Excited-State Proton Transfer:  Indication of Three Steps in the Dissociation and Recombination Process

Abstract

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.

Deprotonation Dynamics and Stokes Shift of Pyranine (HPTS)

“…In these solvents, excited-state proton transfer (ESPT) does not occur. Similar results have been observed in previous studies 11 of HPTS and by Pines and co-workers for the closely related molecule HPTA in dimethyl sulfoxide (DMSO) mixtures. 23 The trends seen in these spectra are representative of all solvents that can hydrogen bond strongly with HPTS but do not permit ESPT.…”

“…6,7 The results presented above demonstrate that to obtain the excellent agreement between experiment and calculation displayed in Figures 7-9 requires an accurate description of the ∼1 ps Stokes shift and a biexponential (2.5 and 88 ps in H 2 O; 4.5 and 210 ps in D 2 O) description of the HPTS deprotonation. The biexponential decay constants found for the deprotonation are in relatively good agreement with previous measurements, 11,14 but the quality of the data and fits obtained here indicate that the current numbers are more quantitative. However, the amplitude of the fast deprotonation component is approximately a factor of 2 smaller than previous reports.…”

“…11,14 They correlated the fast decay seen in the pumpprobe signal to an initial offset in the TCSPC signal of the deprotonated state. 11 They further suggested that the amplitude * To whom correspondence should be addressed. E-mail: fayer@ stanford.edu.…”

Long‐Range Proton Conduction across Free‐Standing Serum Albumin Mats

“…The proton can then diffuse to bulk water or recombine with the excited anion to form the ROH* form in what is known as geminate recombination 33. Since the ROH* and RO −* forms have different emission wavelengths (for HPTS, 440 and 535 nm, respectively), it is relatively easy to follow their time‐resolved and steady‐state fluorescence 34, 35, 36, 37. In pure water, the proton diffuses rapidly from the photoanion, which results in a predominant RO −* species, as can be seen in the steady‐state emission spectra (Figure 2a).…”

“…However, the decay of the ROH* form is much slower on the BSA mat, even in fully hydrated samples, which might imply that protons diffuse along the BSA mat and not into bulk water. The time‐resolved emission of the ROH* form can be used to get an estimation of the dimensionality of the proton diffusion space, where the fluorescent tail obeys a power‐law of t −d/2 , where d is the diffusion space dimensionality (see further discussion in Supporting Information) 34, 36, 37. By plotting a log–log plot (Figure S5, Supporting Information) of the lifetime corrected ROH* decay, we could estimate the fractal space dimensionality of the proton diffusion by linear fitting the first nanoseconds of the decay.…”