Weak-Coupling Scheme Ions

At first sight, cncigy transfer between identical rare earth ions seems to be a process with a low rate, because their interaction will be weak in view of the well-shielded character of the 4/electrons. However, although the radiative rates are small, the spectral overlap can be large. This originates from the fact that AR 0, so that the absorption and emission lines will coincide. Further the transfer rate will easily surpa.ss the radiative rate, since the latter is low. In fact energy migration has been 

This type of research uses pulsed and tunable la.sers as an excitation source. The rare earth ion is excited selectively with a laser pulse,and its decay is analyzed. The shape of the decay curve is characteristic of the physical processes in the compound under study. For a detailed review the reader is referred to the literature [1-3]. Here we give some results for specific situations. We assume that the object of our study consists of a compound of a rare earth ion S which contains also some ions A which are able to trap the migrating excitation energy of S by SA transfer. 

In Fig. 5.2, we have given some of these decay curves. The temperature dependence of Pss is very complicated. Due to inhomogeneous broadening, the S ions 

Dible 5.2. Energy migration characteristics in some Eu compounds at 300 K (data from Ref. 

Compound Shottesi Eu Eu distance Diffusirm rxrnstant (ernes ) Hopping time (s) 

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Energy migration in concentrated systems has been an issue of research in the last decade. Especially since lasers became easily available, the progress has been great. In Section VIII we will first consider the case that S is an ion to which the weak-coupling scheme applies. In practice this case consists of the trivalent rare earth ions. Subsequently we will deal with the case where S is an ion to which the intermediate-or strong-coupling scheme applies. 

In conclusion, there are several types of nonradiative processes in luminescent materials. The two most important ones are energy transfer between two luminescent centers and intracenter nonradiative decay. The former are reasonably well understood and predictable. The latter are only understood and predictable in the case of ions for which the weak-coupling scheme is valid. For the others it is nevertheless possible to arrive at reasonable predictions by applying simple models to a series of luminescent compositions (see Section IV). 

Oxidative coupling is the decisive step in the Cr-based ethylene oligomerization according to the metallacycle mechanism. As shown in Scheme 6.16.7, two ethylene units of the ligand sphere of the Crmetallacyclic intermediate. Assuming a cationic Cr-complex in the first place (with the MAO anion as very weakly coordinating counter-ion), the oxidation state of chromium increases in the process from +I to +III. 

The conductometric results of Meerwein et al. (1957 b) mentioned above demonstrate that, in contrast to other products of the coupling of nucleophiles to arenediazonium ions, the diazosulfones are characterized by a relatively weak and polarized covalent bond between the p-nitrogen and the nucleophilic atom of the nucleophile. This also becomes evident in the ambidentate solvent effects found in the thermal decomposition of methyl benzenediazosulfone by Kice and Gabrielson (1970). In apolar solvents such as benzene or diphenylmethane, they were able to isolate decomposition products arising via a mechanism involving homolytic dissociation of the N — S bond. In a polar, aprotic solvent (acetonitrile), however, the primary product was acetanilide. The latter is thought to arise via an initial hetero-lytic dissociation and reaction of the diazonium ion with the solvent (Scheme 6-11). 

There are amines such as A-methylnitroamine, that are too weakly nucleophilic to be able to form covalent adducts with arenediazonium ions. The products of the latter appear to be those of salts ArNJ N(N02)CH3, as found by Baranchik et al. (1957). Amides also appear not to be sufficiently nucleophilic, but thioamides are, as is shown by the reaction of A-phenylthiourea in the presence of NaOH (Scheme 13-12 Nesynov et al., 1970). First a (probably homolytic) phenylation-de-diazoniation takes place, followed by A-coupling. Selenourea also reacts a mixture of products is formed, which indicates a reaction of the same type as with thiourea (Nesynov and Aldokhina, 1976). 

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