Lanthanide ions lifetimes

It should be emphasized that, because small molecules in usual solvents have diffusion coefficients <10 cm2 s-1, the rapid diffusion limit can be attained only for donors with lifetimes of 1 ms. This is the case for lanthanide ions for instance, the lifetime of Tb3+ chelated to dipicolinate is 2.2 ms. Stryer and coworkers (1978) showed that using Tb3+ as a donor and rhodamine B as an acceptor, the concentration of rhodamine B resulting in 50% transfer was 6.7 x 10 6 M, which is three orders of magnitude less than the concentration corresponding to 50% transfer in the static limit. 

By redesigning the above acyclic podand-type ligand 3 into a cyclic cryptate, the issue of stability can be resolved resulting in kinetically stable complexes (Scheme 4) [102]. The Tb(III) and Eu(III) complexes of cryptate 5 show an increase in lanthanide emission lifetimes of 0.72 ms and 0.41 ms, respectively, upon excitation at 310 nm. Similar results are found with the phenanthroline analogue 6 with Eu(III). A large number of modifications of these cryptates have been reported, all showing enhancements in the lanthanide ion emission [103-106]. 

Experimental lifetimes (ps) of some lanthanide ions in various solvents (from Kimura et al. (2001b)). Experimental conditions T = 25 °C, perchlorate salt, [R(III)] = 10 2 M. Theoretical radiative lifetimes (rra(j, in ps) from Carnall... 

The nanostructure dependence of the excited state dynamics can be derived from the interaction of the electronic excitation with the surrounding environment and its phonon modes. A variety of nanophenomena, particularly, the lifetime of excited states of lanthanide ions in nanostructures may exhibit strong size-dependence (Prasad, 2004). Energy transfer rate and luminescence efficiency in lanthanide activated phosphors are also sensitive to particle size and surrounding environment. 

Following the introduction to size-dependent nanophenomena presented in the previous sections, we now focus our attention on the luminescence properties of lanthanide ions at additional sites or distorted structure existing in nanophases. Phenomena of prolonged luminescence lifetime, anomalous thermalization, upconversion luminescence, dynamics of long-range interaction with two-level-systems (TLS), and quantum efficiency are to be discussed. 

Luminescence lifetime depends upon radiative and nomadiative decay rates. In nanoscale systems, there are many factors that may affect the luminescence lifetime. Usually the luminescence lifetime of lanthanide ions in nanociystals is shortened because of the increase in nomadiative relaxation rate due to surface defects or quenching centers. On the other hand, a longer radiative lifetime of lanthanide states (such as 5Do of Eu3+) in nanocrystals can be observed due to (1) the non-solid medium surrounding the nanoparticles that changes the effective index of refraction thus modifies the radiative lifetime (Meltzer et al., 1999 Schniepp and Sandoghdar, 2002) (2) size-dependent spontaneous emission rate increases up to 3 folds (Schniepp and Sandoghdar, 2002) (3) an increased lattice constant which reduces the odd crystal field component (Schmechel et al., 2001). 

Energy transfer, particularly, phonon-assisted energy transfer processes must be considered in evaluating lanthanide luminescence decays, because they contribute in many cases to a major part of the observed lifetime. Based on the theoretical models described in section 3, we have conducted Monte Carlo simulations of energy transfer and its effect on luminescence decay for lanthanide ions in nanociystals and compared the calculated results with experi-... 

Although Bhargava s mistakes on the shortening of TM lifetime or lanthanide-doped ZnS have been pointed out by other researchers, many scientists still expect that lanthanide-doped II-VI semiconductor nanocrystals may form a new class of luminescent materials. Numerous papers on the luminescence of II-VI semiconductor nanocrystals doped with TM or lanthanide ions have appeared in an effort to achieve high efficient luminescence via ET from II-VI host to lanthanide ions. 

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