Excited-state proton transfer and proton reactions of 6-Hydroxyquinoline and 7-Hydroxyquinoline in water and ice

Time-resolved and steady-state emission spectroscopies as well as absorption UV-vis spectroscopy were employed to study the photoprotolytic cycle and other protic processes of the bifunctional 6-hydroxy- and 7-hydroxyqunoline molecules in methanol-doped ice over a wide range of temperatures. In ice at high temperatures of T > 173 K, the excited-state proton transfer rate decreases as the temperature decreases. The emission band of the H ⁺NRO^-* zwitterion, where the imine nitrogen is protonated and the hydroxyl is deprotonated, is observed. At T > 173 K, the formation rate of the H⁺NRO^-* emission band is approximately that of the decay rate of the neutral form, NROH*. Below 173 K, the rate of the photoprotolytic process is much slower than the radiative and the nonradiative rates, and the excited-state proton transfer could not be clearly observed. Addition of a small concentration of acetic acid increases the proton transfer rate significantly at temperatures below 235 K. The reaction rate in the presence of acetic acid is temperature-independent over a wide range of temperatures (80-235 K). We propose as an explanation for this observation that there exists a direct proton transfer from the hydroxyl group to water-acetic acid complexes at temperatures below 235 K.