Lanthanide ions phosphorescence

The nature of the emission by these three lanthanide ions is phosphorescence, since the emission of light is accompanied by a change in spin multiplicity. For example, the emission by the Eu3+ cation involves a change in the spin multiplicity from 5 to 7 on going from the excited state to the ground state (5Eu —> 7Eu). 

Fig. 8 Energy level diagrams for the dansyl units of dendrimer 11 and the investigated lanthanide ions. The position of the triplet excited state of 11 is uncertain because no phosphorescence can he observed...

Sensitized luminescence in inorganic analysis will be discussed below in the section on lanthanides. Fluorescence, phosphorescence and sensitized luminescence processes are independent of the electronic structure of the organic reagent and the metal ion alone. Of importance are the composition of the complex, the nature, strength, and spatial orientation of metal-ligand bonds, and conditions under which the luminescence reaction proceeds (such as pH and the nature of solvent). All these factors significantly influence the detection limit, sensitivity and selectivity of determination. 

For phosphorescence, lanthanide metal ions can be used in a similar manner. Lanthanide ions have very interesting photophysical properties, but often exhibit weak absorption bands, and aggregate to form clusters, which limit their applications. Thus, a dendrimer that can provide a protective shell to isolate a cation and at the same time enhance the emission by transfer from the periphery to the lanthanide ion at the core could be of great interest. Self-assembled lanthanide-cored dendrimers have been prepared to prove such an assumption synthesis was carried out by mixing three equivalents of polyaryl ether dendrons bearing carboxylic acid entity at the focal point with Ln(III) cations [Er(III), Tb(III), and Eu(III)] (Fig. 5.4) [34]. The authors demonstrated that the enhancement of the lanthanide cation emission associated with the dendritic core shell was observed, and an antenna effect from the periphery to the core was shown to promote this process. 

Anomalous S2 -> Sq fluorescence and T2 -> Sq phosphorescence have been found in the triphenyl methane derivatives (14)—(20) the results are shown in Table 17. The gap between the S2 and S, levels is large, 14000 cm. The coordination of a lanthanide ion quenches in several cases the -+ Sq fluorescence, but the S2 S o emission is not significantly affected. 

In practice, those selection rules are not strict, and some couplings can make forbidden transitions happen. However, they remain weak, slow, or of low probabiUty. Phosphorescence, for example, is a manifestation of a forbidden singlet triplet transition favored by a spin-orbit coupling, whereas the luminescence of the trivalent lanthanide ions is a manifestation of forbidden f-f transitions favored by the disruption of the spherical (centrosymmetric) symmetry of the free ion once coupled to hgands. 

So, what is the difference between fluorescence, phosphorescence, and the liuninescence of the lanthanide ions In fact, the mechanism is not the same. The fluorescence is an allowed transition without a change of the total spin. The fluorescent emission is quick because the transition is allowed, usually in the nanosecond timescale ns = 10 s). 

Concerning the luminescence of the lanthanides, the f f transitions are actually described by the spectroscopic levels of the ion, either down to the groimd state or down to an intermediate level. Usually, the visible emissions of the lanthanide ions have transitions that change the total spin number of the ion, i.e., the (2 5 + 1) multiplicity, whereas the NIR emissions do not change the spin. Since several lanthanides exhibit both mechanisms, the term litminescence is preferred over fluorescence or phosphorescence for the lanthanide ions. In this way, the common mistake of calling fluorescence all kind of emissions is avoided. Becatrse of their forbidden character, f-f transitions are slow and the lanthanide luminescence may take up to a few miUiseconds (jns = 10 s). 

The limiting point is often (5). A sensitizer must possess the correct triplet energy in order to sensitize the desired lanthanide ion. It requires then a triplet emission or phosphorescence of the ligand that is higher in energy than a threshold limit fixed by the energy of the lower luminescent excited spectroscopic level of the lanthanide ion. 

In the course of the pioneering work on sensitised luminescence, Yuster and Weissman smdied the promotion of intersystem crossing due to spin-orbit coupling with heavy atoms [69]. They found that dibenzoylmethanide coordinated to the non-emissive La, Lu and Gd displayed different efficiencies of intersystem crossing, as seen by different ratios of fluorescence versus phosphorescence intensity and changes in phosphorescence lifetimes, directly related to spin-orbit coupling and to the magnetic moment of the lanthanide ion. 

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