Bifunctional Zn<sup>II</sup>Ln<sup>III</sup> Dinuclear
Complexes Combining Field Induced SMM Behavior and Luminescence: Enhanced
NIR Lanthanide Emission by 9‑Anthracene Carboxylate Bridging
Ligands
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Abstract
There
were new dinuclear Zn<sup>II</sup>–Ln<sup>III</sup> complexes
of general formulas [Zn(μ-L)(μ-OAc)Ln(NO<sub>3</sub>)<sub>2</sub>] (Ln<sup>III</sup> = Tb (<b>1</b>), Dy (<b>2</b>), Er (<b>3</b>), and Yb (<b>4</b>)), [Zn(μ-L)(μ-NO<sub>3</sub>)Er(NO<sub>3</sub>)<sub>2</sub>] (<b>5</b>), [Zn(H<sub>2</sub>O)(μ-L)Nd(NO<sub>3</sub>)<sub>3</sub>]·2CH<sub>3</sub>OH (<b>6</b>), [Zn(μ-L)(μ-9-An)Ln(NO<sub>3</sub>)<sub>2</sub>]·2CH<sub>3</sub>CN (Ln<sup>III</sup> =
Tb (<b>7</b>), Dy (<b>8</b>), Er (<b>9</b>), Yb(<b>10</b>)), [Zn(μ-L)(μ-9-An)Yb(9-An)(NO<sub>3</sub>)<sub>3</sub>]·3CH<sub>3</sub>CN (<b>11</b>), [Zn(μ-L)(μ-9-An)Nd(9-An)(NO<sub>3</sub>)<sub>3</sub>]·2CH<sub>3</sub>CN·3H<sub>2</sub>O
(<b>12</b>), and [Zn(μ-L)(μ-9-An)Nd(CH<sub>3</sub>OH)<sub>2</sub>(NO<sub>3</sub>)]ClO<sub>4</sub>·2CH<sub>3</sub>OH (<b>13</b>) prepared from the reaction of the compartmental
ligand <i>N,N</i>′<i>,N</i>″-trimethyl-<i>N,N</i>″-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine
(H<sub>2</sub>L), with ZnX<sub>2</sub>·<i>n</i>H<sub>2</sub>O (X = NO<sub>3</sub><sup>–</sup> or OAc<sup>–</sup>) salts, Ln(NO<sub>3</sub>)<sub>3</sub>·<i>n</i>H<sub>2</sub>O, and, in some instances, 9-anthracenecarboxylate anion (9-An).
In all these complexes, the Zn<sup>II</sup> ions invariably occupy
the internal N<sub>3</sub>O<sub>2</sub> site whereas the Ln<sup>III</sup> ions show preference for the O<sub>4</sub> external site, giving
rise to a Zn(μ-diphenoxo)Ln bridging fragment. Depending on
the Zn<sup>II</sup> salt and solvent used in the reaction, a third
bridge can connect the Zn<sup>II</sup> and Ln<sup>III</sup> metal
ions, giving rise to triple-bridged diphenoxoacetate in complexes <b>1</b>–<b>4</b>, diphenoxonitrate in complex <b>5</b>, and diphenoxo(9-anthracenecarboxylate) in complexes <b>8</b>–<b>13</b>. Dy<sup>III</sup> and Er<sup>III</sup> complexes <b>2</b>, <b>8</b> and <b>3</b>, <b>5</b>, respectively, exhibit field induced single molecule magnet
(SMM) behavior, with <i>U</i><sub>eff</sub> 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<sup>2‑</sup> is able to sensitize Tb<sup>III</sup>- and Dy<sup>III</sup>-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<sup>III</sup>, Nd<sup>III</sup>, and Yb<sup>III</sup> 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 <b>2</b>, <b>3</b>, <b>5</b>, and <b>8</b> can be considered as dual materials
as they combine SMM behavior and luminescent properties
