Excited-state proton transfer: indication of three steps in the dissociation and recombination process.
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.
Leiderman
Pavel
P
Raymond and Beverly Sackler Faculty of Exact Sciences, School of Chemistry, Tel Aviv University, Tel Aviv 69978, Israel.
Genosar
Liat
L
Huppert
Dan
D
D000143:Acids / Q000737:chemistry*; D001190:Arylsulfonates / Q000737:chemistry*; D007700:Kinetics; D017785:Photons; D011522:Protons; D012997:Solvents / Q000737:chemistry*; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
D000143:Acids; D001190:Arylsulfonates; D011522:Protons; D012997:Solvents; D014867:Water; C005047:pyranine