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. Upon irradiation, benzene rather isomerize to fulvene and the very strained benzvalene than stay [4n + 2] π-electron antiaromatic. In this talk, I will give examples of proton and electron transfer reactions that are driven by the relief of excited-state antiaromaticity. The discussion will cover excited-state proton transfer (ESPT) reactions, including excited-state intramolecular proton transfers (ESIPT), biprotonic transfers, dynamic catalyzed transfers, and proton relay transfers. Implications of excited-state antiaromaticity for proton-coupled electron transfer (PCET), for example, in DNA base pairs, photolysis, and water-splitting reactions, also will be discussed.

Molecules in a hurry to escape antiaromaticity

ACS Fall 2021

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