Direct observation of proton transfer from the excited S2 state in the 3-hydroxyflavone molecule

The spectra of dual fluorescence of 3-hydroxyflavone molecules excited by 44-ps pulses in the region of the S1 and S2 absorption bands are measured with a picosecond resolution. The dynamics of the spectra directly demonstrates the time development of the proton transfer from the carboxyl to the carbonyl group of the molecule. Upon excitation into the main absorption band, the transfer process occurs for about 210 ps. The excitation into the region of the S2 band results in a faster (∼170 ps) process, and the relative contribution made to the total spectrum by the long-wavelength band, which belongs to the proton-transfer state, is higher in this case for all the time ranges of luminescence recording. The data obtained directly point to an additional channel of proton transfer via the S2 state. The probability of this process is estimated to be 0.84 × 1012 s−1.