SALT EFFECT ON TRANSIENT PROTON-TRANSFER TO SOLVENT AND MICROSCOPIC PROTON MOBILITY

The effect of an inert salt on the time-resolved fluorescence from exc ited hydroxypyrene-trisulfonate is measured and compared quantitativel y to the solution of the transient Smoluchowski equation for proton di ffusion-reversible recombination in the excited-state. The data in wat er 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 p roton mobility in water and methanolic mixtures is discussed. Since th e rate-limiting step for proton mobility is a hydrogen-bond cleavage, percolation properties of the hydrogen-bond network might lead to a di scontinuous drop in proton mobility, possibly reflected in our data.