Salt effect on transient proton transfer to solvent and microscopic proton mobility

The effect of an inert salt on the time-resolved fluorescence from excited hydroxypyrene-trisulfonate is measured and compared quantitatively to the solution of the transient Smoluchowski equation for proton diffusion-reversible recombination in the excited-state. The data in water can be explained by the screening and mobility-diminishing effects of the added salt. In methanol-water mixtures we observe additionally an enhancement of proton dissociation. The microscopic mechanism for proton mobility in water and methanolic mixtures is discussed. Since the rate-limiting step for proton mobility is a hydrogen-bond cleavage, percolation properties of the hydrogen-bond network might lead to a discontinuous drop in proton mobility, possibly reflected in our data.