Exploring excited state potential energy profile and luminescence properties of uranyl-based complexes by TRLFS and ab initio method
Résumé
Uranyl complexes have been the subject of many research works for fundamental chemistry of actinides, environmen-tal issues, or nuclear fuel cycle processes. The formation of various uranyl complexes, with organic and inorganic ligands in solution must be characterized for a better understanding of U(VI) speciation. As uranyl-ligand interactions and the symmetry of the complexes affect the electronic structure of U(VI) and thus its luminescence properties, time-resolved laser induced fluorescence spectroscopy (TRLFS) is one of the major techniques to characterize U(VI) com-plexes, with high sensitivity and selectivity. However, most of the relevant systems have complex chemical composi-tion in solution and the identification of each species from spectroscopic data is challenging.
In our study, the synergy between TRLFS and ab initio-based interpretation appears as a promising route for com-plexation data. Luminescence spectra of uranyl complexes in solution show in general a narrow energetical range about 6000 cm-1 and we can identify a single electronic transition between the initial and target states with the vibrationally resolved band [1]. The main challenge consists in exploiting a computationally cheap and effective theoretical ap-proach, in a relativistic context, to characterize the main spectral parameters of the ground and luminescent states of symmetrical uranyl compounds (i.e. UO2Cl42-, UO2F53-, UO2(CO3)34-, UO2(NO3)L2) with different organic or inorganic counter ions after the photo-excitation. We will illustrate that TD-DFT with the CAM-B3LYP functional is able to provide accurate excitation/emission energies for these systems, together with accurate vibronic progressions allowing the assignment of experimental data.
As a benchmark system serving the purpose of assessing the accuracy of our theoretical protocol, the uranyl tetrachloride UO2Cl42- was selected because of the extensive amount of structural and spectroscopic data available [2]. A good agreement was found between ours and previously obtained theoretical data (structural parameters, orbitals nature, excitation energies) [3]; the final luminescence spectrum is in remarkable agreement with our TRLFS measurements [4]. We will also quantify the effects of organic or inorganic counterions [5, 6], along with that of first-sphere ligands that might perturb the uranyl(VI) moiety.
This work showcases how one can predict vibrationally resolved spectra to assign the recorded TRLFS data, and shed light on the relationship between the uranyl coordination and its luminescence properties.
In our study, the synergy between TRLFS and ab initio-based interpretation appears as a promising route for com-plexation data. Luminescence spectra of uranyl complexes in solution show in general a narrow energetical range about 6000 cm-1 and we can identify a single electronic transition between the initial and target states with the vibrationally resolved band [1]. The main challenge consists in exploiting a computationally cheap and effective theoretical ap-proach, in a relativistic context, to characterize the main spectral parameters of the ground and luminescent states of symmetrical uranyl compounds (i.e. UO2Cl42-, UO2F53-, UO2(CO3)34-, UO2(NO3)L2) with different organic or inorganic counter ions after the photo-excitation. We will illustrate that TD-DFT with the CAM-B3LYP functional is able to provide accurate excitation/emission energies for these systems, together with accurate vibronic progressions allowing the assignment of experimental data.
As a benchmark system serving the purpose of assessing the accuracy of our theoretical protocol, the uranyl tetrachloride UO2Cl42- was selected because of the extensive amount of structural and spectroscopic data available [2]. A good agreement was found between ours and previously obtained theoretical data (structural parameters, orbitals nature, excitation energies) [3]; the final luminescence spectrum is in remarkable agreement with our TRLFS measurements [4]. We will also quantify the effects of organic or inorganic counterions [5, 6], along with that of first-sphere ligands that might perturb the uranyl(VI) moiety.
This work showcases how one can predict vibrationally resolved spectra to assign the recorded TRLFS data, and shed light on the relationship between the uranyl coordination and its luminescence properties.