A femtosecond pump-probe, with approximately 150 fs resolution, as well as time-correlated single photon counting with approximately 10 ps resolution techniques are used to probe the excited-state intermolecular proton transfer from HPTS to water. The pump-probe signal consists of two ultrafast components (approximately 0.8 and 3 ps) that precede the relatively slow (approximately 100 ps) component. From a comparative study of the excited acid properties in water and methanol and of its conjugate base in basic solution of water, we propose a modified mechanism for the ESPT consisting of two reactive steps followed by a diffusive step. In the first, fast, step the photoacid dissociates at about 10 ps to form a contact ion pair RO-*...H3O+. The contact ion pair recombines efficiently to re-form the photoacid with a recombination rate constant twice as large as the dissociation rate constant. The first-step equilibrium constant value is about 0.5 and thus, at short times, <10 ps, only approximately 30% of the excited photoacid molecules are in the form of the conjugated base-proton contact ion pair. In the second, slower, step, of about 100 ps, the proton is separated by at least one water molecule from the conjugate base RO-. The separated proton and the conjugated base can recombine geminately as described by our previous diffusion-assisted model. The new two-step reactive model predicts that the population of the ROH form of HPTS will decrease with two time constants and the RO- population will increase by the same time constants. The proposed model fits the experimental data of this study as well as previous published experimental data.

We have measured the direct proton-transfer rate between a photoacid, 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) and a base, acetate anion in water and in D 2 O, using a pump-probe technique with ∼150 fs time resolution. The acid-base reaction can be formulated by the kinetic scheme ROH* + Bf RO -* + BH, where ROH* and RO -* are the excited-state protonated and deprotonated forms of HPTS, respectively, and Band BH are the deprotonated and protonated species of the acetate base, respectively. We have analyzed the experimental data within the framework of the diffusive model. We assume that both species are spherically symmetric and interact via a screened Coulomb potential of mean force. The reaction occurs with a certain rate whenever ROH* and Bdiffuse together and come into contact. Our data analysis of H 2 O and D 2 O solutions of 0.5-4 M sodium acetate shows a good agreement with the diffusive model. The intrinsic rate constants at contact of the reactive species were found to be 1.6 × 10 11 and 4 × 10 10 M -1 s -1 for H 2 O and D 2 O solutions, respectively. At 4 M salt concentration, the reaction rate is about 3 ps, one of the fastest intermolecular chemical reactions observed by time-resolved techniques.

In a previous work, we proposed an extended model for intermolecular excited-state proton transfer to the solvent. The model invoked an intermediate species, the contact ion-pair RO(-)...H(3)O(+), where a proton is strongly hydrogen bonded to the conjugated photabase RO(-). In this study we tested the extended model by measuring the transient absorption and emission of 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) in an aqueous solution in the presence of a large concentration of mineral acids. In a neutral pH solution, the pump-probe signal consists of three time components, <1, 4, and 100 ps. The 4 ps time component, with a relative amplitude of about 0.3, was attributed to the formation of the contact ion-pair and the long 100 ps component to the dissociation of the ion-pair to a free proton and RO(-). In the presence of acid, the recombination of an excess proton competes with the geminate recombination. At a high acid concentration, the recombination process alters the time-dependent concentrations of the reactant, product and intermediate contact ion-pair. We observed that when the acid concentration increases, the amplitude of both the long and intermediate time components decreases. At about 3 M of acid, both components almost disappear. Model calculations of the acid effect on the transient HPTS signal indeed showed that the amplitude of the intermediate time component decreases as the excess proton concentration increases.

The time-resolved optical response of the wild-type green fluorescent protein (WT-GFP) in water and D2O was measured at room temperature by two optical techniques. The pump probe technique, with about 150 fs resolution, was used to measure the short-time response up to 150 ps. The short-time signals are similar to previously reported measurements. Time-correlated single photon counting (TCSPC) was used to measure the fluorescence of both the protonated and deprotonated emission bands of WT-GFP. The long-time fluorescence decay of the protonated form of WT-GFP decays nonexponentially. When this decay curve is multiplied by exp(t/τf) where τ f is the lifetime of deprotonated form, the long-time tail decays as a power law of about t -3/2. Such a long-time fluorescence decay behavior represents the general emission decay pattern of excited photoacids in solutions and microemulsions and adds additional previously unrevealed information on the dynamics of this reaction. We attribute the long-time behavior to a diffusion-assisted geminate recombination process of the proton with the deprotonated species to reform the protonated chromophore.