Ultrafast Transient Absorption Spectroscopy of UO₂²⁺ and [UO₂Cl]⁺

Abstract

For the only water coordinated "free" uranyl(VI) aquo ion in perchlorate solution we identified and assigned several different excited states and showed that the Δ state is the luminescent triplet state from transient absorption spectroscopy. With additional data from other spectroscopic methods (TRLFS, UV/vis) we generated a detailed Jabłoński diagram and determined rate constants for several state transitions, like the inner conversion rate constant from the Φ state to theΔ state transition to be 0.35 ps. In contrast to luminescence measurements, it was possible to observe the highly quenched uranyl(VI) ion in highly concentrated chloride solution by TAS and we were able to propose a dynamic quenching mechanism, where chloride complexation is followed by the charge transfer from the excited state uranyl(VI) to chloride. This proposed quenching route is supported by TD-DFT calculations.

Quenching Mechanism of Uranyl(VI) by Chloride and Bromide in Aqueous and Non-Aqueous Solutions

“…Previously, we were not able to differentiate between the association step of the second chloride ion and the redox step. 23 We can now clearly conclude from our kinetic data that at first the second stage complex forms, which then undergoes the actual redox step to uranyl(V) and the dihalide radical X 2…”

“…From the spectra, it is visible that both ions seem to be quantum-mechanically well separated (typical dichloride radical peak visible as if it would be fully solvated), which indicates a somewhat own solvation sphere for each ion in the complex (solvent-separated ions). Furthermore, the fact that ACN has a much lower dielectric constant but is still quite high compared to many other solvents (Gould and Farid calculated that SSRIPs dominate in ACN 34 ) and our previously found theoretical evidence for an outer-sphere complex formation 23 speak for the radical ion pair existing in a solvent-separated (SSRIP) configuration. We assume that these findings are consistent with the luminescence data in acetone (Figure S5, vide supra), where luminescence is as well present due to the formation of an SSRIP in the low dielectric medium acetone (ε = 21.01).…”

“…As one can see in the free uranyl(VI) spectrum (0 M chloride), the TA spectroscopy of uranyl(VI) is determined by two major features: After excitation by UV radiation a broad absorption appears, which we previously assigned as the absorption from the 3 Φ into the 3 Γ state of the uranyl(VI). 23 A few picoseconds later, the signal is replaced via internal conversion by an absorption spectrum with a defined vibronic structure, which is blue-shifted in comparison to the absorption from the 3 Φ state. This absorption was previously assigned to the absorption due to a transition from the 3 Δ state into the 3 Π state of the uranyl(VI) and is the energetically lowest excited state from which the uranyl(VI) luminescence with several microsecond lifetime occurs.…”

Transient absorption spectroscopy

“…This transition from a more outer-lying MO to the metal center AO leads to an asymmetric U-O bond elongation on one side of the molecule, reducing its symmetry [65]. The 3 Δ state has been identified as the origin of the luminescence of uranyl(VI) in water [66] -a property that is known for uranium ores and compounds for more than hundred years [67]. This emission is present at around 500 nm, depending on the observed complex or mineral, giving it a green color.…”

“…As expected, this hot-band disappears when cooling the sample as the population of this first vibronic state vanishes. Higher excited states are the 1 Δ, 1 Φ, 3 Π, and 3 Γ that can be reached via excitations in the UV range [66]. Both absorption and emission are sensitive to the ligand or crystal field surrounding the ion.…”