The near-infrared luminescence and magnetism of dinuclear complexes with different local symmetries constructed from a β-diketonate co-ligand and bis-Schiff base ligand

Article abstract of DOI:10.1039/c9nj05571a

Five new phenoxo-O bridged rare-earth (RE) dinuclear complexes, obtained by adopting the auxiliary ligand Hdbm (dibenzoylmethane) and a bis-Schiff base ligand H₂L (N,N′-bis(2-hydroxy-3-methoxybenzylidene)-propane-1,2-diamine), [RE₂(d

Full text of DOI:10.1039/c9nj05571a

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Near-infrared luminescence and magnetism of dinuclear complexes with
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different local symmetries constructed by β-diketonate coligand and bis-Schiff
base ligand
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Huan-Huan Chen,a Dong-Fang Wu,a Yao-Yao Duan,a Ling Li,a Yun-Juan Wang,a
Xue-Mei Zhang,a Jian-Zhong Cui*a,b and Hong-Ling Gao*a,b
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Abstract
Five new phenoxo-O bridged rare-earth (RE) dinuclear complexes by adopting the identical
auxiliary ligand Hdbm (dibenzoylmethane) and a bis-Schiff base ligand H2L (N, N'- bis(2-
hydroxy- 3- methoxybenzylidene)- propane- 1, 2- diamine), [RE2(dbm)4L·CH3OH] (RE = Y (1),
Dy (2), Ho (3), Er (4) Yb (5)), were prepared and studied in terms of the structures, magnetism
and NIR luminescence. Complexes 1–5 crystallize isomorphically in a space group Pī. More
significantly, two central ions of the complexes adopt different coordination modes and
configurations. The symmetries of two central rare-earth ions are D2d and D4d, respectively. The
results of magnetic studies demonstrate that complex 2 shows frequency-dependent signals with
Ueff = 28 K. It is a typical feature of single-molecule magnet (SMM). For complexes 3, 4 and 5,
the characteristic peaks were found in near-infrared (NIR) fluorescent measurements, resulting
from the antenna effect. The photoluminescent lifetime (τav = 4.99 μs) and quantum yield (ΦLn
0.25%) for complex 5 were confirmed utilizing the luminescence decay curve.
=
1 Introduction
The complex ([Mn12O12(OAc)16(H2O)4]) was reported as the single-molecule magnet by R.
Sessoli et al. in 1993, which made the SMM begin to apply in quantum computation.1-3
Subsequently, the importance of the single ion magnetic anisotropy in the construction of SMMs
has brought attention of scientists. Then, lanthanides with high ground spin state and stronger
magnetic anisotropy enter the field of SMMs.4-6 Lanthanide single molecule magnets (Ln-SMMs)
have inspired many researchers since the first Ln-SMM [TbPc2]- entered the field with a high
relaxation energy (Ueff) of 331 K, which far exceeds the Ueff of 3d-SMMs.7 So far, SMMs have
been successfully constructed by some lanthanide ions including Ce3+, Nd3+, Tb3+, Dy3+, Ho3+,
Er3+, Yb3+ and so on.8-12 However, the Dy3+ ion is the most ideal choice of constructing SMMs
among them, which may be attributable to the following some elements. First and foremost, Dy3+
is a Kramers ion and the ground state is doubly degenerate. Second, from the view of ground state
and the first excitation level, there is a large energy gap (ΔmJ) between them. The third respect is
that Dy3+ ion has a high magnetic anisotropy.13 So far, SMMs have not been put into practical
application because the blocking temperature is so low that hysteresis can be observed only under
aDepartment of Chemistry, Tianjin University, Tianjin, 300354, China.
* Corresponding authors, E-mail: cuijianzhong@tju.edu.cn, ghl@tju.edu.cn.
bKey Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai
University, Tianjin, 300071, China.
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liquid-helium cooling.14-16 In 2017, C. A. P. Goodwin et al. reported a hexaDteOrIt:-1b0u.1t0y3l9/C9NJ05571A
DyIII-complex [Dy(Cpttt)2][B(C6F5)4] exhibiting magnetic hysteresis at 60 K that was 40 K higher
than the previous ones.17 Nevertheless, it is obvious that the threshold is still significantly lower
than the actual achievable 77K temperature of nitrogen liquefaction. Recently, the study of a
DyIII-SMM ([(η5-Cp*)Dy(η5-CpiPr5)][B(C6F5)4]) made by M. L. Tong et al. indicated magnetic
hysteresis above 80K and Ueff = 1541cm-1.18 Compared with [Dy(Cpttt)2][B(C6F5)4], the higher
blocking temperature of [(η5-Cp*)Dy(η5-CpiPr5)][B(C6F5)4] is due to the fact that its ligands not
only give a much stronger axial crystal field and axiality but also weak equatorial crystal field. In
addition, from the perspective of the crystal field, the different symmetry of the molecules has
different effects on the crystal field term. When the molecule is in D4d or D5h symmetry, the
quantum tunneling caused by the crystal field effect can be eliminated, and further improve the
properties of SMMs.19-21 Therefore, it is of great significance to regulate the coordination
configuration of central ion and axial and equatorial ligands in order to get excellent SMMs.
The NIR luminescence is the increasing concern of scientists owing to the significant
application value in electronic communication and fluorescence immunoassay.22-23 In 1958, G. A.
Crosby reported that dibenzoylmethane can sensitize the Yb3+ luminescence.24 Subsequently, the
influences of β-diketonates on Ln3+ luminescence were studied systematically. In 2013, M. R.
Pablo reported a series of complexes with β-diketonates confirming that the luminescence
intensity of Yb3+ is influenced by the β-diketonate chain length.25 β-diketonates with aryl groups
are appropriate for achieving sensitized luminescence of lanthanides, which can be attributed to
the strong absorption. In general, the HoIII, ErIII and YbIII ions produce NIR fluorescence because
of the transitions (5F5→5I7 and 5F5→5I6, 4I13/2 → 4I15/2, 2F5/2 - 2F7/2 ). The realization of the antenna
effect includes two steps: First, the energy is absorbed by the ligands. Second, the energy is
transferred from the lowest triplet energy of ligands to the excited state energy of lanthanide ions
by the method of nonradiative transition.26 Thus, the energy transfer can be accomplished utilizing
the antenna effect in LnIII-complexes with organic ligands.
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In this study, we select a bis-Schiff base ligand and β-diketonate to devise and construct
dinuclear rare-earth complexes [RE2(dbm)4L·CH3OH] (RE = Y (1), Dy (2), Ho (3), Er (4) Yb (5)).
Then, the paper presents the systematic investigation for the synthesis and the properties including
the near-infrared luminescence and magnetism of five phenoxo-O bridged complexes. Magnetic
studies reveal the SMM behavior of complex 2. The Ueff = 28 K (τ0 = 3.39 × 10-7 s) is got in 3000
Oe dc field. The studies of NIR luminescence indicate that the characteristic peaks of the Ho3+,
Er3+ and Yb3+ are observed for complexes 3, 4 and 5. Remarkably, complex 5 shows the
photoluminescent lifetime (τav = 4.99 μs) and quantum yield (ΦLn = 0.25%).
Scheme 1 The structures of N, N'- bis(2- hydroxy- 3- methoxybenzylidene)- propane- 1, 2-
diamine (H2L) and dibenzoylmethane (Hdbm).
2 Experiment
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DOI: 10.1039/C9NJ05571A
2.1 Materials and instruments
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The original materials including solvents and chemicals were purchased directly. Five
organic salts RE(dbm)3·2H2O (RE = Y, Dy, Ho, Er, Yb) were synthesized in accordance with the
reported preparation methods.27 The synthetic method of the bis-Schiff base ligand (H2L) was got
in term of the published literature.28 The instruments and test conditions used in all experiments
are summarized in ESI†.
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2.2 Synthesis of N, N'- bis(2- hydroxy- 3- methoxybenzylidene)- propane- 1, 2- diamine (H2L)
The mixture of o-vanillin (3.04 g, 20 mmol) in absolute EtOH (30 mL) was stirred constantly
and 1,2-diaminopropane (0.85 mL, 10 mmol) was added at the same time. Then the solution was
refluxed for 10 h. The yellow precipitate was obtained by filtering from the solution after cooling
to room temperature. (Scheme 1). Yield: ca. 93%. Elemental analysis (%) calcd for C19H22N2O4
(FW = 342.39): C, 66.59; H, 6.43; N, 8.18. Found: C, 66.43; H, 6.55; N, 8.04. 1H NMR (d6-DMSO,
δ/ppm): 1.15-1.33 (3H, C-H), 3.56-3.75 (3H, C-H), 3.76-3.93 (6H, OMe), 6.78-7.03 (6H, ArH),
8.54, 8.57 (2H, HC=N), 13.58, 13.59 (2H, O-H) (Fig. S1, ESI†). IR (cm-1): 3435 (w), 3057 (w),
3004 (w), 2963 (w), 2930 (w), 2897 (w), 2837(w), 1627 (s), 1465 (s), 1419(m), 1254 (s), 1143 (m),
1086 (m), 1038 (m), 969 (w), 834 (w), 777 (w), 734 (m), 637(w), 613 (w), 579 (w), 440 (w). (Fig.
S2, ESI†).
2.3 Synthesis of the complexes
The colour, shape, yield and elemental analysis of all the following complexes are listed in
Table 1.
[Y2(dbm)4L·CH3OH] (1). Y(dbm)3·2H2O (0.0795 g, 0.1 mmol) and H2L (0.0342 g, 0.1
mmol) were added in the mixing solvent of methanol (10 mL) and acetonitrile (15 mL), and then
triethylamine was added. The solution can be obtained by stirring for 4 h and then yellow
prism-shaped crystals were obtained by the filter and slow evaporation after 7 days.
C80H68N2O13Y2 (FW = 1443.18) IR (cm-1): 3789 (w), 3057 (w), 2961 (w), 2313 (w), 1601 (s), 1554
(s), 1518 (s), 1464 (s), 1411 (s), 1287 (m), 1221 (m), 1071 (w), 1023 (m), 969 (w), 938 (w), 852
(w), 723 (m), 687 (w), 610 (w), 515 (w), 433 (w).
[Dy2(dbm)4L·CH3OH] (2) The method for the preparation of complex 2 is consistent with 1.
We only need to replace Y(dbm)3·2H2O with Dy(dbm)3·2H2O (0.0868 g, 0.1 mmol).
C80H68N2O13Dy2 (FW = 1590.36) IR (cm-1): 3716 (w), 2968 (w), 2361(w), 1600 (s), 1549 (s), 1518
(s), 1469 (s), 1410 (s), 1287(m), 1221 (m), 1069 (w), 1022 (m), 971 (w), 935 (w), 854 (w), 723
(m), 686 (w), 612 (w), 505 (w), 459 (w).
[Ho2(dbm)4L·CH3OH] (3) The method for the preparation of complex 3 is consistent with 1.
Ho(dbm)3·2H2O (0.0522 g, 0.06 mmol), H2L (0.0205 g, 0.06 mmol) and triethylamine in the
mixing solvent of 5 mL methanol and 15 mL acetonitrile were stirred for 4 h. Finally, the crystals
were obtained after 5 days. C80H68N2O13Ho2 (FW = 1595.22) IR (cm-1): 3757 (w), 3057 (w), 2961
(w), 1600 (s), 1553 (s), 1518 (s), 1462 (s), 1409 (s), 1286 (m), 1221 (m), 1175 (m), 1089 (w),
1022 (m), 968 (w), 937 (w), 851 (w), 721 (m), 686 (w), 609 (w), 512 (w), 432 (w).
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DOI: 10.1039/C9NJ05571A
[Er2(dbm)4L·CH3OH] (4) The method for the preparation of complex 4 is consistent with 1,
Er(dbm)3·2H2O (0.0873 g, 0.1 mmol), H2L (0.0342 g, 0.1 mmol) and triethylamine in the mixing
solvent of 5 mL methanol and 10 mL acetonitrile were stirred. The crystals were obtained after 5
days. C80H68N2O13Er2 (FW = 1599.89) IR (cm-1): 3727 (w), 3057 (w), 2958 (w), 1600 (s), 1563 (s),
1518 (s), 1463 (s), 1410 (s), 1286 (m), 1220 (m), 1071 (w), 1022 (m), 969 (w), 938 (w), 853 (w),
736 (m), 687 (w), 608 (w), 512 (w).
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[Yb2(dbm)4L·CH3OH] (5) The method for the preparation of complex 5 is consistent with 1.
The change is that Y(dbm)3·2H2O (0.0795 g, 0.1 mmol) and H2L (0.0342 g, 0.1 mmol) was
substituted by Yb(dbm)3·2H2O (0.0702 g, 0.08 mmol) and H2L (0.0274 g, 0.08 mmol). Finally,
the crystals were obtained after 7 days. C80H68N2O13Yb2 (FW = 1611.44) IR (cm-1): 3759 (w),
3057 (w), 2961 (w), 1601 (s), 1555 (s), 1518 (s), 1470 (s), 1412 (s), 1286 (m), 1222 (m), 1071 (w),
1023 (m), 969 (w), 939 (w), 852 (w), 812 (w), 782 (w), 722 (m), 687 (w), 611 (w), 518 (w), 439
(w).
Table 1 Crystal colour and shape, yield, elemental analysis for 1-5.
Complex Crystal
colour
Crystal Yield
Elemental analysis
(calcd) (%)
Elemental analysis
(Found) (%)
shape
(%)
C
H
N
C
H
N
1
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4
5
yellow
yellow
yellow
yellow
yellow
prism
prism
prism
prism
prism
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57
66.52
60.36
60.18
60.00
59.57
4.71
4.28
4.26
4.25
4.21
1.94
1.76
1.76
1.75
1.74
66.31
60.12
59.98
59.79
59.34
4.55
4.45
4.07
4.14
4.08
2.05
1.89
1.87
1.86
1.83
2.4 Crystal data
The parameters of the crystal structures for all complexes are summarized in Table 2. The
measuring instruments are introduced in ESI†. A number of bond angles and lengths for all
complexes are depicted in Table S1 (ESI†). CCDC 1939555 (1), 1939551 (2), 1939553 (3),
1939552 (4), 1939554 (5).
3 Results and discussion
3.1 Crystal structures
The data confirms that 1-5 are isomorphous dinuclear complexes. All complexes belong to
the triclinic space group Pī. Hence the detailed description was made for the structure of 2. The
structure is composed of the two asymmetric units for 2. The complex 2 is made up of two Dy3+
ions, one L2-, four dbm- and one solvent molecule (CH3OH) in Fig. 1(a). The two Dy (III) cations
that coordinate with N atoms and O atoms are all eight-coordinated. The planar quadrilateral is
formed between the two central Dy (III) cations and two μ-Ophenoxide atoms (O2, O3) from one L2-
ligand, which can be regarded a Dy2O2 core. The bond angles of Dy1-O2-Dy2 and Dy1-O3-Dy2
are 110.73(10) and 113.30(10) in the planar quadrilateral, respectively. Besides, for the Dy-O,
their bond lengths are situated in 2.294(2)-2.690(3) Å and the two Dy–N have slight distinction
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which are 2.487(3) and 2.494(3) Å. As shown in Fig. 1(b), Dy1 and Dy2 adopt DdOiIf:f1e0r.1e0n3t9/C9NJ05571A
geometries. The Dy1 center is described as a distorted triangular dodecahedron geometry (D2d), in
which N1, N2, O2 and O3 as coordination sites from one L2- ligand and O5, O6, O7 and O8 as
coordination sites from two dbm- ligands, adopting a N2O6 sphere. The Dy2 center is considered as
a distorted square antiprism geometry (D4d), in which eight O atoms as coordination site. Within
the geometry, three O atoms (O2, O3, O4) are from L2- ligand. Four O atoms (O9, O10, O11, O12)
are from two dbm- and one O atom (O13) is from CH3OH. The slightly varying coordination
environment can influence the geometries of complexes.
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Ⅲ
Fig. 1 (a) The noncentrosymmetric structure for 2; (b) The geometric configurations for the Dy
ions in 2.
3.2 PXRD and TGA
The characterizations of the purity have been introduced for complexes 1-5 at room
temperature by means of PXRD which are depicted in Fig. S3(ESI†). The experimental data is
essentially coincide with the simulative result of the crystal data. So, the products with the great
purity have been fully demonstrated.
In order to test the air-stability of complexes 1-5, the samples that were exposed to the air for
three days and six days were characterized for the purity by PXRD. The positions and shapes of
the peaks of samples are in good agreement with the simulative curves as shown in Fig. S4(ESI†),
indicating that complexes 1-5 have relatively good air-stability.
To confirm the solvent-stability of the samples, the purity of complexes 1-5 in different
organic solvents (methanol, ethanol, acetonitrile) were explored by PXRD in Fig. S5(ESI†). The
PXRD curves of complexes 1-5 in different solvents are consistent with the theoretical curves,
which show that complexes 1-5 can maintain stability in the above solvents and have favorable
solvent-stability.
It is for the purpose of studying the thermal stability of all complexes that crystallized
samples of 1-5 were measured in air atmosphere by the thermal gravimetric analysis (TGA) with a
given amount of heating rate of 10 oC min-1. In Fig. S6(ESI†), all curves of 1-5 exhibit analogous
o
profiles with the thermal stability up to around 284 C in TGA analysis. Thus, complex 2 is
analyzed in detail in the following sections. Complex 2 experiences two distinct weight loss
procedures in the process of the whole skeleton decomposition. The first loss of the weight is the
o
solvent molecules (CH3OH) loss in the range of 284-340 C and the second is due to the
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o
decomposition of ligands (L2-) and β-diketonates (dbm-) after 340 C. Finally, theDOreI:le10v.1a0n3t9/C9NJ05571A
lanthanide oxides are obtained as the final residue.
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Table 2 Crystallographic data and structure refinements for 1-5.
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Complex
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5
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Formula
C80H68N2O13Y2
1443.18
C80H68N2 O13Dy2 C80H68N2O13Ho2 C80H68N2O13Er2
C80H68N2O13Yb2
1611.44
Mr (g mol-1)
1590.36
triclinic
Pī
1595.22
triclinic
Pī
1599.89
triclinic
Pī
Crystal system
triclinic
Pī
triclinic
Pī
Space group
T (K)
113(2)
113(2)
113(2)
113(2)
113(2)
a (Å)
13.546(3)
13.841(3)
19.646(4)
80.79(3)
83.10(3)
67.97(3)
3363.2(14)
2
13.547(3)
13.789(3)
19.612(4)
80.69(3)
83.18(3)
68.42(3)
3354.7(14)
2
13.528(3)
13.842(3)
19.619(4)
80.83(3)
83.21(3)
68.06(3)
3357.2(14)
2
13.545(3)
13.888(3)
19.666(4)
80.90(3)
83.11(3)
67.71(3)
3372.6(15)
2
13.568(3)
13.924(3)
19.669(4)
81.13(3)
82.97(3)
67.30(3)
3378.9(14)
2
b (Å)
c (Å)
α (o)
β (o)
γ (o)
V (Å3)
Z
Dx (g cm-3)
μ (mm-1)
1.425
1.574
1.578
1.576
1.584
1.785
2.279
2.408
2.540
2.819
F (000)
1488
1596
1600
1604
1612
θ (o)
1.600 to 25.020
32807
1.801 to 27.768
39047
1.599 to 25.017
32513
1.051 to 25.019
32643
1.631 to 25.017
29159
Reflections collected
Independent reflections
11827[R(int)=
0.0482]
15684[R(int)=
0.0524 ]
11833[R(int)=
0.0443]
11885[R(int)=
0.0458]
11804[R(int)=
0.0491]
Max. and min. transmission 1 and 0.8715
1 and 0.6652
1 and 0.8876
1 and 0.779
1 and 0.7743
Data/restraints/parameters
GOF on F2
11827/6/ 878
1.091
15684/52/ 924
1.013
11833/0/878
1.086
11885/6/878
1.023
11804/0/879
1.036
Final R indices [I>2σ(I)]
R1 a = 0.0505,
wR2 b = 0.1041
R1 a = 0.0707,
wR2 b = 0.1259
R1 a = 0.0407,
wR2 b = 0.0790
R1 a = 0.0570,
wR2 b = 0.0857
R1 a = 0.0441,
wR2 b = 0.0933
R1 a = 0.0542,
wR2 b = 0.0993
.
R1 a = 0.0438,
wR2 b = 0.1245
R1 a = 0.0537,
wR2 b = 0.1439
R1 a = 0.0479,
wR2 b = 0.1264
R1 a = 0.0623,
wR2 b = 0.1455
R indices (all data)
a R1= ∑(||Fo| − |Fc||)/∑|Fo|. b wR2= [∑w(|Fo|2− |Fc|2)2/∑w(Fo ) ]
2 2 1/2
3.3 UV-vis spectra
The bis-Schiff base ligand (H2L), Dy(dbm)3·2H2O and complexes 1-5 were discussed more
fully for UV-vis absorption spectra in dichloromethane (10-5 mol L-1) in Fig. S7 (ESI†). It can be
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seen from the curve of the ligand (H2L) that the chief bands are located at 231, 263 and 3D3O2I:n1m0.1.0I3t9/C9NJ05571A
is noteworthy that the bands which are located at 230, 251 and 347 nm are observed according to
the characteristic curve of Dy(dbm)3·2H2O. This kind of phenomenon is caused by the transitions
of dibenzoylmethane (dbm-): π → π* and n → π*. In contrast, there is a difference between the
spectra of the complexes and dibenzoylmethane. The curves of the complexes appear two main
bands that are situated around 232 and 350 nm. More significantly, the peak at 263 nm does not
display from the spectra of complexes 1-5 in contrast to the bis-Schiff base ligand (H2L), which
probably be ascribed to the interaction between the H2L and RE3+ cations.
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3.4 Photoluminescent properties
3.4.1 Photoluminescent properties in the visible region
The photoluminescent properties of complexes 1-2 and ligands (H2L, dbm-) were analyzed in
dichloromethane (10-5 mol L-1) in Fig. S8 (ESI†). The emission spectra of H2L, dbm-, complexes 1
and 2 display a band from 410 nm to 450 nm with the maximum wavelengths at 429, 433, 430 and
431 nm, respectively. The emission spectrum of complex 1 is similar to those of the ligands (H2L,
dbm-), which is ascribed to the absence of electron filling in the ƒ orbits of Y3+ ions. The state of
the special electron filling leads to the fact that complex 1 can not show the characteristic peak of
Y3+.29 The fluorescence curve of complex 2 is closely resemble those of the ligands, suggesting
that the energy absorbed by the ligands can not be effectively transferred to Dy3+, which may be
due to the following two reasons. First, the energy gap between the triplet energy level of dbm-
and the lowest excited energy level of Dy3+ is too small. The triplet energy level of dbm- is 20520
cm-1 and the lowest excited energy level (4F9/2) of Dy3+ is 20875 cm-1.30-31 The energy gap ΔE
(Dy3+) is 355 cm-1 by calulation. According to Dexter's luminescence theory, the optimal range of
the energy gap between the triplet energy level (T1) of the ligand and the excited energy level of
the rare-earth ion is 2000-4500 cm-1.32 By comparison, due to the small energy gap (ΔE), the
energy can not be efficaciously transferred from the dbm- to the Dy3+. Second, the triplet energy
level of the ligand (H2L) does not match the lowest excited state energy level of Dy3+.
3.4.2 Photoluminescent properties in the NIR region
To investigate the luminescent properties of complexes, both solid samples of 3, 4 and 5 were
determined by NIR and the test results were analyzed in detail as shown in Fig. 2, Fig. 3 and Fig. 4.
The characteristic peaks of metal ions were selected as emission wavelengths (974nm (3), 1540
nm (4) and 973 nm (5)) to determine the excitation spectra of 3, 4 and 5. The emission spectrum
of 3 was tested according to the maximum excitation wavelength at 448 nm. The emission
spectrum of 4 was investigated by the corresponding maximum excitation wavelength at 447 nm.
Similarly, the emission spectrum of 5 was studied with 410 nm as the maximum excitation
wavelength. For complex 3, the two emission bands at 974 nm and 1180 nm are attributed to the
transition: 5F5→5I7 and 5F5→5I6 (Fig. 2(b)). Compared with the theoretical value (961nm), the red-
shift of this transition (5F5→5I7) is probably considered as the influence of the crystal field. For 4,
one broad emission band can be observed from the emission spectrum in 1450-1635 nm. The
maximum value at 1540 nm is obtained from the curve, which is ascribe to the transition: 4I13/2
→
4I15/2 (Fig. 3(b)). For 5, the emission spectrum exhibits three bands in the range of 950-1080 nm
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with a maximum peak at 973 nm, which is considered to be the electronic transition: 2F5/2 → 2F7/2.
As shown in Fig. 4(b), the complex 5 can split into three peaks at 973 nm, 1002 nm, 1030 nm. The
primary reason is that the ligand field effect can result in the energy level splitting of the Yb3+
ions .33-34
According to the theory of NIR luminescence, the energy transfer mechanism from ligands to
LnIII cations is proposed. It is well known that the energy transfer is carried out by the method of
non-radiative transition. The path of the energy transfer is implemented from the lowest triplet
energy of ligands to the excited state energy of lanthanide ions, which clearly indicates that both
are matched.35 If the energy gap does not match, two situations may occur. One is the decline of
rate constant due to the large energy gap, the other is the energy back-transfer owing to the
relatively minor energy gap. In this paper, the results of luminescent investigations illustrate that
the energy transfer is effective.
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Fig. 2 The NIR Excitation (a) and Emission (b) spectra of complex 3.
Fig. 3 The NIR Excitation (a) and Emission (b) spectra of complex 4.
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Fig. 4 The NIR Excitation (a) and Emission (b) spectra of complex 5.
3.4.3 Luminescence lifetime and quantum yield
The fluorescence decay curve of the solid powder of complex 5 was measured at 973 nm as
the emission wavelength at room temperature in Fig. 5. The double-exponential function is used to
fit the experimental data. By using the following formula (1), the average lifetime (τav) can be
calculated. In this formula, the pre-exponential factors (A1, A2) exhibit the emission intensity,
indicating that there are two diverse amplitudes. The A1 and A2 correspond to the lifetime τ1 and τ2
(A1 = 0.2702, τ1 = 1.23 μs; A2 = 0.7298, τ2 = 5.31 μs). For complex 5, the average fluorescence
lifetime (τav) of the transition (2F5/2 → 2F7/2) is 4.99 μs.
2
2
τav = (A1τ1 + A2τ2 )/(A1τ1 + A2τ2) (1)
Subsequently, the quantum yield (ΦLn) can be obtained according to formula (2), where τobs
and τrad represent the observed lifetime and radiative lifetime, respectively. In view of the reported
literatures, the τrad value of Yb3+ is 2ms.36 So, the quantum yield (ΦLn) of complex 5 is 0.25% by
calculated, which is considered as the nonradiative deactivation of the excited Ln3+. The
fluorescence parameters of some reported YbIII-complexes are listed in Table 3. The fluorescence
lifetime and quantum yield of complex 5 are higher than those previously reported complexes.37-38
For YbIII-complexes, the fluorescence lifetime and quantum yield change obviously with the
diversification of β-diketone ligands. When β-diketone ligands vary from Hhpfnp, Hhtta to Hhfth,
the luminescence lifetime values and quantum yields increase as shown in Table 3. As mentioned
above, the luminescence lifetime values of complexes are affected by different β-diketone ligands.
ΦLn = τobs/τrad (2)
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Fig. 5 The fluorescence decay curve of complex 5.
Table 3 The fluorescence parameters of some YbIII-complexes
YbIII-complexes
Lifetime (μs)
Quantum yield (%)
Ref.
[Yb2(dbm)4L·CH3OH] (5)
τ1 = 1.23 μs; τ2 = 5.31 μs
τ1 = 0.031μs; τ2 = 0.51 μs
0.25
Our work
37
YbIII–TBAPy
<0.01
Yb(pfnp)3phen
τ1 = 0.85 μs; τ2 = 4.87 μs
-
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[Yb(tta)3phen]
12.01μs
14.7 μs
39 μs
1.1ꢀ
1.28
8
[Yb(hfth)3phen]
[Yb(bta)3(biphepo)]
[Yb(hfa)3(biphepo)]
49 μs
8.5
TBAPy = 1,3,6,8-tetrakis(p-benzoate)pyrene;
Hpfnp = 4,4,5,5,5-pentafluoro-1-(2-naphthyl)-1,3-butanedione;
Htta = thenoyltrifluoroacetonate;
Hhfth = 4,4,5,5,6,6,6-heptafluoro-1-(2-thienyl)hexane-1,3-dione;
Hbta = 1-benzoyl-3,3,3-trifluoroacetonate;
Hhfa = hexafluoroacetylacetonate;
Phen = 1,10-phenanthroline;
Biphepo = 2,2-bis(diphenylphosphoryl)biphenyl.
3.5 Magnetism
3.5.1 Direct current (dc) magnetism
The magnetism of complex 2 was studied and characterized by dc magnetic susceptibility
measurements below 300 K. The plot is obtained with temperature (T) as the abscissa and the
molar magnetic susceptibility (χMT) as the ordinate in Fig. 6. The curve displays that the
relationship of temperature to the molar magnetic susceptibility decreased at first and increased
later on. For 2, the experimental value of χMT is 27.84 cm3 K mol−1 at 300 K. Based on published
literature, the theoretical value is 28.34 cm3 K mol-1 for two dysprosium ions (6H15/2, g = 4/3),
which indicate that both are considerably approximate by the comparison of the theoretical value
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with experimental data.42 The χMT value drops when the temperature gradually deDcOliIn: e10s.10i3n9/C9NJ05571A
110-300 K. Subsequently, the value shows a sharp decline to 24.32 cm3 K mol-1 at about 12 K.
Finally, the χMT value quickly increases with decreasing temperature in 2-12 K. The χMT value is
26.39 cm3 K mol-1 at 2 K. The analysis result reveals the sharp rise in low temperature region,
reflecting the intramolecular ferromagnetic interactions.43
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Fig. 6 ΧMT versus T of 2 (Hdc = 1000 Oe).
3.5.2 Alternating current (ac) magnetism
The measurements of the ac magnetic susceptibility were completed in the zero dc field to
study the dynamic magnetic behavior of 2 as depicted in Fig. S9 (ESI†). The results show that the
in-phase (χ′) signal value decreases with the temperature increases without frequency dependence.
On the contrary, the frequency-dependent signal can be seen in χ′′ curves below 6ꢀK, which better
explain the slow magnetic relaxation at low temperature. It is remarkable that the out-of-phase (χ′′)
signal value shows notable rise when the frequency increases, yet none of the peak values emerge
under various frequencies as shown in Fig. S9(b), which indicate the intense quantum tunneling at
low temperature or additional fast relaxation processes in some polynuclear lanthanide
systems.44-49
In order to further illustrate the impact of the quantum tunneling relaxation of the
magnetization (QTM) effect, we studied the temperature dependencies of ac susceptibility from 2
K to 20 K in 3000 Oe dc field in Fig. 7. Focusing on the complex 2, as opposed to the
phenomenon of the zero dc field, a apparent peak is observed in χ′ and χ′′ signals from 111 to 2611
Hz. The dependence on frequency is clearly seen from 3 K to 7 K in Fig. 7(b), which is the feature
of SMM behavior. The subtle difference can be assigned to the effective inhibition of the quantum
tunneling effect.
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Fig. 7 Temperature-dependent of χ′ (a) and χ′′ (b) signals of the ac susceptibilities of complex 2
(Hac = 3 Oe, Hdc = 3000 Oe).
In addition, to better explore the dynamic magnetism of complex 2, the patterns were
acquired with frequency as the abscissa and χ′ or χ′′ as the ordinate under 3000 Oe dc field in 3
K-7 K. In Fig. 8, the apparent temperature-dependent signal is observable, which also demonstrate
slow magnetic relaxation behavior. The existence of the evident peak can reveal a relaxation
process in χ′′ patterns. As depicted in Fig. 9(a), the linear correlativity between the ln(τ) versus T-1
is obvious at high temperature. However, red fitting line departs from the linearity at low
temperature. In term of the equation (1) to fit the data, we can obtain the Ueff = 28 K, τ0 = 3.39 ×
10-7 s. This τ0 value of complex 2 corresponds to the values (10-6-10-12) that have been published
in the literatures.50-53 The optimum fitting is dominated by Orbach and QTM processes. In the
region of temperature 3 K-7 K, because of the generalized Debye model, Cole-Cole patterns are
obtained with an asymmetrical semi-circular shape with α = 0.35-0.42 as shown in Fig. 9(b).
Based on above discussion, the relaxation time is situated in narrow range.
-1
-1
τ-1 = τ0 exp(-Ueff/T) + τQTM
(1)
Fig. 8 Frequency-dependent of χ′ (a) and χ′′ (b) signals of the ac susceptibilities of complex 2 (Hac
= 3 Oe, Hdc = 3000 Oe).
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Fig. 9 (a) The ln(τ) versus T-1 pattern of 2. The red line is fitted with the τ-1 = τ0-1 exp(-Ueff/T) +
τQTM-1. (b) The Cole-Cole plots of 2. The solid lines are fitted with the generalized Debye model.
4 Magnetism-structure correlationship
As we know, the magnetism of DyIII-SMMs can be influenced by the slightly varying
coordination atmosphere of Dy3+ ions. There are various geometrical symmetries, they have been
noticed that the most ideal symmetry in Dy2 SMMs. In general, the higher local symmetry can
make SMMs with fascinating magnetic properties, such as D4d. Some Dy2 complexes based on
different bis-Schiff base ligands exhibit different energy barriers due to the various symmetries in
Table 4. Complexes 6 and 7 display similar symmetry (D4d), but there is a tiny difference for S
values with 1.063 and 0.826, respectively. It is nothing that complex 7 shows the higher
coordination symmetry due to the smaller deviation of S value. So, the energy barrier is influenced
by coordination symmetry. Complexes 12 and 13 display the higher symmetry (D4d). Under the
model of symmetry, the energy barriers of the complexes 12 and 13 have a significant increase
compared with the remaining Dy2 SMMs. This method to tune the magnetism of some complexes
is feasible. Based on the above analysis, enhancing the local symmetry of the DyIII cations is a
effective way to adjust the magnetism.
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Table 4 The parameters of some reported Dy2 complexes.
DyIII-complexes
Cryst. syst./space group
Geometrical
symmetry
S
Ueff/K
Ref.
6
7
8
9
[Dy2(dbm)4L·CH3OH] (2)
Triclinic/Pī
D2d (Dy1)
D4d (Dy2)
D4d
-
28
Our
work
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Dy2(L1)(hmac)4(H2O) (6)
Triclinic/Pī
Triclinic/Pī
Triclinic/Pī
Triclinic/Pī
Monoclinic/C2
Triclinic/Pī
Triclinic/Pī
1.063
14.1
30.7
28
[Dy2(L1)(BTFA)4(H2O)]·(Et2O) (7)
[Dy(H2L2)(OAc)2]2(PF6)2·4CH2Cl2 (8)
[Dy(H2L2)2(NO3)]2(PF6)4·4CH2Cl2·0.5C6H14 (9)
(Et3NH)2[Dy((R,R)-3-NO2salcy)2]2 (10)
[Dy(H2L3)2(NO3)3]2·2CH2Cl2·2CH3OH (11)
[Dy2(valdien)2(NO3)2] (12)
D4d
0.826
54
-
-
-
-
-
-
55
C4v
32.5
39.9
43
55
D2d
56
D3h
57
D4d
76
58
[Dy2(valdien)2](CF3COCHCOCF3)2] (13)
Monoclinic/P21/n
D4d
-
110
58
H2L1 = N,N’-bis(salicylidene)-o-phenylenediamine;
H2L3= N,N-(1,3-propylene)bis(salicylideneimine)];
H2L2ꢀ=ꢀN,N’-bis(2-hydroxy-3-methoxybenzylidene)-1,3-diaminopropane);
H2valdien = N1,N3-bis(3-methoxysalicylidene) diethylenetriamine.
5 Conclusion
The isostructural series of noncentrosymmetric rare-earth complexes [RE2(dbm)4L·CH3OH]
(RE = Y, Dy, Ho, Er, Yb) were prepared with the bis-Schiff base ligand and the structures and
properties have been examined. The structural analysis reveals all complexes are binuclear with
phenoxo-O bridged structures and the central RE3+ ions all adopt eight-coordinated mode. It is
notable that the crystal structure is composed of the two asymmetric units. As far as we know,
compared with the NIR luminescence of ErIII-complexes and YbIII-complexes, the luminescent
properties of HoIII-complexes with β-diketonate ligands are rarely reported in NIR region. The
characteristic near-infrared fluorescence peaks are observed for complexes 3, 4 and 5. The three
complexes exhibit the transitions 5F5→5I7 and 5F5→5I6 for complex 3, 4I13/2 → 4I15/2 for complex 4
2
2
and F5/2 → F7/2 for complex 5. It is noteworthy that complex 5 shows the photoluminescent
lifetime (τav = 4.99 μs) and quantum yield (ΦLn = 0.25%). Magnetic investigation confirms that
QTM can affect their magnetic relaxation behavior at a zero field. More in-depth research
demonstrates that slow magnetic relaxation can be clearly observed for 2. The Ueff = 28 K and τ0 =
3.39 × 10-7 s are obtained by calculation in 3000 Oe field, suggesting typical of SMM behavior.
According to the analysis of magnetic-structure relationship, it is a significant way to improving
the local symmetry of DyIII cation for tuning the magnetism.
Acknowledgements
We thank the National Natural Science Foundation of China (No. 21571138), the Key
Laboratory of Advanced Energy Materials Chemistry (Ministry of Education, Nankai University,
Opening Foundation), and the 111 Plan project (B12015) for financial support.
Conflicts of interest
There are no conflicts of interest to declare.
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