Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 116, Issue 41, Pages 20303-20308Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1908516116
Keywords
excited-state proton transfer; Baird's rule; aromaticity; antiaromaticity; hydrogen bonding
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Funding
- National Science Foundation (NSF) [CHE-1751370]
- National Institute of General Medical Sciences (NIGMS) of the NIH [R35GM133548]
- NSF [MRI-1531814]
- Swedish Research Council [2015-04538]
- Swedish Research Council [2015-04538] Funding Source: Swedish Research Council
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Baird's rule explains why and when excited-state proton transfer (ESPT) reactions happen in organic compounds. Bifunctional compounds that are [4n + 2] pi-aromatic in the ground state, become [4n + 2] pi-antiaromatic in the first (1)pi pi* 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 S-1 ((1)pi pi*) state, but not in the aromatic S-2 ((1)pi pi*) 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.
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