Synthesis and infrared and fluorescent properties of rare earth complexes with a new aryl amide podand

Article abstract of DOI:10.1016/j.saa.2005.11.013

Solid complexes of rare earth nitrates and picrates with a new aryl amide podand, 1,4-bis{[(2′-benzylamino-formyl)phenoxyl]methyl}-naphthaline (L) were synthesized. The complexes were characterized by elemental analysis, IR and molar conductivity measurem

Full text of DOI:10.1016/j.saa.2005.11.013

Spectrochimica Acta Part A 65 (2006) 372–377
Synthesis and infrared and fluorescent properties
of rare earth complexes with a new
aryl amide podand
Yu Zhao, Yu Tang∗, Wei-Sheng Liu, Ning Tang, Min-Yu Tan∗
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
Received 2 September 2005; received in revised form 7 November 2005; accepted 9 November 2005
Abstract
Solid complexes of rare earth nitrates and picrates with a new aryl amide podand, 1,4-bis{[(2ꢀ-benzylamino-formyl)phenoxyl]methyl}-
naphthaline (L) were synthesized. The complexes were characterized by elemental analysis, IR and molar conductivity measurements. At the
same time, the fluorescent properties of the Eu(III) and Tb(III) nitrates and picrates complexes in solid state were also investigated. Under the
excitation, these complexes exhibited characteristic emissions of europium and terbium ions. The counter anion factor influencing the fluorescent
intensity was also discussed.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Aryl amide podand; Rare earth complexes; Fluorescent properties
1. Introduction
should be able (1) to encapsulate and protect the rare earth
ion from the solvent molecules and (2) to absorb energy and
transfer it efficiently to the central metal [4]. Podand-type lig-
ands [6], macrocyclic compounds [7], carboxylic acid deriva-
tives [8] and pyrazolone derivatives [9] have been extensively
used for these purposes. Among these ligands, podand-type lig-
ands have drawn much attention in recent years, mainly due
to their possessing spheroidal cavities and hard binding sites,
therefore stabilizing their complexes and shielding the encapsu-
lated ion from interaction with the surroundings [10]. Among
numerous podands which have demonstrated their potential
use in functional supramolecular chemistry [11], amide type
podands are important for preparing the rare earth complexes
possessing strong fluorescent properties. Our group is inter-
ested in the coordination chemistry and the fluorescent prop-
erties of rare earth(III) ions with the amide type podands.
As a part of our systematic studies, this paper reports the
preparation and fluorescent properties of the rare earth com-
plexes with a new aryl amide podand 1,4-bis{[(2ꢀ-benzylamino-
formyl)phenoxyl]methyl}-naphthaline (L, Scheme 1). The flu-
orescent properties of the solid complexes of europium(III) and
terbium(III) ions with this new ligand were studied in detail.
And the results indicated that counter anion notably affected
the fluorescence characteristics of europium and terbium
ions.
The specific spectroscopic and magnetic properties of rare
earth ions have made them one of the essential components
in the preparation of new materials and ideal as probes in
studies of biological systems [1]. Among these studies, much
attention has been paid to the fluorescent rare earth complexes
that are found use as labels and sensors for natural and med-
ical science [2], mainly due to their large Stokes shifts, nar-
row emission profiles, etc. [3]. More and more chemists are
attracted to design the organized molecular architectures con-
taining trivalent rare earth ions (Eu3+, Tb3+) working as efficient
light conversion devices [4]. The emission of light from rare
earth ions arises from f–f transitions, which result in emission
bands with extremely narrow bandwidth and no theoretical cap
on the quantum efficiency. Since the f–f transitions are spin-
and parity-forbidden, the excited state of the rare earth ion is
populated through intramolecular energy transfer from the lig-
and or ligands, which therefore serve as sensitizers (antenna
effect) [5]. In order to obtain strongly fluorescent complexes,
the chromophoric ligands which chelate to rare earth metals
∗
Corresponding authors. Tel.: +86 9318912552; fax: +86 9318912582.
E-mail address: tangyu@1zu.edu.cn (Y. Tang).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.11.013

Y. Zhao et al. / Spectrochimica Acta Part A 65 (2006) 372–377
373
Scheme 1. The synthetic route for the ligand L.
2. Experimental
spectrumwasmeasuredonaVarianMercury-300Bspectrometer
in CDCl3 solution with TMS as internal standard. Fluorescence
and phosphorescence spectra were obtained on a Hitachi F-4500
spectrophotometer.
2.1. Materials
N-Benzylsalicylamide [12], rare earth nitrates [13] and
picrates [14] were prepared according to the literature meth-
ods. Other chemicals were obtained from commercial sources
and used without further purification.
2.3. Synthesis of the ligand L
The synthetic route for the ligand L is shown in
Scheme 1. N-Benzylsalicylamide (5.0 mmol), potassium car-
bonate (9.0 mmol) and DMF (100 cm3) were warmed to ca.
90 ◦C and 1,4-bischloromethyl-naphthaline (2.0 mmol) was
added. The reaction mixture was stirred at 90–95 ◦C for
16 h. After cooling down, the mixture was poured into water
(100 cm3) and extracted by CHCl3. The CHCl3 layer was evap-
orated to dryness to give the crude product, which was recrys-
tallized in acetone to afford pure ligand L as a white solid, yield
2.2. Methods
Carbon, nitrogen and hydrogen were determined using an
Elementar Vario EL (see Table 1). Conductivity measurements
werecarriedoutwithaDDSJ-308typeconductivitybridgeusing
10−3 mol cm−3 solutions in DMF at 25 ◦C. The IR spectra were
recorded in the 4000–400 cm−1 region using KBr pellets and
1
1
Nicolet 360 FT-IR instrument (see Tables 2 and 3). H NMR
72%, mp 191–193 ◦C; H NMR (CDCl3, 300 MHz): δ: 4.25
Table 1
Analytical and molar conductance data of the complexes (calculated values in parentheses)
Complexes
Analysis (%)
C
Λm (cm2 ꢀ−1 mol−1
)
H
N
[La(NO3)3L]·H2O
[Eu(NO3)3L]·H2O
[Gd(NO3)3L]·H2O
[Tb(NO3)3L]·H2O
[Y(NO3)3L]·H2O
50.79 (50.59)
50.23 (49.90)
49.32 (49.63)
49.13 (49.55)
53.44 (53.40)
4.36 (3.82)
3.86 (3.77)
3.65 (3.75)
3.55 (3.74)
4.44 (4.03)
6.96 (7.37)
7.39 (7.27)
6.99 (7.23)
7.65 (7.22)
7.39 (7.78)
62.8
58.7
55.4
60.5
59.8
[La(Pic)3L]·(H2O)3
[Eu(Pic)3L]·(H2O)3
[Gd(Pic)3L]·(H2O)3
[Tb(Pic)3L]·(H2O)3
[Y(Pic)3L]·(H2O)3
46.58 (46.94)
46.27 (46.54)
46.57 (46.37)
46.03 (46.32)
48.65 (48.58)
3.57 (3.12)
3.50 (3.10)
3.43 (3.09)
3.04 (3.08)
3.47 (3.23)
10.25 (10.38)
10.37 (10.29)
9.97 (10.26)
9.93 (10.24)
10.98 (10.74)
50.3
64.5
56.7
61.5
60.8
Table 2
The most important IR bands of the rare earth nitrate complexes (cm−1
)
−
ν(NO3 )
Compounds
ν(C O)
ν(C
O
C)
ν1
ν4
ν2
ν3
|ν1–ν4|
L
1642
1609
1610
1605
1608
1607
1107
1108
1108
1107
1107
1108
[La(NO3)3L]·H2O
[Eu(NO3)3L]·H2O
[Gd(NO3)3L]·H2O
[Tb(NO3)3L]·H2O
[Y(NO3)3L]·H2O
1486
1489
1486
1490
1488
1300
1297
1295
1298
1298
1030
1029
1029
1028
1027
816
813
814
815
813
186
192
191
192
190

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Y. Zhao et al. / Spectrochimica Acta Part A 65 (2006) 372–377
Table 3
The most important IR bands of the rare earth picrate complexes (cm−1
)
Compounds
ν(C O)
ν(C
O
C)
ν(C O)
νas(–NO2)
νs(–NO2)
ν(O H)
HPic
1265
1555
1342
L
1642
1615
1107
1108
[La(Pic)3L]·(H2O)3
1272
1273
1272
1273
1273
1577
1542
1363
1326
3340
3342
3341
3340
3342
[Eu(Pic)3L]·(H2O)3
[Gd(Pic)3L]·(H2O)3
[Tb(Pic)3L]·(H2O)3
[Y(Pic)3L]·(H2O)3
1616
1615
1615
1613
1108
1108
1109
1108
1573
1542
1363
1333
1573
1542
1363
1330
1574
1542
1363
1326
1574
1542
1366
1330
(d, 4H, –CH2–N), 5.56 (s, 4H, –CH2–O), 6.96–7.96 (m, 24H,
Ar–H), 8.30 (s, 2H, N–H); IR (KBr): 3398 (m), 3320 (m), 1642
(s, C O), 1600 (m), 1534 (m), 1486 (m), 1451 (m), 1299 (m),
1229 (m, Ar–O), 1107 (m, C O C), 807 (w), 755 (s), 700 (m).
the complexes in DMF (see Table 1) indicate that all complexes
also act as noneletrolytes [15].
3.2. IR spectra
2.4. Preparation of the complexes
The main infrared bands of the ligand and their complexes
are presented in Tables 2 and 3. The IR spectra of the complexes
are similar to each other.
An ethyl acetate solution (5 cm3) of RE(NO3)3·6H2O
(RE = La, Eu, Gd, Tb, Y) (0.1 mmol) was added dropwise to
a solution of 0.1 mmol ligand L in the chloroform (5 cm3).
The mixture was stirred at room temperature for 4 h. And then
the precipitated solid complex was filtered, washed with ethyl
acetate and chloroform, dried in vacuo over P4O10 for 48 h and
submitted for elemental analysis, yield: 70%.
The IR spectrum of the free ligand shows strong band at 1642
and 1107 cm−1, which are attributable to the stretch vibration
of the carbonyl group [ν(C O)] of amide group and ν(C O C),
respectively. In the nitrate complexes, the low-energy band
remains unchanged, but the high-energy band red shifts to about
1608 cm−1 (ꢁν = 34 cm−1) 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 rare earth ion.
An ethyl acetate solution (5 cm3) of RE(Pic)3·6H2O
(RE = La, Eu, Gd, Tb, Y) (0.1 mmol) was added dropwise to
a solution of 0.1 mmol ligand L in the chloroform (5 cm3).
The mixture was stirred at room temperature for 4 h. And then
the precipitated solid complex was filtered, washed with ethyl
acetate and chloroform, dried in vacuo over P4O10 for 48 h and
submitted for elemental analysis, yield: 70%.
The absorption bands assigned to the coordinated nitrate
groups (C2v) are observed at about 1490 cm−1 (ν1), 1298 cm−1
(ν4), 1028 cm−1 (ν2) and 815 cm−1 (ν3) [16] for the nitrate com-
plexes. In addition, the separation of two strongest frequency
bands |ν1–ν4| is approximately 190 cm−1, clearly establishing
−
that the NO3 groups in the solid complexes coordinate to the
3. Results and discussion
rare earth ion as bidentate ligands [17]. And no bands at 1380,
820 and 720 cm−1 in the spectra of complexes indicates that free
nitrate groups (D3h) are absent, in agreement with the results
of the conductivity experiments. Furthermore, broad bands at
3340 cm−1 could be attributed to the O H stretching vibrations
of H2O molecule.
In the picrate complexes, after forming the complex, the char-
acteristic frequency of the free ligand ν(C O) at 1642 cm−1
shifts ca. 28 cm−1 towards lower wave numbers and the
ν(C O C) absorption band also remains unchanged, indicat-
ing that only the oxygen atom of C O take part in coordination
to the metal ion.
The OH out-of-plane bending vibration of the free HPic at
1151 cm−1 disappearsinthespectraofthecomplexes, indicating
that the H-atom of the OH group is replaced by RE(III). The
vibration ν(C O) of Pic− at 1265 cm−1 is shifted toward higher
frequency by ca. 8 cm−1 in the complexes. This can be explained
3.1. Properties of the complexes
Analytical data for the newly synthesized complexes, listed
in Table 1, indicate that the 10 complexes of rare earth nitrates
and picrates all conform to a 1:1 metal-to-ligand stoichiometries
[RE(NO3)3L]·H2O and [RE(Pic)3L]·(H2O)3.
All the rare earth nitrates complexes are soluble in DMF,
DMSO and THF, but slightly soluble in methanol, ethanol,
acetone and ethyl acetate. The molar conductances of the com-
plexes in DMF (see Table 1) indicate that all complexes act as
noneletrolytes [15], implying that all nitrate groups are in coor-
dination sphere.
All the rare earth picrates complexes are soluble in DMF,
DMSO and acetonitrile, but slightly soluble in methanol,
ethanol, ethyl acetate and acetone. The molar conductances of

Y. Zhao et al. / Spectrochimica Acta Part A 65 (2006) 372–377
375
Table 4
by the following effects. First, the hydrogen atom of the OH
Fluorescence data for the ligand and Eu and Tb complexes
group is replaced by RE(III), increasing the ␲-bond character
in the C O bond. Secondly, coordination of the oxygen atom
of L to RE(III) causes the ␲-character to be weakened. The
Compounds
λex (nm)
λem (nm)
RFIa
Assignment
L
320
343
452
2119
1367
free HPic has νas(–NO2) and νs(–NO2) at 1555 and 1342 cm−1
,
respectively, which split into two bands at ca. 1577, 1542 cm−1
and ca. 1360, 1330 cm−1. This indicates that some of the nitryl O
atoms take part in coordination [18]. Additionally, broad bands
at 3340 cm−1 indicates that H2O is existent in the complexes,
confirming the elemental analysis.
5D0 → F0
7
[Eu(NO3)3L]·H2O
[Tb(NO3)3L]·H2O
397
308
466
580
593
616
119
532
913
5D0 → F1
7
5D0 → F2
7
5D4 → F6
7
492
545
583
3973
8705
462
5D4 → F5
7
7
5D4 → F4
[Eu(Pic)3L]·(H2O)3
580
591
615
88
135
545
5D0 → F0
7
3.3. Fluorescent properties of the complexes
7
5D0 → F1
7
5D0 → F2
The fluorescence characteristics of the ligand, europium and
terbium complexes in solid state were measured at room temper-
ature. Among these four complexes, fluorescence of the com-
plexes of Eu, Tb nitrates and Eu picrate were observed but not
for the Tb picrate complex. Under identical experimental condi-
tions, the fluorescence 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. 1.
a
RFI: relative fluorescence intensity.
Excited by the absorption band at 320 nm, the “free” ligand
exhibits broad emission bands (λmax = 343 and 452 nm) in solid
state. The efficient energy transfer from ligand to centre ions
(antenna effect) is one of key factors to achieve rare earth char-
acteristic fluorescence. It is shown in Fig. 1 that the complexes
show the characteristic emissions of Eu3+ or Tb3+. This indi-
The ligand has multiple aromatic rings with a rigid pla-
nar skeleton structure, so it is a strong fluorescence substance.
Fig. 1. (a) Emission spectrum of L in solid state; (b) emission spectrum of [Eu(NO3)3L]·H2O in solid state; (c) emission spectrum of [Tb(NO3)3L]·H2O in solid
state; (d) emission spectrum of [Eu(Pic)3L]·(H2O)3 in solid state.

376
Y. Zhao et al. / Spectrochimica Acta Part A 65 (2006) 372–377
cates that the ligand L is a good organic chelator to absorb and
transfer energy to metal ions.
earth nitrates and picrates. When the ligand formed the rare earth
complexes, obvious changes in IR spectra were observed. In the
complexes, rare earth ions were coordinated to the C O oxygen
atoms of the ligand L. The fluorescent properties of the Eu and
Tb complexes in solid state were investigated. Under the exci-
tation, the complexes exhibited characteristic fluorescence of
europium and terbium ions. And the lowest triplet state energy
levels of the ligand in the nitrate and picrate complexes indi-
cate that the counter anion of the complexes is very essential
in determining the fluorescent properties of the rare earth com-
plexesbyinfluencingtheelectrostaticfactorsintheligand–metal
bonding.
In the spectra of Eu complexes, the relative intensity of
5D0 → F2 is more intense than that of 5D0 → F1, showing that
the Eu(III) ion does not lie in a centro-symmetric coordination
site [19]. Additionally, the Eu(NO3)3 and Eu(Pic)3 complexes
show ligand-based emission bands, but this emission band in the
Tb(NO3)3 complex is almost invisible, which means the energy
transfer in the Tb(NO3)3 complex is more efficient than that in
the Eu(NO3)3 and Eu(Pic)3 complexes. 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 influ-
encing the fluorescence quantum yields of RE(III) complexes
[20]. The energy difference between the triplet state energy level
of the ligand and the lowest excited state level of RE(III) can-
not be too large or too small. A triplet excited state T1 which is
localized on one ligand only and is independent of the rare earth
nature [20].
7
7
Acknowledgements
We are grateful to the National Natural Science Foundation
of China (Project 20401008) and the Research Foundation for
the Young Teachers Possessing Doctor’s Degree of Lanzhou
University.
In order to acquire the triplet excited state T1 of the ligand L,
the phosphorescence spectra of the Gd(III) nitrate and picrate
complexes were measured at 77 K in a methanol–ethanol mix-
ture (v:v, 1:1). The triplet state energy levels T1 of the ligand
L in the nitrate and picrate complexes, which were calculated
from the shortest wavelength phosphorescence bands [21] of the
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