Excited-state proton transfer relieves antiaromaticity in molecules

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 116; no. 41; pp. 20303 - 20308
• Main Authors: Wu, Chia-Hua, Karas, Lucas José, Ottosson, Henrik, Wu, Judy I-Chia
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 08.10.2019
• Subjects: antiaromaticity; Aromatic compounds; aromaticity; Baird's rule; Bonding strength; Chemical bonds; Electrons; Excitation; excited-state proton transfer; Hydrogen; Hydrogen bonding; Hydrogen bonds; Hydroxyquinolines - chemistry; Light effects; Models, Molecular; Molecular Structure; Organic chemistry; Organic compounds; Photoexcitation; Physical Sciences; Protons; Quantum Theory; Salicylic acid; Salicylic Acid - chemistry; Substrates; Tautomers; antiaromaticity; hydrogen bonding; Baird’s rule; excited-state proton transfer; aromaticity
• ISSN: 0027-8424; 1091-6490; 1091-6490
• DOI: 10.1073/pnas.1908516116

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 (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. 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. 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.