Investigation of the Luminescence of [UO2X4]2- (X = Cl, Br) Complexes in the Organic Phase Using Time-Resolved Laser-Induced Fluorescence Spectroscopy and Quantum Chemical Simulations

The luminescence properties of the [UO₂Cl₄]^2- complex in an organic phase, especially the influence of large organic countercations, 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 methods on the basis of a combination of 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 in the luminescence properties depending on the first and second uranyl coordination spheres was predicted theoretically for the [UO₂Br₄]^2- and [R₄N]₂[UO₂Cl₄] ([R₄N] = [Bu₄N], [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 peak attributions are in good accordance with TRLFS and magnetic circular dichroism (MCD) data, respectively.