Effect of Salt Ions on the Proton-Transfer Rate in 3-Hydroxyflavone

The effects of lithium and chloride ions on the spectral characteristics of the two-band fluorescence of 3-hydroxyflavone solutions were studied. The obtained absorption, emission, and fluorescence excitation spectra showed that the salt ions effectively modulated intramolecular proton transfer in the excited state, which led to increases in the shortwavelength band intensity and the fluorescence quantum yield. Thus, the quantum yield for 3-hydroxyflavone in acetonitrile with salt concentrations 0.005, 0.01, and 0.02 M increased by 1.5, 2.0, and 3.0 times, respectively, with standard excitation in the principal absorption band. The intensity ratio of the short-wavelength and longwavelength bands also increased as the salt concentration increased. The increase was so significant that the color change of the fluorescence was easily noticed visually. The good sensitivity of the fluorescence-band intensity ratio to the salt concentration could provide a basis for solving the inverse problem, i.e., determining the salt concentration in solutions and biological objects. An explanation allowing the obtained changes in the fluorescence spectral properties after adding LiCl to the solution to be interpreted was proposed.