University of Houston
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. 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, including excited-state intramolecular proton transfers (ESIPT), biprotonic transfers, dynamic catalyzed transfers, and proton relay transfers. o-Salicylic acid undergoes ESPT only in the “antiaromatic” S1 (1ππ*) state, but not in the “aromatic” S2 (1ππ*) state. Stokes’ shifts of structurally-related compounds (e.g., derivatives of 2-(2-hydroxyphenyl)benzoxazole and hydrogen-bonded complexes of 2-aminopyridine with protic substrates) vary depending on the antiaromaticity of the photoinduced tautomers. Implications of excited-state antiaromaticity for proton-coupled electron transfer in DNA base pairs also will be discussed.