The interaction of rare earth chlorides with 4,4′-bipyridine for the reversible formation of template based luminescent Ln-N-MOFs

Abstract

By solvent free syntheses of the rare earth trichlorides LnCl₃, Ln = Pr, Nd, Sm, Eu, Tb with melts of 4,4′-bipyridine two-dimensional frameworks of the formula ²_∞[Ln₂Cl₆(4,4′-bipy)₃]·2(4,4′-bipy) are obtained, with 4,4′-bipy = C₁₀H₈N₂, 4,4′-bipyridine. 4,4′-Bipyridine acts both as a linker ligand as well as a template and populates all cavities in the structures. The template can be evaporated at temperatures >200 °C giving a new high temperature compound with a yet unknown structure. Further heating results in the release of the remaining linking equivalents of the ligand and reformation of LnCl₃. Thus the reaction can be run in cycles if the evaporated ligand is collected. Luminescence in the visible range without quenching by concentration is observed for the lanthanides Eu and Tb, which is identified for Eu^III with the transitions ⁵D₀→⁷F_J and ⁵D₄→⁷F_J for Tb^III. The hybrid character of the material is reflected by the additional strong antenna effect of the ligand: main excitation is observed via the 4,4′-bipyridine linkers followed by an energy transfer to the metal centres. It is remarkable that both the template containing MOFs as well as the new high temperature compounds exhibit luminescence properties.