Self-Assembly Syntheses, Structural Characterization, and

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Self-Assembly Syntheses, Structural Characterization and Luminescent Properties of Lanthanide Coordination Polymers Constructed by Three Triazole-Carboxylate Ligands Yan Yang, Feilong Jiang, Caiping Liu, Lian Chen, Yanli Gai, Jiandong Pang, Kongzhao Su, Xiuyan Wan, and Maochun Hong Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00060 • Publication Date (Web): 23 Feb 2016 Downloaded from http://pubs.acs.org on February 23, 2016

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Crystal Growth & Design

Self-Assembly Syntheses, Structural Characterization and Luminescent Properties of Lanthanide Coordination Polymers Constructed by Three Triazole-Carboxylate Ligands Yan Yang,a,b Feilong Jiang,a Caiping Liu,a,b Lian Chen,*a Yanli Gai,c Jiandong Pang,a,b Kongzhao Su,a Xiuyan Wan,a,b and Maochun Hong*

a

Laboratory of Photochemical and Optical Physics, Fujian Institute of Research

on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, China b

University of the Chinese Academy of Sciences, Beijing, 100049, China

c

School of Chemistry and Chemical Engineering, Jiangsu Normal University,

Jiangsu, 221116, China

Abstract: Herein, fifteen lanthanide coordination complexes based on three di-, triand tetra-carboxylic ligands with imidazolyl groups, have been solvothermally designed, synthesized and characterized. All the complexes exhibit high thermal stabilities and can keep stable in the open air more than 7 days. Luminescent spectra of Eu3+ and Tb3+ complexes at room temperature indicate that the three carboxylic ligands H2L1, H3L2 and H4L3 are promising potential light sensitizers for lanthanide ions based on their suitable triplet energy levels. Luminescent lifetimes and quantum yields of these compounds were further measured and compared. The correlations between their structural features and photophysical properties are discussed in detail.

Introduction Since last century, scientists have focused extensive attention on the design and applications of the lanthanide coordination complexes due to their attractive photophysical properties, such as characteristic luminescent emissions, high fluorescence quantum yields and long observed lifetimes, which enable them to be

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potential functional materials in light-emitting devices, chemical sensors, biomedical and cell imaging, etc.1-16 Especially terbium and europium complexes, two most useful lanthanide complexes with extraordinary luminescent characteristics, consisting of sharp typical emission bands and millisecond lifetimes, are regarded to be promising potential optical materials.17-22 Despite the efficient photoluminescence processes of lanthanide ions, all Ln3+ ions suffer from weak light absorption (the molar absorptivities generally less than 10 M−1 cm−1) due to the ‘‘Laporte forbidden’’ feature of the transitions between the 4fn configurations of the Ln3+ ions.23-25 ‘‘Antenna effect’’, founded by Weissman, is an effective way to overcome the weak light absorption by introducing organic chromophores (L) as antennas.23-27 Therefore, many different varieties of ligands, including β-diketones28-32 and carboxylic acid derivatives,33-37 have been employed to construct highly luminescent lanthanide complexes. Although a great amount of lanthanide coordination complexes with fascinating structures and excellent luminescent properties have been reported, establishing structure–function correlations within these materials still remains a challenging task in crystallography and photochemistry.38-46 For rational design of luminescent lanthanide materials with high performance, a comprehensive study of the relationship between the structures of lanthanide complexes and their photophysical properties is urgently required. Roughly speaking, the energy transfer efficiency of ligand-to-metal and the probability of nonradiative decay are two major influential factors in photophysical properties of lanthanide coordination complexes. Given that the introduced organic chromophore sensitizes the luminescence of Ln3+ ions by means of transferring the absorbed energy from the lowest triplet energy level (T1) of the ligand to the Ln3+ centers, the triplet state of the organic ligand plays a significant role on the photophysical properties. The triplet state (T1) should match well with the emissive levels of Ln3+ ions (the 5D0 emitting level of Eu3+ or 5D4 emitting level of Tb3+), allowing for efficient energy transfer as well as high luminescence quantum yields and lifetimes.47-50 On the other hand, the water molecules in the coordination spheres of Ln3+ centers, which can induce the occurrence of the nonradiative decay,


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also make crucial passive influence on the lifetimes and quantum yields.51-53 In consequence, selecting ligands with well-matched triplet energy levels and reducing the amount of water molecules in the coordination environments are two strategies to the design of highly luminescent lanthanide coordination complexes. Besides, the other factors in the structures, including the amount of OH groups introduced by the ligands, hydrogen bonding interactions and π···π stacking interactions, can also affect the photophysical properties of lanthanide coordination complexes. Recently, we have reported some work54 on four lanthanide coordination complexes

constructed

by

a

V-typed

tetracarboxylate

3-(3,5-dicarboxylphenyl)-5-(4-carboxylphenyl)-1-H-1,2,4-triazole

(H3L2),

ligand, and

discussed their characteristic luminescence simply. The result demonstrates that such a ligand can sensitize lanthanide ions efficiently. In this regard, we have employed the other

two

similar

ligands

with

different

triplet

energy

levels,

3-(3,5-dicarboxylphenyl)-5-(pyrid-4-yl)-1-H-1,2,4-triazole

namely, (H2L1),

3,5-bis(3,5-dicarboxylphenyl)-1-H-1,2,4-triazole (H4L3), along with the previous ligand to synthesize and characterize a series of lanthanide coordination polymers. We attempt to inquiry the impact of relevant factors in structure, such as coordination modes of central Ln3+ ions, the ligand-centered triplet state energies and water molecules in the inner coordination spheres, on the photophysical properties of the lanthanide coordination polymers. In this study, fifteen lanthanide coordination polymers, [LnL1(HCOO)H2O] (Ln3+ = Sm3+ 1, Eu3+ 2, Gd3+ 3, Tb3+ 4 and Dy3+ 5), [Me2NH2][Ln3(L2)3(HCOO)]⋅DMF⋅15H2O (Ln3+ = Eu3+ 6, Gd3+ 7, Tb3+ 8 and Dy3+ 9) and [Ln(HL3)(H2O)3]⋅1.5H2O (Ln3+ = Sm3+ 10, Eu3+ 11, Gd3+ 12, Tb3+ 13, Dy3+ 14 and Er3+ 15), have been synthesized. The luminescent properties of these complexes, consisting of observed and radiative luminescence lifetimes, radiative and nonradiative decay rates, quantum yields and intrinsic quantum yields, have been measured and compared. The relationships between the crystal structures of these lanthanide complexes and their photophysical properties are discussed detailedly. Note that the syntheses and structures of 6-9 have been discussed in our previous work54, thus, they are only described here briefly for systematic comparisons.



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Experimental Section Material and Methods. All relevant chemicals and solvents were used in the synthetic processes as purchased without any further purification. Powder X-ray diffraction (PXRD) patterns were collected by a Rigaku MiniFlex 600 diffractometer using Cu Kα radiation (λ= 1.5406 Å). Elemental analyses (C, H and N) were performed on a German Elementary Vario EL III instrument. Thermogravimetric analyses (TGA) were recorded in the temperature range of 30-900℃, with a heating rate of 10℃/min under flowing nitrogen atmosphere on a Netzsch Model STA 449C instrument. Emission and excitation spectra in solid state were investigated on a Horiba Jobin-Yvon Fluorolog-3 spectrofluorometer. Time-resolved fluorescence (transient decays) were carried out on a FLS920 spectrophotometer analyzer with a continuous xenon lamp (450 W). The overall photoluminescence quantum yields were obtained on an integrating sphere covered with barium sulfate at room temperature. Synthesis of [LnL1(HCOO)H2O] (Ln3+ = Sm3+ 1, Eu3+ 2, Gd3+ 3, Tb3+ 4 and Dy3+ 5). Ln(NO3)3.6H2O (45 mg， 0.10 mmol), H2L1 (31 mg, 0.10 mmol) in 10 mL mixed solvent of DMF/H2O (v/v = 9/1) were sealed in a 23 mL Teflon cup and heated at 150℃. The mixture was cooled to room temperature after four days, and the colorless crystals were obtained. The yields based on H2L1 are ca. 46%, 58%, 63%, 61% and 54% for complexes 1-5, respectively. [SmL1(HCOO)H2O] (1) Elemental analysis (%): Calcd for C16H11SmN4O7, C, 36.89; H, 2.21; N, 10.69. Found: C, 36.94; H, 2.17; N, 10.73. [EuL1(HCOO)H2O] (2) Elemental analysis (%): Calcd for C16H11EuN4O7, C, 36.73; H, 2.12; N, 10.71. Found: C, 36.58; H, 2.31; N, 10.67. [GdL1(HCOO)H2O] (3) Elemental analysis (%): Calcd for C16H11GdN4O7, C, 36.36; H, 2.10; N, 10.60. Found: C, 36.25; H, 2.35; N, 10.53. [TbL1(HCOO)H2O] (4) Elemental analysis (%): Calcd for C16H11TbN4O7, C, 36.24; H, 2.09; N, 10.57. Found: C, 36.16; H, 2.26; N, 10.48. [DyL1(HCOO)H2O] (5) Elemental analysis (%): Calcd for C16H11DyN4O7, C, 36.28; H, 2.16; N, 10.49. Found: C, 36.20; H, 2.27; N, 10.41.


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Synthesis of [Ln(HL3)(H2O)3]⋅⋅2H2O (Ln3+ = Sm3+ 10, Eu3+ 11, Gd3+ 12, Tb3+ 13, Dy3+ 14 and Er3+ 15). Ln(NO3)3.6H2O (45 mg, 0.10 mmol), H4L3 (40 mg, 0.10 mmol) in DEF (N,N-diethylformamide) (2.5 mL)/H2O (2.5 mL) solvent were sealed in approximately 23 mL glass vials and heated at 85℃. The mixture was cooled to room temperature after three days, and the colorless crystals were obtained. The yields based on H4L3 are ca. 49%, 56%, 50%, 54%, 52% and 44% for complexes 10-15, respectively. [Sm(HL3)(H2O)3]⋅⋅1.5H2O

(10)

Elemental

analysis

(%):

Calcd

for

C18H17SmN3O12.5, C, 34.61; H, 2.79; N, 6.73. Found: C, 34.72; H, 2.71; N, 6.82. [Eu(HL3)(H2O)3]⋅⋅1.5H2O

(11)

Elemental

analysis

(%):

Calcd

for

C18H17EuN3O12.5, C, 34.46; H, 2.73; N, 6.69. Found: C, 34.35; H, 2.81; N, 6.72. [Gd(HL3)(H2O)3]⋅⋅1.5H2O

(12) Elemental

analysis

(%):

Calcd

for

C18H17GdN3O12.5, C, 34.18; H, 2.69; N, 6.61. Found: C, 34.21; H, 2.75; N, 6.67. [Tb(HL3)(H2O)3]⋅⋅1.5H2O

(13)

Elemental

analysis

(%):

Calcd

for

C18H17TbN3O12.5, C, 34.08; H, 2.75; N, 6.54. Found: C, 34.16; H, 2.70; N, 6.60. [Dy(HL3)(H2O)3]⋅⋅1.5H2O

(14)

Elemental

analysis

(%):

Calcd

for

C18H17DyN3O12.5, C, 34.13; H, 2.63; N, 6.47. Found: C, 34.20; H, 2.58; N, 6.56. [Er(HL3)(H2O)3]⋅⋅1.5H2O

(15)

Elemental

analysis

(%):

Calcd

for

C18H17ErN3O12.5, C, 34.01; H, 2.58; N, 6.51. Found: C, 34.11; H, 2.64; N, 6.43. Single-Crystal

X-ray

Crystallography.

Crystal

structure

data

for

complexes 1-15 were obtained on a SuperNova diffractomete using a ω scan. All the structures of complexes 1-15 were solved using direct methods and the full-matrix least-squares refinement on F2 were finished using the SHELX-97.55 All non-hydrogen atoms were refined with anisotropic thermal refinement. Hydrogen atoms of the organic ligands were generated geometrically, while those of the water molecules could not be determined. PLATON/SQUEEZE56,57 was employed on complexes 6-15, because of the highly disordered solvent molecules of those complexes. The final chemical formulas were obtained from the refined data combined with the elemental analysis and TGA data. The



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CCDC numbers for complexes 1-15 are 1437623, 1430881, 1430895, 1430894, 1437624, 1400776, 1400775, 1045953, 1400777, 1437625, 1430897, 1430898, 1430899, 1437558, 1437559, respectively. Crystal data and structure refinements for complexes 1-15 are summarized in Table 1 and Table S1.


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Table 1. Crystal data and refinement results for complexes 1-5 and 10-15. 1

2

3

4

5

10

Formula

C16H11SmN4O

C16H11EuN4O7

C16H11GdN4O7

C16H11TbN4O7

C16H11DyN4O7

C18H17SmN3O12.5

Formula weight

521.65

523.25

528.54

530.22

533.79

625.70

Crystal system

monoclinic

monoclinic

monoclinic

monoclinic

monoclinic

orthorhombic

7

space group

P21/c

P21/c

P21/c

P21/c

P21/c

Fdd2

a (Å)

12.6415 (2)

12.8065 (3)

12.6340 (5)

12.8206(5)

12.8173(4)

11.5324(4)

b (Å)

16.5106(3)

16.5864 (3)

16.5070 (6)

16.5270(8)

16.5251(4)

40.8665(16)

c (Å)

8.1446 (1)

8.1792 (1)

8.1412 (3)

8.1363(3)

8.1335(2)

17.0473(9)

α (°)

90

90

90

90

90

90

β (°)

104.506 (2)

104.997 (2)

104.324(4)

104.878(4)

104.905(3)

90

γ (°)

90

90

90

90

90

90

Volume ( Å3)

1645.74 (5)

1678.20 (6)

1645.06(11)

1666.17(12)

1664.77(8)

8034.2 (6)

T (K)

100

100

100

100

100

100

Z

4

4

4

4

4

16

F (000)

1008

1012

1016

1020

1024

4592

R1 (I>2σ(I))

0.0403

0.0596

0.0576

0.0389

0.0469

0.0851

ωR2 (reflections)

0.1119

0.1699

0.1627

0.1077

0.1364

0.2335

Goodness of fit on F2

1.026

1.047

1.044

1.027

1.042

1.044

11

12

13

14

15

Formula

C18H17EuN3O12.5

C18H17GdN3O12.5

C18H17TbN3O12.5

C18H17DyN3O12.5

C18H17ErN3O12.5

Formula weight

627.30

632.59

634.78

637.84

642.60

Crystal system

orthorhombic

orthorhombic

orthorhombic

orthorhombic

orthorhombic

space group

Fdd2

Fdd2

Fdd2

Fdd2

Fdd2

a (Å)

11.5538(3)

11.5486(3)

11.5002(4)

11.5293(5)

11.5229(5)

b (Å)

41.0118(10)

40.8635(10)

40.7068(13)

40.471(2)

40.4992(19)

c (Å)

16.7789(6)

16.7777(5)

17.0238(6)

17.1304(8)

17.1514(15)

α (°)

90

90

90

90

90

β (°)

90

90

90

90

90

γ (°)

90

90

90

90

90

Volume ( Å3)

7950.6 (4)

7917.7 (4)

7965.5 (5)

7993.1 (6)

8005.5(9)

T (K)

100

100

100

100

100

Z

16

16

16

16

16

F (000)

4592

4624

4640

4656

4672

R1 (I>2σ(I))

0.0555

0.0346

0.0307

0.0445

0.0565

ωR2(reflections)

0.1465

0.0923

0.0813

0.1193

0.1634

1.050

1.037

1.080

1.048

1.005

2

Goodness of fit on F a

b

R1 = Σ||Fo| - |Fc||/Σ|Fo|. ωR2 =

[Σω(Fo2

-

Fc2)2/Σω(Fo2)2]1/2.

Results and Discussion



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Fig. 1 Coordination modes of the three ligands ([L1]2-: MODE I; [L2]3-: MODE II and III; [HL3]3-: MODE IV).

Crystal structure analysis data reveal that complexes 1-5, 6-9 and 10-15 are isomorphous, respectively. Therefore, Tb-complexes (complexes 4, 8 and 13) are selected to discuss in detail as representative. Crystal Structure Description of [LnL1(HCOO)H2O] Complexes (Ln3+ = Sm3+ 1, Eu3+ 2, Gd3+ 3, Tb3+ 4 and Dy3+ 5). Crystal structure analysis data reveal that complex 4 crystallizes in monoclinic space group P21/c. One crystallographically independent Tb3+ ion, one fully deprotonated [L1]2- ligand, one formate anion playing a significant role in balancing charge, and one coordinated water molecule composes the asymmetric unit. As shown in Fig. 2a, the coordination environment of eight-coordinated terbium center of complex 4 consists of four oxygen atoms of µ2-bridging carboxylate groups from four [L1]2- ligands, three oxygen atoms of two formate anions and one oxygen atom of coordinated water molecule. The Tb1 atom and its corresponding symmetry generated one are bridged backward and forward by two bidentate carboxylic groups of two [L1]2- ligands and one formate anion to extend a terbium-carboxylate chain [Tb2(µ2-COO)2(COO)]n, in which the separation of Tb···Tb is 4.295Å (Fig. 2b). The lengths of Tb-O bonds are among 2.325 Å 2.501 Å, which are in line with those corresponding terbium-oxygen complexes


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Fig. 2 (a) Coordination environment of the Tb 3+ ion in complex 4. All H atoms and solvent molecules are omitted. Insert: coordination polyhedron of Tb 3+ ion. (Symmetry codes: A, x, 3/2-y, z-1/2; B, 2-x, 1/2+y, 5/2-z; C, 2-x, 1-y, 2-z.). (b) 1D [Tb2(µ2-COO)2(COO)]n metal-carboxylate chain along the [001] direction. (c) 2D ladder-like structure in 4 along the c axis. (d) 2D layer architecture in 4.

reported before (Table S2).58,59 The fully deprotonated ligand [L1]2- exhibits a µ4-bridging coordination mode (Fig. 1, MODE I): two carboxylate groups both act as µ2-bridging bidentate linkers. And the adjacent terbium-carboxylate chains are ligated by [L1]2- ligands along the a axis, manifesting a 2D ladder-like structure, in which, the landings are composed of two almost paralleled aromatic rings of two ligands, and the remaining moieties of two ligands serve as the side rails (Fig. 2c). The ladder-like structure can convert to a typical 2D layer architecture in the suitable direction in space (Fig. 2d). Then, as presented in Fig. 3, the 2D layers are connected by the almost paralleled aromatic rings to generate a 3D supramolecular structure by means of two ways: one is the π···π stacking interactions between partially overlapping aromatic rings with the distance of the centroid being 3.965 Å; the other is hydrogen bonding interactions, which refer to O3-H3A···N4 (2.768 Å) and O3-H3B···N4



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(3.430 Å) belonging to the coordinated water molecules and pyridyl nitrogen atoms of ligands.

Fig. 3 (a) The modes of the hydrogen bonding interactions and partial π···π stacking interactions of partially overlapping aromatic rings in complex 4 along the c axis. (N atoms involved in the hydrogen bondings are showed in ball-stick model with blue and the H atoms of the coordinated waters involved are also presented to understand the generation of the hydrogen bondings.). (b) 3D space-filling model of complex 4 along the c axis.

Crystal

Structure

Description

of

[Me2NH2][Ln3(L2)3(HCOO)]⋅⋅DMF⋅⋅15H2O Complexes (Ln3+ = Eu3+ 6, Gd3+ 7, Tb3+ 8 and Dy3+ 9). The asymmetric unit of complex 8 contains three crystallographically independent Tb3+ ions, three fully deprotonated [L2]3ligands and one formate anion, in which two Tb3+ ions are nine-coordinated and the other one is eight-coordinated with no water molecules in their coordination spheres (Fig. S1a). As depicted in Fig. 1, the [L2]3- ligand shows two coordination modes: µ6-bridging mode (MODE II) and µ9-bridging mode (MODE III). Three Tb3+ ions are connected through eight carboxylate groups to generate a trinuclear Tb3(COO)8 unit, which is further ligated upward and downward by means of the space ligands to develop a 3D network with an open channel (Fig. S1e). Crystal Structure Description of [Ln(HL3)(H2O)3]⋅⋅1.5H2O Complexes (Ln3+ = Sm3+ 10, Eu3+ 11, Gd3+ 12, Tb3+ 13, Dy3+ 14 and Er3+ 15). Crystal structure analysis data reveal that complex 13 crystallizes in orthorhombic space group Fdd2. One crystallographically independent Tb3+ ion, one


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Fig. 4 (a) Coordination environment of the Tb3+ ion in complex 13. All hydrogen atoms and solvent molecules are omitted. Insert: coordination polyhedron of Tb3+ ion. (Symmetry codes: A, 9/4-x, 1/4+y, 1/4+z; B, 1-x, 1-y, z; C, 2-x, 1-y, z. Tb3+, green; O, red; N, blue; C, gray). (b) The binuclear unit [Tb2(µ2-COO)2] in 13. (c) 1D chain substructure viewed along the c axis in 13. (d) 2D layer network constructed from the 1D chain viewed along the c axis in 13. (e) 3D framework supported by the spacer ligands in 13.

incompletely deprotonated [HL3]3- ligand and three coordinated water molecules composes the asymmetric unit of complex 13. As illustrated in Fig. 4a, the coordination environment of eight-coordinated terbium center of complex 13 consists of two oxygen atoms of µ2-bridging carboxylate groups, two oxygen atoms of one chelating bidentate carboxylate group, one oxygen atom of one monodentate carboxylate group and three oxygen atoms of three coordinated water molecules. In complex 13, the [HL3]3- ligand exhibits a µ4-bridging coordination mode (Fig. 1, MODE IV), in which three carboxylate groups adopt the µ2-bridging bidentate mode, chelating bidentate mode and monodentate mode, respectively, leaving a carboxylate group uncoordinated. The Tb1 atom and its corresponding symmetry generated one are bridged by two µ2-bridging carboxylate groups to form a binuclear cluster



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[Tb2(µ2-COO)2]n, with the separation of Tb···Tb is 5.473Å (Fig. 4b). The Tb-O bond lengths are ranging from 2.258 Å - 2.505 Å, consistent with those of the terbium-oxygen complexes reported before (Table S2).58,59 The neighboring Tb2(µ2-COO)2 clusters are linked into a 1D chain architecture through two µ2-bridging bidentate carboxylate groups and two monodentate carboxylate groups supported by four ligands (Fig. 4c). The adjacent 1D chains are further connected by chelating bidentate carboxylic groups of the ligands to form a 2D layer network as Fig. 4d shown. Then, these layers are cross-ligated to construct a 3D skeleton through the linkers (Fig. 4e). The hydrogen bonding interactions referring to O-H···N and O-H···O exist in the structure of complex 13 as well (Fig. S2).

Fig. 5 (a) The simplified 6-connected node of the binuclear unit [Tb2(µ2-COO)2]. (b) The simplified 3-connected node of the ligand. (c) The (3,6)-connected topological two-nodal net with the Schläfli symbol {4.62}2{42.610.83} in complex 13.

Topology analysis is applied in order to better understand the structure of complex 13. In complexes 13, if the binuclear motif [Tb2(µ2-COO)2] and the [HL3]3- ligand are considered to be a 3-connected node and a 6-connected node, respectively, the


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structure of complex 13 can be simplified to a binodal (3, 6)-connected net with the stoichiometry (3-c)2(6-c), which is a new topology with the point symbol {4.62}2{42.610.83} (Fig. 5).

Fig. 6 The TGA curves of complexes 4 and 13.

Thermal analysis (TGA) and Powder X-ray diffraction (PXRD) properties. Complexes 1-5 and 10-15 were subjected to thermogravimetric analyses (TGA) from 30 to 900 ℃, under nitrogen atmosphere, to observe the thermal stability of these structures (Fig. S5). The TGA curves of complexes 4 and 13 are selected to describe detailedly since complexes 1-5 and 10-15 are isomorphous and have similar thermal behavior respectively (Fig. 6). For complex 4, 3.75% weight loss is found from 30 to 380 ℃ resulted from the liberation of one coordinated water molecule (calculated, 3.40%), and the remained framework begins to decompose after further heated at about 420 °C, indicating that complex 4 exhibits a high thermal stability. For complex 13, the process of weight loss shows two obvious steps. The first weight loss of 4.71% below 105 °C is attributed to the departure of one and a half lattice water molecules (calculated, 4.26%), and the second weight loss of 8.98% (calculated, 8.51%) from 105 °C to 235 °C is assigned to the loss of three coordinated water molecules. Then, the gradual decomposition of the residual occurs above 425 °C, demonstrating the high thermal stability of complex 13.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 7 PXRD patterns of complex 4 (a) and complex 13 (b) after exposed in open air with medial humidity of 45 % for 24 h, 48 h, 72 h and 7 days.

The phase purities of complexes 1-5 and 10-15 were confirmed by powder X-ray diffraction patterns, as shown in Fig. S3. Simultaneously, the stabilities of these complexes in the open air are also validated. Specifically, the prepared complexes 4 and 13 were exposed in open air, the medial humidity of which is 45 % approximately, for 24 h, 48 h, 72 h and 7 days, respectively. Then, the PXRD patterns were used to confirm the air stabilities (Fig. 7). The results reveal that all complexes are stable when exposed in air containing water vapor and they can keep their original skeletons completely in open air more than 7 days. Solid-State Photophysical Studies of Eu3+ and Tb3+ Complexes. Room-temperature luminescent spectra of all complexes of Eu3+ (2, 6 and 11) and Tb3+ (4, 8 and 13) were recorded in solid-state. Broad band in 250-450 nm are observed in the excitation spectra (dot lines, Fig. 8) of all complexes, which may be ascribed to the n → π* or π → π* transitions of ligands H2L1, H3L2 and H4L3. Besides, four sharp characteristic peaks of Eu3+ ions centered at about 363 nm, 382 nm, 395 nm and 417 nm, are detected in all excitation spectra of Eu3+ complexes (Fig. 8a-8c), resulted from the series of transitions between 7

F0,1 and 5G6, 5H4, 5L6 and 5D3 energy levels.60 Meanwhile, weak peaks ascribed


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to the 7F0 → 5L10 and 7F0 → 5G6 transitions of Tb3+ ions exist in the excitation spectra of Tb3+ complexes as well (Fig. 8d and 8e).

Fig. 8 The excitation spectra (dot, λem= 614 nm for Eu3+ complexes (a-c) and 543 nm for Tb3+ complexes (d-f)) and emission spectra (solid, λex= 321 nm for Eu3+ complexes (a-c) and 338 nm for Tb3+ complexes (d-f)).

Solid-state emission spectra of Eu3+ ions and Tb3+ ions at room temperature are presented in solid lines in Fig. 8. Under excitation at 321 nm, complexes 2, 6 and 11 display the characteristic red luminescence of Eu3+ ions with similar emission bands among 500-750 nm region and maximum intensities at 590 nm, 614 nm, 650 nm and 699 nm, arising from the 5D0 → 7FJ (J=1-4) transitions, respectively. The absence or weak intensity of ligand-centered emissions in



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

400-450 nm demonstrates that the energy adsorbed by three ligands can be transferred to the emitting levels of Eu3+ centers efficiently.60,61 According to the ED rule,62,63 the symmetry forbidden transition between 5D0 and 7F0 is only involved with the site symmetry of Cs, Cn and Cnv. The presence of the 5D0 → 7

F0 transitions in the emission spectra of Eu3+ complexes indicates a fact that

the Eu3+ centers in complexes 2, 6 and 11 locate the sites free from inversion symmetry, which matches with the results of X-ray crystallographic analysis. From the other side, on the basis of the Judd-Ofelt theory64,65, the intensity of 5

D0 → 7F2 transition (electric-dipole) can be considered to be the probe of

crystalline filed. The 5D0 → 7F2 transition is supersensitive to the coordination sphere of Eu3+ center, while the 5D0 → 7F1 transition (magnetic-dipole) is nearly independent of the ligand field.66 If the 5D0 → 7F2 transition is dominant in the luminescent spectrum, the Eu3+ ion is located a site free from inversion symmetry, on the contrary, the Eu3+ ion is located a symmetry site within inversion center.67-69 From the emission spectra of Eu3+ complexes depicted in Fig. 8a-8c, it is found that the 5D0 → 7F2 transitions are stronger than those of the 5D0 → 7F1 transitions, implying the asymmetric coordination environments of the Eu3+ centers in complex 2, 6 and 11, in agreement with the result discussed above. As show in Fig. 8d-8f, the solid-state emission spectra of complexes 4, 8 and 13 excited at 338 nm show typical peaks of Tb3+ ions at 489 nm, 543 nm, 584 nm and 620 nm, resulted from the transitions between 5D4 and 7FJ (J=6, 5, 4 and 3). The inexistence of ligand-centered emission bands in all luminescence spectra of Tb3+ ions in 400-450 nm demonstrates that Tb3+ centers can be sensitized by the ligands H2L1, H3L2 and H4L3 efficiently.60,61 The intensities of 5D0 → 7F2 transitions (Eu3+) and 5D4 → 7F5 transitions (Tb3+), the most intense characteristic peaks in the emission spectra of the corresponding complexes, were monitored to estimate the time-resolved luminescence lifetimes (τobs). The observed transient decay curves of these complexes, as shown in Fig. 9, are well-fitted with monoexponential functions


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Fig. 9 The luminescence decay curves for Eu3+ complexes (2 (a), 6 (b) and 11 (c)) excited at 321 nm and Tb3+ complexes (4 (d), 8 (e) and 13 (f)) excited at 338 nm at 298 K (red) and 77 K (blue) (Scattering points: the experimental data; Solid lines: the fitting results.).

both at 298 K and 77 K. The lifetimes of Eu3+ complexes fall in the range of 0.28-0.68 ms at 298 K and 0.36-0.93 ms at 77 K, and the corresponding values for Tb3+ complexes are 0.61-1.04 ms at 298 K and 0.74-1.16 ms at 77 K (Table 2). The presence of the coordinated water molecules, causing the main nonradiative decay involved with OH oscillators, is considered to be the dominant influencing factor of the lifetimes.70,71 Thus, owing to the absence of water molecules in the coordination environments, the luminescence lifetimes for complexes 6 and 8 are observed up to 0.68 ms and 0.72 ms, in which the coordination sites are occupied by oxygen atoms of the carboxylic groups and formate anion completely.70,71 For the same reason, the existence of three coordinated water molecules in the inner coordination spheres



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increases the nonradiative decay, resulting in relatively shorter lifetimes in complexes 11 and 13. It is worth to underline, although one water molecule resides in the coordination environments of Ln3+ centers in complexes 2 and 4, they also exhibit long lifetimes (0.62 ms and 1.04 ms). It may be ascribed to the π···π stacking interactions between partially overlapping aromatic rings in the two complexes, which may make conjugated chromophores closer to each other and induce electronic interactions, resulting in the enhancement of the observed luminescence lifetimes consequently.72,73 Furthermore, compared with the other four complexes, the obvious variation of lifetimes between 298 K and 77 K are observed in complexes 6 and 8, with the values up to 0.93 ms and 1.16 ms at 77 K respectively. It may be ascribed to the existence of notable thermal-activated deactivation processes,62,63 such as back energy transfer or other thermodynamic interactions. Quantum yield (Φ) is also a significant parameter to estimate the efficiency of sensitized emission process from the ligand to Ln3+ centers. The ligand used to construct the objective structure should possess a main ability: protect the metal-center from external nonradiative deactivation processes induced by external sources, like solvent molecules, and then guarantee the high-effective energy transfer from ligand to metal and high quantum yield.74 To further investigate the energy transfer processes from the ligands to Eu3+ centers in complexes 2, 6 and 11, the relationship between the sensitization efficiencies (ηsense) and the observed luminescence lifetimes (τobs) is necessary to be described in terms of the following equation (i):74,75 Φoverall = Φintr × ηsense

(i)

Where Φoverall and Φintr are ligand-sensitized overall luminescence quantum yields and intrinsic quantum yields of Eu3+, respectively; ηsense is sensitization efficiency of ligand-to-metal energy transfer. Different from the overall quantum yields, the intrinsic quantum yields could not be obtained through experimental determination, considering the very low absorption intensity of f-f transition. Therefore, the equation (ii) is introduced to estimate the intrinsic quantum yields, after calculating the radiative lifetimes (τrad) using equation


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Table 2. The data of observed (τobs) and radiative (τrad) luminescence lifetimes at 298 K and 77 K, radiative (Аrad) and nonradiative (Аnrad) decay rates, quantum yields (Φoverall), intrinsic quantum yields (Φintr) and sensitization efficiencies (ηsense), the triplet state energy (T1) and the energy gaps (∆E) between the energy of the triplet states of ligands and the emitting levels of Ln3+ ions for Eu3+ and Tb3+ complexes. τobs (ms) complex

Φoverall(%)

298 K

77 K

τrad (ms)

Аrad(s-1)

Аnrad(s-1)

Φintr(%)

ηsense(%)

T1(cm-1)

∆E

2

10.1

0.62

0.66

5.95

168

1445

10.5

96.2

24450

7183

4

34.6

1.04

1.09

24450

3950

6

14.3

0.68

0.93

4.15

241

834

16.4

87.2

24154

6887

8

40.2

0.72

1.16

24154

3654

11

4.8

0.28

0.36

21552

4285

13

14.9

0.61

0.74

21552

1052

3.51

185

3286

8.0

60

(iii):75 Φintr = Аrad/(Аrad + Аnrad) = τobs /τrad Аrad = 1/τrad = АMD,0 × n3 × (Itot/IMD)

(ii) (iii)

In these equations, τobs and τrad are observed and radiative luminescence lifetimes of emitting state (5D0); Аrad and Аnrad are radiative and nonradiative decay rates, respectively; АMD,0 is the spontaneous emission probability for the 5D0 → 7F1 transition in vacuo, 14.65 s-1; n is the refractive index of the solvent (1.5 for the solid sample of metal-organic complexes); Itot represents the total integrated emission intensity of the 5D0 → 7FJ transitions (J = 0-4) in the emission spectra of Eu3+, and IMD signifies the integrated intensity of the 5D0 → 7F1 transition (magnetic dipole). Table 2 sums up the observed luminescence lifetimes (τobs), quantum yields (Φ) and other pertinent optical parameters of Eu3+ and Tb3+ complexes. The data indicate the quantum yields for these complexes show an increase in sequence of 6 > 2> 11 for Eu3+ and 8 > 4 > 13 for Tb3+. When the coordinated water molecules are replaced by the chelating carboxylate oxygen atoms in the coordination spheres of Ln3+ centers, the overall luminescence quantum yields could be observed improved notably. Therefore, complex 6 for Eu3+ and complex 8 for Tb3+ exhibit the largest overall quantum yields (14.3% and 40.2%) among the six complexes, which are the outcome



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of the absence of vibronic couplings in the coordinated environments of Ln3+ centers, as aforementioned. In contrary, owing to the presence of three water molecules in the inner coordinated spheres, the smallest overall quantum yields appear on the complex 11 for Eu3+ and complex 13 for Tb3+.

Fig. 10 Room-temperature emission spectra of ligands (H3L2 (a), H2L1 (b) and H4L3 (c)) and phosphorescence spectra of complexes [Me2NH2][Gd3(L2)3(HCOO)]⋅DMF⋅15H2O (7) (d), [GdL1(HCOO)H2O] (3) (e) and [Gd(HL3)(H2O)3]⋅1.5H2O (12) (f) at 77 K.

Energy transfer process studies Efficient energy transfer between the triplet state of the ligand and the emitting levels of central Ln3+ ions is key to obtain highly luminescent lanthanide compounds.76,77 To illuminate the mechanism of energy transfer in the six Eu3+ and Tb3+ complexes, there is a need to establish the relevant electronic energy levels of the triplet states of the ligands H2L1, H3L2 and H4L3. The ligand-centered triplet states T1 were calculated from the shortest wavelength emission edges of the corresponding phosphorescence

spectra

of

complexes

[GdL1(HCOO)H2O]

(3),

[Me2NH2][Gd3(L2)3(HCOO)]⋅DMF⋅15H2O (7) and [Gd(HL3)(H2O)3]⋅1.5H2O (12) at 77 K. Because the lowest excited state of the Gd3+ ion is too high, the energy transfer could not be occurred from the ligand to the Gd3+ ion. Hence, the energy levels of Gd3+ ion have no significant influence on the energy levels of ligand-centered triplet states.78,79 As illustrated in Fig. 10, the overlap between the low-temperature emission


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spectra of Gd3+ complexes and the corresponding room-temperature emission spectra of the ligands allows for the ligand-to-metal energy transfer, indicating that the three ligands (H2L1, H3L2 and H4L3) exhibit efficient antennae effect to the central Ln3+ ions. The phosphorescence spectra of three Gd3+ complexes reveal that the triplet state energy levels T1 of three ligands are 24450 cm-1 (409 nm), 24154 cm-1 (414 nm) and 21552 cm-1 (464 nm), respectively, all of which are obviously higher than the lowest excited states of Eu3+ (5D0: 17267 cm-1) and Tb3+ (5D4: 20500 cm-1). Latva’s empirical rule76,80 states that an optimal ligand-to-metal energy transfer process for Ln3+ needs (∆E = T1 - 5DJ) 2500-4000 cm-1 for Eu3+ and 2500-4500 cm-1 for Tb3+. The energy gaps between the experimentally triplet state energy levels of three ligands and central Ln3+ ions are calculated subsequently: ∆E(T1 - 5D0) for Eu3+ are 7183, 6887 and 4285 cm-1, and ∆E(T1 - 5D4) for Tb3+ are 3950, 3654 and 1052 cm-1 in sequence of H2L1, H3L2 and H4L3. The result indicates that, compared with Eu3+ complexes, Tb3+ complexes exhibit more suitable energy gaps and Tb3+ ions can be sensitized by the ligands more effectively. It gives a reasonable theoretical explanation of the relatively higher observed overall quantum yields of Tb3+ complexes than Eu3+ complexes.81 In three Tb3+ complexes, the triplet energy level T1 of ligand H4L3 (21552 cm-1) is extremely close to the resonant level of Tb3+ ion (20500 cm-1) in complex 13, and the energy gap (1052 cm-1) is too small to prevent the back energy transfer from excited state of Tb3+ ion to triplet state of the ligand.82,83 Therefore, a relatively lower efficiency of ligand-to-metal energy transfer takes place in complex 13, leading to a low observed luminescence quantum yield of complex 13. Likewise, it is not difficult to understand that, in the Eu3+ complexes, the observed luminescence quantum yields of complexes 2 and 6 (10.1% and 14.3%) are higher than that of complex 11 (4.8%), since the triplet state of the ligand H4L3 (21552 cm-1) is very close to the 5D2 emitting state of Eu3+ ion (212000 cm-1), which offers opportunity for the thermal-assisted back energy transfer from excited state of central Eu3+ ion in complex 11.84,85

Conclusion In conclusion, we have solvothermally designed, synthesized and characterized fifteen



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lanthanide complexes utilizing three new di-, tri- and tetra-carboxylic ligands with imidazolyl groups. Complexes 1-5 exhibit 3D supramolecular structures based on 2D layer skeletons through π···π stacking and hydrogen bonding interactions. Complexes 6-9 stretch to 3D networks by the coordination between 1D metal-chains [Ln3(COO)8]n and rigid ligand H3L2 with an open one-dimensional channel. Complexes

10-15

show

3D

architectures

constructed

by

binuclear

units

[Ln2(µ2-COO)2]n with (3,6)-connected two-nodal {4.62}2{42.610.83} nets. Luminescent spectra of Eu3+ and Tb3+ complexes at room temperature indicate that the three carboxylic ligands H2L1, H3L2 and H4L3 are promising potential light sensitizers for lanthanide ions in view of the well-matched relationship between the triplet energy levels of three ligands and the emitting levels of Eu3+/Tb3+ centers. Further studies on the luminescent properties reveal that Tb3+ complexes show higher overall quantum yields and longer luminescence lifetimes than the Eu3+ complexes, which can be ascribed to the more appropriate energy gaps between the triplet states of the ligands and the lowest excited states of Tb3+ centers. Furthermore, the relationships between the coordinated water molecules and the luminescent properties of Eu3+/Tb3+ complexes are discussed. The result shows, due to the absence of water molecules in inner coordination spheres, the quantum yields and luminescence lifetimes for complexes 6 and 8 are observed up to 14.3% and 40.2%, 0.68 ms and 0.72 ms, respectively. In general, the work gives some references on the strategy towards the design of highly luminescent lanthanide materials, and further studies on the structures and photophysical properties of lanthanide coordination complexes with different organic ligands are still under way.

Acknowledgements This work was financially supported by the 973 Program (2014CB932101, 2013CB933203) and the National Natural Science Foundation of China (21131006, 21390392).

Notes and references


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a

Laboratory of Photochemical and Optical Physics, Fujian Institute of Research

on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, Fujian, 350002, China b

University of the Chinese Academy of Sciences, Beijing, 100049, China

c

School of Chemistry and Chemical Engineering, Jiangsu Normal University,

Jiangsu, 221116, China *To whom correspondence should be addressed: E-mail: [email protected]; [email protected]; Fax: +86-591-63173149; Tel: +86-591-63173151 Supporting Information. The hydrogen bonding interactions in complexes 10-15, XRD patterns, TGA curves, selected bond lengths and angles of complexes 1-5 and 10-15, and additional experimental details. This information is available free of charge via the Internet at http://pubs.acs.org/.

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For Table of Contents Use Only

Table of Contents Self-Assembly Syntheses, Structural Characterization and Luminescent Properties of Lanthanide Coordination Polymers Constructed by Three Triazole-Carboxylate Ligands Yan Yang,a,b Feilong Jiang,a Caiping Liu,a,b Lian Chen,*a Yanli Gai,c Jiandong Pang,a,b Kongzhao Su,a Xiuyan Wan,a,b and Maochun Hong* Fifteen lanthanide coordination complexes based on three carboxylic ligands with imidazolyl groups, have been solvothermally synthesized. Luminescent spectra of Eu3+/Tb3+ complexes indicate that three ligands are promising light sensitizers for lanthanide ions. Luminescent lifetimes and quantum yields of these compounds were furthered measured and compared. The correlations between their structures and photophysical properties are discussed detailedly.


Self-Assembly Syntheses, Structural Characterization, and

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