Dexter-Forster model

Based on the classical Dexter-Forster model of inter-ionic interaction induced energy transfer, the excitation probability of ion i can be expressed as (Dexter, 1953 Forster, 1948 ... 

It is important to note that the Dexter and Forster models involve a number of simplifications, in particular in treating the systems as involving point dipoles. More recent theoretical studies have concentrated on overcoming these limitations [65, 66]. 

The next two chapters are devoted to ultrafast radiationless transitions. In Chapter 5, the generalized linear response theory is used to treat the non-equilibrium dynamics of molecular systems. This method, based on the density matrix method, can also be used to calculate the transient spectroscopic signals that are often monitored experimentally. As an application of the method, the authors present the study of the interfadal photo-induced electron transfer in dye-sensitized solar cell as observed by transient absorption spectroscopy. Chapter 6 uses the density matrix method to discuss important processes that occur in the bacterial photosynthetic reaction center, which has congested electronic structure within 200-1500cm 1 and weak interactions between these electronic states. Therefore, this biological system is an ideal system to examine theoretical models (memory effect, coherence effect, vibrational relaxation, etc.) and techniques (generalized linear response theory, Forster-Dexter theory, Marcus theory, internal conversion theory, etc.) for treating ultrafast radiationless transition phenomena. 

In contrast, EET has been historically modelled in terms of two main schemes the Forster transfer [15], a resonant dipole-dipole interaction, and the Dexter transfer [16], based on wavefunction overlap. The effects of the environment where early recognized by Forster in its unified theory of EET, where the Coulomb interaction between donor and acceptor transition dipoles is screened by the presence of the environment (represented as a dielectric) through a screening factor l/n2, where n is the solvent refractive index. This description is clearly an approximation of the global effects induced by a polarizable environment on EET. In fact, the presence of a dielectric environment not only screens the Coulomb interactions as formulated by Forster but also affects all the electronic properties of the interacting donor and acceptor [17],... 

Electrochemical and photochemical processes are the most convenient inputs and outputs for interfacial supramolecular assemblies in terms of flexibility, speed and ease of detection. This chapter provides the theoretical background for understanding electrochemical and optically driven processes, both within supramolecular assemblies and at the ISA interface. The most important theories of electron and energy transfer, including the Marcus, Forster and Dexter models, are described. Moreover, the distance dependence of electron and energy transfer are considered and proton transfer, as well as photoisomerization, are discussed. 

The Forster dipole-dipole mechanism for energy transfer from the forbidden 2A level seems to be ruled out. Therefore we have used Dexter s ele tron-exchange model (7). With reasonable coupling parameters we have estimated the distance between the carotenoid and bacteriochlrophyll to be 4-5 A. 

Depending on the electromagnetic nature of Ti , a double-electron exchange (Dexter) mechanism or an electrostatic multipolar (Forster) mechanism have been proposed and theoretically modeled. They are sketched on Fig. 8 for the simple S - T -Ln path. Their specific dependences on the distance d separating the donor D from the acceptor A, i.e., for double-electron exchange and for dipole-dipolar processes, respectively, often limit Dexter mechanism to operate at short distance (typically 30-50 pm) at which orbital overlap is significant, while Forster mechanism may extend over much longer distances (up to 1,000 pm). 

F re 25.4 (a) Forster dipole-coupling and (b) Dexter electron-exchange models for energy transfer (from D to A). 

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