Unusual Temperature Dependence of Proton Transfer 3. Classical Kramers' Theory versus the Landau−Zener Curve-Crossing Formulation

Femtosecond and temperature-dependent picosecond dynamics of ultrafast excited-state proton transfer in water-dioxane mixtures

Synthetic flavylium salts like the 7-hydroxy-4-methylflavylium (HMF) cation have been used as prototypes to study the chemistry and photochemistry of anthocyanins, the major group of water-soluble pigments in the plant kingdom. In this work, a combination of fluorescence upconversion with femtosecond time resolution and time-correlated single photon counting (TCSPC) with picosecond time resolution have been employed to investigate in details the excited-state proton transfer (ESPT) of HMF in water and in binary water/1,4-dioxane mixtures. TCSPC measurements as a function of temperature provide activation parameters for all of the individual rate constants involved in the proton transfer, including those for dissociation and recombination of the geminate excited base-proton pair (A*···H(+)) that can be detected in the water/dioxane mixtures (but not in water). Unlike the other rate constants, the deprotonation rate constant kd shows a non-Arrhenius dependence on temperature in both water and water/dioxane mixtures. At low temperatures kd is close to the dielectric relaxation rate of the solvent with a barrier of ca. 8 kJ mol(-1), suggesting that the solvent reorganization is the rate-limiting step. At higher temperatures (>30 °C) the proton transfer process is nearly barrierless and solvent-dependent. Fluorescence upconversion results in H2O, D2O, and water/dioxane mixtures confirm the two-step model for the ESPT of HMF and provide additional details of the early events prior to the onset of proton transfer, attributed to conformational relaxation and solvent reaccommodation around the initially formed excited state. The results are consistent with DFT calculations that indicate that charge redistribution occurs after rather than prior to the onset of the ESPT process.

Excited-state proton transfer in N-methyl-6-hydroxyquinolinium salts: solvent and temperature effects

The excited-state proton transfer (ESPT) reaction of the "super"photoacid N-methyl-6-hydroxyquinolinium (MHQ) was studied using both fluorescence upconversion and time-correlated single photon counting (TCSPC) techniques. The ultrafast ESPT kinetics were investigated in various alcohols and water and determined to be solvent-controlled. The ESPT temperature dependence of MHQ was also studied in various alcohols and compared to that observed for another "super"photoacid, 5,8-dicyano-2-naphthol (DCN2). A full set of kinetic and thermodynamic parameters describing the ESPT was obtained. The protolytic photodissociation rate constant for MHQ was higher than that for DCN2, while the ESPT activation energies of MHQ were smaller. These findings are attributed to the approximately 3 orders of magnitude differences in excited-state acidities of MHQ and DCN2.

Study of the excited-state proton-transfer reaction of 5-cyano-2-naphthol in sub- and supercritical water

The excited-state proton-transfer (ESPT) reaction of 5-cyano-2-naphthol (5CN2) has been investigated in sub- and supercritical water using time-resolved fluorescence measurements. Under ambient conditions, a very fast decay of the fluorescence from the excited state of normal 5CN2 (ROH*) and a simultaneous increase of the fluorescence from the excited state of the anion species (RO(-)*) were observed, as reported previously. The very high ESPT rate was evaluated as 0.12 ps(-1). With increasing temperature at a constant pressure of 39.0 MPa, the proton transfer became slow. At 615 K and 39.0 MPa, another fluorescence from a new unknown chemical species appeared, which was assigned to the contact ion pair (CIP) of RO(-)* and the hydronium ion. With decreasing pressure at 664 K, the fluorescence from RO(-)* disappeared, and the fluorescence from ROH* and CIP was observed. At the very low density of supercritical water, only the fluorescence decay of ROH* was detected. The reaction dynamics was analyzed with the help of singular value decomposition and spectral decomposition using model functions. The ESPT rate was correlated with the solvent dielectric constant and/or the hydrogen-bonding ability.