Electron transfer in the inverted region

Metal Tunneling Redox couple Barrier In an elecrolyte solution 

7 Charge Transfer Processes at the Semiconductor-Liquid Interface 

This method was recently also applied by Hamann et al. [113]. They studied quantitatively the electron transfer between ZnO and two different redox systems, namely [CoCblpylgl / (A = = 0.04eV) and 

Metal Tunneling Redox couple barrier in an elecrolyte solution 

In the case of semiconductor electrodes, it is impossible to obtain the same information because the energy bands are fixed on the surface and any potential variation occurs only across the space charge layer. Here the maximum rate constant is expected if the peak of the distribution curve occurs at the lower edge of the conduction band of an n-type semiconductor. Therefore, the experimental results obtained with the modified metal electrodes are of great importance for the quantitative analysis of rate constants from current-potential curves measured with semiconductor electrodes (see, e.g.. Section 7.3.4). 

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The inverted region was initially predicted by Marcus and the decrease in the electron transfer rate constant with —AG° has been observed experimentally many times.18 This is an important and remarkable result both for natural and artificial photosynthesis and energy conversion it predicts that, following electron transfer quenching of the excited A -B, the back electron transfer in the inverted region for the charge-separated state A + -B becomes slower as the energy stored increases. 

The theoretical results obtained for outer-sphere electron transfer based on self-exchange reactions provide the essential background for discussing the interplay between theory and experiment in a variety of electron transfer processes. The next topic considered is outer-sphere electron transfer for net reactions where AG O and application of the Marcus cross reaction equation for correlating experimental data. A consideration of reactions for which AG is highly favorable leads to some peculiar features and the concept of electron transfer in the inverted region and, also, excited state decay. 

The imbedded nature of the potential curves in Figure 6 for electron transfer in the inverted region is a feature shared with the nonradiative decay of molecular excited states. In fact, in the inverted region another channel for the transition between states is by emission, D,A -> D+,A + hv, which can be observed, for example, from organic exciplexes,74 chemiluminescent reactions,75 or from intramolecular charge transfer excited states, e.g. (bipy)2Rum(bipyT)2+ - (bipy)2Run(bipy)2+ + hv. 

The fact that the Marcus equation (see above, Eq. 3) predicts a positive increase in AG12 with an increasingly negative AG, beyond AG = Ai2, is well known, as is the fact that the quantitative effect predicted has never been observed. What is usually seen is a leveling in rate at the diffusion-controlled limit, as in a recent electrochemical study. Alternatively, products in long-lived electronically excited states may occur. The lifetimes are themselves sometimes evidence of slow electron transfer in the inverted region, since deactivation paths by electron transfer may be available thermodynamically, but not effective kinetically. In a recent study by Meyer and co-workers, deactivation rate constants fc r of chemiluminescent states of [OsLs] " (L = various bipyridyls and phenanthrolines) have been shown to follow the energy-gap law... 

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