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

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 (3Δg) 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-Oax) in the ground state (Re,g) and U-Oax bond length elongation upon excitation (ΔRe), and (2) between Re,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 Re,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.