Ultrafast electron transfer solutions

X 10 cm where Vth = 10 cm is the thermal velocity. Both of these numbers indicate ultrafast electron transfer dynamics. To provide such a high rate for the electron transfer from the conduction band of GaAs electrode to the adsorbed Co(Cp)2 molecules, the resulting Red species must be oxidized at an equivalent rate. It was shown that a self-exchange reaction in which an electron is exchanged between an adsorbed Red species and an Ox species in the solution, fulfilled this condition [33]. 

Ultrafast Electron Transfer and Short-Lived Prereactive Steps in Solutions... 

With the intensive development of ultrafast spectroscopic methods, reaction dynamics can be investigated at the subpicosecond time scale. Femtosecond spectroscopy of liquids and solutions allows the study of sol-vent-cage effects on elementary charge-transfer processes. Recent work on ultrafast electron-transfer channels in aqueous ionic solutions is presented (electron-atom or electron-ion radical pairs, early geminate recombination, and concerted electron-proton transfer) and discussed in the framework of quantum theories on nonequilibrium electronic states. These advances permit us to understand how the statistical density fluctuations of a molecular solvent can assist or impede elementary electron-transfer processes in liquids and solutions. 

Figure 8. Energy-level diagram of ultrafast electron-transfer processes in aqueous sodium chloride solution. Transitions (eV) correspond to experimental spectroscopic data obtained for different test wavelengths. The abscissa represents the appearance and relaxation dynamics of nonequilibrium electronic populations (CTTS ", CTTS, (e hyd) fCl e pairs). The two channels involved in the formation of an s-like ground hydrated electron state (e hyd, c hyd ) (dso reported in the figure. From these data, it is clear that the high excited CTTS state (CTTS ) corresponds to an ultrashort-lived excited state of aqueous chloride ions preceding an electron photodetachment process.

Aqueous ionic solutions represent a paradigm for the study of early branching between ultrafast nonadiabatic and adiabatic electron transfers. The very recent experimental observations of specific counterion effects on electronic dynamics provide direct evidence of complex influences of inhomogeneous ion-ion distributions on ultrafast electron-transfer processes. These microscopic effects are particularly evident in IR electronic traiectories in sodium chloride solution (86, 92). 

Photogenerated electron transfer reactions in chemistry typically occur on a timescale significantly greater than the decay timescale of coherent processes. Electron transfer reactions fail this criterion in poly(phenylen-evinylene) based polymers, for which the photogenerated coherent wave-function persists for 25 fs (in solution), and electron transfer occurs in =45 fs in the solid BHJ material. In P3HT, the coherent wavefunction persists for 100 fs in solution, while the electron transfer timescale is <100 fs in the BHJ material. It is therefore necessary to develop testable hypotheses regarding the mechanism of electron transfer in situations where a coherent photoexcited state is involved in the ultrafast electron transfer in bulk heterojunction solar cells. 

The behavior of CTTS states is dependent on energy levels of the ion-solvent molecular couphngs. These levels can lead to internal relaxation and/or complete electron detachment via adiabatic or nonadiabatic electron transfer. The ultrafast spectroscopic investigations of electronic dynamics in ionic solutions would permit us to learn more about the primary steps of an electron-transfer reaction within a cationic atmosphere. The influence of counterions on early electron photodetachment trajectories from a hahde ion can be considered as prereactive steps of an electron transfer. 

Figure 11. Schematic representation of sequential events of an SNj ionization reaction in a polar liquid. Elementary events involve contact ion pairs (CIP) and solvent-separated ion pairs (SSIP). In ionic aqueous solutions, the influence of different ion-pair configurations on early electron-transfer trajectories can be considered through the investigation of ultrafast electronic dynamics and radical ion-...

The classical, Marcus/Hush, limit corresponds to Equation (1) with /Cei= 1 and = (FC)r=o-This condition is achieved if either (i) the structural differences between the reactants and products do not implicate high-frequency vibrational modes or (ii) the exchange of energy (heat) between the high-frequency vibrational modes and the solvent is fast on the time scale for electron transfer. Statement (ii) is equivalent to the equilibrium assumption of transition-state theory. That this assumption is not always correct for reactions in solution has been demonstrated in ultrafast kinetic studies of reactions that vary ... 

Y)shihara, K. (1999). Ultrafast intermolecular electron transfer in solution. ddv. Chem. Phys. 107,371. 

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