Acidity Dependence of the Absorption and Fluorescence Spectra of Isoflavone and 7‐hydroxyisoflavone

Abstract ‐The pH and H0 dependence of the absorption and fluorescence spectra of isoflavone and 7‐hydroxyisoflavone are reported. Isoflavone is fluorescent in acidic solution only, whereas 7‐hydroxyisoflavone is fluorescent in all acidity ranges under investigation. Ground and first excited singlet state pKa's have been determined spectrophotometrically and fluorimetrically, respectively. Excited state protolytic equilibration processes via a second order reaction (proton gain) are found to be too slow to compete efficiently with fluorescence. This is deduced from the close agreement between the pKa's of the conjugate acids obtained by absorption and fluorescence titrations. On the other hand, photodissociation of 7‐hydroxyisoflavone proceeds faster than its fluorescence decays. The experimental pKa(S1) is in fair agreement with the calculated one. 7‐Hydroxyisofiavone forms a phototautomer (or exciplex) in the pH 2 to H0‐1 acidity range, which is characterized by its long wavelength emission. Quantum efficiencies are given for isoflavone and 7‐hydroxyisoflavone in aqueous solutions of various acidities. Deuteration effects thereon are discussed.