Bifunctional ZnIILnIII Dinuclear Complexes Combining Field Induced SMM Behavior and Luminescence: Enhanced NIR Lanthanide Emission by 9‑Anthracene Carboxylate Bridging Ligands

There were new dinuclear ZnII–LnIII complexes of general formulas [Zn­(μ-L)­(μ-OAc)­Ln­(NO3)2] (LnIII = Tb (1), Dy (2), Er (3), and Yb (4)), [Zn­(μ-L)­(μ-NO3)­Er­(NO3)2] (5), [Zn­(H2O)­(μ-L)­Nd­(NO3)3]·2CH3OH (6), [Zn­(μ-L)­(μ-9-An)­Ln­(NO3)2]·2CH3CN (LnIII = Tb (7), Dy (8), Er (9), Yb­(10)), [Zn­(μ-L)­(μ-9-An)­Yb­(9-An)­(NO3)3]·3CH3CN (11), [Zn­(μ-L)­(μ-9-An)­Nd­(9-An)­(NO3)3]·2CH3CN·3H2O (12), and [Zn­(μ-L)­(μ-9-An)­Nd­(CH3OH)2(NO3)]­ClO4·2CH3OH (13) prepared from the reaction of the compartmental ligand N,N′,N″-trimethyl-N,N″-bis­(2-hydroxy-3-methoxy-5-methylbenzyl)­diethylenetriamine (H2L), with ZnX2·nH2O (X = NO3– or OAc–) salts, Ln­(NO3)3·nH2O, and, in some instances, 9-anthracenecarboxylate anion (9-An). In all these complexes, the ZnII ions invariably occupy the internal N3O2 site whereas the LnIII ions show preference for the O4 external site, giving rise to a Zn­(μ-diphenoxo)­Ln bridging fragment. Depending on the ZnII salt and solvent used in the reaction, a third bridge can connect the ZnII and LnIII metal ions, giving rise to triple-bridged diphenoxoacetate in complexes 1–4, diphenoxonitrate in complex 5, and diphenoxo­(9-anthracenecarboxylate) in complexes 8–13. DyIII and ErIII complexes 2, 8 and 3, 5, respectively, exhibit field induced single molecule magnet (SMM) behavior, with Ueff values ranging from 11.7 (3) to 41(2) K. Additionally, the solid-state photophysical properties of these complexes are presented showing that ligand L2‑ is able to sensitize TbIII- and DyIII-based luminescence in the visible region through an energy transfer process (antenna effect). The efficiency of this process is much lower when NIR emitters such as ErIII, NdIII, and YbIII are considered. When the luminophore 9-anthracene carboxylate is incorporated into these complexes, the NIR luminescence is enhanced which proves the efficiency of this bridging ligand to act as antenna group. Complexes 2, 3, 5, and 8 can be considered as dual materials as they combine SMM behavior and luminescent properties.