Excited-State
Proton Transfer and Proton Diffusion near Hydrophilic Surfaces
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Abstract
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 H<sub>2</sub>O and D<sub>2</sub>O, 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<sup>–</sup>*, 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