Evolution of Luminescent Supramolecular Lanthanide M_2<i>n</i>L_3<i>n</i> Complexes from Helicates and Tetrahedra to Cubes

Abstract

Lanthanide-containing molecules have many potential applications in material science and biology, that is, luminescent sensing/labling, MRI, magnetic refrigeration, and catalysis among others. Coordination-directed self-assembly has shown great power in the designed construction of well-defined supramolecular systems. However, application of this strategy to the lanthanide edifices is challenging due to the complicated and greatly labile coordination numbers and geometries for lanthanides. Here we demonstrate a sensitive structural switching phenomenon during the stereocontrolled self-assembly of a group of Ln_2nL_3n (Ln for lanthanides, L for organic ligands, and n = 1, 2, 4) compounds. Systematic variation of the offset distances between the two chelating arms on the bis(tridentate) ligands dictated the final outcomes of the lanthanide assembly, ranging from Ln₂L₃ helicates and Ln₄L₆ tetrahedra to Ln₈L₁₂ cubes. Remarkably, the borderline case leading to the formation of a mixture of the helicate and the tetrahedron was clearly revealed. Moreover, the concentration-dependent self-assembly of an unprecedented cubic Ln₈L₁₂ complex was also confirmed. The luminescent lanthanide cubes can serve as excellent turn-off sensors in explosives detection, featuring high selectivity and sensitivity toward picric acid. All complexes were confirmed by NMR, ESI-TOF-MS, and single crystal X-ray diffraction studies. Our results provide valuable design principles for the coordination self-assembly of multinuclear functional lanthanide architectures.