The standard theoretical description used to describe electron transfer is Marcus theory, which maps the polarization of the solvent surrounding the reactants onto a reaction coordinate, q. The questions we address in this paper are: How many and what types of solvent degrees of freedom constitute q? Is it appropriate to treat the solvent as a dielectric continuum? Our approach to answer these questions relies on the study of the simplest possible charge transfer systems: we choose atomic systems that have only electronic degrees of freedom so that any spectroscopic changes that occur during the course of the reaction directly reflect the motions of the surrounding solvent. Our methods for characterizing these systems consist of both molecular dynamics (MD) simulations and femtosecond pump-probe experiments. Using MD, we find that even though solvent rotational motions appear to dominate the electronic relaxation when only the solute's charge changes, the slow translational motions of the few closest solvent molecules control the solvation dynamics when realistic reactant size changes are taken into account. Moreover, we see that the linear response approximation, an assumption inherent in the use of dielectric continuum theories, fails when reactant size changes and solvent translational motions are involved. Our experimental approach focuses on the study of the chargetransfer-to-solvent (CTTS) transition of the sodium anion (Na -). We find that the charge-transfer rate of photoexcited sodide in tetrahydrofuran is ∼3 times slower than what would be expected by assuming that dielectric solvation was the dominant driving force for electron transfer. This suggests that the slow solvent translational motions needed to accommodate the reactant size change are rate-limiting for the charge transfer process, consistent with the simulations. The electron appearance and recombination kinetics also show that even though the charge transfer rate is roughly independent of excitation energy, the distance over which the electron is ejected depends sensitively on the excitation energy. Moreover, the detached electrons recombine with their Na atom partners to regenerate the parent sodide ions on two vastly different time scales. The best way to explain the electron recombination dynamics invokes the existence of two kinds of solvated electron: geminate sodium atom contact pairs. Our molecular picture of the charge-transfer process is that low-energy excitation produces a CTTS excited-state wave function confined within the original anion solvent cavity, leading to production of a sodium atom:solvated electron contact pair that can recombine in about one picosecond. The use of high excitation energies produces CTTS excited-state wave functions with greater curvature and spatial extent, allowing the electron to localize further from the parent in a long-lived (g200 ps) solvent-separated contact pair, or to be ejected into the solvent. Independent of the excitation energy, it is the relatively slow translational motions of first-sh...

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In ''outer sphere'' electron transfer reactions, motions of the solvent molecules surrounding the donor and acceptor govern the dynamics of charge flow. Are the relevant solvent motions determined simply by bulk solvent properties such as dielectric constant or viscosity? Or are molecular details, such as the local solvent structure around the donor and acceptor, necessary to understand how solvent motions control charge transfer? In this paper, we address these questions by using ultrafast spectroscopy to study a photoinduced electron transfer reaction with only electronic degrees of freedom: the charge-transfer-to-solvent ͑CTTS͒ reaction of Na Ϫ ͑sodide͒. Photoexcitation of NaϪ places the excited CTTS electron into a solvent-bound excited state; motions of the surrounding solvent molecules in response to this excitation ultimately lead to detachment of the electron. The detached electron can then localize either in an ''immediate'' contact pair ͑in the same cavity as the Na atom͒, which undergoes back electron transfer to regenerate Na Ϫ in ϳ1 ps, or in a ''solvent-separated'' contact pair ͑one solvent shell away from the Na atom͒, which undergoes back electron transfer in tens to hundreds of picoseconds. We present detailed results for the dynamics of each step of this reaction in several solvents: the ethers tetrahydrofuran, diethyl ether and tetrahydropyran and the amine solvent hexamethylphosphoramide ͑HMPA͒. The results are interpreted in terms of a kinetic model that both incorporates spectral shifting of the reaction intermediates due to solvation dynamics and accounts for anisotropic spectral diffusion in polarized transient hole-burning experiments. We find that the rate of CTTS detachment does not correlate simply with any bulk solvent properties, but instead appears to depend on the details of how the solvent packs around the solute. In contrast, the rate for back electron transfer of solvent-separated contact pairs varies inversely with solvent polarity, indicating a barrier to recombination and suggesting that this reaction lies in the Marcus inverted regime. For immediate contact pairs, the rate of recombination varies directly with solvent polarity in the ethers but is slowest in the highly polar solvent HMPA, suggesting that the spatial extent of the solvated electron in each solvent is one of the major factors determining the recombination dynamics. The fact that each step in the reaction varies with solvent in a different way implies that there is not a single set of solvent motions or spectral density that can be used to model all aspects of electron transfer. In addition, all of the results and conclusions in this paper are compared in detail to related work on this system by Ruhman and co-workers; in particular, we assign a fast decay seen in the near-IR to solvation of the CTTS p-to-p excited-state absorption, and polarization differences observed at visible probe wavelengths to anisotropic bleaching of the Na Ϫ CTTS ground state.

The processes by which solvated electrons are generated and undergo recombination are of great interest in condensed phase physical chemistry because of their relevance to both electron transfer reactions and radiation chemistry. Although most of the work in this area has focused on aqueous systems, many outstanding questions remain, especially concerning the nature of these processes in low polarity solvents where the solvated electron has a fundamentally different structure. In this paper, we use femtosecond spectroscopic techniques to explore the dynamics of solvated electrons in tetrahydrofuran ͑THF͒ that are produced in two different ways: ejection by multiphoton ionization of the neat solvent, and detachment via the charge-transfer-to-solvent ͑CTTS͒ transition of sodide (Na Ϫ). Following multiphoton ionization of the solvent, the recombination of solvated electrons can be well described by a simple model that assumes electrons are first ejected to a given thermalization distance and then move diffusively in the presence of the Coulombic attraction with their geminate cation. The short-time transient absorption dynamics of the THF radical cation in the visible region of the spectrum do not match the kinetics of the solvated electron probed at ϳ2 m, indicating that caution is warranted when drawing conclusions about recombination based only on the dynamics of the solvent cation absorption. With ϳ4 eV of excess energy, geminate recombination takes place on the hundreds of picoseconds time scale, corresponding to thermalization distances у40 Å. The recombination of solvated electrons ejected via CTTS detachment of Na Ϫ , on the other hand, takes place on two distinct time scales of р2 and ϳ200 ps with kinetics that cannot be adequately fit by simple diffusive models. The fraction of electrons that undergo the fast recombination process decreases with increasing excitation energy or intensity. These facts lead us to conclude that electrons localize in the vicinity of their geminate Na atom partners, producing either directly overlapping or solvent-separated contact pairs. The distinct recombination kinetics for the two separate electron generation processes serve to emphasize the differences between them: multiphoton ionization produces a delocalized electron whose wave function samples the structure of the equilibrium fluid before undergoing localization, while CTTS is an electron transfer reaction with dynamics controlled by the motions of solvent molecules adjacent to the parent ion. All the results are compared to recent experiments on the photodetachment of electrons in aqueous systems where contact pairs are also thought to be important, allowing us to develop a qualitative picture for the mechanisms of electron generation and recombination in different solvent environments.

Articles you may be interested inSearching for solvent cavities via electron photodetachment: The ultrafast charge-transfer-to-solvent dynamics of sodide in a series of ether solventsThe roles of the solute and solvent cavities in charge-transfer-to-solvent dynamics: Ultrafast studies of potasside and sodide in diethyl etherThe role of electronic symmetry in charge-transfer-to-solvent reactions: Quantum nonadiabatic computer simulation of photoexcited sodium anions Solvent effects on the ultrafast dynamics and spectroscopy of the charge-transfer-to-solvent reaction of sodide Charge-transfer-to-solvent ͑CTTS͒ transitions have been the subject of a great deal of interest recently because they represent the simplest possible charge transfer reaction: The CTTS electron transfer from an atomic ion to a cavity in the surrounding solvent involves only electronic degrees of freedom. Most of the work in this area, both experimental and theoretical, has focused on aqueous halides. Experimentally, however, halides make a challenging choice for studying the CTTS phenomenon because the relevant spectroscopic transitions are deep in the UV and because the charge-transfer dynamics can be monitored only indirectly through the appearance of the solvated electron. In this paper, we show that these difficulties can be overcome by taking advantage of the CTTS transitions in solutions of alkali metal anions, in particular, the near-IR CTTS band of sodide (Na Ϫ ) in tetrahydrofuran ͑THF͒. Using femtosecond pump-probe techniques, we have been able to spectroscopically separate and identify transient absorption contributions not only from the solvated electron, but also from the bleaching dynamics of the Na Ϫ ground state and from the absorption of the neutral sodium atom. Perhaps most importantly, we also have been able to directly observe the decay of the Na Ϫ * excited CTTS state, providing the first direct measure of the electron transfer rate for any CTTS system. Taken together, the data at a variety of pump and probe wavelengths provide a direct test for several kinetic models of the CTTS process. The model which best fits the data assumes a delayed ejection of the electron from the CTTS excited state in ϳ700 fs. Once ejected, a fraction of the electrons, which remain localized in the vicinity of the neutral sodium parent atom, recombine on a ϳ1.5-ps time scale. The fraction of electrons that recombine depends sensitively on the choice of excitation wavelength, suggesting multiple pathways for charge transfer. The spectrum of the neutral sodium atom, which appears on the ϳ700-fs charge-transfer time scale, matches well with a species of stochiometry (Na ϩ , e Ϫ ) that has been identified in the radiation chemistry literature. All the results are compared to previous studies of both CTTS dynamics and alkali metal solutions, and the implications for charge transfer are discussed.