Long-range transfer processes

In the remainder of this section, the general equations and theory appropriate for an incorporation of these long-range transfer effects into the diffusion-controlled reaction process are discussed. Later in this chapter, the specific cases of interest outlined above are developed and their relation to experimental work commented upon. 

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The width of the encounter pair reactivity zone, 672, is to be considered small. There is no reason for this choice, save convenience. Probably rather larger widths would be more appropriate following work on gas-phase collision kinetics or long-range transfer processes (Chap. 4). In such circumstances, the partially reflecting boundary condition is no longer suitable and other techniques have to be used (see Chap. 8 Sect. 2.4 and Chap. 9 Sect. 4). 

As discussed in Chap. 3 Sect. 2.5, while observation of time-dependent rate coefficients does enable reliable estimates of the diffusion coefficient appropriate to reaction between donors and acceptors, the very ease of observation of these time-dependent effects masks much detail of diffusive motion in liquids. Estimates of i eff reflect more on the parameters appropriate to long-range transfer processes than on collisional events in... 

To show this connection, consider an ion-pair as above (Sect. 2.1). Not only may the ion-pair diffuse and drift in the presence of an electric field arising from the mutual coulomb interaction, but also charge-dipole, charge-induced dipole, potential of mean force and an external electric field may all be included in the potential energy term, U. Both the diffusion coefficient and drift mobility may be position-dependent and a long-range transfer process, Z(r), may lead to recombination of the ion-pair. Equation (141) for the ion-pair density distribution becomes... 

Meeting these design criteria requires a sound knowledge of the fundamental mechanisms of electron transfer processes. Although ET is the most fundamental of all chemical reactions, it is by no means simple This realization is particularly evident in the case of long-range ET processes which forms the basis of this essay. 

The second class of processes also corresponds to a long-range transfer of the first valence electron, as in the reaction... 

This process requires a somewhat unusual experimental condition with the acceptor already present in its triplet state. Nevertheless, with perylene as the donor and phenanthrene-dio the acceptor in rigid solution, this process has been established to occur as long-range transfer . ... 

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 ... 

With this convention, we can now classify energy transfer processes either as resonant, if IA defined in equation (A3.13.81 is small, or non-resonant, if it is large. Quite generally the rate of resonant processes can approach or even exceed the Leimard-Jones collision frequency (the latter is possible if other long-range potentials are actually applicable, such as by pennanent dipole-dipole interaction). 

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