Investigation of the luminescence of mathrm{[UO₂X₄]²⁻} (X=Cl, Br) complexes in organic phase using time-resolved laser-induced fluorescence spectroscopy and quantum chemical simulations

The luminescence properties of the $\mathrm{[UO_2Cl_4]^{2-}}$ complex in an organic phase, especially the influence of large organic counter cations, have been studied by time-resolved laser-induced fluorescence spectroscopy (TRLFS) and ab initio modeling. The experimental spectrum was assigned by vibronic Franck-Condon calculations on quantum chemical models based on relativistic density functional approaches. The shape of the luminescence spectrum of the uranyl tetrachloride complex is determined by symmetrical vibrations and geometrical change upon emission. The possible change of the luminescence properties depending on the first and second uranyl coordination sphere was predicted theoretically for $\mathrm{[UO_2Br_4]^{2-}}$ and $\mathrm{[R_4N]_2[UO_2Cl_4]}$ ($\mathrm{R_4N}$ = $\mathrm{[Bu_4N]}$, $\mathrm{[{A336}]}$) systems. The computations reveal that for U(VI), the second coordination sphere has little influence on the spectrum shape, making speciation of uranyl complexes with identical first coordination-sphere ligands tedious to discriminate. The computed structural changes agreed well with experimental trends; theoretical spectra and peaks attribution are in a good accordance with TRLFS and magnetic circular dichroism (MCD) data respectively.