Energy transfer by electron exchange

Based on luminescence studies, we postulated triplet-triplet energy transfer by electron exchange as the mechanism of photostabilization and we calculated an active quenching sphere with a radius, R0, of 19.7 A for 2,6-ND. Because the value of R0 is larger than 15 A, we postulated that energy migration was occurring. 

Calixarene containing a dioxotetraaza unit, PET-18, is responsive to transition metal ions like Zn2+ and Ni2+. Interaction of Zn2+ with the amino groups induces a fluorescence enhancement according to the PET principle. In contrast, some fluorescence quenching is observed in the case of Ni2+. PET from the fluorophore to the metal ion is a reasonable explanation but energy transfer by electron exchange (Dexter mechanism) cannot be excluded. 

We need not be concerned with the quantum mechanical treatment of electron exchange. All we need to know is that triplet energy transfer by electron exchange, the Dexter mechanism, requires appreciable overlap between the molecular orbitals of D and A, so that the critical transfer distance becomes essentially equal to the sum of the van der Waals radii of D and A. 

From Eq. (3.6) we observe that the rate of energy transfer by electron exchange mechanism decreases exponentially with 2R/L. Thus, it will be negligibly small as R increases more than on the order of one or two molecular diameters. Hence the effective distances for Dexter energy transfer range between 10 and 15 A. 

Figure 8.2 Energy transfer by electron exchange as represented in a HQckel orbital diagram.

Figure 6.15 Electron movements occurring in short-range triplet-triplet energy transfer by the exchange mechanism. Note that an electron initially on D moves to A and an electron initially on A moves to D ...

In the Collins-Kimball theory, the rate constant for the reaction was assumed to be distance-independent. Further refinement proposed by Wilemski and Fix-manc) consists of considering that the reaction rate constant has an exponential dependence on distance, which is indeed predicted for electron transfer reactions and energy transfer via electron exchange (see Dexter s formula in Section 4.6.3). The rate constant can thus be written in the following form ... 

Because the dependence on the distance of separation of donor and acceptor is similar for both energy transfer by the exchange effect and for electron transfer by tunnelling, this section can be abbreviated, the more so since the subject has been reviewed recently by Rice and Pilling [39]). 

The energy transfer by this exchange process occurs when the molecules have spin conservation, that is, the total electron spin does not change after the energy transfer. 

The occurrence of energy transfer requires electronic interactions and therefore its rate decreases with increasing distance. Depending on the interaction mechanism, the distance dependence may follow a 1/r (resonance (Forster) mechanism) or e (exchange (Dexter) mechanisms) [ 1 ]. In both cases, energy transfer is favored by overlap between the emission spectrum of the donor and the absorption spectrum of the acceptor. 

These rules also predict the nature of photoproducts expected in a metal-sensitized reactions. From the restrictions imposed by conservation of spin, we expect different products for singlet-sensitized and triplet-sensitized reactions. The Wigner spin rule is utilized to predict the outcome of photophysical processes such as, allowed electronic states of triplet-triplet annihilation processes, quenching by paramagnetic ions, electronic energy transfer by exchange mechanism and also in a variety of photochemical primary processes leading to reactant-product correlation. 

The efficiency of Forster energy transfer depends mainly on the oscillator strength of the A -> A and B -> B radiative transitions, whereas the efficiency of energy transfer by the electron exchange interaction cannot be directly related to an experimental quantity. 

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