Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1613
Hyperconjugation in Group 14 Organic Compounds
Design and Property Investigations
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.
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 Radialene. Preliminary manuscript.
Reprints were made with permission from the respective publishers.
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.
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 radialene (Paper V) . 47 Concluding remarks ...... 52 Svensk sammanfattning ...... 54 Acknowledgements ...... 57 References ...... 59
AO Atomic Orbital bAB Bond order CC Coupled Cluster CI Configuration Interaction CSF Configuration State Function DFT Density Functional Theory EDG Electron Donating Group eV Electron volt EWG Electron Withdrawing Group GGA Generalized Gradient Approximation HF Hartree-Fock method HOMA The Harmonic Oscillator Model of Aromaticity HOMO Highest Occupied Molecular Orbital HOMO-n The orbital n levels below HOMO HOMO-LUMO gap Energy gap between HOMO and LUMO ISE Isomerization Stabilization Energy IUPAC International Union of Pure and Applied Chemistry LCAO-MO Linear Combination of Atomic Orbitals to Molecu- lar Orbitals LDA Local Density Approximation LSDA Local Spin-Density Approximation LUMO Lowest Unoccupied Molecular Orbital MBPT Many-Body Perturbation Theory MO Molecular Orbital MCSCF Multiconﬁguration Self-Consistent Field nm Nanometer NICS(1)zz The tensor component of Nucleus-Independent Chemical Shift 1.0 Å above the ring plane NRT Natural Resonance Theory OFET Organic field-effect transistor OLED Organic light-emitting diode OPVC Organic photovoltaic cell SA Shannon Aromaticity SCF Self-Consistent Field method S0 Singlet ground state S1 First excited singlet state
T1 First excited triplet state TDDFT Time-Dependent Density Functional Theory UPS Ultraviolet Photoelectron Spectroscopy Å Ångström ΔES-T Energy gap between S0 and T1 states ΔμS-T Dipole moment difference between S0 and T1 states
The ability of an organic molecule to conduct electricity is a property of a great interest today. Such molecules can be used in molecular electronics or as components in materials used in organic electronics. Electron delocaliza- tion is the phenomenon that causes conductivity in a molecule. Going further, conjugation provides electron delocalization. However, the most well-known type of conjugation, π-conjugation, usually leads to molecular rigidness. This is an undesired property as it causes lower solubility and, as a result, limits the potential applications. Meanwhile, other types of conjugation do not cause this disadvantage. Conjugation types other than π-conjugation are not equally well investigated nowadays. This thesis deals with hyperconjugated compounds, their electronic and optical properties, and possible ways to tune these prop- erties.
1.1 Organic electronics Organic semiconductors is a class of organic molecules of high importance nowadays.1 Organic electronics uses these compounds as materials for various devices such as organic light-emitting diodes (OLEDs), photovoltaic cells (OPVCs), field-effect transistors (OFETs), sensors etc. The interest to the miniaturization of electronic devices is still growing and these materials are widely spread now. The first works on organic conducting materials appeared in the 1950s and early 1960s. At that point, research in this field was mostly performed on π-conjugated polymers such as polyacetylene,2 polypyrrole,3 and polyaniline.4 Nevertheless, other types of compounds were investigated as well, for in- stance, charge-transfer complexes based on aromatic compounds.4 In 2002 Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa received the No- bel Prize in Chemistry for their work on conductive π-conjugated polymers.5 The effects of hyperconjugation on properties of conductive polymers and ma- terials are also significant.6 For example, polymers and individual molecules based on siloles are widely used in this field (Figure 1).7-10
Figure 1. Various siloles applied in organic electronics.
1.2 Molecular electronics The first paper reporting on electron transport through a single molecule was published in 1974.11 However, it was a theoretical investigation, while the first paper on measurements of single-molecule transport was published in 1997 by Tour et al.12 To perform described experiment benzene-1,4-dithiol mole- cule was used, where benzene acted as the conducting π-conjugated unit while the sulfur atoms linked the molecule to the surface of the gold electrodes. Now, besides commonly used π-conjugated molecules, there are molecules with other types of conjugation involved into electron transport measure- ments. Nuckolls et al. constructed σ-conjugated α,ω-bis(4-methylthio)-phe- nyloligosilanes that conduct electricity comparable to that of a conjugated chain of C=C π bonds (Figure 2a).13 Later they showed how the strained sili- con molecular wire conducts through two different pathways (Figure 2b).14 Finally, in a recent paper they explored how π-σ-π interaction influences the conduction in molecular wires. In that work, they considered molecules con- taining heavier germanium atoms besides carbon and silicon (Figure 2c).15
12 Figure 2. Molecular wires containing silicon and germanium atoms.
1.3 Aim of this work In this thesis, I review the results from my research, which was done in the projects devoted to the hyperconjugation phenomena. The aim of this research is to find possible ways to design molecules that demonstrate sufficiently strong hyperconjugation. For this purpose, we investigated both small mole- cules and oligomers with the goal in mind to further apply this knowledge in design of hyperconjugated polymers. We investigated possibilities to tune hy- perconjugation strength with a few different approaches: 1. Involvement of group 14 elements starting from light C atoms go- ing to heavy Pb atoms both incorporated into the rings of cyclic molecules and included as substituents on them; 2. Substitution with electron donating and electron withdrawing groups with the focus on aromatic properties in the ground and the first excited states; 3. Gradual elongation going from monomers to oligomers using a few different types of linkages; 4. Substitution of small cyclic molecules with electron donating and electron withdrawing groups at the various positions of the ring; 5. Omni-conjugated pathways in radialene and π-conjugated com- pounds based on it, and further in their hyperconjugated analogs.
13 2. Types of conjugation
2.1 π-Conjugation The term “conjugation” in organic chemistry is used to define a phenomena found in molecules with alternating single and multiple bonds. According to IUPAC “conjugation is interaction of one p atomic orbital with another across an intervening σ bond”.16 Conjugation leads to a delocalization of π electrons across all the adjacent aligned p atomic orbitals.17 As a result, con- jugated molecules often appear to be stabilized and less reactive than non- conjugated ones. However, there are some exceptions, such as antiaromatic compounds that are conjugated and destabilized at the same time.18 Finally, as discussed earlier, delocalization of electrons allows the molecule to conduct electricity. It is worth mentioning that the term “conjugation” is not always used now to denote particularly π-conjugation. There are a few other types of conjuga- tion and to distinguish them from π-conjugation the interaction type should always be accentuated. Other types of conjugation can include σ and δ orbitals in the interaction.19,20 Orbitals of the same type or combination of various types can be involved into the conjugation in this case.
2.2 σ-Conjugation There are few types of conjugation where the orbital interaction occurs be- tween other types of orbitals than local π orbitals. One of these conjugation types is σ-conjugation involving local σ orbitals only. Linear tetrasilane chain is a good example to describe σ bond delocalization in saturated molecules. In this case, according to molecular orbital theory calculations include the inter- action integrals between the two-center σ or σ* bond orbitals. The primary resonance integrals βprim generate the primary electronic structure of the sys- tem. They describe the interaction between the local sp3 hybrid orbitals at the neighboring atoms when these orbitals point to each other (Figure 3). Then the primary resonance integrals produce the corresponding local bonding (nodeless) and antibonding (with one node) σ orbitals, respectively. However, instead of the primary resonance integrals other smaller integrals describe σ-delocalization, such as the geminal resonance integrals, βgem, and vicinal resonance integrals, βvic. The geminal resonance integrals define the
14 interaction between two sp3 hybrid orbitals at the same atom, and the vicinal resonance integrals the interaction between two lobes at adjacent atoms that are not pointing towards each other. While primary resonance integrals are large and always negative, geminal and vicinal are smaller and their sign de- pends on the geometry of the molecule. The geminal integrals provide for σ-conjugation and if larger lobes have the same sign they are negative. Mean- while, the sign and value of the vicinal integrals depend on the angle of rota- tion about the silicon-silicon bond. These integrals are negative in syn-peri- planar alignment and positive in anti-periplanar; they go through zero while twisting between these two positions. Vicinal resonance integrals provide for σ-hyperconjugation. As it was mentioned before primary integrals are larger than geminal and for oligosilanes the difference is two to three times. Further- more, geminal integrals appear to be larger than the vicinal integrals in syn-periplanar position. When going from C to Si and further down group 14 the importance of σ-hyperconjugation declines while that of σ-conjugation grows.21,22
βvic > 0 βvic < 0
βprim < 0 βprim < 0
β < 0 βgem < 0 βgem < 0 gem βgem < 0
Figure 3. Resonance integrals in a linear tetrasilane chain.
2.3 Hyperconjugation Hyperconjugation is another type of conjugation that involves the interaction of both σ and π orbitals. In 1937 Kistiakowsky et al. found that the heat of hydrogenation for cyclopentadiene is only 50.9 kcal/mol, which is 6.2 kcal/mol less than for 1,3-butadiene.23 Mulliken in his pioneering work proposed that this stabilizing effect takes place due to a phenomenon, which he called hyperconjugation. 24,25 He showed that cyclopentadiene could be viewed as an analog to fulvene (Figure 4). In this case, the CH2 group in cy- clopentadiene is represented as a double bond comparable to the C=CH2 moi- ety in fulvene. Similarly, Mulliken demonstrates the analogy between a me- thyl group and an ethynyl group by displaying the methyl group as C≡H3.
15 Figure 4. Cyclopentadiene and fulvene representation by Mulliken.
Mulliken also introduces various classifications of hyperconjugation depend- ing on the type of bonds involved. First-order hyperconjugation includes one quasi-multiple bond and one true multiple bond. This is the case for cyclopen- tadiene where CH2 and CH=CH groups are involved in the hyperconjugation. Second-order hyperconjugation involves two quasi-multiple bonds.24 The ef- fects of first-order hyperconjugation are usually more pronounced than the second-order one. Later Mulliken modified the classification of hyperconjugation. He men- tioned that π-conjugation and hyperconjugation are qualitatively the same types of interaction. Considering this, he denoted π-conjugation, hyperconju- gation, and σ-conjugation as first-order conjugation, second-order conjuga- tion, and third-order conjugation, respectively (Table 1).25 Undoubtedly, it is not always possible to distinguish between various types of conjugation. Lone pairs are often hybridized and show notable s-character. Such as in phenyla- mine, conjugation involves a p orbital and as the result, it is not possible to distinguish between hyperconjugation and π-conjugation in such cases.26
Table 1. Mulliken’s classification of conjugation types.
H2C=CH‒CH=CH2 Ordinary (first-order) conjugation Second-order conjugation H3≡C‒CH=CH2 (first-order hyperconjugation) Third-order conjugation H3≡C‒C≡H3 (second-order hyperconjugation)
Resonance theory describes hyperconjugation by the so called “double bond/no-bond resonance” structures and there are two different ways of clas- sification. The first classification divides hyperconjugative interactions into two types: heterovalent and isovalent hyperconjugation (Figure 5). Heterova- lent hyperconjugation is typical for neutral molecules. In the contributing res- onance structure, one two-electron bond disappears compared with the regular Lewis formula. Isovalent hyperconjugation is typical for cations. In this case, the contributing structure has the same number of two-electron bonds as the
16 main Lewis formula.26 Mulliken denoted heterovalent hyperconjugation as or- dinary or sacrificial hyperconjugation.27
Figure 5. Contributing resonance structures in heterovalent and isovalent hypercon- jugation.
According to another type of classification, hyperconjugation is divided into neutral, positive, and negative hyperconjugation (Figure 6). Neutral hypercon- jugation is observed if there are no dominating directions for the interaction. Interaction between filled π or p orbitals and antibonding σ* orbitals leads to negative hyperconjugation. Donation of electron density from filled σ orbitals into π* orbitals or p type unfilled orbitals results in positive hyperconjuga- tion.26
σc-x → π* π → σ*c-x
Figure 6. Contributing resonance structures in positive, negative, and neutral types of hyperconjugation.
2.4 Cross-hyperconjugation Various types of conjugation were described in details in previous sections. However, not only the type of conjugation but also the type of connectivity in conjugated molecules influences their properties. There are three possible types of connectivity in π-conjugated molecules: linear or through-conjuga- tion, cross-conjugation, and omni-conjugation (Figure 7). A succession of sin- gle and double bonds in a straight line is characteristic for linear conjugation.28
17 In the case when there are three parts of the molecule, two of which are con- jugated to the third one but not to each other, such type of connection is typical for cross-conjugation.29 Finally, if all groups in a non-linear molecule are con- jugated in a linear fashion, this will result in omni-conjugated pathway.30
Figure 7. Various connection pathways in π-conjugated molecules.
As hyperconjugation is qualitatively the same type of orbital interaction as π-conjugation, the connectivity patterns are applicable also to hyperconju- gated systems. The concept of cross-hyperconjugation was first proposed by Ottosson and co-workers in 2013. It is described as “the fusion of two neutral hyperconjugated paths to a cross-hyperconjugated molecule with geminal connectivity between the two paths”.31 There is valence isolobal analogy be- tween structure of cross-hyperconjugated systems and regular cross-π-conju- gation (Figure 8). The term “valence isolobal” is applied to the moieties with the similar energy and orbital shape. Additionally, they should contain the same number of electrons.32
Figure 8. Valence isolobal analogy between a C=C double bond in a cross-π-conju- gated molecule and an ER2 group in a cross-hyperconjugated one.
18 2.5 Quantification of hyperconjugation
2.5.1 UV/Vis absorption spectroscopy The measurement of transitions from the electronic ground state to electroni- cally excited states of molecules is performed with UV/Vis spectroscopy. In an experiment, the molecule absorbs photons of a certain energy matching the energy difference between the states whereby the corresponding transitions become possible. These transitions involve only valence electrons (Figure 9). From the results of UV/Vis spectroscopy measurements, i.e., the absorption bands and their intensities, one can to a certain extent conclude how strongly the electrons are bonded in a molecule.33 However, ultraviolet photoelectron spectroscopy (UPS) is more suitable for this purpose as it deals with photoion- ization process.34
Figure 9. Transitions that are included into UV/Vis spectroscopy measurements.
Most electronic transitions can be included in the measurements, as UV/Vis spectroscopy operates between 160 nm and 2500 nm (from 0.5 to 7.7 eV). The particular operational range depends on the type of a lamp used in spectropho- tometer. In some cases, transitions can be found at shorter wavelengths. For example, in alkanes the only possible transitions are of σ→σ* type which are of very high energy and only absorb at wavelengths shorter than 160 nm: the absorption maxima of methane and ethane are found at 122 and 135 nm, re- spectively.35,36 Chromophores in organic molecules are functional groups that are responsible for the absorption at certain wavelengths.37 UV/Vis absorption spectroscopy is one of the typical methods used for in- vestigation of π-conjugation strength. The highest occupied molecular orbital (HOMO) is placed higher in energy in a π-conjugated molecule than that of a non-conjugated system, while the lowest unoccupied molecular orbital
19 (LUMO) is lower in energy. As a result, the HOMO-LUMO gaps decrease and conjugated systems have smaller excitation energies and therefore absorb at longer wavelengths. For example, 1,3-butadiene absorbs at 220 nm (5.6 eV), while ethylene has an absorption maximum at 185 nm (6.7 eV). Con- sidering longer polyenes it is clear that a greater wavelength of the absorption is characteristic of a longer conjugated carbon chain (with higher degree of conjugation). Thus, the degree of conjugation can be evaluated with the help of UV/Vis absorption spectroscopy. This is applicable to all types of conjuga- tion. However, the correlation between the conjugation strength and the values of absorption maxima is not always obvious. For instance, molecules with strong electron donor and acceptor substituents reveal such behavior.38-40 Moreover, the lowest transition can be forbidden and thus it is not visible in the UV/Vis spectrum. This should be taken into account when analyzing re- sults obtained with the UV/Vis spectroscopy.
2.5.2 Bond lengths It is common knowledge that multiple bonds are moderately elongated while single bonds are significantly shortened in π-conjugated molecules.41,42 This consideration is valid for hyperconjugation too, as it is qualitatively the same type of interaction as π-conjugation. Nevertheless, hyperconjugation is much weaker than π-conjugation, and therefore the bond lengths are not changed considerably for multiple bonds but primarily for single bonds involved.24 In this fashion, comparison between bond lengths in a hyperconjugated molecule and its purely π-conjugated analog can help to figure out the strength of hy- perconjugation.
2.5.3 Bond orders Resonance theory is widely used in organic chemistry. Besides other applica- tions within organic chemistry, it can be useful for understanding hyperconju- gation concept. Resonance theory represents the electronic structure of a mol- ecule as a combination of contributing (or resonance, or mesomeric) struc- tures. In this case the wavefunction is a mixture of different contributing struc- tures. Delocalization is often described by resonance structures,43 and amide resonance in formamide is a classical example illustrating the basic resonance theory (Figure 10). The properties of this molecule are intermediate between those of the localized structures I and II, and the structure has therefore frac- tional bond orders. The relative weightings wI and wII are 0.6 and 0.4, respec- tively.44
Figure 10. Resonance structures of formamide.
In general, the weighted average of idealized values Pα related to localized structural formulas α can describe a molecular property P. In such a case, weighting factors should be nonnegative and their sum should be equal to 1: