Forster and Dexter 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 ... 

Both Forster and Dexter transfers can also be used to gather the excited states for obtaining an optimized emission from devices [2]. These processes are essential in the photophysics of composites and will be discussed in detail in the following sections. 

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 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.)... 

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. 

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. 

Luminescence of rare earth ions can be understood, based on transitions between (almost) atomic eigenstates of the system [5.220, 5.221]. Forster and Dexter first described energy transfer between localized centers in luminescent material [5.222-5.224]. Besides orbital theory, semiconductor theory has also contributed to the understanding of radiative transitions Both band-to-band transitions and transitions involving localized donor and/or acceptor states fit within this framework. Nevertheless, there are also still open questions concerning the theoretical aspects. 

If luminescent centers come closer together, they may show interaction with each other that results in new phenomena. Consider two centers, S and A, with a certain interaction. The relaxed-excited state of S may transfer its energy to A. This energy transfer has been treated by Forster and Dexter and is now well understood (8). 

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-... 

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 ... 

Copper Complexes A copper complex is a triplet emitting material. It has been used with PVK and PBD for LEDs. Both Forster and Dexter energy transfer are involved in the device. ... 

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. 

The value of can also be calculated by using the quantitative theories for D-A multipolar energy transfer given by Forster and Dexter. For instance, the transfer rate for dipole -dipole interaction is given by the Dexter equation [2] ... 

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... 

Figure 3 Schematic illustration of Forster and Dexter types of energy transfers.

Forster and Dexter properly identified the microscopic interionic interaction leading to the communication between an excited and an unexcited ion as arising from the multipolar electrostatic fields produced by the excited state. In order to explain transfer in lanthanide doped systems where often there is no direct overlap in the sharp line spectra of acceptor and donor ions, it becomes necessary to invoke phonon assistance to conserve energy which entail the emission and absorption of one or more phonons. As in the case of relaxation, the various phonon mediated transfer processes have characteristic temperature dependences which allow their identification. The microscopic processes have also definite parametric dependences such as on Rj, the distance between interacting centers, on the amount of energy to be taken up or supplemented by... 

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. 

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