Eu(Ш)-based Coordination Polymers: Ligand-Dependent Morphology and Photoluminescence Properties

Research Article 

Eu(Ш)-based Coordination Polymers: Ligand-Dependent Morphology and Photoluminescence Properties

Corresponding author:  Dr. Shengliang Zhong, College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, Jiangxi 330022, The People’s Republic of China, Tel: 86-791-88120386,
Fax: 86-791-88120386; E-mail: slzhong@jxnu.edu.cn
Abstract
A facile template-free solvothermal strategy has been developed to synthesize Eu(III)-based coordination polymer (CP) hollow spheres, using N, N-dimethylformamide (DMF) as a solvent and 2-aminoterephthalic acid (H2ATPA) as a ligand. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDX), elemental analyses (EA), Fourier transform infrared (FT-IR), Brunauer− Emmett−Teller (BET), and thermogravimetric analysis (TGA). SEM and TEM results show that the Eu-ATPA hollow microspheres have diameters of 200–400 nm and a thickness of about 15–30 nm. Besides, the photoluminescence (PL) properties of the Eu-ATPA hollow microspheres have been investigated in 100-300 K. Results show that the luminescence intensity of Eu-ATPA hollow microspheres decreases dramatically with the increase of temperature between 100 K and 300 K. To investigate the ligand effect on luminescence properties and morphology of the products, a series of Eu-based CPs built from 1,4-benzenedicarboxylate (BDC) and its derivatives have been fabricated. The results demonstrate that the morphologies of the products are sharply different from each other, indicating that their morphologies are ligand-dependent. The emission spectra, quantum yields, and the fluorescence decay curves reveal that the product has the strongest fluorescence intensity when BDC was used as the ligand. Moreover, the luminescence intensity of the products built from electron-withdrawing groups substituted BDC is stronger than those built from electron-donating groups substituted BDC. The order of the fluorescence lifetime and measured photoluminescence quantum yield (PLQY) of the samples is fundamentally the same as the order of the fluorescence intensity.

Keywords: Coordination polymer; Hollow spheres; Ligands effect; Luminescence

Introduction
Micro/nanostructured CPs have demonstrated their superiorities in magnetic resonance imaging, gas adsorption and storage, computed tomography, biosensing, drug delivery, and membranes [1]. The morphologies of micro/nanostructured CPs reported including zero-dimensional (0D) nanosphere [2-4], one-dimensional (1D) nanotubes, nanorods, nanofibers, nanowires [5-9] and two-dimensional (2D) nanoplates, nanobelt, nanofilm [10-13]. Among them, hollow sphere has been of continuous research interests in material science, physics, chemistry and biology due to their high surface-to-volume ratio, low density, high loading capacity, and high permeability for gasses and liquids [14]. In the past several years, many approaches have been developed for the construction of hollow microspheres, such as hard or soft template method, self-template method, Ostwald ripening, galvanic replacement, Kirkendall effect, and reduction of metal oxides[15]. Recently, our group has successfully prepared terbium-based CP hollow microspheres with white-light emission [16].

The luminescence of lanthanide CPs has unique properties with high color pureness, sharp emission profiles, large stokes shifts, and long luminescence lifetimes making them particularly valuable in many potential applications such as optical communications, biomedical sensors, fluoroimmunoassay light-emitting devices, and so on. However, the luminescence properties of rare-earth ions were forbidden f–f transitions by parity selection rules. Therefore, many research groups selected organic ligands coordination to metal ions to circumvent this shortcoming. For instance, Li and his co-workers have synthesized two novel binary and ternary Eu (III) perchlorate complexes, and the fluorescent lifetime and the PLQY of the Eu (III) ternary and binary complexes. It turned out that the ternary complex getting longer lifetime properly was helpful in increasing the fluorescence intensity than the binary system [17].

It is well known that luminescence is temperature-dependent. The temperature-dependent luminescence of inorganic compounds has been intensively studied in the past. For instance, Meng et al. have investigated the luminescence properties of Ba3BP3O12 doped with divalent Eu2+ in 10–525 K. The results show that the luminescence intensity of Eu II (λem= 535 nm) does not decrease drastically in 10–300 K, while that of Eu I (λem=425 nm) decreases dramatically and Ba2.99Eu0.01BP3O12 sample has poor thermal stability at temperature higher than 300 K [18]. Chen et al. have successfully synthesized Ca14Al10Zn 6O35: Dy3+ and Ca14Al10Zn6O35: Dy3+/Mn4+ phosphors through a conventional solid-state reaction and have studied the temperature- dependent luminescence of Ca14Al10Zn6O35 from 303 to 463 K. The result shows that the phosphors have good resistibility to thermal quenching, and the temperature could cause the opposite change in the emission intensity of Stokes and anti-Stokes sidebands of Mn4+ [19].

To date, few studies have reported about ligands effect on fluorescence properties and the morphology of CPs[20]. It is well known that H2ATPA is a dipotic linear aromatic carboxylate molecule, and the steric effect of the uncoordinated amino group may affect the hydrophily or electrostatics of frameworks and therefore affect their luminescence behaviors, which led to it be antenna ligand for preparing stable lanthanide architectures. In addition, BDC has the equally spaced carboxylate groups, the rigidity of the phenyl skeleton and especially its various bridging abilities, it becomes one of the best spacers for the construction of MOFs. Thus, many CPs built from H2ATPA and BDC were reported [21-26]. In this work, we report a one-pot template-free, facile and low-cost solvothermal method for preparing Eu-based CP hollow microspheres containing Eu(III) and H2ATPA. The formation of hollow structures is unusual because no surfactants or templates are intentionally used during this synthesis process. A self-assembly process coupled with an Ostwald ripening process for the hollow structures formation was proposed [27]. The luminescence properties of Eu-ATPA CP were investigated in 100-300 K. In order to study ligands effect on fluorescence properties, a series of CPs micro/nanostructures made from Eu(III) and BDC and its derivatives were prepared. Besides, the ligand effect on the morphology of the Eu-ATPA CPs was studied, which may provide a way to tune the morphologies of other CPs materials.

Experimental section


Materials and synthesis

All chemicals used in this study are of analytical grade and used without further purification. In a typical process for the preparation of sample A: 0.1 mmol H2ATPA (ligand A) was dissolved in 5 ml DMF under stirring in a 40 ml Teflon-lined autoclave, and 0.1 mmol Eu(NO3)3.6H2O was dissolved in 20 ml DMF in a beaker with vigorous stirring. After being stirred for 10 min, the Eu(NO3)3 solution was poured into the H2ATPA solution and the mixture was stirred for another 10 min. The autoclave was sealed and maintained at 160 °C for 6 h in an oven. After that, the autoclave cooled to room temperature naturally. The precipitates were separated by centrifugation and washed several times with deionized water and absolute ethanol to remove impurities. Finally, the products were dried in a vacuum oven at 80 °C for further characterization.

Characterization

XRD was carried out using a Rigaku X-ray diffractometer with Cu-Kα radiation (λ = 1.54178 Å). SEM photos were taken on a scanning electron microscope coupled with energy dispersive X-ray spectroscopy (EDX) (Hitachi S-3400). TEM was performed on a JEOL-2010 TEM with an accelerating voltage of 200 kV. EA (C, H, and O) were performed on an EA 3000 elemental analyzer. FT-IR spectra were measured on a Perkin- Elmer SP one FT-IR spectrometer using KBr disks, scanning from 4000 to 400 cm−1 under ambient conditions. The nitrogen adsorption and desorption isotherms were measured on a BELSORP-mini II apparatus. BET surface area was derived from the desorption branches of the isotherm with the Barrett-Joyner-Halenda (BJH) model. TGA was carried out at a constant heating rate of 10 °C min-1 in air from room temperature to 1000 °C using a Diamond TG/DTA simultaneous thermal analyzer. PL excitation and emission spectra were recorded on an Edinburgh FLS-980 instrument in 550–750 nm. Lifetime measurements were carried out at room temperature the same apparatus equipped with a 450 W Xe lamp and a μF2 microsecond flash lamp as the excitation sources. PLQY of the samples (under optimal excitation wavelength) were measured using a Hamamatsu absolute PL quantum yield spectrometer C11347.

Materials and synthesis of the other Eu-CPs with various ligands

Sample B-F were made from Eu(III) and BDC (ligand B) and its derivatives such as a 2-bromoterephthalic acid (ligand C), 2,5- dichloro terephthalic acid (ligand D), 2-nitroterephthalic acid (ligand E), 2-dihydroxyterephthalic acid (ligand F), respectively. And they were prepared by similar procedures with sample A just by replacing ligand A with another ligand, while the other reaction parameters unchanged. The detailed molecular structure formula for different ligands is shown in Scheme 1.

Scheme 1. Molecular structure formula for the different ligands.

Results and Discussion

Characterization of the Eu-ATPA CP hollow microspheres

Figure 1. (a, b) SEM images and (c, d) TEM images of sample A (the scale bar of insets in d is 50 nm).

The morphology of Eu-ATPA CP was studied by SEM and TEM. Figure 1(a) and (b) show the SEM images of Eu-ATPA CP. It can be seen that a lot of spheres with a size distribution of 200–400 nm in diameter can be observed in Eu-ATPA CP. Furthermore, some of the spheres are broken, disclosing their hollow structure. By close observation, TEM investigation was performed. Fig. 1(c) and (d) show the TEM images of the microspheres. Each hollow nanosphere is composed of numerous sub-20 nm nanoparticles (Fig. 1d, inset). The obvious contrast between the dark edge and the pale center of the spheres confirms their hollow nature or internal porous structure. The edge thickness of the hollow spheres is about 15–30 nm [16].

Figure 2. XRD pattern of sample A.

Figure 3. FT-IR spectra of (a) sample A and (b) H2ATPA.

Figure. 2 shows the powder XRD pattern of the final product. It can be seen that the sample A prepared under the moderate reaction conditions is amorphous material but the typical infinite coordination polymer particles (ICPs) [28-30]. In coordination chemistry, the formation of CP from metal ions and bifunctional building blocks can be conveniently characterized by FT-IR spectroscopy. The FTIR spectrum of sample A (Fig. 3(a)) confirmed that the complete deprotonation of the carboxyl groups of H2ATPA coordinated to Eu3+ ions, suggesting by the disappearance of the band at about 1700 cm-1, which is the characteristic peak of protonated carboxylic groups (as observed in curve b). The broad band at about 3410 cm-1 indicates the presence of water molecules, as seen in Fig. 3(b). The primary aromatic amino group displays two medium absorptions, at 3507 and 3386 cm-1. In addition, the presence of DMF molecules is showed by the weak absorption bands at 2919 and 2850 cm-1, corresponding to the aliphatic stretching vibrations of C-H groups. This may due to that some DMF solvents are residual in the product [31-33].

Figure 4. TG curves of sample A.

Figure 5. XRD patterns of the cubic-phase Eu2O3 hollow spheres.

TG test under the atmosphere was performed to investigate the composition of Eu-ATPA CP. As shown in Fig. 4, there is two rapid weight loss in 28-900°C. The initial weight loss (about 11.14%) in 28-169°C may be attributed to the loss of uncoordinated water molecules plus adsorbed water molecules. The second weight loss (about 49.13%) in 169-800°C corresponds to the combustion of the organic ligand and subsequent formation the cubic phase of Eu2O3. No additional XRD peaks found, indicating the formation of a purely cubic Eu2O3 phase ( Fig. 5) [34, 35].

Figure 6. EDX spectrum of sample A.

To determine the composition of sample A, the chemical composition was further investigated by EDX (Fig. 6) and elemental
analysis. The peaks for Eu, O, N and C can be easily found from EDX spectrum, which is in agreement with the composition analysis. Elemental analyses gave the contents of C, N, and H as 27.793%, 3.628%, and 3.580%, respectively. The molar ratio of metal to the ligand in the complex is calculated to be 1:1.5. On the basis of above analysis, the product is proposed to be Eu(APTA)1.5(H2O)3.

Figure 7. a) Nitrogen adsorption–desorption isotherms of sample A measured at 77 K. b) Nitrogen adsorption–desorption isotherms and TEM images of Eu2O3 obtained after calcination of sample A. c) SEM images and d) TEM images of Eu2O3, respectively.

Figure 8. SEM photos of the products prepared at 160°C for 6 h with different ligands: (a, b) sample B; (c, d) sample C; (e, f) sample D; (a, b) sample E and (c, d) sample F.

The nitrogen adsorption−desorption isotherms of sample A and Eu2O3 obtained after calcination of sample A in air at800°C for 4 h are shown in Fig. 7( a, b), respectively. BET surface area sample A were calculated to be 27.337 m2 g−1 but decreased to 9.8594 m2 g−1 in the purely cubic Eu2O3 phase. The decreased surface area (Eu2O3 vs sample A) is consistent with the hollow structure has collapsed and sintered observed in the TEM images (Fig. 7d) [36]. Combined with the result in Fig.7c, obviously, Eu2O3 obtained after calcination of sample A, it consists of uniform nanosphere with an average diameter of around 200 nm, and nearly half of the sphere were broken. TEM image shows that these nanospheres are constructed by many small nanoparticles (smaller than 45 nm). Compared to the constituent particle of sample A, the constituent particle of Eu2O3 is much larger, this may also be one of the reasons why the BET surface area decreased after calcination.

Ligands effect on morphology and photoluminescence of a series of Eu-based CPs

The morphology and size of the Eu-based CPs were examined using SEM. As shown in Fig. 8, sample C, E and F are microspheres, but not so uniform as sample A, especially for sample C and E. Besides, the morphologies of sample B and D are very different form the morphology of sample A. Obviously, sample D is straw-like sheaves particles and sample B is block-like particles. The results demonstrate that the products are spherical when BDC has only one substituent. On the contrary, when there is no substituent or more substituents on BDC, the morphologies of the products are not spherical. It is not difficult to conclude that the structure of ligand plays a crucial role in the morphologies of the final products. The above results, indicating that we can tune the morphologies of other CPs materials via a similar method.

Figure 9. PL spectra of the Eu-ATPA CP microspheres. (a) excitation spectrum; and (b) emission spectrum.

In order to explore the luminescent properties of these rare earth CPs, the luminescence of Eu-ATPA was determined at the
excitation of 394 nm under ambient temperature. As shown in Fig. 9, Sample A exhibits characteristic transitions of the Eu- (III) ion at 579, 591, 615, 651, and 699 nm, which correspond to 5D0→7F0, 5D0→7F1, 5D0→7F2, 5D0→7F3, 5D0→7F4 transitions, respectively [37]

Figure 10. The emission spectra (λex = 394 nm) of the as-prepared Eu-based CPs.

The spectrum is dominated by the intense 5D0→7F2 electric dipole transition, which is split into two peaks at 612 and 615nm. The relative intensity ratio (R) of 5D0→7F2/5D0→7F1 transition was 4.51,

Figure 11. The excitation spectra (the inset indicates the corresponding emission wavelength) of the as-prepared Eu-based CPs.

Figure 12. The emission spectra (the inset indicates the corresponding excitation wavelength) of the as-prepared Eu-based CPs.

which was much greater than 0.67, a typical value for a centrosymmetric Eu(III) center. This high ratio, therefore, signifiesthat the Eu(III) ion adopted an asymmetric coordination environment [38, 39].

The intensity of the intramolecular energy transfer between the triple level of the ligands and the emitting level of europium ions is closely associated with the luminescence properties of Eu-based CPs [40]. And the specific energy pathway in CPs is a tricky process, the excitation may occur through the ligand states followed by intersystem crossing, energy transfer from ligand to lanthanide(III) excited levels, the emission occurs through the lanthanide f–f transitions, on the basis of the traditional energy transfer from triplet states to lanthanide [41]. The room temperature solid-state emission spectra (λex = 394 nm) of the as-prepared Eu-based CPs are shown in Fig. 10. Among the five Eu-based CPs, it is clearly observed that the luminescence intensities of sample B is much stronger than that of other Eu-based CPs, indicating that all the BDC ligands with different functional groups on benzene ring would lead to luminescence intensity decrease, which may be caused by increasing the resistance of C=O group which coordination to the Eu(III) ions. When an electrophilic group (-OH) is present, sample F, the fluorescence intensity of is the weakest. It is well known that the presence of -OH would greatly quench the luminescence intensity [42]. Whereas, the fluorescence intensity of sample C and D are relatively stronger in above samples except for sample B, which is attributed to the presence of electron-withdrawing ─ groups of -Br and -Cl. In order to investigate whether the order of luminescent intensity would change, the excitation spectra and their emission spectra obtained under optimal excitation wavelength were provided (Fig. 11 and Fig. 12). It can be seen that the luminescent intensity order are still in the order when excited with 394 nm, except that the intensities of sample C and sample D became somewhat strengthened. Based on above results, it is clearly observed that -H on the benzene ring of BDC substituted by the electron-withdrawing group have stronger luminescence intensities, this may be due to the electron-withdrawing groups have lowered the triplet level of the ligands, then the energy of the ligands triplet level transfer to the resonance level of Eu- (III) more easily.

Figure 13. Luminescence decay curves and the corresponding fittingcurves of all samples emission excited by 394 nm (the inset indicates the corresponding emission wavelength): (a) sample A; (b) sample B; (c) sample C; (d) sample D; (e) sample E; (f) sample F.

Table 1. The luminescence lifetime data of sample A-F.

Figure 14. PL spectra of Eu-ATPA CP (λex = 394 nm) for various temperatures
(100–300 K).

Whereas, the introduction of electron-withdrawing groups leads to the opposite. This is probably caused by increasing ofthe triplet energy level, and thus the triplet energy level is far from the lowest excited state level of Eu(III). Our results arethe same with those reported by Yan’s group [43]. In addition, The PLQY record for sample A-F (under optimal excitationwavelength) are 0.3%, 25.8%, 20.7%, 20.8%, 0.7%, 0.4%, respectively.

Moreover, ligand effect on fluorescence decay of the CPs is investigated. The luminescence data of samples A-F are show in Table 1. Luminescence decay of Eu-CPs powder in the solid state at λex = 394 nm can be described as a bi-exponentialfunction with the contributions of the τ1 and τ2 components, respectively. The fluorescence decay of sample B in this article was fitted to the bi-exponential profile (λex = 394 nm), and the fast component τ2 (518.96 μs) had an amplitude of about 17.30%, whereas the contribution of the τ1 ( 926.96 μs) component was about 82.70%. The third lifetime could not be determined, which may be due to that it had an almost negligible contribution corresponding to the decay time. The fluorescence decay for sample A and sample C-F are also revealed a bi-exponential profile and the main component from 677.96 μs to 17.22 μs. For the decay, traces displayed in Fig. 13, it was clear that the fluorescence lifetime of sample B is the longest,the fluorescence lifetimes of sample A and sample F are the shortest. Besides, it is not hard to find that the organic ligands with diffierent functional groups all have different degrees of shortening and the order of the fluorescence lifetimes are fundamentally similar with the order of the fluorescence intensity [44].

Furthermore, the luminescence properties of Eu-ATPA CP were investigated in 100 to 300 K for investigating the influence of temperature on the fluorescence intensity, as summarized in Fig. 14. At 100 K, the characteristics sharp bands are the strongest. However, by elevating temperature, the intensity weakens monotonously due to the temperature quenching [19, 45]. In other words, lower temperature lead to fluorescence performance of Eu-ATPA CP enhanced. Due to the 5D0→7F1 transition is allowed by magnetic dipole and its intensity is independent of the environment, it often used as a reference. The relative intensity ratio (R) of 5D0→7F2 to 5D0→7F1 transition values for 100K, 150 K, 200 K, 250 K and 300 K are 6.16, 5.91, 5.30, 3.75 and 2.27, respectively. The values of R ratio did show dramatically decreasing with the elevating of temperature, indicating that the coordination environment of the Eu3+ ion was getting closer to centrosymmetric when the temperature is increased gradually. However, it was different from what observed by Hao et al. They had synthesized a europium complex and incorporated it in poly(methyl methacrylate), The Eu3+ is protected by the high bulkiness ligands sheath around the Eu3+ during the increase of temperature [46].

Conclusions

In summary, a facile one-pot, template-free, and low-cost solvothermal method for preparation of Eu -based CP hollowmicrospheres was developed. The SEM images of Eu-ATPA indicate that the uniform hollow microspheres have diameters of 200–400 nm and the TEM images show edge thickness of the hollow spheres is about 15–30 nm. The micro/nanostructuresmade from Eu(III) and BDC as well as its derivatives have different morphology and luminescence properties. It turns out that the fluorescence intensity of the CPs built from electron- withdrawing groups are stronger than those built fromelectron-donating groups. And the order of the PLQY and the emission lifetimes were fundamentally the same as the order of the fluorescence intensity. Besides, the temperature-dependent fluorescence of Eu-ATPA CP indicates that lower temperature can enhance fluorescence emission intensity of Eu-ATPA CP.

Acknowledgements

S. L. Zhong acknowledges the supporting projects from the National Natural Science Foundation of China (No. 21261010), Jiangxi Provincial Education Department (No. KJLD13021) and Jiangxi Provincial Department of Science and Technology (No.
20144BCB23039, 20151BDH80049).

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