Lanthanide ions excitation

A good understanding of the way or paths of deexcitation of lanthanide ion excited states in complexes with simple ligands like water can throw light on the coordination sphere of the lanthanide ion. Techniques based on this principle have been used in the determination of the number of water molecules coordinated to the lanthanide ion [121]. 

Lanthanide ion" Excited state Transition energy (cm" ) Calculated t/(2) Actinide ion" Excited state Transition energy (cm" ) Calculated U(2)... 

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]. 

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. 

Stevens formalism turned out to be very powerful, and works easily as long as only the ground 2S+1Lj multiplet ofthe lanthanide ion is considered. As such, it has been widely used in studies on EPR properties of lanthanide-based inorganic systems [6, 22], while it is not well suited for optical spectroscopy. Indeed, when starting to include excited multiplets the Stevens formalism becomes much too involved. This is the reason why a more general formalism, developed by Wybourne [3], is of widespread use in optical studies - naturally dealing with excited multiplets - and... 

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...

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... 

It is also possible, however, to excite the lanthanide ions indirectly. This is done by building into the lattice another ion or group of ions that absorbs strongly the exciting radiation and, subsequently, transfer this energy to the lanthanide ions. 

Many of the trivalent lanthanide ions exhibit long-lived excited states. These excited states cannot be populated directly, since the lanthanide(III) ions themselves have very low absorption coefficients, due to the fact that the f-f transitions are formally forbidden by the LaPorte rule. In addition, a number of transitions are also forbidden by the spin crossover rule. Typically, these extinction coefficients are of the order of 1 M-1 cm-1 (20). 

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