Electron transfer Dexter-type

Finally, when compared to intervalence electron transfer, Dexter-type energy transfer gives widely different decay laws (expressed in terms of In Ftt vs. R), i.e., from -0.085 to -0.33 A. No simple explanation appears at present, due to the higher complexity of the process. One can only remark that, since the matrix element Ftt appears as the product of matrix elements for electron transfer and hole transfer (Eq. (7)), one can expect the rate of decay with distance to be the sum of the rates of decay for these two processes [46a, 83]. This would explain why the decay is generally faster than for electron transfer, with the curious exception of Ziessel s acetylene-bridged complexes (see Section 1.5.4). 

Baldo et al. [ 164] used the platinum complex of 2,3,7,8,12,13,17,18-octaethyl-21 //,23//-porphine (PtOEP, 66) as efficient phosphorescent material. This complex absorbs at 530 nm and exhibits weak fluorescence at 580 nm but strong phosphorescence from the triplet state at 650 nm. Triplet transfer from a host like Alq3 was assumed to follow the Dexter mechanism. Dexter-type excitation transfer is a short-range process involving the exchange of electrons. In contrast to Forster transfer, triplet exciton transfer is allowed. 

Figure 3. a) Electron transfer and b) Dexter-type energy transfer. 

The second possibility of energy transfer is known as exchange type or Dexter energy transfer. Dexter ET is based on quantum mechanical exchange interactions, therefore it needs strong spatial overlap of the involved wavefunctions of D and A. Since the overlap of electronic wavefunctions decays exponentially with distance, it is expected that the rate constant koA decreases even more rapidly with distance R than observed in the case of singlet transfer. A schematic presentation of Dexter ET is shown in Fig. 21. Dexter ET occurs typically over distances which are similar to the van-der-Waals distance, i.e. R = 0.5 - Inm. The rate constant drops exponentially with the distance Rda between D and A ... 

Fig. 11.6 Scheme illustrating various transfer processes from the host to the guest electron (hole) transfer from the host LUMO (HOMO) to the guest LUMO (HOMO), Forster-type energy transfer between singlet states and Dexter-type energy transfer from the host to the guest triplet state. The nature of the excitonic state is simplified for better clarity. 

Figure 4.6 Simplified representation of Dexter-type triplet energy transfer as a correlated two-electron exchange process.

Os and Re complexes 32-35 (Chart 5.7) are systems in which the metal centers are attached in a pendant fashion to a bipyridine-containing conjugated oligomer [45]. In the case of the mixed Ru-Os complex 33, energy transfer from Ru to Os is efficient and results in Os-based emission. The authors ascribed this to a Dexter-type double-electron transfer , mediated by the conjugated ligand. 

As mentioned earlier, triplet energy transfer can occur by way of Forster-type through-space interactions [100] or via Dexter-type electron exchange [101], In the former case, the rate constant for Forster-type triplet energy transfer (kp) can be expressed as follows ... 

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