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

• Published in: Chemistry : a European journal Vol. 19; no. 51; pp. 17369 - 17378
• Main Authors: Matthes, Philipp R., Nitsch, Jörn, Kuzmanoski, Ana, Feldmann, Claus, Steffen, Andreas, Marder, Todd B., Müller-Buschbaum, Klaus
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 16.12.2013; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Charge transfer; Chemistry; Dysprosium; Emission; Energy transfer; Erbium; Gadolinium; lanthanides; Ligands; Luminescence; Metal-organic frameworks; Optical properties; Phosphorescence; photoluminescence; Praseodymium; Pyridines; rare earths; Salmon; Terbium; Ultraviolet radiation; Ytterbium; Yttrium; energy transfer; photoluminescence; lanthanides; metal-organic frameworks; rare earths
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201302504

A series of 12 dinuclear complexes [Ln2Cl6(μ‐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 LnCl3 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 ${\hbox{}{{\hfill 2\atop \hfill \infty }}}$[Ln2Cl6(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 (TbIII: green, EuIII: red, SmIII: salmon red, DyIII: yellow). For the GdIII‐ and YIII‐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 EuIII, were observed for the EuIII compound 5 including energy‐transfer processes to the EuIII 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. Rare Ln complex versus MOF: A series of 12 new dinuclear rare earth complexes [Ln2Cl6(μ‐4,4′‐bipy)(py)6] (in which Ln=Y, Pr, Nd, Sm–Yb) is presented based on LnCl3, 4,4′‐bipyridine, and pyridine (see figure, LMET=ligand‐to‐metal energy transfer, GYOR=green, yellow, orange, red; B = blue; IL = intra ligand). Their photoluminescence properties were extensively studied and compared to the related metal–organic framework (MOF) model systems [Ln2Cl6(μ‐4,4′‐bipy)4]⋅2(4,4′‐bipy), Ln=Eu, Gd, Tb.