Diffusion-assisted acid–base non-exponential dynamics in the short time domain

We measured the direct proton transfer between a photoacid, 8-hydroxypyrene-1,3,6-trisulfonate (HPTS), and a base, acetate anion, in water and water–glycerol mixtures. We used the pump–probe technique with ∼150 fs time resolution to excite the sample at 400 nm and monitor the change in the sample transmittance as a function of both time and wavelength. We analyzed the experimental data using the irreversible diffusion-controlled Smoluchowski model. As the solution viscosity increases, from about 1 cP (neat water) to 10 cP, at 0.27 mole fraction of glycerol, the probe signal decay becomes longer and increasingly non-exponential.