  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.
