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

There were new dinuclear Zn^II–Ln^III complexes of general formulas [Zn­(μ-L)­(μ-OAc)­Ln­(NO₃)₂] (Ln^III = 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 (Ln^III = 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 Zn^II ions invariably occupy the internal N₃O₂ site whereas the Ln^III ions show preference for the O₄ external site, giving rise to a Zn­(μ-diphenoxo)­Ln bridging fragment. Depending on the Zn^II salt and solvent used in the reaction, a third bridge can connect the Zn^II and Ln^III metal ions, giving rise to triple-bridged diphenoxoacetate in complexes 1–4, diphenoxonitrate in complex 5, and diphenoxo­(9-anthracenecarboxylate) in complexes 8–13. Dy^III and Er^III complexes 2, 8 and 3, 5, respectively, exhibit field induced single molecule magnet (SMM) behavior, with U_eff values ranging from 11.7 (3) to 41(2) K. Additionally, the solid-state photophysical properties of these complexes are presented showing that ligand L^2‑ is able to sensitize Tb^III- and Dy^III-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 Er^III, Nd^III, and Yb^III 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.