A Multifunctional Nanocage-based MOF with Tri- and Tetranuclear Zinc Cluster Secondary Building Units

A new Zn-cluster based MOF, [Zn21(BTC)11(μ3-OH)3(μ4-O)3(H2O)18]·21EtOH (1) (H3BTC = 1,3,5-benzenetricarboxylic acid), with two different types of cluster nodes has been successfully synthesized from Zn2+ and H3BTC under the solvothermal conditions. Single crystal X-ray diffraction studies reveal that 1 is a 3D trinodal (3,5,6)-c framework which features a large octahedral cage organized by nine Zn3O and nine Zn4O clusters SBUs and twenty-four triangular BTC3− linkers. The Eu3+/Tb3+-incorporated derivative of 1 with 0.251% Eu3+ and 0.269% Tb3+ exhibits tunable luminescence from yellow to white and then to blue-green by changing the excitation wavelength from 308 to 315 nm. Metal ion exchange with Cu2+ affords isomorphous Cu-based MOF with enhanced N2 and CO2 adsorption capacity. In addition, 1 can act as a selective luminescent sensor for Cu2+ and Al3+ ions.

In the past decade, luminescent MOFs have emerged as promising candidates for the rapid, sensitive and accurate recognition of metal ions [22][23][24][25][26][27] . The recognition of metal ions plays a very important role in many aspects, including our life 28,29 . The Cu 2+ and Al 3+ ions, for instance, are necessary for maintenance of human metabolism. Nevertheless, high concentrations of Cu 2+ and Al 3+ can lead to many adverse health effects. Therefore, the design and synthesis of luminescence MOFs capable of sensing Cu 2+ and Al 3+ is very important 30,31 . Herein we report a novel Zn-cluster based MOF, [Zn 21 (BTC) 11 (µ 3 -OH) 3 (µ 4 -O) 3 (H 2 O) 18 ]·21EtOH (1) (H 3 BTC = 1, 3, 5-benzenetricarboxylic acid) built from a triangular Zn cluster SBU, a tetrahedral Zn cluster SBU and a tritopic linker BTC 3− .Tunable colour and white light emission can be achieved by varying the excitation wavelength and incorporating appropriate amount of Eu 3+ /Tb 3+ in the pore of 1. In addition, compound 1 also exhibits a great potential as a luminescence sensing material for Cu 2+ and Al 3+ ions.

Results and Discussion
Synthesis and description of crystal structure. Colourless crystals of 1 were synthesized by the solvothermal reaction of Zn(NO 3 ) 2 ·6H 2 O,H 3 BTC and 4-cyanopyridine in a 1:1:2 molar ratio, in ethanol (10 ml) at 110 °C for three days. Single crystal X-ray diffraction studies reveal that 1 crystallizes in the trigonal space group R3. The asymmetric unit of 1 contains seven Zn 2+ ions, 11/3 BTC 3− ligands, one μ 3 -OH − anion, one μ 4 -O 2 − anion and six coordinated water molecules (Fig. S1). The structure contains two types of Zn clusters. One is the trinuclear cluster [Zn 3 (µ 3 -OH)(COO) 5 (H 2 O) 3 ] (simplified as Zn 3 O) and the other is the tetranuclear cluster [Zn 4 (µ 4 -O)(COO) 6 (H 2 O) 3 ] (simplified as Zn 4 O). In the Zn 3 O cluster, there is a µ 3 -OH group located at the centre of the cluster. Three Zn ions in Zn 3 O adopt different coordination geometries: Zn5 resides in a distorted tetrahedral geometry, whereas Zn6 and Zn7 adopt a square pyramidal and an octahedral geometry respectively (Fig. 1a). The tetranuclear cluster consists of two ZnO4 tetrahedra, a ZnO5 square pyramid and a ZnO6 octahedron sharing a central µ 4 -O atom (Fig. 1b). The Zn-O bond lengths and angles varied in the normal ranges of 1.885(9)-2.382(16) Å and 83.0(5)-176.6(5)°, respectively. The average Zn···Zn separation in the Zn 3 O cluster is 3.357 Å which is slightly larger than that in the Zn 4 O cluster (3.196  A better insight into this cage can be achieved through connecting the Zn clusters which generates a slightly distorted octahedron whose faces are each composed of four small triangular faces. The size of the octahedral cage is ca. 30.7 × 21.5 × 21.5 Å (Fig. 1e). It has been known that the C 3 -symmetric ligand H 3 BTC is useful for the construction of Zn-BTC octahedral cages. However, the short spacer of H 3 BTC usually leads to small cages [32][33][34][35] . While increasing the lengths of C 3 -symmetric ligands can afford large cages 36 , this work demonstrates that the Zn-BTC octahedral cage can also be expanded with Zn 3 O and Zn 4 O cluster nodes. In 1, each octahedral cage serves as a 6-connected octahedral node and shares two edge-fused triangles of six faces with six surrounding octahedra (Fig. 1f), generating a complicated 3D microporous framework (Fig. 1g). Topologically, the Zn 3 O and Zn 4 O clusters can be considered as distorted square pyramidal and octahedral SBUs (5-and 6-connected nodes, respectively), and the BTC 3− linker as a triangular unit (3-connected node) ( Fig. S2a and b). Thus, the 3D framework of 1 can be viewed as a (3, 5, 6)-connected net ( Fig. S2c and d).
To prove the phase purity of the bulk sample, PXRD analysis is performed. The peak positions of the simulated pattern closely match those of the experimental one, indicating phase purity of the as-synthesized sample (Fig. S3). Thermogravimetric analysis (TGA) of 1 shows a mass loss of ca. 25.2% from 30 to 400 °C, which is corresponding to the loss of lattice solvent molecules and the coordinated water molecules (calcd. 25.6%). Upon further heating the framework starts to decompose (Fig. S4).
Tunable luminescence and white light emission. Compounds with d 10 metal centres and organic ligands are desirable candidates for luminescence-emitting materials. Hence luminescence excitation and emission spectra of 1 and H 3 BTC were investigated at room temperature ( Fig. S5). Compound 1 and H 3 BTC exhibit emission bands at 422 nm (λ ex = 355 nm) and 430 nm (λ ex = 340 nm). By comparison with the free ligand, the emission of 1 is blue shifted by ca. 8 nm. Such behaviour could be due to the strong electrostatic interaction between the Zn 2+ ion and BTC 3− . The solid-state luminescence of 1 excited with various wavelengths was also investigated. As shown in Fig. 2a, as the excitation wavelength varies from 310 to 471 nm, the luminescence colour changes from light-blue to blue-green (Fig. 2b). This result gives us an opportunity to obtain white light emission by incorporating red and green emitting components such as Eu 3+ and Tb 3+ into the pore of 1.
In order to make Ln 3+ -incorporated complexes, compound 1 was immersed in turn in an ethanol solution of Eu 3+ and an ethanol solution of Tb 3+ , then the solid was filtered and washed by ethanol and diethyl ether several times to remove any residual Eu 3+ and Tb 3+ ions on the surface. By adjusting the immersion time, the encapsulated amount of Eu 3+ and Tb 3+ can be optimized to achieve white light emission. The resultant Ln 3+ -incorporated complex, namely Eu 3+ /Tb 3+ -1 contains 0.251% of Eu 3+ and 0.269% of Tb 3+ , as confirmed by ICP results. The solid-state emission spectrum of Eu 3+ /Tb 3+ -1 exhibits the characteristic emission peaks of Eu 3+ ( 5 D 0 to 7 F J , J = 0-4) and Tb 3+ ( 5 D 4 to 7 F J , J = 6-0) (Fig. 2c). Notably, the CIE coordinates of Eu 3+ /Tb 3+ -1 excited at 312 nm is (0.32, 0.33), which are very close to those for pure white light (0.333, 0.333), according to the 1931 CIE coordinate diagram. Meanwhile, the emission of Eu 3+ /Tb 3+ -1 under different excitation wavelengths was also investigated. When excited at 308 nm, the CIE index of Eu 3+ /Tb 3+ -1 is (0.38, 0.40), and it shows yellow light emission. As the excitation wavelength increases gradually, the main emission peaks of Eu 3+ and Tb 3+ gradually weaken. When excited at 315 nm, the CIE of Eu 3+ /Tb 3+ -1 is (0.27, 0.29), and it displays a blue-green light. As a result, the luminescence colour of Eu 3+ /Tb 3+ -1 at different excitation wavelengths changes from yellow to white, and eventually becomes blue-green (Fig. 2d).
Luminescence sensing for metal ions. The existence of a porous structure makes compound 1 a promising candidate for sensing and detecting metal ions. To investigate the luminescence quenching or enhancement for one hour and then ultrasonically agitated for 20 min to form a metal-ion-incorporated MOF suspension. The corresponding luminescence spectra are recorded and are compared in Fig. 3a. The emission spectra show that the luminescence intensity of M n+ -1 excited at 355 nm varies significantly depending on the identity of the metal ions. For example, Li + , Mg 2+ , Ca 2+ , In 3+ , Zn 2+ , Cd 2+ and Gd 3+ have only a slight effect on the luminescence intensity after incorporation into the pores, whereas the other metal ions have varying degrees of effects. Among them the Cu 2+ ion has a significant quenching effect on the emission of 1. The descending order of the quenching efficiencies of the metal ions is as follows: 3+ . In contrast to Cu 2+ , the Al 3+ and Ga 3+ ions show significant enhancement on the emission intensity. Particularly in the presence of Al 3+ , the emission intensity is about three times than the metal-ion free 1 (Fig. 3b). These results clearly indicate that 1 shows a high selectivity towards Cu 2+ and Al 3+ .
The relationship between the luminescence intensity and the concentration of Cu 2+ has been investigated by measuring the emission spectra of 1 after immersion in solutions of various concentrations of Cu 2+ ions (Fig. S6a). The results show that the luminescence intensity of Cu 2+ -incorporated complex is greatly dependent on the concentration of the metal ion. The luminescence intensity decreases quickly as the concentration of Cu 2+ increases and it remains unchanged when the Cu 2+ concentration is greater than 0.03 M (Fig. 4a). Unlike Cu 2+ ion concentration, the immersion time seems to have no influence on the luminescence intensity. As shown in Fig. 4b, the luminescence intensity of 1 after being immersed in 0.03 M Cu 2+ ethanol solution for less one minute decreases sharply and it is also observed that prolongation of immersion time up to 60 min does not cause any further decrease of the luminescence intensity (Fig. S6b). Furthermore, this selective detection of Cu 2+ is not influenced by the existence of other metal ions such as Li + , Zn 2+ , Cd 2+ and Gd 3+ (Figs 4c and S6c). A good linear correlation between (I 0 −I)/I and the concentration of Cu 2+ is observed with the K sv value of 286.1 M −1 (Fig. S6d). The detection limit is calculated on the basis of 3σ/k to be 1.34 × 10 −3 M.
The possible mechanism of luminescence quenching by Cu 2+ could involve the binding of Cu 2+ through Lewis acid-base interaction, as suggested for the selective sensing of Cu 2+ ion with microporous frameworks, such as [Cd 2 (PAM) 2 (dpe) 2 (H 2 O) 2 ]·0.5(dpe) 37   decomposition of 1 occurred due to hydrolysis of Al 3+ , which made the solution acidic. In 0.03 M Al 3+ solution, 1 was partially dissolved, which released BTC 3− in solution and thereby enhancing the ligand fluorescence 41 .
The luminescence quenching of Cu 2+ may also result from partially exchange of the metal ions from Zn 2+ to Cu 2+ in the framework. To check the results, 1 was immersed in ethanol containing 0.03 M Cu 2+ for varying periods of time. The filtered powder was washed thoroughly with ethanol until the filtration became colourless. The colour of crystals changes from colourless to green after exchange with Cu 2+ . The Cu 2+ -exchanged samples thus obtained were then subjected to ICP analysis. The results showed that with immersion time increasing from 1 min to 24 h, the Cu-exchange level on the framework gradually increased accompanied by a reduction of Zn content in the compound. As indicated in Table S1, approximately 50% of framework was replaced with Cu 2+ ion within one day and nearly complete replacement of Cu 2+ (96%) took place after two weeks. Surprisingly, the reversed ion exchange failed and so did the exchange with other transition metal ions like Co 2+ and Ni 2+ . SEM images (Fig. S7) reveal that after Cu 2+ exchange, the large crystals of 1 (ca. 300 μm) with a regular shape collapse into microcrystalline solid. However, the XRD pattern of the Cu 2+ -exchanged samples shows similar peaks to those of 1, suggesting that the framework structure remains intact after MOF exchange with Cu 2+ ion (Fig. S3). Moreover, XPS measurements were also carried out to confirm the existence of Cu in the Cu 2+ -exchanged sample. Fig. S8 shows that the Cu 2p 3/2 and 1/2 spectra of Cu 2+ -exchanged sample were located at 934 and 954 eV, respectively, both of which suggest the presence of Cu 2+ in the Cu 2+ -exchanged sample.
Gas adsorption properties of the Cu 2+ -exchanged compound. Adsorption experiments were carried out to investigate the porosity. The samples were degassed at 100 °C for 12 h under vacuum prior to gas adsorption/desorption measurements. The activated sample 1 shows no significant adsorption for N 2 and CO 2 , presumably due to the pore collapse during sample activation. Interestingly, the adsorption capacity for either N 2 or CO 2 substantially increases by replacing Zn 2+ with Cu 2+ in the framework. The N 2 adsorption of the Cu 2+ -exchanged samples with 50% (1a) and 96% (1b) exchange ratios at 77 K exhibits a type I isotherm, typical for materials that show permanent microporosity. The highest adsorbed amount of N 2 is 174.8 cm 3 g −1 for 1a and 324.6 cm 3 g −1 for 1b, and the corresponding pore volumes are 0.245 and 0.485 cm 3 g −1 respectively (Fig. 5a). The Langmuir and Brunauer-Emmett-Teller (BET) surface areas are 649.02 and 589.05 m 2 g −1 for 1a and 1299.33 and 1179.73 m 2 g −1 for 1b. The CO 2 adsorption capacity increases from 38.5 cm 3 g −1 for 1a to 136.1 cm 3 g −1 for 1b. In both case, the amount of CO 2 uptake decreases by 40.3% as the temperature increases from 273 to 298 K, indicating a typical physisorption behavior (Figs 5b and S9). These results demonstrate that the increase of Cu-exchange ratio dramatically enhance the adsorption capacity for N 2 and CO 2 . The main reason for this may be due to the fact that the replacement of Zn 2+ by Cu 2+ enhances the framework robustness thereby improving the adsorption properties 42 .

Conclusion
In summary, a new cage-based MOF with two different types of Zn cluster SBUs has been synthesized and structurally characterized. This compound features a large octahedral cage constituted by nine Zn 3 O and nine Zn 4 O clusters and twenty-four triangular BTC 3− ligands. Tunable luminescence and white light emission can be achieved by changing the excitation wavelength and by incorporation of Eu 3+ /Tb 3+ ions into the compound. While other transition metal ions such as Mn 2+ , Co 2+ and Ni 2+ displayed relatively weak quenching effects, only Cu 2+ and Al 3+ ions showed significant changes in the emission spectra, which demonstrates that 1 could be regarded as a potential material for selective sensing of Cu 2+ and Al 3+ ions. In addition, the facile ion exchange with Cu 2+ without loss of structural integrity as described herein provide an post-synthesis route to construct isomorphous Cu-MOF that cannot be obtained by direct synthesis.

Materials and instrumentation.
All chemicals were purchased commercially and used as received. TGA was performed using a TGA/NETZSCH STA449C instrument heated from 30-800 °C (heating rate of 10 °C/min, nitrogen stream). IR spectrum using a KBr pellet was recorded on a Spectrum-One FT-IR spectrophotometer in the range 4000-400 cm −1 . The powder X-ray diffraction (PXRD) patterns were recorded on crushed single crystals in the 2θ range 5-55° using Cu Kα radiation. ICP elemental analyses for the metal ions were performed with an ultima2 X-ray ICP optical emission spectrometer. Elemental analyses for C and H were measured with Elemental Vairo EL III Analyser. Luminescence spectra for the solid samples were recorded on an Edinburgh Analytical instrument FLS920. Luminescence spectra for the liquid samples were recorded on a HITACHI F-7000. Gas adsorption measurements were performed in an ASAP (Accelerated Surface Area and Porosimetry) 2020 System. SEM images were obtained using a Phenom G2 SEM microscope.  Single-crystal structure determination. Single-crystal X-ray diffraction data were collected on a Rigaku Diffractometer with a Mercury CCD area detector (Mo Kα; λ = 0.71073 Å) at room temperature. Empirical absorption corrections were applied to the data using the Crystal Clear program 43 . The structure was solved by direct methods using SHELXS-97 44 and refined by full-matrix least-squares on F 2 using SHELXL-2016 program 45 . Metal atoms were located from the E-maps, and other non-hydrogen atoms were located in successive difference Fourier syntheses. All non-hydrogen atoms were refined anisotropically. The organic hydrogen atoms were positioned geometrically. Since the position of the disorder water molecules could not be resolved from Fourier maps, PLATON/SQUEEZE 46 was used to compensate the data for their contribution to the diffraction patterns. The SQUEEZE calculations showed a total solvent accessible area volume of 10178 Å 3 in 1 and the residual electron density amounted to 1744 e per unit cell, corresponding to about seven ethanol molecules per asymmetric unit. The final formula was then calculated from the TGA result combined with elemental analysis data. Crystallographic data and other pertinent information for 1 are summarized in Table S2. Selected bond distances and angles are listed in Table S3. CCDC number for 1 is 1542054.