Synthesis and characterization of the luminescent lanthanide complexes with two similar benzoic acids

Article abstract of DOI:10.1016/j.saa.2009.06.062

2-((4-Methoxyphenoxy) carbonyl) benzoic acid, 2-(1-methoxyvinyl) benzoic acid and their rare earth complexes LnL₂(OH)·3H₂O (Ln = La, Eu, Tb) were synthesized and characterized by means of elemental analysis, FTIR, ¹H NMR, UV and luminescence spectroscopy. The FTIR and ¹H NMR results show that the carboxylic groups in the complexes coordinated to the rare earth ions in the form of one dentate, and the ester carboxylic groups have taken part in the coordination. Among these complexes, Eu(III) complexes and Tb(III) complexes exhibit characteristic fluorescence with comparatively high brightness and good monochromaticity, which indicated that the ligands of HL^I and HL^II are good organic chromophore to absorb and transfer energy to metal ions.

Full text of DOI:10.1016/j.saa.2009.06.062

Spectrochimica Acta Part A 74 (2009) 843–848
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis and characterization of the luminescent lanthanide complexes with
two similar benzoic acids
Tonghuan Liua,b, Guojian Duanb, Yaping Zhangb, Jian Fangb, Zhengzhi Zengb,∗
a Nuclear Science and Technology, Lanzhou University, Lanzhou 730000, China
b College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
2-((4-Methoxyphenoxy) carbonyl) benzoic acid, 2-(1-methoxyvinyl) benzoic acid and their rare earth
complexes LnL2(OH)·3H2O (Ln = La, Eu, Tb) were synthesized and characterized by means of elemental
analysis, FTIR, 1H NMR, UV and luminescence spectroscopy. The FTIR and 1H NMR results show that the
carboxylic groups in the complexes coordinated to the rare earth ions in the form of one dentate, and the
ester carboxylic groups have taken part in the coordination. Among these complexes, Eu(III) complexes
and Tb(III) complexes exhibit characteristic fluorescence with comparatively high brightness and good
monochromaticity, which indicated that the ligands of HLI and HLII are good organic chromophore to
absorb and transfer energy to metal ions.
Received 28 May 2008
Received in revised form 20 June 2009
Accepted 30 June 2009
Keywords:
2-((4-Methoxyphenoxy) carbonyl) benzoic
acid
2-(1-Methoxyvinyl) benzoic acid
© 2009 Elsevier B.V. All rights reserved.
FTIR
1H NMR
UV
Luminescent properties
1. Introduction
2. Experimental
Luminescent lanthanide complexes [1–3] have attracted intense
research interest from both material and biological science [4,5]
mainly due to their very narrow emission bands and large Stokes
shifts, long luminescence lifetime [6]. Especially europium(III) and
terbium(III) ions have excellent luminescence properties, but their
absorption coefficients are very small. In order to overcome these
shortages, lanthanide ions are usually chelated with ligands that
have broad intense absorption bands. Recently, podand-type lig-
ands [7], macrocyclic compounds [8], carboxylic acid derivatives [9]
and pyrazolone derivatives [10] have been attracted much attention
mainly because they can form highly stable and strongly lumines-
cent Eu(III) and Tb(III) ion complexes.
The reactions of precursors with carboxylic acids have been
studied in considerable detail because they show higher thermal
and luminescent stabilities for practical application than other
lanthanide complex systems. Theirusehas openedup alotofoppor-
tunities in growing fields of large social and economical impact
[11–14]. As part of our continuing investigation into the prepara-
tion of lanthanide compounds with the derivatives of benzoic acid,
we report here the synthesis and spectroscopic study of the 2-
((4-methoxyphenoxy) carbonyl) benzoic acid, 2-(1-methoxyvinyl)
benzoic acid and their complexes with La(III), Eu(III) and Tb(III).
2.1. Materials
Phthalic anhydride (99%) and p-hydroxyanisole (99%) were used
as received from Chemical Reagent, Shanghai. And other chemicals
were of analytical reagent grade used without further purification.
2.2. Methods
The metal ions were determined by EDTA titration using xylenol
orange as an indicator. Nitrogen, carbon and hydrogen analyses
were determined using a Vario EL elemental analyzer. IR spectra
were recorded on a Nicolet Avatar 360 FT-IR instrument using KBr
discs in the 400–4000 cm−1 region. 1H NMR spectra were measured
on a Varian Mercury-400BB NMR spectrometer. The UV–vis spec-
tra were recorded on a Varian Cary 100 UV-Vis spectrophotometer.
Luminescence measurements were made on a Hitachi F-4500 spec-
trophotometer at room temperature. The excitation and emission
slit widths were 2.5 nm.
2.3. Synthesis of the ligands (HLI and HLII)
2.3.1. Synthesis of the ligand (HLI) (Fig. 1a)
The solution of phthalic anhydride (10 g and 67.5 mmol) in anhy-
drous methanol (60 mL) was heated for 8 h while refluxing under
N2. The reaction solution was cooled to room temperature and con-
centrated in vacuum to give the crude product of phthalic acid
∗
Corresponding author. Tel.: +86 931 8912582; fax: +86 931 8912582.
E-mail address: zengzhzh@yahoo.com.cn (Z. Zeng).
1386-1425/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2009.06.062

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T. Liu et al. / Spectrochimica Acta Part A 74 (2009) 843–848
Fig. 1. Scheme of the synthesis of the ligand (a: HLI and b: HLII).
Table 1
monomethyl ester as a white solid, the crude product was purified
by re-crystallization from chloroform (yield 92%) [15].
Elemental analyses of the ligands and complexes (wt%).
Complexes
C (%)
H (%)
Ln (%)
2.3.2. Synthesis of the ligand (HLII) (Fig. 1b)
HLI
59.87 (60.00)
37.52 (37.91)
36.67 (37.06)
35.99 (36.62)
66.02 (66.17)
47.09 (47.76)
46.13 (46.95)
46.22 (46.52)
4.53 (4.48)
4.32 (4.07)
4.11 (3.97)
4.14 (3.93)
4.51 (4.44)
4.36 (4.14)
4.33 (4.07)
4.21 (4.03)
0
LaLI2(OH)·3H2O
EuLI2(OH)·3H2O
TbLI2(OH)·3H2O
HLII
25.31 (24.36)
27.11 (26.05)
33.19 (32.52)
0
19.05 (18.41)
20.83 (19.80)
29.67 (28.92)
A mixture of p-hydroxyanisole (3.72 g and 30 mmol) and sodium
(1.2 g and 30 mmol) in toluene (100 mL) was heated for 3 h while
refluxing. The reaction mixture was cooled to 85 ◦C and phthalic
anhydride (4.44 g and 30 mmol) was added. After stirring at 85 ◦C
for 1 h, the reaction mixture was poured over ice water (30 mL),
and the aqueous layer was collected. And then the aqueous layer
was acidified with concentrated hydrochloric acid to pH 2–4, and
the aqueous layer extracted with ethyl acetate (50 mL 3 times). The
combined organic layers were washed with water and dried over
MgSO4, the filtrate was concentrated in vacuum to give the crude
product of phthalic acid mono-(4-methoxy-phenyl) ester as a white
solid, and the crude product was purified by re-crystallization from
ethyl acetate–hexane (yield 75%).
LaLII2(OH)·3H2O
EuLII2(OH)·3H2O
TbLII2(OH)·3H2O
main stretching frequencies of the IR spectra of the ligands and
their complexes are presented in Table 2.
The infrared spectra of ligands show strong bands at 1737 cm−1
and 1691 cm−1 (HLI), and 1752 cm−1 and 1704 cm−1 (HLII), which
are assigned to ꢀ(C O) of ester carbonyl group and carboxylic
group, respectively. In the complexes, the band for ꢀ(C O) of car-
boxylic group is disappeared and the band for ꢀ(C O) of eater
carbonyl group is shifted by 18–19 cm−1 (HLI) or 35–37 cm−1
(HLII) towards lower wave numbers, it is clear that the car-
boxylic group and ester carbonyl group takes part in coordination,
at the same time [16]. Two strong absorption bands lying at
1407–1412 cm−1 and 1550–1560 cm−1 are observed in complexes,
2.4. Synthesis of the complexes
The complexes were prepared by the same method.
Triethyl-amine (3.0 mmol) was added to a solution of ligand
(3 mmol) in methanol (15 mL), the reaction solution was heated
at refluxed for 1 h, to which a solution of Ln(NO3)3·6H2O (Ln = La,
Eu, Tb) (0.5 mmol) in water (5 mL) was added dropwise, the solu-
tion was kept stirring until the white precipitate was formed. The
precipitate was collected and washed three times with the mixture
of methanol/water (v:v, 1:1), further drying in a vacuum afforded a
pale white powder, yield: 55–65%.
which are attributed to the symmetric vibration absorption (ꢀs
)
COO
and asymmetric vibration absorption (ꢀasCOO ) of the carboxylic
groups. The ꢁꢀ(ꢀas − ꢀs) values of all complexes are in the range
of 141–160 cm−1, which indicates that the Ln ions are in the form
of one dentate [17]. The far infrared spectra of all these deriva-
tives show medium bands at about 455–468 cm−1, which may be
assigned to ꢀ(Ln–O) [18]. There is also a strong and wide peak of
ꢀ(OH) (3400–3429 cm−1) and the in-plane and out-of-plane vibra-
tion of water at ca. 800 cm−1 and 570 cm−1 in all the complexes,
which are associated with the coordinated water [19].
3. Results and discussion
3.1. Properties of the complexes
3.3. 1H NMR spectra
Analytical data for the complexes are listed in Table 1. The results
show that the formulas of the complexes are LnLI2(OH)·3H2O and
LnLII2(OH)·3H2O. All complexes are soluble in DMSO and DMF.
Metal ion coordination with ligand by means of oxygen carboxyl
atoms was shown owing to data of 1H NMR spectra. Proton spectra
of the ligand and La(III) complex recorded in DMSO-d6, confirmed
the formation of the complexes. The typical chemical shift of the
1H NMR spectra in DMSO-d6 at room temperature is presented in
Table 3 and Fig. 3.
3.2. IR spectra
The IR spectra of the complexes are obviously different from the
ligand, but the IR spectra of the complexes resemble (Fig. 2). The

T. Liu et al. / Spectrochimica Acta Part A 74 (2009) 843–848
845
3.3.1. 1H NMR spectra of HLI and La(III) complex
the ester carbonyl group was coordinated to the La(III) ion. All the
change of proton signal is due to the inductive effect of La(III) ions
in the complexes.
In the 1H NMR spectrum of the La(III) complexes, there is no pro-
ton peak of the carboxylic group, which suggests that the carboxylic
group is coordinated to the La(III) ion through the oxygen. The pro-
ton signal of H-6, H-4 and H-5 is shifted to higher field 0.24 ppm,
0.1 ppm and 0.1 ppm, respectively. Which is because that the effect
of ester carbonyl group to H-4, H-5 and H-6 is decreased when the
ester carbonyl group was coordinated to the La(III) ion. The proton
signal of H-3 is shifted to lower field 0.125 ppm, this is because of
the electron cloud of the phenyl is lean to carboxylic group after
3.3.2. 1H NMR spectra of HLII and La(III) complex
In the 1H NMR spectrum of the La(III) complex, there is no proton
signal of the carboxylic group, which suggests that the carboxylic
group is coordinated to the La(III) ion through the oxygen. At the
same time, the proton signals of H-10, H-11, H-13 and H-14 are
changed from two group peaks to one group peak. And the proton
Fig. 2. 1H NMR spectra of free ligand and La(III) complexes (a-1: HLI, a-2: LaLI2(OH)·3H2O, b-1: HLII and b-2: LaLII2(OH)·3H2O).

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T. Liu et al. / Spectrochimica Acta Part A 74 (2009) 843–848
Fig. 2. (Continued ).
signals are all shifted to higher field, which is due to the effect of
ester carbonyl group to this phenyl is obvious decreased when the
ester carbonyl group coordinated to the La(III) ion. The proton sig-
nals change the regularity of the H-3, H-4, H-5 and H-6 are similar
with LnLI2(OH)·3H2O, which indicates that the two complexes have
the same chemical condition.
water in the La(III) complex is stronger than the ligand obvi-
ously.
On the basis of above evidence and analyses, the structure of the
complexes is shown in Fig. 4.
3.4. UV spectra
The proton signals of the water molecules in the complexes
were observed at ca. 3.3 ppm, it is combined with the proton
signals of water contained in the solvent, so the signal of the
The study of the electronic spectra in the ultraviolet and vis-
ible ranges for the ligands and the complexes was carried out

T. Liu et al. / Spectrochimica Acta Part A 74 (2009) 843–848
847
Table 2
FTIR spectra data of ligands and complexes (cm−1).
Complexes
ꢀO–H
ꢀC
ꢀas
ꢀs
ꢁꢀas−s
ꢀH
ꢀH
ꢀLn–O
O
O (in-plane)
O (out-of-plane)
2
−
−
COO
2
COO
LI
3451
3408
3426
3422
3440
3418
3425
3429
1737, 1691
1718
1717
1718
1752, 1704
1715
LaLI2(OH)·3H2O
EuLI2(OH)·3H2O
TbLI2(OH)·3H2O
LII
1562
1550
1560
1407
1409
1410
155
141
150
795
797
791
571
575
568
468
466
465
LaLII2(OH)·3H2O
EuLII2(OH)·3H2O
TbLII2(OH)·3H2O
1559
1558
1568
1412
1412
1408
147
146
160
801
805
799
566
572
576
455
457
460
1717
1716
ꢁꢀas−s = ꢀas − ꢀs.
Table 3
1H NMR spectra of ligands and La(III) complexes.
OH
H-3
7.765–7.785
7.889–7.910
7.773–7.849
7.780–7.801
H-4, H-5
H-6
H-9
H-10, H-14
H-11, H-13
H-15
HLI
13.253
7.634–7.654
7.524–7.547
7.662–7.694
7.432–7.450
3.798
3.789
LaLI2(OH)·3H2O
7.393–7.414
7.773–7.849
7.333–7.354
HLII
13.470
7.145–7.167
6.638–6.709
6.692–6.992
3.742
3.702
LaLII2(OH)·3H2O
Fig. 3. The suggested structure of the complexes (a: LnLI2(OH)·3H2O and b: LnLII2(OH)·3H2O).
in DMSO. The results are shown in Table 4. The ligand (HLI) has
an intensive band at ꢂmax = 274 nm (n → ␲*) and a less intensive
band at ꢂmax = 248 nm (␲ → ␲*). But in the complexes, the band
at ꢂmax = 274 nm was shifted to 272 nm; the band at ꢂmax = 248 nm
was shifted to 260 nm and the band was became to the first inten-
sive band. The ligand (HLII) has an intensive band at ꢂmax = 276 nm
(n → ␲*) and a less intensive band at ꢂmax = 233 nm (␲ → ␲*). But in
the complexes, the band at ꢂmax = 276 nm was shifted to 291 nm, the
band at ꢂmax = 233 nm was shifted to 259 nm. These results suggest
that the complexes have formed.
3.5. Luminescence studies
Under identical experimental conditions, the excitation and
emission spectra, as well as the luminescence intensity, were mea-
sured on dried and finely powdered samples at room temperature
(Fig. 4). The major luminescence spectral data are summarized in
Table 5.
Table 4
UV–vis data of the ligands and their complexes.
Compound
HLI
Max (nm)
␧ × 10−3 (mol−1 cm−1
)
Transition
248
274
7.88
9.20
␲ → ␲*
n → ␲*
Table 5
Fluorescence spectra data of the solid complexes.
LaLI2(OH)·3H2O
EuLI2(OH)·3H2O
TbLI2(OH)·3H2O
HLII
259
272
6.44
6.97
␲ → ␲*
n → ␲*
Complexes
ꢂex (nm)
ꢂem (nm)
RFIa
237.1
Transition
5D0
5D0
→
→
7F1
7F2
EuLI2(OH)·3H2O
396
592.8
615.2
260
272
9.85
9.53
␲ → ␲*
n → ␲*
351.1
TbLI2(OH)·3H2O
353
491.2
546
586.4
621.0
534.7
1162
168.1
73.9
5D4
5D4
5D4
5D4
→
→
→
→
7F6
7F5
7F4
7F3
260
272
8.28
7.88
␲ → ␲*
n → ␲*
233
276
3.42
10.47
␲ → ␲*
n → ␲*
396
353
592.6
615
931
1601
5D0
5D0
→
→
7F1
7F2
EuLII2(OH)·3H2O
TbLII2(OH)·3H2O
LaLII2(OH)·3H2O
EuLII2(OH)·3H2O
TbLII2(OH)·3H2O
259
291
3.97
10.55
␲ → ␲*
n → ␲*
491
1113
2495
189.1
65.6
5D4
5D4
5D4
5D4
→
→
→
→
7F6
7F5
7F4
7F3
259
291
11.99
5.66
␲ → ␲*
n → ␲*
546.2
585.4
620.8
260
291
10.68
4.58
␲ → ␲*
n → ␲*
a
RFI is relative fluorescence intensity.

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T. Liu et al. / Spectrochimica Acta Part A 74 (2009) 843–848
energy level (T1) of the ligand matches better to the resonance level
of Tb(III) than Eu(III).
At the same time, the luminescence intensities of
LnLII2(OH)·3H2O complex are stronger than the correspond-
ing LnLI2(OH)·3H2O complex. We consider this is because of the
HLII complexes have the larger conjugated structure than HLI
complexes, HLII could absorb the more energy and transfer it to
the Ln(III) ions.
4. Conclusion
According to the data and discussion above, the new ligands can
form stable solid complexes with rare earth ions, obvious changes
in IR spectra and 1HNMR spectra were observed. The result changes
indicated that the Ln(III) ions were coordinated to the ester carbonyl
group and carboxylic group oxygen atoms of the ligands. The flu-
orescent properties of the Eu and Tb complexes in the solid state
were investigated under the excitation, the complexes exhibited
characteristic fluorescence of europium and terbium ions. Based
on those results, a series of benzoic acid ligands could be designed
and synthesized to optimize the luminescent properties of these
lanthanide ions.
Acknowledgments
This work was supported by the National Natural Science Foun-
dation of China (No. 20171019) and Natural Science Foundation of
Gansu Province (No. 3ZS041-A25-009).
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Fig. 4. The emission spectra of the complexes (a: Eu(III) complex and b: Tb(III)
complex) (1): EuLII2(OH)·3H2O, (2) EuLI2(OH)·3H2O, (3) TbLII2(OH)·3H2O and (4)
TbLI2(OH)·3H2O.
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7
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