Excited-state proton transfer and proton diffusion near hydrophilic surfaces

Time-resolved emission techniques were employed to study the reversible proton photoprotolytic properties of surface-attached 8-hydroxypyrene-1,3,6- trisulfonate (HPTS) molecules to hydrophilic alumina and silica surfaces. We found that the excited-state proton transfer rate of the surface-linked HPTS molecules, in H₂O and D₂O, is nearly the same as of HPTS in the bulk, while the corresponding recombination rate is significantly greater. Using the diffusion-assisted proton geminate-recombination model, we found that the best fit of the time-resolved fluorescence (TRF) signal is obtained by invoking a two-dimensional diffusion space for the proton to recombine with the conjugated basic form, RO^-*, of the surface-linked HPTS. However, we obtain an excellent fit by a three-dimensional diffusion space for diffusional HPTS in bulk water. These results indicate that the photoejected solvated protons are confined to the surface for long periods of time. We suggest two plausible mechanisms responsible for two-dimensional proton diffusion next to hydrophilic surfaces.