The study on the effect and mechanism of the second ligands on the luminescence properties of terbium complexes

Article abstract of DOI:10.1016/j.saa.2007.08.014

The binary complex of Tb(III) with N-phenylanthranilic acid (N-HPA) was synthesized, and the ternary complexes were synthesized by introducing 1,10-phenanthroline (Phen), 2,2′-dipyridyl (Bipy), trioctylphosphine oxide (TPPO) as the second ligand, respecti

Full text of DOI:10.1016/j.saa.2007.08.014

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 70 (2008) 646–650
The study on the effect and mechanism of the second ligands on the
luminescence properties of terbium complexes
Yali Fu, Jingchang Zhang∗, Yuguang Lv, Weiliang Cao
Institute of Modern Catalysis, Beijing University of Chemical Technology, State Key Laboratory of Chemical Resource Engineering,
Beijing 100029, China
Received 9 June 2007; received in revised form 15 August 2007; accepted 18 August 2007
Abstract
The binary complex of Tb(III) with N-phenylanthranilic acid (N-HPA) was synthesized, and the ternary complexes were synthesized by introduc-
ing 1,10-phenanthroline (Phen), 2,2ꢀ-dipyridyl (Bipy), trioctylphosphine oxide (TPPO) as the second ligand, respectively. These complexes were
characterized by infrared spectra, UV spectra and fluorescence spectra. The effect and mechanism of different second ligands on the fluorescent
intensity of the terbium N-phenylanthranilic acid complexes was discussed. It showed that all the complexes exhibited ligand-sensitized green
emission. The luminescence intensity increased in the sequence of Tb(N-PA)3Phen < Tb(N-PA)3(H2O)2 < Tb(N-PA)3Bipy < Tb(N-PA)3(TPPO)2.
The second ligands TPPO and Bipy enhanced the luminescence intensity of the complexes while the Phen quenched it.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Tb3+complexes; N-Phenylanthranilic acid; Second ligand; Luminescence
and 2,2ꢀ-dipyridyl (Bipy) can minimize the contribution of the
radiationless processes [7].
1. Introduction
Recently, much attention has been paid to the fluorescent
rare earth complexes, which not only can be used as probes
and sensors for natural and medical science, but also can
be used as the active center for electroluminescent devices
[1–5].
In order to improve the luminescence properties of rare earth
complexes, the choosing of the second ligands is very important.
Thoughtherewassomeresearchwork, theeffectandmechanism
of the second ligands on the luminescence properties of terbium
complexes still need to be further studied.
The luminescence property of lanthanide complexes depends
on the central metal ions, the energy level and the structure of the
ligands [4–7]. Tb3+ ion, the complexes of which emit green, is
one of the rare earth ions that show the best luminescence prop-
erty. The ligands called “antennas” absorb and transfer energy to
the rare earth ions and consequently increase their luminescence
intensity. The aromatic carboxylic acids and ␤-diketones ligands
have attracted intense research interest due to their high ligand-
to-metal ions energy transfer efficiency, and the energy levels
of the aromatic carboxylic acids match the 5D4 energy level of
Tb3+ ion better [1–6]. Moreover, in the hydrated complexes the
water molecules act as effective luminescence quencher due to
O–H oscillators, so substituting the water molecules by a sec-
ond kind of organic ligands such as 1,10-phenanthroline (Phen)
In this work, Tb(N-PA)3(H2O)2 and the ternary complexes of
terbium N-phenylanthranilic acid with the second ligand (Phen,
Bipy, TPPO) were synthesized. The spectra property of the com-
plexes was studied. The results indicated that the impact of the
secondligandsonthefluorescencecharacteristicsofterbium(III)
ions due to the energy level and the structure of them.
2. Experiments
2.1. Reagents and apparatus
Tb4O7 (99.99%), N-phenylanthranilic acid (N-HPA), 1,10-
phenanthroline (Phen), 2,2ꢀ-dipyridyl (Bipy), trioctylphosphine
oxide (TPPO) and other chemicals were analytical reagent grade
and used without further purification.
TbCl3·6H2O was prepared by dissolving terbium oxides in
hot hydrochloric acid, and evaporating excess hydrochloric acid
and water.
∗
Corresponding author. Tel.: +86 1 64434904; fax: +86 1 64434898.
E-mail address: zhangjc1@mail.buct.edu.cn (J. Zhang).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.08.014

Y. Fu et al. / Spectrochimica Acta Part A 70 (2008) 646–650
647
Fig. 1. The structures of Tb(N-PA)3Phen, Tb(N-PA)3(H2O)2, Tb(N-PA)3
(TPPO)2 and Tb(N-PA)3Bipy.
Infrared spectra were recorded in the range of
4000–400 cm−1 by a prostige-21IR spectrophotometer in
KBr flake. UV–vis spectra were performed on a UV-2501PCS
double spectrophotometer among 200–400 nm. The excitation
and emission spectra (5 × 10−5 mol/L DMF solution) were
obtained with a Shimadzu 5301 spectrofluorophotometer
equipped with a 150 W xenon lamp as the excitation source,
PMT 20 KV, scanning velocity 1200 nm/min. Spectra were
recorded using monochromator slit widths of 3 nm on both
excitation and emission sides.
Fig. 2. IR absorption spectra of the Tb(III) complexes and the first ligand: 1,
N-HPA; 2, Tb(N-PA)3Phen; 3, Tb(N-PA)3Bipy; 4, Tb(N-PA)3(TPPO)2; 5, Tb(N-
PA)3(H2O)2.
the Tb3+ complexes are shown in Fig. 2. It is obviously that
the spectra of the Tb3+ complexes are similar. The charac-
teristic absorption bands for N-HPA at 1661 cm−1(υC O) and
3200–2500 cm−1 (υ–OH) disappeared in the Tb3+ complexes.
The presence of carboxylate groups in the various complexes
was definitely confirmed by both the asymmetric stretching
bands (υasCOO–) at about 1550 cm−1 and the symmetric stretch-
ing (υsCOO–) at about 1400 cm−1
.
2.2. Synthesis of the complexes
Table 1 also shows the shift of twisting bending vibrations
(δC–H) of Bipy from 754 cm−1 to approximately 749 cm−1
for Tb(N-PA)3Bipy, which indicated the coordination between
Bipy and Tb3+. The twisting bending vibrations (δC–H) of Phen
also shifted from 737 cm−1 to 724 cm−1 compared to Tb(N-
PA)3Phen, which suggested Phen was coordinated with Tb3+.
The displacement of υP=O stretching from 1187 cm−1, in free
TPPO ligand, to approximately 1159 cm−1 in the complexes,
indicating that Tb(III) ion was coordinated with the oxygen
atom of P O. Furthermore, the broad bands at 3100–3500 cm−1
for Tb(N-PA)3(H2O)2 originated from the presence of the O–H
stretching vibrations of H2O molecule.
The ternary Tb(III) complexes were synthesized by mixing
the TbCl3·6H2O, N-HPA and the second ligand in ratio 1:3:1
(Phen, Bipy), and in ratio 1:3:2 (TPPO) in ethanol under stir-
ring, which was adjusted to pH 6–7 by adding ammonia. The
precipitation was then filtered, washed with water and ethanol
and dried, then stored in a silica-gel drier.
The binary complex was prepared in similar process except
adding the second ligand . The structures of the complexes are
shown in Fig. 1.
3. Results and discussion
3.1. IR spectra analysis
3.2. UV absorption spectra analysis
Table 1 gives the characteristic bands of the ligands and
UV absorption spectra of the ligand N-HPA and the com-
the complexes. The infrared spectra for the ligand N-HPA and
plexes in DMF solution are shown in Fig. 3. They all exhibit
Table 1
Attribution of IR main peaks of the ligands and Tb(III) complexes (cm−1
)
N-HPA
Tb(N-PA)3(H2O)2
Tb(N-PA)3 Bipy
Tb(N-PA)3Phen
Tb(N-PA)3(TPPO)2
Bipy
Phen
TPPO
υC
1661
3200–2500
–
–
–
–
–
–
–
–
–
–
–
–
754
–
–
–
–
–
–
–
–
–
O
υ–OH(m)
υasCOO–
υsCOO–
δC–H
–
–
–
–
–
1547
1401
–
1551
1400
749
–
1552
1398
843
724
–
1550
1402
–
–
1159
–
851
737
–
–
–
–
υP
–
–
1187
O

648
Y. Fu et al. / Spectrochimica Acta Part A 70 (2008) 646–650
Fig. 4. The excitation spectra of Tb(III) complexes 1, Tb(N-PA)3(TPPO)2; 2,
Tb(N-PA)3Bipy; 3, Tb(N-PA)3(H2O)2; 4, Tb(N-PA)3Phen.
Fig. 3. The UV absorption spectra of the Tb(III) complexes and the first
ligand:1, Tb(N-PA)3(TPPO)2; 2, Tb(N-PA)3Bipy; 3, Tb(N-PA)3Phen; 4, Tb(N-
PA)3(H2O)2; 5, N-HPA.
domain absorption peaks in the ultraviolet region (200–400 nm).
The maximum absorption peaks are listed in Table 2. The results
show that UV spectra of the Tb(III) complexes and the ligand
N-HPA are very similar, and the absorbance of the ternary com-
plexes just a little stronger than that of the binary complex, which
implies N-phenylanthranilic acid absorbed energy mainly either
in binary or ternary complexes.
Furthermore, a red shift (from 291 to 295 nm) is observed
comparing Tb(N-PA)3(H2O)2 with N-HPA, which illuminates
the formation of more extensive conjugated systems between
Tb3+ and N-HPA. The small blue shift of the ternary complexes
compared with the binary complex Tb(N-PA)3(H2O)2 is due to
the coordination interaction between lanthanide ions and second
ligands, which confirming the second ligands have some impact
on the conjugated systems between the Tb3+ and N-HPA. And
the blue shift of the Tb(N-PA)3Phen complex is the greatest, sug-
gesting that when Phen is coordinated to Tb3+ the ␲* orbital of
the complex is destabilized and shifted to higher energy. How-
ever, this destabilization is not obvious when the second ligands
TPPO and Bipy are coordinated.
Fig. 5. The emission spectra of Tb(III) complexes: 1, Tb(N-PA)3(TPPO)2; 2,
Tb(N-PA)3Bipy; 3, Tb(N-PA)3(H2O)2; 4, Tb(N-PA)3Phen.
of Tb(N-PA)3(TPPO)2 and Tb(N-PA)3Bipy is stronger while the
intensity of Tb(N-PA)3Phen is weaker.
The emission spectra were recorded in the range of
400–700 nm by monitoring the maximum excitation wave-
length, as shown in Fig. 5. Note that the Tb3+ complexes
Tb(N-PA)3(TPPO)2, Tb(N-PA)3Bipy, Tb(N-PA)3(H2O)2 and
Tb(N-PA)3Phen all emitted the typical sharp emission bands at
489, 544, 584 and 621 nm, corresponding to the transitions of the
3.3. Luminescent properties of the complexes
Fig. 4showstheexcitationspectraforthecomplexesrecorded
by monitoring the Tb3+ luminescence at 545 nm in the range
of 200–450 nm. The excitation spectra of the four complexes
(Tb(N-PA)3(TPPO)2, Tb(N-PA)3Bipy, Tb(N-PA)3(H2O)2 and
Tb(N-PA)3Phen) exhibit similar profile but with different inten-
sity, and with the maximum excitation wavelength of 367, 365,
358 and 359 nm, respectively. Moreover, compared with the
excitation band of Tb(N-PA)3(H2O)2, the peak position of the
ternary complexes presents red shift, the intensity of the spectra
5
7
5
7
5
7
5
7
Tb3+ ion D4 → F6, D4 → F5, D4 → F4 and D4 → F3,
respectively. No emission bands from the ligands were observed,
indicating that the ligands transferred the excitation energy effi-
ciently to the Tb3+ ion.
The intensity of the peaks of the complexes is listed in
Table 3. It is obvious that compared with Tb(N-PA)3(H2O)2 the
emission intensity of Tb(N-PA)3(TPPO)2 and Tb(N-PA)3Bipy
Table 2
The domain ultraviolet absorption of Tb(III) complexes
N-HPA
Tb(N-PA)3(H2O)2
Tb(N-PA)3Phen
Tb(N-PA)3(TPPO)2
Tb(N-PA)3 Bipy
λ (nm)
291.2
295.4
292.4
294.8
294.2

Y. Fu et al. / Spectrochimica Acta Part A 70 (2008) 646–650
649
Table 3
Relative intensity of fluorescence of Tb(III) complexes
Complexes
λex (nm)
λem (nm) (relative intensity)
7
7
7
7
5D4 → F6
5D4 → F5
5D4 → F4
5D4 → F3
Tb(N-PA)3(H2O)2
Tb(N-PA)3Phen
Tb(N-PA)3Bipy
358
359
365
367
489(106.386)
489(76.718)
489(133.036)
489(146.452)
544(271.525)
544(197.961)
544(358.338)
544(391.969)
584(17.535)
584(11.999)
584(20.693)
584(23.102)
621(7.034)
621(4.834)
621(8.020)
621(8.710)
Tb(N-PA)3(TPPO)2
is stronger, while that of complex Tb(N-PA)3Phen is weaker,
which is ascribed to the second ligands.
[7]. Thus, the luminescence intensity of Tb(N-PA)3(TPPO)2
and Tb(N-PA)3Bipy is much stronger than Tb(N-PA)3(H2O)2.
Since the energy level of the triplet state of TPPO matches
The result shows that substituting the water molecules with
the second ligands TPPO and Bipy intensifies the luminescence
of Tb(N-PA)3(H2O)2, while substituting with Phen weakens it.
The quenching effect of Phen is consistent with the report of
Wang et al. [8].
5
the D4 energy level of Tb3+ ion better than Bipy, the lumi-
nescence intensity of Tb(N-PA)3(TPPO)2 is stronger than
Tb(N-PA)3Bipy.
However, the second ligand Phen quenched the luminescence
of the complex. On one hand, the energy level of the triplet state
of Phen (22,100 cm−1) is lower than that of Bipy and TPPO
[12], and it is too near to the energy level of 5D4(20,400 cm−1) of
Tb3+ ion, which will make the inverse energy transfer 5D4 → T1
happen easily. Thus, most of the excitation energy of itself and
the energy transferred from the ligand N-HPA is consumed by
this to-and-fro energy transfer pattern, which leads to the poor
photoluminescence property of the complex Tb(N-PA)3Phen.
On the other hand, the coordination of N-phenylanthranilic acid
with Tb3+ ion makes the space around very narrow, so Phen
with a large conjugate ␲-bond system may change the dis-
tribution of ␲-electronic density in the complexes even leads
to the change of bond strength and distances of “RE-ligand”.
Thus Phen will destabilize the conjugate system between the
N-phenylanthranilic acid and Tb3+ ion, which has been con-
firmed by the result of the UV spectra, thus it will weaken
the energy transfer efficiency between N-phenylanthranilic acid
and Tb3+ ion. Hence, Phen quenches the luminescence inten-
sity of the complexes of Tb(III) with N-phenylanthranilic acid
and results in the poor luminescence property of the complex
Tb(N-PA)3Phen.
The intramolecular energy transfer efficiency from organic
ligands to RE3+ ion is the most important factor which influ-
encing the luminescence properties of rare earth complexes [9].
According to the intramolecular energy mechanism [10], the
intramolecular energy transfer efficiency depends chiefly on two
energy transfer processes: the first one comes from the triplet
level of ligands to the emissive energy level of the RE3+ ion
by Dexter’s resonant energy transfer interaction; the second
one is just an inverse energy transfer by a thermal deactivation
mechanism. Both energy transfer rate constants depend on the
energy differences between the triplet level of the ligands and
the resonant emissive energy level of RE3+.
According to Dexter luminescence theory [11], intramolec-
ular energy transfer efficiency is directly proportional to the
overlap between the luminescence spectrum of energy donor
(ligand) and the absorption spectrum of energy acceptor (Tb3+).
As energy gap between the resonance energy level of Tb3+
and the triplet state energy of ligand decreases the overlap
increases. Consequently, the intramolecular energy transfer effi-
ciency increases. Thus, ligands with a large energy difference
such as N-HPA cannot sensitize Tb3+ effectively because the
overlap integral is too small to produce effective intramolecular
energy transfer.
4. Conclusion
There also exists an inverse energy transfer process, which
affected by temperature [10]. The activation energy is approx-
imately equal to the energy difference between the triplet level
of the ligands and the resonant emissive energy level of RE3+
in the inverse energy transfer process, so as energy difference
decreases the energy inverse transfer ratio will increases, then
the luminescence intensity decreases.
A series of Tb3+ N-phenylanthranilic acid complexes were
synthesized. It showed that the complexes all emitted ligand-
sensitized green emission, and the intensity of the complexes
followed the decreasing sequence of Tb(N-PA)3(TPPO)2,
Tb(N-PA)3Bipy, Tb(N-PA)3(H2O)2, Tb(N-PA)3Phen. The intro-
duction of TPPO enhanced the luminescence intensity of the
complex while the introduction of Phen weakened it, which was
due to the energy level and structure of the second ligands.
The second ligands TPPO and Bipy, of which the energy level
of the triplet state are lower than that of N-HPA but higher than
5
the D4 energy level of Tb3+ ion, work as an energy transfer
bridge helping decrease the overlap integral between the Tb3+
ion and ligands. As a consequence, the energy transfer efficiency
increases.
Acknowledgments
In addition, the water molecules act as effective lumines-
cence quencher due to O–H oscillators. The second ligands
can prevent Tb3+ ion from coordinating water around. This
shielding effect can reduce considerably radiationless processes
This work was supported by Research Fund for the Doc-
toral Program of Higher Education (No. 20050010014) and the
National High Technology Research and Development Program
of China (863 Program) under Grant No. 2006AA03z412.

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