Lanthanide complexes, phosphorescent

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. 

FIGURE 75 Schematic representation of energy absorption, emission, and dissipation processes in a bimetallic (R R ) lanthanide complex. F, fluorescence P, phosphorescence et, energy transfer r, radiative nr, nonradiative. 

Fig. 7 Schematic representation of energy absorption, migration, emission (plain arrows) and dissipation (dotted arrows) processes in a lanthanide complex. S or S = singlet state, T or T = triplet state, A = absorption, F = fluorescence, P = phosphorescence, k = rate constant, r = radiative, nr = nonradiative, IC = internal conversion, ISC = intersystem crossing, ILCT (or IL) = intra-ligand charge transfer, LMCT (or LM) = ligand-to-metal charge transfer. Back transfer processes are not drawn for the sake of clarity...

There is no reason why the same principle cannot be applied for light-emitting polymers as host materials to pave a way to high-efficiency solution-processible LEDs. In fact, polymer-based electrophosphorescent LEDs (PPLEDs) based on polymer fluorescent hosts and lanthanide organic complexes have been reported only a year after the phosphorescent OLED was reported [8]. In spite of a relatively limited research activity in PPLEDs, as compared with phosphorescent OLEDs, it is hoped that 100% internal quantum efficiency can also be achieved for polymer LEDs. In this chapter, we will give a brief description of the photophysics beyond the operation of electrophosphorescent devices, followed by the examples of the materials, devices, and processes, experimentally studied in the field till the beginning of 2005. 

The remaining lanthanides for which both the fluorescence of the cations and the fluorescence and phosphorescence of the complexes are weak. This behavior is the result of efficient singlet-to-triplet energy transfer and closely spaced energy levels in the cations which enhance nonradiative transitions. 

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