Molecules in a Hurry To Get Rid of Antiaromaticity

Judy I. Wu University of Houston, Houston, USA [email protected]

Antiaromatic molecules, unless kinetically trapped, fused to aromatic frameworks, or stabilized by chemical modifications, often are short-lived and difficult to work with experimentally—they always find ways of escaping the state of being called “antiaromatic.” Cyclobutadiene, cyclopentadiene, pentalene, and other cyclic, π-conjugated compounds, with formal [4n] ring π-electrons, easily dimerize to get rid of antiaromaticity. In photoexcited states, the rules of aromaticity and antiaromaticity can reverse according to Baird’s rule: [4n+2] compounds are antiaromatic and [4n] compounds are aromatic, but the urge for photoexcited molecules to get rid of antiaromaticity is the same. Upon irradiation, benzene rather isomerize to fulvene and the very strained benzvalene than stay [4n + 2] π-electron antiaromatic. Many photochemical reactions can be understood by relief of excited-state antiaromaticity. My talk will focus on the effects of antiaromaticity relief on excited-state proton transfer (ESPT) reactions (see cartoon). Implications for proton-coupled electron transfer in DNA base pairs also will be discussed.

In the past 50 years, the term “antiaromaticity” has evolved quickly from a concept that picks theoretical interest and invites synthetic challenges, to a poster child for many modern applications of chemistry. The relationship between excited-state antiaromaticity and ESPT is another celebration of the antiaromaticity concept, and recognizing this link can have tremendous interpretive value for understanding the photochemistry of many organic and biological systems.

Keywords: Antiaromatic, Baird’s Rule, Excited-State Proton Transfer

Suggested reading: C.-H Wu, L. J. Karas, H. Ottosson, J. I. Wu, Proc. Natl. Acad. Sci. 2019, 116, 20303–20308.