Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 137 (2015) 1265–1269
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Synthesis, characterization and luminescent properties of lanthanide complexes with a novel multipodal ligand Zhen-Zhong Yan a,b,⇑, Na Hou b, Cong-Min Wang a,⇑ a b
Department of Chemistry, Zhejiang University, Zhejiang, Hangzhou 310027, China Department of Pharmaceutics and Chemical Engineering, Taizhou University, Zhejiang, Taizhou 318000, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Solid complexes of lanthanide
Solid complexes of lanthanide nitrates with an novel multipodal ligand, 1,2,4,5-tetramethyl-3,6-bis{N,Nbis[((20 -furfurylaminoformyl)phenoxy)ethyl]-aminomethyl}-benzene (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 investigated. Under the excitation of UV light, these complexes exhibited characteristic emission of central metal ions.
nitrates have been synthesized and characterized. The ligand L is a good organic chelator to absorb and transfer energy to lanthanide ions. The triplet state energy level (T1) of the ligand matches better the resonance level of Tb(III) than other lanthanide ions.
a r t i c l e
i n f o
Article history: Received 4 June 2014 Received in revised form 13 September 2014 Accepted 15 September 2014 Available online 28 September 2014 Keywords: Multipodal ligand Lanthanide complexes Synthesis Luminescent properties
a b s t r a c t Solid complexes of lanthanide nitrates with an novel multipodal ligand, 1,2,4,5-tetramethyl-3,6-bis{N,Nbis[((20 -furfurylaminoformyl)phenoxyl)ethyl]-aminomethyl}-benzene (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 investigated. Under the excitation of UV light, these complexes exhibited characteristic emission of central metal ions. The lowest triplet state energy level of the ligand indicates that the triplet state energy level (T1) of the ligand matches better the resonance level of Tb(III) than other lanthanide ions. Ó 2014 Elsevier B.V. All rights reserved.
Introduction ⇑ Corresponding authors at: Department of Chemistry, Zhejiang University, Zhejiang, Hangzhou 310027, China (Z.-Z. Yan). E-mail addresses: [email protected] (Z.-Z. (C.-M. Wang). http://dx.doi.org/10.1016/j.saa.2014.09.035 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
Yan),
[email protected]
It is well known that the excited states of europium (III) and terbium (III) complexes have strong fluorescence emission, large stokes’ shifts, narrow emission profiles and long fluorescence
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lifetimes [1–4]. And they have been widely used in many aspects, such as chemosensors, and as probes and labels in a variety of biological and chemical devices [5–11]. However, the application of lanthanide-based luminescence suffers from two serious drawbacks: (1) the low absorption coefficients because of the f–f electronic transitions which are forbidden and (2) the efficient nonradiative deactivation of their excited states by OAH oscillators such as water [12]. In order to avoid these obvious problems for the application of lanthanides, according to the modeling of luminescent lanthanide complexes, there are three strategies, (1) addition of effective photo-sensitized ligands, (2) suppression of vibrational frequency via high vibrational OAH and CAH bonds [13], and (3) formation of asymmetric coordination structure for enhancement of the electric dipole transition [14,15]. In this approach, the lanthanide ion is linked via complexation with a ligand, an organic chromophore which serves as an antenna or sensitizer. In the subsequent process, this chromophore can absorb the excitation light energy and transfer the energy from its lowest triplet state energy level (T1) to the resonance level of lanthanides [16]. Macrocyclic, macrobicyclic (cryptand) and podand type ligands have been extensively used for these purposes [17–20]. Podand type ligands have drawn much attention in recent years, mainly due to their possessing spheroidal cavities and binding sites that are hard, therefore, stabilizing their complexes and shielding the encapsulated ion from interaction with the surroundings [21]. Ligands containing many of the common donor groups have been synthesized. Among these numerous podands which have demonstrated their potential use in functional supramolecular chemistry [22–24], amide type podands have been attracted more attention in preparing the lanthanide complexes possessing strong luminescent properties. It is expected that the amide type podands, which are flexible in structure and have ‘terminal-group effects’ [25], will shield the encapsulated lanthanide ion from interaction with the surroundings effectively, and thus to achieve strong luminescent properties. In the present work, we synthesized the lanthanide nitrates complexes of a novel multipodal ligand, 1,2,4,5-tetramethyl-3,6-bis {N,N-bis[((20 -furfurylaminoformyl)phenoxyl)ethyl]-amino methyl}benzene (L) (Scheme 1), and studied the luminescent properties of samarium, europium, terbium and dysprosium complexes with this ligand. Under the excitation of UV light, Sm, Eu, Tb and Dy 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 Gd complex at 77 K. The result indicate that the triplet state energy level of the ligand matches better to the resonance level of Tb(III) than other lanthanide ions. Experimental Materials N-Furfurylsalicylamide [26], b,b0 -dichlordiethylamine hydrochloride salt [27] and lanthanide nitrates [28] were prepared according to the literature methods. Other chemicals were obtained from commercial sources and used without further purification. Methods The Ln(III) ion was determined by EDTA titration using xylenolorange as an indicator. Carbon, nitrogen and hydrogen were determined using an Elementar Vario EL. Conductivity measurements were carried out with a DDS-307 type conductivity bridge using 1.0 103 mol cm3 solutions at 25 °C. 1H NMR spectra were
measured on a Varian Mercury 300 spectrometer in d-DMSO solution, with TMS as internal standard. The IR spectra were recorded in the 4000–400 cm1 region using KBr pellets and a Nicolet Nexus 670 FTIR spectrometer. Differential thermal analysis/thermogravimetry (DTA/TG) of the complex was made on a TG-DTA analyzer (DSC-200 F3 MAIA, Netzsch, Germany) using about 10 mg powder sample and working at a heating rate of 5 °C min1 in static air. Luminescence spectra were obtained at room temperature on a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. Phosphorescence spectra were obtained on a Hitachi F-4500 spectrophotometer at 77 K. Fluorescence quantum yields were determined by using eosin solution (U = 0.190 in water) as standard for the Eu3+ complex, and quinine sulfate (U = 0.558 in 0.5 mol cm3 H2SO4) for the Tb3+ complex. Synthesis of the ligand L The synthetic route for the multipodal ligand is shown in Scheme 1. The b,b0 -dichlordiethylamine hydrochloride salt (2 mmol) and potassium carbonate (4 mmol) were refluxed in acetone (25 cm3) for 30 min, and then the 1,4-bis(bromomethyl)2,3,5,6-tetramethyl-benzene (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-furfurylsalicylamide (4.0 mmol), potassium carbonate (8 mmol) and DMF (20 cm3) which was warmed to ca. 90 °C. And the reaction mixture was stirred at 92–95 °C 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 (2:3)] to get the ligand L, yield 78%, m.p. 132–134 °C; Anal. Calcd for C68H72O12N6: C, 70.09; H, 6.23; N, 7.21; Found: C, 71.03; H, 6.34; N, 7.18%. 1H NMR (d-DMSO, 300 MHz): 2.09 (s, 12H), 2.65 (t, 8H), 3.61 (s, 4H), 3.88 (t, 8H), 4.54– 4.62 (d, 8H), 6.58–7.29 (m, 28H), H (NAH) not detected. IR (KBr pellet, cm1): 1648 (s, C@O), 1236 (m, ArAO), 1106 (w, ArAO). Synthesis of the lanthanide complexes An ethyl acetate solution (5 cm3) of Ln(NO3)36H2O (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er) (0.1 mmol) was added drop wise to a solution of 0.1 mmol ligand L in the ethyl acetate (5 cm3). 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 72%. Elemental analytical data and molar conductance values of the complexes are given in Table 1. All the complexes are white powders and stable in air. Result and discussion Properties of the complexes Elemental analytical data for the newly synthesized complexes, listed in Table 1, indicate that the eight nitrate complexes conform to a 1:1 metal-to-ligand stoichiometry, [LnL(NO3)3]3H2O (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er). All the complexes are soluble in DMF and DMSO, slightly soluble in methanol, ethanol, acetone, ethyl acetate and acetonitrile, but sparingly soluble in chloroform and ethyl ether. The molar conductances of the complexes in acetone (see Table 1) indicate that all complexes act as nonelectrolytes [29], implying that all nitrate groups are in coordination sphere. The TG and DTA curves for the Sm and Tb complexes show that the complexes have no melting point and the water molecules containing in the complex are all crystallized water molecules,
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Scheme 1. The synthetic route for the multipodal ligand L.
Table 1 Elemental analytical and molar conductance data of the complexes (calculated values in parentheses).
Km (cm2 X1 mol1)
Complexes
Analysis (%) C
H
N
Ln
[PrL(NO3)3]3H2O [NdL(NO3)3]3H2O [SmL(NO3)3]3H2O [EuL(NO3)3]3H2O [GdL(NO3)3]3H2O [TbL(NO3)3]3H2O [DyL(NO3)3]3H2O [ErL(NO3)3]3H2O
52.64(52.82) 52.88(52.71) 52.36(52.50) 51.98(52.44) 52.64(52.27) 52.97(52.21) 52.62(52.09) 51.72(51.93)
5.31(5.08) 4.93(5.07) 4.89(5.05) 4.98(5.05) 4.87(5.03) 4.76(5.03) 4.96(5.01) 4.83(5.00)
8.05(8.15) 8.62(8.13) 8.35(8.10) 8.42(8.09) 8.36(8.07) 8.56(8.06) 8.08(8.04) 8.48(8.02)
8.92(9.11) 9.13(9.31) 9.37(9.66) 9.44(9.76) 9.96(10.06) 10.02(10.16) 10.17(10.36) 10.44(10.64)
because the endothermic peak appear at ca. 94 °C. And the percent of water is consistent with the results of elemental analysis. So the formula of these complexes could be indicated as [LnL(NO3)3]3H2O. Thus the lanthanide ion could be effectively encapsulated and protected by the coordinated ligands. 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 1648 and 1106 cm1, which are attributable to the stretch vibration of the carbonyl group [m(C@O)] of amide group and m(CAOAC), respectively. In the complexes, the low-energy band remains unchanged, but the high-energy band red shifts to about 1610 cm1 (Dm = 38 cm1) as compared to its counterpart for the ‘‘free’’ ligand, thus indicating that only the oxygen atom of C@O takes part in coordination to the lanthanide ions. The characteristic frequencies of the coordinating nitrate groups (C2v) appear at ca 1484 cm1 (m1), 1300 cm1 (m4), 1045 cm1 (m2) and 812 cm1 (m3), and the difference between two strongest
L [PrL(NO3)3]3H2O [NdL(NO3)3]3H2O [SmL(NO3)3]3H2O [EuL(NO3)3]3H2O [GdL(NO3)3]3H2O [TbL(NO3)3]3H2O [DyL(NO3)3]3H2O [ErL(NO3)3]3H2O
m(C@O) m(CAOAC) m 1648 1611 1608 1607 1608 1610 1609 1610 1612
1106 1112 1113 1111 1112 1114 1112 1110 1109
absorptions (m1 and m4) of the 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 [30,31]. 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. 3395 cm1 indicate that water molecules are existent in the complexes, confirming the elemental and thermal analysis results. Absorption spectra The Absorption spectra in the visible region of the Ln(III) complexes exhibit alternations in intensity and shifts in position of the absorption bands relative to the corresponding Ln(III) aquoions. The shift has been attributed by Jørgensen to the effect on the crystal field of interelectronic repulsion between the 4f electrons, and is related to the covalent character of the metal–ligand bond, assessed by Sinha’s parameter (d), the nephelauxetic ration (b) and the bonding parameter (b1/2) [32–34]. Absorption spectra of the Pr(III), Nd(III) and Er(III) complexes were registered in methanol solutions at room temperature and the covalent parameters were calculated and listed in Table 3. Table 3 Absorption spectral data and covalent parameters of Pr, Nd and Er complexes.
Table 2 The most important IR spectral data of the ligand L and complexes (cm1). Compounds
8.7 7.9 8.4 7.8 7.5 7.2 6.9 7.1
Complexes
Frequency (cm1)
Assignment
Covalent parameters
[PrL(NO3)3]3H2O
22,318 21,244 20,706 16,592 19,240 17,186 13,542 12,568 20,432 19,247 15,334
3
b = 0.9957 d = 0.4319 b1/2 = 0.0464
(NO 3)
m1
m4
m2
m3
m1–m4
1485 1481 1486 1483 1483 1483 1484 1481
1301 1300 1301 1302 1298 1299 1302 1297
1046 1044 1045 1046 1044 1043 1042 1040
814 815 814 812 813 814 812 814
184 181 185 181 185 184 182 184
[NdL(NO3)3]3H2O
[ErL(NO3)3]3H2O
(22,290) (21,255) (21,085) (16,595) (19,414) (17,203) (13,435) (12,608) (20,313) (19,147) (15,136)
H4 ? 3P2 H4 ? 1I6 3 H4 ? 3P1 3 H4 ? 1D2 4 I9/2 ? 4G9/2 4 I9/2 ? 2G7/2 4 I9/2 ? 4S3/2 4 I9/2 ? 2H9/2 4 I15/2 ? 4F7/2 4 I15/2 ? 2H11/2 4 I15/2 ? 4F9/2 3
b = 0.9987 d = 0.1302 b1/2 = 0.0255 b = 1.0085 d = -0.8428
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Table 4 Luminescence data for the ligand and complexes in solid state at room temperature.
a
Compounds
kex (nm)
kem (nm)
RIa
L [SmL(NO3)3]3H2O
321 319
426 566 597 645 591 616 490 545 584 483 573
2131 34 38 29 351 1032 3419 6715 431 374 340
[EuL(NO3)3]3H2O
390
[TbL(NO3)3]3H2O
311
[DyL(NO3)3]3H2O
314
Assignment 4
G5/2 ? 6H5/2 G5/2 ? 6H7/2 4 G5/2 ? 6H9/2 5 D0 ? 7F1 5 D0 ? 7F2 5 D4 ? 7F6 5 D4 ? 7F5 5 D4 ? 7F4 4 F9/2 ? 6H5/2 4 F9/2 ? 6H13/2 4
RI: relative intensity.
The values of b, d and b1/2 indicate that the interaction between the trivalent lanthanide ions and the ligands is essentially electrostatic and that there is a very small participation of 4f orbitals in bonding [35]. Moreover, the magnitudes of covalency and bonding parameters increase from Pr(III) to Er(III), indicating that the extent of electrostatic character of the metal–ligand bond increases with atomic number (in agreement with the lanthanide contraction).
Luminescent properties of the complexes Excited by the absorption band at 332 nm, the ‘‘free’’ unsymmetrical tripodal ligand exhibits broad emission bands (kmax = 466 nm) in solid state (the excitation and emission slit widths were 2.5 nm).
The luminescence emission spectra of [LnL(NO3)3]3H2O (Ln = Sm, Eu, Tb, Dy) in solid state (the excitation and emission slit widths were 2.5 nm, Table 4, Fig. 1a–d) were recorded at room temperature. The efficient energy transfer from ligand to center ions (antenna effect) is one of key factors to achieve lanthanide characteristic luminescence [36,37]. It is shown in Fig. 1 that these four complexes all can show the characteristic emissions of Sm3+, Eu3+, Tb3+ or Dy3+. This indicates that the ligand L is a good organic chelator to absorb and transfer energy to lanthanide ions. The ligand has multiple aromatic rings with a semirigid skeleton structure, so it is a strong luminescence substance. In the spectrum of Eu complex, the relative intensity of 5D0 ? 7F2 is more intense than that of 5 D0 ? 7F1, the most intensity ratio value g(5D0 ? 7F2/5D0 ? 7F1) is 2.94, showing that the Eu(III) ion does not lie in a centro-symmetric coordination site [38]. A triplet excited state T1, which is localized on one ligand only and is independent of the lanthanide nature. In order to acquire the triplet excited state T1 of the ligand L, the phosphorescence spectrum (Fig. 2) of the Gd(III) complex was measured at 77 K in a methanol–ethanol mixture (v:v = 1:1). And the triplet state energy level T1 of the ligand L, which was calculated from the shortest-wavelength phosphorescence band [39], is 23,810 cm1. This energy level is above the lowest excited resonance level 4G5/2 of Sm(III) (17,924 cm1), 5D0 of Eu(III) (17,286 cm1), 5D4 of Tb(III) (20,545 cm1) and 4F9/2 of Dy(III) (21,144 cm1). Thus, the absorbed energy could be transferred from ligand to the Sm, Eu, Tb or Dy ions. And we may deduce that the triplet state energy level T1 of this ligand L matches better to the lowest resonance level of Tb(III) (Dm = 3265 cm1) than to Sm(III) (Dm = 5886 cm1), Eu(III) (Dm = 6524 cm1) and Dy(III) (Dm = 2666 cm1) ions, because such
Fig. 1. Emission spectra of the complexes [LnL(NO3)3]3H2O in solid state at room temperature: (a) [SmL(NO3)3]3H2O; (b) [EuL(NO3)3]3H2O; (c) [TbL(NO3)3]3H2O; (d) [DyL(NO3)3]3H2O.
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Acknowledgments This work was supported by the Educational Foundation of Taizhou University (project no. 2013PY30) and the advanced programs of the post doctor of the Zhejiang province (project no. Bsh1202009). And supported by Zhejiang Provincial Natural Science Foundation of China under Grant (project no. LQ13B010001, LY12B01006). References
Fig. 2. Phosphorescence spectrum of [GdL(NO3)3]3H2O at 77 K.
large or small Dm could result in the non-radiative deactivation of the lanthanide emitting state and quench the luminescence of the complexes [40]. The quantum yields of the metal-centered luminescence have been measured upon ligand excitation, we find UTb = 11.2% and UEu = 3.4%. The value of U for the Eu complex is lower than that for the Tb complex. Therefore, although the ligands appear to transfer the energy of their excited states onto the europium ion, efficient deexcitation processes take place within the complex molecule. Some of these processes may occur within the bonded ligand molecules themselves, in particular radiationless energy degradation through interactions between the ligand strands. Other radiationless energy degradation mechanisms involve the metal ion, for instance the 5D0(Eu) level is easily quenched by high energy vibrations [41], or by mixing of the 4f levels with either the ligand-to-metal charge-transfer state [42], as reported in a lanthanide dinuclear complexes with p-tert-Butylcalix[8]arenes [43]. Conclusion According to the data and discussion above, the novel multipodal ligand 1,2,4,5-tetramethyl-3,6-bis{N,N-bis[((20 -furfurylamino formyl)phenoxyl)ethyl]-aminomethyl}-benzene (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 C@O oxygen atoms of the ligand L. It is noteworthy that the characterization of these complexes demonstrate 1:1 (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.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
Synthesis, characterization and luminescent properties of lanthanide complexes with a novel multipodal ligand Zhen-Zhong Yan a,b,⇑, Na Hou b, Cong-Min Wang a,⇑ a b
Department of Chemistry, Zhejiang University, Zhejiang, Hangzhou 310027, China Department of Pharmaceutics and Chemical Engineering, Taizhou University, Zhejiang, Taizhou 318000, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Solid complexes of lanthanide
Solid complexes of lanthanide nitrates with an novel multipodal ligand, 1,2,4,5-tetramethyl-3,6-bis{N,Nbis[((20 -furfurylaminoformyl)phenoxy)ethyl]-aminomethyl}-benzene (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 investigated. Under the excitation of UV light, these complexes exhibited characteristic emission of central metal ions.
nitrates have been synthesized and characterized. The ligand L is a good organic chelator to absorb and transfer energy to lanthanide ions. The triplet state energy level (T1) of the ligand matches better the resonance level of Tb(III) than other lanthanide ions.
a r t i c l e
i n f o
Article history: Received 4 June 2014 Received in revised form 13 September 2014 Accepted 15 September 2014 Available online 28 September 2014 Keywords: Multipodal ligand Lanthanide complexes Synthesis Luminescent properties
a b s t r a c t Solid complexes of lanthanide nitrates with an novel multipodal ligand, 1,2,4,5-tetramethyl-3,6-bis{N,Nbis[((20 -furfurylaminoformyl)phenoxyl)ethyl]-aminomethyl}-benzene (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 investigated. Under the excitation of UV light, these complexes exhibited characteristic emission of central metal ions. The lowest triplet state energy level of the ligand indicates that the triplet state energy level (T1) of the ligand matches better the resonance level of Tb(III) than other lanthanide ions. Ó 2014 Elsevier B.V. All rights reserved.
Introduction ⇑ Corresponding authors at: Department of Chemistry, Zhejiang University, Zhejiang, Hangzhou 310027, China (Z.-Z. Yan). E-mail addresses: [email protected] (Z.-Z. (C.-M. Wang). http://dx.doi.org/10.1016/j.saa.2014.09.035 1386-1425/Ó 2014 Elsevier B.V. All rights reserved.
Yan),
[email protected]
It is well known that the excited states of europium (III) and terbium (III) complexes have strong fluorescence emission, large stokes’ shifts, narrow emission profiles and long fluorescence
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lifetimes [1–4]. And they have been widely used in many aspects, such as chemosensors, and as probes and labels in a variety of biological and chemical devices [5–11]. However, the application of lanthanide-based luminescence suffers from two serious drawbacks: (1) the low absorption coefficients because of the f–f electronic transitions which are forbidden and (2) the efficient nonradiative deactivation of their excited states by OAH oscillators such as water [12]. In order to avoid these obvious problems for the application of lanthanides, according to the modeling of luminescent lanthanide complexes, there are three strategies, (1) addition of effective photo-sensitized ligands, (2) suppression of vibrational frequency via high vibrational OAH and CAH bonds [13], and (3) formation of asymmetric coordination structure for enhancement of the electric dipole transition [14,15]. In this approach, the lanthanide ion is linked via complexation with a ligand, an organic chromophore which serves as an antenna or sensitizer. In the subsequent process, this chromophore can absorb the excitation light energy and transfer the energy from its lowest triplet state energy level (T1) to the resonance level of lanthanides [16]. Macrocyclic, macrobicyclic (cryptand) and podand type ligands have been extensively used for these purposes [17–20]. Podand type ligands have drawn much attention in recent years, mainly due to their possessing spheroidal cavities and binding sites that are hard, therefore, stabilizing their complexes and shielding the encapsulated ion from interaction with the surroundings [21]. Ligands containing many of the common donor groups have been synthesized. Among these numerous podands which have demonstrated their potential use in functional supramolecular chemistry [22–24], amide type podands have been attracted more attention in preparing the lanthanide complexes possessing strong luminescent properties. It is expected that the amide type podands, which are flexible in structure and have ‘terminal-group effects’ [25], will shield the encapsulated lanthanide ion from interaction with the surroundings effectively, and thus to achieve strong luminescent properties. In the present work, we synthesized the lanthanide nitrates complexes of a novel multipodal ligand, 1,2,4,5-tetramethyl-3,6-bis {N,N-bis[((20 -furfurylaminoformyl)phenoxyl)ethyl]-amino methyl}benzene (L) (Scheme 1), and studied the luminescent properties of samarium, europium, terbium and dysprosium complexes with this ligand. Under the excitation of UV light, Sm, Eu, Tb and Dy 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 Gd complex at 77 K. The result indicate that the triplet state energy level of the ligand matches better to the resonance level of Tb(III) than other lanthanide ions. Experimental Materials N-Furfurylsalicylamide [26], b,b0 -dichlordiethylamine hydrochloride salt [27] and lanthanide nitrates [28] were prepared according to the literature methods. Other chemicals were obtained from commercial sources and used without further purification. Methods The Ln(III) ion was determined by EDTA titration using xylenolorange as an indicator. Carbon, nitrogen and hydrogen were determined using an Elementar Vario EL. Conductivity measurements were carried out with a DDS-307 type conductivity bridge using 1.0 103 mol cm3 solutions at 25 °C. 1H NMR spectra were
measured on a Varian Mercury 300 spectrometer in d-DMSO solution, with TMS as internal standard. The IR spectra were recorded in the 4000–400 cm1 region using KBr pellets and a Nicolet Nexus 670 FTIR spectrometer. Differential thermal analysis/thermogravimetry (DTA/TG) of the complex was made on a TG-DTA analyzer (DSC-200 F3 MAIA, Netzsch, Germany) using about 10 mg powder sample and working at a heating rate of 5 °C min1 in static air. Luminescence spectra were obtained at room temperature on a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. Phosphorescence spectra were obtained on a Hitachi F-4500 spectrophotometer at 77 K. Fluorescence quantum yields were determined by using eosin solution (U = 0.190 in water) as standard for the Eu3+ complex, and quinine sulfate (U = 0.558 in 0.5 mol cm3 H2SO4) for the Tb3+ complex. Synthesis of the ligand L The synthetic route for the multipodal ligand is shown in Scheme 1. The b,b0 -dichlordiethylamine hydrochloride salt (2 mmol) and potassium carbonate (4 mmol) were refluxed in acetone (25 cm3) for 30 min, and then the 1,4-bis(bromomethyl)2,3,5,6-tetramethyl-benzene (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-furfurylsalicylamide (4.0 mmol), potassium carbonate (8 mmol) and DMF (20 cm3) which was warmed to ca. 90 °C. And the reaction mixture was stirred at 92–95 °C 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 (2:3)] to get the ligand L, yield 78%, m.p. 132–134 °C; Anal. Calcd for C68H72O12N6: C, 70.09; H, 6.23; N, 7.21; Found: C, 71.03; H, 6.34; N, 7.18%. 1H NMR (d-DMSO, 300 MHz): 2.09 (s, 12H), 2.65 (t, 8H), 3.61 (s, 4H), 3.88 (t, 8H), 4.54– 4.62 (d, 8H), 6.58–7.29 (m, 28H), H (NAH) not detected. IR (KBr pellet, cm1): 1648 (s, C@O), 1236 (m, ArAO), 1106 (w, ArAO). Synthesis of the lanthanide complexes An ethyl acetate solution (5 cm3) of Ln(NO3)36H2O (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er) (0.1 mmol) was added drop wise to a solution of 0.1 mmol ligand L in the ethyl acetate (5 cm3). 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 72%. Elemental analytical data and molar conductance values of the complexes are given in Table 1. All the complexes are white powders and stable in air. Result and discussion Properties of the complexes Elemental analytical data for the newly synthesized complexes, listed in Table 1, indicate that the eight nitrate complexes conform to a 1:1 metal-to-ligand stoichiometry, [LnL(NO3)3]3H2O (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er). All the complexes are soluble in DMF and DMSO, slightly soluble in methanol, ethanol, acetone, ethyl acetate and acetonitrile, but sparingly soluble in chloroform and ethyl ether. The molar conductances of the complexes in acetone (see Table 1) indicate that all complexes act as nonelectrolytes [29], implying that all nitrate groups are in coordination sphere. The TG and DTA curves for the Sm and Tb complexes show that the complexes have no melting point and the water molecules containing in the complex are all crystallized water molecules,
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Scheme 1. The synthetic route for the multipodal ligand L.
Table 1 Elemental analytical and molar conductance data of the complexes (calculated values in parentheses).
Km (cm2 X1 mol1)
Complexes
Analysis (%) C
H
N
Ln
[PrL(NO3)3]3H2O [NdL(NO3)3]3H2O [SmL(NO3)3]3H2O [EuL(NO3)3]3H2O [GdL(NO3)3]3H2O [TbL(NO3)3]3H2O [DyL(NO3)3]3H2O [ErL(NO3)3]3H2O
52.64(52.82) 52.88(52.71) 52.36(52.50) 51.98(52.44) 52.64(52.27) 52.97(52.21) 52.62(52.09) 51.72(51.93)
5.31(5.08) 4.93(5.07) 4.89(5.05) 4.98(5.05) 4.87(5.03) 4.76(5.03) 4.96(5.01) 4.83(5.00)
8.05(8.15) 8.62(8.13) 8.35(8.10) 8.42(8.09) 8.36(8.07) 8.56(8.06) 8.08(8.04) 8.48(8.02)
8.92(9.11) 9.13(9.31) 9.37(9.66) 9.44(9.76) 9.96(10.06) 10.02(10.16) 10.17(10.36) 10.44(10.64)
because the endothermic peak appear at ca. 94 °C. And the percent of water is consistent with the results of elemental analysis. So the formula of these complexes could be indicated as [LnL(NO3)3]3H2O. Thus the lanthanide ion could be effectively encapsulated and protected by the coordinated ligands. 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 1648 and 1106 cm1, which are attributable to the stretch vibration of the carbonyl group [m(C@O)] of amide group and m(CAOAC), respectively. In the complexes, the low-energy band remains unchanged, but the high-energy band red shifts to about 1610 cm1 (Dm = 38 cm1) as compared to its counterpart for the ‘‘free’’ ligand, thus indicating that only the oxygen atom of C@O takes part in coordination to the lanthanide ions. The characteristic frequencies of the coordinating nitrate groups (C2v) appear at ca 1484 cm1 (m1), 1300 cm1 (m4), 1045 cm1 (m2) and 812 cm1 (m3), and the difference between two strongest
L [PrL(NO3)3]3H2O [NdL(NO3)3]3H2O [SmL(NO3)3]3H2O [EuL(NO3)3]3H2O [GdL(NO3)3]3H2O [TbL(NO3)3]3H2O [DyL(NO3)3]3H2O [ErL(NO3)3]3H2O
m(C@O) m(CAOAC) m 1648 1611 1608 1607 1608 1610 1609 1610 1612
1106 1112 1113 1111 1112 1114 1112 1110 1109
absorptions (m1 and m4) of the 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 [30,31]. 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. 3395 cm1 indicate that water molecules are existent in the complexes, confirming the elemental and thermal analysis results. Absorption spectra The Absorption spectra in the visible region of the Ln(III) complexes exhibit alternations in intensity and shifts in position of the absorption bands relative to the corresponding Ln(III) aquoions. The shift has been attributed by Jørgensen to the effect on the crystal field of interelectronic repulsion between the 4f electrons, and is related to the covalent character of the metal–ligand bond, assessed by Sinha’s parameter (d), the nephelauxetic ration (b) and the bonding parameter (b1/2) [32–34]. Absorption spectra of the Pr(III), Nd(III) and Er(III) complexes were registered in methanol solutions at room temperature and the covalent parameters were calculated and listed in Table 3. Table 3 Absorption spectral data and covalent parameters of Pr, Nd and Er complexes.
Table 2 The most important IR spectral data of the ligand L and complexes (cm1). Compounds
8.7 7.9 8.4 7.8 7.5 7.2 6.9 7.1
Complexes
Frequency (cm1)
Assignment
Covalent parameters
[PrL(NO3)3]3H2O
22,318 21,244 20,706 16,592 19,240 17,186 13,542 12,568 20,432 19,247 15,334
3
b = 0.9957 d = 0.4319 b1/2 = 0.0464
(NO 3)
m1
m4
m2
m3
m1–m4
1485 1481 1486 1483 1483 1483 1484 1481
1301 1300 1301 1302 1298 1299 1302 1297
1046 1044 1045 1046 1044 1043 1042 1040
814 815 814 812 813 814 812 814
184 181 185 181 185 184 182 184
[NdL(NO3)3]3H2O
[ErL(NO3)3]3H2O
(22,290) (21,255) (21,085) (16,595) (19,414) (17,203) (13,435) (12,608) (20,313) (19,147) (15,136)
H4 ? 3P2 H4 ? 1I6 3 H4 ? 3P1 3 H4 ? 1D2 4 I9/2 ? 4G9/2 4 I9/2 ? 2G7/2 4 I9/2 ? 4S3/2 4 I9/2 ? 2H9/2 4 I15/2 ? 4F7/2 4 I15/2 ? 2H11/2 4 I15/2 ? 4F9/2 3
b = 0.9987 d = 0.1302 b1/2 = 0.0255 b = 1.0085 d = -0.8428
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Table 4 Luminescence data for the ligand and complexes in solid state at room temperature.
a
Compounds
kex (nm)
kem (nm)
RIa
L [SmL(NO3)3]3H2O
321 319
426 566 597 645 591 616 490 545 584 483 573
2131 34 38 29 351 1032 3419 6715 431 374 340
[EuL(NO3)3]3H2O
390
[TbL(NO3)3]3H2O
311
[DyL(NO3)3]3H2O
314
Assignment 4
G5/2 ? 6H5/2 G5/2 ? 6H7/2 4 G5/2 ? 6H9/2 5 D0 ? 7F1 5 D0 ? 7F2 5 D4 ? 7F6 5 D4 ? 7F5 5 D4 ? 7F4 4 F9/2 ? 6H5/2 4 F9/2 ? 6H13/2 4
RI: relative intensity.
The values of b, d and b1/2 indicate that the interaction between the trivalent lanthanide ions and the ligands is essentially electrostatic and that there is a very small participation of 4f orbitals in bonding [35]. Moreover, the magnitudes of covalency and bonding parameters increase from Pr(III) to Er(III), indicating that the extent of electrostatic character of the metal–ligand bond increases with atomic number (in agreement with the lanthanide contraction).
Luminescent properties of the complexes Excited by the absorption band at 332 nm, the ‘‘free’’ unsymmetrical tripodal ligand exhibits broad emission bands (kmax = 466 nm) in solid state (the excitation and emission slit widths were 2.5 nm).
The luminescence emission spectra of [LnL(NO3)3]3H2O (Ln = Sm, Eu, Tb, Dy) in solid state (the excitation and emission slit widths were 2.5 nm, Table 4, Fig. 1a–d) were recorded at room temperature. The efficient energy transfer from ligand to center ions (antenna effect) is one of key factors to achieve lanthanide characteristic luminescence [36,37]. It is shown in Fig. 1 that these four complexes all can show the characteristic emissions of Sm3+, Eu3+, Tb3+ or Dy3+. This indicates that the ligand L is a good organic chelator to absorb and transfer energy to lanthanide ions. The ligand has multiple aromatic rings with a semirigid skeleton structure, so it is a strong luminescence substance. In the spectrum of Eu complex, the relative intensity of 5D0 ? 7F2 is more intense than that of 5 D0 ? 7F1, the most intensity ratio value g(5D0 ? 7F2/5D0 ? 7F1) is 2.94, showing that the Eu(III) ion does not lie in a centro-symmetric coordination site [38]. A triplet excited state T1, which is localized on one ligand only and is independent of the lanthanide nature. In order to acquire the triplet excited state T1 of the ligand L, the phosphorescence spectrum (Fig. 2) of the Gd(III) complex was measured at 77 K in a methanol–ethanol mixture (v:v = 1:1). And the triplet state energy level T1 of the ligand L, which was calculated from the shortest-wavelength phosphorescence band [39], is 23,810 cm1. This energy level is above the lowest excited resonance level 4G5/2 of Sm(III) (17,924 cm1), 5D0 of Eu(III) (17,286 cm1), 5D4 of Tb(III) (20,545 cm1) and 4F9/2 of Dy(III) (21,144 cm1). Thus, the absorbed energy could be transferred from ligand to the Sm, Eu, Tb or Dy ions. And we may deduce that the triplet state energy level T1 of this ligand L matches better to the lowest resonance level of Tb(III) (Dm = 3265 cm1) than to Sm(III) (Dm = 5886 cm1), Eu(III) (Dm = 6524 cm1) and Dy(III) (Dm = 2666 cm1) ions, because such
Fig. 1. Emission spectra of the complexes [LnL(NO3)3]3H2O in solid state at room temperature: (a) [SmL(NO3)3]3H2O; (b) [EuL(NO3)3]3H2O; (c) [TbL(NO3)3]3H2O; (d) [DyL(NO3)3]3H2O.
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Acknowledgments This work was supported by the Educational Foundation of Taizhou University (project no. 2013PY30) and the advanced programs of the post doctor of the Zhejiang province (project no. Bsh1202009). And supported by Zhejiang Provincial Natural Science Foundation of China under Grant (project no. LQ13B010001, LY12B01006). References
Fig. 2. Phosphorescence spectrum of [GdL(NO3)3]3H2O at 77 K.
large or small Dm could result in the non-radiative deactivation of the lanthanide emitting state and quench the luminescence of the complexes [40]. The quantum yields of the metal-centered luminescence have been measured upon ligand excitation, we find UTb = 11.2% and UEu = 3.4%. The value of U for the Eu complex is lower than that for the Tb complex. Therefore, although the ligands appear to transfer the energy of their excited states onto the europium ion, efficient deexcitation processes take place within the complex molecule. Some of these processes may occur within the bonded ligand molecules themselves, in particular radiationless energy degradation through interactions between the ligand strands. Other radiationless energy degradation mechanisms involve the metal ion, for instance the 5D0(Eu) level is easily quenched by high energy vibrations [41], or by mixing of the 4f levels with either the ligand-to-metal charge-transfer state [42], as reported in a lanthanide dinuclear complexes with p-tert-Butylcalix[8]arenes [43]. Conclusion According to the data and discussion above, the novel multipodal ligand 1,2,4,5-tetramethyl-3,6-bis{N,N-bis[((20 -furfurylamino formyl)phenoxyl)ethyl]-aminomethyl}-benzene (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 C@O oxygen atoms of the ligand L. It is noteworthy that the characterization of these complexes demonstrate 1:1 (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.
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