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Steady-state and time-resolved fluorescence techniques were employed to study the excited-state proton transfer (ESPT) from a reversibly dissociating photoacid, 2-naphthol-6,8-disulfonate (2N68DS). The reaction was carried out in water and in acetonitrile–water solutions. We find by carefully analyzing the geminate recombination dynamics of the photobase–proton pair that follows the ESPT reaction that there are two targets for the proton back-recombination reaction: the original O− dissociation site and the SO3− side group at the 8 position which is closest to the proton OH dissociation site. This observation is corroborated in acetonitrile-water mixtures of χwater < 0.14, where a slow intramolecular ESPT occurs on a time scale of about 1 ns between the OH group and the SO3− group via H-bonding water. The proton-transferred R*O− fluorescence band in mixtures of χwater < 0.14 where only intramolecular ESPT occurs is red shifted by about 2000 cm−1 from the free R*O− band in neat water. As the water content in the mixture increases above χwater = 0.14, the R*O− fluorescence band shifts noticeably to the blue region. For χwater > 0.23 the band resembles the free anion band observed in pure water. Concomitantly, the ESPT rate increases when χwater increases because the intermolecular ESPT to the solvent (bulk water) gradually prevails over the much slower intramolecular via the water-bridges ESPT process. [ABSTRACT FROM AUTHOR]

A time-resolved fluorescence spectroscopy study supplemented by molecular modeling and dynamics simulations is conducted to determine the reversible proton dissociation and geminate recombination of the common photoacid, 8-hydroxypyrene-1,3,6-trisulfonate(pyranine), either in dilute aqueous solution or when forming a complex with y-cyclodextrin (y-CD). The results show that the pyranine in the complex can assume more than one level of interaction with the solvent, as the polysugar torus surrounding the pyranine perturbs the hydrogen bond in the dye's immediate vicinity and deforms the electrostatic potential inside the Coulomb cage, resulting in significant changes from a simple spheric symmetry.

Time-resolved emission was used to measure the photoprotolytic cycle of an excited photoacid as a function of temperature in water as well as ice in the presence of an inert salt. The inert salt affects the geminate recombination between transferred proton with the conjugate base of the photoacid and the proton diffuses in the ice under the influence of the Coulomb potential between [H.sup.}-[RO.sup.-] that enhances the geminate recombination.

A study of the fluorescent spectra of the green fluorescence protein (GFP) and the anionic form of p-hydroxybenzylidenemethylimidazolone (p-HBDI) reveals a well-resolved vibronic structure but broad and structureless chromophore spectra at lower temperatures. The results show that the cis-trans isomerization exhibited by the protein significantly narrows the fluorescence spectra, which is otherwise consistent with the decay of the torsional deformation (TD).

An extended model for intermolecular excited-state proton transfer (ESPT) to the solvent is developed on the basis of pump-probe measurements of both the POH and RO(super -) signals. The model includes two reactive steps followed by a diffusion-assisted step.

Time-resolved and steady-state emission characterization of 10-hydroxycamptothecin reveals a rich but less complex proton-transfer behavior. The electronic effect of the additional electron-withdrawing ring makes the excited state both less basic and more acidic than the parent.

The proton dissociation and reversible geminate recombination from the photoacid 2-naphthol-6-sulfonate to water as a function of pressure are studied using time-resolved emission techniques. The result reveal that the proton dissociation rate constant, k(sub pt), of excited 2-naphthol-6-sulfonate in water up to the pressure of the freezing point, increases by about a factor of 8 with pressure.

The calculation of the proton-transfer rate constant associated with prototropic mobility in water as a function of temperature is reported. The results found a strong correlation between the proton-transfer rate constant as low temperature and the dielectric relaxation time.

The solvation statics and dynamics of coumarin 153 and 102 dyes were studied in hexane-methanol mixtures. The solvation of the coumarin dyes, in ground and in excited electronic states, exhibited a nonlinear solvatochromic shift in the methanol mole fraction.

The classical Kramers' theory in the medium and strong-damping limits was used to calculate the transmission coefficient and rate constant for strong photoacids to several solvents such as alcohols and water as a function of temperature. The results indicated good agreement between both the models.

The calculation of the proton-transfer from a super photoacid to a protic solvent is presented. The model used for the calculations is based on diffusive propagation of the solvent configuration.

Analysis of the excited-state proton transfer in an aqueous/surfactant AOT reverse micelles containing 2-naphthol-6,8-disulfonate incates that the naphthol is located about the center of the water pool. The proton transfer in large-diameter water spheres is similar to that of bulk water.

A theoretical prediction of reversible bimolecular chemical reactions of the A + B = AB type to deviate from exponential kinetics, obeying convolution kinetics at intermediate times and ultimately approaching equilibrium as a power law is presented. The results demonstrate that there is significant information concealed in the transient behavior of fast bimolecular reactions.

The reversible proton dissociation and geminate recombination of photoacids as a function of temperature in monols, diols, and glycerol is studied. The experimental data are analyzed and solved numerically.

The direct proton-transfer rate is measured 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. It is concluded that at 4 M salt concentration, the reaction rate is about 3 ps, one of the fastest intermolecular chemical reactions observed by time-resolved techniques.

Issues concerning the reversible proton dissociation and geminate recombination of photoacids are presented. It was established that the proton dissociation rate constant of excited 5,8-dicyano-2-naphthol in neat alcohols at temperatures above 10 degrees C is almost temperature independent.

The solvatochromism of 2-naphthol (2OH) and 2-methoxynaphthalene (2OMe) have been studied by measuring the excitation and emission fluorescence spectra in a series of pure solvents. The empirical solvatochromic scale of Kamlet and Taft was used since it provides accurate values for dipolarity interactions as well as the HB accepting and donating power of the solvent. Results reveal that solvatochromism of 2OH is dominated by HBing rather than polarity effects. Both molecules have a small dipole moment in their ground electronic state which increases the excited (S1) state.