Excited-State Proton Transfer: Molecules in a Hurry to Get Rid of Antiaromaticity

Baird’s rule explains why and when excited-state proton transfer (ESPT) reactions happen in organic compounds. Bifunctional compounds that are [4n+2] π-aromatic in the ground state, become [4n+2] π-antiaromatic in the first ¹ππ* states, and proton transfer (eitherinter-or intra-molecularly) helps relieve excited-state antiaromaticity. Computed nucleus independent chemical shifts (NICS) for several ESPT examples (including excited-state intramolecular proton transfers (ESIPT), biprotonic transfers, dynamic catalyzed transfers, and proton relay transfers) document the important role of excited-state antiaromaticity. 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 pro tic substrates) vary depending on the antiaromaticity of the photoinduced tautomers. Remarkably, Baird’s rule predicts the effect of light on hydrogen bond strengths; hydrogen bonds that enhance (and reduce) excited-state antiaromaticity in compounds become weakened (and strengthened) upon photoexcitation.