Energy Dexter mechanism

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. 

Murphy CB, Zhang Y, Troxler T, Ferry V, Martin JJ, Jones WE Jr (2004) Probing Forster and Dexter energy-transfer mechanisms in fluorescent conjugated polymer chemosensors. J Phys Chem B 108 1537-1543... 

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. 

As well as returning to the ground state by radiative or radiationless processes, excited states can be deactivated by electronic energy transfer. The principal mechanisms for this involve dipole-dipole interactions (Forster mechanism) or exchange interactions (Dexter mechanism). The former can take place over large distances (5 nm in favourable cases) and is expected for cases where there is good overlap between the absorption spectrum of the acceptor and the emission spectrum of the donor and where there is no change in the spin... 

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

Dexter mechanism Energy transfer by radiative emission... 

Fig. 5.3 Energy transfer according to the Dexter mechanism (schematic)...

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. 

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. 

The Dexter mechanism can be thought of as electron tunneling, by which one electron from the donor s LUMO moves to the acceptor s LUMO at the same time as an electron from the acceptor s HOMO moves to the donor s HOMO. In this mechanism, both singlet-singlet and triplet-triplet energy transfers are possible. This contrasts with the Forster mechanism, which operates in only singlet states. 

FIGURE 26. Some donor-bridge-acceptor systems by which energy transfer occurs through both Forster and Dexter mechanisms. (Modified from Ref. 78.)... 

Through-bond energy transfer was also observed for porphyrin systems (regardless whether it occurs via a Forster or a Dexter mechanism). Through-bond energy transfer was reported for the rhodium meso-tetraphenylporphyrin-tin (2,3,7,13,17,18-hexamethyl-8,12-diethylcorrole), which exhibits a Rh-Sn bond... 

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 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...

Thermodynamic estimates of the driving force for energy transfer, by either the Forster or Dexter mechanism, may be made from the following ... 

Experimentally, one of the main methods of distinction between the Forster and Dexter mechanisms in an energy transfer is a study of the distance dependence of the observed process. From Equation (2.32) it is evident that the rate of dipole-induced energy transfer, kfen/ decreases as d 6. This is typical of dipolar interactions and is reminiscent of the distance dependence of other such mechanisms, e.g. London dispersion forces. Therefore, the Forster mechanism can operate over large distances, whereas, in contrast, the rate of Dexter energy transfer, kden, falls off exponentially with distance. 

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