Synthesis and spectroscopic properties of rare earth picrate complexes with a new biphenylamide

Article abstract of DOI:10.1016/j.saa.2006.08.019

The new ligand N-benzyl-2-{2′-[(benzyl-ethyl-carbamoyl)-methoxy]-biphenyl-2-yloxy}-N-ethyl-acetamide (L) and its complexes of rare earth picrates were synthesized. The complexes were characterized by elemental analysis, IR, UV-vis spectra and conductivity

Full text of DOI:10.1016/j.saa.2006.08.019

Spectrochimica Acta Part A 67 (2007) 624–627
Synthesis and spectroscopic properties of rare earth picrate
complexes with a new biphenylamide
Yan-Ling Guo, Ya-Wen Wang, Wei-Sheng Liu∗, Wei Dou, Xia Zhong
Department of Chemistry and National Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000, China
Received 9 June 2006; accepted 15 August 2006
Abstract
The new ligand N-benzyl-2-{2ꢀ-[(benzyl-ethyl-carbamoyl)-methoxy]-biphenyl-2-yloxy}-N-ethyl-acetamide (L) and its complexes of rare earth
picrates were synthesized. The complexes were characterized by elemental analysis, IR, UV–vis spectra and conductivity measurements. The
fluorescence properties of the europium complex in solid state and in CHCl3, ethyl acetate, acetone, acetonitrile and DMF were investigated. Under
the excitation, the europium complex exhibited characteristic emissions of europium. The lowest triplet state energy level of the ligand indicates
that the triplet state energy level of the ligand matches better to the resonance level of Eu(III) than Tb(III) ion.
© 2007 Published by Elsevier B.V.
Keywords: Biphenylamide; Rare earth picrate complexes; Spectroscopic properties
1. Introduction
In this work, we introduced biphenyl groups as the
basic molecular frame and obtained a new open-chain
crown ether ligand, N-benzyl-2-{2ꢀ-[(benzyl-ethyl-carbamoyl)-
methoxy]-biphenyl-2-yloxy}-N-ethyl-acetamide (L). The fluo-
rescent properties of the complex of europium ions with this
new ligand were studied in detail. The results indicated that
the organic solvent affected the fluorescence characteristics of
europium ions. And the lowest triplet state energy level of the
ligand which was calculated from the phosphorescence spec-
trum of the Gd(III) at 77 K indicates that the triplet state energy
level of the ligand matches better to the resonance level of Eu(III)
than Tb(III) ion.
Luminescent lanthanide complexes [1–4] are used as labels
and sensors for natural and medical science [5,6] mainly due
to their large Stokes shifts, narrow emission profiles, and
long fluorescence lifetimes [7]. They have attracted more and
more chemists to design the organized molecular architectures
containing trivalent lanthanide ions (Eu3+, Tb3+) working as effi-
cientlightconversiondevices[1,8]. Soitisessentialtodesignthe
multifunctional ligand to optimize the luminescent properties of
these lanthanide ions by facilitating the well-known light con-
version process, the antenna effect [9], and forming highly and
strongly luminescent europium and terbium ion complexes [10].
Amide-based crown ethers offer many advantages in extrac-
tion and analysis of the rare earth ions because of their ring-like
coordination structure and terminal group effects [11,12]. How-
ever, luminescent properties on open-chain crown ethers with
lanthanide complexes have been rarely reported [13]. Due to
their low luminescence, recently, we have designed a series of
polyfunctional ligands having both selective ability to coordi-
nate lanthanide ions and enhanced luminescence of lanthanide
complexes, by providing proper conjugate absorption groups
suitable for energy transfer.
2. Experimental
2.1. Materials
The
lanthanide
picrates
[14]
and
N-ethyl-N-
benzylchloroacetamide [15] were prepared according to
literature method. All commercially available chemicals were
of A.R. grade and all solvents used were purified by standard
methods.
2.2. Methods
∗
The metal ions were determined by EDTA titration using
xylenal orange as indicator. C, H and N were determined
Corresponding author. Tel.: +86 931 8912552; fax: +86 931 8912582.
E-mail address: liuws@lzu.edu.cn (W.-S. Liu).
1386-1425/$ – see front matter © 2007 Published by Elsevier B.V.
doi:10.1016/j.saa.2006.08.019

Y.-L. Guo et al. / Spectrochimica Acta Part A 67 (2007) 624–627
625
Table 1
Analytical and molar conductance data for the complexes (calculated values in parentheses)
Complex
Analysis (%)
C
∧m (cm2 ꢀ−1 mol−1
)
H
N
Ln
La(pic)3L
Nd(pic)3L
Eu(pic)3L
Gd(pic)3L
Tb(pic)3L
Y(pic)3L
45.77 (45.92)
45.57 (45.75)
45.21 (45.48)
45.17 (45.32)
45.01 (45.25)
47.59 (47.67)
3.32 (3.09)
3.22 (3.08)
3.21 (3.06)
3.14 (3.05)
3.34 (3.05)
3.24 (3.20)
10.89 (11.33)
10.91 (11.29)
10.99 (11.22)
10.86 (11.18)
10.79 (11.17)
11.61 (11.76)
10.01 (10.23)
10.38 (10.56)
10.84 (11.08)
11.17 (11.40)
11.33 (11.53)
6.65 (6.80)
42.7
37.0
37.8
48.8
34.6
42.9
using an Elementar Vario EL. Conductivity measurements were
carried out with a DDS-307 type conductivity bridge using
10−3 mol dm−3 solutions in acetone at 25 ◦C. IR spectra were
recorded on Nicolet FT-170SX instrument using KBr discs in
the 400–4000 cm−1 region, 1H NMR spectra were measured on
a Varian Mercury plus 300BB spectrometer in CDCl3 solution
with TMS as internal standard. Electronic spectra were recorded
with a Varian Cary 100 spectrophotometer in chloroform solu-
tion. Fluorescence and phosphorescence measurements were
made on a Hitachi F-4500 spectrophotometer.
of ethanol. The mixture was stirred at room temperature for 5 h.
The precipitated solid complex was filtered, washed with ethanol
and dried in vacuo over P2O5 for 48 h. All the complexes were
obtained as yellow powders, yield: 80–85%.
3. Results and discussion
3.1. Properties of the discussion
Analytical data for the complexes listed in Table 1, indicate
that the complexes conform to a 1:3:1 metal-to-picrate-to-L
stoichiometry Ln(pic)3L. All complexes are soluble in DMF,
DMSO, acetonitrile, acetone, ethyl acetate and CHCl3, but
slightly soluble in ethanol. The molar conductances of the com-
plexes in acetone (see Table 1) indicate that all complexes act as
nonelectrolytes [16], implying that all the picrate groups are in
coordination sphere.
2.3. Synthesis of the ligand
The synthetic route for the ligand is shown in Scheme 1.
Anhydrous K2CO3 (5.6 g, 41 mmol) was added into the
15 mL DMF solution of 2,2ꢀ-dihydroxybiphenyl (1.86 g,
10 mmol) at 100 ◦C. After 1 h, a solution of N-ethyl-N-
benzylchloroacetamide (6.35 g, 30 mmol) in 10 mL DMF was
added dropwise to the mixture and maintained at 110 ◦C
for 7 h. When cooled, 60 mL distilled water was poured and
the turbid solution was extracted by 40 mL chloroform three
times. Organic phase combined was washed with water and
dried with anhydrous Na2SO4. Solvent removed, the residue
was chromatographed to afford the ligand L; yield: 85%.
1H NMR (CDCl3, 300 MHz) δ: 7.34–7.23(m, 9H, ArH and
C12H8–), 7.17–6.93(m, 9H, ArH), 4.62(m, 4H, –O–CH2–CO–),
4.51(d, 2H, –N–CH2–), 4.35(d, 2H, –N–CH2–), 3.29(q, 2H,
–CH2–CH3), 3.13(q, 2H, –CH2–CH3), 1.14 (t, 3H,–CH3),
0.87(t, 3H,–CH3).
3.2. IR spectra
The main infrared bands of the ligand and its complexes are
presented in Table 2. The IR spectra of the complexes are sim-
ilar. The “free” ligand L exhibit two absorption bands at 1656
and 1119 cm−1 which are assigned to v(C O) and v(C–O–C),
respectively. In the complexes, the bonds shift by ca. 42 and
37 cm−1 towards lower wave numbers, thus indicating that the
C O and ether O-atoms take part in coordination to the metal
ions. The larger shifts v(C O) in the spectra of the complexes
suggest that the Ln–O (carbonyl) bond is stronger than the Ln–O
(ether) bond. The OH out-of-plane bending vibration of the
free Hpic at 1151 cm−1 disappears in the spectra of the com-
plexes indicating that the H atom of the OH group is replaced by
Ln(III). The vibration ν(C–O) at 1265 cm−1 is shifted towards
2.4. Synthesis of the complexes
To a solution of 0.2 mmol lanthanide picrate in 5 mL of
ethanol was added dropwise the solution of 0.2 mmol L in 10 mL
Scheme 1. The synthetic route for the ligand L.

626
Y.-L. Guo et al. / Spectrochimica Acta Part A 67 (2007) 624–627
Table 2
The most important IR bands (cm−1
)
Complex
ν(C = O)
ν(C–O–C)
ν(C–O)
νas(–NO2)
νs(–NO2)
Hpic
L
–
–
1265
–
1555
–
1342
–
1656
1614
1614
1614
1615
1615
1616
1119
1082
1081
1082
1082
1080
1082
La(pic)3L
Nd(pic)3L
Eu(pic)3L
Gd(pic)3L
Tb(pic)3L
Y(pic)3L
1273
1273
1274
1274
1271
1274
1576, 1543
1577, 1542
1578, 1542
1578, 1544
1577, 1544
1578, 1543
1361, 1329
1360, 1330
1360, 1332
1360, 1331
1361, 1332
1360, 1331
higher frequency by ca. 8 cm−1 in the complexes. This is due to
the following two effects. First, the hydrogen atom of the OH
group is replaced by Ln(III), increasing the ␲-bond character
in the C–O bond. Secondly, coordination of the oxygen atom
of L to Ln(III) causes the ␲-character to be weakened. The free
Hpic has νas(NO2) and νs(NO2) at 1555 and 1342 cm−1, respec-
tively, which splits into two bands at 1577, 1543 cm−1 and 1360,
1331 cm−1 in the complexes. This indicates that some of the O-
atoms in the nitro group of Pic− take part in coordination [17].
On the basis of the similarity of their IR spectra, it may be
assumed that the complexes have the similar structures.
3.3. Electronic spectra
Fig. 1. The emission spectrum of the europium complex in solid state.
The electronic spectra in the visible region of the Ln(III) com-
plexes exhibit alternations in intensity and shifts in position of
the absorption bands relative to the corresponding Ln(III) aquo
ions. The shift has been attributed by Jorgensen to the effect on
the crystal field of interelectronic repulsion between the 4f elec-
trons, and is related to the covalent character of the metal–ligand
bond, assessedbySinha’sparameter(δ), thenephelauxeticration
(β) and the bonding parameter (b1/2) [18]. Absorption spectra
of the Nd(III) complex was registered in chloroform solution at
room temperature and the covalent parameters were calculated
(Table 3). The values of β, which are less than unity, and posi-
tive values of δ and b1/2 support the existence of partial covalent
bonding between metal and ligand [19].
state (the excitation and emission slit widths were 2.5 nm, Fig. 1)
and in solution (the excitation and emission slit widths were
10.0 nm, Fig. 2) were recorded at room temperature. The fluo-
rescence characteristics of the europium complex in solid state
andinCHCl3, ethylacetate, acetone, acetonitrile, andDMFsolu-
tions (concentration: 1.0 × 10−3 mol L−1) are listed in Table 4.
It can be seen that the Eu complex shows strong emission when
excited with 451 nm in the solid state. This indicates that the lig-
and L is a comparative good organic chelator to absorb energy
and transfer them to Eu ion. The most intensity ratio value η
7
7
(5D0 → F2/5D0 → F1) is 7.8, showing that the Eu(III) ion does
not lie in a centro-symmetric coordination site [20]. It also could
be seen that in DMF solution, the fluorescence of europium was
quenched which is contributed to the decomposition of the com-
3.4. Fluorescence studies
Monitored by the emission band at 600 nm, the europium
complex exhibits broad excitation bands around 450 nm. The
luminescence emission spectra of the europium complex in solid
Table 3
Covalent parameters for the neodymium complex
Complex
Frequency (cm−1
)
Assignment
Covalent parameter
Nd(pic)3L
11,160
11,441
12,469
13,387
13,514
17,182
17,391
19,011
4I9/2–4F3/2
β = 0.9986
δ = 0.1402
b
1/2 = 0.02646
4F3/2
4F5/2
4F7/2
4S3/2
4G5/2
2G7/2
4G7/2
Fig. 2. The emission spectrum of the europium complex in different solutions
at room temperature in: (1) CHCl3, (2) ethyl acetate, (3) acetone and (4) ace-
tonitrile.

Y.-L. Guo et al. / Spectrochimica Acta Part A 67 (2007) 624–627
627
Table 4
IR spectrum, the lanthanide ions were coordinated to the C O
oxygen atoms and C–O–C oxygen atoms. The europium com-
plex exhibited characteristic fluorescence of europium ion. The
different solvents may affect the fluorescence of europium ions.
The lowest triplet state energy level of the ligand indicates that
the triplet state energy level of the ligand matches better to
the resonance level of Eu(III) than Tb(III) ion. Based on those
results, a series of new ligands could be designed and synthe-
sized to optimize the luminescent properties of these lanthanide
ions.
Fluorescence data for the europium complex
Complex
Solvent
λex (nm)
λem (nm)
RFI
967
Assignment
7
Eu(pic)3L
Solid state
451
592
616
5D0 → F1
7
7524
5D0 → F2
7
CHCl3
461
461
461
461
–
592
615
781
5596
5D0 → F1
7
5D0 → F2
7
AcOEt
592
615
413
2360
5D0 → F1
7
5D0 → F2
7
Acetone
Acetonitrile
DMF
592
615
240
1537
5D0 → F1
7
5D0 → F2
Acknowledgements
7
592
615
131
611
5D0 → F1
7
5D0 → F2
We are grateful to the NSFC (Grants 20371022, 20431010
and 20021001), the Specialized Research Fund for the Doctoral
Program of Higher Education (200307300015), and the Key
Project of the Ministry of Education of China (Grant 01170)
for financial support.
–
–
–
plex in this solvent, and in the other four solvents, the complex
has the similar excitation and emission wavelengths.
In CHCl3 solution the Eu complex has the strongest lumi-
nescence, and then in ethyl acetate, acetone, acetonitrile. This
is due to the coordinating effects of solvents, namely solvate
effect [21]. Together with the raising coordination abilities of
CHCl3, ethyl acetate, acetone, acetonitrile for the lanthanide
ions, the oscillatory motions of the entering molecules consume
more energy which the ligand triplet level transfer to the emit-
ting level of the lanthanide ion. Thus, the energy transfer could
not be carried out perfectly.
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5
energy level is above the lowest excited resonance level D0
5
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5
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4. Conclusion
According to the data and discussion above, the new lig-
and can form stable complexes with lanthanide picrate. From