Lanthanide excited states

Lanthanides also have potential as DEFRET energy donors. Selvin et al. have reported the use of carbostyril-124 complexes (53) with europium and terbium as sensitizers for cyanine dyes (e.g., (54)) in a variety of immunoassays and DNA hybridization assays.138-140 The advantage of this is that the long lifetime of the lanthanide excited state means than it can transfer its excitation energy to the acceptor over a long distance (up to 100 A) sensitized emission from the acceptor, which occurs at a wavelength where there is minimal interference from residual lanthanide emission, is then measured. 

Due to the competing non-radiative decay routes for the lanthanide excited state, there is an intrinsic limit to the overall quantum yield in luminescent lanthanide complexes. It has been estimated that these values are 0.50 and 0.75 for europium and terbium, respectively (27). Although quantum yields exceeding these have been reported (31,32), care should be taken in analyzing quantum yield results in the literature, as these are often given for the energy transfer process alone, and not the overall quantum yield, and in other cases it is unclear as to which process(es) the quoted quantum yield refers to. 

Recently, two cryptands and their lanthanide complexes have been synthesized which include either a bipyridyl (L56) or pyridyl (L57) chromophore (89). These have proved effective at populating the lanthanide excited states. Aqueous luminescence lifetimes of up to... 

Spontaneous emission and radiative lifetime of lanthanide excited state in condensed phases is determined by the electromagnetic field and the index of refraction as shown in eq. (3). In nanocrystals, spontaneous emission of photons is influenced by two mechanisms (1) the non-solid medium surrounding the nanoparticles that changes the effective index of refraction thus influences the radiative lifetime (Meltzer et al., 1999 Schniepp and Sandoghdar, 2002), (2) size-dependent spontaneous emission rate due to interferences (Schniepp and Sandoghdar, 2002). 

As described previously, nonradiative decay due to solvent interactions can severely reduce lanthanide luminescence through energy dissipation by vibronic modes, with the O—H oscillator being the most common and eflBcient quencher. However, if these O—H oscillators are replaced with lower-frequency O—D oscillators, the eflBciency of vibronic deactivation decreases substantially. Therefore, the rate constants for luminescence lifetimes (th o) of lanthanide excited states in water or alcoholic solvents are often much shorter than those in analogous deuterated solvents (td o)- This property can be utilized to determine the degree of solvation for luminescent lanthanides. 

Poole, R.A., Kielar, R, Richardson, S.L., et al. (2006) A ratiometric and non-enzymatic luminescence assay for uric acid differential quenching of lanthanide excited states by anti-oxidants. Chemical Communications, 4084-4086. 

Laser flash photolysis has also been used to study the photophysics and photochemistry of metallocene-containing cryptands and their complexes with rare earth eations [69]. The metallocene moiety was shown to act as an efficient centre for the radiationless deactivation of the lanthanide excited state. Detailed time-resolved studies permitted the characterisation of the coordination chemistry about Dy " within the cryptate and showed, once again, that the functions within the host cryptand primarily responsible for coordination of the guest cation were the amide carbonyl groups. 

A study has been made of the emission of some related Tb and Eu macrocylic complexes, immobilized in a sol-gel glass, which is made pH-dependent either by perturbing the energy of the aryl singlet or triplet state, or by modulating the degree of quenching of the lanthanide excited state. The effect of bicarbonate chelation on the polarized luminescence from chiral... 

Obviously, many structural variations are possible in the design of ferrocene oxa-aza cryptands. Some of the oxa-azaferrocene cryptands form alkali metal and lanthanide complexes [90]. FAB mass spectrometry experiments have shown that the cryptands have strong selectivity for the potassium cation compared with Li+, Na, or Cs" " [94], In these complexes the macrocycle functions as a host, but in Mg + complexes the cation is coordinated by the amide carbonyl groups [95]. In the lanthanide complexes the metallocene moiety acts as an efficient center for radiationless deactivation of the lanthanide excited state [96]. 

Once the lanthanide excited state has formed, luminescence is not an inevitable outcome. Non-radiative quenching of the excited state can occur through vibrational modes of the array, and particularly through vibrational quenching by X-H oscillators in the ligand backbone or in aqueous media. To optimize emission from the lanthanide it is necessary to minimize the non-radiative quenching pathways... 

Photosensitized ligand reactions by lanthanide excited states. 

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... 

The sensitization efficiency is calculated by measm-ing the observed lifetime of the lanthanide excited state, the... 

The observed hfetime usually significantly depends on the temperature (whereas the radiative lifetime only depends on the small variation of the retractive index as a function of temperature if the coordination is unaffected by the lower temperature). This dependence comes from the fact that some nonradiative deactivations of the lanthanide ion are vibrationally assisted (i.e., needs some heat to take place, e.g., back-transfer to the triplet state of the ligand). Nevertheless, the observed lifetime of the lanthanide does not depend on the excitation wavelength. A direct excitation through an f-f transition or through a sensitizer results in the same observed lifetime. In other words, this hfetime, and thus the deactivation of the lanthanide excited state only depends on the chemical environment and on the temperature, not on how this excitation was achieved. 

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 triplet state of the hgand has to be located not too far in energy from one of the excited state of the lanthanide ions, in order to have an optimal rate of the energy transfer yet not too close to avoid energy back transfer from the lanthanide excited state to the triplet state, which would drastically decrease the luminescence intensity. 

In the case of tetra-methyl-l,2-dioxetane [(6.75)], the luminescence originates from triplet acetone which is formed directly from the starting material. In the presence of oxygen, the decomposition reaction is first order in dioxetane in the absence of oxygen in hydrocarbon solvents, the destruction of dioxetane is second order in dioxetane and appears to be autocatalytic in triplet acetone. Impurities in hydroxylic solvents (presumably transition metal ions) can catalytically quench the luminescence however, in the presence of lanthanide ions, the chemiluminescence is quite efficient, with emission occurring from lanthanide excited states. 

Fluorescence intensities and lifetimes of lanthanide excited states in H O... 

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