Ultrafast Transient Absorption Spectroscopy of UO₂²⁺ and [UO₂Cl]

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 ³Δ 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 ³Φ state to the ³Δ state transition to be 0.35 ps^-1. 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.