Dexter-type transfer

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. 

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. 

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. 

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 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 ... 

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 ... 

The absence of an absorption cross section for the exciplex means that it cannot be excited optically. Instead, an exciplex is formed by complexation of a ground-state molecule with an excited-state molecule, i.e. by Dexter-type energy transfer from a bulk exciton. Figure 2.10 plots the photoluminescence excitation spectra of the PFB, the F8BT, and the exciplex emission, all measured from the same 50 50 PFB F8BT blend. The PLE signature of the exciplex is a superposition of those of the two excitons. Hence, the exciplex is excited via energy transfer from the two bulk excitons. 

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. 

More than a decade ago, Frank et al. (1987) reported on the light-modulated EMR ofa number of synthetic porphyrin-polyene model systems that were linked by an amide group and in some cases by methylene spacers of various lengths. The triplet transfer efficiency increased as the link between the two pigments became shorter which is what one would expect, especially for Dexter type triplet exchange... 

Energy transfer is typically measured by a decrease in the donor lifetime and, in the rapid diffusion limit, is a sensitive function of closest approach. Energy transfer based on a Forster dipole-dipole mechanism is proportional to a in three dimensions and a in two dimensions (e.g., on membranes), where a is the distance of closest approach. If the distance of closest approach is less than 10 or 11 A, energy transfer is dominated by a Dexter type exchange mechanism owing to overlapping wave functions between the donor and acceptor. The exact distance of closest approach then becomes difficult to measure. 

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