Field-Induced SMM and Vis/NIR Luminescence on Mononuclear Lanthanide Complexes with 9-Anthracenecarboxylate and 2,2 (cid:48) :6,2 -Terpyridine

: Five new mononuclear lanthanide complexes are synthesized by adding the several lanthanide nitrate hexahydrate salts, which for lanthanide (Ln) are Eu, Tb, Dy, Er, and Yb, with 9-anthracenecarboxylic acid (9-Hanthc) and 2,2 (cid:48) :6,2 (cid:48)(cid:48) -terpyridine (TPY) in mixed solution of methanol and dimethylformamide (DMF). The general formula is [Eu(9-anthc) 3 (TPY)(DMF)] · H 2 O ( 1Eu ) where Eu(III) is ennea-coordinated or [Ln(9-anthc) 3 (TPY)(H 2 O)] · H 2 O · DMF (Ln = Tb ( 2Tb ), Dy ( 3Dy ), Er ( 4Er ), and Yb ( 5Yb )) where Ln(III) is octa-coordinated. For compounds 3Dy , 4Er , and 5Yb , the dynamic ac magnetic study indicated ﬁeld-induced single molecule magnet (SMM) behavior. The photoluminescence studies in the solid state of these complexes show the sensitization of 4 f -4 f transitions for 4Er and 5Yb in the NIR region.


Introduction
In recent years, coordination complexes with lanthanide ions have been extensively studied because of their interesting magnetic and luminescent properties. The magnetic interest arises from the high magnetic momentum of the lanthanide ions and strong magnetic anisotropy of its ground state widely used in the molecular magnetism area to design single-molecule magnets (SMMs) that provide potential in information storage, quantum computation, and spintronics [1][2][3][4][5][6][7][8]. In SMMs lanthanide compounds, the energy needed to reverse the magnetization, and the gaps energy between the emission contributions, result in the sublevel structure of the Stark components. In addition, the magnetic relaxation occurs through different process such as Orbach one, direct relaxation, Raman mechanism, or Quantum Tunneling of the Magnetization (QTM). In many cases, these processes can occur conjunctly. Therefore, to optimize the SMMs characteristics, the involved mechanism in the magnetic relaxation and the factors that affect them have to be understood. One of the used strategies to better understand the magnetic properties is to take advantage of the correlation between the magnetic and optical properties of the luminescent SMM compounds. Photoluminescent studies may allow spectroscopic detection of the energy splitting of the m J states of Ln(III) that can be compared with the magnetic results obtained [9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25].
Regarding the optical aspect, Ln(III)-based materials manifest exceptional luminescence characteristics, with high emission quantum yields, narrow bandwidths, ligandinduced large Stokes shifts, long-lived emission, and ligand-dependent luminescence

X-ray Crystal Structures
Complexes 1Eu-5Yb present two different structures depending on the atomic radii of the lanthanide ion. For the largest Eu(III), the lanthanide ion is ennea-coordinated and presents a type I structure. On the other hand, for small lanthanides, the coordination is eight and presents a type II structure. As complexes 2Tb-5Yb are isostructural, only the structures of compounds 1Eu and 4Er will be discussed in detail. The most relevant parameters for single crystal determination are collected in Table 1 in the Materials and Methods section.

(TPY)(DMF)]·H 2 O (1Eu)
Complex 1Eu of formula [Eu(9-anthc) 3 (TPY)(DMF)]·H 2 O crystallizes in the monoclinic space group P2 1 /c. A partially labeled plot of the Eu(III) mononuclear complex is presented in Figure 1a. Selected bond distances are listed in Table 2. The structure of 1Eu consists of [Eu(9-anthc) 3 (TPY)(DMF)] molecules in which each Eu(III) ion is ennea-coordinated with a EuN 3 O 6 coordination environment formed by a tridentate TPY ligand, two chelating 9-anthc ligands, one monodentate 9-anthc ligand, and a coordinated DMF solvent molecule. Three of the coordination sites around the central atom are occupied by the N1, N2, and N3 atoms of a TPY ligand, with Eu-N bond distances equal to 2.552 (3), 2.606 (3), and 2.545 (3) Å, respectively. In addition, the Eu(III) central atom is coordinated to five oxygen atoms from three different 9-anthc ligands. Two of the 9-anthc ligands show a chelating coordination mode with Eu1-O1 and Eu1-O2 bond distances of 2.421 (3)      The estimation of the coefficients of distortion of the EuN 3 O 6 coordination polyhedron for 1Eu in reference to the ideal nine vertex polyhedral was made by employing the continuous shape measure theory and SHAPE software [51,52], showing that the N 3 O 6 disposition for 1Eu is intermediate between different coordination polyhedra. The best SHAPE estimation led to Muffin (MFF-9, C 2v ), spherical-capped square antiprism (CSAPR-9, C 4v ), and spherical-tricapped trigonal prism (TCTPR-9, D 3h ) geometries with continuous shape measure (CShM) values of 1.678, 2.006, and 2.407, respectively. A graphical representation of the metal coordination geometry of 1Eu is shown in Figure 1b.
The coordination of [Eu(9-anthc) 3 (TPY)(DMF)] forms a hydrogen bond with a molecule of water through the hydrogen atom H1W and the oxygen atom O6 of the monodentate 9anthc ligand (Supplementary Materials, Table S2). In addition, each [Eu(9-anthc) 3 (TPY)(DMF)] molecule forms a π-π type interaction with a neighboring molecule between the ring containing the N1 atom (Cg(1) from the Supplementary Materials, Table S3) and the one containing the N2 atom (Cg(2) from the Supplementary Materials, Table S3) of the respective TPY ligands. In addition, the mononuclear compound also forms an intermolecular interaction of the same nature with another neighboring molecule via two anthracene groups of the two corresponding 9-anthc ligands containing O1-O2 oxygens (contact Cg(4)-Cg(4)' from the Supplementary Materials, Table S3). The sum of the two intermolecular contacts forms chains of [Eu(9-anthc) 3   The estimation of the coefficients of distortion of the EuN3O6 coordination polyhedron for 1Eu in reference to the ideal nine vertex polyhedral was made by employing the continuous shape measure theory and SHAPE software [51,52], showing that the N3O6 disposition for 1Eu is intermediate between different coordination polyhedra. The best SHAPE estimation led to Muffin (MFF-9, C2v), spherical-capped square antiprism (CSAPR-9, C4v), and spherical-tricapped trigonal prism (TCTPR-9, D3h) geometries with continuous shape measure (CShM) values of 1.678, 2.006, and 2.407, respectively. A graphical representation of the metal coordination geometry of 1Eu is shown in Figure 1b.
The coordination of [Eu(9-anthc)3(TPY)(DMF)] forms a hydrogen bond with a molecule of water through the hydrogen atom H1W and the oxygen atom O6 of the monodentate 9-anthc ligand (Supporting information, Table S2). In addition, each [Eu(9anthc)3(TPY)(DMF)] molecule forms a π-π type interaction with a neighboring molecule between the ring containing the N1 atom (Cg(1) from the Supplementry Information,Table S3) and the one containing the N2 atom (Cg(2) from the Supplementary Information, Table S3) of the respective TPY ligands. In addition, the mononuclear compound also forms an intermolecular interaction of the same nature with another neighboring molecule via two anthracene groups of the two corresponding 9-anthc ligands containing O1-O2 oxygens (contact Cg(4)-Cg(4)' from the Supporting Information, Table S3). The sum of the two intermolecular contacts forms chains of [Eu   Complexes 2Tb-5Yb with formula [Ln(9-anthc) 3 (TPY)(H 2 O)]·H 2 O·DMF (Ln = Tb (2Tb), Dy (3Dy), Er (4Er), and Yb (5Yb)) crystallize in the monoclinic space group P21/n. Selected bond distances for compounds 2Tb-5Yb are listed in Table 1. As an example of the structural type II, 4Er will be described below ( Figure 3).
The structure of compound 4Er consists of [Er(9-anthc) 3 (TPY)(H 2 O)] molecules in which each Er(III) ion is octa-coordinated. The Er(III) ion has an ErN 3 O 5 octa-coordination environment formed by a tridentate TPY ligand, a chelating 9-anthc ligand, two monodentate 9-anthc ligands, and a water molecule. The octa-coordinated environment of the Er(III) ion corresponds to four oxygen atoms of three different 9-anthc ligands (O1 and O5 from two monodentate 9-anthc ligands and O3 and O4 from the chelating 9-anthc ligand, with Er-O average distance of 2.314 Å), three nitrogen atoms of the TPY ligand (N1, N2 and N3, with an average Er-N distance of 2.492 Å), and the oxygen atom from the coordinated water molecule (O7, Er1-O7 bond distance of 2.344 Å).
The estimation of the coefficient of distortion of the ErN3O5 coordination polyhedron for 4Er in reference to the ideal eight vertex polyhedral was made by employing the continuous shape measure theory and SHAPE software and shows that the N3O5 distribution in 4Er is intermediate between different coordination polyhedra. The best SHAPE estimation led to triangular dodecahedron (TDD-8, D2d), biaugmented trigonal prism (BTPR-8, C2v), or Johnson-biaugmented trigonal prism (JBTP-8, C2v) geometries with continuous shape measure (CShM) values of 2.342, 2.426, and 2.516, respectively. A graphical depiction of the metal coordination geometry of 4Er is shown in Figure 3b. The result of the estimation of the coefficient of distortion of the LnN3O5 coordination polyhedron for 2Tb-5Yb in reference to the ideal eight vertex polyhedral are reported in Table S1 in the Supplementary Information.
Compound 4Er presents an intramolecular hydrogen bond between H7A from the water molecule coordinated to Er(III) and O6 from one of the 9-anthc ligands coordinated in the monodentate mode. On the other hand, the other hydrogen of this H2O molecule, H7B, forms another H bond with the crystallization water molecule (O1W). At the same time, both hydrogen atoms of this water molecule also form hydrogen bonds, H1WA with O2 of a monodentate 9-anthc ligand and H1WB with O1D from a DMF crystallization molecule ( Figure 4). The contact distances and angles for compounds 2Tb-5Yb are listed in Table S2 from the Supporting Information.  Table S1 in the Supplementary Materials.
Compound 4Er presents an intramolecular hydrogen bond between H7A from the water molecule coordinated to Er(III) and O6 from one of the 9-anthc ligands coordinated in the monodentate mode. On the other hand, the other hydrogen of this H 2 O molecule, H7B, forms another H bond with the crystallization water molecule (O1W). At the same time, both hydrogen atoms of this water molecule also form hydrogen bonds, H1WA with O2 of a monodentate 9-anthc ligand and H1WB with O1D from a DMF crystallization molecule ( Figure 4). The contact distances and angles for compounds 2Tb-5Yb are listed in Table S2   The compounds also present π-π stacking interactions between different aromatic rings. One of them is given within each molecule [Ln(9-anthc)3(TPY)(H2O)] between the TPY ligand rings (Cg(1) from the Supporting Information, Table S3) and the anthracene group of the monodentate ligand containing the oxygen atoms O1 and O2 (Cg (6)'or Cg (7)' from the Supporting Information, Table S3). The intermolecular contacts vary slightly according to the Ln(III) ion. In the case of the Tb(III) and Dy(III) compounds, the TPY rings of each mononuclear unit interact with two different neighbor molecules via the corresponding terpyridine ligands. Thus, there is an interaction between the ring that contains The compounds also present π-π stacking interactions between different aromatic rings. One of them is given within each molecule [Ln(9-anthc) 3 Table S3) and the anthracene group of the monodentate ligand containing the oxygen atoms O1 and O2 (Cg (6) or Cg (7) from the Supplementary Materials, Table S3). The intermolecular contacts vary slightly according to the Ln(III) ion. In the case of the Tb(III) and Dy(III) compounds, the TPY rings of each mononuclear unit interact with two different neighbor molecules via the corresponding terpyridine ligands. Thus, there is an interaction between the ring that contains the N1 atom with the ring that contains the N2 atom of a neighboring molecule (Cg (1)-Cg (2) from the Supplementary Materials, Table S3) and another supramolecular contact of the TPY ligand ring that contains the N3 nitrogen with the same ring of another adjacent molecule (contact Cg (3)-Cg (3) from the Supplementary Materials, Table S3). On the other hand, the compounds [Ln(9-anthc) 3 (TPY)(H 2 O)] with Ln (III) = Er and Yb have a single supramolecular contact type π-π, which is also through the TPY ligands, aromatic ring containing the N1 atom of the reference molecule with the ring containing the nitrogen N3 of the neighboring molecule (Supplementary Materials, Figure S1).

Structural Discussion
The key to obtaining the structural types I and II for the described compounds by using the same synthetic method is the contraction of the ionic radius within the series of the lanthanide elements. As described, for the Eu(III) ion, with the largest ionic radius, the nona-coordinate coordination sphere is due to the coordination of three carboxylate ligands 9-anthc, two in chelating mode and one in monodentate mode, and for the coordination of a DMF molecule. For the Tb(III), Dy(III), Er(III), and Yb(III) ions, the compounds have been obtained with an octa-coordinate environment since the decrease in ionic radius causes two of the three 9-anthc anions to be coordinated in monodentate mode and the last to be coordinated in chelating mode. This structural change is also accompanied by the coordination of a molecule of water instead of a DMF molecule. At 300 K, the χMT values for compounds 1Eu-5Yb are 1.16, 11.55, 12.15, 9.52, and 2.05 cm 3 mol −1 K, respectively. These data are in good agreement with the expected room temperature values for the corresponding Ln(III) centers [53]. In the case of the 1Eu compound, the non-zero value of χMT reveals the presence of excited states that are very close in energy to the fundamental one [54]. For compounds 2Tb and 4Er, the χMT values remain practically constant as the samples are cooled to T ≈ 150 K. Below this temperature, the susceptibilities gradually decrease to values of χMT = 9.12 cm 3 mol −1 K a T = 11 K for 2Tb and χMT = 7.68 cm 3 mol −1 K a T = 30 K for 4Er. From these temperatures, the fall of the χMT values accelerates to reach minimum values equal to 5.69 cm 3 mol −1 K for 2Tb and 4.70 cm 3 mol −1 K for 4Er at T = 2 K. For the 3Dy complex, it is observed that the χMT values are At 300 K, the χ M T values for compounds 1Eu-5Yb are 1. 16, 11.55, 12.15, 9.52, and 2.05 cm 3 mol −1 K, respectively. These data are in good agreement with the expected room temperature values for the corresponding Ln(III) centers [53]. In the case of the 1Eu compound, the non-zero value of χ M T reveals the presence of excited states that are very close in energy to the fundamental one [54]. For compounds 2Tb and 4Er, the χ M T values remain practically constant as the samples are cooled to T ≈ 150 K. Below this temperature, the susceptibilities gradually decrease to values of χ M T = 9.12 cm 3 mol −1 K a T = 11 K for 2Tb and χ M T = 7.68 cm 3 mol −1 K a T = 30 K for 4Er. From these temperatures, the fall of the χ M T values accelerates to reach minimum values equal to 5.69 cm 3 mol −1 K for 2Tb and 4.70 cm 3 mol −1 K for 4Er at T = 2 K. For the 3Dy complex, it is observed that the χ M T values are practically constant when decreasing the temperature. From T = 8 K, the χ M T values decrease quickly to χ M T = 9.28 cm 3 mol −1 K at 2 K.

Magnetic Properties
Magnetization measurements at 2 K were also performed for complexes 1Eu-5Yb and are depicted in Figure 5b. The magnetization saturation under an applied field of 5T is only observed in compounds 3Dy and 5Yb, with M values equal to 4.75 and 1.85 Nµ B , respectively.

ac Magnetic Studies
Magnetic susceptibility measurements were performed on compounds 2Tb-5Yb at a 4 G ac field oscillating between 10 and 1488 Hz. Under zero dc magnetic field, no maxima for the in-phase (χ M ) and out-of-phase (χ M ) susceptibility components were observed for any of the compounds, which was probably due to an important quantum tunneling of the magnetization (QTM) process present in these systems. To suppress the QTM relaxation process, an optimal external dc field of 1000G was applied for the complexes 2Tb-5Yb after checking with low fields and basing on recent works reported by the authors [41][42][43]. Then, slow relaxation of the magnetization was revealed ( Figure 6 and Supplementary Materials, Figure S2) for the compounds 3Dy-5Yb [55,56]. For 2Tb, even under applied dc fields up to 4000 G, no frequency or thermal depend ency of χM' and/or χM'' were observed.
The ac magnetic susceptibility of compound 3Dy has been measured between 2 and 14 K. Figure 6a shows the out-of-phase component of the susceptibility (χM") vs. the fre quency of the ac field, with maxima for temperatures below 6.4 K. The curves measured at temperatures between 6.2 and 3.4 K have been adjusted to the generalized Debye equa tion [57]. The corresponding Cole-Cole diagrams (Figure 6b) show slightly elongated semi-circular shapes, with α values between 0.13 and 0.06. The low distribution of the relaxation times (Figure 6c from the Supporting Information, Table S4) indicates the pres For 2Tb, even under applied dc fields up to 4000 G, no frequency or thermal dependency of χ M and/or χ M were observed.
The ac magnetic susceptibility of compound 3Dy has been measured between 2 and 14 K. Figure 6a shows the out-of-phase component of the susceptibility (χ M ) vs. the frequency of the ac field, with maxima for temperatures below 6.4 K. The curves measured at temperatures between 6.2 and 3.4 K have been adjusted to the generalized Debye equation [57]. The corresponding Cole-Cole diagrams (Figure 6b) show slightly elongated semi-circular shapes, with α values between 0.13 and 0.06. The low distribution of the relaxation times (Figure 6c from the Supplementary Materials, Table S4) indicates the presence of a single relaxation process. The fitting of the thermal dependence of τ with the Arrhenius law (Orbach relaxation mechanism) for temperatures higher than 4.5 K reveals values of τ 0 = 1.43 × 10 −6 s and U eff = 21.7 cm −1 . To fit the values throughout the range of temperatures, equation (Equation (1) was used, which also takes into account a direct relaxation. The obtained parameters were τ 0 = 8.47 × 10 −7 s, U eff = 24.0 cm −1 , and For compound 4Er, the representation of χ M as a function of the frequency between temperatures of 5.0 and 1.8 K shows maxima for temperatures below 2.7 K (Figure 6d). The fitting of the Cole-Cole diagrams (Figure 6e) shows a narrow distribution of relaxation times with α values between 0.10 and 0.05 (from the Supplementary Materials, Table S5). The relaxation time follows a linear dependence at temperatures above 2.3 K and can be fitted according to the Arrhenius law. The parameters obtained were τ 0 = 8.68 × 10 −7 s and U eff = 9.33 cm −1 (Figure 6f). On the other hand, the data can be fitted throughout all the range of temperatures, taking into account Orbach and direct relaxation processes (Figure 6f). The parameters obtained from this second fitting are τ 0 = 4.67 × 10 −8 s, U eff = 15.4 cm −1 , and A = 903 K −1 s −1 .
Compost 5Yb shows a maxima of χ M frequency dependents for temperatures lower than 2.7 K (Figure 6g). The Cole-Cole plots for temperatures between 2.7 and 1.8 K (Figure 6h) have a semi-circular shape. The fitting with the generalized Debye model indicates a narrow distribution of the relaxation times (from the Supplementary Materials, Table S6) with values of α between 0.12 and 0.24. Figure 6i shows the thermal dependence of the relaxation times. For temperatures between 3.2 and 2.5 K, the τ values can be modeled considering an Orbach relaxation mechanism with fitting parameters of τ 0 = 7.94 × 10 −6 s and U eff = 6.79 cm −1 . At temperatures below 2.5 K, the values are deviated from linearity, but they can be fitted considering a relaxation of the magnetization by Orbach and direct processes. The parameter obtained are τ 0 = 4.16 × 10 −6 s, U eff = 8.6 cm −1 , and A = 323 K −1 s −1 .

Photoluminescence Properties
The emission properties of 1Eu-5Yb were studied in solid state at r.t. and 77 K.

Visible Emission
When compound 1Eu is excited at λ ex = 319 nm, the resulting emission spectrum shows the characteristic Eu(III) transitions centered at 578 nm ( 5 D 0 → 7 F 0 ), 591 nm ( 5 D 0 → 7 F 1 ), 616 nm ( 5 D 0 → 7 F 2 ), and 692 nm ( 5 D 0 → 7 F 4 ), as well as the residual emission of the 9-anthc ligand as a broad band centered at 470 nm (Figure 7a). Meanwhile, the emission spectra of compounds 2Tb and 3Dy (Supplementary Materials, Figure S3) only show a broad band between 400 and 500 nm that corresponds to the 9-anthc ligand. This may be because there is a water molecule directly bonded to the lanthanide atom, which normally quenches the luminescence of such metals.
shows the characteristic Eu(III) transitions centered at 578 nm ( 5 D0 → 7 F0), 591 nm ( 5 D0 → 7 F1), 616 nm ( 5 D0 → 7 F2), and 692 nm ( 5 D0 → 7 F4), as well as the residual emission of the 9anthc ligand as a broad band centered at 470 nm (Figure 7a). Meanwhile, the emission spectra of compounds 2Tb and 3Dy (Supporting Information, Figure S3) only show a broad band between 400 and 500 nm that corresponds to the 9-anthc ligand. This may be because there is a water molecule directly bonded to the lanthanide atom, which normally quenches the luminescence of such metals.

NIR Emission
Complex 4Er shows a principal peak corresponding to the 4 I13/2 → 4 I15/2 transition with a maximum at 1580 nm (Figure 7b)). Decreasing the temperature to 77 K does not permit us to increase sufficiently the resolution to have access to the crystal field splitting. Com-

NIR Emission
Complex 4Er shows a principal peak corresponding to the 4 I 13/2 → 4 I 15/2 transition with a maximum at 1580 nm (Figure 7b). Decreasing the temperature to 77 K does not permit us to increase sufficiently the resolution to have access to the crystal field splitting. Compound 4Er, when measured at room temperature and at 77 K under an excitation wavelength of 360 nm, shows broad peaks of residual emission of the ligands between 390 and 550 nm, in the absorption range of the 9-anthc ligand ( Figure S4). As can be seen in Figure 7b, under this same excitation energy and at T = 77 K, the compound also emits in the area of the near IR, with a maximum of 1580 nm, which correspond to the transition 4 I 13/2 → 4 I 15/2 .
Complex 5Yb shows the characteristic Yb(III) transition emission profile, with the principal transition 2 F 5/2 → 2 F 7/2 at 1005 nm (Figure 7c, black spectrum). When decreasing the temperature to 77 K (Figure 7c, red line), the emission band splits into four bands, with the maxima at 980, 1005, 1014, and 1035 nm, which correspond to the four expected Stark levels for a J = 7/2 in low symmetry [56,58]. This transition offers us information of the crystal field energy diagram of the ytterbium ion in complex 5Yb and permits us to estimate the energy between the ground and the first excited m J states of the fundamental 2 F 7/2 level of about 210 cm −1 (Figure 7d). The difference between this value and the effective energy barrier value obtained from the Arrhenius fit of the ac data (≈7.0 cm −1 ) confirms that relaxation of the magnetization takes place by direct and/or Raman processes instead of only the Orbach one [59].

General Syntheses
Compounds 1Eu-5Yb have been synthetized by following the same experimental procedure. To a solution consisting of 1.5 mmol 9-Hanthc and 0.3 mmol TPY in 15 mL of MeOH, a solution of 10 mL DMF containing 0.25 mmol of Ln(NO 3 ) 3 ·xH 2 O is added (Ln(III) = Eu (1Eu), Tb (2Tb), Dy (3Dy), Er (4Er), and Yb (5Yb). In all the cases, slow evaporation of the methanol/dimethylformamide solution of the product gave prismshaped yellow monocrystals suitable for X-ray determination.
Data ( a ) is 9-anthcencarboxylate in monodentate coordination mode and ( b ) is 9-anthcencarboxylate in chelate coordination mode.

Physical Measurements
Elemental analyses of the compounds were done at the Serveis Científics i Tecnològics of the Universitat de Barcelona.
Luminescence spectra were registered on a Horiba Jobin Yvon SPEX Nanolog fluorescence spectrophotometer at rt. and at 77 K.
Magnetic measurements were done on polycrystalline samples in a Quantum Design MPMS-XL SQUID magnetometer at the Unitat de Mesures Magnètiques (CCiTUB) of the Universitat de Barcelona. To estimate the diamagnetic corrections, Pascal's constants were used and subtracted from the experimental susceptibilities to give the corrected molar magnetic susceptibilities.

X-ray Crystallography
Crystal data, conditions retained for the intensity data collection, and some features of the structure refinements for 1Eu-5Yb are summarized in Table 2. Crystals of 1Eu-5Yb were selected and mounted in air on a D8VENTURE (Bruker) diffractometer with a CMOS detector. All the structures were refined by the least-squares method. Intensities were collected with a multilayer monochromated Mo-Kα radiation. Lorentz polarization and absorption corrections were made in all the compounds. The structures were solved by direct methods, using the SHELXS-97 software [60] and refined by the full-matrix leastsquares method, using the SHELXL software [61]. The non-H atoms were located in successive difference Fourier syntheses and refined with anisotropic thermal parameters on F 2 . In the case of H atoms, isotropic temperature factors have been assigned as 1.2 or 1.5 times of the respective parent.

Conclusions
Here, we have presented the structural, magnetic, and luminescence studies of new five mononuclear Ln(III) complexes by employing 9-anthracenecarboxylic acid (9-Hanthc) and 2,2 :6,2 -terpyridine ligands. We have used a straightforward room-temperature synthetic procedure, and the viability of the experimental processes adopted to obtain low-nuclear coordination compounds with Ln(III) has been corroborated.
To the best of our knowledge, the complexes 1Eu-5Yb reported in this work are the first examples of mononucleares complexes of lanthanide(III) in combination with secondary organic ligands.
Not all the compounds discussed in this work show the f -f emission luminescence. The Eu(III) (1Eu), Tb(III) (2Tb), and Dy(III) (3Dy) compounds emit in the visible range, while the Er(III) (4Er) and Yb(III) (5Yb) compounds emit in the NIR range.
Thus, compounds 4Er and 5Yb present both field-induced SMM and luminescent behaviors; therefore, they can be considered as multifunctional compounds with possible potential biomedical applications.
The results reported certainly will attract the interest of scientists working in the area of multifunctional lanthanide compounds.
More and further investigations in this area with other lanthanide metal ions and chiral organic ligands could be very interesting. Such investigations are under development in our laboratories with promising results.