Near-infrared luminescent erbium complexes with 8-hydroxyquinoline-terminated hyperbranched polyester

Article abstract of DOI:10.1016/j.polymer.2017.02.061

Novel 8-hydroxyquinoline-terminated hyperbranched polyesters (1-HBPQ and 2-HBPQ) were synthesized and characterized by ¹H-NMR and UV-Vis analyses. Then, two erbium complexes (1-HBPQ-Er³⁺-HQ and 2-HBPQ-Er³⁺-HQ) were prepare

Full text of DOI:10.1016/j.polymer.2017.02.061

Accepted Manuscript
Near-infrared luminescent erbium complexes with 8-hydroxyquinoline-terminated
hyperbranched polyester
Dan Liu, Chengjian Li, Yaling Xu, Dapeng Zhou, Hongmei Wang, Ping Sun,
Huasheng Jiang
PII:
S0032-3861(17)30185-4
10.1016/j.polymer.2017.02.061
JPOL 19462
DOI:
Reference:
To appear in: Polymer
Received Date: 5 October 2016
Revised Date: 26 January 2017
Accepted Date: 20 February 2017
Please cite this article as: Liu D, Li C, Xu Y, Zhou D, Wang H, Sun P, Jiang H, Near-infrared luminescent
erbium complexes with 8-hydroxyquinoline-terminated hyperbranched polyester, Polymer (2017), doi:
10.1016/j.polymer.2017.02.061.
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Near-infrared Luminescent Erbium Complexes with
8-Hydroxyquinoline-terminated
Polyester
Hyperbranched
Dan Liu , Chengjian Li, Yaling Xu, Dapeng Zhou, Hongmei Wang, Ping
Sun, Huasheng Jiang
College of Biological, Chemcial Sciences and Engineering, Jiaxing University,
Jiaxing 314001, P. R. China
Corresponding author.
E-mail address: liudan1029@126.com
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ABSTRACT
Novel 8-hydroxyquinoline-terminated hyperbranched polyesters (1-HBPQ and
1
2-HBPQ) were synthesized and characterized by H-NMR and UV-Vis analyses.
Then, two erbium complexes (1-HBPQ-Er3+-HQ and 2-HBPQ-Er3+-HQ) were
prepared by using 1-HBPQ or 2-HBPQ as the macromolecular ligand and
8-hydroxyquinoline (HQ) as a second ligand. The FTIR spectra and UV-Vis
absorption spectra confirmed that both the macromolecular ligand and the second
ligand were successfully coordinated with Er3+ ions. The erbium complexes emitted
intense near-infrared luminescence at about 1530nm, which corresponds to the
4I13/2-4I15/2 transition of Er3+ ions. Moreover, their full width at half maximum (FWHM)
values were greater than 80 nm. The erbium complexes exhibited good solubility in
conventional solvents and excellent near-infrared luminescence properties, indicating
their promising potential as gain medium materials in optical amplification devices.
Keywords: Near-infrared; Erbium complexes; Hyperbranched polyester;
8-Hydroxyquinoline; Luminescence
1 Introduction
Optical waveguide amplifier is an important device in the optical transmission
field, which can be used to amplify light signals [1, 2]. Gain medium is a key material
in optical waveguide amplifier device and has been widely investigated [3-5].
Luminescence materials containing trivalent erbium ion (Er3+) are regarded as
excellent gain medium materials due to their near-infrared luminescence emission
4
around 1550 nm attributed to the I13/2→4I15/2 transition of Er3+ ion exactly
corresponding to the low-loss transmission window of optical fibers (1500-1650 nm)
[6-10]. Thus, it is crucial to prepare luminescence materials with high Er3+ ion content
and strong absorption to obtain high gain. At present, Er3+ ions are usually doped
directly in inorganic matrixes (glass, oxides films) while fabricating amplifiers [11,
12]. In these Er3+-doped inorganic materials, Er3+ ion content is very low due to the
poor compatibility between Er3+ ions and the inorganic bases. Moreover, the 4f-4f
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transitions of the Er3+ ions are partly forbidden, leading to low absorption intensity of
Er3+-doped inorganic materials. Consequently, Er3+-doped inorganic gain materials
often show weak luminescence and low gain. A useful strategy to overcome these
drawbacks is to design and prepare suitable organic ligands to coordinate with Er3+
ions to obtain erbium complexes. In these complex systems, the organic ligands can
absorb energy efficiently and excite the Er3+ ions via an energy transfer from the
organic ligands to Er3+ ions, and then the complexes can emit strong luminescence of
Er3+ ions, which is known as the “antenna effect”.
8-Hydroxyquinoline (HQ) is a highly suitable chelating ligand for the Er3+ ions
as its good energy matches well with Er3+ ion, and the corresponding Er3+ complex
exhibits strong near-infrared emission [13-16]. However, the low-molecular
lanthanide complexes often display poor physicochemical stability, poor
processability and limited photostability. On the other hand, polymer materials are
ideal as organic ligands for lanthanide ions due to their many outstanding properties,
such as excellent thermal and chemical stability, versatility, processability, etc. [17,
18]. Thus, developing new polymer ligands covalently bonded with HQ and then
coordinating with Er3+ ions to achieve erbium complexes would resolve problems
associated with ErQ3 alone. Some attempts have been made to synthesize polymer
molecules bonded with HQ structure and investigate luminescence properties of their
corresponding lanthanide complexes in recent years. For example, Meyers et al. [19]
reported the preparation of poly(norbornene) and poly(cyclooctene) bearing
8-hydroxyquinoline side chains. The corresponding ytterbium complex showed the
characteristic luminescence of Yb3+ ions in the near-infrared region. In addition, Luo
et al. [20] synthesized a polymer ligand by copolymerization of a polymerizable HQ
derivative with N-vinylcarbazole (NVK), which was then coordinated with Er3+ ions
to form erbium complexes. The resulting erbium complexes displayed intense
near-infrared luminescence of the Er3+ ions when excited by UV-Vis light. However,
the reported polymer ligands reported containing HQ structures are usually based on
linear polymer backbones [19-22].
Different from linear polymers, hyperbranched polymers have globular
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molecular structures, which make them show excellent solubility and low melt
viscosities due to lack of restrictive interchain entanglements [23-27]. More
importantly, hyperbranched polymers usually have a multitude of highly reactive ends,
such as hydroxyl group and amino-group, which could be easily modified by
covalently attaching desirable functional groups. Therefore, our aim here is to attach
HQ functional groups to the ends of a hyperbranched polymer by chemical reactions
so as to produce a hyperbranched polymer ligand with a large number of HQ
end-groups, which are expected to chelate more Er3+ ions than traditional linear
polymers. Also, the new erbium complexes based on hyperbranched polymer are
expected to have intense near-infrared luminescence, as well as good solubility.
Among hyperbranched polymers, hyperbranched polyesters (HBP) are the most
extensively investigated type due to their ease of synthesis and large number of highly
reactive hydroxyl end-groups [28-31]. Here, We developed a new approach to prepare
8-hydroxyquinoline-terminated hyperbranched polyesters (1-HBPQ and 2-HBPQ) by
attaching HQ groups on the end branches of HBP by the reaction of sulphonyl
chloride groups of 8-hydroxyquinoline-5-sulphonyl chloride (HQ-SO2Cl) and
hydroxyl end groups of HBP. Furthermore, two novel erbium complexes
(1-HBPQ-Er3+-HQ or 2-HBPQ-Er3+-HQ) were prepared and characterized. The
near-infrared luminescence properties of these erbium complexes were further
investigated.
2 Experimental
2.1. Materials
8-Hydroxyquinoline (HQ) was obtained from Shenyang Chemical Reagent
Factory. Chlorosulfonic acid (ClSO3H), p-toluenesulfonic acid (p-TSA), pyridine,
N,N-dimethylformamide (DMF) N,N-dimethlyacetamide (DMAc), tetrahydrofuran
(THF), dimethyl sulfoxide (DMSO), acetone, anhydrous ether, ethyl acetate, and
ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd. 1,1,1-T ris
(hydroxymethyl) propane (TMP), 4,4-bis(4-hydroxyphenyl) valeric acid (DPA),
2,2-bis(hydroxymethyl) propionic acid (DMPA), tetra-n-butyl titanate (TBT), sussinic
anhydride and tin chloride were obtained from Aladdin Reagent Co.. Erbium nitrate
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hexahydrate (Er(NO3)3·6H2O) was purchased from Shandong Qingda Fine Chemical
Factory. All chemical reagents were of analytical reagent grade.
2.2. Measurements
Fourier transform infrared spectra (FTIR) were recorded on
a
Thermo Nicolet Nexus 470 FT-IR spectrophotometer in the 4000-400 cm-1 region
using KBr pellets at room temperature. Nuclear magnetic resonance spectra (1H-NMR
and 13C-NMR) were obtained in deuterated dimethyl sulfoxide (DMSO-d6) on a
Varian ADVENCE 400M spectrometer with tetramethylsilane (TMS) as internal
reference. The ultraviolet-visible (UV-Vis) absorption spectra were measured by a
Cary5000 spectrophotometer at room temperature. The Er3+ ion contents were
determined by a Leeman Prodigy inductively coupled plasma spectrometer (ICP).
Elemental analyses were performed on an Elementar Vario EL III elemental analyzer.
The fluorescence excitation and emission spectra were measured on a QM/TM steady
state and transient state fluorescence spectrometer. The concentrations of DMF
solution of the Er3+ complexes were 1×10-4 mol/L.
2.3. Synthesis procedure
2.3.1. Synthesis of 8-hydroxyquinoline-5-sulphonyl chloride (HQ-SO2Cl)
A mixture of HQ (0.4000 g, 2.76 mmol) and ClSO3H (5.00 ml, 75.22 mmol) was
heated to 45 °C under nitrogen atmosphere and stirred for 4 h. Then, the reaction
mixture was poured into ice water and a yellow precipitate was formed. The
precipitate was filtered off and dried at 80 °C for 12 h in the vacuum oven to obtain
HQ-SO2Cl as a pale yellow powder. IR (KBr) ν/cm-1 3404 (OH), 3081 (C-H), 1627,
1603, 1556, 1503, 1416 (8-HQ ring), 1384 (S=O, asym.), 1229 (C-O), 1191 (S=O,
1
sym.); H-NMR (DMSO-d6) δ 7.32 (d, 1H, 8-HQ), 8.05 (d, 1H, 8-HQ), 8.11 (q, 1H,
8-HQ), 9.06 (d, 1H, 8-HQ), 9.77 (d, 1H, 8-HQ), 12.09 (s, 1H, OH); 13C-NMR
(DMSO-d6) δ 113.88, 122.89, 126.54, 128.69, 130.08, 135.80, 144.63, 145.57,
149.60.
2.3.2. Synthesis of hydroxyl-terminated hyperbranched polyester (1-HBP and 2-HBP)
1-HBP was synthesized by the following procedure: A mixture of TMP (2.0000 g,
14.91 mmol), DMPA (17.9946 g, 134.16 mmol) and p-TSA (0.1000 g, 0.53 mmol)
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was heated to 140 °C under nitrogen atmosphere and stirred for 1 h. Then, the
reaction mixture was stirred continuously for 1 h under reduced pressure, and then
was purified by dissolution and precipitation with acetone and anhydrous ether. The
precipitate was filtered off, and dried at 80 °C for 12 h in the vacuum oven to obtain
1-HBP as a white powder.
2-HBP was synthesized by the following procedure: A mixture of TMP (2.0000 g,
14.91 mmol), DPA (12.8041 g, 44.72 mmol) and TBT (2.00 ml, 5.85 mmol) were
heated to 200 °C under nitrogen atmosphere and stirred for 5 h. Then, the reaction
mixture was stirred continuously for 2 h under reduced pressure, and then was
purified by dissolution and precipitation with THF and deionized water. The
precipitate was filtered off, and dried at 80 °C for 12 h in the vacuum oven to obtain
2-HBP as a yellow powder.
2.3.3. Synthesis of 8-hydroxyquinoline-terminated hyperbranched polyester (1-HBPQ
and 2-HBPQ)
The synthesis procedure for 1-HBPQ was as follows: 1-HBP (0.2000 g, 0.17
mmol) was dissolved in 6.00 ml dry pyridine, and then a solution of HQ-SO2Cl
(0.4960 g, 2.04 mmol) in 4.00 ml dry pyridine was added dropwise with stirring. The
resulting mixture was heated to 30 °C under nitrogen atmosphere and stirred for 16 h,
and then was precipitated in ethyl acetate. The precipitate was filtered off, and dried at
80 °C for 12 h in the vacuum oven to obtain 1-HBPQ as a yellow powder.
The synthesis procedure for 2-HBPQ was as follows: 0.2000 g 2-HBP (0.2000 g,
0.21 mmol) was dissolved in 8.00 ml dry pyridine, and then a solution of (0.3114 g,
1.28 mmol) HQ-SO2Cl in 4.00 ml dry pyridine was added dropwise with stirring. The
resulting mixture was heated to 30 °C under nitrogen atmosphere and stirred for 16 h,
and then was precipitated in anhydrous ether. The precipitate was filtered off, and
dried at 80 °C for 12 h in the vacuum oven to obtain 2-HBPQ as a yellow powder.
2.3.4. Synthesis of erbium complexes base on 8-hydroxyquinoline-terminated
hyperbranched polyester (1-HBPQ-Er3+-HQ and 2-HBPQ-Er3+-HQ)
The synthesis procedure for 1-HBPQ-Er3+-HQ was as follows: 1-HBPQ (0.2000
g, 0.05 mmol) was dissolved in 3.00 ml dry DMF, and then a solution of
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Er(NO3)3·6H2O (0.3021 g, 0.65 mmol) and HQ (0.2831 g, 1.95 mmol) in 2.00 ml dry
DMF was added under constant stirring. The reaction solution was stirred for 2 h at 80
°C, and then precipitated in ethyl acetate. The precipitate was filtered off, and dried at
80 °C for 12 h in the vacuum oven to obtain 1-HBPQ-Er3+-HQ as a yellow powder.
The synthesis procedure for 2-HBPQ-Er3+-HQ was as follows: 2-HBPQ (0.2000
g, 0.09 mmol) was dissolved in 3.00 ml dry DMF, and then a solution of
Er(NO3)3·6H2O (0.2537 g, 0.55 mmol) and HQ (0.2394 g, 1.65 mmol) in 2.00 ml dry
DMF was added under constant stirring. The reaction solution was stirred for 2 h at 80
°C, and then precipitated in ethyl acetate. The precipitate was filtered off, and dried at
80 °C for 12 h in the vacuum oven to 2-HBPQ-Er3+-HQ as a yellow powder.
2.3.5. Synthesis of other erbium complexes (1-HBPQ-Er3+, 2-HBPQ-Er3+ and
Er3+-HQ)
For comparison purposes, 1-HBPQ-Er3+, 2-HBPQ-Er3+ and Er3+-HQ were also
prepared. The synthesis procedure for 1-HBPQ-Er3+ was as follows: 1-HBPQ (0.2000
g, 0.05 mmol) was dissolved in 3.00 ml dry DMF. Then a solution of Er(NO3)3·6H2O
(0.3021 g, 0.65 mmol) in 2.00 ml dry DMF was added. The mixture was stirred for 2
h at 80 °C, and then precipitated in ethyl acetate. The precipitate was filtered off, and
dried at 80 °C for 12 h in the vacuum oven to obtain 1-HBPQ-Er3+ as a yellow
powder.
The synthesis procedure for 2-HBPQ-Er3+ was as follows: 2-HBPQ (0.2000 g,
0.09 mmol) was dissolved in 3.00 ml dry DMF. Then a solution of Er(NO3)3·6H2O
(0.2537 g, 0.55 mmol) in 2.00 ml dry DMF was added. The mixture was stirred for 2
h at 80 °C, and then precipitated in ethyl acetate. The precipitate was filtered off, and
dried at 80 °C for 12 h in the vacuum oven to obtain 2-HBPQ-Er3+ as a yellow
powder.
The synthesis procedure for Er3+-HQ was as follows: HQ (0.2831 g, 1.95 mmol)
was dissolved in 3.00 ml dry DMF. Then a solution of Er(NO3)3·6H2O (0.3021 g,
0.65 mmol) in 2.00 ml dry DMF was added. The reaction solution was stirred for 2
h at 80 °C, and then precipitated in ethanol. The precipitate was filtered off, and dried
at 80 °C for 12 h in the vacuum oven to obtain Er3+-HQ as a light yellow powder.
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3. Results and discussion
3.1. Design, synthesis and characterization of 8-hydroxyquinoline-terminated
hyperbranched polyester (1-HBPQ and 2-HBPQ)
In order to obtain new organic ligands with many HQ end-groups capable of
coordinating with Er3+ ions, two novel hyperbranched polyester ligands, 1-HBPQ and
2-HBPQ, have been designed and synthesized as shown in Scheme 1.
Scheme 1. Synthesis of 1-HBPQ and 2-HBPQ.
HQ-SO2Cl was prepared by chlorosulfonation reaction of HQ with ClSO3H as a
chlorosulfonation agent. The structure of HQ-SO2Cl was characterization by FTIR,
13
1H-NMR and C-NMR spectra, as shown in Fig. S1 to Fig. S3, which are consistent
with the reported literature [32]. It can be confirmed that HQ-SO2Cl is successfully
synthesized from HQ.
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Fig. 1. FTIR spectra of (a) TMP, (b) DMPA and (c) 1-HBP.
Fig. 1 displays the FTIR spectrum of 1-HBP, as well as the spectra of the two
reactants, TMP and DMPA. The absorption peak located at 1691 cm-1 corresponding
to the C=O stretching vibration of carboxyl group of DMPA was absent in the
spectrum of 1-HBP; while a new peak at 1727 cm-1 appeared, which belonged to the
C=O stretching vibration of ester group of 1-HBP. Fig. 2 presents the FTIR spectra of
TMP, DPA and 2-HBP, respectively. Similarly, the absorption peak at 1703 cm-1
attributed to the C=O stretching vibration of carboxyl group of DPA disappeared in
the spectrum of 2-HBP, while a new peak at 1712 cm-1 corresponding to the C=O
stretching vibration of eater group of 2-HBP was observed. Furthermore, the 1H-NMR
spectra of 1-HBP and 2-HBP and the peak assignments are given in Fig. 3a and Fig.
1
4a, respectively. The above FTIR and H-NMR results confirmed that 1-HBP and
2-HBP were successfully synthesized.
Then, 1-HBP and 2-HBP were reacted with HQ-SO2Cl to afford the desired
products, 1-HBPQ and 2-HBPQ, respectively. Fig. 3b and Fig. 4b exhibit the
1H-NMR spectra of 1-HBPQ and 2-HBPQ, respectively. In Fig. 3b, the peaks at 4.952
ppm and 4.639 ppm corresponding to the chemical shifts of terminal hydroxyl group
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proton of 1-HBP were absent; while new peaks between 7.1 ppm to 12.5 ppm
attributed to HQ structure appeared. The results demonstrated that the terminal
hydroxyl groups of 1-HBP reacted with the -SO2Cl groups of HQ-SO2Cl, and HQ
groups were successfully connected to the ends of 1-HBP molecules. The similar
phenomenon was also observed in Fig. 4b. The new peaks in the range of 7.1 ppm to
12.5 ppm corresponding to HQ structure appeared, while the peak at 9.203ppm
attributed to terminal hydroxyl group proton of 2-HBP disappeared, showing that
2-HBPQ was also successfully synthesized.
Fig. 2. FTIR spectra of (a) TMP, (b) DPA and (c) 2-HBP.
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Fig. 3. 1H-NMR spectra of (a) 1-HBP and (b) 1-HBPQ.
Fig. 4. 1H-NMR spectra of (a) 2-HBP and (b) 2-HBPQ.
It is speculated that the UV-Vis absorption behaviors of 1-HBP and 2-HBP would
change after attaching a large number of HQ groups with conjugated structures on the
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hyperbranched polyester molecular structures. Hence, the UV-Vis absorption spectra
of 1-HBPQ and 2-HBPQ were measured, as well as those of HQ, 1-HBP and 2-HBP.
The results were presented in Fig. 5 and Fig. 6. As shown in Fig. 5, 1-HBP exhibits an
absorption band from 200 nm to 270 nm due to the n→π* electronic transition of
carbonyl group of 1-HBP. HQ had a wide band from 200 nm to 400 nm
corresponding to the π→π* and n→π* electronic transitions of quinoline ring. Relative
to 1-HBP and HQ, the absorption band of 1-HBPQ became obviously wider and was
red-shifted. This could be attributed to the decrease in electron density around 1-HBP
due to the introduction of a large number of the conjugated quinoline rings and
the decrease of transition energy, which also indicates that the HQ structures had been
successfully attached onto 1-HBP. Similarly, as shown in Fig. 6, 2-HBPQ displays a
wider absorption band in comparison with 2-HBP because of the formation of a more
conjugated system by the introduction of HQ structures into 2-HBP molecule. The
1H-NMR spectra and the UV-Vis absorption spectrum results of 1-HBPQ and
2-HBPQ confirmed that HQ structures had been successfully attached onto 1-HBP
and 2-HBP.
Fig. 5. UV-Vis spectra of (a) 1-HBP, (b) HQ and (c) 1-HBPQ.
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Fig. 6. UV-Vis spectra of (a) 2-HBP, (b) HQ and (c) 2-HBPQ.
3.2. Synthesis and characterization of erbium complexes based on
8-hydroxyquinoline-terminated hyperbranched polyester (1-HBPQ-Er3+-HQ and
2-HBPQ-Er3+-HQ)
Scheme 2. Synthesis of 1-HBPQ-Er3+-HQ and 2-HBPQ-Er3+-HQ.
Two novel erbium complexes based on 8-hydroxyquinoline functional
hyperbranched polyester (1-HBPQ-Er3+-HQ and 2-HBPQ-Er3+-HQ) were prepared by
using 1-HBPQ and 2-HBPQ as the macromolecular ligands, respectively, and HQ as a
second ligand. The synthesis route of 1-HBPQ-Er3+-HQ and 2-HBPQ-Er3+-HQ is
given in Scheme 2. The Er3+ contents in the complexes prepared were measured by
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ICP, and were founded to be 13.91 wt% for 1-HBPQ-Er3+-HQ and 13.49 wt% for
2-HBPQ-Er3+-HQ, as shown in Table 1. The results suggested that the obtained
complexes have high Er3+ contents, which are much higher than the Er3+ complexes
based on the linear polymer with 8-hydroxyquinoline end-groups (lower than 1.6 wt%)
[20]. This can be due to the fact that the globular molecular structure ends of
hyperbranched polyesters have abundant 8-hydroxyquinoline groups, which can
chelate more Er3+ ions than traditional linear polymers. High Er3+ ion content is
benefit for the Er3+ complexes to yield intense near-infrared luminescence.
Table 1. ICPa and Elemental analysis dataa of 1-HBPQ-Er3+-HQ and
2-HBPQ-Er3+-HQ.
Complexes
Element
Found
1-HBPQ-Er3+-HQ
2-HBPQ-Er3+-HQ
C
H
N
Er
C
H
N
Er
45.13
2.80 8.58
13.91
48.49
3.02
8.19 13.49
a All the values are given in percentages.
Fig. 7. FTIR spectra of (a) HQ, (b) Er3+-HQ, (c) 2-HBPQ and (d)
2-HBPQ-Er3+-HQ.
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Fig. 8. UV-Vis spectra of (a) HQ, (b) Er3+-HQ, (c) 2-HBPQ, (d) 2-HBPQ-Er3+
and (e) 2-HBPQ-Er3+-HQ.
Fig. 9. UV-Vis spectra of (a) HQ, (b) Er3+-HQ, (c) 1-HBPQ, (d) 1-HBPQ-Er3+
and (e) 1-HBPQ-Er3+-HQ.
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In order to confirm the existence form of Er3+ ions in the complexes, the FTIR
spectra of the complexes were investigated. Fig. 7 shows the FTIR spectra of HQ,
Er3+-HQ, 2-HBPQ and 2-HBPQ-Er3+-HQ. Compared with the spectrum of HQ, a new
peak at 494 cm-1 attributed to the Er3+-O stretching vibration appeared in the spectrum
of Er3+-HQ. Moreover, the peak ascribed to the C-O stretching vibration of HQ
shifted to 1110cm-1 from 1093 cm-1. These results indicated that the O atom of HQ
coordinated with Er3+ ion [33, 34]. In addition, the peak at 740 cm-1 corresponding to
the Er3+-N stretching vibration appearing in the spectrum of Er3+-HQ suggested that
the N atom of HQ also coordinated with Er3+ ion [35]. These results verified that both
O and N atoms of HQ coordinated with Er3+ ion in the Er3+-HQ system. Similar to the
spectrum of Er3+-HQ, a new peak at 495 cm-1 attributed to the Er3+-O stretching
vibration was observed in the spectrum of 2-HBPQ-Er3+-HQ. Also, the relative
intensity of the peak at 742 cm-1 in the spectrum of 2-HBPQ-Er3+-HQ increased
significantly in comparison with that of 2-HBPQ, which might be due to the
formation of the Er3+-N bonds. The above results verified that the Er3+ ions in the
2-HBPQ-Er3+-HQ system were not simply doped in the organic ligands, but
coordinated with the O and N atoms of the ligands. However, it was difficult to
ascertain whether the O and N atoms coordinated with Er3+ ions were from the second
ligand HQ or the HQ groups of the macromolecular ligand 2-HBPQ. To further
investigate the coordinated environment of Er3+ ions in the 2-HBPQ-Er3+-HQ system,
the UV-Vis absorption spectrum of 2-HBPQ-Er3+-HQ was measured, as shown in Fig.
8. The UV-Vis absorption spectra of HQ, Er3+-HQ and 2-HBPQ-Er3+ are also
presented in Fig. 8 for comparison. Relative to HQ, the absorption band of Er3+-HQ
was obviously red-shifted due to the decrease electron density around HQ as a result
of the formation of the coordination bonds between Er3+ ion and HQ [36]. In addition,
two new weak peaks at 520 and 653 nm appeared in the absorption spectrum of
Er3+-HQ, which were attributed to the 4f-4f transitions of Er3+ ions corresponding to
4
the I15/2→2H11/2 and 4I15/2→4F9/2 transitions, respectively. The results suggested that
the absorption of Er3+ ions themselves was rather weak and the absorption of Er3+-HQ
was mainly from organic ligand HQ. Moreover, the absorption band of 2-HBPQ-Er3+
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was red-shifted in comparison with that of 2-HBPQ, which indicated that 2-HBPQ
coordinated successfully with Er3+ ion in the 2-HBPQ-Er3+ system. Compared with
2-HBPQ-Er3+, the absorption band of 2-HBPQ-Er3+-HQ was further red-shifted,
indicating that HQ coordinated with Er3+ ion in the 2-HBPQ-Er3+-HQ system. On the
other hand, 2-HBPQ-Er3+-HQ also showed a red-shifted absorption band in
comparison with that of Er3+-HQ, suggesting that 2-HBPQ was also involved in
coordination reaction with Er3+ ion in the 2-HBPQ-Er3+-HQ system. Therefore, it can
be concluded that both the HQ groups of 2-HBPQ and the second ligand HQ
coordinated with Er3+ ions in the 2-HBPQ-Er3+-HQ system. Fig. 9 presents the
UV-Vis spectra of 1-HBPQ-Er3+-HQ, Er3+-HQ, 1-HBPQ-Er3+, HQ and 1-HBPQ, from
which the similar conclusion could be drawn.
Based on the above results, it is evident that both the HQ groups of the
macromolecular ligands and the second ligand HQ coordinated with Er3+ ions in the
complexes. In order to ascertain the molecular structures of the complexes, the C, H
and N contents in the complexes were measured by elemental analysis, and the results
are listed in Table 1. Combining the results from FTIR, UV-Vis, ICP and elemental
analysis, the possible molecular formula of the complexes is depicted in Scheme 2.
The x/y values were calculated approximately and were close to 3:1 for
1-HBPQ-Er3+-HQ and close to 4:1 for 2-HBPQ-Er3+-HQ. The results indicated that
the HQ groups of the macromolecular ligands could not be fully coordinated with Er3+
ions due to steric hindrance [20].
Furthermore, the solubility properties of 1-HBPQ-Er3+-HQ and 2-HBPQ-Er3+-HQ
were investigated. As seen from Table 2, the coordination reactions of
8-hydroxyquinoline-terminated hyperbranched polyesters and Er3+ ions had some
influence on the solubilities of these polyesters in 7 conventional solvents.
1-HBPQ-Er3+-HQ and 2-HBPQ-Er3+-HQ displayed a slightly decreased solubility in
comparison with non-coordinated polyester 1-HBPQ and 2-HBPQ. The results could
be attributed to the increased rigidity of the polyester molecule due to the formation of
many chelate rings after coordinating with Er3+ ions, which led to the difficult
diffusion of solvent molecules in the polyester molecules. Nevertheless,
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1-HBPQ-Er3+-HQ and 2-HBPQ-Er3+-HQ could still easily dissolve in DMF, DMAc
and DMSO at room temperature, suggesting that the erbium complexes prepared have
good solubility. As is well known, the excellent solubility is very important for the
practical application of these luminescence materials. In addition, 1-HBPQ-Er3+-HQ
could also dissolve in THF and acetone on heating. The above results confirmed that
two new erbium complexes 1-HBPQ-Er3+-HQ and 2-HBPQ-Er3+-HQ with stable
coordination structures, high Er3+ ion contents and excellent solubilities were
successfully prepared.
Table 2 Solubilitiesa of 1-HBPQ-Er3+-HQ and 2-HBPQ-Er3+-HQ.
Samples
DMF DMAc DMSO THF Acetone Ethanol Water
1-HBPQ
＋
＋
＋
＋
＋
＋
＋
＋
＋
＋ ＋
＋－ ＋－
＋ －
－ －
－
－
－
－
－
－
－
－
1-HBPQ-Er3+-HQ ＋
2-HBPQ
＋
2-HBPQ-Er3+-HQ ＋
a
＋, soluble; －, insoluble; ＋－, soluble on heating.
3.3. Near-infrared luminescence properties of erbium complexes based on
8-hydroxyquinoline-terminated hyperbranched polyester (1-HBPQ-Er3+-HQ and
2-HBPQ-Er3+-HQ)
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Fig. 10. (a) Excitation and (b) emission spectra of 1-HBPQ-Er3+-HQ.
Fig. 10 presents the excitation and emission spectra of 1-HBPQ-Er3+-HQ. The
strong and broad excitation band centered at 403 nm in the excitation spectrum of
1-HBPQ-Er3+-HQ could be attributed to the absorption of ligands, while the weak and
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sharp peaks at 527, 654 and 987 nm are due to the 4f-4f transitions of Er3+ ions,
4
4
corresponding to the I15/2→2H11/2
,
4I15/2→4F9/2 and I15/2→4I11/2 transitions,
respectively. This result indicated the emission of 1-HBPQ-Er3+-HQ was mainly
sensitized by the absorption of ligands rather than that of Er3+ ions themselves. Under
excitation at 403nm, 1-HBPQ-Er3+-HQ exhibited a broad emission band centered at
4
1528 nm attributed to the I13/2→4I15/2 transition of Er3+ ion. This emission band
corresponds to the wavelength of the low-loss transmission window in silica
telecommunication fibers, which suggests that 1-HBPQ-Er3+-HQ would have the
potential applications in the optical amplification field. Moreover, it is worth pointing
out that the full width at half maximum (FWHM) of 1-HBPQ-Er3+-HQ was as wide as
85 nm, which is much wider than that of other reported Er3+-containing materials,
such as Er3+-doped tellurite glass (66 nm) [37] and polydentate hemispherand organic
cage complexes (70 nm) [38]. Such wide FWHM could enable a wide gain bandwidth
for optical amplification.
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Fig. 11. (a) Excitation and (b) emission spectra of 2-HBPQ-Er3+-HQ.
The excitation and emission spectra of 2-HBPQ-Er3+-HQ are shown in Fig. 11.
The maximum excitation peak of 2-HBPQ-Er3+-HQ located at 428 nm corresponding
to the absorption of the ligands, and three other small peaks at 522, 653 and 984 nm
were attributed to the 4f-4f transitions of Er3+ ions, which were very similar to the
excitation spectrum of 1-HBPQ-Er3+-HQ. Under excitation at 428 nm,
2-HBPQ-Er3+-HQ emitted the characteristic luminescence of Er3+ ion centered at
1533 nm. It could be observed that 2-HBPQ-Er3+-HQ showed much higher
luminescence intensity than 1-HBPQ-Er3+-HQ, which could be because 2-HBPQ has
more rigid polyester backbone structures than 1-HBPQ, as shown in Scheme 1,
resulting in the more stable coordination structure of 2-HBPQ-Er3+-HQ. This was
beneficial for the enhancement of the emission intensity of Er3+ ions. Besides, the
FWHM of 2-HBPQ-Er3+-HQ was 88 nm, which was also wider than that of
1-HBPQ-Er3+-HQ. Therefore, 2-HBPQ-Er3+-HQ with strong emission at around 1530
nm and wide FWHM is very promising as a new optical amplification luminescence
material.
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Fig. 12. Emission spectra of (a) 2-HBPQ-Er3+-HQ, (b) Er3+-HQ and (c)
2-HBPQ-Er3+.
In order to further investigate the effects of the macromolecular ligand and second
ligand on the luminescence emission of 2-HBPQ-Er3+-HQ, the emission spectra of
Er3+-HQ and 2-HBPQ-Er3+ were measured, as presented in Fig. 12. Obviously, the
emission intensity of 2-HBPQ-Er3+-HQ was much higher compared to Er3+-HQ and
2-HBPQ-Er3+. This could be due to the fact that the simultaneous coordination of both
2-HBPQ and HQ with Er3+ ions could not only meet the coordination number of Er3+
ion but also form a more stable coordination structure. This synergistic effect of
2-HBPQ and HQ was beneficial for the decrease in the non-radiative transition of Er3+
ions and the increase in the emission intensity of the complex, thus resulting in the
intense near-infrared luminescence of 2-HBPQ-Er3+-HQ. Therefore, a kind of novel
Er3+ complexes base on 8-hydroxyquinoline-terminated hyperbranched polyester was
successfully prepared, which had high Er3+ contents and showed quite strong
near-infrared luminescence.
4. Conclusions
Two new hyperbranched polymer ligands containing HQ end-groups 1-HBPQ
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and 2-HBPQ were designed and synthesized. Furthermore, the corresponding erbium
complexes 1-HBPQ-Er3+-HQ and 2-HBPQ-Er3+-HQ were successfully prepared. In
these complex systems, both the macromolecular ligand 1-HBPQ or 2-HBPQ and the
second ligand HQ participated in the coordination reaction with Er3+ ions. Due to the
synergistic effects of the two ligands, the as-prepared erbium complexes showed
intense near-infrared luminescence at around 1530 nm corresponding to the
4I13/2→4I15/2 transition of Er3+ ion. The FWHM values of 1-HBPQ-Er3+-HQ and
2-HBPQ-Er3+-HQ were as wide as 85 nm and 88 nm, respectively. These erbium
complexes with excellent near-infrared luminescence properties are promising
materials for optical amplification applications.
Acknowledgements
This work was supported by the National Natural Science Foundation of China
(21306062, and 51402126); Zhejiang Provincial Natural Science Foundation of China
(LQ13E030007, and LY16E050010); Program for Science and Technology of Jiaxing
(2014AY21013); and National Training Programs of Innovation and Entrepreneurship
for Undergraduates (201510354010).
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ò Two novel 8-hydroxyquinoline-terminated hyperbranched polyesters
were synthesized.
ò Erbium complexes based on modified hyperbranched polyesters were
prepared.
ò Erbium complexes show intense near-infrared luminescence and wide
FWHM.