An investigation of the effect of ligands on thermal stability of luminescent samarium complexes

Article abstract of DOI:10.1016/j.jallcom.2014.05.211

Three kinds of Sm(L)3PhennH2O complexes (L: a-thenoyltrifluoroacetone (HTTA), 4-fluorobenzoicacid (p-FBA) and 4-aminobenzoicacid (p-ABA)) were designed and synthesized by the wet chemical method. Their thermal stabilities were compared through thermogravimetric analysis (TGA) and high temperature photoluminescence measurements. The results indicated that [Sm(p-FBA)3Phen]2 presented the best thermal stability, and the photoluminescence intensity could remain 50% even after being heat-treated at 280 C for 20 min. Not only the rigidity of the organic ligand and complexes structures, but also the coordinating ability between rare earth ions and ligand played a crucial role in the stability of the complexes. In this article, p-fluorobenzoicacid has been proven to be a promising ligand for the preparation of highly luminescent and stable rare earth complexes.2014 Elsevier B.V. All rights reserved.

Full text of DOI:10.1016/j.jallcom.2014.05.211

Accepted Manuscript
An investigation of the effect of ligands on thermal stability of luminescent
samarium complexes
Yaru Ni, Jing Tao, Junyang Jin, Chunhua Lu, Zhongzi Xu, Feng Xu, Jiamei
Chen, Zhitao Kang
PII:
S0925-8388(14)01318-8
DOI:
http://dx.doi.org/10.1016/j.jallcom.2014.05.211
JALCOM 31400
Reference:
To appear in:
Received Date:
Revised Date:
Accepted Date:
20 March 2014
27 May 2014
28 May 2014
Please cite this article as: Y. Ni, J. Tao, J. Jin, C. Lu, Z. Xu, F. Xu, J. Chen, Z. Kang, An investigation of the effect
of ligands on thermal stability of luminescent samarium complexes, (2014), doi: http://dx.doi.org/10.1016/
j.jallcom.2014.05.211
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An investigation of the effect of ligands on thermal stability of luminescent
samarium complexes
a
a
b
Yaru Ni a,b,*, Jing Tao a, Junyang Jin , Chunhua Lu , Zhongzi Xu a, *, Feng Xu ,
Jiamei Chen a, Zhitao Kang c,d
a
Materials Science and Engineering, Nanjing Tech University, Nanjing 210009,
China
b Key Laboratory of MEMS of Ministry of Education, Southeast University, Nanjing
210096, China
c
School of Material Science and Engineering, Georgia Institute of Technology,
Atlanta, GA 30332, USA
d Georgia Tech Research Institute, Georgia Institute of Technology, Atlanta, GA 30332,
USA
Abstract
Three kinds of Sm(L)3Phen •nH2O complexes (L: α-thenoyltrifluoroacetone
(HTTA), 4-fluorobenzoicacid (p-FBA) and 4-aminobenzoicacid (p-ABA)) were
designed and synthesized by the wet chemical method. Their thermal stabilities were
* Corresponding author.
Tel.: +86 25 83587220; fax: +86 25 83587220.
Email address: nyr@njtech.edu.cn (Y. Ni), zzxu@njtech.edu.cn (Z. Xu).
1

compared through thermogravimetric analysis (TGA) and high temperature
photoluminescence measurements. The results indicated that [Sm(p-FBA)3Phen]2
presented the best thermal stability, and the photoluminescence intensity could remain
50% even after being heat-treated at 280 °C for 20 min. Not only the rigidity of the
organic ligand and complexes structures, but also the coordinating ability between
rare earth ions and ligand played a crucial role in the stability of the complexes. In
this article, p-fluorobenzoicacid has been proven to be a promising ligand for the
preparation of highly luminescent and stable rare earth complexes.
Keywords
Effect of ligands; Samarium complexes; Luminescent thermal stability; Substituent
1. Introduction
The interest in the development of highly luminescent polymer-based rare earth
complexes has continuously grown in the last decades [1-4]. This type of complex can
provide both advantages of polymers, such as molding and shaping capability,
excellent UV absorption, mechanical strength and flexibility, and the luminescence
characteristics of rare earth complexes [5, 6], including sharp emission bands, large
Stokes shift, high quantum yields [7-10], temperature-sensitivity [11] and general
versatility [12-16]. However, it is difficult to uniformly incorporate particles of rare
earth compounds into a polymer matrix, which typically results in inhomogeneous
dispersion and poor mechanical strength of polymer phosphors. Therefore, work in
this area has focused on the development of various rare earth complexes [17], where
the organic ligand promotes the compatibility between the rare earth element and the
2

polymer [18]. In order to improve dispersion capability of fluorescent particles into
polycarbonate (PC), twin-screw extruding accompanied by a high temperature
blending above 280 °C is normally used. However, it should be noted that some of the
rare earth complexes may be phase transformed or decomposed during the blending
process. In this case, the enhancement of the thermal resistance of the complexes
becomes very critical [19,20]. Constable et al.[5] pointed out that ligand configuration
plays a fundamental role on the thermal resistance and luminescent properties of rare
earth complexes [21-23].
Based on our previous study and understanding of thermal resistance and
luminescent properties of samarium complexes [24], this article extensively focuses
on the design and synthesis of highly thermo-stable fluorescent complexes using
different kinds of ligands and substituents. Sm(L)3Phen•nH2O (L: HTTA, p-FBA,
p-ABA) complexes were prepared in this article. The effects of different ligand
configurations and substituents in the benzoic acid ring on the photoluminescence and
thermal resistance of samarium complexes were also compared in this study.
Characterization techniques including infrared (FT-IR) spectroscopy, elemental
analysis (EA), X-ray fluorescence (XRF), and photoluminescence (PL) spectroscopy
were used to analyze and confirm the synthesis of these complexes. Through
thermogravimetric analyses (TGA) and high temperature photoluminescence
experiments, the thermal resistance and luminescence stability of these complexes
were investigated and compared.
3

2. Experimental section
2.1. Materials and reagents
SmCl3·6H2O (99.9%) was purchased from Funing Rare Earth Industrial Company
Ltd. α-thenoyltrifluoroacetone (HTTA) (99.00%), 4-Fluorobenzoicacid (p-FBA)
(99.00%), 4-aminobenzoic acid (p-ABA) (99.00%) and 1,10-phenanthroline (Phen)
(99.00%) were obtained from Sinopharm Chemical Reagent Company Ltd.
2.2. Instruments and methods
Infrared (IR) spectra were measured at room temperature between 4000 and 400
cm-1 using a Nexus 670 FT-IR spectrophotometer with KBr pellet technique. The
photoluminescence (PL) spectra were recorded by a Horiba Jobin-Yvon FL3-221
spectrofluorometer with the band pass for the excitation and emission
monochromators setting at 2.0 nm. A TAP-02 instrument with a temperature
controlled sample holder was used to heat products from room temperature to 300 °C
while high temperature photoluminescence spectra of the complexes were measured
with a FL3-221 spectrofluorometer. Thermogravimetric analyses (TGA) of the
samples were performed by a STA 449C/6F analyzer from room temperature to
900 °C at a heating rate of 20 °C/min in air. Elements (C, H and N) analysis was
conducted by an Elementar Vario EL Ⅲ(Elementar Analyzer, Germany). Elements of
O, F, S and Sm were detected by X-ray fluorescence spectrometry (XRF, Thermo
ARL Advant XP’, America).
2.3. Syntheses of Sm(L)3Phen•nH2O complexes
Three Sm(L)3Phen•nH2O (L: p-FBA, HTTA, p-ABA) complexes were synthesized
4

according to the previously reported procedure with minor modifications [24]. They
were obtained in the following steps. First, SmCl3·6H2O (2.0 mmol), L (6.0 mmol)
and Phen (2.0 mmol) were dissolved by stirring in 40, 70 and 30 ml ethanol,
respectively. Subsequently, the ethanol solution of SmCl3 was added dropwise into L
ethanol solution within 3 min, with the molar ratio (L to Sm ion) of 3:1. An ethanol
solution of Phen with the molar ratio of Phen to Sm ion being 1:1 was slowly added to
the mixture within 5 min. The pH value of the mixture was adjusted between 5 and 6
by adding sodium hydroxide solution (1 mol/L). Then the mixture was stirred and
refluxed at 60 °C in a water bath until an orange precipitate appeared. Finally, a
suction filter was used to take a sample from the precipitation, which was then
washed with water and ethanol. Powders of these complexes were stored in a
silica-gel dryer after drying in a vacuum oven at 50 °C.
A comprehensive elemental analysis was carried out to evaluate the complex
structure. Anal. Calc. for Sm(TTA)3Phen (C36H20N2F9O6S3Sm): N, 2.82; C, 43.49; H,
2.03; O, 9.66; S, 9.67; F, 17.20; Sm, 15.13%; Found: N, 2.80; C, 43.07; H, 1.93; O,
10.53; S, 11.04; F, 15.78; Sm, 14.77%; Anal. Calc. for [Sm(p-FBA)3Phen]2
(C66H40N4F6O12Sm2): N, 3.75; C, 52.99; H, 2.69; O, 12.83; F, 7.62; Sm, 20.11%;
Found: N, 3.68; C, 52.74; H, 2.69; O, 11.52; F, 6.73; Sm, 22.24%; Anal. Calc. for
Sm(p-ABA)3H2OPhen•H2O (C33H30N5O8Sm): N, 9.04; C, 51.14; H, 3.90; O, 12.99;
Sm, 20.35%; Found: N, 8.57; C, 49.22; H, 4.00; O, 12.72; Sm, 22.39%.
The molecular structure and synthetic route of the Sm(TTA)3Phen [25],
[Sm(p-FBA)3Phen]2 [26], and Sm(p-ABA)3H2OPhen•H2O [27] complexes are shown
5

in Fig.1. Unlike ligand HTTA with a thiophene structure, p-FBA and p-ABA have a
benzene ring structure.
3. Results and discussion
3.1. FT-IR spectra
The FT-IR spectra of Sm(L)3Phen•nH2O (L: HTTA, p-FBA, p-ABA) complexes
before and after heat treatment at 280 °C for 20 min are shown in Fig. 2. The IR
absorption peaks of the complexes in Fig. 2(b), can be compared with the raw
material shown in Fig. 2(a). The IR absorption peaks of the Sm(TTA)3Phen complex
are similar to those reported in the literature [24, 28]. The appearance of peaks at 581
cm-1, which corresponds to the stretching vibration of νSm-O, indicates the presence of
chemical bonding between HTTA and Sm3+. The stretching vibration of νC=N at 1560
cm-1 belongs to the second ligand of Phen as shown in Fig. 2. After forming
coordination bonds with Sm3+, these peaks moved to lower frequencies of 1540 cm-1
[29]. It can be observed that stretching vibration peaks of νC=N in [Sm(p-FBA)3Phen]2
and Sm(p-ABA)Phen also shifted to lower wavenumber 1539 cm-1 and 1552 cm-1,
respectively.
The
FT-IR
spectra
of
the
[Sm(p-FBA)3Phen]2
and
Sm(p-ABA)3H2OPhen•H2O proved intense absorption bands at 1416, 1625 cm-1 and
1401, 1623 cm-1, which were attributed to the characteristics of the carboxylate group
including symmetric νs (C=O) and asymmetric νas (C=O) vibrations, respectively [30].
The 3500 cm-1 peak is the stretched vibration of νH-O because the organic complex
easily absorbs water in air.
6

After heat treatment at 280 °C, the spectra in Fig. 2 obviously shows that the 581
cm-1 stretching vibration of νSm-O in the Sm(TTA)3Phen complex disappeared. It
indicated that the Sm-O bond was unstable and the HTTA group dissociated from the
center samarium ions after heat treatment, which can be further confirmed in the
following discussions on DSC/TG and high temperature luminescence results.
Meanwhile, the peaks of [Sm(p-FBA)3Phen]2 and Sm(p-ABA)3H2OPhen •H2O
exhibited no change after heat treatment at 280℃. These observations proved that
complexes of [Sm(p-FBA)3Phen]2 and Sm(p-ABA)3H2OPhen•H2O were more stable
than Sm(TTA)3Phen.
3.2. Thermal stability
Thermogravimetric analyses (TGA) of Sm(TTA)3Phen, [Sm(p-FBA)3Phen]2 and
Sm(p-ABA)3H2OPhen•H2O were carried out to explore their degradation behavior
and water content. As shown in Fig. 3, [Sm(p-FBA)3Phen]2 exhibited a higher thermal
stability than Sm(TTA)3Phen and Sm(p-ABA)3H2OPhen•H2O. The peak at 252°C
corresponds to the phase transition of Sm(TTA)3Phen. Its initial decomposition
temperature occurs at 318 ± 10 °C, whereas Sm(TTA)3Phen and
Sm(p-ABA)3H2OPhen•H2O begin to lose weight at 318±10 °C. For Sm(TTA)3Phen
complex, there is a ~66.8% mass loss between 318~374 °C，which corresponds to the
dissociation of the TTA component. Beyond 374 °C, the SmPhen structure also
decomposes. The thermal decomposition of [Sm(p-FBA)3Phen]2 took place in almost
one step at much higher temperatures from 340 to 530 °C. However, TGA themogram
of Sm(p-ABA)3H2OPhen•H2O shows that ~5% mass loss at 100 °C is attributed to
7

the disassociation of physisorbed water molecules. At the end of the total
decomposition of Sm(p-ABA)3H2OPhen•H2O, the rest of the molecules give a weight
plateau at above 410 °C, which corresponds to Sm2O3. Herein, it is interesting to note
that TGA curves of [Sm(p-FBA)3Phen]2 and Sm(TTA)3Phen exhibit that their
anhydrous nature may lead to enhanced thermal resistance. In addition, p-FBA has a
benzene ring structure, which is more stable than the thiophene ring, corresponding to
its higher thermal resistance than Sm(TTA)3Phen. The main reason for the poor
stability of Sm(p-ABA)3H2OPhen•H2O was assigned to the –NH2 group which is a
hydrophilic group as well as an electron donating group. Its hydrophilic nature makes
it easy to absorb water. Moreover, its electron donating capability will activate the
carboxyl group on the para position of the benzene ring, which makes the electron
density of Sm-O coordinating bonds increase and the bond stability decrease.
3.3. Photoluminescence properties
Fluorescence emission spectra of Sm(L)3Phen (L: (a) HTTA, (b) p-FBA, (c)
p-ABA) complex powders were measured. It is expected that the emission peak wave
lengths of samarium complexes which was tested at room temperature were the same
for different ligands and did not change after heat treatment at 280 °C. As can be seen
in Fig. 4, three emission peaks existed at 564, 600 and 645 nm with different
intensities, which belong to the transitions of 4G5/2→6H5/2, 4G5/2→6H7/2 and
4G5/2→6H9/2 of Sm3+ [31]. The strongest peak near 645 nm is the characteristic red
fluorescence of Sm3+ resulting from 4G5/2→6H9/2 electron dipole transition. In addition,
8

there is a weaker wide emission band (400-480 nm) in the [Sm(FBA)3Phen]2 complex,
which belongs to the ligand of p-FBA.
The luminescent spectra of samples heat-treated at 280 °C for 20 min revealed that
different samarium complex structures resulted in different thermal resistivity. It is
obvious that [Sm(FBA)3Phen]2 possessed the strongest thermal resistivity, which
remained almost 50% of the original intensity after a heat treatment at 280 °C for 20
min, while the intensity of Sm(TTA)3Phen and Sm(p-ABA)3H2OPhen•H2O only
preserved 39% and 16%, respectively.
To investigate the high temperature luminescence properties, the luminescence
intensities of different samples as a function of testing temperature were also
measured. Photoluminescence results up to 280 °C are displayed in Fig. 5 and Fig. 6.
Excitation measurements were also taken to find out the pump wavelengths for
maximum emission intensity and to investigate the effects of the host lattice on the
orbitals of the Sm3+ ion.
For all the complexes, it is clear that the luminescence intensity decreased
dramatically with the increase of testing temperature. A simple relationship has been
reported by Weber et al for the temperature dependency of nonradiative transition
rates based on multiphonon emission (MPE) (Eq. (1)) [32]:
hυ
−n
)]
Wnr (T) =Wnr(T =0)[1− exp(
(1)
kBT
where Wnr(T) is the Tn dependence nonradiative rate, Wnr(T=0) is a constant
nonradiative rate at low temperature, n is the number of phonons involved in the
transition, hv is the photon energy, kB is the Boltzmann constant, and T is the
9

temperature. As the temperature increases, more phonons become available for
multiphonon relaxation of electrons to low energy levels. From Eq. (1), we can expect
that increasing temperature will result in a drastically increased nonradiative rate,
which depends on Tn. This dependence of fluorescence intensity on temperature for
[Sm(FBA)3Phen]2 and Sm(TTA)3Phen complexes is shown in Fig. 6. While
Sm(ABA)3Phen makes an exception from room temperature to 100 °C before the H2O
molecules dissociate away from the NH2 group, the luminescence intensity increases
slightly below 100 °C, then drops drastically with further increase of temperature.
As shown in Fig. 5(a) and Fig. 6, the luminescence peaks of the Sm(TTA)3Phen
complex were very weak when the temperature reached 280 °C. According to the
TGA testing results, Sm(TTA)3Phen complex began to transform its phase at 252 °C.
Therefore, in this case we expected that not only the nonradiative effects, but also
complex structure has caused a significant decrease of luminescence intensity when
the Sm(TTA)3Phen complex was heated above 252 °C. The matastable crystal
structure may forbid the energy transfer from the ligand to the center samarium ions.
In the case of the [Sm(FBA)3Phen]2 complex vs. the Sm(p-ABA)3H2OPhen•H2O
complex, different para substituents were used on the phenyls of benzoic acid. The
results exhibit that para electron withdrawing groups (as –F in FBA) deactivate
phenyls’ rings as their π* electron density decreases [33], and make the carboxyl
oxygen become reduction potentials [34], which leads to a stronger coordinate ability
of Sm3+ [35]. The Sm3+ are bridged by four p-FBA ligands [26], making
[Sm(FBA)3Phen]2 become ultra-stable. In contrast to the case of electron withdrawing
10

substituents, para electron donating groups (as –NH2 in ABA) activate the carboxyl
oxygen and weaken the combination with samarium ions, which results in an unstable
complex.
4. Conclusions
The main objective of this work was to find out whether the category of ligand
itself or the type of substituent on the benzene ring will significantly affect the
complex luminescent properties and their thermal resistance. The results demonstrated
that both of these would happen. Three interesting conclusions were made: First, the
[Sm(p-FBA)3Phen]2 complex presented the best thermal resistance in comparison to
the Sm(p-TTA)3Phen complex and the Sm(p-ABA)3H2OPhen•H2O complex. Second,
although the Sm(TTA)3Phen complex showed the strongest luminescence intensity,
phase transition begins at 252 °C, which makes it unsuitable to be dispersed into PC,
while [Sm(p-FBA)3Phen]2 presents a promising thermo stable complex. Finally, the
type of substituent on the benzene ring plays an important role in the complex
stability. Owing to the electron withdrawing group of –F, which deactivates carboxyl
at the para position of the beneze ring, the substituent leads to the stable luminescent
properties observed from the dinuclear [Sm(p-FBA)3Phen]2 complex, and vice versa,
for the Sm(p-ABA)3H2OPhen•H2O complex. With a stable p-fluorobenzoicacid,
together with the down conversion capability of the Sm3+ ion, [Sm(p-FBA)3Phen]2
shows potential applications in photovoltaic devices or light emitting fields as a
thermally stable phosphor material.
11

Acknowledgements
This work was supported by the National Science Foundation of China (NSFC, Grant
Nos. 61106055, 51372039) and the Fundamental Research Funds for the Central
Universities (2242013R30004, 2242014K40031), the Natural Science Foundation of
Jiangsu Province of China (BK2010553), the University Natural Science Grand Basic
Research Project of Jiangsu Province of China (Grant No. 10KJA430016), the
Innovation Scholars National ‘climbing’ Program of Jiangsu Province of China (Grant
No. SBK200910148), Jiangsu Province Postdoctoral Fund (Grant No. 1302096C) and
the Priority Academic Program Development of Jiangsu Higher Education Institutions
Fund.
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Figure captions
Fig. 1. Molecular structure and synthetic route of the Sm(L)3Phen•nH2O (L: (a) HTTA, (b)
p-FBA, (c) p-ABA) complexes.
Fig. 2. The FT-IR spectra of (a) Raw materials (SmCl3·6H2O, Phen, HTTA, p-FBA, p-ABA), (b)
Sm(L)3Phen•nH2O (L: HTTA, p-FBA, p-ABA) complexes before and after heat treatment at
280°C for 20 min.
Fig. 3. DSC/TG curves of (a) Sm(TTA)Phen, (b) [Sm(p-FBA) Phen]2 and (c)
Sm(p-ABA)3H2OPhen•H2O.
Fig. 4. Fluorescence excition (λem=647nm) and emission (λex=360nm) spectra of Sm(L)3Phen•
nH2O (L: (a), (b) HTTA; (c), (d) p-FBA; (e), (f) p-ABA) complex before and after 280 °C heat
treatment (holding 0 min, 10 min, 20 min).
Fig. 5. High temperature photoluminescence spectra of Sm(L)3Phen•nH2O complexes. (a)
Sm(TTA)3Phen (λex=380nm), (b) [Sm(FBA)3Phen]2 (λex=360nm), (c) Sm(p-ABA)3H2OPhen•H2O
(λex=360nm).
Fig. 6. Emission intensity as a function of testing temperature for Sm(L)3Phen•nH2O complexes
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Highlight:
ꢀ Designed classical complexes of Sm(L)3Phen•nH2O have been synthesized
by wet chemical method to compare their thermal and photoluminescent
stability.
ꢀ Substitutes on ligands play a crucial role on the stability of the complexes.
ꢀ p-fluorobenzoicacid works well as a superior thermal stability ligand because
of Sm3+ were bridged by four p-FBA during the synthesis process.
ꢀ [Sm(p-FBA)3Phen]2 complex can well serve as a thermally stable
down-converting phosphor.
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