Emission of lanthanides

Initially, interest for NIR emission of lanthanide ions stemmed from the development of optical libers, lasers and amplifiers for telecommunication (Kido and Okamoto, 2002 Kuriki et al., 2002) and there are a wealth of theoretical and technical papers published in this area. Up-conversion processes have also been the subject of intense investigations (Auzel, 2004). These two areas of research and development mostly deal with purely inorganic compounds or, more recently, with luminescent polymers they will not be covered in this chapter, with the exception of the latter, which will be partly described. 

Because of the optical quality and chemical stability of crystals, the low probability of non radiative processes and the wide transmission range, fluorides are the most appropriate materials for solid state lasers with specific wavelengths. Thus the 4f - 4f line emissions of lanthanide ions have been used in order to obtain infrared laser radiation up to 4.34 fxm and blue or green radiation by up-conversion pumping. Tunable laser operation in the ultraviolet has been demonstrated using the broad 5d 4f emission of Ce3+. Tunable lasers in the UV or IR ranges have also been experimented using Ag+, Pb+, 3d ions. 

Table 3.14 Summary of the main visible emissions of lanthanide ions, and typical intensities of their visible emission (Andres and Chauvin 2012)...

To analyze the influence of high hydrostatic pressure on the emission of lanthanide ions, the semiempirical crystal-field approach is used where the total Hamiltonian of the 4f system is given by the sum of a free ion and the crystal-field terms as follows [113]. 

Light Amplification hy Stimulated Emission of Lanthanide Radiation—Laser 142... 

LIGHT AMPLIFICATION BY STIMULATED EMISSION OF LANTHANIDE RADIATION—LASER... 

The use of strongly absorbing d-block chromophores as sensitizers for the NIR emissions from lanthanide ions has attracted increasing attention since the first report by van Veggel and coworkers on the Nd + and Yb + luminescence sensitized by Ru(bpy)j+ and ferrocene in 2000. Subsequently, many d-block chromophores, based on related transition metals, such as R +, Ru +, Re+, Os +, Pd, Zn +, Cr +, and Co +, have been extensively studied in sensitizing the NIR emissions of lanthanide ions (i.e., Nd +,... 

Ward and coworkeis (Shavaleev et al., 2003a) showed that the near-infrared emission of lanthanide complexes can be sensitized by visible light excitation of the electronic transitions localized on the d-metal metal chromophore of heterodinuclear d-f complexes,... 

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]. 

Homogeneous Time Resolved Fluorescence (HTRF) (Cisbio International) is an assay based on the proximity of a lanthanide cryptate donor and a fluorescent acceptor molecule whose excitation wavelength overlaps that of the cryptate s emission. The utility of this technique is based on the time resolved fluorescence properties of lanthanides. Lanthanides are unique in the increased lifetime of their fluorescence decay relative to other atoms, so a delay in collection of the emission intensity removes the background from other fluorescent molecules. An example of the HTRF assay is a generic protein-protein interaction assay shown in Fig. 2. 

The sol-gel co-immobilization of a non-fluorescent blue indicator bromothymol blue (BTB) with an europium (Ill)-complex intense antenna mediated lanthanide dye represents a new scheme for the fluorescence analysis38. Luminescence spectra of europium (Ill)-complex shown in Figure 12 were found to be independent of pH changes in the range 1-10. Therefore, BTB, a non-fluorescent pH indicator with alkaline absorption maximum close to main europium emission band was added to the sol-gel mixture to shield reversibly the emission of the europium (Ill)-complex at different pH s without quenching of the antenna function. 

Lanthanide chelates also can be used in FRET applications with other fluorescent probes and labels (Figure 9.51). In this application, the time-resolved (TR) nature of lanthanide luminescent measurements can be combined with the ability to tune the emission characteristics through energy transfer to an organic fluor (Comley, 2006). TR-FRET, as it is called, is a powerful method to develop rapid assays with low background fluorescence and high sensitivity, which can equal the detection capability of enzyme assays (Selvin, 2000). 

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). 

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