Bifunctional Zniiln

The relaxation times, , for compounds 2, 8 and 5 were extracted from the fit of the frequency dependence of _M" at each temperature to the generalized Debye model (Figures S2-S3). In the case of compound 3, that does not exhibit any maximum in the _M" vs frequency plot (the same fit as for compounds 2, 8 and 5 does not lead to reliable values of ), the temperatures and frequencies of the maxima in the _M" vs T plot were used to extract the relaxation times. The results were then used in constructing the Arrhenius plot,  = ₀ exp(/k_BT), shown in Figures S2-S3. The fit of the high temperature linear portions of the data afforded the effective energy barriers for the reversal of the magnetization  and _o values indicated in Table 1. The Arrhenius plots, constructed from the temperatures and frequencies of the maxima observed for the "_M signals in Figures 2 and 3 for compounds 2, 5 and 8 lead virtually to the same  and _ (flipping rate) parameters, as expected. The fact that the out-of-phase susceptibility for these compounds tends to zero after the maxima is a clear indication that the QTM is suppressed. Only for compound 3 is the suppression of the QTM incomplete, which may be due to the existence of significant intermolecular interactions. In some cases, the effects of these interactions, that favour the fast QTM process, cannot be eliminated by application of a small magnetic dc field.

The Cole-Cole diagrams for these complexes are shown in Figure S4. The Cole-Cole plots for complexes 2, 5 and 8 are rather symmetrical and in the high temperature regions corresponding to the linear portion of the data in the Arrhenius plots they exhibit semicircular shapes with  values in the ranges 0.02 (8 K)-0.10 (5 K), 0.1(3 K)-0.15 (2.2 K), 0.06 (6.75 K)-0.19 (5 K) for 2, 5 and 8, respectively. These low values indicate very narrow distribution of slow relaxation in these regions, which would be compatible with the existence of only one relaxation process. In the case of 3 the Cole-Cole diagram is non-symmetrical with  values are in the range 0.06 (3 K)-0.29 (2.4 K), thus indicating that the crossover from the thermally activated relaxation process to a quantum regime occurs below ~2.6 K.

Table 1. and_values for compounds 2, 3, 5 and 8.

It should be noted that Er^III based SMMs are rare and, as far as we know, only three such examples have been reported. One of them, Na₉[Er(W₅O₁₈)₂]·xH₂O¹⁸ (the Er^III ion is encapsulated between two diamagnetic polyoxometalate cages) exhibits SMM behavior at zero field with a thermal energy barrier of 55.3 K. The other two are Zn₃Er complexes of very similar hexaimine planar macrocyclic ligands with six oxygen atoms bridging the Zn^II and Er^III ions.¹⁹ The former displays a flattened D_4d antiprismatic LnO₈ coordination sphere leading to a M_J = ± 13/2 ground state that is separated from the first excited state by ~30 cm^-1, whereas the other two have LnO₉^19a and suspected LnO₁₀^19b coordination spheres that conserve a strong equatorial ligand field. Er^III has prolate electron density^1a and, to generate single-ion anisotropy and SMM behavior, an equatorial ligand field is, in principle, required. The two Zn₃Er^III complexes are examples of systems having this kind of ligand field, whereas the Er-POM system exhibits a ligand field that can be considered as intermediate between axial and equatorial. For complexes 3 and 5, with LnO₉ coordination spheres very similar to that of 2 and 8, the absence of a clear axial field can allow an easy-axis anisotropy leading to the observed SMM behavior.

In the last few years, several examples of lanthanide complexes coordinated by compartmental Salen-type Schiff-base ligands have been reported to exhibit interesting photophysical properties. These ligands act as antenna groups, sensitizing Ln^III-based luminescence through an intramolecular energy transfer process.¹⁵ Considering the similarity between ligand H₂L and those Salen-derivates, the photophysical properties of samples 1 – 6 have been studied to determine the ability of ligand L^2- to act as sensitizer. The photophysical properties of complexes 7 – 13, which additionally contain 9-anthracene carboxylate ligands in their structure, have also been analyzed. The reflectance spectrum of ligand H₂L (Figure S5) shows intense absorption bands in the UV region located at 240 and 290 nm, which are typical of intraligand π-π^* electronic transitions in the aromatic groups. Excitation at 290 nm resulted in the appearance of a weak ligand-centered emission (Fig. S5, inset) with a maximum located at λ_em = 391 nm and a shoulder located at higher energy (λ_em = 365 nm).

The 9-Anthracene carboxylate ligand is a well-known luminophore and anthracene derivates have been previously used as antenna groups to sensitize Ln^III-based luminescence in the NIR region.²⁰ This ligand shows characteristic π-π^* absorption bands that extend well into the visible region and displays an intense and broad emission band centered at c.a. 500 nm.

In all cases, we examined the emissive properties of the complexes as microcrystalline powders, their poor solubility preventingdetailed study of their photophysical properties in solution.

Firstly, we examined the emissive properties of complexes 1 and 2 where the respective Tb^III and Dy^III ions are potential emitters in the visible region which can be sensitized by an energy transfer from the L^2- ligand. In both cases, excitation into the UV π-π^* absorption band of ligand L^2- at 290 nm resulted in the appearance of the characteristic Tb^III (⁵D₄→⁷F_J; J = 3, 4, 5, 6) and Dy^III (⁴F_9/2→⁶H_J/2; J = 15/2, 13/2) emission bands in the visible region respectively (Figure 4). This sensitized Ln^III-based emission can only occur through a L→Ln photoinduced energy transfer process, which probes the ability of ligand L^2- to act as an antenna group. However, a significant residual ligand-centered emission is still observed which indicates that the energy transfer process is not complete.

(a)	(b)

Figure 4. (a) Sensitized emission spectra of complexes 1 (green) and 2 (blue) in the solid state at room temperature. (b) Jablonski’s diagram of complexes 1 and 2; approximate energy values of S₁ and T₁ were determined from the UV-Vis absorption and emission spectra of ligand H₂L.

Similarly, the photophysical properties of samples 7 and 8 were studied. Both complexes contain the bridging ligand 9-anthracene carboxylate directly linked to the lanthanide ions. In these cases, excitation of the complexes into the UV absorption manifold of the 9-anthracene carboxylate unit (λ_ex. = 355 nm) did not result in sensitized Ln^III emission in the visible region. In both cases, only the characteristic emission of the anthracene moiety was observed. This is due to the fact that the energy of the emissive ³ππ^* state is lower than that of the emissive ⁵D₄ and ⁴F_9/2 excited states of ions Tb^III and Dy^III respectively.

This demonstrates that ligand L^2- sensitizes Ln^III-based emission, although the efficiency of the energy transfer process is rather low. To improve the NIR Ln^III-based emissive properties of this kind of molecule, the organic ligand 9-anthracene carboxylate was introduced as a bridging ligand in complexes 9 – 13. In addition, for samples 11 and 12, a second anthracene unit is directly chelated to the lanthanide ion. As expected, in all these cases, excitation at 355 nm resulted in the appearance of intense sensitized NIR emission from ions Er^III (9), Nd^III (12, 13) and Yb^III (10, 11) at their characteristic wavelengths (Figure 5).

(a)	(b)

These results confirm that the use of 9-Anthracene carboxylate ligands as bridging and/or chelate ligands directly coordinated to the Ln^III ions significantly improves their NIR luminescent properties.

We have demonstrated that the compartmental ligand H₂L (N, N’, N’’-trimethyl-N,N’’-bis(2-hydroxy-3-methoxy-5-methylbenzyl)-diethylenetriamine) allows the preparation of four series of dinuclear Zn^II-Ln^III complexes, in which the Zn^II ion occupies the internal N₃O₂ site and the oxophylic Ln^III ion occupies the external O₄ site, leading to diphenoxo-bridged species. The sixth position in the Zn^II coordination sphere is occupied by either a water molecule or the oxygen atom belonging to acetate, nitrate or 9-anthracenecarboxylate bridging groups, leading to doubly-bridged diphenoxo or triply bridged diphenoxo-acetate, diphenoxo-nitrate and diphenoxo-antracene carboxylate complexes, respectively. Dynamic ac magnetic susceptibility measurements as a function of temperature and frequency show that the Dy^III complexes 2 and 8 and the Er^III derivates 3 and 5 exhibit field-induced SMM behavior with effective thermal energy barriers of 41 K, 32.1 K, 11.7 K and 22 K respectively. These are larger than those found for isostructural complexes with paramagnetic 3d ions such as Ni^II, and Co^II, suggesting that the replacement of paramagnetic ions by diamagnetic ions is one strategy for increasing U_eff in 3d/4f systems. The increase in U_eff appears to due to the elimination of the weak magnetic exchange coupling between 3d and 4f ions that leads to a small energy separation between the ground and first excited state. Moreover, we believe that, as observed in other systems containing diamagnetic ions, the existence of a diamagnetic Zn^II ion linked to the Ln^III ions mitigates the intermolecular interactions between the Ln^III, thus diminishing the QTM process and favoring the observation of slow relaxation of the magnetization. Complexes 3 and 5 are very rare examples of Er^III-containing SMMs.

Finally, the photophysical properties of these complexes have been studied and the ability of the ligand L^2- to sensitize Tb^III and Dy^III-based luminescence in the visible region has been demonstrated. For complexes 1 and 2, excitation of the ππ^* absorption of the ligand results in concomitant sensitized emission in the visible region from the Tb^III and Dy^III units respectively. However, the efficiency of this ligand to sensitize the characteristic emission from Er^III, Nd^III and Yb^III ions in the NIR region is rather low due to the poor spectroscopic overlap existing between the ligand emission and the f-f excited states of these ligands. NIR emission in this kind of system was enhanced by introducing 9-Anthracene carboxylate luminophores as bridging and/or chelating ligands coordinated to the lanthanide ions.

Dy^III complexes 2 and 8 and Er^III complexes 3 and 5 combine field-induced SMM behavior and luminescent properties, and therefore can be considered as examples of dual magnetic-luminescent materials.

Elemental analyses for all the complexes, X-ray crystallographic data for 1, 3-13, including data collection, refinement and selected bond lengths and angles. Variable-temperature frequency dependence of the ac out-of-phase χ_M" signal for complexes 2, 5 and 8, Cole-Cole plots for complexes complexes 2, 3, 5 and 8, reflectance spectrum of the ligand and photoluminisence spectrum of compound 4. This material is available free of charge via the Internet at http://pubs.asc.org.

*Email: ecolacio@ugr.es

The authors declare no competing financial interest.

Financial support from the Spanish Ministerio de Ciencia e Innovación (MICINN) (Project CTQ-2011-24478), the Junta de Andalucía (FQM-195, the Project of excellence P11-FQM-7756), and the University of Granada is acknowledged. S. T.-P. thanks the Junta de Andalucía for a research grant. EKB thanks the EPSRC for funding.

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