Forster and Dexter mechanisms

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. 

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

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. 

There are two possible excited state interfacial electron transfer processes that can occur from a molecular excited state, S, created at a metal surface (a) the metal accepts an electron from S to form S+ or (b) the metal donates an electron to S to form S . Neither of these processes has been directly observed. The two processes would be competitive and unless there is some preference, no net charge will cross the interface. In order to obtain a steady-state photoelectrochemical response, back interfacial electron transfer reactions of S+ (or S ) to yield ground-state products must also be eliminated. Energy transfer from an excited sensitizer to the metal is thermodynamically favorable and allowed by both Forster and Dexter mechanisms [20, 21]. There exists a theoretical [20] and experimental [21] literature describing energy transfer quenching of molecular excited states by metals. How-... 

Systematic studies about distance-dependent energy transfer was reported on linked porphyrin molecules by Osuka et al. [527]. Both Forster and Dexter mechanisms depend on the intermolecular distance and the rate constant ksT is expressed as... 

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... 

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 ... 

From Table 13.11, the variation of the lifetimes and fluorescence quantum yields in the series of compounds shows the clear increase of homo-chromophore interactions in the excited states when the distance between the chromophores diminishes. The rate and efficiency of the energy transfer in hetero-dimers does not seem to be metal dependent. The distance dependence of the energy transfer rate has been analyzed using Forster and Dexter theories. Harvey and Guilard have established that in 135-Zn-H2 and 136-Zn-H2, energy transfer is dominated by a Forster mechanism, while in the case of hetero-dimers 137, 138, and 139, it proceeds mainly via a Dexter mechanism. The critical distance at which the Dexter mechanism becomes inoperative is estimated between 5 and 6 By analogy with what has been discussed earlier in the case of linearly arranged covalent dimers, it should be noted that for compounds 135-139, no electron density should be present on the meso carbons involved in the covalent connection to the spacer. 

An important optical process in organic materials concerns the energy transfer between molecules via excitons. Two mechanisms can be distinguished Forster and Dexter transfer. In the Forster mechanism, the energy released from the exciton upon its dissociation is nonradiatively transferred to a molecule, which in turn creates another exciton by a process previously described. This mechanism occurs between the donor and acceptor molecules and is a long-range distance process. Several conditions should be fulfilled to expect an efHcient energy transfer by the Forster 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. 

Figure 23. Two principal mechanisms of excitation energy transfer (EET). (a) The Forster dipole-dipole mechanism, in which the active electrons, one and two, remain, respectively, on D and A throughout the process, (b) In the (Dexter) exchange mechanism, electrons one and two exchange locations.

Energy-transfer processes in which free photons exist as intermediates are sometimes referred to as trivial transfer mechanism. This term is misleading in the sense that such processes (e.g., in combination with internal reflection) can cause very complex and interesting phenomena [61, 65-67]. Radiationless energy-transfer processes have been studied extensively since the pioneering work of Forster [68, 69] and Dexter [70] (see, e.g., [40, 67, 71-73]). Here, we concentrate on the description of one-photon events, specifically with respect to radiationless energy-transfer processes. 

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. 

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... 

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. 

In these graphs a Bohr radius value (L) of 4.8 A (the value for porphyrin) is used in the Dexter equation 33.18 Also, the solid lines correspond to hypothetical situations in which only the Forster mechanism operates the dotted lines are hypothetical situations for when the Dexter mechanism is the only process.18 The curved lines are simulated lines obtained with equation 32 (Forster) or 33 (Dexter) but transposed onto the other graph (i.e., Forster equation plotted against Dexter formulation and vice versa). 

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

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