Excited States and Luminescent Properties of Uranyl Carbonates and Their Hydrated Complexes in Aqueous Solution: A...

Understanding the luminescence properties of uranyl complexes is essential for predicting their coordination chemical behavior. This study systematically investigates the low-energy excited states of [(H₂O)_xUO₂(CO₃)_n]^2-2n (x = 0-3, n = 1-3) using CCSD(T) and CASPT2 methods. The results show that the ligand field exerts comparable effects on both the ground and lowest excited states (³Δ_g) at the scalar relativistic level, whereas the spin-orbit coupling effects on the luminescence spectral profile are minimal in (near-)saturated coordination environments. Based on these findings, key correlations are established: (1) between the equilibrium bond length of uranium and axial oxygen (U-O_ax) in the ground state (R_e,g) and U-O_ax bond length elongation upon excitation (ΔR_e), and (2) between R_e,g and the excitation energy difference (ΔΔE) of vertical and adiabatic one. These relationships are further validated in previously reported uranyl-dichloride-water and uranyl-difluoride-water systems. Leveraging these correlations, a predictive approach is developed to estimate the luminescence spectral parameters of structurally intricate complexes using the calculation results of simple uranyl compounds in the same primary-ligand coordination environment and only the R_e,g and the lowest vertical excitation energy of the intricate complexes, thus avoiding extensive computational cost. This predictive framework provides a robust tool for predicting the spectral properties of complex uranyl systems.