Hierarchical Self-Assembly and Chiroptical Studies of Luminescent 4d-4f Cages

Multinuclear lanthanide-containing supramolecular cages have received increasing attention recently because of their unique electroptical and magnetic properties. Here we report the hierarchical self-assembly and chiroptical studies of a group of 4d-4f heterometallic cages synthesized from a preformed dimetalloligand [(bpy)₂Pd₂1₂]²⁺ (2) (bpy = 2,2-bipyridine) and a variety of trivalent lanthanide ions (Ln = Nd^III, Eu^III, Yb^III). The programmable self-assembly process leading to the trigonal bipyramidal cages formulated as {Ln₂[(bpy)₂Pd₂1₂]₃}¹²⁺ (3) has been confirmed by one- and two-dimensional NMR, electro-spray-ionization time-of-flight mass-spectroscopy, and in one typical case by single-crystal X-ray diffraction studies. Circular dichroism and circular polarized luminescence spectra confirmed the strict control of stereoselectivity on the heterometallic cages, dictated by the chiral amide groups on the ligands. Excitation (up to 420 nm) on the dipalladium chromophores on these cages leads to the characteristic lanthanide luminescence at both the visible and the near-infrared regions, depending on the lanthanide ions used. Through the assembly-disassembly process, luminescent turn-off sensing toward penicillin among several widely used antibiotics has also been demonstrated with the Europium cage, featuring a limit of detection as low as 0.88 ppb (S/N = 3). Our results pave the way for the construction of chiral 4d-4f supramolecular cages which may find potential applications in luminescent sensing and/or labeling reagents.