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

<p>Baird’s rule explains why and when excited-state proton transfer (ESPT) reactions happen in organic compounds. Bifunctional compounds that are [4<i>n</i>+2] π-aromatic in the ground state, become [4<i>n</i>+2] π-antiaromatic in the first <sup>1</sup>ππ* states, and proton transfer (either<i>inter-</i>or <i>intra-</i>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. <i>o-</i>Salicylic acid undergoes ESPT only in the “antiaromatic” S<sub>1</sub>(<sup>1</sup>ππ*) state, but not in the “aromatic” S<sub>2</sub>(<sup>1</sup>ππ*) state. Stokes’ shifts of structurally-related compounds (<i>e.g.</i>, 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.</p>