Solvent-controlled electron transfer dynamics

Rips I., Klafter J. and Jortner J. Solvation dynamics and solvent-controlled electron transfer, in Photochemical Energy Convertion, J. R. Norris and D. Meisel (Eds.), Elsevier, Amsterdam, (1989). 

Second, we note that the dynamical aspect of the dielectric response is still incomplete in the above treatment, since (1) no information was provided about the dielectric response frequency and (2) a harmonic oscillator model for the local dielectric response is oversimplified and a damped oscillator may provide a more complete description. These dynamical aspects are not important in equilibrium considerations such as our transition-state-theory level treatment, but become so in other limits such as solvent-control electron-transfer reactions discussed in Section 16.6. 

The wide diversity of the foregoing reactions with electron-poor acceptors (which include cationic and neutral electrophiles as well as strong and weak one-electron oxidants) points to enol silyl ethers as electron donors in general. Indeed, we will show how the electron-transfer paradigm can be applied to the various reactions of enol silyl ethers listed above in which the donor/acceptor pair leads to a variety of reactive intermediates including cation radicals, anion radicals, radicals, etc. that govern the product distribution. Moreover, the modulation of ion-pair (cation radical and anion radical) dynamics by solvent and added salt allows control of the competing pathways to achieve the desired selectivity (see below). 

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

In the excited state, the redistribution of electrons can lead to localized states with distinct fluorescence spectra that are known as intramolecular charge transfer (ICT) states. This process is dynamic and coupled with dielectric relaxations in the environment [16]. This and other solvent-controlled adiabatic excited-state reactions are discussed in [17], As shown in Fig. 1, the locally excited (LE) state is populated initially upon excitation, and the ICT state appears with time in a process coupled with the reorientation of surrounding dipoles. 

The reader is also referred to the innovative nonphotochemical electron transfer studies of Weaver et al. [147], These authors have been exploring dynamical solvent effects on ground state self-exchange kinetics for or-ganometallic compounds. This work has explored many aspects of solvent control on intermediate barrier electron transfer reactions, including the effect on a distribution of solvation times. The experimental C(t) data on various solvents have been incorporated into the theoretical modeling of the ground state electron transfer reactions studied by Weaver et al. [147]. 

In summary, solvent interactions play a very important role that control photoinduced electron-transfer processes. It is mainly the high impact on the stabilization energies of the generated high-energy species that are decisive. We have to distinguish between two different effects—the static solvent influence and the dynamic solvent influence. 

Experiments aimed at probing solvent dynamical effects in electrochemical kinetics, as in homogeneous electron transfer, are only of very recent origin, fueled in part by a renaissance of theoretical activity in condensed-phase reaction dynamics [47] (Sect. 3.3.1). It has been noted that solvent-dependent rate constants can sometimes be correlated with the medium viscosity, t] [101]. While such behavior may also signal the onset of diffusion-rather than electron-transfer control, if the latter circumstances prevail this finding suggests that the frequency factor is controlled by solvent dynamics since td and hence rL [eqn. (23), Sect. 3.3.1] is often roughly proportional to... 

There is little experimental information on possible solvent dynamical effects for electron transfer in aqueous solution. However, water is a dynamically "fast solvent, vos being determined by "solvent inertial effects so that the usual transition-state formula [eqn. (22)] should be applicable for determining vn (Sect. 3.2.1). Consequently, solvent dynamical effects in this and other "low friction media (e.g. acetonitrile) should be controlled by the rotational frequency of individual solvent molecules and limited to reactions involving only very small inner-shell barriers (Sect. 3.3.1). 

Though much research on the influence of the solvent on the rate of electrode reactions has been done in recent years the problem is still far from a profound understanding. The basic question is the role of the dynamic and energetic terms in the control of the kinetics of simple electron-transfer electrode reactions. To answer this question it is essential to have reliable kinetic data for analysis. Unfortunately some kinetic data are too low and should be redetermined, preferably using submicroelectrodes. 

In view of the great importance of chemical reactions in solution, it is not surprising that basic aspects (structure, energetics, and dynamics) of elementary solvation processes continue to motivate both experimental and theoretical investigations. Thus, there is growing interest in the dynamical participation of the solvent in the events following a sudden redistribution of the charges of a solute molecule. These phenomena control photoionization in both pure liquids and solutions, the solvation of electrons in polar liquids, the time-dependent fluorescence Stokes shift, and the contribution of the solvent polarization fluctuations to the rates of electron transfer in oxidation-reduction reactions in solution. 

The universality of intermolecular electron transfer (ET) makes the mechanism one of the central questions to be solved in chemistry and biology. ET is strongly influenced by the nature of solvent and its dynamics in solution. Dynamical solvent effects on the course of reaction have recently studied both theoretically [1-5] and experimentally [6-15]. For ET with a rate comparable to the solvent flucmation rate, the motion and structure of the solvent can determine the rate of ET and it becomes the "solvent-controlled" reaction [2,3,6-9]. 

The heterogeneous rate constant [236] of electrochemical reduction of [Co "(bpy)3] + to [Co"(bpy)3] + is relatively slow, about 0.1 cm s in CH3CN or CH2CI2. Detailed studies of solvent and pressure effects [236, 237] have revealed that the rate of heterogeneous electron transfer is controlled by solvent dynamics. This implies that the electron transfer is adiabatic. 

Equation (49) describes the rate of formation of the rubrene geminate triplet-triplet pair in a similar way. Strongly exergonic formation of the excited triplets and ground states lies in the inverted Marcus region and therefore Eq. (24) must be applied. In both cases, vibronic excitation of the reaction products can take place, leading to an increase in the electron transfer rate. The electron transfer rate constitutes a superposition of a solvent-dynamics-controlled contribution and a... 

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