Abstract
Chemical reactions in confined environments are important in areas as diverse as heterogenous catalysis, environmental chemistry and biochemistry, yet they are much less well understood than the equivalent reactions in either the gas phase or in free solution. The understanding of chemical reactions in solution was greatly enhanced by real time studies of model reactions, through ultrafast spectroscopy (especially when supported by molecular dynamics simulation). Here we review some of the efforts that have been made to adapt this approach to the investigation of reactions in confined media. Specifically, we review the application of ultrafast fluorescence spectroscopy to measure reaction dynamics in the nanoconfined water phase of reverse micelles, as a function of the droplet radius and the charge on the interface. Methods of measurement and modelling of the reactions are outlined. In all of the cases studied (which are focused on ultrafast intramolecular reactions) the effect of confinement was to suppress the reaction. Even in the largest micelles the result in the bulk aqueous phase was not usually recovered, suggesting an important role for specific interactions between reactant and environment, for example at the interface. There was no simple one-to-one correspondence with direct measures of the dynamics of the confined phase. Thus, understanding the effect of confinement on reaction rate appears to require not only knowledge of the dynamics of the reaction in solutions and the effect of confinement on the medium, but also of the interaction between reactant and confining medium.
Highlights
Ultrafast spectroscopy has long been established as a key tool in the investigation of chemical reactions in the condensed phase
Much of the progress in understanding reaction dynamics in solution has been driven by progress in ultrafast experimentation, and a top level review of this area has recently been presented, which serves as a useful introduction.[7]
transient absorption spectroscopy (TA) is an extremely powerful tool for investigating excited state dynamics. It does present the experimenter with the challenge of separating all of these spectrally distinct contributions. This is usually met by application of one of the established global analysis methods, which fit the evolution of the entire transient spectrum to a kinetic model.[14]
Summary
Ultrafast spectroscopy has long been established as a key tool in the investigation of chemical reactions in the condensed phase. In bimolecular reactions the molecular dynamics on the reactive potential energy surface are often obscured by slower diffusive process This limitation applies in the present review of ultrafast reactions in confined media. It does present the experimenter with the challenge of separating all of these spectrally distinct contributions This is usually met by application of one of the established global analysis methods, which fit the evolution of the entire transient spectrum to a kinetic model.[14] Despite its obvious power the TA method is less widely applied to the study of reactions in confinement than time resolved fluorescence. This will be followed by a detailed look at ultrafast reaction dynamics in some of those confined liquids, with the focus on inverse micelles (inverse and reverse will be used interchangeably in this work) stabilizing water nanodroplets
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