Synthesis, characterizations and luminescent properties of three novel aryl amide type ligands and their lanthanide complexes

Article abstract of DOI:10.1016/j.saa.2006.01.031

Three new aryl amide type ligands, N-(phenyl)-2-(quinolin-8-yloxy)acetamide (L¹), N-(benzyl)-2-(quinolin-8-yloxy)acetamide (L²) and N-(naphthalene-1-yl)-2-(quinolin-8-yloxy)acetamide (L³) were synthesized. With these ligands, three series of lanthanide(III) complexes were prepared: [Ln(L¹)₂(NO₃)₂]NO₃, [Ln(L²)₂(NO₃)₂(H₂O)₂]NO₃·H₂O and [Ln(L³)₂(NO₃)₂(H₂O)₂]NO₃·H₂O (Ln = La, Sm, Eu, Gd). The complexes were characterized by the elemental analyses, molar conductivity, ¹H NMR spectra, IR spectra and TG-DTA. The fluorescence properties of complexes in the solid state and the triplet state energies of the ligands were studied in detail, respectively. It was found that the Eu(III) complexes have bright red fluorescence in solid state. The energies of excited triplet state for the three ligands are 20 325 cm^-1 (L³), 21 053 cm^-1 (L²) and 22 831 cm^-1 (L¹), respectively. All the three ligands sensitize Eu(III) strongly and the order of the emission intensity for the Eu(III) complexes with the three ligands is L³ > L² > L¹. It can be explained by the relative energy gap between the lowest triplet energy level of the ligand (T) and ⁵D₁ of Eu(III). This means that the triplet energy level of the ligand is the chief factor, which dominates Eu(III) complexes luminescence.

Full text of DOI:10.1016/j.saa.2006.01.031

Spectrochimica Acta Part A 65 (2006) 912–918
Synthesis, characterizations and luminescent properties of three
novel aryl amide type ligands and their lanthanide complexes
Wei-Na Wu, Wen-Bing Yuan, Ning Tang, Ru-Dong Yang,
Lan Yan∗, Zi-Hua Xu
Department of Chemistry, Lanzhou University, Lanzhou 730000, PR China
Received 25 October 2005; received in revised form 16 January 2006; accepted 23 January 2006
Abstract
Three new aryl amide type ligands, N-(phenyl)-2-(quinolin-8-yloxy)acetamide (L1), N-(benzyl)-2-(quinolin-8-yloxy)acetamide (L2) and N-
(naphthalene-1-yl)-2-(quinolin-8-yloxy)acetamide (L3) were synthesized. With these ligands, three series of lanthanide(III) complexes were
prepared: [Ln(L1)2(NO3)2]NO3, [Ln(L2)2(NO3)2(H2O)2]NO3·H2O and [Ln(L3)2(NO3)2(H2O)2]NO3·H2O (Ln = La, Sm, Eu, Gd). The complexes
1
were characterized by the elemental analyses, molar conductivity, H NMR spectra, IR spectra and TG–DTA. The fluorescence properties of
complexes in the solid state and the triplet state energies of the ligands were studied in detail, respectively. It was found that the Eu(III) complexes
have bright red fluorescence in solid state. The energies of excited triplet state for the three ligands are 20 325 cm−1 (L3), 21 053 cm−1 (L2) and
22 831 cm−1 (L1), respectively. All the three ligands sensitize Eu(III) strongly and the order of the emission intensity for the Eu(III) complexes
with the three ligands is L3 > L2 > L1. It can be explained by the relative energy gap between the lowest triplet energy level of the ligand (T) and
5D1 of Eu(III). This means that the triplet energy level of the ligand is the chief factor, which dominates Eu(III) complexes luminescence.
© 2006 Elsevier B.V. All rights reserved.
Keywords: Lanthanide complexes; Aryl amide; Synthesis; Fluorescence properties; Triplet state energy
1. Introduction
ion occurs from the lowest triplet state energy level (T) of
the ligand to the resonance level of Ln(III) [7]. This energy
transfer process is one of the most important processes hav-
ing influence on the luminescence properties of the lanthanide
complexes.
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 fluo-
rescence lifetimes [1]. And they have been widely used in
many aspects, such as in biological medicine, especially in the
fluorescence imagery cancer radiation treatment, fluorescence
mark, and fluorescence analysis [2–5]. Since the f–f transitions
are spin- and parity-forbidden, the excited state of the lan-
thanide ion is populated through intramolecular energy transfer
from the ligand or ligands, which therefore serve as sensitiz-
ers (antenna e.ect) [6]. The role of the ligands is on one hand
to collect the photons provided by the light source in order
to allow an energy transfer to the energy levels of the Ln(III)
ion, and on the other hand to shield it against the solvent in
order to avoid non-radioactive deactivation processes. It is gen-
erally accepted that the energy transfer from ligand to Ln(III)
Aryl amide type ligands, which are flexible in structure and
have ‘terminal-group effects’ [8], will shield the encapsulated
ion from interaction with the surroundings effectively. And they
have stronger antenna effect to Eu(III) and Tb(III) [9]. Amide-
based open-chain crown ethers offer many advantages in the
extraction and analysis of the lanthanide ions [10,11] because of
their ring-like coordination structure and terminal group effects
[11,12]. And little work has been done on the luminescence
mechanism of their complexes [13–15]. So, we have designed
and synthesized three aryl amide type ligands and their lan-
thanum(III), samarium(III), europium(III) and gadolinium(III)
nitrate complexes. The luminescence of these lanthanide com-
plexes were enhanced by providing some of the proper conjugate
absorption groups suitable for energy transfer, i.e. naphthalene.
At the same time, we have determined the energies of excited
triplet state for the three ligands in order to discuss the mecha-
nism of luminescence [16].
∗
Corresponding author. Tel.: +86 931 8639816; fax: +86 931 8912582.
E-mail address: wuwn03@st.lzu.edu.cn (L. Yan).
1386-1425/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.01.031

W.-N. Wu et al. / Spectrochimica Acta Part A 65 (2006) 912–918
913
2. Experimental
2.3. Synthesis of ligands (Fig. 1)
2.3.1. Preparation of 1–3
2.1. Materials
The preparation was similar to the previously reported proce-
dure [17]. A chloroform solution containing chloroacetyl chlo-
ride (2.26 g, 0.02 mol) was added dropwise to a solution of
phenylamine (1.86 g, 0.02 mol) and pyridine (2.60 g, 0.03 mol)
in chloroform (20 mL) under stirring on a ice-water bath. Then,
the reaction mixture was stirred at room temperature for 3.5 h. A
solid product was separated from the solution by suction filtra-
tion, purified by washing with water, 0.5 mol/L HCl, 0.5 mol/L
NaOH and distilled water, respectively. Then, the product 1 was
The lanthanide(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.
2.2. Physical measurements
1
dried in a vacuum. Yield, 85%. m.p. 137–138 ◦C; H NMR
The melting points of the compounds were determined on
a XT4-100× microscopic melting point apparatus (made in
Beijing, China). Elemental analyses were carried out on an
Elemental Vario EL analyzer. The metal contents of the com-
plexes were determined by titration with EDTA. The infrared
spectra (IR, ν = 4000–400 cm−1) were determined by the KBr
pressed disc method on a Nicolet-170SX FT-IR spectropho-
tometer. 1H NMR spectra were recorded on a Varian VR 300-
MHz spectrometer in CDCl3 and 200 MHz on Bruker DRX-200
spectrometer in CD3COCD3 solutions, with TMS as inter-
nal standard. All conductivity measurements were carried out
with a DDS-11A conductometer (made in China) bridge using
1.0 × 103 mol/L solution in DMF at 25 ◦C. The thermal behav-
ior was monitored on a PCT-2 differential thermal analyzer,
and ␣-Al2O3 was used as a reference in static air. The FAB
mass spectrum was obtained on a VG ZAB-HS mass spectrom-
eter using m-nitrobenzyl alcohol (NBA) as a matrix. Fluores-
cence emission and excitation spectra were determined on a
Hitachi F-4500 FL spectrophotometer. Phosphorescence spec-
tra at 77 K were taken on a Hitachi-850 fluorescence spec-
trophotometer equipped with phosphorescence measurement
apparatus.
(CDCl3, ppm): ␦ 4.21 (2H, s, CH2) 7.07–7.59 (5H, m, phenyl).
2 and 3 were prepared according to the same procedure as
1
1. 2: yield, 70%. m.p. 92–93 ◦C; H NMR (CDCl3, ppm): ␦
4.11 (2H, s, CH2Cl), 4.49–4.51 (2H, d, CH2), 6.90 (1H, s, NH),
7.27–7.40 (5H, m, phenyl). 3: yield 91%. m.p. 156–157 ◦C; 1H
NMR (CDCl3, ppm): ␦ 4.35 (2H, s, CH2), 7.25–8.00 (7H, m,
naphthyl), 8.78 (1H, s, NH).
2.3.2. Preparation of Ln (n = 1–3)
A quality of 8-hydroxyquinoline (0.73 g, 5 mmol) was dis-
solved in absolute DMF (15 mL), and to it was added solid
K2CO3 (0.76 g, 5.5 mmol). After stirring at 90 ◦C for 0.5 h, a
DMF solution containing 1 (0.85 g, 5 mmol) was added drop-
wise. Then, the reaction mixture was stirred at the same temper-
ature for 3 h. After cooling to the room temperature, 150 mL
water was added and then stirred for 15 h. The brown solid
was obtained by suction filtration and treated with column chro-
matography on silica gel [petroleum ether–ethyl acetate (20:1)]
1
to get 0.55 g of L1, yield 40%. m.p. 146–148 ◦C; H NMR
(CDCl3, ppm): ␦ 3.68 (2H, s, CH2), 5.84–6.21 (5H, m, phenyl),
6.277–7.793 (6H, m, oxyquinoline), 9.300 (H, s, NH); FAB MS:
m/z = 279.2 (M + H).
Fig. 1. Scheme of the synthesis of the ligands.

914
W.-N. Wu et al. / Spectrochimica Acta Part A 65 (2006) 912–918
L2 and L3 were synthesized using procedures similar to those
a long time. The molar conductivity values of the complexes in
DMF are in the range of 87–120 s cm2 mol−1, indicating that the
complexes are 1:1 electrolytes [18].
1
for L1. L2: yield 36%. m.p. 104–106 ◦C; H NMR (CDCl3,
ppm): ␦ 3.507–3.526 (2H, d, NCH2), 3.853 (2H, s, CH2O),
6.196–6.246 (5H, m, phenyl), 6.363–7.590 (6H, m, oxyquino-
line), 8.760 (H, s, NH); FAB MS: m/z = 293.2 (M + H). L3: yield
3.2. 1H NMR spectra
1
31%. m.p. 129–130 ◦C; H NMR (CDCl3, ppm): ␦ 5.09 (2H,
1
s, CH2), 8.97–7.26 (13H, m, naphthyl and oxyquinoline), 11.04
The H NMR spectra of the free ligands and their La(III)
(1H, s, NH); FAB MS: m/z = 329.2 (M + H).
complexes were measured in CD3COCD3 at room tempera-
ture. The complexes were dried at 100 ◦C to constant weights.
Owing to the solvent effect, the 1H NMR spectrum of free lig-
ands in CD3COCD3 is slightly different from that in CDCl3.
Some 1H NMR data are shown in Table 2. In CD3COCD3, for
L1, the signal of H1, H2–H5 (and C6H5– protons), H6, and H7 are
at 9.05–9.08, 7.06–7.88, 8.41–8.46 and 4.90 ppm, respectively.
Upon coordination, the protons signals move to lower field. The
signals of them shift to 9.08–9.10, 7.08–7.84, 8.45–8.49 and
5.03 ppm, respectively. And for L2, the signals of H1, H2–H5
(and C6H5– protons), H6, H7 and H9 are shifted by 0.16,
0.04–0.11, 0.1, 0.26and0.08 ppmintheLa(III)complex, respec-
tively. While for L3, the signals of H1, H2–H5 (and C10H7–
protons), H6, and H7 are at 8.91–8.94, 7.47–8.44, 8.58–8.62
and 5.05 ppm, respectively. In the La(III) complex, they are
shifted by 0.03, –(0.12–0.49), –0.41 and 0.09, respectively. We
can see that the protons signals of H2–H5 (and C10H7– pro-
tons) and H6 move to higher field. The abnormal phenomenon
was reported before [19], this is probably due to the reduc-
tion of the conjugation in naphthalene ring and quinoline ring
and tortility in the conformation of the ligand in the complex
[20]. It can be concluded that the oxygen atom of the carbonyl
2.4. Synthesis of the complexes
The ligand (Ln) (0.1 mmol) was dissolved in ethyl acetate
(2 mL). To this solution was added dropwise a ethyl acetate
solution (2 mL) containing Ln(NO3)3·nH2O (0.05 mmol). After
stirring for 4 h at room temperature, the precipitate was sepa-
rated from the solution by suction filtration, purified by washing
several times with ethyl acetate, and dried for 24 h in a vacuum.
The yields were in the range of 40–60%.
3. Results and discussion
3.1. Properties of the complexes
Analytical data for the complexes are presented in Table 1.
The elemental analyses show that the formulas of the complexes
are Ln(L)2(NO3)3·nH2O (L = L1, n = 0; L = L2–3, n = 3). All of
complexes are white powders. They are soluble in DMF, DMSO,
acetone, methanol and ethanol, slightly soluble in ethyl acetate,
insoluble in water and diethyl ether, and may be kept in air for
Table 1
Analytical and molar conductance data for the complexes
Complexes
C% Found (calc.)
H% Found (calc.)
N% Found (calc.)
Ln% Found (calc.)
ꢀm (s cm2 mol−1
)
La(L1)2(NO3)3
46.04 (46.32)
45.32 (45.73)
44.96 (45.35)
45.13 (45.38)
44.56 (44.87)
44.01 (44.34)
44.05 (44.27)
43.76 (44.03)
48.34 (48.71)
47.88 (48.17)
48.31 (48.10)
47.96 (47.86)
10.93 (11.12)
11.09 (10.98)
10.79 (10.94)
10.77 (10.90)
10.15 (10.17)
10.19 (10.06)
10.22 (10.04)
10.14 (9.98)
9.38 (9.47)
3.26 (3.20)
3.24 (3.16)
2.88 (3.15)
3.46 (3.14)
3.72 (3.97)
3.65 (3.93)
3.58 (3.92)
3.65 (3.90)
3.35 (3.70)
3.59 (3.66)
3.49 (3.65)
3.65 (3.63)
16.1 (15.8)
17.2 (16.8)
17.4 (17.0)
17.6 (17.5)
14.8 (14.4)
15.7 (15.4)
15.8 (15.6)
16.3 (16.0)
13.8 (13.4)
14.7 (14.4)
14.6 (14.5)
15.3 (14.9)
120.3
115.3
105.3
108.3
127.8
87.8
Sm(L1)2(NO3)3
Eu(L1)2(NO3)3
Gd(L1)2(NO3)3
La(L2)2(NO3)3·3H2O
Sm(L2)2(NO3)3·3H2O
Eu(L2)2(NO3)3·3H2O
Gd(L2)2(NO3)3·3H2O
La(L3)2(NO3)3·3H2O
Sm(L3)2(NO3)3·3H2O
Eu(L3)2(NO3)3·3H2O
Gd(L3)2(NO3)3·3H2O
90.3
101.8
108.6
103.2
110.4
96.7
9.16 (9.36)
9.41 (9.35)
9.19 (9.30)
Table 2
200 MHz 1H NMR chemical shifts ␦ (ppm/TMS) for the protons of free ligands and their complexes in CD3COCD3
Ligands and their complexes
H1
H2–H5 and C6H5–a/C10H7–b protons
H6
H7
H8
H9
L1
9.05–9.08d
9.08–9.10d
8.73–8.75d
8.89–8.91d
8.91–8.94d
8.94–8.96d
7.06–7.88mt
7.08–7.84mt
7.23–7.61mt
7.27–7.72mt
7.47–8.44mt
7.35–7.95mt
8.41–8.46d
8.45–8.49d
8.34–8.39d
8.44–8.49d
8.58–8.62d
8.17–8.20d
4.90s
5.03s
4.76s
5.02s
5.05s
5.14s
11.02s
La(L1)2(NO3)3
L2
9.47s
9.48s
11.08s
4.47–4.50d
4.55–4.57d
La(L2)2(NO3)3·3H2O
L3
La(L3)2(NO3)3·3H2O
d = doublet; s = singlet; mt = multiplet.
a
The phenyl of L1 and L2.
The naphthalene of L3.
b

W.-N. Wu et al. / Spectrochimica Acta Part A 65 (2006) 912–918
915
betw−een the two bands lie in 187–216 cm−1, suggesting that the
NO3 groups in the complexes are bidentate ligands (Table 3)
[30].
group, ethereal oxygen atom and phen nitrogen atom take part
in coordination in La(L1)2(NO3)3, La(L2)2(NO3)3·3H2O and
La(L3)2(NO3)3·3H2O [19–21].
Furthermore, about four proton resonance peaks of
La(L2)2(NO3)3·3H2O and La(L3)2(NO3)3·3H2O are at 3.04 and
2.96 ppm, respectively. Excluding the influence of crystal water
via the constant weight procedure, we can conclude that there
should be two coordinating water molecules in the complexes.
3.4. TG–DTA analysis
Some thermal analyses data are listed in Table 4. The DTA
curves of the free ligands have an endothermic peak at 148 ◦C
(L1), 107 ◦C (L2) and 131 ◦C (L3), respectively. But there are no
weight loss on the corresponding TG curves, showing that these
are phase transition processes, which are the same as the melting
points of the ligands. There are endothermic peaks at 60–68 ◦C
and 174–192 ◦C for the L2 complexes, while endothermic peaks
of the L3 complexes appear at 63–78 ◦C and 160–212 ◦C. The
corresponding TG curves show that the weight losses are equal
to one and two water molecules, indicating the presence of one
lattice water molecule and two coordinating water molecules in
the complexes. But the L1 complexes has no endothermic peaks.
The result is in accordance with some of the assumptions made
on the basis of 1H NMR spectra, infrared spectral and elemental
analyses studies. For all complexes, two or three exothermic
peaks appear around 204–508 ◦C. The final products were found
to be Ln2O3 when the temperature was above 850 ◦C, and the
total weight loss of the complexes was found to be closed to the
calculated value.
According to the data of the elemental analyses, molar
conductivity, 1H NMR spectra, IR spectra and TG–DTA,
composition of the three series complexes can be inferred:
[Ln(L1)2(NO3)2]NO3, [Ln(L2)2(NO3)2(H2O)2]NO3·H2O and
[Ln(L3)2(NO3)2(H2O)2]NO3·H2O. And the coordination num-
ber for the L1, L2 and L3 complexes may be 10, 12 and 12,
respectively. The possible molecular structures of the complexes
are shown in Fig. 2.
3.3. IR Spectra
The IR spectrum of the free ligands show strong band
at 1650–1699 cm−1, which is attributable to stretch vibra-
tions of the carbonyl group of amide (ν(C O)). The peak at
1555–1563 cm−1 can be assigned to the ν(C N) of amide,
the peak at 1609–1630 cm−1 to ν(C N) and the peak at
1212–1247 cm−1 to ν(Ar–O). In the IR spectra of their lan-
thanide(III) complexes, the ν(C O), ν(C N), ν(C N) and
ν(Ar–O) shift by 3–48, 8–18, 10–32 and 1–39 cm−1, respec-
tively; thus indicating that carbonyl, ethereal oxygen atom and
phen nitrogen atom take part in coordination to the metal ion
[22–25]. The result is in accordance with some of the assump-
1
tions made on the basis of the H NMR spectra studies. In
addition, the very smaller shift of 3–4 cm−1 in ν(C O) of the
L2 complexes and 1–3 cm−1 in ν(Ar–O) of the L3 complexes is
mainly due to the fact that the molecules have large sterically
hinderedeffect, whichpreventscarbonyloxygenatomsinL2 and
ethereal oxygen atom in L3 from tight coordinating lanthanide
ion [26].
For the L2 and L3 complexes, the aqueous ν(OH) bands
appear at 3401–3442 cm−1, this shows that there is some water
in the complexes. And a weak intensity band at 606–626 cm−1
in the complexes is due to the rocking vibration of coordina-
tive water [27]. Additionally, for all the complexes, bands at
1384 cm−1 in the spectra of complexes indicate that free nitrate
groups (D3h) exist [28]. The two intense absorptions associated
with the asymmetric stretching in the range of 1293–1317 cm−1
(ν4) and −1504–1509 cm−1 (ν1) appear, clearly establishing that
the NO3 groups (C2v) are coordinated [29]. The differences
3.5. Luminescence properties of the complexes
3.5.1. Fluorescence
For comparison, the fluorescence spectrum data of the Eu(III)
and Sm(III) complexes in solid state are given in Table 5. The
Table 3
Major IR data of the free ligands and their complexes (cm−1
)
Compounds
␯ (OH)
␯ (C O)
␯ (C N)
␯ (C–N)
␯ (Ar–O–C)
␯ (NO3)
1
␯ (NO3)
4
␯ (NO3)
0
␳ (OH)
L1
1699
1664
1651
1651
1651
1650
1653
1646
1646
1647
1694
1660
1651
1651
1651
1609
1598
1594
1599
1594
1620
1600
1601
1601
1600
1630
1599
1599
1598
1600
1555
1566
1571
1572
1567
1563
1576
1577
1577
1577
1555
1563
1573
1567
1569
1247
1211
1208
1215
1215
1244
1207
1210
1210
1209
1212
1209
1210
1210
1211
La(L1)2(NO3)3
Sm(L1)2(NO3)3
Eu(L1)2(NO3)3
Gd(L1)2(NO3)3
L2
1504
1507
1507
1507
1317
1317
1317
1317
1384
1384
1384
1384
La(L2)2(NO3)3·3H2O
Sm(L2)2(NO3)3·3H2O
Eu(L2)2(NO3)3·3H2O
Gd(L2)2(NO3)3·3H2O
L3
La(L3)2(NO3)3·3H2O
Sm(L3)2(NO3)3·3H2O
Eu(L3)2(NO3)3·3H2O
Gd(L3)2(NO3)3·3H2O
3442
3418
3421
3401
1508
1508
1508
1509
1317
1314
1314
1316
1384
1384
1384
1384
626
616
611
617
3405
3438
3422
3426
1508
1509
1509
1509
1316
1293
1315
1297
1384
1384
1384
1384
614
612
606
608

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W.-N. Wu et al. / Spectrochimica Acta Part A 65 (2006) 912–918
Table 4
Thermal analyses data of the compounds
Compound
Tendo (◦C)
H2O loss (calc.)%
Texo (◦C)
Residue (calc.)%
t1
t2
t3
L1
148
305
204
227
227
237
230
547
336
347
338
354
330
341
339
343
365
587
344
335
334
342
La(L1)2(NO3)3
Sm(L1)2(NO3)3
Eu(L1)2(NO3)3
Gd(L1)2(NO3)3
L2
470
486
470
480
481
483
498
473
485
663
508
455
481
499
19.21 (18.50)
20.44 (19.54)
20.53 (19.69)
20.89 (20.13)
107
60
68
64
67
131
76
63
78
71
La(L2)2(NO3)3·3H2O
Sm(L2)2(NO3)3·3H2O
Eu(L2)2(NO3)3·3H2O
Gd(L2)2(NO3)3·3H2O
L3
La(L3)2(NO3)3·3H2O
Sm(L3)2(NO3)3·3H2O
Eu(L3)2(NO3)3·3H2O
Gd(L3)2(NO3)3·3H2O
188
190
192
174
1.72 (1.87)
1.74 (1.85)
1.67 (1.84)
1.71 (1.83)
3.69 (3.74)
3.34 (3.69)
3.47 (3.69)
3.21 (3.67)
17.69 (16.93)
18.26 (17.90)
18.89 (18.03)
19.05 (18.45)
275
274
273
321
290
221
273
237
212
162
160
171
1.71 (1.74)
1.93 (1.72)
1.91 (1.72)
2.01 (1.71)
3.42 (3.48)
3.68 (3.44)
3.81 (3.44)
3.74 (3.42)
15.14 (15.75)
16.07 (16.67)
17.63 (16.79)
18.06 (17.19)
other complexes exhibit the free ligands band emission. From
Table 5, it is found that the emission intensities of Eu(III) com-
plexes are stronger than those of the Sm(III) complexes in solid
state, indicating an efficiently intermolecular transfer from the
ligands to the Eu(III). Furthermore, the order of the emission
intensity for the Eu(III) complexes with the three ligands is
L3 > L2 > L1. It shows that the proper conjugate terminal group
can enhance the emission intensities of the complexes. The emis-
sion spectra of the Eu(III) complexes are shown in Fig. 3. From
3.5.2. Phosphorescence
At 77 K in a solution of methanol–ethanol (1:1), all Gd(III)
complexesdisplayonlyoneweakphosphorescentemissionband
at 438, 475 and 492 nm, respectively (shown in Fig. 4). And they
can be assumed to be the 0–0 transition [15]. The triplet state
energy was determined and listed in Table 6.
The triplet states of the ligands are above the resonance levels
of Eu(III) ion. Thus, the excited ligands could transfer energy
to Eu(III) ion. The intramolecular transfer efficiency depends
mainly on two energy transfer processes: one is from the low-
est triplet level of ligand to the resonance level of Ln(III) by
resonant exchange interaction, and the other is just an inverse
energy transfer by the thermal de-excitation mechanism [16].
Both energy transfer rate constants are dependent on the energy
7
Table 5, it can be seen that the intensity of 5D0 → F2 transition
5
7
is far stronger than that of D0 → F1 transition, and a higher
7
7
value for ηEu = 5D0 → F2/5D0 → F1 (ηEu = 2.6–3.2) indicates
that the line-like emission spectrum results from intra-4f transi-
tions of predominantly electric dipole character and thus Eu(III)
lies in a non-centrosymmetric ligand field [31].
5
difference[ꢁE(T − D1)]betweenthelowesttripletenergylevel
Table 5
Fluorescence spectrum data of the solid complexes at room temperature (nm)
Compounds
Ex slit
5
Em slit
5
λEx
λEm
Emission intensity
Transition
7
Eu(L1)2(NO3)3
332
557
593
618
24
364
1007
5D1 → F2
7
5D0 → F1
7
5D0 → F2
Sm(L1)2(NO3)3
5
5
5
5
406
337
597
643
37
18
4G5/2 → H7/2
6
6
4G5/2 → H9/2
Eu(L2)2(NO3)3·3H2O
555
592
617
48
464
1470
5D1 → F2
7
7
5D0 → F1
7
5D0 → F2
Sm(L2)2(NO3)3·3H2O
Eu(L3)2(NO3)3·3H2O
5
5
5
5
394
325
595
642
58
23
4G5/2 → H7/2
6
6
4G5/2 → H9/2
7
579
592
614
397
2116
6628
5D0 → F0
7
5D0 → F1
7
5D0 → F2
6
Sm(L3)2(NO3)3·3H2O
5
5
413
594
619
643
95
77
47
4G5/2 → H7/2
6
4G5/2 → H3/2
6
4G5/2 → H9/2

W.-N. Wu et al. / Spectrochimica Acta Part A 65 (2006) 912–918
917
Table 6
5
Triplet state energies of the Gd(III) complexes and ꢁE(T − D1) for the ligands
5
Complexes
λEx (nm)
0–0 transition (nm)
Triplet state energy level (cm−1
)
ꢁE(T − D1a) (cm−1
)
Gd(L1)2(NO3)3
280
300
290
438
475
492
22831
21053
20325
3811
2033
1305
Gd(L2)2(NO3)3·3H2O
Gd(L3)2(NO3)3·3H2O
a
5D1: 19020 cm−1 [32].
Fig. 3. The fluorescence emission spectra of Eu(III) complexes in solid state at
room temperature: (1) Eu(L1)2(NO3)3 λEx = 332 nm; (2) Eu(L2)2(NO3)3·3H2O
λEx = 337 nm; (3) Eu(L3)2(NO3)3·3H2O λEx = 325 nm.
5
of the ligand (T) and 5D1 [Eu(III)]. ꢁE(T − D1) for L1, L2 and
L3 are given in Table 6.
5
Sato and Wada [16] have estimated that, when ꢁE(T − D1)
for Eu(III) is greater than 4000 cm−1, the fluorescence yield
of the Eu(III) complexes is approximately zero at room tem-
5
perature; when ꢁE(T − D1) for Eu(III) is in the range of
1000–2000 cm−1, the fluorescence yield of the Eu(III) com-
plexes is very high at room temperature. According to this, the
strong luminescence of Eu(III) with L3 should be due to the
5
optimal ꢁE(T − D1), and the weak luminescence of Eu(III)
Fig. 4. The phosphorescence spectra of Gd(III) complexes in methanol–ethanol
(1:1) at room temperature: (1) Gd(L1)2(NO3)3 (1 × 10−5 mol/L) λEx = 280; (2)
Gd(L2)2(NO3)3·3H2O (1 × 10−5 mol/L) λEx = 300; (3) Gd(L3)2(NO3)3·3H2O
(1 × 10−5 mol/L) λEx = 290: a.u., arbitrary units.
Fig. 2. Possible molecular structures of the complexes: (1)
[Ln(L1)2(NO3)2]NO3;
(2)
[Ln(L2)2(NO3)2(H2O)2]NO3·H2O;
(3)
[Ln(L3)2(NO3)2(H2O)2]NO3·H2O.

918
W.-N. Wu et al. / Spectrochimica Acta Part A 65 (2006) 912–918
5
with L1 and L2 should be due to the larger ꢁE(T − D1). So it
could be explained that the order of the emission intensity for
the Eu(III) complexes with the three ligands is L3 > L2 > L1.
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4. Conclusions
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According to the data and discussion above, it is obvious
that the ligands formed complexes with lanthanide(III) ions.
The europium(III) complexes exhibited characteristic fluores-
cence of europium ions, and the terminal group may affect the
fluorescence.
Comparing the fluorescence spectra of the Eu(III) complexes,
we can find that the emission intensity order for the Eu(III)
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means that the triplet energy level of the ligands is the chief
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comparing the structure of three ligands, we can conclude that
high emission intensity may be attributed to suitable flexibility
and proper conjugate absorption groups of the ligands. Based
on those results, a series of new aryl amide type ligands could
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Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at doi:10.1016/j.saa.2006.01.031.
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