Ultrafast solvation, electron-transfer

An ultrafast intermolecular electron transfer (ET) from electron donating solvent to an excited dye molecule was found. A temperature-dependent non-exponential time dependence was observed in aniline, and a temperature-independent single exponential process for Nile blue (160 fs) and oxazine 1 (260 fs) was observed in N,N-Smethylaniline. The solvation times of solvent anilines were obtained by dynamic Stokes shift measurements. The rate of ET in some systems was observed to be much greater than the solvation time of anilines. The dynamic behavior was simulated by the 2-dimen ional potential energy surface for reaction, taking into account of the effects of both solvent reorientation and nuclear motion of reactants. 

Recent interest in the role of the solvent in electron transfer reactions has focused on ultrafast photoinduced electron transfers and theoretical modeling of solvation.[l-... 

The several theoretical and/or simulation methods developed for modelling the solvation phenomena can be applied to the treatment of solvent effects on chemical reactivity. A variety of systems - ranging from small molecules to very large ones, such as biomolecules [236-238], biological membranes [239] and polymers [240] -and problems - mechanism of organic reactions [25, 79, 223, 241-247], chemical reactions in supercritical fluids [216, 248-250], ultrafast spectroscopy [251-255], electrochemical processes [256, 257], proton transfer [74, 75, 231], electron transfer [76, 77, 104, 258-261], charge transfer reactions and complexes [262-264], molecular and ionic spectra and excited states [24, 265-268], solvent-induced polarizability [221, 269], reaction dynamics [28, 78, 270-276], isomerization [110, 277-279], tautomeric equilibrium [280-282], conformational changes [283], dissociation reactions [199, 200, 227], stability [284] - have been treated by these techniques. Some of these... 

Solvation effects at the transition-state level of electron-transfer reactions is a field that is opening up by means of clusters reactions. Reactions within clusters (see Section 2.7) or at the surface of clusters (see Section 2.8) provide an experimental tool to observe the effect of gradual solvation on electron-transfer reactions. Femtosecond methods appear highly valuable for characterizing the electron transfer in reactions within clusters, since in neutral clusters the electrostatic effects of solvation will show up intensely at the electron-transfer level and for a short period of time, before the products are fully developed. In the gas phase the solvation of electrons is directly sensed by photoelectron spectroscopy, as exemplified in the (nonreactive) case of ultrafast solvation of electrons in water clusters [305]. 

This deuterium isotope effect cannot be explained by a purely electronic process but could be explained by a proton-coupled electron transfer. The population decay rate of the excited state at a fixed energy is successfully decomposed into two components an isotope independent solvation term and a proton-coupled electron transfer term with a marked deuterium effect. The latter terms for the CH3OH overlayer are found to be about twice those for the CH3OD overlayer. Thus, with time-resolved 2PPE, the ultrafast dielectric response of a protic/solvent metal-oxide interface has been revealed. 

Tianquan Lian received his BS degree from Xiamen University in 1985, his MS degree from the Chinese Academy of Sciences in 1988 and his PhD from the University of Pennsylvania in 1993. After postdoctoral training in the University of California at Berkeley, he joined the faculty of chemistry department at Emory University in 1996. He was promoted to associate professor in 2002 and full professor in 2005. He has been a recipient of the NSF CAREER award and the Sloan fellowship. His research interest is focused on the ultrafast dynamics of nanomaterials and interfaces. He is particularly interested in fundamental physical chemistry problems related to nanomaterials-based solar energy conversion concepts and devices. These problems include the dynamics of electron transfer, energy transfer, vibrational energy relaxation and solvation at interfaces and in nanomaterials. 

There have been two reviews of photoinduced electron transfer. The subject is dealt with generally in one " whilst ultrafast photochemical charge transfer and excited state solvation are considered specifically in the other. ... 

The direct photoexcitation of water molecules by ultrashort laser pulses is used for the investigation of primary events occurring from 10 s (thermal orientation of water molecules and ultrafast proton transfer) to 10" s (primary reactions of a solvated electron with protic species) (57,58,61-65). The nonlinear interaction of ultrashort UV pulses (typically less than 100 fs in duration and having a power of 10 W cm" ) with water molecules triggers multiple electron photodetachment channels within a hydrogen bond network (see equations 4-7). An initial energy deposition via a two-photon absorption process (2 X 4 eV) leads to the formation of nonequilibrium states of an excess electron... 

In this chapter we will highlight recent experimental data on the picosecond dynamics of electron localization and solvation in polar liquids and on the ultrafast radiationless transitions that accompany laser excitation of e in the same systems. The specific issues we address concern (1) the mechanism for electron localization in polar liquids, (2) the molecular description of the solvation process in forming the cluster, and (3) the dynamics of electron transfer following photodetachment of an electron from its cluster. 

Since the solvation time correlation function is known both from experiments and from computer simulations, we can easily carry out the above exercise. When this is done, the theory predicts a lack of, or weak, dependence of the electron transfer rate on solvent dynamics, for weakly adiabatic reactions the reason being the dominance of the ultrafast component in SD of water, so the solvent moves too fast to offer any retardation ... 

Spectroelectrochemical methods have been used in recent years to study fast-photoinduced electron transfer at the liquid/liquid interface.- "- - Of particular importance is extending the idea of employing solvent (typically N,N-dimethylaniline or DMA) as an electron donor to the liquid/liquid interface.The advantage of this approach is that complications due to ion transfer across the interface and to diffusion are obviated. Several studies of ET between coumarin dyes and electron-donating solvents in micelles, reverse micelles, at the surface of proteins, and in nanocavities have demonstrated ultrafast electron transfer that is faster than solvation due to the close proximity of the redox pair. These experiments provided additional evidence for the existence of the Marcus-inverted region at liquid interfacial sy stems. ... 

J. Stabler, U. Bovensiepen, M. Meyer, and M. Wolf, Chem. Soc. Rev., 37,2180-2190 (2008). A Surface Science Approach to Ultrafast Electron Transfer and Solvation Dynamics at Interfaces. 

The photodynamics of electronically excited indole in water is investigated by UV-visible pump-probe spectroscopy with 80 fs time resolution and compared to the behavior in other solvents. In cyclohexane population transfer from the optically excited La to the Lb state happens within 7 ps. In ethanol ultrafast state reversal is observed, followed by population transfer from the Lb to the La state within 6 ps. In water ultrafast branching occurs between the fluorescing state and the charge-transfer-to-solvent state. Presolvated electrons, formed together with indole radicals within our time resolution, solvate on a timescale of 350 fs. 

The population transfer between the excited La and Lb states of 6.5 2 ps is determined from indole dissolved in ethanol and cyclohexane. In water the appearance of presolvated elections within the time resolution of our experiment and the fluorescence quantum yield of 0.09 indicate an ultrafast branching between the fluorescing state and the CTTS state immediately after photoexcitation. The solvation of the generated electrons shows the same initial dynamics of 350 fs for solvated indole and for pure water but differs on longer timescales. 

In the last part of this chapter, intramolecular charge transfer (ICT) in anthryl derivatives with linked donor-acceptor parts was discussed. Ultrafast spectroscopy has been applied both for structural characterization and for real-time probing of the ICT in this case. Microscopic solvation effects on the torsional motions and the ICT in the molecules have been examined by the use of their clusters with polar solvents. One of the most important problems which awaits for further studies is an ambiguous description of the electronic character of the charge-separated states in the systems. So far, high-level quantum-mechanical calculations have not been able to deal with such large molecular systems, but reliable evaluation of electronically... 

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