Excited State Aromaticity and Antiaromaticity

Ayub, R. 2017. Excited State Aromaticity and Antiaromaticity. Fundamental Studies and Applications. Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 1594. 61 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-0138-9. 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.