A Series of LanthanideâGermanate Oxo Clusters Decorated by 1,10

Article pubs.acs.org/IC

A Series of Lanthanide−Germanate Oxo Clusters Decorated by 1,10Phenanthroline Chromophores Xiao-Feng Tan,†,‡ Jian Zhou,*,†,‡ Hua-Hong Zou,*,§ Lianshe Fu,*,⊥ and Qiuling Tang† †

Chongqing Key Laboratory of Inorganic Functional Materials, College of Chemistry, Chongqing Normal University, Chongqing 401331, P. R. China § State Key Laboratory for Chemistry and Molecular Engineering of Medicinal Resources, School of Chemistry & Pharmacy, Guangxi Normal University, Guilin 541004, P. R. China ⊥ Department of Physics and CICECO, Aveiro Institute of Materials, University of Aveiro, Aveiro 3810-193, Portugal S Supporting Information *

ABSTRACT: A series of lanthanide−germanate oxo clusters, [Ln8(phen)2Ge12(μ3-O)24T12(H2O)16]·2H2O [Ln = Dy (1a) and Er (1b); T = −CH2CH2COO− group; phen = 1,10-phenanthroline], [Ln8(phen)2Ge12(μ3O)24T12(H2O)16]·2phen·16H2O [Ln = Sm (2a), Eu (2b), and Gd (2c)], and [Ho8(phen)2Ge12(μ3-O)24T12(H2O)14]·2phen·13H2O (3), have been hydrothermally synthesized from the reactions of bis(carboxyethylgermanium sesquioxide) and Ln2O3 with auxiliary phen chromophores. Compounds 1a and 1b consist of cage clusters [Ln8(phen)2Ge12(μ3-O)24T12(H2O)16] and free H2O molecules, where cage clusters are arranged in a CsCl type, while compounds 2a−2c consist of cage clusters [Ln 8 (phen) 2 Ge 12 (μ 3 O)24T12(H2O)16], free phen, and free H2O molecules, where cage clusters are arranged in a NaCl type. Compound 3 consists of the one-dimensional neutral chain [Ho8(phen)2Ge12(μ3-O)24T12(H2O)14]n and free H2O molecules. These compounds provide the first examples of p−f heterometallic [Ge−O−Ln] oxo clusters decorated by phen chromophores. The photoluminescent and magnetic properties of all compounds have been investigated.

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INTRODUCTION The incorporation of other metal ions into the frameworks of lanthanide (Ln) oxo clusters is an efficient route to synthesizing a new class of heterometallic clusters that can be closely related to their fascinating structures and their unique magnetism and optical properties associated with their 4f1−14 electronic configurations. 1 So far, a large number of the d−f heterometallic oxide clusters are built up from the assembly of different d−f metal ions and multidentate chelating organic ligands with both N- and O-donor atoms under different reaction conditions because the soft-donor N atom tends to coordinate to d-block metal ions and the hard-donor O atom prefers to bind Ln ions.1b−h The combination of heavy Ln (such as Tb3+, Dy3+) and first-row d-block metal (such as Cr3+, Mn2+/3+, Fe2+, Co2+, Ni2+, Cu2+) ions can produce magnetic 3d−4f heterometallic oxo clusters as single-molecule magnets1f−h,2 mainly because the couplings between the 3d and 4f ions are usually stronger than f−f interactions, and two kinds of spin carriers combined into a single material may give better single-molecule-magnet properties. Moreover, light-absorbing d-block metal (such as Zn2+ and Cd2+ with a d10 configuration) chromophores can serve as sensitizers for lanthanide luminescence following efficient d → f energy transfer. As a result, the d-block metal chromophores incorporating Ln ions (Ln3+) can give various luminescent d−f heterometallic oxo clusters,3 where the intensities of characteristic lanthanide © XXXX American Chemical Society

emissions are significantly enhanced. Compared with numerous d−f heterometallic oxo clusters, the p−f heterometallic oxo clusters containing lighter p-block metal and Ln3+ are less explored,4 mainly because hard Lewis acidic p-block metal ions coexist with difficulty with Ln3+ in the same framework of heterometallic oxide clusters in solution, which could result from the obvious difference in the radii and oxophilic of lighter p-block metal and Ln3+. The lighter p-block Ge ion is often used for constructing good phosphor hosts of germinates with high density and luminescent conversion efficiency because of the high defect concentration of the germanate matrix.5 The combination of Ge and Ln metal atoms in the same framework might obtain a new class of lanthanide−germanate oxo clusters with useful luminescent properties. However, lanthanide− germinate oxo clusters prepared by mild hydrothermal methods are relatively rare, and the limited examples with luminescent properties were reported by Yang’s group.4b,c On the other hand, high-nuclearity lanthanide oxo clusters have usually been obtained from aqueous environments by organic ligand control of the hydrolysis of Ln3+,6 but their solids are not ideal for luminescent applications because the presence of an OH oscillator of the H2O molecule or −OH group in these clusters has a tendency to reduce emission intensities by Received: May 19, 2017

A

DOI: 10.1021/acs.inorgchem.7b01271 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 1. Crystallographic Data for 1−4 formula fw cryst syst space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z T, K calcd density, Mg m−3 F(000) 2θ(max), deg total reflns collected unique reflns no. of param R1 [I > 2σ(I)] wR2 (all data) GOF on F2

1a

1b

2a

2b

2c

3

C60H100Dy8Ge12N4O66 4104.52 monoclinic P21/n 14.6471(8) 22.8090(13) 15.3079(8) 90 90.549(2) 90 5113.9(5) 2 292(2) 2.665

C60H100Er8Ge12N4O66 4142.60 monoclinic P21/n 14.5751(17) 22.748(3) 15.2770(18) 90 90.728(2) 90 5064.7(10) 2 296(2) 2.716

C84H144Ge12N8O80Sm8 4620.03 triclinic P1̅ 14.6468(14) 15.1942(16) 16.3470(17) 70.424(2) 75.723(2) 82.445(2) 3317.1(6) 1 296(2) 2.313

C84H144Eu8Ge12N8O80 4632.83 triclinic P1̅ 14.7035(8) 15.3286(9) 16.3702(9) 70.278(2) 75.737(2) 82.563(2) 3361.8(3) 1 294(2) 2.288

C84H144Gd8Ge12N8O80 4675.15 triclinic P1̅ 14.719(4) 15.292(3) 16.372(4) 70.148(7) 75.507(9) 82.882(8) 3352.7(15) 1 286(2) 2.315

C84H134Ge12Ho8N8O75 4646.52 triclinic P1̅ 14.3952(4) 14.6078(5) 16.4226(5) 79.287(3) 75.457(3) 85.972(2) 3283.46(18) 1 293(2) 2.350

3784 50.20

3816 50.20

2208 50.20

2168 50.20

2176 50.20

3388 50.20

32745

28191

45860

25982

33229

24708

8883

8929

11758

11737

11839

11683

676

677

982

862

871

865

0.0363

0.0213

0.0225

0.0469

0.0403

0.0405

0.0726

0.0608

0.0537

0.0845

0.0831

0.1148

1.060

1.070

1.014

0.958

0.965

1.096

heterometallic [Ge−O−Ln] oxo clusters decorated by neutral phen chromophores.

the nonradiative deactivation of lanthanide excited states. To decrease the nonradiative deactivation, chelating π-conjugated aromatic chromophores are generally used as sensitizers for lanthanide luminescence via efficient energy transfer,7 where they directly chelate to a Ln3+ ion and prevent the Ln3+ ion from the H2O molecules. Bidentate neutral phen is one of the most widely used chromophoric ligands in the design of luminescent lanthanide complexes8 because it can not only chelate to a Ln3+ ion and protect the Ln3+ ion from the H2O molecules but also absorb and efficiently transfer energy to the Ln3+ excited states. It is expected that the integration of phen into the skeletons of high-nuclearity lanthanide oxo clusters may generate novel lanthanide oxo clusters with useful luminescent properties. However, high-nuclearity lanthanide oxo clusters decorated by the neutral phen ligand and its derivatives are very scarce9 mainly because the phen ligand acting as a terminating group prevents further connections, which easily produce mononuclear and dinuclear lanthanide complexes.7,10 Moreover, no heterometallic [Ge−O−Ln] oxo clusters decorated by neutral phen chromophores have been reported to the best of our knowledge. In this work, we set out to explore the reaction system Ln2O3/L/phen/H2O [Ln = Sm, Eu, Gd, Dy, Ho, and Er; L = bis(carboxyethylgermanium sesquioxide)] under hydrothermal conditions with the aim of obtaining high-nuclearity p−f heterometallic oxo clusters and have successfully prepared a series of lanthanide−germanate oxo clusters, [Ln8(phen)2Ge12(μ3-O)24T12(H2O)16]·2H2O [Ln = Dy (1a) and Er (1b); T = −CH2CH2COO− group; phen = 1,10-phenanthroline], [Ln8(phen)2Ge12(μ3-O)24T12(H2O)16]· 2phen·16H2O [Ln = Sm (2a), Eu (2b), and Gd (2c)], and [Ho8 (phen)2Ge 12 (μ3 -O) 24 T12(H 2O) 14 ]·2phen·13H 2O (3), which present the first examples of high-nuclearity p−f

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EXPERIMENTAL SECTION

Materials and Methods. All analytical-grade chemicals were obtained commercially and used without further purification. Fourier transform infrared spectra were obtained from a powdered sample pelletized with KBr on an ABB Bomen MB 102 series IR spectrophotometer in the range of 400−4000 cm−1. Elemental analyses (C, H, and N) were performed on an Vario EL III elemental analyzer. Photoluminescence spectra were recorded at room temperature with a modular double-grating excitation spectrofluorimeter with a TRIAX 320 emission monochromator (Fluorolog-3, Horiba Scientific) coupled to an R928 Hamamatsu photomultiplier with an excitation source of a 450 W xenon arc lamp. Variable-temperature magnetic susceptibility measurements were carried out in the temperature range of 2−300 K with a Quantum Design MPMS-XL5 magnetometer. Powder X-ray diffraction (XRD) patterns were obtained using a Bruker D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å) in the range of 2θ = 5−50° with a scan step width of 0.02°. Synthesis of [Dy8(phen)2Ge12(μ3-O)24T12(H2O)16]·2H2O (1a). A mixture of L (0.0421 g, 0.124 mmol), phen (0.0356 g, 0.197 mmol), Dy2O3 (0.0374 g, 0.100 mmol), and water (2 mL) was stirred for 0.5 h, and then the pH of the mixed solution was adjusted to 2 by HCl (12 mol L−1). The final mixture was sealed in a 20 mL Teflon-lined autoclave and heated at 150 °C for 7 days. After slow cooling to room temperature, yellow block crystals were obtained. The yield of 1a was 45% based on Dy2O3. Anal. Calcd for1a (C60H100Dy8Ge12N4O66): C, 17.56; H, 2.46; N, 1.37. Found: C, 17.61; H, 2.52; N, 1.41. Synthesis of [Er8(phen)2Ge12(μ3-O)24T12(H2O)16]·2H2O (1b). The pink crystals of 1b were prepared similarly from Er2O3. The yield was 57% based on Er2O3. Anal. Calcd for 1b (C60H100Er8Ge12N4O66): C, 17.40; H, 2.43; N, 1.35. Found: C, 17.43; H, 2.47; N, 1.40. B


Article

Inorganic Chemistry Synthesis of [Sm8(phen)2Ge12(μ3-O)24T12(H2O)16]·2phen·16H2O (2a). A mixture of L (0.0422 g, 0.124 mmol), phen (0.0463 g, 0.257 mmol), Sm2O3 (0.0395 g, 0.113 mmol), and water (2 mL) was stirred for 0.5 h, and then the mixed solution was adjusted to pH 2 by HCl (12 mol L−1). The final mixture was sealed in a 20 mL Teflon-lined autoclave and heated at 150 °C for 7 days. After slow cooling to room temperature, yellow block crystals. The yield was 86% based on Sm2O3. Anal. Calcd for 2a (C84H144Ge12N8O80Sm8): C, 21.84; H, 3.14; N, 2.43. Found: C, 21.87; H, 3.17; N, 2.47. Synthesis of [Eu8(phen)2Ge12(μ3-O)24T12(H2O)16]·2phen·16H2O (2b). The colorless crystals of 2b were prepared by a similar method used in the synthesis of the crystals of 2a except that Sm2O3 was replaced by Eu2O3. The yield was 83% based on Eu2O3. Anal. Calcd for 2b (C84H144Eu8Ge12N8O80): C, 21.78; H, 3.13; N, 2.42. Found: C, 21.82; H, 3.17; N, 2.47. Synthesis of [Gd8(phen)2Ge12(μ3-O)24T2(H2O)16]·2phen·16H2O (2c). The colorless crystals of 2b were prepared by a similar method used in the synthesis of the crystals of 2a except that Sm2O3 was replaced by Gd2O3. The yield was 43% based on Gd2O3. Anal. Calcd for 2c (C84H144Gd8Ge12N8O80): C, 21.58; H, 3.10; N, 2.40. Found: C, 21.61; H, 3.13; N, 2.43. Synthesis of [Ho8(phen)2Ge12(μ3-O)24T12(H2O)14]·2phen·13H2O (3). A mixture of L (0.0455 g, 0.134 mmol), phen (0.0495 g, 0.275 mmol), Ho2O3 (0.0515 g, 0.136 mmol), and water (2 mL) was stirred for 0.5 h. The final mixture (pH 6) was sealed in a 20 mL Teflon-lined autoclave and heated at 120 °C for 7 days. After slow cooling to room temperature, orange-red rodlike crystals were obtained. The yield was 32% based on Ho2O3. Anal. Calcd for 3 (C84H134Ge12Ho8N8O75): C, 21.71; H, 2.91; N, 2.41. Found: C, 21.74; H, 2.93; N, 2.43. X-ray Crystallography. Single-crystal XRD data for all compounds were collected on a Rigaku Mercury CCD diffractometer using a ω-scan method with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Routine Lorentz polarization and absorption corrections were applied using a multiscan technique. The structures of all compounds were solved by direct methods of SHELXS-9711 and refined by full-matrix least-squares methods on F2 using the SHELXL97 program package.12 The positions of H atoms from the phen ligands and −CH2CH2COO− groups were geometrically placed, and H atoms were refined isotropically as a riding mode using the default SHELXTL parameters. No H atoms associated with H2O molecules were located from the difference Fourier map, but H atoms were added in the formula. A summary of the crystallographic data is listed in Table 1. CCDC 1547508−1547513 contains the supplementary crystallographic data for this paper.

Figure 1. Structures of the wheel-shaped ring [Ge6O12T6] (a), [Ln(phen)(H2O)2] group (b), cap-shaped cluster [Ln(phen)(H2O)2Ge6O12T6] (c), circular fragment [Ln6O36] (d), and cage cluster [Ln8(phen)2Ge12(μ3-O)24T12(H2O)16] (e). All H atoms bonded to C/O atoms are omitted for clarity.

coordinated by eight O atoms to form distorted bicapped trigonal-prism geometries, which are further joined together to give a circular [Ln6O36] fragment via the sharing of O atoms (Figure 1d). This circular fragment [Ln6O36] is sandwiched by two cap-shaped clusters [Ln(phen)(H2O)2Ge6O12T6] via the s h a r i n g o f O a t o m s t o f o rm t h e ca g e c l u s t e r [Ln8(phen)2Ge12(μ3-O)24T12(H2O)16] (Figure 1e). If the cage backbones had full symmetry, the entire cage clusters would have the point group C2h. Compounds 1a and 1b are isostructural and crystallize in the monoclinic centrosymmetric space group P21/n with two formula units in the unit cell. Compounds 1a and 1b consist of the cage clusters [Ln8(phen)2Ge12(μ3-O)24T12(H2O)16] and free H2O molecules. Each cage cluster connects eight other cage clusters via intermolecular O−H···O hydrogen bonds between H2O molecules and carboxylic groups, leading to a three-dimensional (3D) hydrogen-bonded network structure (Figure 2a). The O···O distances are in the range of 2.720− 2.847 Å, which are in agreement with the values reported in the literature.13 If the cage clusters can be regarded as 8-connected nodes, the 3D hydrogen-bonded network in compounds 1a and 1b can be classified as the CsCl-type topological network (Figure 2b). Although each cage cluster in compounds 1a and 1b has two phen ligands, π−π aromatic stacking interactions between the phen ligands of adjacent cage clusters are not observed. Compounds 2a−2c are isostructural and crystallize in the triclinic space group P1̅ with one formula unit in the unit cell. So, only 2a is discussed in detail. 2a consists of neutral cage clusters [Sm8(phen)2Ge12(μ3-O)24T12(H2O)16], free phen ligands, and free H2O molecules. The coordinated H2O molecules of the cage clusters in 2a are involved in

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RESULTS AND DISCUSSION Descriptions of [Ln8(phen)2Ge12(μ3-O)24T12(H2O)16]· 2H2O [Ln = Dy (1a) and Er (1b)] and [Ln8(phen)2Ge12(μ3-O)24T12(H2O)16]·2phen·16H2O [Ln = Sm (2a), Eu (2b), and Gd (2c)]. Compounds 1a, 1b, and 2a−2c contain neutral cage clusters [Ln8(phen)2Ge12(μ3O) 2 4 T 1 2 (H 2 O) 1 6 ]. The cage [Ln 8 (phen) 2 Ge 1 2 (μ 3 O)24T12(H2O)16] backbones are built up from the combination of two [Ln(phen)O8] units, two [Ge6O12T6] rings, and one circular [Ln6O36] fragment via the sharing of O atoms. Two GeO double bonds of the L (T2Ge2O32−) ligand are opened under hydrothermal conditions to be converted into a [T2Ge2O5] unit, where each Ge atom adopts a tetrahedral geometry comprised of one C and three O atoms. Three [T2Ge2O5] units are held together to give a wheel-shaped [Ge6O12T6] ring in a corner-sharing fashion (Figure 1a). The [Ln(phen)(H2O)2] group (Figure 1b) is capped over the center of the [Ge6O12T6] ring to give a cap-shaped cluster [Ln(phen)(H2O)2Ge6O12T6] (Figure 1c), whose Ln3+ ion adopts an irregular tetracapped trigonal prism comprised of six O atoms from a cap-shaped cluster, two N atoms of the phen ligand, and two H2O molecules. The other Ln3+ ions are C


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Inorganic Chemistry

one H2O molecule. The cage clusters in 3 are connected to each other by the coordination of two carboxylic groups of the T groups, resulting in the 1D neutral chain [Ho8(phen)2Ge12(μ3-O)24T12(H2O)16]n (Figure 3a). These

Figure 2. (a) 3D hydrogen-bonded network of 1a, (b) CsCl-type topological network (green: cage clusters). (c) 3D hydrogen-bonded network of 2a. (d) NaCl-type topological network (green: cage clusters). (e) 1D H2O chain in 2a. All H atoms bonded to C/O atoms are omitted for clarity. Figure 3. (a) 1D neutral chain [Ho 8 (phen) 2 Ge 1 2 (μ 3 O)24T12(H2O)14]n in 3. (b) View of the layer constructed by O− H···O hydrogen bonds (shown as dashed lines). (c) 3D supramolecular network structure of 3. (d) π−π-stacking interactions between phen ligands. All H atoms bonded to C/O atoms are omitted for clarity.

intermolecular O−H···O hydrogen bonds with carboxylic groups of adjacent cage clusters, resulting in a 3D hydrogenbonded network structure with one-dimensional (1D) tunnels filled by free phen ligands and H2O molecules (Figure 2c). The π−π aromatic stacking interactions between free and coordinated phen ligands play a stabilizing role in the 3D hydrogen-bonded network of 2a. Each cage cluster is surrounded by six other cage clusters. If the cage clusters can be regarded as 6-connected nodes, the 3D hydrogen-bonded network in 2a can be classified as the NaCl-type topological network (Figure 2d). In addition, an interesting feature of 2a is the presence of a 1D H2O chain (Figure 2e). The complete H2O chain structure is constructed by six repeating crystal lattice H2O molecules, namely, O9W, O11W, O12W, O13W, O14W, and O15W. Such a H2O chain is fixed within the 3D network via O−H···O hydrogen bonds between free H2O molecules (O13W and O14W) and coordinated H 2O molecules (O3W and O7W). The O···O distances varying from 2.674 to 2.846 Å are slightly shorter than the values observed in liquid water (2.85 Å).14 Description of 3. Compound 3 crystallizes in the triclinic space group P1̅ and contains the 1D neutral chain [Ho8(phen)2Ge12(μ3-O)24T12(H2O)14]n, free phen ligands, and free H2O molecules. The cage cluster in 3 is obviously different from that of 1a, 1b, and 2a−2c. First, the 24 OCOO atoms of the carboxylic groups from two [Ge6O12T6] rings in 1a, 1b, and 2a−2c link back to the circular fragment [Ln6O36], while the 24 OCOO atoms of the carboxylic groups in 3 can be classified into two types according to the coordination modes: 22 OCOO atoms of 22 carboxylic groups link back to the circular fragment [Ho6O36], and two carboxylic groups act as linkers. Second, the Ln33+ or Ln3A3+ ion of the circular fragment [Ho6O36] has two H2O molecules, whereas the Ho33+ or Ho3A3+ ion of the circular fragment [Ho6O36] in 3 only has

chains are arranged in a parallel manner with the same orientation and further connected via O−H···O hydrogen bonds between coordinated H2O molecules and carboxylic groups to give a layer (Figure 3b). The phen template molecules are located at the interlayer spaces and form extensive π−π aromatic stacking interactions between free phen molecules and free phen and phen ligands of adjacent cage clusters. The centroid−centroid distances between adjacent phen rings are in the range of 3.707−3.798 Å, and the dihedral angles of adjacent phen rings vary from 0.0 to 4.4°, which indicates the parallelism of the phen rings (Figure 3d). The interconnection of these layers with the template molecules via π−π interactions leads to a 3D supramolecular network structure (Figure 3c). Photoluminescent Properties. The interesting electronic features of Ln3+ ions containing valence-shell electrons of [Xe]4f0−14 are excellent photoluminescent properties with high color purity. Therefore, the photoluminescent properties of all compounds in the solid state have been investigated at room temperature. The solid-state excitation spectrum (Figure S1) of 1a, which was obtained by monitoring the emission of the Dy3+ ion (4F9/2 → 6H13/2 transition) at 572 nm, consists of a broad band with the maximum excitation wavelength centered at 345 nm and some weak peaks in the range of 300−500 nm. The strong broad band is attributed to the electronic transitions from the ground-state level (π) S0 to the excited-state level (π*) S1 of the phen ligand, while the weak peaks are ascribed to the f−f transitions of Dy3+ ions and their transition assignments D


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Inorganic Chemistry

Figure 4. (a) Emission spectrum (λex = 345 nm) of 1a [the inset shows the emission spectrum (λex = 245 nm) of the phen ligand]. (b) Emission spectrum (λex = 360 nm) of 1b in the UV−visible region. (c) Emission spectrum (λex = 380 nm) of 1b in the near-IR region. (d) Emission spectrum of 2a (λex = 347 nm). (e) Emission spectrum for 2b at λex = 280 nm (the inset shows the enlarged wavelength range from 520 to 570 nm). (f) CIE chromaticity diagrams for 1a, 1b, 2a−2c, and 3.

are summarized in Table S1.15 Compared to the f−f transition bands from Dy3+ ions, the excitation spectrum shows the dominated band from the organic ligand, indicating that a sensitization of Dy3+ ion luminescence is mainly through an indirect energy-transfer process from the phen ligand to Dy3+ ions. Upon excitation at 345 nm, 1a displays strong emission bands in the visible region (Figure 4a). The band at 479 nm is assigned to the blue insensitive transitions 4F9/2 → 6H15/2, whereas the more intensive emission band at 572 nm belongs to the yellow sensitive transitions 4F9/2 → 6H13/2 of Dy3+ ions. The very weak emission band at 661 nm corresponds to the 4 F9/2 → 6H11/2 transition of Dy3+ ions. Regarding the emission bands of the phen ligand (inset of Figure 4a), nearly no emission bands from 1a can be observed, suggesting that an efficient intramolecular energy transfer from the phen ligand to the Dy3+ ions. Because of the strong emission in the yellow region, the overall emission shows a yellow color to the eyes. The CIE chromaticity coordinate was calculated to be (0.394, 0.446), as shown in Figure 4f. For 1b, upon excitation at 360 nm, it shows a very broad band. The green peaks at 536 and 553 nm correspond to the electron transition 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 of Er3+ ions, while the bands in the blue region mainly arise from the phen ligand (Figure 4b). The CIE chromaticity coordinate was calculated to be (0.210, 0.254), as shown in Figure 4f. Compared to the ligand, this band is red-shifted, which is mainly considered to originate from the strong interaction between the ligand and metal centers.16 It should be noted that this ligand-centered emission band exhibits several indentations that can be attributed to the partial reabsorption of the emitted luminescence by direct excitation based on the 4f−4f transitions of Er3+ ions within the same compound,17 and this phenomenon was also observed especially for Er3+ complexes with N-donor ligands.18 Compared with Dy3+, Sm3+, and Eu3+ ions, because the energy levels of the excited

states of Er3+ ions are very close to one another,1b compound 1b can give weak emission from the f−f transition of Er3+ in the visible region and also can present emission in the near-IR region. Upon excitation at 380 nm, 1b shows a broad band centered at 1541 nm originating from the transition of 4I13/2 → 4 I15/2 of Er3+ ions (Figure 4c). Therefore, this kind of material has potential applications in the optical amplification and biomedical fields. The excitation spectrum for 2a was measured at room temperature and presented in Figure S2. The excitation spectrum was obtained by monitoring the emission wavelength of Sm3+ at 643 nm. It shows a broad band ranging from 240 to ∼380 nm attributed to electronic transitions from the groundstate level (π) S0 to the excited-state level (π*) S1 of the organic ligand. The excitation spectrum also displays some weak sharp peaks that are the characteristic f−f electronic transitions of Sm3+ ions (Table S2).15 Because the excitation band is dominated by ligand absorption, the sensitization of Sm3+ luminescence is mainly through an indirect energy-transfer process from ligand to Sm3+ ions. Upon excitation at 347 nm, the emission spectrum exhibits several characteristic emission bands corresponding to 4G5/2 → 6Hx/2 (x = 5, 7, 9 and 11; Table S3 and Figure 4d). There is no ligand-based emission in the whole determined wavelength range, which verifies that the energy-transfer process is effective, results similar to those reported.19 For 2a, the 4G5/2 → 6H9/2 transition might be of electric dipole character and the relative intensity of the4G5/2 → 6 H5/2 and 4G5/2 → 6H9/2 transitions, which indicates magneticdipole-forbidden and electric-dipole-allowed character, supports low site symmetry for the Sm3+ ion.20 The CIE chromaticity coordinate was calculated to be (0.545, 0.370), as shown in Figure 4f. The luminescence decay curve of Sm3+ related to the 4 G5/2 → 6H9/2 emission is shown in Figure S3. The decay curve is single-exponential, confirming that all Sm3+ ions lie in the E


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Inorganic Chemistry

coordinates are (0.158, 0.022) and (0.169, 0.007) for 2c and 3, respectively, as shown in Figure 4f. Magnetic Properties. Variable-temperature magnetic susceptibilities of all compounds were performed under an applied magnetic field of 1 kOe in the range of 2−300 K. The plot of χMT versus T is shown in Figures 5 and S8 and S9. The

same average environment. The luminescence decay lifetime was determined to be 9.6 μs. The excitation spectrum of 2b was obtained by monitoring the most intense emission wavelength of Eu3+ at 614 nm. It shows a broad band ranging from 240 to ∼370 nm attributed to electronic transitions from the ground-state level (π) S0 to the excited-state level (π*) S1 of the phen ligand. As shown in Figure S4, the excitation spectrum also displays some weak sharp peaks assigned to intraligand f−f transitions,15 which are characteristic of the absorption of Eu3+ ions. Because the excitation band is dominated by phen ligand absorption, the sensitization of Eu3+ luminescence is mainly through an indirect energy-transfer process from the phen ligand to Eu3+ ions. Upon excitation at 280 nm, the emission spectrum exhibits several characteristic emission bands corresponding to 5D0 → 7 FJ (J = 0−4; Table S4), with the 5D0 → 7F2 emission as the dominant band (Figure 4e). Besides 5D0 → 7FJ transitions, three very weak bands at 525, 535, and 555 nm are also observed (inset of Figure 4e), which are tentatively assigned to the transitions 5D1 → 7F0, 5D1 → 7F1, and 5D1 → 7F2, respectively. Because all of these bands are from Eu3+ ion emission and no emission from the phen ligand is observed, it is further demonstrated that there is an intramolecular energy transfer from the phen ligand to Eu3+ ions. The CIE chromaticity coordinate was calculated to be (0.656, 0.340), as shown in Figure 4f. The luminescence decay curve of Eu3+ related to the 5D0 → 7F2 emission is shown in Figure S5. The decay curve is single-exponential, confirming that all Eu3+ ions lie in the same average environment. The luminescence lifetime (τexp) was determined to be 0.443 ms. In order to further discuss the luminescent features for 2b, the luminescence quantum efficiency q and the number of H2O molecules nw coordinated to the Eu3+ ion of 2b in the first coordination sphere were calculated (see the Supporting Information for details), and the results are listed in Table 2.

Figure 5. Plot of χMT versus T for compounds 1a, 1b, 2c, and 3.

χMT values at 300 K are 113.2 cm3 K mol−1 for 1a, 92.9 cm3 K mol−1 for 1b, 63.3 cm3 K mol−1 for 2c, and 112.8 cm3 K mol−1 for 3, which are close to the theoretical values for eight noninteracting Ln3+ ions (Table S5). Upon cooling, the observed χMT values of compounds 1a, 1b, 2c, and 3 gradually decrease, as a consequence of depopulation of the sublevels of the ground-state J multiplet split by the crystal field and antiferromagnetic coupling between the Ln3+ ions.22 Antiferromagnetic interactions between Ln3+ ions can also be confirmed by the smaller Ln−O−Ln angle values of 108.8(2)− 110.5(2)° for 1a, 108.98(13)−110.44(13)° for 1b, 109.7(2)− 110.5(2)° for 2c, and 108.59(19)−111.0(2)° for 3 because the rule is that Ln−O−Ln angles below 113.50° are assumed to cause an antiferromagnetic exchange in the literature.23The χM−1 versus T plot over the entire temperature range for compounds 1a, 1b, 2c, and 3 can be fitted to the Curie−Weiss law, χM = C/(T − θ), with the Curie constant C = 18.93,15.91, 39.73, and 12.55 cm3 K mol−1 and the Weiss constant θ = −0.16, −0.17, −0.63, and −0.11, respectively (Figure S10); the negative θ values further indicate antiferromagnetic interactions between the neighboring Ln3+ ions. For 2a, the χMT value of 3.15 cm3 K mol−1 at 300 K is much higher than the expected value for eight magnetically isolated Sm3+ ions in the ground state, mainly because not only the ground state but also the first exited state (6H7/2) and above for the Sm3+ ion can be populated at room temperature.24 The χMT value (9.48 cm3 K mol−1) of 2b at 300 K is also much higher than the value for eight isolated Eu3+ ions in the ground state because of some contributions from thermally accessible levels (such as 7F1 and 7F2).24b,25 As the temperature decreases, the χMT values of 2a and 2b decrease rapidly, which could result from thermal depopulation of the excited-state levels.24a Syntheses and IR Spectra. Although the coordination numbers of Ln3+ ions in these lanthanide−germanate oxo clusters are all the same, they show three types of crystal structures, which might be related to the reactant quantity and pH value. Initially, the crystals of [Ln8(phen)2Ge12(μ3O)24T12(H2O)16]·2H2O [Ln = Dy (1a) and Er (1b)] were o b t a i n e d b y t h e h yd r o t he r m a l re a c t i o n of bi s -

Table 2. Experimental (kexp) and Calculated Radiative (kr) and Nonradiative (knr) 5D0 Decay Rates (ms−1), Decay Time (τexp, ms), Quantum Efficiency (q), and Number of H2O Molecules Coordinated to Eu3+ Ions (nw) 2b

τexp/ms

kexp/ms−1

kr/ms−1

knr/ms−1

q/%

nw

0.443

2.257

0.417

1.840

18.5

1.7

The quantum efficiency (q) for 2b is 18.5%, which is comparable to the results from other compounds.21 The number of H2O molecules coordinated to Eu3+ ions (nw) is around 2, which is in agreement with the crystal structure. The excitation and emission spectra of 2c are shown in Figure S6. Upon excitation at 275 nm, 2c exhibits a broad band from 340 to 500 nm with a maximum emission wavelength of 370 nm. Because the first excited state 6P7/2 (32000 cm−1) of the Gd3+ ion is too high,1b the Gd3+ ion cannot accept the energy from the excited state of the phen ligand and thus no intramolecular energy transfer from the phen ligand to Gd3+ ions occurs. Considering the similarity of the emission spectra between 2c and a pure phen ligand, the emission can be assigned to intraligand transitions, belongs to the perturbation luminescence of the phen ligand, and has no metal-centered emission bands. Upon excitation at 275 nm, 3 also shows an emission spectrum similar to that of 2c, which originated from the organic phen ligand (Figure S7).The CIE chromaticity F


Article

Inorganic Chemistry

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(carboxyethylgermanium sesquioxide), phen, and Ln2O3 in water at a lower pH value (pH 2). When the quantity of phen ligands was slightly increased, other lanthanide−germanate oxo clusters [Ln8(phen)2Ge12(μ3-O)24T12(H2O)16]·2phen·16H2O [Ln = Sm (2a), Eu (2b), and Gd (2c)] under similar conditions were obtained, which contain free phen molecules. When the mixed solution was increased to pH 6 and other parameters were unchanged, crystals of 3 were obtained. As can be seen from the IR spectra shown in Figure S11, the strong and broad bands in the region of 3020−3530 cm−1 can be assigned to the characteristic OH stretching of H2O molecules. The weaker bending bands of the −CH2 groups are present at about 2898−2970 cm−1. The stronger absorption bands within the range of 1520−1600 cm−1 can be indicative of the antisymmetric stretching of −COO− groups, and the absorption bands in the region of 1320−1460 cm−1 can belong to the symmetric stretching of −COO− groups. The absorption bands at about 770 and 540 cm−1 can be attributed to the asymmetric and symmetrical stretching of Ge−O bonds, respectively.26

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Corresponding Authors

*E-mail: [email protected] (J.Z.). *E-mail: [email protected] (H.-H.Z.). *E-mail: [email protected] (L.F.). ORCID

Jian Zhou: 0000-0002-8290-8893 Author Contributions ‡

X.-F.T. and J.Z. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NNSF of China (Grants 21671029 and 21601038), the NSF of the Chongqing municipality (Grants cstc2014jcyjA50002 and cstc2015jcyjBX0117), Fundaçaõ para a Ciência e a Tecnologia (FCT, Portugal), the European Union, QREN, FEDER through Programa Operacional Factores de Competitividade (COMPETE), and CICECO,Aveiro Institute of Materials, POCI-01-0145-FEDER-007679 (FCT ref. UID/CTM/50011/ 2013), financed by national funds through the FCT/MEC and when appropriate cofinanced by FEDER under the PT2020 Partnership Agreement, a program for leading talents of scientific and technological innovation in the Chongqing municipality (Grant CSTCCXLJRC201707), and the Program for Excellent Talents in Chongqing Higher Education Institutions. The authors are also grateful to Chongqing Normal University for financial support (Grant 14CSLJ02), Graduate Innovative Research Projects of the Chongqing municipality (Grant CYS16147), and the Innovative Training Program of University Student (Grant 201610637008).

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CONCLUSION The introduction of a chromophoric phen ligand into the bis(carboxyethylgermanium sesquioxide)/Ln2O3/H2O synthetic system under hydrothermal conditions led to the formation of a series of lanthanide−germanate oxo clusters containing phen chromophores, which provide the first examples of high-nuclearity p−f heterometallic [Ge−O−Ln] oxo clusters decorated by neutral phen chromophores. Compounds 1a, 1b, 2a, and 2b exhibit stronger characteristic Ln3+ emissions because they are closely related to the incorporation of phen chromophores into [Ge−O−Ln] oxocluster frameworks; for example, 2b can emit stronger red luminescence at 614 nm with a long luminescent lifetime, which makes 2b a good candidate for potential red-light materials. From the results, it is probable that the Ge ion only acts as a component of the ligand and there is no evident effect of the p−f mixed compounds on the florescence emission. The successful syntheses of these lanthanide−germanate oxo clusters not only provide the diversity of high-nuclearity p−f heterometallic oxo clusters but also provide us with a possible method that can construct other new high-nuclearity p−f heterometallic oxo clusters with luminescent properties by using different chelating π-conjugated aromatic ligands as chromophores.

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AUTHOR INFORMATION

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REFERENCES

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01271. Calculations of the quantum efficiency q and number of H2O molecules nw coordinated to the Eu3+ ion, IR data, excitation/emission spectra of some compounds, plot of χMT versus T for 2a and 2b, plot of χM−1 versus T for 1a, 1b, 2c, and 3, TGA data of 1a, 1b, and 2a, and XRD data (PDF) Accession Codes

CCDC 1547508−1547513 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The G


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