Coordination-Driven Nanosized Lanthanide “Molecular Lantern ” with Tunable Luminescent Properties

Coordination-driven self-assembly of discrete nanosized molec-ular cages has become one of the most active areas of supra-molecular chemistry.1 Over the past decade, extensive studies have been made on discrete molecular assemblies MxLy with various values of x and y.2 Among these metal-ligand clusters, the assembly of M2L4 tetragonal cage remains quite rare, although a few such d-block metal supramolecular complexes have appeared in the literature recently.3 Canonical symmetric Ln2L4 lanthanide cages have, to the best of our knowledge, never been described. This is clearly due to inherent difficulties in harmonizing the subtle relationship between the versatile coordination modes of the lanthanide metals (coordination numbers usually g8) and the ligand conformations in the synthetic cage systems. Lanthanide discrete cage-like assemblies, however, are of considerable interest in broad scientific areas, especially as the luminescent materials. We have recently reported a series of coordination polymers and supramolecular complexes based on bent five-membered hetero-atom-ring-bridged ligands.4 By taking into account the bent geometry of such spacers, we wondered if the five-membered heteroatom ring-bridged 3,3′-biphenylcarboxylate type ligands could be used as an “organic clip”5 to bind lanthanide ions into discrete molecular cages, especially the Ln2L4 tetragonal cages. Depending on this ligand-directed approach,3 a series of lan-thanide nanosized tetragonal cages Ln2L14 (Ln(III) ) La (1), C