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Unique Chiral Interpenetrating d−f Heterometallic MOFs as Luminescent Sensors Zhi-Lei Wu,†,‡ Jie Dong,† Wei-Yan Ni,† Bo-Wen Zhang,† Jian-Zhong Cui,‡ and Bin Zhao*,† †

Department of Chemistry, Key Laboratory of Advanced Energy Material Chemistry, MOE, TKL of Metal and Molecule Based Material Chemistry, and Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Nankai University, Tianjin 300071, China ‡ Department of Chemistry, Tianjin University, Tianjin 300072, China S Supporting Information *

ABSTRACT: One novel three-dimensional (3D) 3d−4f metal−organic framework (MOF), [TbZn(L)(CO3)2(H2O)]n (1) [HL = 4′-(4-carboxyphenyl)-2,2′:6′,2″-terpyridine], has been successfully synthesized and structurally characterized. Structural analysis shows that compound 1 features a unique chiral interpenetrating 3D framework for the first time. The resulting crystals of 1 are composed of enantiomers 1a (P41) and 1b (P43), as was clearly confirmed by the crystal structure and the corresponding circular dichroism (CD) analyses of eight randomly selected crystals. The investigations on CD spectra based on every single crystal clearly assigned the Cotton effect signals. The powder X-ray diffraction measurement of 1 after being immersed in common solvents reveals that 1 possess excellent solvent stability. Furthermore, luminescent studies imply that 1 displays highly selective luminescent sensing of aldehydes, such as formol, acetaldehyde, and propanal.

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INTRODUCTION Chirality, as “a signature of nature”, plays an important part in many aspects of our society. In the past several years, the design and synthesis of chiral metal−organic frameworks (MOFs) have been a frontier research field not only because of their intriguing architectures and topologies but also for their potential applications in the field of nonlinear optics, catalysis, magnetism, and enantiomerically selective separation.1 To synthesize target MOFs with chiral structures, chemists have made great efforts, and various chiral MOFs have been reported by many groups.2 The chiral MOFs were mainly constructed by the following ways: (1) chiral molecule as ligands can introduce their inherent chirality into the frameworks via a “chirality conservation” process;3 (2) with the help of chiral induction © 2015 American Chemical Society

agents in the reaction system to affect the growth of nucleation or crystal, such as camphoric acid and cinchona alkaloids are most frequently employed in the chiral synthesis;4 and (3) using the achiral precursors to construct a homochiral MOFs or a pair of enantiomers by means of spontaneous resolution.5 It should be noted that, most of reported chiral MOFs almost focused on the transition metal-based frameworks, while chiral d−f heterometallic MOFs were little explored. Indeed, the d−f heterometallic MOFs, featuring the intrinsic properties of d- and f-block, have potential application in various fields such as luminescence, adsorption, catalysis, Received: February 1, 2015 Published: May 19, 2015 5266

DOI: 10.1021/acs.inorgchem.5b00240 Inorg. Chem. 2015, 54, 5266−5272

Article

Inorganic Chemistry chemsensor and magnetism.6−8 Nevertheless, the synthesis of d−f frameworks confronts a great challenge in that a fierce competition exists between d and f metals that are coordinated to the same ligand, frequently resulting in the generation of homometallic rather than heterometallic compounds. To date, only several chiral d−f heterometallic MOFs were reported based on chiral ligand. For example, Dang et al. introduced the chiral camphoric acid into a series of 3D homochiral [Mn−Ln] frameworks for the first time.9 Subsequently, Zheng’s group synthesized a family of chiral d−f heterometallic MOFs based on the chiral D-camphoric acid ligand.10 Nevertheless, to our knowledge, the chiral interpenetrated d−f heterometallic MOFs have never been reported hitherto. Inspired by this, we may expect to integrate both chirality and d−f block into one single system not only for fascinating topological structure but also for versatile applications. In this contribution, according to the soft−hard theory that the d and f ions have different affinities for N and O donors, we select the ligand HL (4′-(4-carboxyphenyl)-2,2′:6′,2″-terpyridine) to imbed both 3d and 4f ions into the same MOFs. Simultaneously, the ligand HL with large conjugated π electronic systems serves as an ideal luminescent chromophore, which can transfer the energy to the luminescent centers effectively. As a result, a unique compound [TbZn(L)(CO3)2(H2O)]n (1), was successfully obtained by reaction of HL, Zn(CH3COO)2, Tb(NO3)3 in a mixed DMF/H2O solution under solventhermal condition. 1 represents the first chiral 2-fold interpenetrating d−f MOFs and can serve as luminescent sensors with high sensitivity to detect formol, acetaldehyde, and propanal. The CD spectra of several single crystals were explored, and the Cotton effect of two enantiomers was clearly determined.

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Table 1. Crystallographic Data for Compounds 1a and 1b compound formula Fw crystal system space group a, Å b, Å c, Å α, deg ß, deg γ, deg V, Å3 Z Dc, mg/mm3 μ, mm−1 reflns collected 2θ range, deg F(000) GOF on F2 R1/wR2 (I > 2σ(I)) R1/wR2 (all data) Flack parameter

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1a C24H16N3O9TbZn 714.69 P41 tetragonal 12.1584(5) 12.1584(5) 15.0541(7) 90 90 90 2225.39(17) 4 2.133 4.295 4039 6.36−50.02 1392 1.077 R1 = 0.0302, wR2 = 0.0663 R1 = 0.0315, wR2 = 0.0672 −0.071(18)

1b C24H16N3O9TbZn 714.69 P43 tetragonal 12.1871(3) 12.1871(3) 15.1107(4) 90 90 90 2244.31(10) 4 2.115 4.259 8621 6.34−50 1392 1.054 R1 = 0.0413, wR2 = 0.0743 R1 = 0.0471, wR2 = 0.0765 −0.039(19)

RESULTS AND DISCUSSION Crystal Structure. X-ray crystal structure analysis reveals that the compound 1 crystallizes in two types of space groups P41 (1a) and P43 (1b). The structure of compound 1b as a representative is described in detail. The asymmetric unit of 1b consists of one Tb3+ ion, one Zn2+ ion, one L− ligand, one coordinating water molecule, and two CO32− anions from the decomposition of DMF (Figure 1). The nine-coordinated Tb3+

EXPERIMENT SECTION

Materials and General Methods. All chemicals were of commercial origin without further purification except that the Tb(NO3)3·6H2O was prepared by HNO3 and corresponding rareearth oxide. The C, H, and N microanalyses were carried out at the Institute of Elemental Organic Chemistry, Nankai University. Powder X-ray diffraction measurements were collected on a D/Max-2500 Xray diffractometer using Cu Kα radiation. TGA were performed on a Labsys NETZSCH TG 209 Setaram apparatus under N2 from room temperature to 800 °C at a heating rate of 2 °C·min−1. The solid-state circular dichroism (CD) spectra were recorded on a Jasco J-750 spectropolarimeter using KBr pellets. The fluorescent spectrum were measured on a Varian Cary Eclipse Fluorescence spectrophotometer at room temperature. Preparation of [TbZn(L)(CO3)2(H2O)]n. A mixture of HL (0.1 mmol), Tb(NO3)3·6H2O (0.2 mmol), Zn(CH3COO)2 (0.2 mmol), and 5 mL of DMF/water (4:1) solutions were sealed in a Teflon-lined stainless vessel (25 mL) and heated at 160 °C for 96 h, and then the vessel was cooled slowly to room temperature at 1.5 °C/h, affording the products as yellow needle shaped crystals. The yield was 47% based on Tb. Elemental analysis (%) calcd: C, 40.30; H, 2.24; N, 5.88. Found: C, 40.05 H, 2.13; N, 5.79. Crystal Structure Determination. Single-crystal X-ray diffraction measurement was carried out on a Bruker Smart Aepex CCD single crystal diffractometer equipped with graphite-monchromatic Mo Kα radiation (λ = 0.71073 Å). The structures were solved by direct methods and refined by full-matrix least-squares techniques based on F2 using the SHELXS-97. All the non-hydrogen atoms were refined with anisotropic parameters, while hydrogen atoms were placed in calculated positions and refined using a riding model. The selected crystal parameters, data collection, and refinements are summarized in Table 1.

Figure 1. Asymmetric unit of 1b with an ellipsoid probability of 30% (all H atoms are omitted for clarity).

is surrounded by six oxygen atoms (O3, O4, O7, O8, O4A, and O7A) from CO32−, two oxygen atoms (O1A and O2A) from the carboxylate group of L− ligand, and one oxygen atom (O6) from water molecules, forming a distorted tricapped trigonal prismic coordination geometry (Figure 2a). The Tb−O bond lengths range from 2.301 to 2.503 Å, and the O−Tb−O angles fall in the range from 52.68(17) to 149.2(2)°, all of which are comparable to the reported previously.11 The five-coordinated environments of the Zn2+ ion are completed by three nitrogen 5267

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Figure 2. (a) Nine coordinated environment of Tb3+ in compound 1b. (b) The five coordinated environment of Zn2+ in compound 1b.

atoms (N1, N2, and N3) from L− ligand and two oxygen atoms from CO32−, displaying a square pyramid geometry (Figure 2b). Each CO32− with the μ3-η2,η2-coordination mode (Scheme 1) connects two Tb3+ ions and one Zn2+ ion, extending into a

left-handed helical chain along a axis with a 4-fold screw axis and a pitch of 15.111 Å (Figure 3a). Zn2+ ions wrap around the screw axis with the closest distance of 4.141 Å for Tb3+···Zn2+ and 5.963 Å for Zn2+··· Zn2+. The distance of two nearest Tb3+ is 3.895 Å. The neighboring helical chains are connected by L− anions into a 3D framework with large square grid channels of 10.9 × 10.9 Å2 along c axis (Figure 3b). Interestingly, two sets of the independent 3D frameworks interpenetrate each other so close that there is almost no solvent accessible void (Figure 3c). For clarity, the interpenetrated structure of 1b can be simplified as a model (Figure 3d), in which every upright pillar stands for the Tb-chain bridged by CO32−, and the green tape represents 4-fold screw comprised of Zn2+ ions, as well the line between adjacent pillars represent ligand L−. In the interpenetrated model of 1b, all of helical chains have left-handed characteristic, while the structure of 1a with P41 space group corresponds to right-handed helix (Figure S1, ESI). Compared with lots of

Scheme 1. Coordination Modes of CO32− (a) and L− (b) Observed in Compound 1b

Figure 3. (a) Left-handed helical arrangement in 1b along the b axis. The pitch is highlighted. (b) One set of the 3D framework in 1b along the c axis. (c) 2-fold interpenetrating 3D framework in 1b viewed from the c axis. (d) The simplified interpenetrating model in 1b. 5268

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Inorganic Chemistry Table 2. Summary of Structure Determinations of 1 (Eight Crystals) with Various Refinement Parameters #1 #2 #3 #4 #5 #6 1a 1b

a

b

c

space group

R1

wR2

Flack parameter

12.1642(2) 12.1724(5) 12.1822(9) 12.1343(4) 12.1803(3) 12.1807(3) 12.1584(5) 12 0.1871(3)

12.1642(2) 12.1724(5) 12.1822(9) 12.1343(4) 12.1803(3) 12.1807(3) 12.1584(5) 12.1871(3)

14.9912(4) 15.0581(6) 15.1170(8) 14.9938(6) 15.0789(4) 15.0545(4) 15.0541(7) 15.1107(4)

P43 P43 P41 P43 P43 P41 P41 P43

0.0342 0.0347 0.0535 0.0365 0.0292 0.0325 0.0302 0.0413

0.0776 0.0726 0.1045 0.0698 0.0641 0.0677 0.0672 0.0765

−0.01(3) −0.054(19) −0.03(3) −0.029(19) −0.034(17) −0.020(18) −0.071(18) −0.039(19)

at room temperature. As shown in Figure S2, the simulated and experimental powder X-ray diffraction patterns are well consistent from each other, indicative that 1 has good phase purity. After the sample of 1 was immersed in various solvents for 24 h, including methanol, ethanol, acetonitrile, DMF, DMA, formol (40%), acetaldehyde (40%), propanal, 1,4-dioxane, benzene, CCl4, methylbenzene, propylene oxide, and acetone, the individually experimental powder X-ray diffraction patterns are almost in agreement with the corresponding simulated one (Figure 5), suggesting that 1 possesses high chemical stabilities. Thermogravimetric analyses (TGA) of 1 display that 3D framework starts to collapse at 330 °C, indicative of excellent thermal stability of the compound (Figure S4). Luminescent Properties. The excitation and emission spectra of 1 at room temperature were explored (Figure S5, Supporting Information, SI). The excitation peak about 338 nm can be ascribed to the absorption of the ligand. Upon excited at 330 nm, the compound 1 displays the characteristic emissions of Tb3+ at 487, 544, 585, and 621 nm, belonging to the transition of 5D4 → 7FJ (J = 6−3) of Tb3+ ion, respectively.13 In order to investigate the effect of organic molecules on luminescent density of 1, the luminescence of 1 dispersed into different solvents were performed (Figure 6). The results reveal that the corresponding luminescent densities strongly depend on the solvents. Interestingly, formol, acetaldehyde, and propanal almost make the luminescence completely disappear, while the emission of 1 is still significantly observed under the effect of other solvents. The results imply that 1 can be considered as luminescent sensors of formol, acetaldehyde, and propanal. It should be noted that three types of solvent molecules are harmful to the health and environments, and especially formol, being a fatal pollutant indoors.14 Next, the changes of luminescent intensities with different concentrations of formol, acetaldehyde, and propanal were investigated in ethanol. The luminescent intensities of compound 1 were monitored by increasing the rate of formol/ethanol, acetaldehyde/ethanol, and propanal/ethanol gradually. The luminescence of 1 slowly deceases with increasing the formol (Figure 7). The lowest detection limit is 0.9 vol %, and the luminescent intensities almost quench at the formol content of 51.8 vol %. Amazingly, acetaldehyde and propanal have the same changing trend as that of formol (Figures 8 and 9). The lowest detection limits of them are 0.5 and 0.25 vol %, respectively. Strikingly, all the luminescence response times were very fast within ten seconds. To our knowledge, MOFs materials as luminescent sensors for formol were little reported,15 although many kinds of MOFs as luminescent probes of cations,16 anions,17 and other organic small molecules18 were well reported. The mechanism for the quenching effects of small molecules are also investigated, when 1 was immersed in other aldehyde

interpenetrated MOFs based on the transition metal ions, the interpenetrated d−f MOFs are very rare hitherto. The first interpenetrated d-f MOFs was obtained by us in 2006,12a and the second example was reported based on rigid ligand in 2009.12b To our knowledge, 1 gave the first example of the d−f heterometallic MOFs with both chiral and interpenetrated features. CD Spectra. In order to investigate the absolute configurations of the compound 1, the crystal structure of eight randomly picked crystals of 1 from the same batch were carefully determined. The results reveal that five of them crystallize in space group P43, and three have a space group P41, all of which the Flack parameters are near zero (Table 2). Namely, one single crystal belongs to either P41 or P43 space group in crystallizing, and the mixture of chiral P41 and P43 was not observed in these crystals. The case indicates that 1a and 1b can realize spontaneous resolution during the course of crystallization. Furthermore, the individual solid-state CD spectra were measured on eight single crystals in KBr plates (Figure S3, ESI). The CD spectra of compounds 1a and 1b display two different Cotton signals (Figure 4), which are

Figure 4. Solid-state CD spectrum of 1a and 1b from one batch at room temperature.

mirror images for each other. The single crystal with space group P41 and P43 possesses a positive and negative Cotton signal at 320 nm, respectively, but the inversion appears at 560 nm. Remarkably, in reported chiral MOFs, it is very difficult to clearly assign Cotton effect signals to specifically chiral space group, because a pair of enantiomers often cocrystallizes in one single crystal with a considerable Flack parameter. Powder X-ray Diffraction. In order to confirm the phase purity of 1, the powder X-ray diffraction pattern was performed 5269

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Figure 5. PXRD patterns of compound 1 after immersing in different solvents for 24 h.

Figure 6. PL spectra of 1 dispersed into different solvents when excited at 330 nm.

Figure 8. PL spectra of 1/ethanol suspension in the presence of various amounts of acetaldehyde solvent.

Figure 7. PL spectra of 1/ethanol suspension in the presence of various amounts of formol solvent.

Figure 9. PL spectra of 1/ethanol suspension in the presence of various amounts of propanal solvent.

compounds such as glutaraldehyde and benzaldehyde, the PL spectra also exhibit quenching effects, respectively (Figure S6, 5270

DOI: 10.1021/acs.inorgchem.5b00240 Inorg. Chem. 2015, 54, 5266−5272

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Inorganic Chemistry SI). It indicates that the electron-withdrawing HCO group definitely plays an important role during quenching the luminescence. After the solvent-suspension luminescent measurements, the FTIR spectra of 1 in different aldehydes solvents were investigated. As shown in the SI, Figure S7, the result indicates that the functional groups of 1 before and after aldehydes sensing do not change. Furthermore, the crystal structure of 1 in formol solvent is determined, confirming CO32− still exits. Given the interpenetrating frameworks in 1, it is very difficult for these small guest molecules to enter into the channels of 1. But it should be noted that compound 1 was finely ground and could disperse evenly in the solution, and it is likely for the organic molecules to attach on the surface of the MOFs, decreasing the energy transfer from ligand to the Tb3+ center, resulting in luminescent quenching of 1. Additionally, when 1 was immersed in formol, acetaldehyde, and propanal vapors, the PL spectra of 1 in solid state all deceases in different degree (Figures S8−10, SI), so 1 can also be used as a vapor detection sensor. In summary, the first chiral and interpenetrating d−f heterometallic MOFs was structurally and luminously characterized. The investigations on combined the structures and CD spectra of lots of single crystals clearly assigned the Cotton effect signals. Interestingly, 1 as luminescent sensors can detect small organic molecules formol, acetaldehyde and propanal.

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ASSOCIATED CONTENT

* Supporting Information S

Structural analysis, PXRD patterns, TGA, solid-state CD spectra, PL spectra, and selected bond lengths and angles for 1a and 1b. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.inorgchem.5b00240.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 Program (2012CB821702 and 2011CB935902), NSFC (21421001 and 21473121), 111 project (B12015), and MOE Innovation Team (IRT13022 and IRT-13R30) of China.

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DOI: 10.1021/acs.inorgchem.5b00240 Inorg. Chem. 2015, 54, 5266−5272

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DOI: 10.1021/acs.inorgchem.5b00240 Inorg. Chem. 2015, 54, 5266−5272