Exchange mechanism Dexter

One can see tliat tlie Dexter exchange mechanism is exponentially dependent on tlie distance between tlie donor and acceptor, and as such it begins to play a visible role only at very short distances when tlie electron clouds begin to... 

The rate of energy transfer at a very short donor-acceptor separation R by the exchange mechanism has been given by Dexter (1953) as follows ... 

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.

According to the Dexter theory of energy transfer, the distance dependence of energy transfer by the exchange mechanism falls off rapidly and is given by ... 

Dexter s formulation of exchange energy transfer (very weak coupling) In contrast to the inverse sixth power dependence on distance for the dipole-dipole mechanism, an exponential dependence is to be expected from the exchange mechanism. The rate constant for transfer can be written as... 

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... 

Given that triplet-triplet energy transfer proceeds via a Dexter (electron exchange) mechanism, it is not surprising that electron transfer can also occur via upper triplet states. Two-color experiments with anthracene in acetonitrile in the presence of ethylbromoacetate, a dissociative electron acceptor, showed that excitation to an upper triplet state led to depletion of the T-T absorption and concurrent production of the anthracene cation radical as a result of electron transfer (Scheme 1) [52]. 

Dexter excitation transfer (Electron Exchange Excitation Transfer) Excitation transfer occurring as a result of an electron exchange mechanism. It requires an overlap... 

The energy transfer from Tb + to Eu + ion in the [Eu/Tb(tfa)3(tppo)2] pure chelate and the in situ chelate doped in gel glass indicates that the Dexter exchange mechanism is operative in the energy transfer process between ions" . Pettinari and coworkers" showed terbium 4-acylpyrazol-5-onate and its derivatives are luminescent materials, also pointing to the importance of properties such as hydrophobicity, solubility in non-polar solvents and large size of the molecule. 

It is worth noting that while Dexter s name became synonymous with the exchange mechanism of energy transfer, the main body of his seminal paper is devoted to spin-allowed EET mediated by the various multipole terms of the Coulomb interaction. 

Cf. the Fermi golden rule. Section 5.2.3.) The density of states pg (number of states per unit energy interval) is related to the spectral overlap J, and using the relations for (i given above the expressions derived by Fdrster (1951) and Dexter (1953) for the rate constant of energy transfer by the Coulomb and the exchange mechanism, respectively, may be written as... 

Increasing Q, would decrease the average minimum distance between a self localized PFq.fast excitation and a possible CTS defect, which suggests that Eq. 4, simply describes the qualitative dependence with distance for the Dexter electron exchange mechanism k j = ko e R, where ko is the maximum rate constant for energy transfer, occurring when donor and acceptor are at the collision distance Ro and R is the separation between donor and acceptor when they are further apart than Ro. 

Details on the mechanisms and theories of excitation energy transfer via dipole-dipole interaction (FRET Forster resonance energy transfer) and via exchange interaction (Dexter s mechanism) can be found in B. Valeur, Molecular Fluorescence. Principles and Applications, Wiley-VCH, Weinheim, 2002, chap. 4 and 9. 

We need not be concerned with the quantum mechanical treatment of electron exchange. All we need to know is that triplet energy transfer by electron exchange, the Dexter mechanism, requires appreciable overlap between the molecular orbitals of D and A, so that the critical transfer distance becomes essentially equal to the sum of the van der Waals radii of D and A. 

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