Synthesis and infrared and fluorescent spectra of rare earth complexes with a new amide ligand

Article abstract of DOI:10.1016/j.saa.2006.12.060

Solid complexes of rare earth nitrates and picrates with a new amide ligand, 1,6-bis[(2′-benzylaminoformyl)phenoxyl]hexane (L) have been prepared. These complexes are characterized by elemental analysis, UV-vis spectra and IR spectra. The fluorescent and

Full text of DOI:10.1016/j.saa.2006.12.060

Spectrochimica Acta Part A 68 (2007) 478–483
Synthesis and infrared and fluorescent spectra of rare earth
complexes with a new amide ligand
Haixia Cuia,b, Jianmin Chena,∗, Huidi Zhoua, Yanhua Lua,b
a State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China
b Graduate School of The Chinese Academy of Sciences, Beijing 100039, China
Received 14 September 2006; received in revised form 7 December 2006; accepted 14 December 2006
Abstract
Solid complexes of rare earth nitrates and picrates with a new amide ligand, 1,6-bis[(2ꢀ-benzylaminoformyl)phenoxyl]hexane (L) have been
prepared. These complexes are characterized by elemental analysis, UV–vis spectra and IR spectra. The fluorescent and luminescent properties
of the Eu(III) and Tb(III) nitrates and picrates complexes in solid state are also investigated. Under the excitation of UV light, these complexes
except Tb(III) picrate complex exhibit characteristic emission of europium and terbium ions. The influence of the counter anion on the fluorescent
intensity is also discussed.
© 2007 Published by Elsevier B.V.
Keywords: Amide ligand; Rare earth complexes; Fluorescent properties
1. Introduction
sensitizers (antenna effect) [6,7]. In order to obtain strong flu-
orescent complexes, the chromophoric ligands which chelate
to rare earth metals should be able: (1) to encapsulate and
protect rare earth ions from the solvent molecules and (2) to
absorb energy and transfer it efficiently to the central metal [8,9].
Podand-type ligands [10], macrocyclic compounds [11,12], car-
boxylic acid derivatives [13] and pyrazolone derivatives [14]
have been extensively used for these purposes. Among these
ligands, podand-type ligands have drawn much attention in
recent years, mainly due to their possessing spheroidal cavi-
ties and hard binding sites, therefore stabilizing their complexes
and shielding the encapsulated ion from interaction with the
surroundings [15–17]. Among numerous podands which have
demonstrated their potential use in functional supramolecu-
lar chemistry [18,19], amide type podands are important for
preparing the rare earth complexes possessing strong fluores-
cent properties. It is expected that the amide type podands,
which are flexible in structure and have ‘terminal-group effects’
[20], will shield the encapsulated rare earth ion from interac-
tion with the surroundings effectively, and thus achieve strong
luminescent properties. Our group is interested in organic light
emitting devices (OLED). In order to find strong fluorescent
rare earth complexes and use them in OLED furthermore, a new
amide ligand 1,6-bis[(2ꢀ-benzylaminoformyl)phenoxyl]hexane
(L, Scheme 1) and its rare earth complexes have been designed
The study of the coordination compounds of the trivalent rare
earth ions (RE3+) continues to be an active research area, which
may be attributed to the specific spectroscopic and magnetic
propertiesofrareearthionsandtheirapplicationsassupramolec-
ular device, contrast-enhancing agents in magnetic resonance
imaging, effective chelators in the separation and purification
of rare earth, optical signal amplifiers, electroluminescent (EL)
devices and luminescent probes in biological systems [1–3].
Among these compounds, the fluorescent rare earth complexes
are of special interest because these complexes could show nar-
row emission bands, a large Stokes’ shift, long luminescence
decay times and especially the possibility of obtaining improved
efficiency over the more conventional materials [4,5]. The emis-
sion of light from rare earth ions arises from f–f transitions,
which result in emission bands with extremely narrow band-
width and no theoretical cap on the quantum efficiency. The f–f
transitions are spin and parity-forbidden, so the excited state
of rare earth ions is populated through intramolecular energy
transfer from the ligand or ligands, which therefore act as
∗
Corresponding author. Tel.: +86 931 4968018; fax: +86 931 8277088.
E-mail address: chenjm@lzb.ac.cn (J. Chen).
1386-1425/$ – see front matter © 2007 Published by Elsevier B.V.
doi:10.1016/j.saa.2006.12.060

H. Cui et al. / Spectrochimica Acta Part A 68 (2007) 478–483
479
Table 1
Analytical data of the complexes (calculated values in parentheses)
2.2. Methods
Carbon, nitrogen and hydrogen were determined using a
Vario EL CHN (see Table 1). 1H NMR spectrum was measured
on a Bruker-400 FT NMR (400 MHz) spectrometer in CDCl3
solution with TMS as internal standard. The IR spectra were
recorded in the 4000–400 cm−1 region using KBr pellets and
a Bruker FTS66V/S instrument (see Tables 2 and 3). UV–vis
spectra were recorded on a Hitachi U-3010 spectrophotome-
ter. Fluorescence and luminescence spectra were obtained on a
Hitachi F-4500 spectrophotometer at room temperature. Each
sample in solid state is about 90 mg and is ground roughly
before measurement. The excitation and emission slit widths
were 1.0 nm and the PMT voltage is 700 V.
Complexes
Found (calculated) (%)
C
H
N
L
76.30 (76.09)
53.23 (53.36)
53.13 (53.27)
51.76 (51.96)
51.55 (51.72)
51.54 (51.65)
51.30 (51.29)
45.06 (44.74)
44.27 (44.33)
44.51 (44.11)
46.31 (46.41)
7.67 (6.76)
4.41 (4.92)
4.99 (4.91)
4.96 (4.96)
4.84 (4.94)
4.59 (4.93)
4.59 (4.89)
3.12 (3.32)
3.43 (3.29)
3.71 (3.27)
3.72 (3.44)
5.20 (5.22)
7.48 (7.32)
7.29 (7.31)
7.06 (7.13)
7.16 (7.10)
6.98 (7.09)
7.02 (7.04)
10.90 (11.04)
11.07 (10.94)
10.66 (10.88)
11.30 (11.45)
2La(NO3)3·3L·2H2O
2Pr(NO3)3·3L·2H2O
2Eu(NO3)3·3L·4H2O
2Gd(NO3)3·3L·4H2O
2Tb(NO3)3·3L·4H2O
2Er(NO3)3·3L·4H2O
La(Pic)3·L·2H2O
Eu(Pic)3·L·2H2O
Tb(Pic)3·L·2H2O
Y(Pic)3·L·2H2O
2.3. Preparation of amide ligand L
and prepared. The fluorescent and luminescent properties of
the europium(III) and terbium(III) complexes were studied in
detail.
The synthetic route for the ligand L is shown in Scheme 1.
N-Benzylsalicylamide (4.80 g, 21.1 mmol), potassium carbon-
ate (1.50 g, 10.9 mmol) and DMF (100 cm3) were warmed
to ca. 80 ◦C and 1,6-dibromohexane (2.44 g, 10.0 mmol) was
added. The reaction mixture was stirred at 80–85 ◦C for 15 h.
After cooling down, the mixture was poured into pure water
(200 cm3). The resulted solid was filtrated and washed by pure
water again and again, then dried to get the white ligand L,
2. Experimental
2.1. Materials
yield 92%, mp 121–122 ◦C; H NMR (CDCl3, 400 MHz): δ,
1
N-Benzylsalicylamide [21], rare earth nitrates [22] and
picrates [23] were prepared according to the literature meth-
ods. Other chemicals were obtained from commercial sources
and used without further purification.
8.259–8.263 (quartet, 2H, N H), 6.903–7.438 (m, 18H, Ar H),
4.622–4.635 (d, 4H, CH2 N), 3.938–3.969 (t, 4H, O CH2),
1.481–1.513 (t, 4H, O CH2 CH2) 1.089–1.126 (quintet, 4H,
Table 2
The most important IR bands of the rare earth nitrate complexes (cm−1
)
Ligand/complexes
υ (C O) (cm−1
)
υ (Ar
O
C) (cm−1
)
υ (NO3−) (cm−1
)
υ1
υ4
υ2
υ3
|υ1 − υ4|
L
1641
1609
1610
1612
1610
1614
1240
1234
1234
1235
1235
1235
1107
1110
1108
1110
1110
1108
2Pr(NO3)3·3L·2H2O
2Eu(NO3)3·3L·4H2O
2Gd(NO3)3·3L·4H2O
2Tb(NO3)3·3L·4H2O
2Er(NO3)3·3L·4H2O
1486
1488
1488
1487
1486
1305
1306
1307
1305
1303
1028
1028
1029
1032
1033
816
815
815
815
815
181
182
181
182
183
Table 3
The most important IR bands of the rare earth picrate complexes (cm−1
)
Compound
υ (C O)
υ (Ar
O
C)
υ (C O)
υas
(
NO2)
υs ( NO2)
HPic
1265s
1555m
1342s
L
1641s
1614s
1615s
1615s
1615s
1107m
1108w
1108w
1108w
1108w
La(Pic)3·L·2H2O
Eu(Pic)3·L·2H2O
Tb(Pic)3·L·2H2O
Y(Pic)3·L·2H2O
1273m
1275m
1276m
1275s
1575m, 1542m
1577m, 1542m
1578m, 1542m
1578s, 1542s
1363m, 1332m
1363m, 1334m
1363m, 1336s
1363m, 1334s
Scheme 1. The synthetic route for the ligand L.

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H. Cui et al. / Spectrochimica Acta Part A 68 (2007) 478–483
O CH2 CH2 CH2); IR (KBr): 3346 (s), 1641 (s, C O), 1600
(m), 1534 (s), 1489 (m), 1450 (m), 1301 (m), 1242 (s, Ar O),
1107 (m, C O C), 806 (w), 755 (s), 695 (m).
2.4. Preparation of the complexes
An ethyl acetate solution (5 cm3) of RE(NO3)3·6H2O
(RE = La, Pr, Eu, Gd, Tb, Er) (0.2 mmol) was added dropwise
to a solution of 0.3 mmol L in chloroform (5 cm3). The mixture
was stirred at room temperature for 4 h. Then the precipitated
solid complex was filtered, washed with ethyl acetate and chlo-
roform, dried in vacuo over P4O10 for 48 h and submitted for
elemental analysis, yield: 80%.
A solution of 0.3 mmol RE(Pic)3·6H2O (RE = La, Eu, Tb,
Y) in 10 cm3 of ethyl acetate was added dropwise to a solution
of 0.3 mmol ligand L in 10 cm3 of ethyl acetate. The mixture
was stirred at room temperature for 4 h. 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%.
Fig. 1. The UV–vis spectra of
L
(A), Eu(Pic)3·L·2H2O (B) and
2Eu(NO3)3·3L·4H2O (C) (in THF, 10−4 mol L−1).
3.3. IR spectra
3. Results and discussion
The main IR peaks of the ligand and their complexes are
presented in Tables 2 and 3. The IR spectra of the complexes
are similar to each other. The IR spectrum of the free ligand L
showstwostrongabsorptionbandat1641and1107 cm−1, which
are assigned to the stretch vibration of the carbonyl group [υ
(C O)] and υ (C O C), respectively. In the nitrate complexes,
the low-energy band remains unchanged, but the high-energy
band red shifts to about 1610 cm−1 (ꢀυ = 31 cm−1) as com-
pared to its counterpart for the free ligand, thus indicating that
only the oxygen atom of C O takes part in coordination to rare
earth ions. The characteristic frequencies of the coordinating
nitrate groups (C2v) appear at about 1488 (υ1), 1305 (υ4), 1028
(υ2) and 815 cm−1 (υ3) [24]. In addition, the difference between
two strongest absorptions of the nitrate groups |υ1 − υ4| is
3.1. Properties of the complexes
Analytical data for the newly synthesized complexes, listed in
Table 1, indicate that six rare earth nitrates complexes conform
to a 2:3 metal-to-ligand stoichiometries [2RE(NO3)3·3L]·nH2O
(n = 2, 4), but the four picrates complexes conform to a 1:1 metal-
to-ligand stoichiometries [RE(Pic)3·L]·2H2O.
The ligand L is easy to dissolve in dimethyl sulphoxide
(DMSO), dimethyl formamide (DMF), chloroform and tetrahy-
drofuran (THF), and can dissolve in acetone, methanol, ethanol,
acetonitrile and ethyl acetate, but hardly dissolve in petroleum
ether.
All the rare earth nitrate complexes are soluble in DMSO,
DMF, acetone, methanol, acetonitrile and THF, but hardly sol-
uble in chloroform, ethanol, ethyl acetate, petroleum ether. All
the rare earth picrate complexes are soluble in DMSO, DMF and
THF easily, and can be soluble in acetonitrile, methanol, ethanol,
acetone and ethyl acetate, but slightly soluble in chloroform,
petroleum ether.
−
approximately 180 cm−1, establishing that the NO3 groups
in the solid complexes coordinate to rare earth ions as biden-
tate ligands [25]. No bands at 1380, 820 and 720 cm−1 in the
spectra of complexes indicate that free nitrate groups (D3h) are
absent.
In the rare earth picrate complexes, after the formation of
the complex, the characteristic frequency of the free ligand υ
(C O) at 1641 cm−1 shifts ca. 26 cm−1 towards lower wave
numbers and the υ (C O C) absorption band also remains
unchanged, indicating that only the oxygen atom of C O coordi-
nate to rare earth ions. The OH out-of-plane bending vibration
of the free HPic at 1151 cm−1 disappears, indicating that the
H-atom of the OH group is replaced by RE(III). The vibration
υ (C O) of Pic− at 1265 cm−1 is shifted toward higher fre-
quency by ca. 10 cm−1 in the complexes. This is due to the
following two effects. First, the hydrogen atom of the OH group
is replaced by RE(III), increasing the л-bond character in the
C O bond. Secondly, coordination of the oxygen atom of L
to RE(III) decreases the л-character. The free HPic has υas
( NO2) and υs ( NO2) at 1555 and 1342 cm−1, respectively,
which split into two bands at ca. 1577, 1542 cm−1 and ca. 1363,
3.2. UV–vis spectra
UV–vis spectra of the ligand and the complexes are seen in
Fig. 1. In THF, two absorption peaks of the free ligand appear at
241 and 291 nm (ε 30640 and 15600 dm3 mol−1 cm−1, respec-
tively), which are assigned to the л–л* and n–л* transition of
the ligand. UV–vis spectra of 2Eu(NO3)3·3L·4H2O is very sim-
ilar to that of the free ligand L. The only difference is that the
absorption intensity of the complex is increased. The first strong
absorption wavelength of Eu(Pic)3·L·2H2O is same to those of
the ligand and 2Eu(NO3)3·3L·4H2O, but the second is shifted to
322 nm, which is assigned to the n–л* transition of the aromatic
and NO2 groups of rare earth picrates.

H. Cui et al. / Spectrochimica Acta Part A 68 (2007) 478–483
481
Table 4
Fluorescence and luminescence data for the ligand and Eu and Tb complexes
Compounds
λex (nm)
Fluorescence data
Luminescence data
λem (nm)
RFIa
Assignment
τ (ms)
0.439
1.305
λem (nm)
RLIb
Assignment
L
320
396
440
591
618
171
9
39
440
579
591
617
. . .c
163
765
7
7
2Eu(NO3)3·3L·4H2O
5D0 → F1
5D0 → F0
7
7
5D0 → F2
5D0 → F1
5D0 → F2
7
3295
7
7
2Tb(NO3)3·3L·4H2O
320
460
490
543
585
620
978
2179
60
5D4 → F6
490
543
585
620
. . .
. . .
5D4 → F6
7
7
5D4 → F5
5D4 → F5
5D4 → F4
3551
2240
5D4 → F4
7
7
7
7
39
5D4 → F3
5D4 → F3
7
7
Eu(Pic)3·L·2H2O
578
591
616
16
4
36
5D0 → F0
578
591
616
1599
447
3680
5D0 → F0
7
7
5D0 → F1
5D0 → F1
7
7
5D0 → F2
0.093
5D0 → F2
a
RFI, relative fluorescence intensity.
RLI, relative luminescence intensity.
. . ., The numeral of relative intensity is larger than 10,000.
b
c
1334 cm−1, respectively, in the complexes. This indicates that
some O-atoms in the nitro group of Pic− take part in coordi-
nation [26]. Additionally, broad bands at 3340 cm−1 indicate
that H2O is existent in the complexes, confirming the elemental
analysis.
3.4. Fluorescence properties of the complexes
The fluorescence spectra of the ligand and its Eu3+ and Tb3+
complexes in solid state were recorded at room temperature.
Except for Tb picrate, fluorescence of the other three complexes
Fig. 2. (A) Fluorescence emission spectrum of L; (B) fluorescence emission spectrum of 2Eu(NO3)3·3L·4H2O; (C) fluorescence emission spectrum of
2Tb(NO3)3·3L·4H2O; (D) fluorescence emission spectrum of Eu(Pic)3·L·2H2O; (E) luminescence emission spectrum of 2Eu(NO3)3·3L·4H2O; (F) luminescence
emission spectrum of Eu(Pic)3·L·2H2O.

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H. Cui et al. / Spectrochimica Acta Part A 68 (2007) 478–483
was observed. Under identical experimental conditions, the fluo-
rescence characteristic emissions of the complexes in solid state
are listed in Table 4. The emission spectra of the ligand and
complexes are shown in Fig. 2.
In the excitation spectrum, the ligand L has a wide absorp-
tion band from 230 to 350 nm (λmax = 320 nm). Excited by the
absorption band at 320 nm, the free ligand exhibits broad emis-
sion bands (λmax = 440 nm). The efficient energy transfer from
ligand to centre ions (antenna effect) is one of key factors to
achieve rare earth characteristic fluorescence. It is shown in
Fig. 2 that the complexes show the characteristic emissions of
Eu3+ or Tb3+. This indicates that the ligand L is a good organic
chelator to absorb and transfer energy to metal ions.
oformyl)phenoxyl]hexane (L) can form stable solid complexes
with rare earth nitrate and picrates. After the formation of the
complexes obvious changes in IR spectra were observed and
rare earth ions were coordinated to the C O oxygen atom of
the ligand. It can be seen from the fluorescence spectra of the
nitrates complexes that the triplet state energy level of the ligand
L is more suitable to the lowest excited state level of Tb than that
of Eu. The relative fluorescence intensity of Tb(NO3)3 complex
is very strong and it is easy to be prepared, so it may be used
for EL devices. The counter anion of the complexes is essential
in determining the fluorescent properties of the rare earth com-
plexes. Under the same conditions, the luminescence emission
of the complexes is much more intense than the fluorescence
emission of the complexes.
It can be seen from Fig. 2, that the complexes of Eu, Tb
nitrates and Eu picrate show no ligand-based emission bands in
ca. 440 nm, which means the energy transfer in these complexes
is efficient. It is also shown in Fig. 2, the relative fluorescence
intensity of Tb(NO3)3 complex is much stronger than that of Eu
complex. Intramolecular energy transfer from the triplet state of
the ligand to the resonance level of the RE(III) ion is one of the
most important processes influencing the luminescence quan-
tum yields of RE(III) complexes [27]. The energy difference
between the triplet state energy level of the ligand and the low-
est excited state level of RE(III) cannot be too large or too small.
So considering the emission spectra of these three complexes, it
can be concluded that the triplet energy of L is in an appropriate
level to Tb3+ in the Tb(NO3)3 complex, which makes the energy
transition from the ligand to Tb(III) more easily. Additionally,
in the spectrum of Eu(NO3)3 complex, the relative intensity of
Acknowledgments
The authors are grateful to the Innovative Group Founda-
tion from NSFC (Grant No.50421502) and the National Natural
Science Foundation of China (Grant No. 50575217).
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