Ultrafast fluorescence investigations of the effects of nanoscale structure and confinement in liquid environments [electronic resource]
Keywords: Chemistry.

The properties of liquid systems confined on nanometer length scales may differ considerably from those of bulk liquids, with consequences for the dynamics of processes taking place in those systems. Proton transfer dynamics in nanoconfined aqueous environments are a topic of considerable scientific interest. A significant area of research within that topic is the influence of the confining interface on dynamics within the confined environment. Similarly, liquids that display relatively long-range structure (nanometer size scales) may show the effects of that structure in the dynamics of dissolved solutes. For example, room temperature ionic liquids (RTIL), a class of compounds that has attracted considerable interest for applications in catalysis, synthesis, and energy storage, have been shown to exhibit complicated nanoscale liquid structure with the formation of hydrophilic and hydrophobic domains. The orientational relaxation dynamics of the fluorescent probe perylene dissolved in solutions of various concentrations of lithium salts in RTILs were studied by measurement of fluorescence anisotropy. Increasing concentrations of lithium salt were found to cause both the in-plane and out-of-plane rotational diffusion of perylene to become slower as the liquid's bulk viscosity increased. However, the rotational diffusion did not slow sufficiently to track the viscosity and both of the corresponding viscosity-independent molecular friction coefficients decreased. This reveals that lithium ions, which are solvated by the ionic regions of the RTILs, change the structure of the alkyl regions of the RTIL. Another series of experiments studied excited-state proton transfer kinetics in both ionic and neutral reverse micelles. TCSPC measurements of the population decay were conducted on the fluorescent photoacid 8-hydroxypyrene-1,3,6-trisulfonate (HPTS) in bulk water, ionic reverse micelles, and non-ionic reverse micelles, and the orientational dynamics of dissolved MPTS, the methoxy derivative of HPTS, in those systems were also investigated by time dependent fluorescence anisotropy measurements. The orientational diffusion data suggest that in ionic reverse micelles, the probe molecules are located in the water core of the reverse micelle, while in non-ionic reverse micelles, the data suggest the probes are located at the water-surfactant interface. HPTS proton transfer in ionic reverse micelles followed kinetics qualitatively like those in bulk water, albeit slower, with the power law time dependence associated with recombination of the proton with the dissociated photoacid having a smaller exponent, suggesting a modified diffusion-controlled process. In neutral reverse micelles, proton transfer kinetics did not show discernible power law behavior and were best represented by a two component model with one relatively water-like population, and a population with a faster fluorescence lifetime and negligible proton transfer. Finally, the analysis of proton transfer dynamics in bulk water resulted in a t-1.1 power law for proton recombination as opposed to the t-1.5 power law predicted by theory and some previously reported observations. The reliability of these newer observations is justified, and possible ways in which existing theory may be incomplete are discussed.