Rare earth complexes with a novel ligand N-(naphthalen-2-yl)-N-phenyl-2-(quinolin-8-yloxy)acetamide: Preparation and spectroscopic studies

Article abstract of DOI:10.1016/j.saa.2008.05.001

Six complexes of rare earth nitrates (Ln = La, Sm, Eu, Gd, Tb, Dy) with a new amide type ligand, N-(naphthalen-2-yl)-N-phenyl-2-(quinolin-8-yloxy)acetamide (L) have been prepared and characterized by elemental analysis, conductivity measurements, IR and and ¹H NMR spectra. Under excitation, Eu(III) and Sm(III) complexes exhibited strong red emissions. And the luminescence intensity of Sm(III) complex is higher than that of Eu(III) complex. Thus the Eu(III) and Sm(III) complexes are the potential light conversion agent. However, the Tb(III) and Dy(III) complexes cannot exhibit characteristic emissions of terbium and dysprosium ions, respectively. The results of phosphorescence spectrum show that the triplet-state energy level of the ligand matches better to the resonance level of Eu(III) than Tb(III) ion. In addition, the luminescence of the Eu(III) complex is also relatively strong in highly diluted tetrahydrofuran solution (2 × 10^-4 mol/L) compared with the powder. This is not only due to the solvate effects but also to the changes of the structure of the Eu(III) complex after being dissolved into the solvents. Furthermore, owing to the co-luminescence effect, the proper La(III) or Gd(III) doped Eu(III) complexes show stronger luminescence than the pure Eu(III) complex.

Full text of DOI:10.1016/j.saa.2008.05.001

Spectrochimica Acta Part A 71 (2008) 1461–1465
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Rare earth complexes with a novel ligand
N-(naphthalen-2-yl)-N-phenyl-2-(quinolin-8-yloxy)acetamide:
Preparation and spectroscopic studies
Wei-Na Wua,b, Ning Tanga,∗, Lan Yana
a College of Chemistry and Chemical Engineering and State Key Laboratory of Applied Organic Chemistry,
Lanzhou University, Lanzhou 730000, PR China
b Department of Physics and Chemistry, Henan Polytechnic University, Jiaozuo 454003, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Six complexes of rare earth nitrates (Ln = La, Sm, Eu, Gd, Tb, Dy) with a new amide type ligand, N-
(naphthalen-2-yl)-N-phenyl-2-(quinolin-8-yloxy)acetamide (L) have been prepared and characterized by
elemental analysis, conductivity measurements, IR and and 1H NMR spectra. Under excitation, Eu(III) and
Sm(III) complexes exhibited strong red emissions. And the luminescence intensity of Sm(III) complex is
higher than that of Eu(III) complex. Thus the Eu(III) and Sm(III) complexes are the potential light conver-
sion agent. However, the Tb(III) and Dy(III) complexes cannot exhibit characteristic emissions of terbium
and dysprosium ions, respectively. The results of phosphorescence spectrum show that the triplet-state
energy level of the ligand matches better to the resonance level of Eu(III) than Tb(III) ion. In addition,
the luminescence of the Eu(III) complex is also relatively strong in highly diluted tetrahydrofuran solu-
tion (2 × 10−4 mol/L) compared with the powder. This is not only due to the solvate effects but also to
the changes of the structure of the Eu(III) complex after being dissolved into the solvents. Furthermore,
owing to the co-luminescence effect, the proper La(III) or Gd(III) doped Eu(III) complexes show stronger
luminescence than the pure Eu(III) complex.
Received 5 August 2007
Received in revised form 30 April 2008
Accepted 4 May 2008
Keywords:
Lanthanide complexes
Aryl amide
Luminescence properties
Triplet-state energy
Doped complex
Complexation constant
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
overcome this problem, the ligand containing the common donor
groups has been synthesized, which serves as an antenna or sen-
Owing to their good luminescent properties, such as the large
stokes shifts, narrow emission profiles, long lifetimes and so on [1],
the rare earth complexes are attracting more and more attention
and have been widely used in many aspects, such as light-emitting
diode (LED), laser materials, optical signal amplification, fluoroim-
munoassay, and so on [2–4]. Among them, the application of Eu(III)
and Sm(III) complexes to light conversion agent has been paid more
and more attention. It is well known that purple and green lights
in the sunlight do harm to plants. The Eu(III) and Sm(III) complexes
could absorb the ultraviolet light and then transform them into
the beneficial red light for photosynthesis efficiently. Furthermore,
the red light exhibited by Sm(III) complex fits the photosynthesis
better, because of the emission peak of Sm(III) complex at about
645 nm [5,6].
sitizer, absorbing the excitation light and transferring the energy
from its lowest triplet-state energy level (T) to the resonance level of
Ln(III) ion [7,8]. Among numerous ligands, the amide type ligands,
which are flexible in structure and have ‘terminal-group effects’
[9–11], will shield the encapsulated lanthanide ion from interaction
with the surroundings effectively, and thus to achieve strong lumi-
nescent emissions. Therefore, an amide type ligand was selected
as ‘antenna’ in this work. The coordination and luminescence
properties of rare earth ions with the ligand have been studied
in detail.
2. Experimental
2.1. Materials
However, direct excitation of Eu(III) or Sm(III) ion is not effi-
cient because of its inherently small absorption cross-section. To
The rare earth (III) nitrates were prepared from their
oxide acquired from Yue Long (PR China). All other chemicals
used were of analytical grade. Absolute chloroform and N,Nꢀ-
dimethylformamide (DMF) were obtained after being distilled by
standard methods.
∗
Tel.: +86 9318911218; fax: +86 9318912582.
E-mail addresses: wuwn03@lzu.cn (W.-N. Wu), tangn@lzu.edu.cn (N. Tang).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.05.001

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W.-N. Wu et al. / Spectrochimica Acta Part A 71 (2008) 1461–1465
Fig. 1. Synthesis scheme of the ligand.
Table 1
2.2. Physical measurements
Analytical and molar conductance data for the complexes
The melting point of the ligand was determined on an XT4-
100x microscopic melting point apparatus (made in Beijing,
China). Elemental analyses were carried out on an Elemental
Complexes
C% (calc.)
H% (calc.)
N% (calc.)
ꢁm
(s cm2 mol−1
)
La2(L)3(NO3)6
Sm2(L)3(NO3)6
Eu2(L)3(NO3)6
Gd2(L)3(NO3)6
Tb2(L)3(NO3)6
Dy2(L)3(NO3)6
51.93(52.21)
51.91(51.58)
51.59(51.49)
51.81(51.21)
51.58(51.12)
51.35(50.92)
3.32(3.25)
3.44(3.21)
3.56(3.20)
3.32(3.18)
3.34(3.18)
3.02(3.17)
8.60(9.02)
8.46(8.91)
8.62(8.90)
8.38(8.85)
8.35(8.83)
8.33(8.80)
227
258
270
275
278
300
Vario EL analyzer. The infrared spectra (IR, ꢀ = 4000–400 cm−1
)
were determined by the KBr pressed disc method on a Nicolet-
170SX FT-IR spectrophotometer. 1H NMR spectra were recorded
on Bruker DRX-200 spectrometer in CD3COCD3 solutions. All
conductivity measurements were carried out with a DDS-11A
conductometer (made in China) bridge using 1.0 × 10−3 mol/L solu-
tions in acetonitrile at 25 ◦C. The mass spectrum was obtained
on a TRACE DSQ GC/MS. The ultraviolet absorption spectra were
measured on Shimadzu UV-240 spectrophotometer. Lumines-
cence emission and excitation spectra were determined on a
Hitachi F-4500 FL spectrophotometer. Phosphorescence spectra
at 77 K were taken on the same spectrophotometer equipped
with phosphorescence measurement apparatus. All the complexes
were dried at 100 ◦C to constant weights before these measure-
ments.
filtration, purified by washing for several times with ethyl acetate,
and dried for 24 h in a vacuum.
4. Result and discussion
Analytical data for the complexes, presented in Table 1, conform
to Ln2(L)3(NO3)6. All complexes are white powders, which are sol-
uble in DMF, DMSO, acetone, methanol, acetonitrile and ethanol,
slightly soluble in ethyl acetate, while hardly soluble in water and
ether. The molar conductivity values of the complexes in acetoni-
trile are in the range of 227–300 s cm2 mol−1, indicating that the
complexes are 2:1 electrolytes [13].
3. Synthesis of ligand
The synthesis scheme is shown in Fig. 1. The compound 1 was
prepared according to the literature [12].
4.1. IR spectra
8-Hydroxyquinoline (1.5 g, 10.3 mmol) and anhydrous potas-
sium carbonate (1.6 g, 11.6 mmol) were added to DMF (15 mL), then
1 (2.96 g, 10.0 mmol) and a small quantity of KI were added. The
reaction mixture was stirred for 5 h at 100–110 ◦C. After cooling
down, 150 mL water was added and stirred for 2 h. The precipitate
was collected by filtration and washed with water. Recrystallization
from ethyl acetate gave the ligand L; Yield: 56.0%. m.p. 84–85 ◦C;
MS, m/z: 404 [M]+.
The complexes have similar IR spectra which indicate that they
have similar coordination structures (Table 2). The characteristic
absorption bands of L due to ꢀ(C O), ꢀ(C N) and ꢀ(Ar
O C) appear
at1684, 1620and1260 cm−1, respectively, aftertheformation of the
complexes which shift by ca. 37, 29 and 2 cm−1, respectively. The
results clearly show that the carbonyl oxygen atom and quinoline
nitrogen in the ligand L participate in the coordination to the metal
The 1H NMR chemical shifts ı (ppm/TMS) for the protons of L in
CD3COCD3 are listed in Table 3.
ions [14,15]. However, the slight shift of ꢀ(Ar
O C) between each
complex and the ligand indicates that the etheric oxygen atom does
not coordinate with the metal ion. It is possibly due to the fact that
the ligand has large sterically hindered effect, which prevents the
etheric oxygen atom from coordinating with lanthanide ion.
The characteristic frequencies of the coordinating nitrate groups
(C2v) appear at about 1507 cm−1 (ꢀ1), 1032 cm−1 (ꢀ2), 821 cm−1
(ꢀ3), and 1312 cm−1 (ꢀ4) and the difference between the two highest
3.1. Synthesis of the complexes
The ligand (L) (0.1 mmol) and Ln(NO3)3·nH2O (0.05 mmol) were
added to ethyl acetate (5 mL). After stirring for 4 h at room temper-
ature, the precipitate was separated from the solution by suction
Table 2
Major IR data of the free ligand and its complexes (cm−1
)
Compounds
ꢀ(C O)
ꢀ(C N)
ꢀ(Ar
O
C)
ꢀ0(NO3)
ꢀ1(NO3)
ꢀ4(NO3)
ꢀ1–ꢀ4(NO3)
L
1684
1647
1648
1642
1638
1637
1638
1620
1591
1592
1592
1592
1591
1591
1260
1260
1261
1260
1262
1262
1263
La2(L)3(NO3)6
Sm2(L)3(NO3)6
Eu2(L)3(NO3)6
Gd2(L)3(NO3)6
Tb2(L)3(NO3)6
Dy2(L)3(NO3)6
1384
1382
1384
1384
1383
1384
1507
1503
1503
1506
1506
1507
1312
1308
1313
1312
1312
1313
195
195
190
194
194
194

W.-N. Wu et al. / Spectrochimica Acta Part A 71 (2008) 1461–1465
1463
Table 3
The 1H NMR data of free ligand and its La(III) complex in CD3COCD3(ppm)
Compounds
H1
H3
H6
H7
H2, 4, 5, 8–19
L
8.85–8.88
8.94–8.96
8.25–8.30
8.18–8.34
7.17–7.21
7.11–7.15
5.03
5.28
7.43–7.98
7.47–8.01
La2(L)3(NO3)6
frequency bands (ꢀ1–ꢀ4) is in the range of 190–195 cm−1, indicating
that the coordinated nitrate groups in the complexes are bidentate
[16,17]. The free nitrate groups (D3h) appear at ca. 1384 cm−1 in the
spectra of the complexes [18], which is in agreement with the result
of the conductivity experiment.
4.2. 1H NMR spectra
The 1H NMR spectra of the free ligand and its La(III) complex
were measured in CD3COCD3 at room temperature (Table 3). For
L, the signals at 8.85–8.88, 7.17–7.21 and 5.03 ppm, are assigned to
H1, H6 and H7, respectively. Upon coordination, they are shifted by
0.08–0.09, −0.06 and 0.25 ppm, respectively. Among them, the sig-
nal of proton H6 moves to a higher field. The abnormal phenomenon
has been reported before [19], this is probably due to the reduction
of the conjugation quinoline ring and tortility in the conformation
of the ligand in the complex [20]. Combining with the assumptions
made on the basis of the IR spectra studies, it can be concluded
that the oxygen atom of the carbonyl group and quinoline nitrogen
atom take part in coordination in the complexes [20,21].
According to the data of the elemental analyses, molar conduc-
tivity, IR spectra and 1H NMR spectra, composition of the complexes
can be inferred: [Ln2(L)3(NO3)4](NO3)2.
4.3. Luminescence properties of the complexes
4.3.1. Solids
The Eu(III) and Sm(III) complexes show strong red luminescence
in solid state (Fig. 2 (solid) and Table 4). However, the Tb(III) and
Dy(III) complexes only exhibit the free ligand band emission. This
indicates that the ligand L is a comparative good organic chelator to
absorb and transfer energy to Eu(III) and Sm(III) ions. Furthermore,
it is interesting that the luminescence of Sm(III) complex is stronger
than that of Eu(III) complex. Thus the Eu(III) and Sm(III) complexes
are the potential light conversion agent. The high ratio value for
Fig. 2. (Solid) The luminescence emission spectra of Eu(III) and Sm(III) com-
plexes in solid state at room temperature: ꢃEx(Eu) = 337 nm; ꢃEx(Sm) = 343 nm; slit
width = 1 nm. (Solution) The emission spectra of Eu(III) complex in different solu-
tions (2 × 10−4 mol/L) at room temperature: (1) tetrahydrofuran; (2) acetone; (3)
acetonitrile; (4) ethanol: ꢃEx = 337 nm; slit width = 1 nm.
7
7
ꢂEu = 5D0 → F2/5D0 → F1 is 2.45, showing that the Eu(III) ion does
not lie in a centro-symmetric coordination site [22].
Table 4
Luminescence spectra data (nm) of the Eu(III) and Sm(III) complexes at room temperature
Compounds
Ex slit
1
Em slit
1
ꢃEx
ꢃEm
Emissin intensity
Transition
Quantum yield (˚)
4G5/2
4G5/2
4G5/2
→
→
→
6H5/2
6H7/2
6H9/2
0.063
a
343
563
596
643
909
852
394
Sm2(L)3(NO3)6
5D0
5D0
5D0
→
7F0
0.027
a
1
1
337
580
593
619
86
275
673
Eu2(L)3(NO3)6
→
→
7F1
7F2
1b
2b
3b
4b
5c
6c
7c
8c
1
1
1
1
5
5
5
5
1
1
1
1
5
5
5
5
337
337
337
337
337
337
337
337
617
617
618
619
618
618
617
617
186
118
112
5D0
5D0
5D0
5D0
5D0
5D0
5D0
5D0
→
→
→
→
→
→
→
→
7F2
7F2
7F2
7F2
7F2
7F2
7F2
7F2
40
380
1428
2408
2506
0.027
0.039
0.081
0.043
a
In solid state.
b
c
In different solutions (1, tetrahydrofuran; 2, acetone; 3, acetonitrile; 4, ethanol), concentration: 2 × 10−4 mol/L.
La(III) or Gd(III) doped Eu(III) complexes in tetrahydrofuran (5, the pure Eu(III) complex; 6, Eu3+:La3+ = 0.5:0.5; 7, Eu3+:Gd3+ = 0.9:0.1; 8, Eu3+:Gd3+ = 0.7:0.3). The concen-
trations of both L and the total lanthanide ions were 1 × 10−5 mol/L.

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W.-N. Wu et al. / Spectrochimica Acta Part A 71 (2008) 1461–1465
Table 5
Data of triplet-state energy and ꢄE (T − 5D)
Complexes
Gd2 (L)3(NO3)6
458
21,834
2814
0–0 transition (nm)
Triplet-state energy level (cm−1
)
ꢄE(T − 5D1a) (cm−1
ꢄE(T − 5D4b) (cm−1
)
)
1334
a
5D1: 19,020 cm−1 (the resonance level of Eu(III) ion) [36].
5D4: 20,500 cm−1 (the resonance level of Tb(III) ion) [24].
b
5
5
ꢄE(T − D1) for Eu(III) is greater than 4000 cm−1 or ꢄE(T − D4) for
Tb(III) is less than 1500 cm−1, the fluorescence quantum yield of the
Eu(III) or Tb(III) complexes is approximately zero at room temper-
ature. According to this idea, the strong fluorescence of the Eu(III)
5
complex is due to the optimal ꢄE(T − D1), and it should be due
5
to the small ꢄE(T − D4) that the Tb(III) complex cannot exhibit its
characteristic luminescence.
Fig. 3. Luminescence spectra of L in tetrahydrofuran (1.0 × 10−5 mol/L) in the pres-
ence of Eu3+ ion (as nitrate): ꢃEX = 337 nm. The insert shows the plot of I0/(I − I0) vs.
4.3.5. Doped complexes
−1
.
[Eu3+
]
The luminescence properties of La(III) or Gd(III) doped Eu(III)
complexes were measured in detail (Fig. 4 and Table 4). The concen-
trations of both L and the total lanthanide ions were 1 × 10−5 mol/L
in tetrahydrofuran. Changing the ratio of Eu(III) to La(III) [or Gd(III)]
(0.9:0.1, 0.7:0.3, 0.5:0.5, 0.3:0.7 and 0.1:0.9), the luminescence
emission spectra of the ten doped complexes were measured after
12 h. It can be found that when the ratios are 0.5:0.5 (Eu3+:La3+),
0.9:0.1 (Eu3+:Gd3+) and 0.7:0.3 (Eu3+:Gd3+), the emission inten-
sities of the doped complexes at 618 nm is enhanced by 3.76,
6.34 and 6.59 times compared with that of the pure Eu(III) com-
plex, respectively. The other doped complexes only exhibit the
free ligand band emission or have no notable intensity changes.
This is probably due to the proper amount of La(III) or Gd(III) ion
acting as an intermediate, which absorbs the triplet-state level
energy of ligand and then transfers such energy to the Eu(III)
ion. Such process optimizes the luminescence property of Eu ion,
and is named co-luminescence effect. In addition, because the
radius of Gd(III) is smaller than La(III), the distance between Gd(III)
and Eu(III) ions is shorter than that between La(III) and Eu(III)
ions, which results in the Gd(III) ion represents more excellent
energy transfer efficiency and sensitization capability to Eu(III)
[31,32].
4.3.2. Solutions
The influences of solvents on the luminescence intensity of
the Eu(III) complex were investigated. From Fig. 2 (solution) and
Table 4, it could be seen that in tetrahydrofuran solution the Eu(III)
complex has the strongest fluorescence, and following by that in
acetone, acetonitrile and ethanol. This is due to the coordinat-
ing effects of solvents, namely solvate effect [23]. Additionally,
it is interesting that the luminescence of the Eu(III) complex is
also relatively strong in highly diluted tetrahydrofuran solution
(2 × 10−4 mol/L). The unusual phenomenon [24,25] may be not only
owing to the solvate effects but also to the changes of the structure
in the Eu(III) complex after adding the solvent.
4.3.3. Stoichiometry of ligand with Eu(III) in solution
Fig. 3 shows the changes in the emission spectrum of L as a
function of Eu3+ concentration in tetrahydrofuran. The insert shows
the plot of I0/(I − I0) versus [Eu3+
]
−1, where I0 and I represent the
emission intensity at 618 nm in the absence and presence of Eu3+
,
respectively. The yielded straight line suggests that the complex-
ation of Eu3+ into the ligand is in a 1:1 ratio and the linearity
correlation coefficient R is 0.992 [26]. It clearly confirms that the
structure of the Eu(III) complex in the solution is different from that
in solid state (Eu3+:L = 2:3). Complexation constant (log K), deter-
mined from such emission spectra data for the cation, is 3.75 0.05
[27,28].
4.3.4. The triplet-state energy
Phosphorescence spectrum of the Gd(III) complex was
measured at 77 K in the methanol–ethanol (1:1) solution
(1 × 10−5 mol/L). The lowest triplet-state level energy (T) of the lig-
and was calculated by the shortest wavelength transition in the
phosphorescence spectrum to be 21,834 cm−1 (458 nm) [29]. This
energy level is above the resonance level 5D1 of Eu(III) and 5D4 of
Tb(III). Thus the absorbed energy could be transferred from L to the
Eu(III) or Tb(III) ion. However, the intramolecular transfer efficiency
depends chiefly on two energy transfer processes: one is from the
lowest triplet level (T) of ligand to the resonance level of Eu(III)
ion (5D1) by resonant exchange interaction, and the other is just an
inverse energy transfer by the thermal de-excitation mechanism
[30]. Established on this theory, the conclusion can be drawn that
energy gap (ꢄE) between T and 5D (Table 5) is of opposite influence
on the two energy transfer processes and an optimal value can be
assumed to exist. In addition, it has been reported that [30], when
Fig. 4. Luminescence spectra of La(III) or Gd(III) doped Eu(III) complexes in tetrahy-
drofuran (5: the pure Eu(III) complex; 6: Eu3+:La3+ = 0.5:0.5; 7: Eu3+:Gd3+ = 0.9:0.1;
8: Eu3+:Gd3+ = 0.7:0.3). The concentrations of both L and the total lanthanide ions
were 1 × 10−5 mol/L: ꢃEx = 337 nm; slit width = 5 nm.

W.-N. Wu et al. / Spectrochimica Acta Part A 71 (2008) 1461–1465
1465
4.3.6. Luminescence quantum yields
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2
correction was used: Rf2(tetrahydrofuran)/Rf (acetonitrile) [35].
The values of ˚c are listed in Table 4.
Acknowledgements
This work is supported by the National Science Foundation
of China (20571035) and the Gansu Science Foundation of China
(3ZS051-A25-003). The authors are also grateful to the Instrumen-
tal Analysis and Research Center of Lanzhou University and State
Key Laboratory of Applied Organic Chemistry for providing instru-
mentation facilities.
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