Absorption and Fluorescence Spectra, pKa Values, and Fluorescence Lifetimes of Monohydroxyflavones and Monomethoxyflavones

The absorption and fluorescence spectra of all isomeric hydroxyflavones and methoxyflavones, except for the photolabile 3‐methoxyflavone, have been measured in organic solvents and in aqueous solutions of various acidity. With the exception of 5‐hydroxyflavone, all are found to be fluorescent in methanol and water solution. On the other hand, there is virtually no fluorescence observed in cyclohexane solution, a fact that is interpreted in terms of high intersystem crossing rates and the El‐Sayed selection rules. – Except for the 3‐hydroxy and 7‐hydroxy isomers, the anions of hydroxyflavones are non‐fluorescent, whereas the protonated forms exhibit strong fluorescence emission. Methoxyflavones have higher fluorescence quantum yields than the corresponding hydroxyflavones and have fluorescence decay times ranging from 1.3 to 6.9 ns in methanol at room temperature. – The ground state pKa values of the hydroxyflavones range from 7.8 to 9.8, except for the 5‐hydroxy isomer (11.6). The pKa's governing the protonation step range from – 1.22 to – 1.55, again with the exception of the 5‐hydroxy isomer (– 3.1). The dissociation constants of the first excited singlet state were calculated with the help of the Förster‐Weller equation. The results predict a reversal of the most basic and most acidic sites of the hydroxyflavones. In aqueous solutions, this should result in the formation of excited state tautomers which, however, could be detected only for the 3‐hydroxy and 7‐hydroxy isomers. Apparently, the lifetimes of the other isomers are too short to allow the establishment of excited state equilibria.