Enhanced luminescence of tris(carboxylato)uranyl(vi) complexes and energy transfer to Eu(iii): a combined spectroscopic and theoretical investigation

Complex formation between uranyl and carboxylate ligands (benzoate, nicotinate and isonicotinate) has been studied extensively by absorption and luminescence spectroscopy in acetonitrile medium. Experimental data had indicated the existence of stable and enhanced luminescent tris(carboxylato) uranyl(vi) complexes i.e. [UO2(L)(3)](-) with D-3h symmetry. The high luminescence of these complexes was due to the sensitization of the O-yl -> U ligand to metal charge transfer (LMCT) emission by extremely intense equatorial (carboxylate ligands) LMCT bands. The variation in the experimentally observed parameters such as intensity of equatorial LMCT bands, luminescence lifetimes, quantum yields and structural parameters among tris(carboxylato) uranyl(vi) complexes are affirmed by quantum chemical calculations using density functional theory and the computational results are found to be in good agreement with experimental findings. Interestingly, in a very dilute mixture of [UO2(L)(3)](-) and Eu(iii), energy transfer from uranyl to Eu(iii) is observed and it leads to the detection of europium at trace levels. This is an intriguing observation as none of the previous studies have reported such a low level of detection limit of Eu(iii) by means of energy transfer from any metal.

Automated summary

This study investigated the complex formation between uranyl and carboxylate ligands and found stable and enhanced luminescent tris(carboxylato) uranyl(vi) complexes. Quantum chemical calculations verified the experimental results, and an energy transfer from uranyl to Eu(iii) was observed, enabling the detection of europium at trace levels.