Lanthanide nonradiative processes

In addition to mastering the various processes leading to electronic excitation of the lanthanide ions, one has to prevent excited states to de-excite via nonradiative processes. The overall deactivation rate constant, which is inversely proportional to the observed lifetime r0bs, is given by ... 

Examples of studies on multiphoton absorption processes and nonlinear second-and third-harmonic generation processes will be discussed along with some possible radiative and nonradiative processes. The selection rules for multiphoton absorption will be mentioned in Section 7.3, and molecular examples will be shown along with their correlating photophysical properties in Section 7.4. The effect of some parameters relating to second-order activity along the lanthanide... 

Keywords Apphcations of luminescence Charge transfer Divalent lanthanide Electronic transitions Lanthanide luminescence Nonradiative processes Upconversion... 

Some other important characteristics of the emission are the rate of the deactivation of the excited state and the rate of the radiative deactivation. If we measure a time-resolved emission spectrum of the emission, we will observe that the emission spectrum loses some intensity as a function of time after a pulsed excitation. This emission decay is usually monoexponential and corresponds to the rate constant of the deactivation of the excited state, or observed deactivation rate constant kobs- It is important here not to confuse this rate constant with the rate constant of the radiative deactivation (in Figure 8, k and for the fluorescence and phosphorescence rate constants of the ligand, respectively, for the radiative rate constant of the lanthanide). Despite the fact that this method measures the decay of the emission, between each time step, the nonradiative processes (the k deactivation rate constants in Figure 8) also deactivate the excited state. To better visualize the decay rates, some equations are helpful. 

The lower vibrational energy of the OD bond relative to the OH bond is responsible for the lower quenching of the excited lanthanide in the deuterated solvent (more OD than OH vibrations are needed to deactivate the same excited state). The vibrational relaxation needs several vibrational quanta of the quenching molecule in order to deactivate an excited state and particularly a lanthanide excited state. Both the match between the vibrational quanta and the excitation energy, and the number of vibrational quanta required to achieve such a relaxation define the efficiency of this nonradiative process. The more quanta, the less efficient the deactivation, because of the selection rules for vibrational transitions. 

The photochemical process will in general not occur in non-molecular solids due to the restriction in the nuclear coordinates. However, the nonradiative return to the ground state does, using MMCT states as an intermediary [35]. This can be nicely illustrated on a molecular solid, viz. the lanthanide-decatung-states which contain complex ions [Ln(III)Wio036] [40, 121]. 

Room-temperature NIR emission has also been reported for the di-urethanesils Ut(600)3-Er(Otf)3 and Ut(900) Nd(Otf)3 ( = 80,60), indicating that this hybrid framework protects the lanthanide ions from nonradiative deactivation processes equally efficiently when compared with the di-ureasils (Carlos et al., 2004). In fact, the most noticeable difference between the Nd111-doped di-ureasils and di-urethanesils is the energy difference between the undoped host and the doped hybrids which, in the case of Ut(600) , for instance, is concentration dependent (Gongalves et al., 2005). 

Nonradiative transitions between the 4f levels of lanthanide ions are caused by multiphonon processes. In the case of band emissions, the quantum efficiency is commonly interpreted by the Mott s model. It should be noted that Struck and Fonger have shown that in fact an unified model can be used for these two types of emission. ... 

Figure 2.18 Schematic representation of photophysical processes in lanthanide(III) complexes (antenna effect). A = absorption, F = fluorescence, P = phosphorescence, L = lanthanide-centred luminescence, ISC = intersystem crossing, ET = energy transfer S = singlet, T = triplet. Full vertical lines radiative transitions dotted vertical lines nonradiative transitions...

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. 

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