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Hyperconjugation in Group 14 Organic Compounds: Design and Property Investigations
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Molecular Biomimetics.
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. , p. 61
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1613
Keyword [en]
1, 4-disilacyclohexa-2, 5-diene, aromaticity, computational chemistry, conjugation, cross-hyperconjugation, hyperconjugation, group 14 elements, organosilicon chemistry, silole
National Category
Organic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-336192ISBN: 978-91-513-0190-7 (print)OAI: oai:DiVA.org:uu-336192DiVA, id: diva2:1165221
Public defence
2018-02-09, Häggsalen, Ångström laboratory, Lägerhyddsvägen 1, Uppsala, 09:15 (English)
Opponent
Supervisors
Available from: 2018-01-19 Created: 2017-12-12 Last updated: 2018-03-07
List of papers
1. 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, p. 2997-3004Article 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
2. Impact of Ground- and Excited-State Aromaticity on Cyclopentadiene and Silole Excitation Energies and Excited-State Polarities
Open this publication in new window or tab >>Impact of Ground- and Excited-State Aromaticity on Cyclopentadiene and Silole Excitation Energies and Excited-State Polarities
Show others...
2014 (English)In: Chemistry - A European Journal, ISSN 0947-6539, E-ISSN 1521-3765, Vol. 20, no 30, p. 9295-9303Article in journal (Refereed) Published
Abstract [en]

A new qualitative model for estimating the properties of substituted cyclopentadienes and siloles in their lowest pi pi* excited states is introduced and confirmed through quantum chemical calculations, and then applied to explain earlier reported experimental excitation energies. According to our model, which is based on excited-state aromaticity and antiaromaticity, siloles and cyclopentadienes are cross-hyperconjugated "aromatic chameleons" that adapt their electronic structures to conform to the various aromaticity rules in different electronic states (Huckel's rule in the pi(2) electronic ground state (S-0) and Baird's rule in the lowest pi pi* excited singlet and triplet states (S-1 and T-1)). By using pen-and-paper arguments, one can explain polarity changes upon excitation of substituted cyclopentadienes and siloles, and one can tune their lowest excitation energies by combined considerations of ground-and excited-state aromaticity/antiaromaticity effects. Finally, the "aromatic chameleon" model can be extended to other monocyclic compound classes of potential use in organic electronics, thereby providing a unified view of the S-0, T-1, and S-1 states of a range of different cyclic cross-pi-conjugated and cross-hyperconjugated compound classes.

Keyword
aromaticity, conjugation, density functional calculations, electronic structure, organic electronics
National Category
Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-231122 (URN)10.1002/chem.201402577 (DOI)000339568800022 ()
Available from: 2014-09-04 Created: 2014-09-04 Last updated: 2018-04-23Bibliographically approved
3. Expanding the (Cross-)Hyperconjugation of 1,4-Disilacyclohexa-2,5-dienes to Larger Monomers and Oligomers: A Computational Investigation
Open this publication in new window or tab >>Expanding the (Cross-)Hyperconjugation of 1,4-Disilacyclohexa-2,5-dienes to Larger Monomers and Oligomers: A Computational Investigation
2016 (English)In: RSC Advances, ISSN 2046-2069, E-ISSN 2046-2069, Vol. 6, no 43, p. 36961-36970Article in journal (Refereed) Published
Abstract [en]

We used density functional theory calculations to examine molecules that can be regarded as expanded 1,4-disilacyclohexa-2,5-dienes as well as oligomers based on these or 1,4-disilacyclohexa-2,5-diene with the aim to identify systems with extended (cross-)hyperconjugation. Among the three "expanded 1,4-disilacyclohexa-2,5-dienes" considered cyclobutadisilole is the most interesting as it has a higher thermodynamic stability than the isomeric 1,6-disilacyclodeca-2,3,4,7,8,9-hexaene and significantly lower first electronic excitation energy than 1,6-disilacyclodeca-2,4,7,9-tetraene. Cyclobutadisilole with trimethylsilyl substituents at Si shows particularly low excitations with the first strong transition at 3.46 eV (358 nm), i.e., similar to 1.1 eV lower than in 1,4-disilacyclohexa-2,5-diene. The monomers were connected into oligomers via their Si atoms using bis(dimethylsilanediyl) linkers, and some extended hyperconjugation was revealed. The first allowed UV/Vis excitation in the cyclobutadisilole-based tetramers is calculated at 2.57 eV (482 nm), although the lowering in excitation energies when going from monomer to tetramer is merely similar to 0.5 eV and hyperconjugation has modest impact on geometries. Yet, the tetra(cyclobutadisilole) has a significantly lower first allowed excitation when compared to a previously studied tetra(1,4-disilacyclohexadiene) with first excitation at 3.9 eV (318 nm).

National Category
Organic Chemistry Engineering and Technology
Research subject
Chemistry with specialization in Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-293564 (URN)10.1039/C6RA02732F (DOI)000374561300088 ()
Funder
Swedish Research Council
Available from: 2016-05-13 Created: 2016-05-13 Last updated: 2017-12-12Bibliographically approved
4. A Computational Investigation of the Substituent Effects on Geometric, Electronic, and Optical Properties of Siloles and 1,4-Disilacyclohexa-2,5-dienes
Open this publication in new window or tab >>A Computational Investigation of the Substituent Effects on Geometric, Electronic, and Optical Properties of Siloles and 1,4-Disilacyclohexa-2,5-dienes
2017 (English)In: Molecules, ISSN 1420-3049, E-ISSN 1420-3049, Vol. 22, no 3, article id 370Article in journal (Refereed) Published
Abstract [en]

Thirty two differently substituted siloles 1a–1p and 1,4-disilacyclohexa-2,5-dienes 2a–2p were investigated by quantum chemical calculations using the PBE0 hybrid density functional theory (DFT) method. The substituents included σ-electron donating and withdrawing, as well as π-electron donating and withdrawing groups, and their effects when placed at the Si atom(s) or at the C atoms were examined. Focus was placed on geometries, frontier orbital energies and the energies of the first allowed electronic excitations. We analyzed the variation in energies between the orbitals which correspond to HOMO and LUMO for the two parent species, here represented as ΔεHL, motivated by the fact that the first allowed transitions involve excitation between these orbitals. Even though ΔεHL and the excitation energies are lower for siloles than for 1,4-disilacyclohexa-2,5-dienes the latter display significantly larger variations with substitution. The ΔεHL of the siloles vary within 4.57–5.35 eV (ΔΔεHL = 0.78 eV) while for the 1,4-disilacyclohexa-2,5-dienes the range is 5.49–7.15 eV (ΔΔεHL = 1.66 eV). The excitation energy of the first allowed transitions display a moderate variation for siloles (3.60–4.41 eV) whereas the variation for 1,4-disilacyclohexa-2,5-dienes is nearly doubled (4.69–6.21 eV). Cyclobutadisiloles combine the characteristics of siloles and 1,4-disilacyclohexa-2,5-diene by having even lower excitation energies than siloles yet also extensive variation in excitation energies to substitution of 1,4-disilacyclohexa-2,5-dienes (3.47–4.77 eV, variation of 1.30 eV).

National Category
Nano Technology
Research subject
Engineering Science with specialization in Nanotechnology and Functional Materials
Identifiers
urn:nbn:se:uu:diva-335095 (URN)10.3390/molecules22030370 (DOI)000398743500031 ()
Note

Title of manuscript in list of papers in Julius Tibbelin´s thesis: A comparative computational investigation of the substituent effects on geometric, electronic, and optical properties of 1,4-disilacyclo-hexa-2,5- dienes and siloles

Available from: 2017-11-30 Created: 2017-11-30 Last updated: 2018-02-16Bibliographically approved
5. Computational Design of Strongly σ/π-Conjugated Compounds with a Start at the Omni-Conjugated [3]Radialene
Open this publication in new window or tab >>Computational Design of Strongly σ/π-Conjugated Compounds with a Start at the Omni-Conjugated [3]Radialene
(English)Manuscript (preprint) (Other academic)
National Category
Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-336170 (URN)
Available from: 2017-12-12 Created: 2017-12-12 Last updated: 2017-12-20

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