Evolution of Luminescent Supramolecular Lanthanide M2nL3n Complexes from Helicates and Tetrahedra to Cubes

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 Ln2nL3n (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.

含镧系元素的分子在材料科学与生物学领域拥有诸多潜在应用，例如发光传感/标记、磁共振成像（MRI）、磁制冷以及催化等。配位导向自组装在精准构建结构明确的超分子体系方面展现出强大的应用潜力。然而，由于镧系元素的配位数与配位几何构型复杂多变且极易波动，将该策略应用于镧系组装体仍颇具挑战。本文报道了一类Ln₂ₙL₃ₙ（其中Ln代表镧系元素，L代表有机配体，n=1、2、4）化合物在立体可控自组装过程中出现的灵敏结构切换现象。双三齿配体上两个螯合臂的偏移距离的系统性调控，决定了镧系组装体的最终产物类型，涵盖Ln₂L₃螺旋体、Ln₄L₆四面体以及Ln₈L₁₂立方体。尤为值得关注的是，本研究清晰观测到了可同时生成螺旋体与四面体混合物的临界反应条件。此外，本研究还证实了一种前所未有的立方结构Ln₈L₁₂配合物的浓度依赖型自组装行为。这类发光镧系立方体可作为性能优异的爆炸物检测淬灭型传感器，对苦味酸表现出极高的选择性与灵敏度。所有配合物均通过核磁共振波谱（NMR）、电喷雾电离飞行时间质谱（ESI-TOF-MS）以及单晶X射线衍射分析得到了表征确认。本研究结果为多核功能性镧系组装体的配位自组装提供了极具价值的设计准则。