Kinetics of Electron Attachment to Reverse Micelles

Observation of Nanoconfinement Effect on the Kinetics of Hydrated Electron in the Nanoscale Water Pools of Water-AOT-Cyclohexane Microemulsions by Picosecond Pulse Radiolysis

The decay kinetics of the hydrated electron (eaq-) in aerosol OT (AOT)-based ternary microemulsions with pool sizes ranging from 0.34 to 4.85 nm were studied using picosecond pulse radiolysis coupled with transient absorption UV-vis spectroscopy. Electron transfer from oil to water and the subsequent solvation occurred within a time resolution of 7 ps. The decay kinetics of eaq- were accurately modeled using a double-exponential decay model, revealing the occurrence of two types of reactions, i.e., the recombination reaction at the water-oil interface and the radical-radical reactions in the water pools. The apparent lifetimes of both types of decays decreased significantly as the size of water pools decreased, indicating the influence of nanoconfinement effects. Moreover, the importance of the water-oil interface increased with decreasing water content, regardless of the presence or absence of NO3- as an electron scavenger in the water pools. Our findings provide a comprehensive understanding on the kinetics of the radiation reaction in AOT-based microemulsions.

Photophysical probes for multiple-redox and multiple-excited-state interactions in molecular aggregates

Photosynthesis takes place through a highly efficient series of energy, electron, and proton transfers initiated by absorption of one or more photons within the visible region of the solar spectrum. Because of the presence of multiple chromophores needed for effective light harvesting, extinction coefficients must be very high. The absorbing multiunit array is held within a rigidly arranged structure that facilitates each electron hop. A fully artificial, yet biomimetic, alternative to photosynthesis that produces fuels directly and efficiently from sunlight and simple low molecular weight molecules would change the world. Achieving this goal requires a detailed understanding of the mechanisms of the key steps of the complex chemical and photochemical processes taking place in natural photosynthesis. One of these mechanisms relies on light harvesting to initiate multiple-step sequences to obtain combustible molecules suitable for burning. In particular, we are devising and testing photophysical models with characteristics that facilitate multiple electron transfers within a single aggregate and are directly relevant to light harvesting. We focus on structural features that promote photoinduced electron transfer at high dye densities, placed for optimal solar utilization and catalysis. The reaction producing oxygen is further complicated by the need for four electrons to complete the sequence, even though the first initiation step is presumably absorption of a single photon. Therefore we explore steps that accumulate charge or have the potential to do so. We also emphasize the synthesis of model systems that probe the complexity of individual steps. This Account examines the factors that influence the efficiency of electron redistribution in multiple-dye, multiple-excited-state, and multiple-redox equivalent arrays. Such knowledge will allow us to optimize the efficiency of electron migration and may contribute to a better understanding of multiple-equivalent light harvesting events by which photosynthetic energy storage takes place.

Observation of Three Behaviors in Confined Liquid Water within a Nanopool Hosting Proton-Transfer Reactions

In this contribution, we report on studies of rotational and diffusional dynamics of 7-hydroxyquinoline (7HQ) within a reverse micelle (RM) containing different amounts of water. Analyzed in terms of the wobbling-in-a-cone model, the data reveal structural and dynamical properties of the nanopool. We clearly observed three regions in the behavior of confined water molecules within the RM hosting a double proton-transfer reaction between the probe and water. This observation remarkably reproduces the change of calculated water density within this life-mimicking medium. The number of water molecules per AOT head in the transition regions changes from 2 to 5, the latter being very near to the full solvation number (6) of the RM heads. Moreover, the H-bonds breaking and making within the RM to give new structures of the probe strongly affect the environment fluidization in different extents, reflected in different relaxation times of these structures; however, they are of similar sizes. We discuss the role of RM confinement and the proton-transfer dynamics on the behavior of water and their relationships to the packing of water molecules in the studied range of concentrations.

Photodetachment of Ferrocyanide in Reverse Micelles

Solvated electrons have been generated in reverse micelles (RMs) through photodetachment of ferrocyanide (Fe(CN)(6)(4-)) in sodium bis(2-ethylhexyl) sulfosuccinate (AOT) RMs. We have measured both bleach recovery of the parent ferrocyanide CN stretch in the infrared and the decay of the solvated electron absorption at 800 nm. The bleach recovery has been fit to a diffusion model for the geminate recombination process. The fit parameters suggest a narrowing of the spatial distribution of ejected electrons due to confinement in the RMs when compared to bulk water. The diffusion coefficient of the solvated electron does not appear to be significantly affected by RM confinement. The decay of the solvated electron absorption exhibits an additional decay component that is not observed in bulk water and is smaller for larger RMs. No corresponding additional component is seen in the parent ferrocyanide IR bleach recovery, which supports our interpretation that the confinement-induced new decay process in RMs is due to electrons reacting with AOT headgroups.