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Can Baird's and Clar's Rules Combined Explain Triplet State Energies of Polycyclic Conjugated Hydrocarbons with Fused 4n pi- and (4n+2)pi-Rings?
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Molecular Biomimetics.
Univ Girona, IQCC, C Maria Aurelia Capmany 6, Girona 17003, Catalonia, Spain..
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Molecular Biomimetics.
Univ Girona, IQCC, C Maria Aurelia Capmany 6, Girona 17003, Catalonia, Spain..
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2017 (English)In: Journal of Organic Chemistry, ISSN 0022-3263, E-ISSN 1520-6904, Vol. 82, no 12, 6327-6340 p.Article in journal (Refereed) Published
Abstract [en]

Compounds that can be labeled as "aromatic chameleons" are pi-conjugated compounds that are able to adjust their pi-electron distributions so as to comply with the different rules of aromaticity in different electronic states. We used quantum chemical calculations to explore how the fusion of benzene rings onto aromatic chameleonic units represented by biphenylene, dibenbzocyclooctatetraene, and dibenzo[a,e]pentalene modifies the first triplet excited states (T-1) of the compounds. Decreases in T-1 energies are observed when going from isomers with linear connectivity of the fused benzene rings to those with cis- or transbent connectivities. The T-1 energies decreased down to those of the parent (isolated) 4n pi-electron units. Simultaneously, we observe an increased influence of triplet State aromaticity of the central 4n ring as given by Baird's rule and evidenced by geometric, magnetic, and electron density based aromaticity indices (HOMA, NICS-XY, ACID, and FLU). Because of an influence of,triplet state aromaticity in the central 4n pi-electron units,, the most stabilized, compounds, retain the triplet excitation in Baird pi-quartets or octets, enabling the outer benzene rings to adapt closed-shell singlet Clar pi-sextet character. Interestingly, the T-1 energies go down as the total number of aromatic cycles within a molecule in the T-1 state increases.

Place, publisher, year, edition, pages
AMER CHEMICAL SOC , 2017. Vol. 82, no 12, 6327-6340 p.
National Category
Organic Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-329665DOI: 10.1021/acs.joc.7b00906ISI: 000403854500031PubMedID: 28535673OAI: oai:DiVA.org:uu-329665DiVA: diva2:1142700
Funder
Swedish Research Council
Available from: 2017-09-20 Created: 2017-09-20 Last updated: 2017-10-27Bibliographically approved
In thesis
1. Excited State Aromaticity and Antiaromaticity: Fundamental Studies and Applications
Open this publication in new window or tab >>Excited State Aromaticity and Antiaromaticity: Fundamental Studies and Applications
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The central theme of this thesis is the ability to tune various molecular properties by controlling and utilizing aromaticity and antiaromaticity in the lowest electronically excited states. This investigation is based on qualitative theory, quantum chemical (QC) calculations and experimental work.

Baird's rule tells that the π-electron count for aromaticity and antiaromaticity is reversed in the ππ* triplet (T1) state when compared to Hückel's rule for the singlet ground state. The excited state aromatic character of [4n]annulenes is probed by usage of two structural moieties, the cyclopropyl (cPr) group and the silacyclobutene (SCB) ring. The results of QC calculations and photoreactivity experiments showed that the cPr group and the SCB ring remained closed when attached to or fused with [4n]annulenes so as to preserve T1 aromatic stabilization. In contrast, both moieties ring-opened when attached to or fused with [4n+2]annulenes as a means for alleviation of T1 antiaromaticity. These two structural moieties are shown to indicate T1 aromatic character of [4n]annulenes except in a limited number of cases.

The T1 antiaromatic character of compounds with 4n+2 π-electrons was utilized for photo(hydro)silylations and photohydrogenations. QC calculations showed that due to T1 antiaromaticity, benzene is able to abstract hydrogen atoms from trialkylsilanes. The photoreactions occurred under mild conditions for benzene and certain polycyclic aromatic hydrocarbons. In contrast, COT was found to be unreactive under similar conditions.

It is further revealed that various properties of molecules can be tailored by rational design using Baird’s rule. Three modes of connectivity (linear, bent, and cyclic) of polycyclic conjugated hydrocarbons (PCH) were explored by DFT calculations. When the PCHs contain a central [4n]unit and 4nπ-electron perimeter, bent isomers have lower triplet state energies than linear ones due to increased T1 aromaticity in the bent isomers. With regard to the cyclic connectivity, macrocyclic compounds are designed by modifying the C20 monocycle through incorporation of monocyclic units (all-carbon as well as heterocyclic) and the impact of macrocyclic T1 aromaticity upon insertion of different units is examined through QC calculations. The results provide insights on excited state aromaticity in macrocyclic systems.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. 61 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1594
Keyword
Baird's rule, Clar's rule, Computational quantum chemistry, Excited state aromaticity, Excited state aromaticity indicators, Organic photochemistry, Polycyclic conjugated hydrocarbons
National Category
Organic Chemistry
Research subject
Chemistry with specialization in Organic Chemistry
Identifiers
urn:nbn:se:uu:diva-332404 (URN)978-91-513-0138-9 (ISBN)
Public defence
2017-12-15, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
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Supervisors
Available from: 2017-11-24 Created: 2017-10-27 Last updated: 2017-11-24

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Ayub, RabiaJorner, KjellOttosson, Henrik

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