Synthesis, characterization and luminescent properties of lanthanide complexes with an unsymmetrical tripodal ligand

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Journal of Luminescence 128 (2008) 1394–1398 www.elsevier.com/locate/jlumin

Synthesis, characterization and luminescent properties of lanthanide complexes with an unsymmetrical tripodal ligand Zhen-Zhong Yan, Yu Tang, Wei-Sheng Liu, Min-Yu Tan State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China Received 14 October 2007; received in revised form 13 January 2008; accepted 15 January 2008 Available online 20 January 2008

Abstract Solid complexes of lanthanide nitrates with a new unsymmetrical tripodal ligand, bis[(20 -benzylaminoformyl)phenoxyl)ethyl](ethyl)amine (L) have been synthesized and characterized by elemental analysis, infrared spectra and molar conductivity measurements. At the same time, the luminescent properties of the Sm(III), Eu(III), Tb(III) and Dy(III) nitrate complexes in solid state were also investigated. Under the excitation of UV light, these complexes exhibited characteristic emission of central metal ions. r 2008 Elsevier B.V. All rights reserved. Keywords: Lanthanide nitrate complexes; Unsymmetrical tripodal ligand; Luminescent properties

1. Introduction There has been much interest in luminescent lanthanide complexes because of their attractive emission properties such as long lifetime, large Stokes shift, and line-like emission [1–4]. These complexes can be used as various analytical tools ranging from luminescent probes in timeresolved measurements [5–9] to luminescent chemosensors for detection of gases, ions, and molecules in solution [10–16]. Since f–f transitions are Laporte forbidden [17], free Ln3+ have low extinction coefficients resulting in low luminescence intensity. For effective excitation of lanthanide ions, organic chromophore, so-called ‘‘antenna’’, is usually attached to lanthanide complexes. In order to obtain strongly luminescent complexes, the chromophoric ligands, which chelate to lanthanide ions, should be able to encapsulate and protect the lanthanide ion from the solvent molecules and to absorb energy and transfer it efficiently to the central metal. More and more chemists are attracted to design the organized molecular architectures containing trivalent lanthanide ions (Sm3+, Eu3+, Tb3+, Dy3+) working as efficient light conversion devices [3,18]. Corresponding authors. Tel.: +86 931 8912552; fax: +86 931 8912582.

E-mail address: [email protected] (Y. Tang). 0022-2313/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.01.012

We have been interested in employing self-assembly strategies for lanthanide complexes, and have recently used tripodal ligands as sensitizers for lanthanide luminescence [19]. Tripodal amines have the advantage of the selective coordinating capacity and hard binding sites, therefore stabilizing their complexes, acquiring novel coordination structure and shielding the encapsulated ion from interaction with the surroundings [20]. Most of these ligands are symmetrical with three identical arms while the unsymmetrical tripodal organic ligands, especially the amide type tripodal ligands, are rare [21]. In our research, we designed and prepared a new conformationally flexible unsymmetrical ethyl- and (20 benzylaminoformyl)phenoxyl-containing amide type tripodal ligand bis[(20 -benzylaminoformyl)phenoxyl)ethyl](ethyl)amine (L) (Scheme 1) and studied the luminescent properties of samarium, europium, terbium and dysprosium complexes with this ligand. Under the excitation of UV light, Sm3+, Eu3+, Tb3+, and Dy3+ complexes exhibited characteristic emission of corresponding lanthanide ions. The lowest triplet state energy level of the ligand was calculated from the phosphorescence spectrum of the Gd3+ complex at 77 K. The result indicates that the triplet state energy level of the ligand matches better to the resonance level of Tb(III) than other lanthanide ions.

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Cl NH·HCl Cl Br

H N

Cl

N O

OH O N K2CO3

K2CO3 acetone

DMF

O

O

N H

Cl

O N H

L

Scheme 1. The synthetic route for the ligand L.

2. Experimental

2.4. Preparation of the complexes

2.1. Materials

An ethyl acetate solution (5 cm3) of Ln(NO3)3  6H2O (Ln ¼ Sm, Eu, Gd, Tb, Dy) (0.1 mmol) was added dropwise to a solution of 0.1 mmol ligand L in 5 cm3 of ethyl acetate. The mixture was stirred at room temperature for 8 h. And then the precipitated solid complex was filtered, washed with ethyl acetate, dried in vacuo over P4O10 for 48 h and submitted for elemental analysis, yield 70%.

N-benzylsalicylamide [22], b,b0 -dichlorodiethylamine hydrochloride salt [23] and lanthanide nitrates [24] were prepared according to the literature methods, respectively. Other chemicals were obtained from commercial sources and used without further purification. 2.2. Methods

3. Result and discussion The Ln(III) ion was determined by EDTA titration using xylenol-orange as an indicator. Carbon, nitrogen and hydrogen were determined using an Elementar Vario EL. Conductivity measurements were carried out with a DDS307 type conductivity bridge using 1.0  103 mol cm3 solutions at 298 K. The IR spectra were recorded in the 4000–400 cm1 region using KBr pellets and a Nicolet Nexus 670 FTIR spectrometer. 1H NMR spectra were measured on a Varian Mercury 300 spectrometer in dchloroform solutions, with TMS as internal standard. Luminescence and phosphorescence spectra were obtained on a Hitachi F-4500 spectrophotometer. 2.3. Preparation of the unsymmetrical tripodal ligand L The synthetic route for the unsymmetrical tripodal ligand L is shown in Scheme 1. The b,b0 -dichlorodiethylamine hydrochloride salt (1 mmol) and potassium carbonate (2 mmol) was refluxed in acetone (25 cm3) for 30 min, then the bromoethane (1 mmol) was added to the solution. The reaction mixture was refluxed for 12 h and the hot solution was filtered off. The collected organic phase was evaporated in vacuum. Then the obtained product was added to a mixture of N-benzylsalicylamide (2.0 mmol), potassium carbonate (4 mmol) and DMF (20 cm3), which was warmed to ca. 363 K. And the reaction mixture was stirred at 363–368 K for 8 h. After cooling down, the mixture was poured into water (100 cm3). The resulted solid was treated with column chromatography on silica gel [petroleum ether: ethyl acetate (3:2)] to get the ligand L, yield 80%, m. p. 88–90 1C; 1H NMR (CDCl3, 300 MHz): 0.88–0.94 (t, 3H), 2.26–2.38 (q, 2H), 2.62–2.70 (t, 4H), 3.89–3.94 (t, 4H), 4.56–4.60 (d, 4H), 6.67–7.42 (m, 18H), 8.20–8.26 (t, 2H); Anal. Calcd. for C34H37O4N3: C, 74.32; H, 6.46; N, 7.42. Found: C, 74.02; H, 6.76; N, 7.62.

3.1. Properties of the complexes Analytical data for the newly synthesized complexes, listed in Table 1, indicate that the five nitrate complexes all conform to a 2:3 metal-to-ligand stoichiometry, [Ln2L3(NO3)6]  H2O (Ln ¼ Sm, Eu, Gd, Tb, Dy). All the complexes are soluble in DMF and DMSO, slightly soluble in methanol, ethanol, acetone and acetonitrile, but sparingly soluble in chloroform, ethyl ether and ethyl acetate. The molar conductances of the complexes in acetone (see Table 1) indicate that all complexes act as noneletrolytes [25], implying that all nitrate groups are in coordination sphere. 3.2. IR spectra The most important IR peaks of the ligand L and the complexes are reported in Table 2. The complexes have similar IR spectra, of which the characteristic bands have similar shifts (see Table 2), suggesting they have a similar coordination structure. The IR spectrum of the free ligand L shows bands at 1635 and 1106 cm1, which are attributable to the stretch vibration of the carbonyl group [n(CQO)] of amide group and n(C–O–C), respectively. In the complexes, the lowenergy band remains unchanged while the high-energy band red shifts to about 1610 cm1 (Dn ¼ 25 cm1) as compared to its counterpart for the free ligand L, indicating that only the oxygen atom of CQO takes part in coordination to the lanthanide ions. The characteristic frequencies of the coordinating nitrate groups (C2v) appear at ca. 1484 cm1 (n1), 1300 cm1 (n4), 1045 cm1 (n2) and 812 cm1 (n3), and the difference between two strongest absorptions (n1 and n4) of the

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Table 1 Analytical and molar conductance data of the complexes (calculated values in parentheses) Complexes

Lm (cm2 O1 mol1)

Analysis (%) C

[Sm2L3(NO3)6]  H2O [Eu2L3(NO3)6]  H2O [Gd2L3(NO3)6]  H2O [Tb2L3(NO3)6]  H2O [Dy2L3(NO3)6]  H2O

51.82 52.44 51.64 52.25 51.82

H (52.23) (52.15) (51.92) (51.85) (51.69)

N

4.81 4.92 4.73 4.76 4.86

(4.86) (4.85) (4.83) (4.82) (4.81)

Table 2 The most important IR spectral data of the ligand L and complexes (cm1) Compounds

L [Sm2L3(NO3)6]  H2O [Eu2L3(NO3)6]  H2O [Gd2L3(NO3)6]  H2O [Tb2L3(NO3)6]  H2O [Dy2L3(NO3)6]  H2O

n(CQO) n(C–O–C) n(NO 3)

1635 1608 1609 1610 1610 1609

1106 1113 1113 1111 1112 1110

n1

n4

n2

n3

n1n4

1484 1482 1483 1484 1485

1301 1302 1298 1299 1302

1045 1046 1044 1043 1042

814 815 813 814 812

183 180 185 185 183

nitrate groups is about 180 cm1, clearly establishing that the NO 3 groups in the solid complexes coordinate to the lanthanide ion as bidentate ligands [26]. Additionally, no bands at 1380, 820 and 720 cm1 in the spectra of complexes indicates that free nitrate groups (D3h) are absent, in agreement with the results of the conductivity experiments. In addition, broad bands at ca. 3400 cm–1 indicate that water molecules are existent in the complexes, confirming the elemental results [26(c)]. 3.3. Luminescent properties of the complexes Excited by the absorption band at 330 nm, the ‘‘free’’ unsymmetrical tripodal ligand exhibits broad emission bands (lmax ¼ 462 nm) in solid state (the excitation and emission slit widths were 1.0 and 2.5 nm, respectively, Fig. 1a). The luminescence emission spectra of [Ln2L3(NO3)6]  H2O (Ln ¼ Sm, Eu, Tb, Dy) in solid state (the excitation and emission slit widths were 1.0 and 2.5 nm, Table 3, Fig. 1b–e) were recorded at room temperature. The efficient energy transfer from ligand to central ions (antenna effect) is one of key factors to achieve lanthanide characteristic luminescence [27,28]. It is shown in Fig. 1 that these four complexes all can show the characteristic emissions of Sm3+, Eu3+, Tb3+ or Dy3+, indicating efficient energy transfers from the ligand to the Ln3+ ion. The ligand has multiple aromatic rings with a semirigid skeleton structure, so it is a strong luminescence substance [29]. It could be seen from the emission spectrum of the Gd complex (Fig. 1f) that the tripodal ligand exhibits broad emission bands (lmax ¼ 458 nm) in the complex. Thus we may deduce that the ligand L is a good organic

9.05 9.03 8.97 8.96 8.93

Ln (8.96) (8.94) (8.90) (8.89) (8.86)

12.87 12.64 12.96 13.98 13.78

(12.82) (12.94) (13.33) (13.55) (13.71)

60.1 53.6 42.8 36.2 32.8

chelator to absorb and transfer energy to the lanthanide ions based on the absence of ligand emission in the spectra of the Sm3+, Eu3+, Tb3+, and Dy3+ complexes. In the spectrum of Eu complex, the relative intensity of 5D0-7F2 is more intense than that of 5D0-7F1, the most intensity ratio value Z(5D0-7F2/5D0-7F1) is 2.7, showing that the Eu3+ ion does not lie in a centro-symmetric coordination site [30]. To demonstrate the energy transfer process, the phosphorescence spectrum of the Gd3+ complex was measured at 77 K in a methanol–ethanol mixture (v:v ¼ 1:1) for the triplet energy level data of the ligand L. From the phosphorescence spectrum (Fig. 2), the triplet energy level T1 of [Gd2L3(NO3)6]  H2O, which corresponds to its shortest-wavelength phosphorescence band, is 23,923 cm1 [31,32]. And the lifetime value for this level, which was determined at 77 K is 0.665 ms. Because the lowest excited state, 6P7/2, of Gd3+ is too high to accept energy from the ligand, the data obtained from the phosphorescence spectrum actually reveal the triplet energy level of the ligand L in lanthanide complexes [33]. In general, the sensitization pathway in luminescent europium complexes consists of excitation of the ligands into their excited singlet states, subsequent intersystem crossing of the ligands to their triplet states, and energy transfer from the triplet state to the 5DJ manifold of the Eu3+ ions, followed by internal conversion to the emitting 5D0 state. Consequently, most europium complexes give rise to typical emission bands at about 580, 590, 615, 655, and 700 nm, corresponding to the deactivation of the 5D0 excited state to the 7FJ ground states (J ¼ 0–4) [33(a)]. In a similar way, the 4f electrons of the Sm3+, Tb3+, and Dy3+ ions are excited to the 4GJ, 5 DJ, and 4FJ ion manifolds from the ground state. Finally, the Sm3+, Tb3+, and Dy3+ ions give rise to their typical emission bands corresponding to the deactivation of the excited state of 4G5/2, 5D4, and 4F9/2 to the 6HJ, 7FJ, and 6 HJ ground states. Latva’s empirical rule [31] states that an optimal ligand-to-metal energy transfer process for Ln3+ needs DE(T1–5DJ) ¼ 2500–4000 cm1 for Eu3+ and 2500–4500 cm1 for Tb3+. The triplet energy level of the ligand L (23,923 cm1) is higher than the lowest excited resonance level 4G5/2 of Sm(III) (17,924 cm–1), 5D0 of Eu(III) (17,286 cm–1), 5D4 of Tb(III) (20,545 cm–1) and 4F9/ –1 2 of Dy(III) (21,144 cm ). This therefore supports the observation of stronger sensitization of the Tb3+ complex

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Fig. 1. Emission spectra of the ligand L and the complexes [Ln2L3(NO3)6]  H2O in solid state at room temperature: (a) L; (b) [Sm2L3(NO3)6]  H2O; (c) [Eu2L3(NO3)6]  H2O; (d) [Tb2L3(NO3)6]  H2O; (e) [Dy2L3(NO3)6]  H2O; (f) [Gd2L3(NO3)6]  H2O.

Table 3 Luminescence data for the ligand and complexes in solid state at room temperature Compounds

lex (nm)

lem (nm)

L

330

462

[Sm2L3(NO3)6]  H2O

319

563 596

4

592 616

5

490 545 582

5

481 573

4

[Eu2L3(NO3)6]  H2O [Tb2L3(NO3)6]  H2O

[Dy2L3(NO3)6]  H2O

395 312

314

Assignment

4

5

G5/2-6H5/2 G5/2-6H7/2 D0-7F1 D0-7F2

D4-7F6 D4-7F5 5 D4-7F4 5

4

F9/2-6H5/2 F9/2-6H13/2

Fig. 2. Phosphorescence spectrum of [Gd2L3(NO3)6]  H2O at 77 K.

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(Dn ¼ 3378 cm–1) than the Sm3+ (Dn ¼ 5999 cm–1), Eu3+ (Dn ¼ 6637 cm–1), and Dy3+ (Dn ¼ 2779 cm–1) complexes because of the larger or smaller overlap between the ligand triplet and lanthanide ion excited states resulting in the non-radiative deactivation of the lanthanide emitting state and quench the luminescence of the complexes [34]. 4. Conclusion According to the data and discussion above, the new unsymmetrical tripodal ligand bis[(20 -benzylaminoformyl)phenoxyl)ethyl](ethyl)amine (L) can form stable solid complexes with lanthanide nitrates. When the ligand formed the lanthanide complexes, obvious changes in IR spectra were observed. In the complexes, lanthanide ions were coordinated to the CQO oxygen atoms of the ligand L. It is noteworthy that the characterization of these complexes demonstrate 2:3 (M:L) type coordination stoichiometries. Thus, the lanthanide ion could be effectively encapsulated and protected by the coordinated ligands. The luminescent properties of the Sm, Eu, Tb and Dy complexes in solid state were investigated. Under the UV light excitation, the complexes exhibited characteristic luminescence of samarium, europium, terbium and dysprosium ions. This indicates that the ligand L is a good organic chelator to absorb and transfer energy to lanthanide ions. The lowest triplet state energy level of the ligand indicates that the triplet state energy level (T1) of the ligand matches better to the resonance level of Tb(III) than other lanthanide ions. Thus, the present results demonstrate that the bis[(20 -benzylaminoformyl)phenoxyl)ethyl](ethyl)amine complex of Tb3+ may find potential applications as a light conversion molecular device. Acknowledgments This work was supported by the National Natural Science Foundation of China (Project no. 20401008), the program for New Century Excellent Talents in University (NCET-06-0902) and the Natural Science Foundation of Gansu Province (Project no. 3ZS061-A25-003). References [1] D. Parker, R.S. Dickins, H. Puschmann, C. Crossland, J.A.K. Howard, Chem. Rev. 102 (2002) 1977. [2] D. Parker, Chem. Soc. Rev. 33 (2004) 156. [3] N. Sabbatini, M. Guardigli, J.-M. Lehn, Coord. Chem. Rev. 123 (1993) 201. [4] J.-C.G. Bu¨nzli, C. Piguet, Chem. Rev. 102 (2002) 1897. [5] N. Weibel, L.J. Charbonnie`re, M. Guardigli, A. Roda, R. Ziessel, J. Am. Chem. Soc. 126 (2004) 4888. [6] J. Yuan, K. Matsumoto, Anal. Chem. 70 (1998) 596. [7] A. Scorilas, A. Bjartell, H. Lilja, C. Moller, E.P. Diamandis, Clin. Chem. (2000) 1450.

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