Aromaticity Across States: A Framework for Reactivity and Structure in Organic Systems
Aromaticity and antiaromaticity are unifying concepts that shape the stability, structure, and reactivity of organic molecules. In the ground state, [4n+2] π-systems are stabilized by aromaticity, while [4n] π-systems are destabilized by antiaromaticity. Upon photoexcitation, these rules invert, as described by Baird’s rule, making aromaticity a dynamic property that can be tuned by light, metal coordination, or electronic effects. This dissertation investigates how changes in aromaticity across electronic states can be harnessed to explain and predict reactivity and molecular structure. In the first part, we identify the criteria for transition metals to promote arene–alkene cycloadditions. Computed reaction profiles show that dearomatization is required for arenes to exhibit ene- and diene-like reactivity. While substituted and η6-coordinated arenes retain high free energy barriers, strongly dearomatized η2- and η4-arenes display substantially lower barriers (~20 kcal mol−1), demonstrating that the extent of aromaticity loss, rather than metal binding alone, governs reactivity. The second part examines how excited-state aromaticity modulates Lewis acid–base interactions. We show that adducts containing [4n] π-electron rings (e.g., boroles) undergo photodissociation, stabilized by aromaticity gain in the excited state, whereas [4n+2] systems (e.g., borepins, pyrroles) favor photoassociation through relief of excited-state antiaromaticity. This framework further explains unusual photochemical behaviors, including bond-cleavage induced charge transfer in borafluorenes. The third part addresses the structural challenge of tetra-tert-butyl-s-indacene (TtB-s-indacene), a kinetically stabilized but strongly antiaromatic compound long debated between delocalized D2h and bond-localized C2h forms. By combining computed and experimental proton chemical shifts with modern density functional methods, we show that TtB-s-indacene adopts a bond-localized C2h structure, consistent with its antiaromatic character, and highlight limitations of widely used functionals such as B3LYP. Together, these studies demonstrate that reactivity, structure, and photochemical response—whether promoted by metal coordination, light, or electronic correlation—can be rationalized through a common principle: molecules adapt to the energetic incentives of losing or regaining aromatic stabilization. This framework bridges fundamental concepts of physical organic chemistry with strategies for designing catalysts, photochemical switches, and functional antiaromatic materials.
Aromaticity, Excited-state chemistry, Organic Chemistry
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