Electronic structure lanthanide ions, luminescence

Exclted states, primary processes Lanthanides, electronic structure Lanthanides, excited states Lanthanides, photochemistry Actinides, electronic structure ActlnldCs, excited states Uranyl ion, photochemistry Uranyl complexes, photochemistry Uranyl Ion, luminescence quenching Photochemistry, actinide alkyls... 

As discussed early in this chapter, quantum confinement has little effect on the localized electronic levels of lanthanide ions doped in insulating nanocrystals. But when the particle size becomes very small and approaches to a few nanometers, some exceptions may be observed. The change of lanthanide energy level structure in very small nanocrystals (1-10 nm) is due to a different local environment around the lanthanide ion that leads to a drastic change in bond length and coordination number. Lanthanide luminescence from the new sites generated in nanoparticles can be found experimentally. The most typical case is that observed in nanofilms ofEu Y203 with a thickness of 1 nm, which exhibits a completely different emission behavior from that of thick films (100-500 nm) (Bar et al., 2003). 

Table 2. Transition metal ions in doped halide lattices for which upconversion luminescence has been demonstrated, including relevant mechanistic and electronic-structural information. The lightest and heaviest lanthanides showing single-ion upconversion are also listed. Adapted from [17] ...

Applications of photophysics in biology and medicine are very extensive and only a few topics can be mentioned in this review. A survey of the use of lanthanide ions as luminescent probes of biomolecular structure and a general account of long distance electron transfer in proteins and model systems are very helpful. The methods applicable to the synthesis and activation of a number of photoactivable fluoroprobes have been described and photoactivation yields measured . 

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. 

The measurement of the absolute quantum yield has to be completed by the determination of the hfetimes tobs and the radiative lifetimes trad, giving access to the intrinsic d>Ln quantum yields, and overall sensitization efficiency jjsens-These values are sometimes neglected however, they allow a better understanding of the electronic structure of the ligand and therefore of the influence of the ligand for sensitizing the lanthanide ion. They can allow a better prediction of the properties of the complexes. For a better definition of each of these values, the reader is referred to Luminescence, and for a discussion of the influence of these parameters within the heUcate famihes, to the article of Biinzli et al ... 

The luminescence of Ln ion from the f-f transitions can be classified as two types of transitions the parity-allowed magnetic dipole transitions and the parity-forbidden electric dipole transitions. When the Ln " ion is inserted into a chemical enviromnent, noncentrosymmetric interactions allow the mixing of electronic states of opposite parity into the 4f wave functions, and electric dipole transitions become partly allowed. The intensity of some of these transitions is particularly sensitive to the nature of the metal ion environment, and these transitions are called hypersensitive transition a typical example is the Dq p2 transition of Eu [106]. Thus, the luminescence of lanthanide ions can provide valuable information about the local enviromnent and make them very suitable for acting as a structural probe deciphering the symmetry of the chemical environment and the coordination sphere. 

Although no quantum confinement should occur in the electronic energy level structure of lanthanides in nanoparticles because of the localized 4f electronic states, the optical spectrum and luminescence dynamics of an impurity ion in dielectric nanoparticles can be significantly modified through electron-phonon interaction. Confinement effects on electron-phonon interaction are primarily due to the effect that the phonon density of states (PDOS) in a nanocrystal is discrete and therefore the low-energy acoustic phonon modes are cut off. As a consequence of the PDOS modification, luminescence dynamics of optical centers in nanoparticles, particularly, the nonradiative relaxation of ions from the electronically excited states, are expected to behave differently from that in bulk materials. 

---