There are only few examples of spectroscopic studies on actinide(IV) compounds both in solution and solid state, principally because an absolute assignment of the electronic transitions in open shell tetravalent actinide compounds is not straightforward. In fact, for tetravalent actinide ions the involvement of 5f orbitals in chemical bonds makes the electronic structure very complex to investigate,3 although the Russell-Saunders coupling
scheme can be used as a good initial approximation.
In actinide(IV) complexes the position and the intensity of the f-f electronic transitions can be significantly influenced by the ligand field, the symmetry and the polarity of the solvent.3 An example explaining the influence of the symmetry is shown in Figure 1.5, where the energies of the electronic states of the free U(IV) ion change considerably moving to an octahedral environment.108
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Figure 1.5. (Left) A qualitative energy-level diagram for the free U4+ ion showing successively the effects of electrostatic repulsion and spin-orbit coupling; (right) an energy level diagram for
the U4+ ion in an octahedral crystal-field.108
Uranium(IV) represents a challenging model for its complex electronic structure; while
it has a strong spin-orbit interaction, its 5f orbitals are delocalized enough to create an LS- like situation due to mixing of the J = 5/2 and 7/2 levels, which are split due to relativistic effects.78 The spin-orbit coupling pushes the 5f electrons toward the j-j coupling scheme,
where the early actinides preferentially fill the J = 5/2 level and the later actinides fill the J = 7/2 level.109 However, in the case of uranium(IV), hybridisation results in increased
mixing of J = 7/2 character and hence reduced spin-orbit interaction. Therefore, while uranium follows LS coupling, it also has strong spin-orbit interaction, which is partially masked by the degree of delocalization of the 5f states.109
U(IV) organic complexes are generally known to be non-emissive. However, there are
few examples in the literature that have tried to elucidate the electronic structure of tetravalent uranium compounds by examining emission profiles. In the solid state, for example, for the doped systems U4+ in LiYF4,110ThCl4 and ThSiO4,111and Cs2ZrBr6,112
with a cubic geometry, the emission bands, observed in the visible region and originated from CT bands in the UV-vis region, were assigned to 5f16d1 → 5f2electronic transitions
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from the lowest excited state to different levels of the 5f2 manifold. These were produced from a redistribution of valence electrons in the 5f sub-shell, with a radiative lifetime of tens of nanoseconds at both room temperature and low temperature (77 K). In addition, for single crystals of LiYF4:U4+, an absorption band observed at 240 nm has seven
corresponding emission bands at 262, 282, 304, 328, 334, 438 and 492 nm.110 They were
assigned as transitions between the 3F2 (5f16d1) excited state and 3H4 (5f2manifold) ground state, with a radiative lifetime of approximately 17 ns at both 300 and 77 K.110
In this regard, Kiplinger et al.113showed that, if U(IV) complexes have ligands that
exhibit CT bands extended until the visible region, then the emission through the 5f orbital manifold is very difficult to observe, being in the picoseconds domain.
A remarkable example is the luminescence study on U4+ ion in aqueous perchlorate
medium reported by Kirishima et al.24b (Figure 1.6). The absorption band observed in the
UV region is assigned to the CT transition 3H4→1S0 and the corresponding emission bands
at 525, 409, 394, 345, 338, 320, 318, 291 and 289 nm as transitions from the 1S0 highest
lying state to lower lying states 1I6, 1G4, 3P0, 1D2, 3F3, 3F4 and 3H5, with a radiative lifetime
for each band of less than 20 ns at room temperature and 149 ns at 77 K.24c The emission
bands mirrored the absorption bands and the excitation spectrum of all emission bands was identical, suggesting that emission arises from de-excitation of the 1S0 state only. This
study underlines the fact that varying the nature of the medium (perchlorate, chloride and sulfate) leads to a small difference in the emission profile as consequence of a change in local symmetry and crystal-field effect. In other words, the spectroscopic profile of uranium(IV) complexes is influenced by the coordinating environment and solvent.
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Figure 1.6. Transition energies of the U4+ ion in aqueous perchlorate medium reported by
Kirishima.24b
It is noteworthy also the spectroscopic study conducted on simple U(IV) coordination compounds by Hashem et al.24a The absorption spectra of the compounds
[Li(THF)4][UX5(THF)] (X = Cl, Br, I), [Et4N]2[UCl6] and UCl4 were acquired in THF and
the electronic transitions were interpreted with the aid of computational methods, Figure 1.7. Excitation into a band of f-d and LMCT character, followed by energy transfer into the 5f-orbital manifold, leads to a radiative de-excitation observed in the UV-vis region of the emission spectrum.24a The emission bands were assigned to transitions from the excited
5f16d1 electronic configuration to a mixture of states arising from the ground state 5f2 electronic configuration.24a, 114 In summary, one can predict that any U(IV) compound,
that does not have CT bands extended in the visible region, could be emissive. A recent spectroscopic study has also reported unusual fluorescence properties from a Th(IV) complex, both in solution and solid state.115
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Figure 1.7. (Left), assignment of intra-configurational f–f transitions of [Li(THF)4][UCl5(THF)] in THF (insert shows bands in the region 1750–2000 nm); (right), emission spectra of (a) [Li(THF)4][UCl5(THF)] (λex = 303 nm), of (b) [Li(THF)4][UBr5(THF)] (λex = 325 nm), of (c)
[UCl4(THF)3] (λex = 290 nm), all measured in THF at 298 K.24a