Evidence for the Inverted Region

A dyad is a supramolecular structure consisting of two distinct linked components such as donor and acceptor groups (see Chapter 12). 

Fullerenes such as C60 are excellent electron acceptors. In a fullerene-porphyrin-based dyad, the photoexcited state of the Qo accepts an electron from the linked zinc porphyrin group to give a charge-separated state. 

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Guldi D M and Asmus K-D 1997 Electron transfer from Cjg D2) and Cjg C2 ) to radical cations of various arenes evidence for the Marcus inverted region J. Am. Chem. See. 119 5744-5... 

One striking prediction of the energy gap law and eq. 11 and 14 is that in the inverted region, the electron transfer rate constant (kjjj. = ket) should decrease as the reaction becomes more favorable (lnknr -AE). Some evidence has been obtained for a fall-off in rate constants with increasing -AE (or -AG) for intermolecular reactions (21). Perhaps most notable is the pulse radiolysis data of Beitz and Miller (22). Nonetheless, the applicability of the energy gap law to intermolecular electron transfer in a detailed way has yet to be proven. 

Figure 2.3 Evidence for the Marcus inverted region from intramolecular electron rate constants as a function of AG° in methyltetrahydrofuran solution at 206 K. Reprinted with permission from G.L. Closs, L.T. Calcaterra, H.J. Green, K.W. Penfield and J.R. Miller, ]. Phys. Chem., 90,3673 (1986). Copyright (1986) American Chemical Society...

Clear-cut experimental evidence for the existence of an inverted region was provided in 1984 by the work Miller et al.352 on long-distance intramolecular electron transfer in the rigidly linked bichromophoric radical anions 9 (Figure 5.4), which were generated by pulsed electron injection. The electrons are initially captured with nearly equal probability by either the donor chromophore biphenyl or the acceptor chromophore mounted on the other side of the saturated 5a-androstane spacer. Electron transfer rates to attain equilibrium were determined by time-resolved observation of the ensuing absorbance changes. 

The Inverted Region The Marcus model predicts that as absolute values of AG° decrease (i.e., as electron transfer becomes more thermodynamically favorable), electron transfer rate constants should decrease. Because energetically more favorable reactions generally occur more rapidly, it is counterintuitive to expect the opposite to occur. However, this is precisely the case in the inverted region where more thermodynamically favorable reactions occur more slowly. The exothermic region in which this occurs is therefore referred to as inverted. Marcus predicted this behavior in I960,13 22 and the first experimental evidence for it was provided more than two decades later.23... 

Two findings are particularly noteworthy. First, the experiments in which the reactivity of water-soluble fullerene derivatives in aqueous media was probed (62-64) Not only, that the intermolecular reactions with hydrated electron and various radicals provided unequivocally evidence for the presence of fullerene clusters. But, furthermore, these investigations helped, in reference to the kinetics of the fullerene monomers, to estimate the agglomeration number for, for example, the mono pyrrolidinium salt in the respective fullerene cluster. Secondly, the intermolecular electron transfer reactions between radiolytically generated arene tt-radical cations and higher fullerenes (25,51) The noted parabolic dependence of the rate constants on the thermodynamic driving force is one of the rare confirmations of the existence of the Marcus-Inverted region in forward electron transfer. 

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. ... 

Becker, H. G. O., Pfeifer, D., and Urban, K., Evidence for the Marcus inverted region of back-electron transfer in solution by means of chemically induced dynamic nuclear polarization, /. Chem. Soc., Faraday Trans. 2, 85,1765, 1989. 

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