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 [4 n +2] π-aromatic in the ground state, become [4 n +2] π-antiaromatic in the first 1 ππ* states, and proton transfer (either inter- 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” S 1 ( 1 ππ*) state, but not in the “aromatic” S 2 ( 1 ππ*) 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.