Dexter-type Energy Transfer

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). 

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 ... 

If the charge distributions of the D and A overlap than a new class of interactions has to be considered, namely the exchange interaction between the electrons on D and on A. This type of energy transfer is called Dexter transfer [80, 96,98], Here we briefly outline the physical principles involved. 

The second type, radiationless energy transfer, is more efficient. There are two different mechanisms used to describe this type of energy transfer the Forster and Dexter mechanisms. 

Schlicke et al. [27] synthesized rod-like compounds, in which two metal-bipyridyl complexes were linked by >/igo(plienylene)s. The longest spacer (seven phenylene units) gave rise to a metal-to-metal distance of 4.2 nm. In this work, Dexter-type energy-transfer mechanism from [Ru(bpy)3]2+ to [Os(bpy)3]2+ was established. The energy transfer mechanism is essentially temperature-dependent and decreases exponentially with an attenuation coefficient of 0.32 A-1. 

Energy transfer requires electronic interactions and therefore its rate decreases with increasing distance, r. Depending on the electronic interaction mechanism, the distance dependence may follow a 1/r (resonance, also called Fdrster-type, mechanism) or e (exchange, also called Dexter-t5rpe, mechanism) (6). In both cases, energy transfer is favored when the emission spectrum of the donor overlaps the absorption spectrum of the acceptor. 

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

Starting from the (terpy)Ru(terpy-) end group, bimetallic Ru(II)-Os(II) dyads with zero to two phenylene units have been studied (cf. Section 1.5.3). Energy transfer from Ru to Os has been observed, and assigned to a Dexter-type process from order-of-magnitude arguments [80]. Unfortunately, only a lower limit for k (>10 ° s for all complexes at 293 and 77 K) could be obtained. 

The energy transfer processes can occur by two mechanisms the Forster-type mechanism (through-space) [55], based on coulombic interactions, and the Dexter-type mechanism (through-bond) [56], based on exchange interactions. The energy transfer rate constants according to the Forster and Dexter treatments can be evaluated by Eqs. (4) [55] and (5) [56], respectively ... 

---