saturated E(E'Me-3)(2) moieties (E and E' as the same or different tetrel (group 14) elements). We reveal that the all-carbon cyclohexadienes with gradually heavier E'Me-3 substituents at the two saturated carbon atoms display significant cross-hyperconjugation. The first electronic excitations in these compounds, which formally have two isolated C=C bonds, are calculated to reach wavelengths as long as 400 nm (excitation energies of 3.1 eV). These transitions are mostly forbidden, and the lowest allowed transitions are found at 387 nm (3.2 eV). The silicon analogues are also cross-hyperconjugated, while a decline is observed in the 1,4-digerma/distanna/diplumbacyclohexa-2,5-diene. Experiments on two substituted 1,4-disilacyclohexa-2,5-dienes confirm the effect of the E'Me3 substituents, with regard to both electronic excitations and geometries as determined by UV absorption spectroscopy and X-ray crystallography, respectively. At the end, we reveal through computations how electron-donating and electron-withdrawing substituents at the C=C double bonds influence the electronic properties of the all-carbon ring. We find that the first calculated excitation, which is forbidden, can be shifted to 440 nm (2.83 eV). This shows to what extent cyclic cross-hyperconjugation can affect the electronic and optical properties of a compound with two formally isolated C=C double bonds.

National Category
Organic Chemistry
Research subject
Chemistry with specialization in Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-221022 (URN)10.1021/om5001875 (DOI)000337936800008 ()
Available from: 2014-03-24 Created: 2014-03-24 Last updated: 2017-12-12Bibliographically approved
5. Using Ground and Excited State Aromaticity to Understand Cyclopentadiene and Silole Excitation Energies and Excited State Polarities
Open this publication in new window or tab >>Using Ground and Excited State Aromaticity to Understand Cyclopentadiene and Silole Excitation Energies and Excited State Polarities
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(English)Article in journal (Other academic) Submitted
National Category
Chemical Sciences
Research subject
Chemistry with specialization in Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-221027 (URN)
Available from: 2014-03-24 Created: 2014-03-24 Last updated: 2014-06-30Bibliographically approved
6. In Search of Flexible Molecular Wires with Near Conformer-Independent Conjugation and Conductance: A Computational Study
Open this publication in new window or tab >>In Search of Flexible Molecular Wires with Near Conformer-Independent Conjugation and Conductance: A Computational Study
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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
National Category
Physical Chemistry
Research subject
Chemistry with specialization in Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-209260 (URN)10.1021/jp409767r (DOI)000333381300003 ()
Available from: 2013-10-16 Created: 2013-10-16 Last updated: 2017-12-06Bibliographically approved
7. Coupling of Disilane and Trisilane Segments Through Zero, One, Two, and Three Disilanyl Bridges in Cyclic and Bicyclic Saturated Carbosilanes
Open this publication in new window or tab >>Coupling of Disilane and Trisilane Segments Through Zero, One, Two, and Three Disilanyl Bridges in Cyclic and Bicyclic Saturated Carbosilanes
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2013 (English)In: Organometallics, ISSN 0276-7333, E-ISSN 1520-6041, Vol. 32, no 2, 396-405 p.Article in journal (Refereed) Published
Abstract [en]

Several six-membered cyclic and [2.2.2]bicyclic organo-silanes with varying proportions of silicon atoms in the bridges have been prepared following a stepwise approach that exploits dianionic polysilanes. Focus in our analysis was placed on the bicyclic compounds which all have silicon atoms at the bridgehead positions. Quantum chemical calculations of these compounds revealed the possibility to enhance the coupling through a single cisoid tetrasilane cage segment by replacing one or two of the other -SiMe2SiMe2- bridges with -CH2CH2- bridges. UV absorption spectroscopy revealed a red shift in the lowest visible transitions when going from a bicyclo[2.2.2]octane with three -SiMe2SiMe2- bridges to those with two or one such bridge. However, these red shifts are deceptive, as the lowest vertically excited singlet states, which are dark according to TD-DFT calculations, do not display the same trend. Still, since these compounds have (i) excellent structural rigidity, (ii) provide potentials for functionalization through their exocyclic trimethylsilyl groups, and (iii) display electronic structure variations with the number of -SiMe2SiMe2- bridges, they could be interesting for further studies: e.g., in single-molecule electronics.

National Category
Natural Sciences
Identifiers
urn:nbn:se:uu:diva-196040 (URN)10.1021/om3006678 (DOI)000314332100007 ()
Available from: 2013-03-04 Created: 2013-03-04 Last updated: 2017-12-06Bibliographically approved
8. Conductance through Carbosilane Cage Compounds: A Computational Investigation
Open this publication in new window or tab >>Conductance through Carbosilane Cage Compounds: A Computational Investigation
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2013 (English)In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 117, no 42, 21692-21699 p.Article in journal (Refereed) Published
Abstract [en]

Silicon is still the dominating material in microelectronics, yet primarily π-conjugated hydrocarbons are investigated in the field of single-molecule electronics even though linear oligosilanes are σ-conjugated. A drawback with the latter is their high conformational flexibility which strongly affects conductance. Here we report on a first principles density functional theory investigation of a series of rigid [2.2.2]bicyclic carbosilanes with 3, 2, 1, or 0 disilanylene bridges, providing all-silicon paths for charge transport. It is explored if these paths can be seen as independent and equivalent current paths acting as parallel resistors. For high conductance through the carbosilanes they need to be anchored to the gold electrodes via groups that are matched with the σ-conjugated paths of the oligosilane cage segment, and we find that silyl (SiH3) groups are better matched than thiophenol groups. Even for the carbosilane with three disilanylene bridges we find that the most transmitting conductance channel is not equally distributed on the three parallel bridges. In addition, there is significant communication between the various pathways, which results in destructive interference lowering the conductance. Taken together, the different disilanylene bridges in the cage compounds do not act as parallel resistors.

Keyword
Molecular electronics, organosilicon chemistry, electronic structure, density functional theory, sigma conjugation
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-209236 (URN)10.1021/jp407485n (DOI)000326260000008 ()
Available from: 2013-10-16 Created: 2013-10-15 Last updated: 2017-12-06Bibliographically approved
9. Configuration- and Conformation-Dependent Electronic Structure Variations in 1,4-Disubstituted Cyclohexanes Enabled by a Carbon-to-Silicon Exchange
Open this publication in new window or tab >>Configuration- and Conformation-Dependent Electronic Structure Variations in 1,4-Disubstituted Cyclohexanes Enabled by a Carbon-to-Silicon Exchange
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2014 (English)In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 20, no 30, 9304-9311 p.Article in journal (Other academic) Published
Abstract [en]

Cyclohexane, with its well-defined conformers, could be an ideal force-controlled molecular switch if it were to display substantial differences in electronic and optical properties between its conformers. We utilize sigma conjugation in heavier analogues of cyclohexanes (i.e. cyclohexasilanes) and show that 1,4-disubstituted cyclohexasilanes display configuration-and conformation-dependent variations in these properties. Cis- and trans-1,4-bis(trimethylsilylethynyl)-cyclohexasilanes display a 0.11 V difference in their oxidation potentials (computed 0.11 V) and a 0.34 eV difference in their lowest UV absorption (computed difference between first excitations 0.07 eV). This is in stark contrast to differences in the corresponding properties of analogous all-carbon cyclohexanes (computed 0.02 V and 0.03 eV, respectively). Moreover, the two chair conformers of the cyclohexasilane trans isomer display large differences in electronic-structure-related properties. This enables computational design of a mechanically force-controlled conductance switch with a calculated single-molecule ON/OFF ratio of 213 at zero-bias voltage.

National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-209259 (URN)10.1002/chem.201402610 (DOI)000339568800023 ()
Note

De 2 sista författarna delar sistaförfattarskapet.

Available from: 2013-10-16 Created: 2013-10-16 Last updated: 2017-12-06Bibliographically approved

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