- Email: [email protected]
A series of luminescent Lnlll-based coordination polymers: Syntheses, structures and luminescent properties
Journal Pre-proofs Research paper A series of Luminescent Lnlll-based coordination polymers: syntheses, structures and luminescent properties Xiaoqing Zhao, Fenhang Zhang, Yajun Liu, Tianhao Zhao, Hongyan Zhao, Shuo Xiang, Yunchun Li PII: DOI: Reference:
S0020-1693(19)31810-9 https://doi.org/10.1016/j.ica.2020.119459 ICA 119459
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
23 November 2019 17 January 2020 17 January 2020
Please cite this article as: X. Zhao, F. Zhang, Y. Liu, T. Zhao, H. Zhao, S. Xiang, Y. Li, A series of Luminescent Lnlll-based coordination polymers: syntheses, structures and luminescent properties, Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.119459
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
A series of Luminescent Lnlll-based coordination polymers: syntheses, structures and luminescent properties Xiaoqing Zhao,a,b†* Fenhang Zhang,a† Yajun Liu,a Tianhao Zhao,a Hongyan Zhao,a Shuo Xiang,a Yunchun Lia Abstract A series of lanthanide coordination polymers (Ln-CPs), with the formula of {[Ln(L)3(CH3OH)(H2O)]n} (Ln = Sm (1), Eu (2), HL = 2-chloroisonicotinic acid) and {[Ln(L)3(H2O)2]n} (Ln =Dy (3), Tb (4)), were synthesized under solvothermal condition. This series display two different 1D chains due to the lanthanide contraction: complexes 1 and 2, based on light Sm and Eu, respectively, consist of paddle-wheel [Ln2(COO)4] dimmers through double μ2-COO bridges, and complexes 3 and 4 with heavy Dy and Tb, respectively, show 1D [Ln(COO)2]n chains. The luminescent measurements indicate the typical emissions of corresponding Ln3+ ions in complexes 1-4. Furthermore, complexes 2 and 4 display significant luminescent response for Fe3+ ions with high quenching constant. The result indicates that complexes 2 and 4 can act as luminescent probes for Fe3+ ions with relatively high sensitivity and selectivity. Keywords: Lanthanide coordination polymers; 2-chloroisonicotinic acid; luminescent probes
1. Introduction As an important class of hybrid materials, lanthanide coordination polymers (Ln-CPs) have received extensive attention due to their potential ability as functional materials, combining their structural features with exceptional optical and/or magnetic properties arising from the unique 4f configuration of Ln3+ ions [1-3]. Among the huge amount of Ln-CPs, there are special attention to luminescent Ln-CPs, which have been considered as attractive luminescent materials in tunable lasers,
a b
College of Science, Sichuan Agricultural University, Ya’an 625014, China. E-mail: [email protected]. Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin
300071, China †
These authors contribute equally. 1
low-energy scintillators, optical storage and chemical sensing, etc
[4-7]
. Because
Ln-CPs always have the advantageous luminescent properties, such as long lifetimes, narrow line-like emission bands and excellent resistance to photo-bleaching. It is well known that direct excitation of Ln3+ ions is usually inefficient, due to the spin- and parity-forbidden nature of the f-f transitions [8-10]. To address this challenge, one strategy is the so-called “antenna effect”, which apply the organic ligands as chromophore to absorb incident photons and transfer energy to the Ln3+ ions, resulting in the significant luminescence of Ln-complexes [11-13]. As a consequence, a lot of organic ligands, i.e. pyridine derivatives, β-diketonate, tetrazolate, aromatic carboxylate, pyrazolonate, etc
[14-17]
, have been developed to construct novel
luminescent Ln-complexes. In addition, the high-energy vibrations groups in the organic ligands, such as C–H and O–H bonds, may significantly decrease the metal excited states energy, resulting in shorter luminescence lifetimes, low luminescent intensities and quantum yields [18-21]. Hence, organic ligands with the low-vibrational groups (eg. C–Cl, C–F, C–Br) may make contribution in synthesizing significant luminescent Ln-complexes
[20-23]
. Thereinto, 2-chloroisonicotinic acid (HL) is a rigid
chlorinated N, O-containing ligand and may provide favorable coordination sites to construct luminescent Ln-CPs. Nevertheless, as we known, there is no Ln-complexes associated with HL reported until now. Moreover, the luminescent Ln-complexes as chemical sensors have been concerned, particularly for the recognition of metal ions, because most of them pose a health and environment threats [24-26]. These promising application encourages researchers to design and synthesize luminescent Ln-complexes for metal ions recognition, for example, the 3D Tb-MOF exhibits significant luminescent selectivity for Fe3+ ion [26]. Based on the above considerations, we herein selected HL as ligand to synthesize luminescent
Ln-complexes,
and
finally,
a
series
of
Ln-CPs,
namely,
{[Ln(L)3(CH3OH)(H2O)]n} (Ln = Sm (1), Eu (2)) and {[Ln(L)3(H2O)2]n} (Ln =Dy (3), Tb (4)), were successfully obtained. All complexes exhibit two types of 1D chains due to the lanthanide contraction. Complexes 1 and 2 with light Sm and Eu exhibit a 1D [Ln(COO)2Ln(COO)4]n chain, while complexes 3 and 4 with heavy Dy and Tb are a 1D 2
[Ln(COO)2]n chain. The luminescent properties indicate that complexes 1-4 exhibit emission characteristics of correlative Ln3+ ions. Furthermore, complexes 2 and 4 exhibit significant luminescence response to Fe3+ ions, suggesting that both complexes should be potential luminescent probes for Fe3+ ions.
2. Experimental section 2.1. Materials and apparatus All chemicals and solvents were commercially purchased and used without further purification. The single-crystal X-ray diffraction data of complexes 1-3 were collected by employing an Agilent Technologies SuperNova single crystal diffractometer (America) equipped with graphite-monochromatic Mo/Ka radiation (λ = 0.71073 Å) at 120 K. Elemental analyses for C, H and N were carried out on a Perkin–Elmer analyzer at the Institute of Elemento–Organic Chemistry, Nankai University. The FT-IR spectra were recorded from KBr pellets in the range 400-4000 cm-1 on a Bio-Rad FTS 135 spectrometer. Powder X-ray diffraction (PXRD) data were acquired using a D/Max-RA diffractometer (DX-2600, Dan-Dong China) with Mo Kα radiation (λ = 1.548 Å) operating at 40 kV and 100 Ma. The luminescent spectra were measured on a luminescent spectrophotometer SHIMADZU RF-5301pc at room temperature. 2.2. Syntheses of complexes 1-4 A mixture of HL (78.7 mg, 0.50 mmol), Ln(Ac)3·4H2O (0.10 mmol) (Ln = Sm (1), Eu (2), Dy (3), Tb (4)), and ZnO (8.2mg, 0.10 mmol), H2O (5 mL) and CH3OH (5 mL) were placed in a 25 mL Teflon-lined steel vessel and heated to 140 oC for 3 days, then cooled to room temperature at a rate of 5°C/h. The resulting rod-shaped crystals were collected after washing with H2O (5mL) and CH3OH (5mL). {[Sm(L)3(CH3OH)(H2O)]n} (1). Yield: 54 mg (78% based on Sm). Anal. Calcd (%): C 34.06, H 2.26, N 6.27; found: C 34.08, H 2.29, N 6.22. IR bands (Fig. S1) (cm-1): 3394 (vs), 1601 (vs), 1540 (s), 1472(w), 1405 (vs), 1174 (m), 1113 (m), 1082 (m), 1021 (m), 990 (m), 881 (m), 777 (vs), 740 (s), 679 (m). {[Eu(L)3(CH3OH)(H2O)]n} (2). Yield: 51 mg (74% based on Eu). Anal. Calcd (%): C 33.98, H 2.25, N 6.26; found: C 33.95, H 2.26, N 6.21. IR bands (Fig. S1) (cm-1): 3391 (m), 1584 (vs), 1543 (vs), 1467 (w), 1415 (vs), 1179 (w), 1113 (w), 1077 (w), 993 (w), 891 (w), 861 (w), 783 (m), 750 (w), 681 (w). 3
{[(Dy(L)3(H2O)2]n} (3). Yield: 48 mg (72% based on Dy) Anal. Calcd (%): C 32.36, H 1.96, N 6.29; found: C 32.39, H 1.93, N 6.24. IR bands (Fig. S1) (cm-1): 3381 (m), 1593 (s), 1533 (s), 1415 (vs), 1174 (w), 1067 (w), 995 (w), 895 (w), 768 (m), 682 (w). {[(Tb(L)3(H2O)2]n} (4). Yield: 46 mg (69% based on Tb). Anal. Calcd (%): C 32.53, H 1.97, N 6.32; found: C 32.58, H 1.95, N 6.28. IR bands (Fig. S1) (cm-1): 3378(m), 1581 (vs), 1573 (vs), 1468 (m), 1418 (vs), 1166 (w), 1116 (w), 1085 (w), 997 (w), 890 (w), 777 (s), 676 (m). 2.3. X-ray crystallography Table 1. Data collection and processing parameters for 1-3. 1 Formula
2
3
C19H15Cl3N3O8Sm
C19H15Cl3N3O8Eu
C18H13Cl3N3O8Dy
M (g mol )
670.07
671.65
668.16
T (K)
143.00(10)
143.00(10)
143.00(10)
Crystal system
Triclinic
Triclinic
Triclinic
Space group
PῙ
PῙ
PῙ
a (Å)
9.6921(6)
9.6945(13)
9.6545(6)
b (Å)
11.2346(9)
11.2440(9)
11.6390(9)
c (Å)
11.9007(7)
11.9536(13)
11.9233(6)
α (º)
87.791(8)
112.086(7)
β (º)
88.000(6) 67.275(6)
67.240(12)
112.658(5)
γ (º)
80.259(6)
80.174(9)
91.941(6)
1177.34(15)
1183.4(2)
1120.37(14)
2 1.887
2
2
1.885
1.981
µ (mm )
2.884
3.039
3.743
F(000)
652
656
646
Refl.collected/unique(R(int))
10359/5371(0.0335)
10148/5407 (0.0445)
6904/6904 (0.0452)
(°)
3.17 to 29.26
3.175 to 29.27
2.797 to 25.01
5371/6/327
5407/26/327
6904/0/301
GOF on F
1.073
1.052
1.019
R1, wR2 (I >2σ(I))
0.0388, 0.0939
0.0379, 0.0799
0.0443, 0.1117
0.0435, 0.0843
0.0487, 0.1131
-1
3
V (Å ) Z -3
ρcalcd (g cm ) -1
Data/restraints/parameters 2
R1, wR2 (all data) a
0.0472, 0.1006 b
2
2 2
2 2 1/2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 ={∑[w(Fo − Fc ) ]/∑[w(Fo ) ]} .
Suitable single crystals of 1-3 (Fig. S2) were selected to analyze the structures. The single crystal diffraction data were acquired by using an Oxford SuperNova single crystal diffractometer which is equipped with graphite-monochromatic Mo/Kα radiation (λ = 0.71073 Å). The structures were solved by direct method using the program SHELXS-97 and subsequent Fourier difference techniques, and refined anisotropically by full-matrix least-squares on F2 using SHELXL-97 4
[27,28]
. Crystal data
and structure refinements are summarized in Table 1, and selected bond lengths and angles are shown in Table S1.
3. Results and discussion 3.1. Syntheses Complexes 1-4 were successfully synthesized by the solvothermal reaction of Ln(Ac)3·4H2O (Ln = Sm (1), Eu (2), Dy (3), Tb (4)), ZnO and HL in H2O and CH3OH (1:1) (Scheme 1). Originally, we aimed to synthesize heterometallic Ln3+-Zn2+ complexes by mixing HL, ZnO, and Ln(Ac)3·4H2O. Nevertheless, only the Ln-based complexes were resulted in. We tried to synthesize complexes (1-4) without ZnO, but no crystalline product was obtained. We can see, hence, that ZnO plays an important role in the synthesis, which may act as base to neutralize HL, and then improve the coordination ability of L-. Complexes 1-4 were synthesized by the similar method, however they display different structures due to lanthanide contraction as usually observed
[29]
. There are three coordination modes of L- ligands in complexes 1-4
(Scheme 2), which contribute to the structural change. complexes 1 and 2 with light Sm and Eu are isomorphic with coordiation mode A, while complexes 3 and 4 with heavy Dy and Tb are isotructural with coordination modes B and C. [Ln(L)3(CH3OH)(H2O)]n (Ln=Sm(1), Eu(2)) Ln= Sm, Eu
1D [Ln(COO)2Ln(COO)4]n chain
ZnO, CH3OH:H2O(1:1) Ln(Ac)3·4H2O Ln= Dy, Tb
1D [Ln(COO)2]n chain
[(Ln(L)3(H2O)2]n (Ln=Dy(3),Tb (4)) Scheme 1 Syntheses of complexes 1-4.
A
B
Scheme 2 The coordination modes of L- ligand in complexes 1-4. 5
C
3.2. Crystal structure of complexes 1-4 Single-crystal X-ray diffraction analyses indicate that complexes 1 and 2 are isomorphic and crystallize in triclinic system with space group PῙ, displaying a 1D chain based on a paddle-wheel-type dimmer-[Ln2(COO)4]. Here, the structure of complex 1, as a representative, will be discussed in detail. In this structure, there is one crystallographic independent Sm3+ ion, which is eight-coordinated by eight O atoms from six L− anions, one H2O, and one CH3OH (Fig. 1a). The coordination geometry around Sm ion is close to a biaugmented trigonal prism (Fig. S3a and Table S2). The bond lengths of Sm–O are in the range of 2.363(3)–2.533(3) Å and angles of O–Sm–O range from 70.68(11) to 146.01(13)°, consistent with those values of reported Sm-complexes [30,31]. (a)
(b)
(c)
Fig. 1. (a) The coordination environment of Sm3+ in complex 1 (symmetry codes: a = 1–x, 1–y, 1–z; b = 2-x, 1-y, 1-z). (b) Paddle wheel type dimer-[Ln2(COO)4]. (c) The 1D chain along c axis. The H atoms are omitted for clarity.
In complex 1, there is one coordination mode of L- ligand (Scheme 2A), whereas there are two kinds of linkages between the neighboring Sm3+ ions: one linkage is four syn-syn carboxylate groups to form a paddle-wheel-like dimer-[Sm2(COO)4] (Fig. 1b), and the other is two syn-syn carboxylate groups to form a grid-like [Sm2(COO)2]. It also can say that these paddle-wheel-like dimers are further linked by two syn-syn carboxylate groups along c axis to form a 1D chain (Fig. 1c). The neighboring Sm···Sm distances are 4.4365(3) and 5.2596(3) Å in [Sm2(COO)4] and [Sm2(COO)2], respectively. The adjacent 1D chains are further extended via the O-H···N hydrogen 6
bonds into a 3D supramolecular network (Fig. S4a). Complexes 3 and 4 were obtained by the similar process as that for 1 and 2. For complex 4, no suitable single-crystal was obtained for single-crystal X-ray diffraction analysis, and the powder X-ray diffraction confirm its isostructural feature with complex 3, as the next section discussed. The single-crystal X-ray diffraction analysis reveals that complex 3 crystallizes in the triclinic space group PῙ, and shows different 1D chain from complexes 1 and 2, which exhibit a regular [Ln(COO)2]n chain. In the unsymmetrical unit, there are one crystallographic independent Dy3+ ion, three L− anions and two coordinated H2O (Fig. 2a). The Dy3+ ion is eight-coordinated by four O atoms from four bridging L- anions, one chelated L- anion, and two H2O, and adopts a square antiprism coordination geometry (Fig. S3b and Table S2). The average distance of Dy–O is 2.372 Å and the angles of O–Dy–O range from 53.08(16) to 155.6(2)°, which are in the normal range [32,33]. As shown in Fig. 2b, the neighboring Dy3+ ions are bridged by double syn-anti carboxylate groups to form a regular [Dy(COO)2]n chain. The neighboring Dy3+···Dy3+ distances are 4.9325(3) and 4.7344 Å for Dy1···Dy1a and Dy1a···Dy1b, respectively. The adjacent chains stack together to form a 2D supramolecular network via π···π interactions along the b axis (Fig. S4b). (a)
(b)
Fig. 2. (a) The coordination environment of Dy3+ in complex 3 (symmetry codes: a= –x, 2–y, 1–z; b= 1-x, 2-y, 1-z). (b) The 1D chain along b axis. The H atoms are omitted for clarity.
Complexes 1-4 with light and heavy Ln3+ show different structural types, indicating the lanthanide contraction. It is also observed in Ln–O bond lengths, decreasing from Sm to Dy (Table S3). 3.3. PXRD and thermal gravimetric analyses (TGA) For complex 4, no suitable single-crystal was obtained for single-crystal X-ray diffraction analysis, and the powder X-ray diffractions (PXRD) were measured to analyse its structural feature of complex 4, as well as the phase purity of complexes 7
1-4 (Fig. 3). The results indicate the isostructural feature of complex 4 with 3. Furthermore, the experimental PXRD patterns of complexes 1-4 are in agreement with those simulated ones from single-crystal data, indicating the phase purity of the synthesized product. Owing to the different orientations of the samples, there are minor difference in the intensity and shape of the peaks between the experimental and simulated data. (a)
(b)
Fig. 3. The experimental and simulated PXRD patterns for complexes 1-4.
To investigate the thermal stability of complexes 1-4, the thermogravemetric analyses for 1-4 were performed and the samples are stable up to about 130°C (Fig. S5). Complexes 1 and 2 show similar thermal behavior. The first weigh loss corresponds to the loss of one H2O and one CH3OH (calcd. 7.5%, obsed. 7.4% for 1; calcd. 7.4%, obsed. 7.0% for 2) before 250 °C. On further heating, it began to decompose gradually. For complexes 3 and 4, the first weight loss from 130 to 270 °C is about 4.7%(3) and 4.9%(4), which is consistent with the weight loss of two H2O molecules (calcd. 5.3% for 3; calcd. 5.4% for 4), and the further weight loss indicate the decomposition of complexes. TGA results imply preferable thermal stability of complexes 1-4. 3.4. Luminescent properties Ln-complexes exhibit characteristic long luminescence lifetimes, large Stokes’ shifts, narrow emission peaks and high purity colors
[34]
. Therefore, the solid-state
luminescence of 1-4 was measured at room temperature.
8
(a)
(b)
(c)
(d)
Fig. 4. The solid-state emission spectra of 1-4 at room temperature.
As shown in Fig. S6, the excitation spectra of 1-4, monitored at the characteristic emissions (594, 616, 575 and 543 nm, respectively), exhibits the excitation peaks at 354, 350, 300 and 308 nm, respectively. Upon excitation at 354 nm, complex 1 displays the weak characteristic emissions of Sm3+ ion at 564, 594 and 630 nm, which are assigned to 4G5/2→6HJ (J = 5/2, 7/2, 9/2) transitions, respectively (Fig. 4a). The weak luminescence efficiency maybe due to poor matching of the triplet state of the HL with that of the emissive excited states of the Sm3+ ion. Meanwhile, an intense emission band centered at 428 nm is observed, which arises from ligand-centred (LC) luminescence
[35]
. Compared to complex 1, complex 2 clearly shows characteristic
emissions of Eu3+ ion at 538, 578, 591, 616, 652 and 659 nm excited at 350 nm, corresponding to the transitions (5D1→7F1) and 5D0→7FJ (J = 0-4), respectively, respectively (Fig. 4b). Moreover, the 5D0→7F2 (electric dipole) transition at 616 nm is stronger than that of the5D0→7F1 (magnetic dipole) transition at 591 nm, indicating Eu3+ ions in the low-symmetry coordination sites
[36-39]
, which is consistent with
single-crystal data. Complex 3 displays typical yellow luminescence of Dy3+ ion under excitation at 300 nm. Two typical intense emissions at 478 and 575 nm correspond to 4
F9/2→6HJ (J = 15/2 and 13/2), and the 4F15/2→6H13/2 transition is weak at 543 nm (Fig. 9
4c). For complex 4, when excited at 308 nm, it displays an intense green light of Tb3+ ion, dominated by the emission at 543 nm (5D4→7F5), similar to the most of reported Tb3+ complexes
[40-42]
correspond to
5
. Other characteristic emissions at 490, 581 and 620 nm
D4→7FJ (J = 6, 4, 3) transitions, respectively (Fig. 4d). As
above-discussed, the energy transfer from ligand to Ln3+ ions is more efficient in complexes 2, 3 and 4 compared with 1. 3.5. Luminescent probe (a)
(c)
(b)
(d)
Fig. 5. (a) and (b) Emission spectra of 2 and 4 in DMF with various cations (1×10−3 mol·L-1). (c) and (d) the 5D0→7F2 transition (616 nm) luminescence intensity for 2 and 5D4→7F5 transition (543nm) for 4 in various metal ions, respectively.
Since complexes 2-4 show intense luminescence, especially for complexes 2 and 4, and we wonder whether complexes 2 and 4 can act as luminescent probes for some metal ions. Thus we explore the sensing abilities of complexes 2 and 4 for some common metal ions. The powder samples of 2 and 4 (2 mg), respectively, were dissolved in DMF (4 mL) with addition of 1×10-3 mol·L-1 MClx (M = Mg2+, Ca2+, Zn2+, Na+, Cd2+, K+, Ni2+, La3+, Ce3+, Mn2+, Co2+, Al3+, Cu2+ and Fe3+; x = 1-3), and then, the luminescent properties were measured. As shown in Fig. 5, the luminescent intensity of 2 and 4 decreases upon adding those metal ions. Notably, the luminescence of complexes 2 and 4 was quenched upon addition of Fe3+ ions, indicating that complexes 2 and 4 maybe act as luminescent probes for Fe3+ ions. Furthermore, the 10
emitting color of 2 and 4 was observed in different analytes under 254 nm. As shown in Fig. S7, the emitting colors change from bright-red/green to dim colorless with increasing Fe3+ ions, respectively for complexes 2 and 4. Generally, there are many different metal ions in one system, whether the other coexisting metal ions will influence the luminescence. Thus, it is valuable to research the effect of mixed metal ions for luminescence
[43]
. Inspired by this point, 2 mg of
Ln-CPs 2 and 4, respectively, were immersed in 4 mL of DMF with Fe3+ and another metal ion (1×10−3 mol·L-1), and then the corresponding luminescence was measured. As shown in Fig 6, the luminescence quenching is still observed for 2 and 4 after introducing other metal ions. These results indicate that complexes 2 and 4 can selectively sense Fe3+ ions. (a)
(b)
Fig. 6. The luminescence intensities of 2 (a) and 4 (b) exposed to mixed metal ions (1×10−3 mol·L-1).
Subsequently, the luminescent titrations were further executed to study the sensitivity of 2 and 4 for Fe3+ ions, respectively. As shown in Fig. 7a and 7b, the luminescent intensity of 2 and 4 decrease gradually with increasing Fe3+ ions, which implies the diffusion-controlled quenching process
[44-47]
. And the luminescence
quenching effect can be quantitatively rationalized by the Stern−Volmer (S−V) equation (1): I0/I = 1 + Ksv[C]
(1)
where I0 and I are the initial luminescence intensity and after the addition of the Fe3+ ions, respectively; Ksv is the quenching constant (L·moL-1) and [C] is the molar concentration of the analyte [48-51]. For complexes 2 and 4, The S–V plots have a favorable linear fit (Fig. 7c and 7d, inset) with Ksv of 2.16 × 104 and 1.89 × 104 L·moL-1 at low concentration (0 − 1.5×10−4 11
mol·L-1), respectively. The high Ksv values are comparable with those reported Eu/Tb-complexes (Table S4), implying high sensitivity of complexes 2 and 4 for Fe3+ ions
[52]
. Whereafter, the S−V curves subsequently deviate from linearity at high
concentration. So an exponential quenching equation (2) is adopted to fit the nonlinear relation: I0/I = a · exp(k[C]) + b
(2)
in which a, b and k are constants [50,54]. The nonlinear S−V equation of 2 and 4 could be described as: I0/I = 0.5964 exp(0.011[C]) + 0.8089 (R2=0.9989) and I0/I = 0.4684 exp(0.0099[C]) + 1.2618 (R2=0.9958), respectively (Fig. 7c and 7d). The phenomenon may be attributed to the energy-transfer process or self-absorption [54-56]. (a)
(c)
(b)
(d)
Fig. 7. Emission spectra of 2 (a) and 4 (b) dispersed in DMF upon incremental addition of Fe3+ ions, and S–V plots of 2 (c) and 4 (d) with Fe3+ ions. Insets: the linear correlation for the plot of I0/I vs concentration.
In previous reports, the mechanism of deactivation of complexes for various metal ions have been studied, including weak interaction and competitive energy absorption.[57] The mechanism of complexes 2 and 4 for Fe3+ detection should be competitive energy absorption (Fig. S6 and S8), exhibiting the extensive overlaps between the UV-Vis spectrum of Fe3+ (Fig. S8) and the excitation spectrum of corresponding complexes (Fig. S6). 12
4. Conclusions In summary, a series of lanthanide coordination polymers (1-4) based on 2-chloroisonicotinic acid have been synthesized successfully under the presence of ZnO. Owing to the lanthanide contraction, complexes 1-4 exhibit two structural types. Complexes 1 and 2 are 1D [Ln(COO)2Ln(COO)4]n chain, while complexes 3 and 4 feature 1D [Ln(COO)2]n chain. Luminescence measurements reveals that all complexes exhibit the characteristic luminescence of corresponding Ln3+ ions. Especially, complexes 2 and 4 can be potential luminescent probes for Fe3+ ions. Meanwhile, this work provides a new insight in making new preferable luminescent materials as luminescent probes by using chloride-derivative ligands.
Acknowledgements This work was supported by the Subject Building Special Fund of Sichuan Agricultural University.
Appendix A. Supporting information Supplementary Information (SI) associated with this article can be found in the online version.
Appendix B. Supplementary data The supplementary crystallographic data for this paper were deposited with the Cambridge Crystallographic Data Centre Centre(CCDC), CCDC-1955102-1955104 (1-3). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/conts/retrieving.html.
References [1] Z. Chen, Z. Y. Liu, Y. Liu, K. Z. Zheng, W. P. Qin, J. Fluorine Chem. 144 (2012) 157-164. [2] W. L. Leong, J. J. Vittal, Chem. Rev. 111 (2011) 688-764. [3] Y. Z. Zheng, Z. Zheng, X. M. Chen, Coord. Chem. Rev. 258 (2014) 1-15. [4] J. Rocha, L. D. Carlos, F. A. A. Paz, D. Ananias, Chem. Soc. Rev. 40 (2011) 926-940. [5] Y. J. Cui, J. Zhang, H. J. He, G. D. Qian, Chem. Soc. Rev. 47 (2018) 5740-5785. [6] K. Binnemans, Chem. Rev. 109 (2009) 4283-4374. [7] K. Staszak, K. Wieszczycka, V. Marturano, B. Tylkowsk, Coord. Chem. Rev. 397 (2019) 76-90. [8] H. Maas, A. Currao, G. Calzaferri, Angew. Chem. Int. Ed. 41 (2002) 2495-2497. [9] J. C. G. Bünzli, Acc. Chem. Res. 39 (2006) 53-61. [10] L. N. Zheng, F. H. Wei, H. M. Hua, C. Bai, X. L. Yang, X. F. Wang, G. L. Xue, Polyhedron 161 (2019) 47-55. [11] E. G. Moore, A. P. S. Samuel, K. N. Raymond, Acc. Chem. Res. 42 (2009) 542-552. 13
[12] T. B. Emelina, I. V. Kalinovskaya, A. G. Mirochnik, Acta Mol. Biomol. Spectrosc. Spectrochim. Acta A 207 (2019) 222-228. [13] M. Mihorianu, M. Leonzio, M. Bettinelli, F. Piccinelli, Inorg. Chim. Acta 438 (2015) 10-13. [14] O. Pietraszkiewicz, S. Mal, M. Pietraszkiewicz, M. Maciejczyk, I. Czerski, T. Borowiak, G. Dutkiewicz, O. Drobchak, L. Penninck, J. Beeckman, K. Neyts, J. Photochem. Photobiol. A‐Chem. 250 (2012) 85-91. [15] A. De Bettencourt‐Dias, P. S. Barber, S. Viswanathan, Coord. Chem. Rev. 165 (2014) 273-274. [16] S. Mal, M. Pietraszkiewicz, O. Pietraszkiewicz, Lumin. 33 (2018) 370-375. [17] M. Pietraszkiewicz, S. Mal, O. Pietraszkiewicz, Opt. Mater. 34 (2012) 1507-1512. [18] J. Heine, K. Muller-Buschbaum, Chem. Soc. Rev. 42 (2013) 9232-9242. [19] B. Li, H. M. Wen, Y. Cui, G. Qian and B. Chen, Prog. Poly. Sci. 48 (2015) 40-84. [20] O. Sun, T. Gao, J. W. Sun, G. M. Li, H. F. Li, H. Xu, C. Wang, P. F. Yan, CrystEngComm 16 (2014) 10460-10468. [21] X. Yao, X. Y. Wang, Y. Q. Han, P. F. Yan, Y. X. Li, G. M. Li, CrystEngComm 20 (2018) 3335-3343. [22] L. Armelao, S. Quici, F. Barigelletti, G. Accorsi, G. Bottaro, M. Cavazzini, E. Tondello, Coord. Chem. Rev. 254 (2010) 487-505. [23] K. Zheng, Z. Q. Liu, Y. F. Jiang, P. H. Guo, H. R. Li, C. H. Zeng, S. W. Ng, S. L. Zhong, Dalton Trans. 47 (2018) 17432-17440. [24] J. Z. Gu, Y. Cai, Y. Liu, X. X. Liang, A. M. Kirillov, Inorg. Chim. Acta 469 (2018) 98-104. [25] R. F. Li, Y. W. Zhang, X. F. Liu, X. H. Chang, X. Feng, Inorg. Chim. Acta 502 (2020) 119370. [26] H. Xu, H. C. Hu, C. S. Cao, B. Zhao, Inorg. Chim., 54 (2015) 4585-4587. [27] G. M. Sheldrick, SHELXS-97, Program for the Solution of Crystal Structures, University of Göttingen (Germany), 1997. [28] G. M. Sheldrick, SHELXL-97, Program for the Refinement of Crystal Structures, University of Göttingen (Germany), 1997. [29] S. Q. Su, W. Chen, C. Qin, S. Y. Song, Z. Y. Guo, G. H. Li, X. Z. Song, M. Zhu, S. Wang, Z. M. Hao, H. J. Zhang, Cryst. Growth Des. 12 (2012) 1808−1815. [30] A. Gusev, V. Shul'gin, E. Braga, E. Zamnius, N. Lyubomirskiy, M. Kryukova, W. Linert, J. Lumin. 212 (2019) 315-321. [31] C. N. Muniz, H. Patel, D. B. Fast, L. E. S. Rohwer, E. W. Reinheimer, M. Dolgos, M. W. Graham, M. Nyman, J. Solid State Chem. 259 (2018) 48-56. [32] W. M. Wang, X. H. Shi, H. X. Zhang, M. M. Wu, Y. L. He, M. Fang, Y. Shi, M. Fang, Polyhedron 141 (2017) 304-308. [33] X. Q. Zhao, D. X. Bao, J. Wang, S. Xiang, Y. C. Li, Inorg. Chim. Acta 466 (2017) 110-116. [34] F. H. Zhang, Y. Y. Wang, C. Lv, Y. C. Li, X. Q. Zhao, J. Lumin. 207 (2019) 561-570. [35] Y. H. Zhang, X. Li, S. Song, H. Y. Yang, D. Ma, Y. H. Liu, CrystEngComm, 43 (2014) 5974-5977. [36] G. Vicentini, L. B. Zinner, J. Zukerman-Schpector, K. Zinner, Coord. Chem. Rev. 196 (2000) 353-382. [37] A. Datcu, N. Roques, V. Jubera, I. Imaz, D. Maspoch, J. P. Sutter, C. Rovira, J. Veciana, Chem. Eur. J. 17 (2011) 3644-3656. [38] M. M. Castaño-Briones, A. P. Bassett, L. L. Meason, P. R. Sshton, Z. Pikramenou, Chem. Commun. (2004) 2832-2833. [39] X. Hu, W. Dou, C. Xu, X. L. Tang, J. R. Zheng, W. S. Liu, Dalton Trans. 40 (2011) 3412-3418. [40] X. P. Yang, R. A. Jones, J. Am. Chem. Soc. 127 (2005) 7686-7687. 14
[41] C. Yang, W. T. Wong, J. Mater. Chem. 11 (2001) 2898-2900. [42] K. Hanaoka, K. Kikuchi, H. Kojima, Y. Urano, T. Nagano, Angew. Chem., Int. Ed. 42 (2003) 2996-2999. [43] H. Xu, M. Fang, C. S. Cao, W. Z. Qiao, B. Zhao, Inorg. Chem. 55 (2016) 4790-4794. [44] G. Y. Wang, L. L. Yang, Y. Li, H. Song, W. J. Ruan, Z. Chang, X. H. Bu, Dalton Trans. 42 (2013) 12865-12868. [45] B. Chen, Y. Yang, F. Zapata, G. Lin, G. Qian, E. B. Lobkovsky, Adv. Mater. 19 (2007) 1693-1696. [46] W. Q. Tong, W. N. Liu, J. G. Cheng, P. F. Zhang, G. P. Li, L. Hou, Y. Y. Wang, Dalton Trans. 47 (2018) 9466-9473. [47] S. Dang, X. Min, W. Yang, F. Y. Yi, H. You, Z. M. Sun, Chem.–Eur. J. 19 (2013) 17172-17179. [48] W. Wei, R. J. Lu, S. Y. Tang and X. Y. Liu, J. Mater. Chem. A 3 (2015) 4604-4611. [49] B. L. Chen, L. B. Wang, Y. Q. Xiao, F. R. Fronczek, M. Xue, Y. J. Cui, G. D. Qian, Angew. Chem. Int. Ed. 48 (2009) 500-503. [50] Y. T. Yan, J. Liu, G. P. Yang, F. Zhang, Y. K. Fan, W. Y. Zhang, Y. Y. Wang, CrystEngComm 20 (2018) 477-486. [51] S. K. Sahoo, D. Sharma, R. K. Bera, G. Crisponi, J. F. Callan, Chem. Soc. Rev. 41 (2012) 7195-7227. [52] J. J. Huang, J. H. Yu, F. Q. Bai, J. Q. Xu, Cryst. Growth Des. 18 (2018) 5353-5364. [53] X. C. Sun, J. K. He, Y. T. Meng, L. C. Zhang, S. C. Zhang, X. Y. Ma, S. Dey, J. Zhao, Y. Lei, J. Mater. Chem. A 4 (2016) 4161-4171. [54] S. Y. Wu, Y. N. Lin, J. W. Liu, W. Shi, G. M. Yang, P. Cheng, Adv. Funct. Mater. 28 (2018) 1707169. [55] Y. Salinas, R. MartinezManez, M. D. Marcos, F. Sancenon, A. M. Castero, M. Parra, S. Gil, Chem. Soc. Rev. 41 (2012) 1261-1296. [56] S. S. Nagarkar, B. Joarder, A. K. Chaudhari, S. Mukherjee, S. K. Ghosh, Angew. Chem., Int. Ed. 52 (2013) 2881-2885. [57] J. L. Du, X. Y. Zhang, C. P. Li, J. P. Gao, J. X. Hou, X. Jing, Y. J. Mu, L. J. Li, Sens. Actuators-B Chem., 257 (2018) 207-213.
Author statement Xiaoqing Zhao: Conceptualization, Methodology, Writing-Review and Editing. Fenhang Zhang: Investigation, Writing-Original draft preparation. Yajun Liu: Resources. Tianhao Zhao: Data Curation. Hongyan Zhao: Validation. Shuo
Xiang: Resources. Yunchun Li: Supervision. Graphical abstract 15
A series of Ln-CPs (1-4) display two kinds of 1D chains due to the lanthanide contraction, which exhibit characteristic emission of corresponding Ln3+ ion. Furthermore, 2 (Eu) and 4 (Tb) act as luminescent probes for Fe3+ ions.
Highlights A series of luminescent Lnlll coordination polymers based on 2-chloroisonicotinic (HL) acid have been synthesized. Complexes 1(Sm) and 2(Eu) exhibit a 1D [Ln(COO)2Ln(COO)4]n chain, while complexes 3(Dy) and 4(Tb) are a 1D [Ln(COO)2]n chain. HL is an effective “antenna” to sensitize the luminescence of Ln3+ ions, and 2(Eu) and 4(Tb) should be potential luminescent probes for Fe3+ ions.
16
S0020-1693(19)31810-9 https://doi.org/10.1016/j.ica.2020.119459 ICA 119459
To appear in:
Inorganica Chimica Acta
Received Date: Revised Date: Accepted Date:
23 November 2019 17 January 2020 17 January 2020
Please cite this article as: X. Zhao, F. Zhang, Y. Liu, T. Zhao, H. Zhao, S. Xiang, Y. Li, A series of Luminescent Lnlll-based coordination polymers: syntheses, structures and luminescent properties, Inorganica Chimica Acta (2020), doi: https://doi.org/10.1016/j.ica.2020.119459
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
A series of Luminescent Lnlll-based coordination polymers: syntheses, structures and luminescent properties Xiaoqing Zhao,a,b†* Fenhang Zhang,a† Yajun Liu,a Tianhao Zhao,a Hongyan Zhao,a Shuo Xiang,a Yunchun Lia Abstract A series of lanthanide coordination polymers (Ln-CPs), with the formula of {[Ln(L)3(CH3OH)(H2O)]n} (Ln = Sm (1), Eu (2), HL = 2-chloroisonicotinic acid) and {[Ln(L)3(H2O)2]n} (Ln =Dy (3), Tb (4)), were synthesized under solvothermal condition. This series display two different 1D chains due to the lanthanide contraction: complexes 1 and 2, based on light Sm and Eu, respectively, consist of paddle-wheel [Ln2(COO)4] dimmers through double μ2-COO bridges, and complexes 3 and 4 with heavy Dy and Tb, respectively, show 1D [Ln(COO)2]n chains. The luminescent measurements indicate the typical emissions of corresponding Ln3+ ions in complexes 1-4. Furthermore, complexes 2 and 4 display significant luminescent response for Fe3+ ions with high quenching constant. The result indicates that complexes 2 and 4 can act as luminescent probes for Fe3+ ions with relatively high sensitivity and selectivity. Keywords: Lanthanide coordination polymers; 2-chloroisonicotinic acid; luminescent probes
1. Introduction As an important class of hybrid materials, lanthanide coordination polymers (Ln-CPs) have received extensive attention due to their potential ability as functional materials, combining their structural features with exceptional optical and/or magnetic properties arising from the unique 4f configuration of Ln3+ ions [1-3]. Among the huge amount of Ln-CPs, there are special attention to luminescent Ln-CPs, which have been considered as attractive luminescent materials in tunable lasers,
a b
College of Science, Sichuan Agricultural University, Ya’an 625014, China. E-mail: [email protected]. Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Nankai University, Tianjin
300071, China †
These authors contribute equally. 1
low-energy scintillators, optical storage and chemical sensing, etc
[4-7]
. Because
Ln-CPs always have the advantageous luminescent properties, such as long lifetimes, narrow line-like emission bands and excellent resistance to photo-bleaching. It is well known that direct excitation of Ln3+ ions is usually inefficient, due to the spin- and parity-forbidden nature of the f-f transitions [8-10]. To address this challenge, one strategy is the so-called “antenna effect”, which apply the organic ligands as chromophore to absorb incident photons and transfer energy to the Ln3+ ions, resulting in the significant luminescence of Ln-complexes [11-13]. As a consequence, a lot of organic ligands, i.e. pyridine derivatives, β-diketonate, tetrazolate, aromatic carboxylate, pyrazolonate, etc
[14-17]
, have been developed to construct novel
luminescent Ln-complexes. In addition, the high-energy vibrations groups in the organic ligands, such as C–H and O–H bonds, may significantly decrease the metal excited states energy, resulting in shorter luminescence lifetimes, low luminescent intensities and quantum yields [18-21]. Hence, organic ligands with the low-vibrational groups (eg. C–Cl, C–F, C–Br) may make contribution in synthesizing significant luminescent Ln-complexes
[20-23]
. Thereinto, 2-chloroisonicotinic acid (HL) is a rigid
chlorinated N, O-containing ligand and may provide favorable coordination sites to construct luminescent Ln-CPs. Nevertheless, as we known, there is no Ln-complexes associated with HL reported until now. Moreover, the luminescent Ln-complexes as chemical sensors have been concerned, particularly for the recognition of metal ions, because most of them pose a health and environment threats [24-26]. These promising application encourages researchers to design and synthesize luminescent Ln-complexes for metal ions recognition, for example, the 3D Tb-MOF exhibits significant luminescent selectivity for Fe3+ ion [26]. Based on the above considerations, we herein selected HL as ligand to synthesize luminescent
Ln-complexes,
and
finally,
a
series
of
Ln-CPs,
namely,
{[Ln(L)3(CH3OH)(H2O)]n} (Ln = Sm (1), Eu (2)) and {[Ln(L)3(H2O)2]n} (Ln =Dy (3), Tb (4)), were successfully obtained. All complexes exhibit two types of 1D chains due to the lanthanide contraction. Complexes 1 and 2 with light Sm and Eu exhibit a 1D [Ln(COO)2Ln(COO)4]n chain, while complexes 3 and 4 with heavy Dy and Tb are a 1D 2
[Ln(COO)2]n chain. The luminescent properties indicate that complexes 1-4 exhibit emission characteristics of correlative Ln3+ ions. Furthermore, complexes 2 and 4 exhibit significant luminescence response to Fe3+ ions, suggesting that both complexes should be potential luminescent probes for Fe3+ ions.
2. Experimental section 2.1. Materials and apparatus All chemicals and solvents were commercially purchased and used without further purification. The single-crystal X-ray diffraction data of complexes 1-3 were collected by employing an Agilent Technologies SuperNova single crystal diffractometer (America) equipped with graphite-monochromatic Mo/Ka radiation (λ = 0.71073 Å) at 120 K. Elemental analyses for C, H and N were carried out on a Perkin–Elmer analyzer at the Institute of Elemento–Organic Chemistry, Nankai University. The FT-IR spectra were recorded from KBr pellets in the range 400-4000 cm-1 on a Bio-Rad FTS 135 spectrometer. Powder X-ray diffraction (PXRD) data were acquired using a D/Max-RA diffractometer (DX-2600, Dan-Dong China) with Mo Kα radiation (λ = 1.548 Å) operating at 40 kV and 100 Ma. The luminescent spectra were measured on a luminescent spectrophotometer SHIMADZU RF-5301pc at room temperature. 2.2. Syntheses of complexes 1-4 A mixture of HL (78.7 mg, 0.50 mmol), Ln(Ac)3·4H2O (0.10 mmol) (Ln = Sm (1), Eu (2), Dy (3), Tb (4)), and ZnO (8.2mg, 0.10 mmol), H2O (5 mL) and CH3OH (5 mL) were placed in a 25 mL Teflon-lined steel vessel and heated to 140 oC for 3 days, then cooled to room temperature at a rate of 5°C/h. The resulting rod-shaped crystals were collected after washing with H2O (5mL) and CH3OH (5mL). {[Sm(L)3(CH3OH)(H2O)]n} (1). Yield: 54 mg (78% based on Sm). Anal. Calcd (%): C 34.06, H 2.26, N 6.27; found: C 34.08, H 2.29, N 6.22. IR bands (Fig. S1) (cm-1): 3394 (vs), 1601 (vs), 1540 (s), 1472(w), 1405 (vs), 1174 (m), 1113 (m), 1082 (m), 1021 (m), 990 (m), 881 (m), 777 (vs), 740 (s), 679 (m). {[Eu(L)3(CH3OH)(H2O)]n} (2). Yield: 51 mg (74% based on Eu). Anal. Calcd (%): C 33.98, H 2.25, N 6.26; found: C 33.95, H 2.26, N 6.21. IR bands (Fig. S1) (cm-1): 3391 (m), 1584 (vs), 1543 (vs), 1467 (w), 1415 (vs), 1179 (w), 1113 (w), 1077 (w), 993 (w), 891 (w), 861 (w), 783 (m), 750 (w), 681 (w). 3
{[(Dy(L)3(H2O)2]n} (3). Yield: 48 mg (72% based on Dy) Anal. Calcd (%): C 32.36, H 1.96, N 6.29; found: C 32.39, H 1.93, N 6.24. IR bands (Fig. S1) (cm-1): 3381 (m), 1593 (s), 1533 (s), 1415 (vs), 1174 (w), 1067 (w), 995 (w), 895 (w), 768 (m), 682 (w). {[(Tb(L)3(H2O)2]n} (4). Yield: 46 mg (69% based on Tb). Anal. Calcd (%): C 32.53, H 1.97, N 6.32; found: C 32.58, H 1.95, N 6.28. IR bands (Fig. S1) (cm-1): 3378(m), 1581 (vs), 1573 (vs), 1468 (m), 1418 (vs), 1166 (w), 1116 (w), 1085 (w), 997 (w), 890 (w), 777 (s), 676 (m). 2.3. X-ray crystallography Table 1. Data collection and processing parameters for 1-3. 1 Formula
2
3
C19H15Cl3N3O8Sm
C19H15Cl3N3O8Eu
C18H13Cl3N3O8Dy
M (g mol )
670.07
671.65
668.16
T (K)
143.00(10)
143.00(10)
143.00(10)
Crystal system
Triclinic
Triclinic
Triclinic
Space group
PῙ
PῙ
PῙ
a (Å)
9.6921(6)
9.6945(13)
9.6545(6)
b (Å)
11.2346(9)
11.2440(9)
11.6390(9)
c (Å)
11.9007(7)
11.9536(13)
11.9233(6)
α (º)
87.791(8)
112.086(7)
β (º)
88.000(6) 67.275(6)
67.240(12)
112.658(5)
γ (º)
80.259(6)
80.174(9)
91.941(6)
1177.34(15)
1183.4(2)
1120.37(14)
2 1.887
2
2
1.885
1.981
µ (mm )
2.884
3.039
3.743
F(000)
652
656
646
Refl.collected/unique(R(int))
10359/5371(0.0335)
10148/5407 (0.0445)
6904/6904 (0.0452)
(°)
3.17 to 29.26
3.175 to 29.27
2.797 to 25.01
5371/6/327
5407/26/327
6904/0/301
GOF on F
1.073
1.052
1.019
R1, wR2 (I >2σ(I))
0.0388, 0.0939
0.0379, 0.0799
0.0443, 0.1117
0.0435, 0.0843
0.0487, 0.1131
-1
3
V (Å ) Z -3
ρcalcd (g cm ) -1
Data/restraints/parameters 2
R1, wR2 (all data) a
0.0472, 0.1006 b
2
2 2
2 2 1/2
R1 = ∑||Fo| − |Fc||/∑|Fo|. wR2 ={∑[w(Fo − Fc ) ]/∑[w(Fo ) ]} .
Suitable single crystals of 1-3 (Fig. S2) were selected to analyze the structures. The single crystal diffraction data were acquired by using an Oxford SuperNova single crystal diffractometer which is equipped with graphite-monochromatic Mo/Kα radiation (λ = 0.71073 Å). The structures were solved by direct method using the program SHELXS-97 and subsequent Fourier difference techniques, and refined anisotropically by full-matrix least-squares on F2 using SHELXL-97 4
[27,28]
. Crystal data
and structure refinements are summarized in Table 1, and selected bond lengths and angles are shown in Table S1.
3. Results and discussion 3.1. Syntheses Complexes 1-4 were successfully synthesized by the solvothermal reaction of Ln(Ac)3·4H2O (Ln = Sm (1), Eu (2), Dy (3), Tb (4)), ZnO and HL in H2O and CH3OH (1:1) (Scheme 1). Originally, we aimed to synthesize heterometallic Ln3+-Zn2+ complexes by mixing HL, ZnO, and Ln(Ac)3·4H2O. Nevertheless, only the Ln-based complexes were resulted in. We tried to synthesize complexes (1-4) without ZnO, but no crystalline product was obtained. We can see, hence, that ZnO plays an important role in the synthesis, which may act as base to neutralize HL, and then improve the coordination ability of L-. Complexes 1-4 were synthesized by the similar method, however they display different structures due to lanthanide contraction as usually observed
[29]
. There are three coordination modes of L- ligands in complexes 1-4
(Scheme 2), which contribute to the structural change. complexes 1 and 2 with light Sm and Eu are isomorphic with coordiation mode A, while complexes 3 and 4 with heavy Dy and Tb are isotructural with coordination modes B and C. [Ln(L)3(CH3OH)(H2O)]n (Ln=Sm(1), Eu(2)) Ln= Sm, Eu
1D [Ln(COO)2Ln(COO)4]n chain
ZnO, CH3OH:H2O(1:1) Ln(Ac)3·4H2O Ln= Dy, Tb
1D [Ln(COO)2]n chain
[(Ln(L)3(H2O)2]n (Ln=Dy(3),Tb (4)) Scheme 1 Syntheses of complexes 1-4.
A
B
Scheme 2 The coordination modes of L- ligand in complexes 1-4. 5
C
3.2. Crystal structure of complexes 1-4 Single-crystal X-ray diffraction analyses indicate that complexes 1 and 2 are isomorphic and crystallize in triclinic system with space group PῙ, displaying a 1D chain based on a paddle-wheel-type dimmer-[Ln2(COO)4]. Here, the structure of complex 1, as a representative, will be discussed in detail. In this structure, there is one crystallographic independent Sm3+ ion, which is eight-coordinated by eight O atoms from six L− anions, one H2O, and one CH3OH (Fig. 1a). The coordination geometry around Sm ion is close to a biaugmented trigonal prism (Fig. S3a and Table S2). The bond lengths of Sm–O are in the range of 2.363(3)–2.533(3) Å and angles of O–Sm–O range from 70.68(11) to 146.01(13)°, consistent with those values of reported Sm-complexes [30,31]. (a)
(b)
(c)
Fig. 1. (a) The coordination environment of Sm3+ in complex 1 (symmetry codes: a = 1–x, 1–y, 1–z; b = 2-x, 1-y, 1-z). (b) Paddle wheel type dimer-[Ln2(COO)4]. (c) The 1D chain along c axis. The H atoms are omitted for clarity.
In complex 1, there is one coordination mode of L- ligand (Scheme 2A), whereas there are two kinds of linkages between the neighboring Sm3+ ions: one linkage is four syn-syn carboxylate groups to form a paddle-wheel-like dimer-[Sm2(COO)4] (Fig. 1b), and the other is two syn-syn carboxylate groups to form a grid-like [Sm2(COO)2]. It also can say that these paddle-wheel-like dimers are further linked by two syn-syn carboxylate groups along c axis to form a 1D chain (Fig. 1c). The neighboring Sm···Sm distances are 4.4365(3) and 5.2596(3) Å in [Sm2(COO)4] and [Sm2(COO)2], respectively. The adjacent 1D chains are further extended via the O-H···N hydrogen 6
bonds into a 3D supramolecular network (Fig. S4a). Complexes 3 and 4 were obtained by the similar process as that for 1 and 2. For complex 4, no suitable single-crystal was obtained for single-crystal X-ray diffraction analysis, and the powder X-ray diffraction confirm its isostructural feature with complex 3, as the next section discussed. The single-crystal X-ray diffraction analysis reveals that complex 3 crystallizes in the triclinic space group PῙ, and shows different 1D chain from complexes 1 and 2, which exhibit a regular [Ln(COO)2]n chain. In the unsymmetrical unit, there are one crystallographic independent Dy3+ ion, three L− anions and two coordinated H2O (Fig. 2a). The Dy3+ ion is eight-coordinated by four O atoms from four bridging L- anions, one chelated L- anion, and two H2O, and adopts a square antiprism coordination geometry (Fig. S3b and Table S2). The average distance of Dy–O is 2.372 Å and the angles of O–Dy–O range from 53.08(16) to 155.6(2)°, which are in the normal range [32,33]. As shown in Fig. 2b, the neighboring Dy3+ ions are bridged by double syn-anti carboxylate groups to form a regular [Dy(COO)2]n chain. The neighboring Dy3+···Dy3+ distances are 4.9325(3) and 4.7344 Å for Dy1···Dy1a and Dy1a···Dy1b, respectively. The adjacent chains stack together to form a 2D supramolecular network via π···π interactions along the b axis (Fig. S4b). (a)
(b)
Fig. 2. (a) The coordination environment of Dy3+ in complex 3 (symmetry codes: a= –x, 2–y, 1–z; b= 1-x, 2-y, 1-z). (b) The 1D chain along b axis. The H atoms are omitted for clarity.
Complexes 1-4 with light and heavy Ln3+ show different structural types, indicating the lanthanide contraction. It is also observed in Ln–O bond lengths, decreasing from Sm to Dy (Table S3). 3.3. PXRD and thermal gravimetric analyses (TGA) For complex 4, no suitable single-crystal was obtained for single-crystal X-ray diffraction analysis, and the powder X-ray diffractions (PXRD) were measured to analyse its structural feature of complex 4, as well as the phase purity of complexes 7
1-4 (Fig. 3). The results indicate the isostructural feature of complex 4 with 3. Furthermore, the experimental PXRD patterns of complexes 1-4 are in agreement with those simulated ones from single-crystal data, indicating the phase purity of the synthesized product. Owing to the different orientations of the samples, there are minor difference in the intensity and shape of the peaks between the experimental and simulated data. (a)
(b)
Fig. 3. The experimental and simulated PXRD patterns for complexes 1-4.
To investigate the thermal stability of complexes 1-4, the thermogravemetric analyses for 1-4 were performed and the samples are stable up to about 130°C (Fig. S5). Complexes 1 and 2 show similar thermal behavior. The first weigh loss corresponds to the loss of one H2O and one CH3OH (calcd. 7.5%, obsed. 7.4% for 1; calcd. 7.4%, obsed. 7.0% for 2) before 250 °C. On further heating, it began to decompose gradually. For complexes 3 and 4, the first weight loss from 130 to 270 °C is about 4.7%(3) and 4.9%(4), which is consistent with the weight loss of two H2O molecules (calcd. 5.3% for 3; calcd. 5.4% for 4), and the further weight loss indicate the decomposition of complexes. TGA results imply preferable thermal stability of complexes 1-4. 3.4. Luminescent properties Ln-complexes exhibit characteristic long luminescence lifetimes, large Stokes’ shifts, narrow emission peaks and high purity colors
[34]
. Therefore, the solid-state
luminescence of 1-4 was measured at room temperature.
8
(a)
(b)
(c)
(d)
Fig. 4. The solid-state emission spectra of 1-4 at room temperature.
As shown in Fig. S6, the excitation spectra of 1-4, monitored at the characteristic emissions (594, 616, 575 and 543 nm, respectively), exhibits the excitation peaks at 354, 350, 300 and 308 nm, respectively. Upon excitation at 354 nm, complex 1 displays the weak characteristic emissions of Sm3+ ion at 564, 594 and 630 nm, which are assigned to 4G5/2→6HJ (J = 5/2, 7/2, 9/2) transitions, respectively (Fig. 4a). The weak luminescence efficiency maybe due to poor matching of the triplet state of the HL with that of the emissive excited states of the Sm3+ ion. Meanwhile, an intense emission band centered at 428 nm is observed, which arises from ligand-centred (LC) luminescence
[35]
. Compared to complex 1, complex 2 clearly shows characteristic
emissions of Eu3+ ion at 538, 578, 591, 616, 652 and 659 nm excited at 350 nm, corresponding to the transitions (5D1→7F1) and 5D0→7FJ (J = 0-4), respectively, respectively (Fig. 4b). Moreover, the 5D0→7F2 (electric dipole) transition at 616 nm is stronger than that of the5D0→7F1 (magnetic dipole) transition at 591 nm, indicating Eu3+ ions in the low-symmetry coordination sites
[36-39]
, which is consistent with
single-crystal data. Complex 3 displays typical yellow luminescence of Dy3+ ion under excitation at 300 nm. Two typical intense emissions at 478 and 575 nm correspond to 4
F9/2→6HJ (J = 15/2 and 13/2), and the 4F15/2→6H13/2 transition is weak at 543 nm (Fig. 9
4c). For complex 4, when excited at 308 nm, it displays an intense green light of Tb3+ ion, dominated by the emission at 543 nm (5D4→7F5), similar to the most of reported Tb3+ complexes
[40-42]
correspond to
5
. Other characteristic emissions at 490, 581 and 620 nm
D4→7FJ (J = 6, 4, 3) transitions, respectively (Fig. 4d). As
above-discussed, the energy transfer from ligand to Ln3+ ions is more efficient in complexes 2, 3 and 4 compared with 1. 3.5. Luminescent probe (a)
(c)
(b)
(d)
Fig. 5. (a) and (b) Emission spectra of 2 and 4 in DMF with various cations (1×10−3 mol·L-1). (c) and (d) the 5D0→7F2 transition (616 nm) luminescence intensity for 2 and 5D4→7F5 transition (543nm) for 4 in various metal ions, respectively.
Since complexes 2-4 show intense luminescence, especially for complexes 2 and 4, and we wonder whether complexes 2 and 4 can act as luminescent probes for some metal ions. Thus we explore the sensing abilities of complexes 2 and 4 for some common metal ions. The powder samples of 2 and 4 (2 mg), respectively, were dissolved in DMF (4 mL) with addition of 1×10-3 mol·L-1 MClx (M = Mg2+, Ca2+, Zn2+, Na+, Cd2+, K+, Ni2+, La3+, Ce3+, Mn2+, Co2+, Al3+, Cu2+ and Fe3+; x = 1-3), and then, the luminescent properties were measured. As shown in Fig. 5, the luminescent intensity of 2 and 4 decreases upon adding those metal ions. Notably, the luminescence of complexes 2 and 4 was quenched upon addition of Fe3+ ions, indicating that complexes 2 and 4 maybe act as luminescent probes for Fe3+ ions. Furthermore, the 10
emitting color of 2 and 4 was observed in different analytes under 254 nm. As shown in Fig. S7, the emitting colors change from bright-red/green to dim colorless with increasing Fe3+ ions, respectively for complexes 2 and 4. Generally, there are many different metal ions in one system, whether the other coexisting metal ions will influence the luminescence. Thus, it is valuable to research the effect of mixed metal ions for luminescence
[43]
. Inspired by this point, 2 mg of
Ln-CPs 2 and 4, respectively, were immersed in 4 mL of DMF with Fe3+ and another metal ion (1×10−3 mol·L-1), and then the corresponding luminescence was measured. As shown in Fig 6, the luminescence quenching is still observed for 2 and 4 after introducing other metal ions. These results indicate that complexes 2 and 4 can selectively sense Fe3+ ions. (a)
(b)
Fig. 6. The luminescence intensities of 2 (a) and 4 (b) exposed to mixed metal ions (1×10−3 mol·L-1).
Subsequently, the luminescent titrations were further executed to study the sensitivity of 2 and 4 for Fe3+ ions, respectively. As shown in Fig. 7a and 7b, the luminescent intensity of 2 and 4 decrease gradually with increasing Fe3+ ions, which implies the diffusion-controlled quenching process
[44-47]
. And the luminescence
quenching effect can be quantitatively rationalized by the Stern−Volmer (S−V) equation (1): I0/I = 1 + Ksv[C]
(1)
where I0 and I are the initial luminescence intensity and after the addition of the Fe3+ ions, respectively; Ksv is the quenching constant (L·moL-1) and [C] is the molar concentration of the analyte [48-51]. For complexes 2 and 4, The S–V plots have a favorable linear fit (Fig. 7c and 7d, inset) with Ksv of 2.16 × 104 and 1.89 × 104 L·moL-1 at low concentration (0 − 1.5×10−4 11
mol·L-1), respectively. The high Ksv values are comparable with those reported Eu/Tb-complexes (Table S4), implying high sensitivity of complexes 2 and 4 for Fe3+ ions
[52]
. Whereafter, the S−V curves subsequently deviate from linearity at high
concentration. So an exponential quenching equation (2) is adopted to fit the nonlinear relation: I0/I = a · exp(k[C]) + b
(2)
in which a, b and k are constants [50,54]. The nonlinear S−V equation of 2 and 4 could be described as: I0/I = 0.5964 exp(0.011[C]) + 0.8089 (R2=0.9989) and I0/I = 0.4684 exp(0.0099[C]) + 1.2618 (R2=0.9958), respectively (Fig. 7c and 7d). The phenomenon may be attributed to the energy-transfer process or self-absorption [54-56]. (a)
(c)
(b)
(d)
Fig. 7. Emission spectra of 2 (a) and 4 (b) dispersed in DMF upon incremental addition of Fe3+ ions, and S–V plots of 2 (c) and 4 (d) with Fe3+ ions. Insets: the linear correlation for the plot of I0/I vs concentration.
In previous reports, the mechanism of deactivation of complexes for various metal ions have been studied, including weak interaction and competitive energy absorption.[57] The mechanism of complexes 2 and 4 for Fe3+ detection should be competitive energy absorption (Fig. S6 and S8), exhibiting the extensive overlaps between the UV-Vis spectrum of Fe3+ (Fig. S8) and the excitation spectrum of corresponding complexes (Fig. S6). 12
4. Conclusions In summary, a series of lanthanide coordination polymers (1-4) based on 2-chloroisonicotinic acid have been synthesized successfully under the presence of ZnO. Owing to the lanthanide contraction, complexes 1-4 exhibit two structural types. Complexes 1 and 2 are 1D [Ln(COO)2Ln(COO)4]n chain, while complexes 3 and 4 feature 1D [Ln(COO)2]n chain. Luminescence measurements reveals that all complexes exhibit the characteristic luminescence of corresponding Ln3+ ions. Especially, complexes 2 and 4 can be potential luminescent probes for Fe3+ ions. Meanwhile, this work provides a new insight in making new preferable luminescent materials as luminescent probes by using chloride-derivative ligands.
Acknowledgements This work was supported by the Subject Building Special Fund of Sichuan Agricultural University.
Appendix A. Supporting information Supplementary Information (SI) associated with this article can be found in the online version.
Appendix B. Supplementary data The supplementary crystallographic data for this paper were deposited with the Cambridge Crystallographic Data Centre Centre(CCDC), CCDC-1955102-1955104 (1-3). These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/conts/retrieving.html.
References [1] Z. Chen, Z. Y. Liu, Y. Liu, K. Z. Zheng, W. P. Qin, J. Fluorine Chem. 144 (2012) 157-164. [2] W. L. Leong, J. J. Vittal, Chem. Rev. 111 (2011) 688-764. [3] Y. Z. Zheng, Z. Zheng, X. M. Chen, Coord. Chem. Rev. 258 (2014) 1-15. [4] J. Rocha, L. D. Carlos, F. A. A. Paz, D. Ananias, Chem. Soc. Rev. 40 (2011) 926-940. [5] Y. J. Cui, J. Zhang, H. J. He, G. D. Qian, Chem. Soc. Rev. 47 (2018) 5740-5785. [6] K. Binnemans, Chem. Rev. 109 (2009) 4283-4374. [7] K. Staszak, K. Wieszczycka, V. Marturano, B. Tylkowsk, Coord. Chem. Rev. 397 (2019) 76-90. [8] H. Maas, A. Currao, G. Calzaferri, Angew. Chem. Int. Ed. 41 (2002) 2495-2497. [9] J. C. G. Bünzli, Acc. Chem. Res. 39 (2006) 53-61. [10] L. N. Zheng, F. H. Wei, H. M. Hua, C. Bai, X. L. Yang, X. F. Wang, G. L. Xue, Polyhedron 161 (2019) 47-55. [11] E. G. Moore, A. P. S. Samuel, K. N. Raymond, Acc. Chem. Res. 42 (2009) 542-552. 13
[12] T. B. Emelina, I. V. Kalinovskaya, A. G. Mirochnik, Acta Mol. Biomol. Spectrosc. Spectrochim. Acta A 207 (2019) 222-228. [13] M. Mihorianu, M. Leonzio, M. Bettinelli, F. Piccinelli, Inorg. Chim. Acta 438 (2015) 10-13. [14] O. Pietraszkiewicz, S. Mal, M. Pietraszkiewicz, M. Maciejczyk, I. Czerski, T. Borowiak, G. Dutkiewicz, O. Drobchak, L. Penninck, J. Beeckman, K. Neyts, J. Photochem. Photobiol. A‐Chem. 250 (2012) 85-91. [15] A. De Bettencourt‐Dias, P. S. Barber, S. Viswanathan, Coord. Chem. Rev. 165 (2014) 273-274. [16] S. Mal, M. Pietraszkiewicz, O. Pietraszkiewicz, Lumin. 33 (2018) 370-375. [17] M. Pietraszkiewicz, S. Mal, O. Pietraszkiewicz, Opt. Mater. 34 (2012) 1507-1512. [18] J. Heine, K. Muller-Buschbaum, Chem. Soc. Rev. 42 (2013) 9232-9242. [19] B. Li, H. M. Wen, Y. Cui, G. Qian and B. Chen, Prog. Poly. Sci. 48 (2015) 40-84. [20] O. Sun, T. Gao, J. W. Sun, G. M. Li, H. F. Li, H. Xu, C. Wang, P. F. Yan, CrystEngComm 16 (2014) 10460-10468. [21] X. Yao, X. Y. Wang, Y. Q. Han, P. F. Yan, Y. X. Li, G. M. Li, CrystEngComm 20 (2018) 3335-3343. [22] L. Armelao, S. Quici, F. Barigelletti, G. Accorsi, G. Bottaro, M. Cavazzini, E. Tondello, Coord. Chem. Rev. 254 (2010) 487-505. [23] K. Zheng, Z. Q. Liu, Y. F. Jiang, P. H. Guo, H. R. Li, C. H. Zeng, S. W. Ng, S. L. Zhong, Dalton Trans. 47 (2018) 17432-17440. [24] J. Z. Gu, Y. Cai, Y. Liu, X. X. Liang, A. M. Kirillov, Inorg. Chim. Acta 469 (2018) 98-104. [25] R. F. Li, Y. W. Zhang, X. F. Liu, X. H. Chang, X. Feng, Inorg. Chim. Acta 502 (2020) 119370. [26] H. Xu, H. C. Hu, C. S. Cao, B. Zhao, Inorg. Chim., 54 (2015) 4585-4587. [27] G. M. Sheldrick, SHELXS-97, Program for the Solution of Crystal Structures, University of Göttingen (Germany), 1997. [28] G. M. Sheldrick, SHELXL-97, Program for the Refinement of Crystal Structures, University of Göttingen (Germany), 1997. [29] S. Q. Su, W. Chen, C. Qin, S. Y. Song, Z. Y. Guo, G. H. Li, X. Z. Song, M. Zhu, S. Wang, Z. M. Hao, H. J. Zhang, Cryst. Growth Des. 12 (2012) 1808−1815. [30] A. Gusev, V. Shul'gin, E. Braga, E. Zamnius, N. Lyubomirskiy, M. Kryukova, W. Linert, J. Lumin. 212 (2019) 315-321. [31] C. N. Muniz, H. Patel, D. B. Fast, L. E. S. Rohwer, E. W. Reinheimer, M. Dolgos, M. W. Graham, M. Nyman, J. Solid State Chem. 259 (2018) 48-56. [32] W. M. Wang, X. H. Shi, H. X. Zhang, M. M. Wu, Y. L. He, M. Fang, Y. Shi, M. Fang, Polyhedron 141 (2017) 304-308. [33] X. Q. Zhao, D. X. Bao, J. Wang, S. Xiang, Y. C. Li, Inorg. Chim. Acta 466 (2017) 110-116. [34] F. H. Zhang, Y. Y. Wang, C. Lv, Y. C. Li, X. Q. Zhao, J. Lumin. 207 (2019) 561-570. [35] Y. H. Zhang, X. Li, S. Song, H. Y. Yang, D. Ma, Y. H. Liu, CrystEngComm, 43 (2014) 5974-5977. [36] G. Vicentini, L. B. Zinner, J. Zukerman-Schpector, K. Zinner, Coord. Chem. Rev. 196 (2000) 353-382. [37] A. Datcu, N. Roques, V. Jubera, I. Imaz, D. Maspoch, J. P. Sutter, C. Rovira, J. Veciana, Chem. Eur. J. 17 (2011) 3644-3656. [38] M. M. Castaño-Briones, A. P. Bassett, L. L. Meason, P. R. Sshton, Z. Pikramenou, Chem. Commun. (2004) 2832-2833. [39] X. Hu, W. Dou, C. Xu, X. L. Tang, J. R. Zheng, W. S. Liu, Dalton Trans. 40 (2011) 3412-3418. [40] X. P. Yang, R. A. Jones, J. Am. Chem. Soc. 127 (2005) 7686-7687. 14
[41] C. Yang, W. T. Wong, J. Mater. Chem. 11 (2001) 2898-2900. [42] K. Hanaoka, K. Kikuchi, H. Kojima, Y. Urano, T. Nagano, Angew. Chem., Int. Ed. 42 (2003) 2996-2999. [43] H. Xu, M. Fang, C. S. Cao, W. Z. Qiao, B. Zhao, Inorg. Chem. 55 (2016) 4790-4794. [44] G. Y. Wang, L. L. Yang, Y. Li, H. Song, W. J. Ruan, Z. Chang, X. H. Bu, Dalton Trans. 42 (2013) 12865-12868. [45] B. Chen, Y. Yang, F. Zapata, G. Lin, G. Qian, E. B. Lobkovsky, Adv. Mater. 19 (2007) 1693-1696. [46] W. Q. Tong, W. N. Liu, J. G. Cheng, P. F. Zhang, G. P. Li, L. Hou, Y. Y. Wang, Dalton Trans. 47 (2018) 9466-9473. [47] S. Dang, X. Min, W. Yang, F. Y. Yi, H. You, Z. M. Sun, Chem.–Eur. J. 19 (2013) 17172-17179. [48] W. Wei, R. J. Lu, S. Y. Tang and X. Y. Liu, J. Mater. Chem. A 3 (2015) 4604-4611. [49] B. L. Chen, L. B. Wang, Y. Q. Xiao, F. R. Fronczek, M. Xue, Y. J. Cui, G. D. Qian, Angew. Chem. Int. Ed. 48 (2009) 500-503. [50] Y. T. Yan, J. Liu, G. P. Yang, F. Zhang, Y. K. Fan, W. Y. Zhang, Y. Y. Wang, CrystEngComm 20 (2018) 477-486. [51] S. K. Sahoo, D. Sharma, R. K. Bera, G. Crisponi, J. F. Callan, Chem. Soc. Rev. 41 (2012) 7195-7227. [52] J. J. Huang, J. H. Yu, F. Q. Bai, J. Q. Xu, Cryst. Growth Des. 18 (2018) 5353-5364. [53] X. C. Sun, J. K. He, Y. T. Meng, L. C. Zhang, S. C. Zhang, X. Y. Ma, S. Dey, J. Zhao, Y. Lei, J. Mater. Chem. A 4 (2016) 4161-4171. [54] S. Y. Wu, Y. N. Lin, J. W. Liu, W. Shi, G. M. Yang, P. Cheng, Adv. Funct. Mater. 28 (2018) 1707169. [55] Y. Salinas, R. MartinezManez, M. D. Marcos, F. Sancenon, A. M. Castero, M. Parra, S. Gil, Chem. Soc. Rev. 41 (2012) 1261-1296. [56] S. S. Nagarkar, B. Joarder, A. K. Chaudhari, S. Mukherjee, S. K. Ghosh, Angew. Chem., Int. Ed. 52 (2013) 2881-2885. [57] J. L. Du, X. Y. Zhang, C. P. Li, J. P. Gao, J. X. Hou, X. Jing, Y. J. Mu, L. J. Li, Sens. Actuators-B Chem., 257 (2018) 207-213.
Author statement Xiaoqing Zhao: Conceptualization, Methodology, Writing-Review and Editing. Fenhang Zhang: Investigation, Writing-Original draft preparation. Yajun Liu: Resources. Tianhao Zhao: Data Curation. Hongyan Zhao: Validation. Shuo
Xiang: Resources. Yunchun Li: Supervision. Graphical abstract 15
A series of Ln-CPs (1-4) display two kinds of 1D chains due to the lanthanide contraction, which exhibit characteristic emission of corresponding Ln3+ ion. Furthermore, 2 (Eu) and 4 (Tb) act as luminescent probes for Fe3+ ions.
Highlights A series of luminescent Lnlll coordination polymers based on 2-chloroisonicotinic (HL) acid have been synthesized. Complexes 1(Sm) and 2(Eu) exhibit a 1D [Ln(COO)2Ln(COO)4]n chain, while complexes 3(Dy) and 4(Tb) are a 1D [Ln(COO)2]n chain. HL is an effective “antenna” to sensitize the luminescence of Ln3+ ions, and 2(Eu) and 4(Tb) should be potential luminescent probes for Fe3+ ions.
16