Orbital Engineering Mediated by Cation Conjugation in Luminescent Uranyl-Organic Hybrid Materials

A series of compounds of the form [HAr]₂ [UO₂ X₄ ] is reported here, wherein Ar is systematically varied between pyridine (1-X), quinoline (2-X), acridine (3-X), 2,5-dimethylpyrazine (4-X), quinoxaline (5-X), and phenazine (6-X), and X=Cl or Br. With greater conjugation in the organic cation, a larger quenching in uranyl luminescence is observed in the solid state. Supporting our luminescence experiments with computation, we map out the potential energy diagrams for the singlet and triplet states of both the [HAr]⁺ cations and [UO₂ Cl₄ ]^2- anion in the crystalline state, and of the assembly. The distinct energy transfer pathways in each compound are discussed.