The Series of Rare Earth Complexes [Ln2Cl6(μ-4,4′-bipy)(py)6], Ln=Y, Pr, Nd, Sm-Yb: A Molecular Model System for Luminescence Properties in MOFs Based on LnCl3 and 4,4′-Bipyridine

A series of 12 dinuclear complexes [Ln(2)Cl(6)(-4,4-bipy)(py)(6)], Ln=Y, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, (1-12, respectively) was synthesized by an anhydrous solvothermal reaction in pyridine. The complexes contain a 4,4-bipyridine bridge and exhibit a coordination sphere closely related to luminescent lanthanide MOFs based on LnCl(3) and 4,4-bipyridine. The dinuclear complexes therefore function as a molecular model system to provide a better understanding of the luminescence mechanisms in the Ln-N-MOFs [Ln(2)Cl(6)(4,4-bipy)(3)]2(4,4-bipy). Accordingly, the luminescence properties of the complexes with Ln=Y, Sm, Eu, Gd, Tb, Dy, (1, 4-8) were determined, showing an antenna effect through a ligand-metal energy transfer. The highest efficiency of luminescence is observed for the terbium-based compound 7 displaying a high quantum yield (QY of 86%). Excitation with UV light reveals typical emission colors of lanthanide-dependent intra 4f-4f-transition emissions in the visible range (Tb-III: green, Eu-III: red, Sm-III: salmon red, Dy-III: yellow). For the Gd-III- and Y-III-containing compounds 6 and 1, blue emission based on triplet phosphorescence is observed. Furthermore, ligand-to-metal charge-transfer (LMCT) states, based on the interaction of Cl- with Eu-III, were observed for the Eu-III compound 5 including energy-transfer processes to the Eu-III ion. Altogether, the model complexes give further insights into the luminescence of the related MOFs, for example, rationalization of Ln-independent quantum yields in the related MOFs.