Ultrafast direct photoacid-base reaction

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 D₂O, using a pump-probe technique with ∼150 fs time resolution. The acid-base reaction can be formulated by the kinetic scheme ROH* + B^- → RO^-* + BH, where ROH* and RO^-* are the excited-state protonated and deprotonated forms of HPTS, respectively, and B^- and 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 B^- diffuse together and come into contact. Our data analysis of H₂O and D₂O 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 × 10¹¹ and 4 × 10¹⁰ M^-1 s^-1 for H₂O and D₂O 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.