Lanthanide ions sensitization

Yb +, andEr +). Specifically, zmc(ll) SchifF-base complexes, which are known to be effective emitters, can act as effective anteima chromophores for lanthanide ion sensitization. 

The role of cerium in these lighting phosphors is not as the emitting atom but rather as the sensitizer. The initial step in the lighting process is the efficient absorption of the 254 nm emission Ce ", with broad absorption bands in the uv, is very suitable. This absorbed energy is then transferred to the sublattice within the crystalline phosphor eventually the activator ion is fed and emission results. Cerium, as a sensitizer ion, is compatible in crystal lattices with other lanthanide ions, such as Eu and Tb, the usual activator atoms. 

Self-assembly of functionalized carboxylate-core dendrons around Er +, Tb +, or Eu + ions leads to the formation of dendrimers [19]. Experiments carried out in toluene solution showed that UV excitation of the chromophoric groups contained in the branches caused the sensitized emission of the lanthanide ion, presumably by an energy transfer Forster mechanism. The much lower sensitization effect found for Eu + compared with Tb + was ascribed to a weaker spectral overlap, but it could be related to the fact that Eu + can quench the donor excited state by electron transfer [20]. 

Figure 4 Cartoon illustrating sensitized emission from a lanthanide ion.

An alternative approach is that adopted by Horrocks and co-workers, where the aromatic residues in metal-binding proteins are used as sensitizers. Since the distance between the metal and the donor is effectively fixed, this provides a rigid scaffold for the experiment, and the absence of a directly conjugated pathway between the metals means that Forster (through space) energy transfer can be assumed. The r-6 distance-dependence of this means that the extent of sensitized emission from the lanthanide ion provides information on the spatial relationship between the metal-ion binding site (lanthanide ions often bind at Ca2+ sites) and nearby aromatic residues. 58-60... 

The photophysical properties of lanthanide ions are influenced by their local environment, the nature of the quenching pathways available to the excited states of sensitizing chromophores, and the presence of any available quenchers (as we have seen when discussing bioassay). All of these factors can be exploited for the sensing of external species. 

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... 

Addition of lanthanide ions to dendrimer solutions showed that (a) the absorption spectrum of the dendrimer is almost unaffected, (b) the fluorescence of the dansyl units is quenched (c) the quenching effect is very large for Nd3+ (Fig. 7) and Eu3+, moderate for Er3+ and Yb3+, small for Tb3+, and very small for Gd3+ (d) in the case of Nd3+ (Fig. 7), Er3+, and Yb3+ quenching of the dansyl fluorescence is accompanied by the sensitized near-infrared emission of the lanthanide ion [42]. 

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 nanostructure dependence of the excited state dynamics can be derived from the interaction of the electronic excitation with the surrounding environment and its phonon modes. A variety of nanophenomena, particularly, the lifetime of excited states of lanthanide ions in nanostructures may exhibit strong size-dependence (Prasad, 2004). Energy transfer rate and luminescence efficiency in lanthanide activated phosphors are also sensitive to particle size and surrounding environment. 

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