Dexter, energy transfer process

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. 

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. 

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. 

In the Forster mechanism, energy transfer occurs through dipolar interactions. This process is not coupled through bond interactions, and therefore orbital overlap and inter-component electronic coupling are unimportant. Dipole-dipole interactions may occur efficiently in systems where the donor and acceptor species are over 100 A apart, whereas Dexter energy transfer is typically efficient only up to distances of approximately 10 A. 

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. 

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 ... 

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