Dexter electron exchange

Figure 2.14 Schematic representations of the mechanisms of photoinduced (a) electron transfer, (b) Dexter (electron-exchange) energy transfer, and (c) Fdrster (dipole-dipole) energy transfer mechanism processes in the supramolecular dyad A-L-B spheres represent electrons, while curved arrows indicate the directions of transfer...

Given that triplet-triplet energy transfer proceeds via a Dexter (electron exchange) mechanism, it is not surprising that electron transfer can also occur via upper triplet states. Two-color experiments with anthracene in acetonitrile in the presence of ethylbromoacetate, a dissociative electron acceptor, showed that excitation to an upper triplet state led to depletion of the T-T absorption and concurrent production of the anthracene cation radical as a result of electron transfer (Scheme 1) [52]. 

Increasing Q, would decrease the average minimum distance between a self localized PFq.fast excitation and a possible CTS defect, which suggests that Eq. 4, simply describes the qualitative dependence with distance for the Dexter electron exchange mechanism k j = ko e R, where ko is the maximum rate constant for energy transfer, occurring when donor and acceptor are at the collision distance Ro and R is the separation between donor and acceptor when they are further apart than Ro. 

ET proceeds by a Dexter electron exchange mechanism and is a weakly coupled non-adiabatic process. Its rate constant kET can be approximated by the Golden Rule " " ... 

F re 25.4 (a) Forster dipole-coupling and (b) Dexter electron-exchange models for energy transfer (from D to A). 

Figure 2.11. Effect of separation distance on the rate constant for intramolecular triplet energy transfer as calculated for (a) Dexter electron exchange with /S = 0.1, (b) Dexter electron...

The electron exchange interaction [see Eq. (6.16)] requires overlap of the electronic wave functions of the donor and acceptor. Dexter<31) evaluated the exchange expectation value equation and obtained the following result ... 

In contrast to the dipole-dipole interaction, the electron-exchange interaction is short ranged its rate decreases exponentially with the donor-acceptor distance (Dexter, 1953). This is expected since, for the electron exchange between D and A, respective orbital overlap would be needed. If the energy transfer is envisaged via an intermediate collision complex or an exciplex, D + A—(D-------A)- D + A, then Wigner s rule applies there must be a spin com-... 

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 ... 

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. 

Figure 3.36 The Dexter mechanism of energy transfer through simultaneous electron exchange.

The electron exchange (Dexter) mechanism which involves the overlap of wavefunctions of the donor and acceptor groups. This is a short-range excitation transfer mechanism that operates by... 

For all the following considerations it is an important fact that within the CC of interest mutual chromophore wave function overlap and electron exchange effects among different chromophores do not take place (absence of the Dexter mechanism). Therefore, we may assume the orthogonality relation Pvnf Pnb) — A/ ,i A,.h to be valid, where A, / - /A,) denotes the electronic... 

The Dexter mechanism is a nonradiative energy transfer process that involves a double electron exchange between the donor and the acceptor (Fig. 12).16 Although the double electron exchange is involved in this mechanism, no charge separated-state is formed. 

Coulombic or dipole-dipole interaction (Forster) and double electron exchange (Dexter) mechanisms. 

---