Lanthanide complexes ligand-absorption

There are difficulties associated with the use of ordinary electronic absorption spectra of lanthanide complexes in solution to provide detailed information regarding coordination number and geometry. However, difference spectra versus NdCl3 are reported for Nd3+-ligand (L) solutions for the 4/9/2— -4G5/2, 4G7,2 transitions (L = dipicolinate, oxydiacetate, iminodiacet-ate, malate, methyliminodiacetate and Ar,Ar -ethylenebis Af-(o-hydroxyphenyl)glycinate ). Hypersensitive behaviour was examined and transition dipole strengths were discussed in terms of the nature of the complex species present.431... 

A few observations of photosubstitution in lanthanide complexes have been reported. Irradiation into the f—f bands of [Pr(thd)3], [Eu(thd)3] and [Ho(thd)3] (thd is the anion of 2,2,6,6-tetramethyl-3,5-heptanedione) results in substitution of thd by solvent.153 The proposed mechanism involves intramolecular energy transfer from an f—f excited state to a reactive IL excited state which is responsible for the observed ligand loss. Photosubstitution has also been observed upon direct excitation into the ligand absorption bands of [Tb(thd)3].154... 

Compound 61 can be depicted in a cartoon manner as A in Scheme 6. Our idea was to form the linear f-d-f assembly (B) by simply using a Cu(II), Fe(II) or Zn(II) ion as the bridging unit where these would coordinate to the phen ligand and as such bring two of the lanthanide complexes together. While the formation of a self-assembly was successful, the desired structure (B in Scheme 6) was not exclusively formed. Using Cu(II) we showed that upon addition of 61, at pH 7.4, the Eu(III) emission was switched off. Moreover, the absorption spectra were shifted to the red and the singlet... 

The use of lanthanides are common for optical purposes because of their narrow and sharp bands, and distinguishable long lifetimes, accomparied by low transition probabilities due to the forbidden nature of the transitions [10-13]. Thus chromophoric sensitization of ligand to metal has been subjected to numerous theoretical and experimental investigations [14—16]. However, only limited classes of organic-lanthanide complexes have been developed and shown to display nonlinear processes [17-19]. Common nonlinear processes from lanthanide complexes include harmonic generation, photon up-conversion and multiphoton absorption induced emission. 

The above series in general is similar to the ligand nephelauxetic series observed in the case of d-transition metals. The greatest nephelauxetic effect has been observed in sulphides [46], cyclopentadienides [47], and oxides [48] of lanthanides. However, attempts to formulate a common and general nephelauxetic series for the lanthanide series have been futile using aquo ions as reference standards. In the case of lanthanide complexes with the same ligand in aqueous solutions, the absorption band positions of light and heavy lanthanides shift in different directions. This unusual behavior of complexes may be due to the differences in structure of aquo ions and the complexes. 

Empirical correlation of intensities of absorption bands with the structure of complexes in solutions have been made for lanthanide complexes. It has been recognized that forced electric-dipole transitions of low intensities, in some cases lower in intensity than those of magnetic-dipole transitions, may indicate that the ligand field has point group symmetry with a center of inversion. This criterion has been used in the determination of the ligand field by symmetry of Eu3+ aquo ion [202], The absorption band intensity ratios have been used to show the octahedral structure [49] of lanthanide hexahalide complexes, LnXg. ... 

Fluorine substitution in nucleosides and nucleic bases resulted in stronger interaction between lanthanides and ligands as compared with unsubstituted nucleosides and nucleic bases. The analysis of the fine structure of the bands, based upon the selection rules for the symmetry allowed transitions enabled the understanding of ligand field symmetry in Pr(III) and Nd(III) complexes of fluorinated nucleosides and nucleic bases. The stepwise complexation of Pr(III) Nd(III) was studied by the red shift of the absorption band which indicated a decrease in Ln-O distance in the complex due to the substitution of water by the fluorinated nucleoside ligand. 

The lighter lanthanides preferentially accumulate in the liver while heavier lanthanides prefer bone [157], This trend is particularly evident when lanthanide citrate complexes are administered. When lanthanides are introduced as chelate complexes, the absorption by the body is complete and the excretion rates also increase [158]. The excretion depends upon the stability of the chelate complex. If there is no exchange with the physiological ligands the excretion is rapid and complete. 

It has been very difficult to unambiguously prove which state is responsible for the energy transfer processes because of the lack of information regarding the emission from the excited states of the coordinated ligand and the difficulties in determining ligand-localized triplet-triplet absorption spectra of lanthanide complexes. All the experimental work conducted seemed to support case (a) in Figure 1.8. 

Dendrimer-type ligand (32) serves as a lanthanide container to exhibit on-off switchable luminescence upon lanthanide complexation in response to external anions [56]. Because of the presence of two classes of coordination sites for the lanthanide cations at the inner and outer spheres, the dendrimer 32 exhibits two different binding modes to afford on-off lanthanide luminescence, in which outer complexation at the tetradentate tripod site offers the on luminescence state upon quinoline excitation whereas, inner complexation at the multidentate core site corresponds to the off luminescence state. Upon complexation of 32 with Yb(CF3 SO3 )3, the quite weak NIR luminescence from the Yb(III) center suggests that the Yb(III) ion is most probably located at the inner coordination sites and apart from the excited quinoline moieties. Nevertheless, addition of SCN anion to the 32-Yb(CF3803)3 system induced remarkable spectral changes around the quinoline absorption band and about ninefold enhancement in luminescence intensity at around 980 nm. As the intense Yb luminescence appeared upon quinoline excitation, the employed SCN anion promoted the tripod-Yb +... 

Absorption and Luminescence Spectra of Lanthanide Complexes with Chelating Nitronyl Nitroxide Ligands... 

Figure 2c shows the near-infrared luminescence spectrum of [Gd(hfac)3NIT-BzImH] compared to its lowest-energy absorption band system. At 5 K, both spectra show well-resolved structure that is similar to the patterns observed for the uncoordinated radical, as summarized in Tables 1 and 2. The corresponding electronic transitions can be observed for many other complexes of lanthanide or d-block metal ions with radical ligands [24-27, 30]. In general, the spectra for lanthanide complexes are very similar to those of the uncoordinated radicals. 

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