Hyperconjugation in Group 14 Organic Compounds
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Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1613 Hyperconjugation in Group 14 Organic Compounds Design and Property Investigations ALEKSANDRA DENISOVA ACTA UNIVERSITATIS UPSALIENSIS ISSN 1651-6214 ISBN 978-91-513-0190-7 UPPSALA urn:nbn:se:uu:diva-336192 2017 Dissertation presented at Uppsala University to be publicly examined in Häggsalen, Ångström laboratory, Lägerhyddsvägen 1, Uppsala, Friday, 9 February 2018 at 09:15 for the degree of Doctor of Philosophy. The examination will be conducted in English. Faculty examiner: Professor Thomas Müller. Abstract Denisova, A. 2017. Hyperconjugation in Group 14 Organic Compounds. Design and Property Investigations. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1613. 61 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-0190-7. Nowadays π-conjugated molecules are widely used as materials for devices in organic and molecular electronics. This is due to the ability of such molecules to conduct electricity. However, π-conjugation leads to molecular rigidness and associated lower solubility, which limits possible applications. Meanwhile, there are other types of conjugation that do not cause molecular rigidness but still provide conductivity. One of them is so called hyperconjugation. While π-conjugation involves only p atomic orbitals, hyperconjugation is characterized by interaction of π and σ orbitals. Hyperconjugation is normally weaker than π-conjugation, thus, in order to get strongly hyperconjugated molecules they should be enhanced in some way. In this thesis, I describe methods for design of strongly hyperconjugated molecules. It is possible to increase the strength of hyperconjugation by various methods and some of them are discussed. We performed quantum chemical calculations in order to investigate optical and geometric properties of the hyperconjugated molecules and evaluate the relative strength of hyperconjugation. In some cases, results of calculations were compared with experimental results aiming to confirm the relevance of the calculations. First, we have investigated how the change of group 14 elements in the 1,4-ditetrelocyclohexa-2,5-dienes influence the hyperconjugation strength. Next, the substituent effect was considered in fulvenes and their hyperconjugated analogs. We showed this effect from the perspective of the substituents influence on the aromatic properties of molecules in the ground and first electronically excited states. Further, the gradual shift when going from monomer to oligomers were investigated. For this hyperconjugated oligomers were constructed from 1,4-disilacyclohexa-2,5-diene and cyclobutadisilole fragments. Additionally we showed the influence of electron withdrawing and electron donating groups on hyperconjugation in siloles and 1,4-disilacyclohexa-2,5-dienes. Finally, hyperconjugation was investigated in a set of silicon-containing omni-hyperconjugated compounds. The results obtained from this research showed that hyperconjugation strength can be increased significantly up to levels comparable to purely π-conjugated molecules. We hope that these results will be useful in development of other hyperconjugated small molecules, oligomers, and polymers, which can be further used as material for electronic devices. Keywords: 1, 4-disilacyclohexa-2, 5-diene, aromaticity, computational chemistry, conjugation, cross-hyperconjugation, hyperconjugation, group 14 elements, organosilicon chemistry, silole Aleksandra Denisova, Department of Chemistry - Ångström, Molecular Biomimetics, Box 523, Uppsala University, SE-75120 Uppsala, Sweden. © Aleksandra Denisova 2017 ISSN 1651-6214 ISBN 978-91-513-0190-7 urn:nbn:se:uu:diva-336192 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-336192) To my little family 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.;† Denisova, A. V.;† Ottosson, H. Optimization of the Cyclic Cross-Hyperconjugation in 1,4-Ditetrelcyclohexa- 2,5-dienes. Organometallics, 2014, 33, 2997–3004. †These authors contributed equally II Jorner, K.;† Emanuelsson, R.;† Dahlstrand, C.; Tong, H.; Den- isova, A. V.; Ottosson, H. Impact of Ground- and Excited-State Aromaticity on Cyclopentadiene and Silole Excitation Energies and Excited-State Polarities. Chem. Eur. J., 2014, 20, 9295– 9303. †These authors contributed equally III Denisova, A. V.; Emanuelsson, R.; Ottosson, H. Expanding the (Cross-)Hyperconjugation of 1,4-Disilacyclohexa-2,5-dienes to Larger Monomers and Oligomers: a Computational Investiga- tion. RSC Adv., 2016, 6, 36961–36970. IV Denisova, A. V., Tibbelin, J., Emanuelsson, R., Ottosson, H. A Computational Investigation of the Substituent Effects on Ge- ometric, Electronic, and Optical Properties of Siloles and 1,4-Di- silacyclohexa-2,5-dienes. Molecules, 2017, 22, 370. V Denisova, A.V., Yadav, S., Ottosson, H. Computational Design of Strongly σ/π-Conjugated Compounds with a Start at the Omni- Conjugated [3]Radialene. Preliminary manuscript. Reprints were made with permission from the respective publishers. Contribution report The author wishes to clarify her contribution to the included papers I – V: I Performed the majority of the calculations. Contributed to the manuscript writing. II Performed calculations on ring size effects. III Performed all the calculations and major part of data analysis. Wrote the manuscript with feedbacks from the middle and last authors. IV Performed all the calculations and major part of data analysis. Wrote the manuscript with feedbacks from the middle and last authors. V Performed part of the calculations and part of the data analysis. Co-wrote the manuscript together with the other two authors. Contents 1. Introduction ............................................................................................... 11 1.1 Organic electronics ............................................................................. 11 1.2 Molecular electronics ......................................................................... 12 1.3 Aim of this work ................................................................................ 13 2. Types of conjugation ................................................................................. 14 2.1 π-Conjugation ..................................................................................... 14 2.2 σ-Conjugation ..................................................................................... 14 2.3 Hyperconjugation ............................................................................... 15 2.4 Cross-hyperconjugation ..................................................................... 17 2.5 Quantification of hyperconjugation .................................................... 19 2.5.1 UV/Vis absorption spectroscopy ................................................ 19 2.5.2 Bond lengths ............................................................................... 20 2.5.3 Bond orders ................................................................................. 20 2.5 Aromaticity and antiaromaticity ......................................................... 21 3. Computational chemistry .......................................................................... 23 3.1 Quantum chemistry ............................................................................ 23 3.2 Qualitative molecular orbital theory ................................................... 23 3.3 Hartree-Fock and post-Hartree-Fock methods ................................... 24 3.4 DFT and TDDFT methods ................................................................. 25 3.5 Aromaticity indices ............................................................................ 26 4. Cross-hyperconjugation in cyclic organic group 14 element compounds (Paper I) ........................................................................................................ 29 4.1 Quantum chemical calculations .......................................................... 29 4.2 Comparison between experimental and calculated data ..................... 32 4.3 Influence of substitution on the C=C double bonds ........................... 33 5. Ground and excited state aromaticity in cyclic cross-π-conjugated and cross-hyperconjugated compounds (Paper II) ............................................... 34 5.1 Aromatic chameleon behavior in five-membered rings ..................... 34 5.2 Ring size effect ................................................................................... 36 6. Conjugation in oligomers composed of cyclic cross-hyperconjugated monomers (Paper III) .................................................................................... 39 7. Substituent effect on properties of small hyperconjugated cycles (Paper IV) ..................................................................................................... 43 8. Omni-hyperconjugation in compounds based on [3]radialene (Paper V) . 47 Concluding remarks ...................................................................................... 52 Svensk sammanfattning ................................................................................ 54 Acknowledgements ....................................................................................... 57 References ..................................................................................................... 59 Abbreviations AO Atomic Orbital bAB Bond order