Dexter process

Developed into a power series in R 1, where R is the intermolecular separation, H exhibits the dipole-dipole, dipole-quadrupole terms in increasing order. When nonvanishing, the dipole-dipole term is the most important, leading to the Forster process. When the dipole transition is forbidden, higher-order transitions come into play (Dexter, 1953). For the Forster process, H is well known, but 0. and 0, are still not known accurately enough to make an a priori calculation with Eq. (4.2). Instead, Forster (1947) makes a simplification based on the relative slowness of the transfer process. Under this condition, energy is transferred between molecules that are thermally equilibriated. The transfer rate then contains the same combination of Franck-Condon factors and vibrational distribution as are involved in the vibrionic transitions for the emission of the donor and the adsorptions of the acceptor. Forster (1947) thus obtains... 

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.

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. 

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... 

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Also of interest in connection with Fig. 16 is a process that has been labeled hot transfer (see, for example, Toyozawa, 1978 Kayanuma and Nasu, 1978 Jortner, 1979). Here it is suggested that in the case AE > A the transition to the ground state can take place during the lattice relaxation, i.e., before the excited state has reached its equilibrium position. This effect was first suggested by Dexter et al. (1955) and further analyzed by Bartram and Stoneham (1975) for F centers. A recent slight modification, in terms of fast capture of a majority carrier subsequent to that of a minority carrier, has been suggested by Sumi (1981) he points out that this process may be active in recombination-enhanced defect reactions. 

The process of nonradiative-resonance exchange has long been recognized to be of great importance in the understanding of luminescence. Numerous studies of the effect have been made. Of particular importance was the work by Dexter (41, 44), Dexter and Schulman (45), Forster (46, 47), and Perrin (48). The potential importance to rare earth-rare earth systems was stressed by Varsanyi and Dieke (49). 

Since the backup ions other than aluminum are of a size similar to the terbium ion, it is reasonable to assume that the structure of the glass matrix over the whole series is the same. Therefore, the concentration dependence of the lifetime is unequivocally due to terbium ions being packed closer and closer together. Pearson and Peterson postulate that, as the ions are situated closer and closer together, the quenching mechanism of Dexter and Schulman (45) becomes operative. That is, the excitation jumps from ion to ion by a resonance process until it reaches a sink. 

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