ACIDITY DEPENDENCE OF THE ABSORPTION AND FLUORESCENCE SPECTRA OF ISOFLAVONE AND 7-HYDROXYISOFLAVONE |
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Authors: | Otto S Wolfbeis Rudolf Schipfer |
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Institution: | Institut für Organische Chemie, Karl Franzens-Universität, A-8010 Graz. Austria |
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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 p K a'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 p K a'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 p K a(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. |
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