A time-resolved emission technique was employed to study the effect of excess protons on the fluorescence quenching process of flavin mononucleotide (FMN) in methanol-doped ice samples. We found that an excess of protons in ice has a very large effect on the fluorescence quenching whereas in liquid water the proton fluorescence quenching is rather small. We analyzed the experimental data using the Smoluchowski diffusionassisted binary collision model. Under certain assumptions and approximations, the calculated proton diffusion constant in ice in the range of 245-265 K is about 10 times that of water at 295 K.

A time-resolved emission technique was employed to study the photoprotolytic cycle of the 2-naphthol-6,8disulfonate (2N68DS) photoacid in the presence of a low concentration of a strong HCl acid. We found that an excess of protons in ice has a very large and profound effect on the photoprotolytic cycle. The excess proton reacts with the deprotonated form of the photoacid. An analysis of the experimental data reveals that the proton diffusion constant in ice in the temperature range of 240-270 K is much larger than in the liquid state. Under certain assumptions and approximations, the calculated proton diffusion constant in ice is 10 times larger than in water at 295 K, i.e., D H + ice ) 1.2 × 10 -3 cm 2 /s.

The temperature dependence of the proton diffusion constant, D H + , in methanol-doped ice was studied over the wide temperature range of 80-260 K. For that purpose, we measured the time-resolved fluorescence of a flavin mononucleotide (FMN) and riboflavin in ice doped with HCl or HF. The analysis of the fluorescence quenching provided the value of D H + . We found that the temperature dependence of D H + at T > 235 K is rather small, whereas at T < 235 K, it is rather large. These temperature-dependence results are similar to previous conductivity measurements. We used a stepwise, two-coordinate qualitative proton transfer model to explain the temperature dependence of D H + in ice.

The reversible proton dissociation and geminate recombination of a photoacid is studied as a function of temperature in water electrolyte solutions and binary water-methanol mixtures, containing 0.1 and 0.2 mole fractions of methanol. 8-Hydroxypyrene-1,3,6-trisulfonate trisodium salt (HPTS) is used as the photoacid. The experimental data are analyzed by the reversible geminate recombination model. We found that the slope of the logarithm of the proton-transfer rate constant as a function of the inverse of temperature (Arrhenius plot) in the liquid phase of these samples are temperature-dependent, while in the solid phase, the slope is nearly constant. The slope of the Arrhenius plot in frozen electrolyte solution is larger than that of the water-methanol mixtures, which is about the same as in pure water. Careful examination of the time-resolved emission in ice samples shows that the fit quality using the geminate recombination model is rather poor at relatively short times. We were able to get a better fit using an inhomogeneous kinetics model assuming the proton-transfer rate consists of a distribution of rates. The model is consistent with an inhomogeneous frozen water distribution next to the photoacid.

The effects of excess protons on the fluorescence quenching process of 1-naphthol-4-sulfonate (1N4S) and 1-naphthol-3-sulfonate (1N3S) in methanol-doped ice samples were studied by employing a time-resolved emission technique. We found that the fluorescence quenching of the deprotonated form RO(-)* of both photoacids by protonation is very efficient in ice, whereas in liquid water the proton fluorescence quenching is rather small. Using the Smoluchowski diffusion-assisted binary collision model under certain assumptions and approximations, we found that the calculated proton diffusion constant in ice in the temperature range of 240-260 K was 10 times greater than that of water at 295 K.