Properties of the lanthanide ions

For the application of lanthanide complexes in medical diagnosis and therapy, a general requirement is that the ion Ln3+ and the ligand should remain associated while the complex is in the body, i. e. their dissociation should be minimal, since the free ligand and Ln3+ are toxic. For the dissociation to be negligible, the complexes must be kinetically inert under physiological conditions. Since the complexation properties of the lanthanide ions and Y3+ are quite similar, it is of interest to compare the results obtained as concerns the kinetic behavior of Gd3+ complexes with those known for the complexes of other lanthanides and Y3+. 

Interest in the lanthanide complex species is centered currently largely in coordination number and stereochemistry, in thermodynamic stability and its interpretation, in the effects of the ligand on the properties of the lanthanide ion, and in practical applications. 

In the case of the lanthanide ions, the electronic repulsion induces the higher shift relative to the hydrogenoid energy level of the subshell. The shift due to the spin-orbit coupling comes next ( 10 times lower than the electronic repulsion), whereas the shift due to the ligand field is weak ( a thousand times lower than the electronic repulsion). As a result, a remarkable property of the lanthanide ions is that the sphtting of these electronic levels remains fairly constant whatever the enviromnent around the lanthanide ion. The reason for this behavior is the inner character of the 4f orbitals. 

In order to understand the photophysical properties of the lanthanide ions, it is necessary to have a clear knowledge about the different kinds of interactions that fight can have with matter as well as about fight itself... 

Cottam, G.L., A.D. Sherry, and K.M. Valentine, 1974a, Catalytic and spectroscopic properties of the lanthanide ion alkaline phosphatase complex, in Haschke, J.M. and H.A. Eick, eds.. Proceedings of the 11th Rare Earth Research Conference, Traverse City, Michigan, Vol. 1 (USAEC Technical Information Center, Oak Ridge, Tennessee) pp. 204-212. 

Coordination compounds composed of tetrapyrrole macrocyclic ligands encompassing a large metal ion in a sandwich-like fashion have been known since 1936 when Linstead and co-workers (67) reported the first synthesis of Sn(IV) bis(phthalocyanine). Numerous homoleptic and heteroleptic sandwich-type or double-decker metal complexes with phthalocyanines (68-70) and porphyrins (71-75) have been studied and structurally characterized. The electrochromic properties of the lanthanide pc sandwich complexes (76) have been investigated and the stable radical bis(phthalocyaninato)lutetium has been found to be the first example of an intrinsic molecular semiconductor (77). In contrast to the wealth of literature describing porphyrin and pc sandwich complexes, re-... 

Trivalent lanthanide cations have luminescent properties which are used in a number of applications. The luminescence of the lanthanide ions is unique in that it is long-lasting (up to more than a millisecond) and consists of very sharp bands. Lanthanide emission, in contrast to other long-lived emission processes, is not particularly sensitive to quenching by oxygen because the 4f electrons found within the inner electron core... 

Finally we note some other properties of these allowed transitions of the lanthanide ions. From Table 1 it becomes clear that in general the 4f—5d bands have a smaller band width than the c.t. transitions, typical values being 1000 and 2000 cm-i, respectively. In this connection it is interesting to find that at low temperatures the 4f- -5d absorption and emission bands often show a distinct and extended vibrational fine structure [Ce3+ (25), Tb + (25), Eu2+ (14, 26), Yb2+ (27)], whereas c.t. transitions do not. From this it seems probable that in the excited c.t. state the interaction between the lanthanide ion and its surroundings is stronger than in the excited 4f 5d state. This is not imexpected. As far... 

Parker and Williams recently reported NAND logic action in the terbium complex 16.[S8] The delayed emission of the lanthanide ion is switched off when H+ and 02 are present simultaneously. Protonation of the phenanthridine side chain causes its triplet excited state to approach theTb(m) 5D4 excited state energetically. This leads to equilibration of these two excited states and sharing of their properties. Thus, the metal-centered state displays the 02 sensitivity usually only found in organic triplets. 

The properties of the liquid lanthanide trihalides depend strongly on the atomic number of the halide. The variation in the heat capacity of the lanthanide fluorides indicates a strongly ionic behaviour of the melts with a concomittent irregular trend related to the electronic configuration of the lanthanide ions. In the lanthanide chlorides, bromides and iodides the trend becomes systematically more constant, indicating an increasing molecular nature of the melts. 

To provide an overview of the rare-earth compounds which have been studied under pressure so far, table 1 lists the compounds, with respect to the doped ion and with the respective references. Obviously, Eu has been studied under pressure in much more host matrices than any of the other elements. This situation is similar to the observations made by Gorller-Walrand and Binnemans (1996), who reviewed the experimental data on spectroscopic properties of trivalent lanthanide ions doped into crystalline host matrices at ambient pressure. They found that Nd and Eu alone built up around 50% of all studies. 

In the past decade, lanthanide ions doped in nanocrystalline semiconductors have been the subject of numerous investigations. Although quantum size effects are not expected on lanthanide energy level structures, influence of quantum confinement in semiconductor on the luminescence properties of the lanthanides is expected. One of the advantages of lanthanide-doped semiconductor nanocrystals is that the lanthanide luminescence can be efficiently sen-... 

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