Sandwich heterometallic coordination polymers consisting of copper-cluster pillars and layered networks of {Ln6} wheels: synthesis, structures, spectroscopic properties and Judd–Ofelt analysis

Two novel 3-D Ln(III)–Cu(I) heterometallic coordination polymers, [Ln2Cu2(L3)4(C2O4)2]·H2O (Ln = Sm, Eu; HL3 = 5-aminonicotinic acid), were hydrothermally produced by reacting lanthanide oxide and copper halide with 5-aminonicotinic acid and orotic acid. In the two isostructural complexes, {Ln6}-wheel clusters link each other via edge-sharing mode, constructing honeycomb-like layers which stack in –ABAB– mode. On the whole, the architectures can be described as sandwich framework constructed by {Ln6}-wheel layers pillared by interlamellar [CuOL32] triangle pillars. The electronic structure was theoretically calculated by the density functional theory, confirming the direct transition character. The steady and transient photoluminescent spectra were measured, from which the intensity parameters, radiative lifetime and quantum efficiency were calculated within the framework of the Judd–Ofelt theory. [Eu2Cu2(L3)4(C2O4)2]·H2O can be regarded as a potential candidate for LED applications.