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

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journal contribution
posted on 14.07.2005, 00:00 by Pavel Leiderman, Liat Genosar, Dan Huppert
A femtosecond pump−probe, with ∼150 fs resolution, as well as time-correlated single photon counting with ∼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 (∼0.8 and 3 ps) that precede the relatively slow (∼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 ∼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.