Tunable 4f/5f Bimodal Emission in Europium-Incorporated Uranyl Coordination Polymers

There have been numerous studies on emission-color regulation by the adjustment of molar amounts of multiple trivalent lanthanide cations, such as Eu³⁺, Tb³⁺, Dy³⁺, and others, in many types of solid host materials. Although uranyl emission originating from charge-transfer transitions has been well-recognized and investigated for many decades, as of now there is no report on tunable 4f/5f bimodal emission based on heterobimetallic lanthanide(III) and uranyl(VI) compounds. In most cases, complete energy transfer between uranyl(VI) and lanthanide(III) centers was observed. In this work, a series of isotypic-europium-incorporated uranyl coordination polymers, Eu@UO₂L(DMF) (L^2- = 3,5-pyridinedicarboxylate, denoted as 1-10, which represent the different Eu contents in UO₂L(DMF); DMF = N,N-dimethylformamide), has been synthesized by solvothermal reactions. Crystallographic evidence of this series unveiled one-dimensional chains of UO₂²⁺ as pentagonal-bipyramidal units bridged by pyridinedicarboxylate with no defined, crystallographically unique site containing Eu, even for the products with high concentrations of Eu in this series. However, emission bands characteristic of Eu³⁺ were clearly observed in every product along with the characteristic uranyl-emission feature when observed with UV-vis fluorescence spectroscopy. Laser-ablation inductively coupled plasma mass spectrometry indicated that europium was concomitant with uranium, corroborating the incorporation of europium into crystals of UO₂L(DMF). Systematic control of the solvent ratio (V_H/V_DMF) in each reaction gives rise to an enrichment of Eu³⁺ in the interior of UO₂L(DMF). In addition, the color of emission of these compounds changed significantly from bright red to bright green with decreasing Eu content. This phenomenon occurs from the highly efficient energy transfer between the UO₂²⁺ and Eu³⁺ centers within each sample, providing the first case of a tunable 4f/5f bimodal emission in a mixed 4f/5f-elements-bearing metal-organic-hybrid material.