Electron transfer inverted limit

Early studies showed tliat tire rates of ET are limited by solvation rates for certain barrierless electron transfer reactions. However, more recent studies showed tliat electron-transfer rates can far exceed tire rates of diffusional solvation, which indicate critical roles for intramolecular (high frequency) vibrational mode couplings and inertial solvation. The interiDlay between inter- and intramolecular degrees of freedom is particularly significant in tire Marcus inverted regime [45] (figure C3.2.12)). 

The energy dependence of charge separation is most readily scrutinized and it was investigated first in considerable detail. Rehm and Weller studied rate constants of fluorescence quenching for a series of more than 60 organic donor-acceptor pairs and found a maximum rate of electron transfer, essentially diffusion limited, without any indication for an inverted region (Fig. 6) [136]. These results were at variance with the existing theories, but they could be rationalized on the basis... 

There are, however initiating systems, which demonstrate the inverted-reyion-like kinetic behavior (Figure 39), e.g., they show that the rate of polymerization is reduced when the thermodynamic driving force (—AGgt) is increased. This specific behavior is a clear indication that the rate of intermolecular electron transfer is not the limiting step for the entire process. 

It is customary to consider electron transfer in three limits, (a) The normal limit, defined by Ex > AE, (b) the activatiordess limit, defined by Ex = AE, and (c) the inverted limit, defined by Ex < AE. These limits are illustrated in Fig. 9.7. 

A hint of the inverted region was suggested by Creutz and Sutin for the bimolecular electron transfer between photo-excited Ru(bpy)3 and M(bpy)3 complexes, where M = Cr, Os and Ru. However, all of the rate constants are within a factor of two of the diffusion-limited value. Marcus and Siders have given a more detailed analysis of these results. 

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