Effect of pH on the fluorescence characteristics of some flavones probes

The photophysical properties such as electronic absorption, molar extinction coefficient, emission spectra, fluorescence quantum yield and lifetime of three different hydroxyflavones (a typical model of flavonols) such as: 3-HF, 3,6-diHF and 3,7-diHF, have been studied in the pH range from 2.5 to 9.2. Both electronic absorption and fluorescence spectra are sensitive to pH. The fluorescence quantum yield at pH 7.4 of the mentioned flavones probes have been determined. The fluorescence lifetime of different emissive species (Normal, Tautomer and Anion forms) as pH dependence have been also estimated. The effect of pH on the intramolecular excited state proton transfer process (ESIPT) has been discussed. The normal and tautomeric forms change as a function of pH, the normal one being more sensitive. The position of the -OH group on the second aromatic ring in the flavonol's structure has been also discussed. The results have relevance to compounds which have photoreactions accompanied by dual fluorescence.