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.