The unusually strong effect of a 4-cyano group upon electronic spectra and dissociation constants of 3-substituted 7-hydroxycoumarins.

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Synthesis, absorption and fluorescence spectra as well as pKa values of 7-hydroxycoumarins with electron-withdrawing substituents in positions 3 are described. Introduction of a 4-cyano group is achieved by oxidative cyanation of coumarins using potassium cyanide and elemental bromine. 7-Hydroxycoumarins without a 4-cyano group are useful indicators for measuring physiological pH values due to their intense fluorescences, longwave absorptions and emissions, as well as pKa values of around 7. The presence of a 4-cyano group gives rise to a dramatic longwave shift in absorption (30–40 nm in methanol) and emission (55–80 nm). In water solution, the fluorescence maxima are at around 570–600 nm, with excitation maxima between 410 and 510 nm, depending on whether the phenol or phenolate species is excited. For all coumarins under investigation, fluorescence is from the anion form even in the pH 2–7 range. This phenomenon is interpreted in terms of excited state dissociation according to the Forster model. The interpretation is corroborated by calculations of the excited state pKa values, which show them to be lower by 4.5–6.4 units than those of the ground state.
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