Article pubs.acs.org/IC
Mixed-Lanthanide Porous Coordination Polymers Showing RangeTunable Ratiometric Luminescence for O2 Sensing Jia-Wen Ye, Jiao-Min Lin, Zong-Wen Mo, Chun-Ting He, Hao-Long Zhou, Jie-Peng Zhang,* and Xiao-Ming Chen MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275, China S Supporting Information *
ABSTRACT: Luminescent porous coordination polymers (PCPs) are emerging as attractive oxygen-sensing materials, but they are mostly based on single-wavelength luminometry. Here, we report a special mixed-lanthanide strategy for selfreferenced ratiometric oxygen sensing. A series of isostructural, pure-lanthanide, or mixed-lanthanide PCPs, MCF-53(Tb/Eux), were synthesized by solvothermal reactions. Single-crystal Xray diffraction revealed that MCF-53(Tb/Eux) is composed of complicated two-dimensional coordination networks, which interdigitate to form a three-dimensional supramolecular structure retaining one-dimensional ultra-micropores. MCF53(Tb/Eux) can undergo multiple single-crystal to single-crystal structural transformations upon desorption/adsorption of coordinative and lattice guest molecules, and the lanthanide metal ions are partially exposed on the pore surface at the guest-free state. Tb(III) ions are not luminescent and only act as separators between Eu(III) ions, and the Tb(III)/Eu(III) mixing ratio can tune the relative emission intensities, luminescence lifetimes of the Eu(III) phosphorescence, and the ligand fluorescence, giving rise to not only ratiometric photoluminescence oxygen sensing but also tunable emission-color-changing ranges.
1. INTRODUCTION Luminescence materials have captured continuous attentions for sensing applications due to their superiority of no analyte consumption, fast response, high sensitivity, and no electrical connection.1−7 Inspecting oxygen level is requisite in plenty of fields, such as medicine, chemical industry, food packaging, biotechnology, and so on.8,9 Some luminescent porous coordination polymers (PCPs) have been recently reported as attractive oxygen-sensing materials,10−22 but they are mostly based on single-wavelength luminometry, which is always troublesome to overcome many challenges like drifts of the photoelectric sources and detectors, inner filter effects, variations of the luminescence probe (photobleaching, leaching, concentration, etc.), and background luminescence.1,2,23−25 In this regard, referenced analytical techniques relying on either external luminescence material or internal luminophor are more attractive. Compared with the external reference method,26 selfreferencing can be more effective, but it needs delicate material design to control energy transfer between multiple luminous building blocks.23,27,28 Lanthanide-based coordination complexes usually show outstanding photoluminescence arising from triplet excited states of f−f transitions.29−35 On account of the low absorption coefficients, the luminescence of lanthanide ions is usually generated through the “antenna effect”, which means that the organic ligands absorb the ultraviolet light and transfer the energy to the lanthanide ions.35,36 Mixing lanthanide ions in © 2017 American Chemical Society
coordination complexes can combine two phosphorescent centers with large emission difference, which is a common strategy to gain self-referenced luminescence probes for sensing temperature,37,38 biomolecules,39 and organic solvents.39−41 However, on the one hand, the triplet phosphorescence emissions of lanthanide ions can be strongly and similarly quenched by the oxygen molecules, which is not beneficial for ratiometric sensing.2 On the other hand, the short-lifetime fluorescence of organic ligand usually shows low sensitivity to oxygen. Therefore, a PCP showing simultaneous lanthanide phosphorescence and ligand fluorescence is a good candidate for ratiometric oxygen sensing. In this context, the key challenge is to balance the emission intensities of the organic ligands and lanthanide ions or the efficiency of energy transfer from the organic ligands to the lanthanide ions.42−45 In this work, we report a series of isostructural, purelanthanide or mixed-lanthanide PCPs, in which the ligand fluorescence and Eu(III) phosphorescence intensities/lifetimes, oxygen sensitivities, and emission-color-changing ranges can be finely tuned by the lanthanide mixing ratio.
2. EXPERIMENTAL SECTION Materials and Physical Measurements. All commercially available reagents and solvents were used as received without further Received: January 30, 2017 Published: March 23, 2017 4238
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Inorganic Chemistry purification. 2-(2-Pyridyl)-1H-imidazol-4,5-di(4-benzoic acid) (H3pidpa) was synthesized according to the literature method.46 Elemental analyses (C, H, and N) were performed with a Vario EL elemental analyzer. Coupled plasma-atomic emission spectrometry (ICP-AES) results were collected by a Optima8300 coupled plasmaatomic emission spectrophotometer. Powder X-ray diffraction (PXRD) data were collected on a Bruker D8 ADVANCE X-ray powder diffractometer (Cu Kα). Thermogravimetry (TG) analyses were performed on a TA Q50 instrument with a ramping rate of 10.0 °C min−1 under a nitrogen atmosphere. The UV-absorption spectra were measured by a UV-3600 UV−vis−near-IR spectro-photometer. Synthesis of [Tb1−0.01xEu0.01x(Hpidba)] (MCF-53(Tb/Eux)). A mixture of Tb(NO3)3·6H2O (1−0.01xfeed mmol), Eu(NO3)3·6H2O (0.01xfeed mmol), and H3pidba (1 mmol) in N,N-dimethylformamide (DMF, 30 mL) and H2O (4 mL) in a Teflon-lined stainless steel vessel (100 mL) was heated at 140 °C for 24 h and then cooled to room temperature at a rate of 5 °C h−1 to give yellow rodlike crystals, which were washed with DMF three times, filtered, and dried in air (yield 50−75%). Elemental analysis calculated for as-synthesized sample of MCF-53(Tb/Eu0) ([Tb(Hpidba)(HCOO)(H2O)0.5(HCOOH)0.5]· 3H2O or C23.5H22N3O10.5Tb): C 41.92%, H 3.29%, N 6.24%. Found: C 41.95%, H 3.50%, N 6.59%; for the activated sample (adsorbed H2O from air, [Tb(Hpidba) (HCOO)(H2O)]·4H2O or C23H24N3O11Tb): C 40.78%, H 3.57%, N 6.20%. Found: C 40.36%, H 3.44%, N 5.94%. The mixing ratios were determined by ICP-AES with the digested samples (in 4:1 HNO3/H2O2 at 190 °C for 30 min), which found x = 3.4, 21.4, 33.7 for xfeed = 4, 20, 40, respectively. X-ray Single-crystal Diffraction Analysis. Diffraction data were collected on a Rigaku XtaLAB P300DS diffractometer (Cu Kα, λ = 1.541 78 Å). Absorption corrections were applied by using the multiscan program CrysAlisPro. The structures were solved with the direct methods and refined with a full-matrix least-squares technique F2 with the SHELXTL program package. In the as-synthesized structure, occupancies of the terminal formic acid and H2O molecules are refined as 0.464(11) and 0.536(11), respectively. For convenience and simplicity, the occupancies were then both set as 0.5. All nonhydrogen atoms were refined anisotropically except 1.5 oxygen atom of lattice H2O in 1a. All hydrogen atoms were generated geometrically. Crystal data and details of data collection and refinements are summarized in Table S1. Additional crystallographic information is available in the Supporting Information. Photoluminescence Measurement. Steady-state photoluminescence spectra and lifetime measurements were performed using an Edinburgh FLS920 spectrometer equipped with a continuous Xe900 xenon lamp, 365 nm laser, and flash xenon lamp. After Soxhlet extraction by methanol, all the samples were heated at 160 °C for 2 h under vacuum before measurements. All instrument parameters such as the excitation slit, emission slit, and scanning speed were fixed during the measurements. O2 responses of photoluminescence were in situ measured by placing the sample inside an ARS Optical Cryostat with a three-way valve that connects the sample chamber to a vacuum pump and an O2 cylinder. The test temperature was controlled by a Lake Shore model 335 Cryogenic Temperature Controller. We also measured the O2-sensing property of 1 in mixed N2/O2. As shown in Figure S1, the Stem−Volmer (SV) plots of the mixed N2/O2 system and the pure O2 system are basically the same.
orthorhombic space group Pbca with one Tb(III) ion, one Hpidba2− ligand, one formate ligand, one terminally coordinated solvent (formic acid with 0.5 occupancy and H2O with 0.5 occupancy) molecule and three lattice guest H2O molecules in its asymmetric unit. The formic acid and formate in 1 should be generated by hydrolysis of DMF during the synthesis. Each Tb(III) ion is coordinated by four oxygen atoms from four Hpidba2− ligands, three oxygen atoms from two formate anions, and one oxygen atom from the coordinated solvent molecule (Figure 1a). Each Hpidba2− ligand coordinates to four Tb(III)
Figure 1. (a) Comparison of the coordination environments of 1a (red), 1b (green), and 1c (blue). For clarity, all hydrogen atoms and the terminal formic acid ligands are omitted, while the terminal H2O ligands are highlighted as spheres. Perspective views of the 2D network structure of 1a (coordinated solvent molecules are omitted for clarity) along the (b) a-, (c) b-, and (d) c-axes.
ions through two carboxylate groups, leaving the pyridyl group and neutral imidazole group uncoordinated. Each formate anion coordinates with two Tb(III) ions. Adjacent Tb(III) ions are bridged by carboxylate groups (Tb···Tb 4.63 Å) to form one-dimensional (1D) chains along the a-axis. These chains are interconnected by the V-shaped Hpidba2− ligands to form a complicated two-dimensional (2D) layer (Figure 1b−d). These complicated 2D layers pack in an interdigitated manner to form the three-dimensional (3D) supramolecular structure, in which narrow 1D channels (void ratio = 19.6%) running along the aaxis can be found between the layers (Figures 2a and S3).
3. RESULTS AND DISCUSSION Synthesis, Structures and Characterization. Solvothermal reaction of pure or mixed lanthanide salt Tb(NO3)3/ Eu(NO3)3 with H3pidba in DMF/H2O produced five samples formulated as [Tb 1 − 0 . 0 1 x Eu 0 . 0 1 x (Hpidba)(HCOO)(H2O)0.5(HCOOH)0.5]·3H2O (MCF-53(Tb/Eux), x = 0, 3.4, 21.4, 33.7, and 100 and hereafter denoted as 1, 2, 3, 4, 5, respectively), which are isostructural according to PXRD (Figure S2). The structures of 1 in different guest-inclusion states were studied by single-crystal X-ray diffraction. The assynthesized crystal of 1 (denoted as 1a) adopts the
Figure 2. Comparison of the pore surface structures of (a) 1a (coordinated solvent molecules are omitted) and (b) 1b. 4239
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Inorganic Chemistry TG analyses showed that the guest molecules and the coordinated solvent molecules in 1a can be completely removed below 220 °C, and after exchange by CH3OH via Soxhlet extraction, the sample can be readily activated at 120 °C (Figure S4). Interestingly, the guest-free sample can retain good single-crystallinity. The guest-free framework of MCF53(Tb/Eu0), that is, [Tb(Hpidba) (HCOO)] (denoted as 1b), adopts a different space group Pbcm. Furthermore, the unit-cell volume of 1b not only decreases by 3.9% but also halves from 1a. Consequently, the asymmetric unit of 1b contains only half a Tb(III) ion, half a Hpidba2− ligand, and half a formate anion. Compared with 1a, the formate ligand shifts a little toward the position originally occupied by the coordinated solvent molecule (Figure 1a), meaning that the open space generated by removal of the coordinated solvent molecule is partially retained.47 In 1b, the effective channel size is 3.8 Å, and the void ratio remains 15.5% (Figure 2b). After it stood in the air for 24 h, 1b reabsorbed water and changed into [Tb(Hpidba)(HCOO)(H2O)]·4H2O (denoted as 1c) with retention of good single-crystallinity. The structure of 1c is almost identical with 1a (Figure 1a and Table S1). It is worth pointing out that examples of multistep single-crystal to single-crystal (SCSC) structural transformation are rare, because the stresses produced during the structural transformation tend to crack the fragile/large crystals, and there are more damages to the single-crystallinity after multiple structural transformation processes.48,49 Optical O2-Sensing Properties. Samples 1−5 possess similar ultraviolet (UV) absorption spectra (Figure S5), which can be assigned to the n→π* transition of the organic ligand. Photoluminescence spectra of guest-free 1−5 were studied at room temperature. For 1 in vacuum, weak blue emission (483 nm) of the ligand was observed rather than the characteristic Tb(III) emission peaks (Figure 3a,b), illustrating that the excited-state energy of the ligand does not match the 7F6∼5D4 energy gap of the Tb(III) ions (425−430 nm, 23 000−24 000 cm−1).36 At 1 bar O2, the blue luminescence of 1 at 483 nm was quenched only by 31% (Figure 3), corresponding to I0/I100 = 1.45. In contrast, 5 displays not only the ligand fluorescence (483 nm) but also the characteristic Eu(III) emission peaks at 578, 590, 617, 650, and 700 nm, corresponding to 5D0−7FJ (J = 0−4) transitions, meaning that the excited-state energy of the ligand fits well that of the 7F0∼5D0 transition of Eu(III) (492− 505 nm, 19 800−20 300 cm−1).36 In 1 bar oxygen, the Eu(III) and ligand emission peaks of 5 were quenched by 50% (at 617 nm) and 11% (at 483 nm), respectively (Figure 3). The relatively high quenching efficiency of the Eu(III) luminescence can be attributed to its triplet excited state, which possesses longer lifetime.2 Because of the dissimilar quenching efficiencies and the large difference of emission colors (Δλ = 134 nm) from the two luminophors, 5 may be used for ratiometric oxygen sensing. Unfortunately, the luminescence color-changing of 5 is quite small (Figure 4), because of the relatively low oxygen sensitivity of the Eu(III) emission. Like 5, the mixed-lanthanide samples 2, 3, and 4 display the ligand and Eu(III) emission peaks simultaneously. With the decreasing concentration (x) of Eu(III), the red emission of Eu(III) becomes weaker compared with the blue emission of the ligand (Figure S6). However, the Eu(III) phosphorescence lifetime increases along with the decrease of x (Figure S7 and Table 1). In the lanthanide systems, the luminescence intensity is often in direct proportion to the luminescence lifetime.29,30 On the one hand, reducing the concentration of Eu(III), that is,
Figure 3. (a) The emission spectrum of (a) H3pidba (1 × 10−4 M) in DMF. (b−f) Emission spectra of 1 (b), 2 (c), 3 (d), 4 (e), 5 (f) at different O2 pressures (excitation at 365 nm).
Figure 4. Chromaticity coordinates (CIE) of 2 (red ●), 3 (blue ■), 4 (green ▲), and 5 (black ★) at different O2 pressures.
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inspire the design, synthesis, and characterization of PCP-based luminescence sensing materials.
Table 1. Luminescence Parameters of MCF-53(Tb/Eux) compound x 483 nm lifetime/ns quenched at 1 bar O2 I0/I100 617 nm lifetime/μs quenched at 1 bar O2 I0/I100
1 0 1.38 31% 1.45
2 3.4 1.06 30% 1.43 416 76% 4.17
3 21.4 1.00 20% 1.25 256 71% 3.42
4 33.7 0.64 19% 1.23 201 64% 2.78
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5 100 0.46 11% 1.13 92 50% 1.99
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00252. TG curves, PXRD patterns, calculation of photoluminescence quenching, luminescence lifetime analysis, 3D aphroid structures of 2D layers, UV−absorption spectra, emission spectra, emission decay profiles, SV plots of luminescence quenching data, phosphorescence/ fluorescence intensity ratios, time-dependent photoluminescence intensity plots, crystallograpy data and structural refinements, luminescence quenching parameters (PDF) X-ray crystallographic information (CIF)
increase the separation between nearest Eu(III) ions by insertion of Tb(III) ions, can reduce the energy transfer and self-quenching effect between adjacent Eu(III) ions.50 On the other hand, it reduces the ligand-to-Eu(III) energy transfer to increase the ligand fluorescence, as demonstrated by its gradually increased lifetime (Figure S7 and Table 1). By decreasing the Eu(III) concentration, the quenching efficiency of the Eu(III) phosphorescence of 4, 3, and 2 for 1 bar oxygen increases to 64%, 71%, and 76%, respectively. At the same condition, the quenching efficiency for the ligand fluorescence slightly increases to 19%, 20%, and 30%, respectively (Figure 3 and Table 1). The relationships of O2 pressures and emission intensities of these samples can be fitted by the two-site SV model (Figures S1 and S8 and Table S2).51 The different degrees of the modulation of quenching efficiencies for the ligand and Eu(III) emissions by changing the Eu(III) concentration can be useful for tuning the emissioncolor-changing range of ratiometric oxygen sensing (Figures 4 and S9). In vacuum, the emission of 2, 3, 4, and 5 can be described as bluish white, yellowish pink, yellowish pink, and orange with CIE coordinates of (0.24, 0.28), (0.41, 0.32), (0.44, 0.34), and (0.50, 0.36), respectively. At 1 bar O2, the emission colors turn into greenish blue, bluish white, pinkish white, and orange yellow with CIE coordinates of (0.20, 0.26), (0.30, 0.30), (0.35, 0.33), and (0.44, 0.36), respectively. The colorchanging range of 3 is the largest, meaning it has a suitable Eu(III) concentration and quenching efficiency. We also measured the photoluminescence responses of the four Eu(III)-containing samples in alternatively changing cycles between 1 bar O 2 and vacuum (Figure S10). Their luminescence intensities can reach equilibrium quickly after the change of oxygen concentration, and there was no obvious luminescence lost after 10 cycles of test.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jie-Peng Zhang: 0000-0002-2614-2774 Xiao-Ming Chen: 0000-0002-3353-7918 Notes
The authors declare no competing financial interest. CCDC 1529217−1529219 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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ACKNOWLEDGMENTS This work was supported by the “973 Project” (2014CB845602), NSFC (91622109 and 21473260), and National Key Scientific Instrument and Equipment Development Project (2013YQ24051107).
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REFERENCES
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4. CONCLUSIONS In summary, a series of pure-lanthanide and mixed-lanthanide PCPs were synthesized and structurally characterized at different guest-inclusion states, which undergo multistep SCSC guest adsorption/desorption and can retain partially open metal sites at the guest-free state. Because the excitedstate energy of the ligand can transfer to Eu(III) instead of Tb(III), Tb(III) ions are not luminescent and only act as separators between Eu(III) ions. The long-lifetime Eu(III) phosphorescence is more sensitive to oxygen than the shortlifetime ligand fluorescence, giving ratiometric photoluminescence against different oxygen concentrations. Decreasing the Eu(III) concentration or increasing the separation between Eu(III) ions significantly increases the phosphorescence lifetime and slightly increases the ligand fluorescence lifetime. Therefore, emission-color-changing ranges of the PCPs can be finely tuned by the lanthanide mixing ratio. These results may 4241
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DOI: 10.1021/acs.inorgchem.7b00252 Inorg. Chem. 2017, 56, 4238−4243
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DOI: 10.1021/acs.inorgchem.7b00252 Inorg. Chem. 2017, 56, 4238−4243
