Excited-State Proton Transfer in Methanol-Doped Ice in the Presence of KF

Abstract

Steady-state and time-resolved emission techniques were employed to study the photoprotolytic cycle of an excited photoacid in ice in the presence of a low concentration of a weak base-like F(-). In previous studies we found that the photoprotolytic cycle in methanol-doped ice (1% mol fraction) is too slow to be observed at temperatures below 190 K. In this study we found that at temperatures below 240 K an additional proton-transfer process occurs in ice doped with 10 mM KF. We attributed this reaction to the creation of a mobile L-defect by F(-) ions. We used a diffusion-assisted reaction model, based on the Debye-Smoluchowski equation, to account for the direct reaction of the L-defect with the excited photoacid at temperatures below T < 240 K. Below 160 K the spectroscopic properties as well as the photoprotolytic cycle change dramatically. We propose that below 160 K the sample enters a new phase. The excited-state proton-transfer (ESPT) process was observed and followed down to a liquid nitrogen temperature of approximately 78 K. In the low-temperature phase the ESPT rate is almost twice as much as at 180 K and the temperature dependence of the rate is very small. The kinetic isotope effect of the ESPT at the low-temperature phase is small of about 1.3.

Excited-State Proton Transfer and Proton Reactions of 6-Hydroxyquinoline and 7-Hydroxyquinoline in Water and Ice

“…For bridged water−acetate complexes, where one water molecule or more acts as a bridge between the hydroxyl group of HPTS and the acetate ion, the rate decreases by more than 1 order of magnitude with respect to the direct transfer. In the current study, we used a similar approach to enhance the proton transfer rate of 6HQ and 7HQ in ice, especially at temperatures below 173 K. In several previous studies, , we found that ESPT to methanol-doped ice for several photoacids is not effective below 173 K. In the current hydroxyquinoline study of 6HQ and 7HQ, we were unable to clearly detect an efficient ESPT process in a sample doped with 0.1% mole ratio of methanol below 173 K. Similar behavior is also observed for hydroxyaromatic acids such as 2-naphtholsulfonate derivatives. As we will show below, an addition of a small concentration of acetic acid increases the ESPT rate by at least 10-fold at temperatures below 173 K.…”

Unusual Temperature Dependence of the Proton Transfer Rate from 8-Hydroxy-1,3,6-pyrenetrisulfonate Photoacid to Methanol-Doped Ice

“…The slope of the plot in Figure s11 in the Supporting Information is nearly −2.4, which means that k PT ( T < 173 K ) ∝ c −2.4 . This strikingly strong dependence of HPTS on the methanol doping level at low temperatures was not observed in our previous studies on HPTS. , In those studies we used higher concentrations of methanol, and therefore we were unable to clearly observe the proton transfer process below 173 K. In the current study, the methanol concentrations in use were between 0.1 and 1% mole ratio. The upper limit in the current experiment was the lowest concentration in our previous experiments.…”

“…The RO − * broad emission band could be fitted to several distinguished vibronic bands. In a previous study we fitted the structured emission spectrum by assigning a log-normal line shape function to each vibration band . This type of spectrum is a result of the Franck−Condon principle.…”

Isotope Effect of Proton/Deuteron Diffusion Constant in Ice

“…In previous studies 53,59 on the properties of the photoprotolytic cycle of photoacids in ice in the presence of KF, we found that the L‐defect mobility is larger than previously reported. According to the model presented here, 4,58 the proton mobility is related to and larger than the L‐mobility.…”