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‐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.

The pH and H,, 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 pK.'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 pK.'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 pK.(S, ) is in fair agreement with the calculated one. 7-Hydroxyisoflavone forms a phototautomer (or exciplex) in the pH 2 to H,, -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.

Absorptionsspektren, sichtbar und ultraviolett / Dksoziationskonstanten / Huoreszenz / Ionisation / I.soto&neffekte The absorption and fluorescence spectra of 4'-hydroxyflavone and 4'-methoxyflavone have been studied as a function of the acidity (PH or Ho) of the solution. Dissociation constants have been determined for the ground and first excited singlet state. The values are compared with those obtained from Forster-Weller calculations. -The pKts obtained by fluorimetry are rather similar to the ground state pK,,' s, which indicates a rapid deactivation of the S,-state. before prototropic equilibration is complete. -On changing from protic to deuterated solvents partially strong effects are observed with respect to the fluorescence intensity, but also to the shape of the curves. It is evident, that proton transfer processes in the excited state significantly contribute to fluorescence quenching, thus making the fluorescence quantum yields strongly acidity dependent.