pH-Dependent fluorescence spectroscopy. Part 12. Flavone, 7-hydroxyflavone, and 7-methoxyflavone

Abstract

Flavone is found to be non-fluorescent in neutral aqueous solution, but strongly fluorescent in sulphuric acid. The pK_a(S₁) value of its conjugated acid was calculated from ground-state data and is found to be in poor agreement with the pK_a value obtained from a plot of fluorescence intensity versus Hammett acidity. This is interpreted in terms of rapid intersystem crossing to the triplet state, before a prototropic process can take place. Conversely, 7-hydroxyflavone is found to be fluorescent in its neutral, conjugate acid, and conjugate base form. Excited state pK_a values have been obtained from Förster–Weller calculations and from fluorescence titration curves, but are in poor agreement. 7-Hydroxyflavone undergoes adiabatic photodissociation in its S₁ state and forms an exciplex (or a phototautomer) in pH 3 to H_o–3 solution, and similarly in acidified methanol. Fluorescence quantum yields are given for alkaline, neutral, and acidic solutions. Compared with 7-hydroxycoumarins, flavones are only weakly fluorescent in alkaline solution. This behaviour is explained by the small energy difference between the S₁ and T₁ states giving rise to more efficient intersystem crossing. To demonstrate the dynamic nature of excited state proton transfer processes and the involvement of water molecules in quenching processes, deuteriation experiments have been performed. Drastic changes are found in both fluorescence intensity and shape of the curves.