Absorption spectra of lanthanide complexes in solutions

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... 

Some salient features of the absorption spectra of lanthanide complexes in solutions.635... 

Many studies on the absorption spectra of lanthanides in alcoholic media have been made and the observations and anomalies have been explained in terms of entry of a chloride ion into the coordination sphere of the lanthanide ion. The composition and stability of halide complexes of lanthanides in alcohol and aqueous alcoholic solutions have been studied by spectral techniques. The halide ions have been found to cause marked changes in the spectra of lanthanides in alcoholic and aqueous media. The observed spectral changes may be attributed to changes in the immediate coordination environment of the lanthanide ion [223]. 

Berkelium exhibits both the hi and iv oxidation states, as would be expected from the oxidation states displayed by its lanthanide counterpart, terbium. Bk(iii) is the most stable oxidation state in non-complexing aqueous solution. Bk(iv) is reasonably stable in solution, undoubtedly because of the stabilizing influence of the half-filled 5f electronic configuration. Bk(iii) and Bk(iv) exist in aqueous solution as the simple hydrated ions Bk (aq) and Bk (aq), respectively, unless complexed by ligands. Bk(iii) is green in most mineral acid solutions. Bk(iv) is yellow in HCl solution and is orange-yellow in H2SO4 solution. A discussion of the absorption spectra of berkelium ions in soluticm can be found in Section 10.4.3. 

Comparison of electronic absorption spectra of the Ln-TTHA complexes in the solid state and in solution has shown that the monomeric species with Ln3+ coordination numbers 10 and 9 also occur in solution for the light and heavy lanthanides, respectively [39,41,43]. In addition, these studies suggest the presence of another species with one uncoordinated N-atom for the Nd3+ and Eu3+ systems. Absorption spectra [39,41,43],luminescence [45] and H Nuclear Magnetic Resonance Dispersion (NMRD) studies [46] have shown that oligomeric species also occur in solution, particularly below pH 5. 

As was stressed above, in case of direct excitation of the lanthanides, the differences in the absorption spectra between the solvated ion and the complex formed are usually considered as slight but allow nevertheless the use of spectrophotometry for speciation purposes (for a recent example, see Giroux et al. (2000)). In the case of U(VI), the absorption changes observed upon complexation have often been used to determine equilibrium constants (for the classical example of U(VI) hydrolysed species, see Dai et al. (1998), Meinrath (1998)). For curium, no information of this kind is available the low molar absorption coefficient implies the use of solutions with high total curium concentrations, which are almost impossible to handle nowadays due to safety reasons. The absorption spectra of curium are known only for aqueous solutions of HCIO4 (Carnall et al., 1958), HC1, H2SO4 and HNO3 (Pascal, 1962). 

The trivalent actinides, like the trivalent lanthanides, form only weak chloride complexes in aqueous solution, and although there is evidence of slight formation of anionic complexes in concentrated LiCl from anion exchange data, no anionic chloride complexes have previously been positively identified. Ryan and j0rgensen have recently prepared the trivalent lanthanide hexachloro and hexabromo complexes and studied their absorption spectra. This paper discusses preliminary results of the extension of this work to the trivalent actinides. 

UV-Vis absorption spectra were recorded on a JASCO V-550 spectrometer. Emission spectra of the lanthanide complexes were measured with a JASCO F-6300-H spectrometer and corrected for the response of the detector system. The intrinsic emission quantum yields (< n) of lanthanide complex solutions degassed with an argon (10 ttiM in toluene, chloroform, acetone, AA -dimethylformamide DMF) were obtained by comparison with the integrated emission signal... 

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