Department of Chemistry, University of Houston, Houston, TX 77004.
Department of Chemistry, Ångström Laboratory, Uppsala University, 751 20 Uppsala, Sweden.
Proc Natl Acad Sci U S A. 2019 Oct 8;116(41):20303-20308. doi: 10.1073/pnas.1908516116. Epub 2019 Sep 25.
Baird's rule explains why and when excited-state proton transfer (ESPT) reactions happen in organic compounds. Bifunctional compounds that are [4 + 2] π-aromatic in the ground state, become [4 + 2] π-antiaromatic in the first ππ* states, and proton transfer (either inter- or intramolecularly) 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. Salicylic acid undergoes ESPT only in the "antiaromatic" S (ππ*) state, but not in the "aromatic" S (ππ*) 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. 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.
贝尔德规则解释了为什么以及何时在有机化合物中发生激发态质子转移（ESPT）反应。在基态下为[4 + 2]π-芳香族的双官能化合物，在第一ππ态下变为[4 + 2]π-反芳香族，质子转移（无论是分子内还是分子间）有助于缓解激发态反芳香性。几个 ESPT 示例（包括激发态分子内质子转移（ESIPT）、双质子转移、动态催化转移和质子中继转移）的计算核独立化学位移（NICS）证明了激发态反芳香性的重要作用。水杨酸仅在“反芳香族”S（ππ）态下发生 ESPT，但不在“芳香族”S（ππ*）态下发生。结构相关化合物的斯托克斯位移[例如，2-（2-羟基苯基）苯并恶唑的衍生物和 2-氨基吡啶与质子供体形成的氢键复合物]取决于光致互变异构体的反芳香性。值得注意的是，贝尔德规则预测了光对氢键强度的影响；在化合物中增强（和降低）激发态反芳香性的氢键，在光激发后会减弱（和增强）。
