Conjugation in Organic Group 14 Element Compounds

Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1140 Conjugation in Organic Group 14 Element Compounds Design, Synthesis and Experimental Evaluation RIKARD EMANUELSSON ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 ISBN 978-91-554-8929-8 UPPSALA urn:nbn:se:uu:diva-221683 2014 Dissertation presented at Uppsala University to be publicly examined in B42, BMC, Husargatan 3, Uppsala, Tuesday, 27 May 2014 at 13:15 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Professor Rik Tykwinski (Friedrich-Alexander-Universität Erlangen-Nürnberg). Abstract Emanuelsson, R. 2014. Conjugation in Organic Group 14 Element Compounds. Design, Synthesis and Experimental Evaluation. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1140. 70 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-554-8929-8. 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. Keywords: conjugation, conductance, electronic structure, Group 14 elements, hyperconjugation, molecular electronics, organosilicon chemistry Rikard Emanuelsson, Department of Chemistry - BMC, Physical Organic Chemistry, Box 576, Uppsala University, SE-75123 Uppsala, Sweden. © Rikard Emanuelsson 2014 ISSN 1651-6214 ISBN 978-91-554-8929-8 urn:nbn:se:uu:diva-221683 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-221683) Till min familj List of papers This thesis is based on the following papers, which are referred to in the text by their Roman numerals. I Emanuelsson, R.; Wallner, A.; Ng, E. A. M.; Smith, J. R.; Nauroozi, D.; Ott, S.; Ottosson, H. Cross- Hyperconjugation: An Unexplored Orbital Interaction between π-Conjugated and Saturated Molecular Segments. Angew. Chem. Int. Ed. 2013, 52, 983–987. II Göransson, E; Emanuelsson, R.; Jorner, K.; Hammarström, L.; Ottosson, H. Charge Transfer through Cross- hyperconjugated versus Cross-π-conjugated Bridges: An Intervalence Charge Transfer Study. Chem. Sci. 2013, 4, 3522–3532. III Tibbelin, J.; Wallner, A.; Emanuelsson, R.; Heijkenskjöld, F.; Rosenberg, M.; Yamazaki, K.; Nauroozi, D.; Karlsson, L.; Feifel, R.; Pettersson, R.; Baumgartner, J.; Ott, S.; Ot- tosson, H. 1,4-Disilacyclohexa-2,5-diene: A Molecular Building Block that Allows for Remarkably Strong Neutral Cyclic Cross-Hyperconjugation. Chem. Sci. 2014, 5, 360– 371. IV Emanuelsson, R.;† Denisova. A. V.;† Baumgartner, J.; Ot- tosson H. Optimization of the Cyclic Cross- Hyperconjugation 1,4-Ditetrelcyclohexa-2,5-dienes. Sub- mitted † These authors contributed equally V Jorner. K.; † Emanuelsson, R.; † Dahlstrand, C.; Tong, H.; Denisova, A. V.; Ottosson, H. Using Ground and Excited State Aromaticity to Understand Cyclopentadiene and Si- lole Excitation Energies and Excited State Polarities. Sub- mitted † These authors contributed equally VI Emanuelsson, R.; Löfås, H.; Zhu, J.; Ahuja, R.; Grigoriev, A.; Ottosson, H. In Search of Flexible Molecular Wires with Near Conformer-Independent Conjugation and Con- ductance: A Computational Study J. Phys. Chem. C. 2014, 118, 5637–5649. VII Wallner, A.;† Emanuelsson, R.;† Baumgartner, J.; Marsch- ner, C.; Ottosson, H. Coupling of Disilane and Trisilane Segments Through Zero, One, Two, and Three Disilanyl Bridges in Cyclic and Bicyclic Saturated Carbosilanes. Or- ganometallics 2013, 32, 396–405. † These authors contributed equally VIII Löfås, H.; Emanuelsson, R.; Ahuja, R.; Grigoriev, A.; Ot- tosson, H. Conductance Through Carbosilane Cage Com- pounds: A Computational Investigation. J. Phys. Chem. C. 2013, 117, 21692–21699 IX Emanuelsson, R.;† Löfås, H.;† Wallner, A.; Nauroozi, D.; Baumgartner, J.; Marschner, C.; Ahuja, R.; Ott, S.; Grigoriev, A.; Ottosson. H. Configuration- and Confor- mation-Dependent Electronic Structure Variations in 1,4- Disubstituted Cyclohexanes Enabled by a Carbon-to- Silicon Exchange. Submitted † These authors contributed equally Paper I reprinted with permission from John Wiley and Sons, Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. Paper II and III reproduced by permission of the Royal Society of Chem- istry. Copyright 2013-2014, Royal Society of Chemistry. Paper VI, VII and VIII reproduced with permission from the American Chemical Society. Copyright 2012-2014, American Chemical Society. Contribution report My contributions to the research performed in the following papers are as follows: I Performed the majority of the calculations, data analysis and synthesis. Co-wrote the manuscript. II Participated extensively in developing the project. Per- formed/supervised the synthesis and contributed to the manuscript writing. III Performed most of the calculations and analyzed the data from these. Joined the project at a late stage when the experimental study was completed and a nearly complete manuscript existed. Participated extensively in reworking the manuscript, performing revisions and adding comparisons with paper I. IV Participated extensively in developing the project. Per- formed part of the synthesis and analyzed the computed re- sults. Co-wrote the manuscript. V Participated in developing of the project. Performed part of the calculations and analysis of these. Contributed to the manuscript writing. VI Participated in developing the project. Performed all isolated molecule calculations and the corresponding data analysis. Co-wrote the manuscript. VII Participated extensively in developing the project. Per- formed part of the synthesis and spectroscopy and all of the calculations. Co-wrote the manuscript. VIII Participated in developing the project. Performed the isolated molecule calculations. Contributed to the data analysis. Wrote part of the manuscript. IX Participated extensively in developing the project. Per- formed the synthesis and UV studies. Performed and analyzed the isolated molecule calculations. Co-wrote the manuscript. Contents 1. Introduction .................................................................................... 13 2. Conjugation .................................................................................... 15 2.1. Conjugation in chemistry ...................................................... 15 2.2. π-Conjugation ....................................................................... 16 2.3. Hyperconjugation .................................................................. 18 2.4. -Conjugation ....................................................................... 20 3. Design, synthesis and evaluation ................................................... 21 3.1. Design and optimization ....................................................... 21 3.1.1. Molecular orbital theory ................................................... 21 3.1.2. Computational chemistry ................................................. 22 3.2. Synthesis and evaluation ....................................................... 28 3.2.1. Synthesis .......................................................................... 28 3.2.2. Ultraviolet/visible spectroscopy ....................................... 29 3.2.3. Cyclic

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