A Dual-Functional Luminescent MOF Sensor for Phenylmethanol

Feb 14, 2018 - A highly luminescent porous metal–organic framework Cd3(L)2.5(4-PTZ)(DMF)3, labeled as NBU-9, has been designedly synthesized based o...
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

A Dual-Functional Luminescent MOF Sensor for Phenylmethanol Molecule and Tb3+ Cation Fei-Yan Yi,*,†,‡ Minli Gu,† Shi-Cheng Wang,† Jia-Qi Zheng,† Luqing Pan,† and Lei Han*,† †

School of Materials Science and Chemical Engineering, Ningbo University, Ningbo, Zhejiang, 315211, P. R. China State Key Laboratory of Structure Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, P. R. China

‡

S Supporting Information *

ABSTRACT: A highly luminescent porous metal−organic framework Cd3(L) 2.5(4-PTZ)(DMF)3, labeled as NBU-9, has been designedly synthesized based on Cd(NO3)2·4H2O and mixed ligands of 4-(1Htetrazol-5-yl)pyridine (4-HPTZ) with N-coordinated sites and thiophene2,5-dicarboxylic acid (H2L) with heteroatomic (S) ring and carboxylate groups in N,N-dimethylformamide (DMF) at 100 °C for 3 days. The interesting result is that this compound NBU-9 can be also obtained via the mixed raw materials of Cd(NO3)2·4H2O, 4-cyanopyridine, NaN3, and H2L under solvothermal condition at a higher temperature of 140 °C for 3 days, involving in situ ligand synthesis of 4-HPTZ. Its structure was indentified by single-crystal X-ray study, powder X-ray diffraction, element analysis, and TGA results. Structural analysis shows that the three-dimensional framework of NBU-9 contains cubic channels of 9.59 × 10.26 Å2 covered by a large number of open S- and O-coordinated sites and can be simplified into a 8connected uninodal eca net with high potential solvent accessible volumes of 34.1%. Its luminescent properties demonstrate that NBU-9 as a multifunctional sensory material realizes the selective detection for the phenylmethanol molecule on the basis of fluorescence quenching mechanism and effectively sensitizing the visible emitting of the Tb3+ cation based on luminescence enhancement.

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etc.5 However, how to control the synthesis of MOFs meeting the requirements of particular luminescence sensors is still a formidable challenge for chemists. For MOFs, judicious selection of metal center and predesignedly functional organic linkers is a vital component for their fluorescent behavior. It is well-known that Zn2+, Cd2+, and Ln3+ were usually adopted as luminescent cations. The introduction of functional organic ligands with guest-accessible sites on the pore surface were proved a simple and effective strategy to improve molecular recognition containing noncoordinated functional group or unsaturated (open) metal sites. Just few successful examples for excellent MOF sensors can be achieved based on functional properties of open metal sites or coordinated sites.6 In this contribution, we present a porous Cd(II)MOF (Cd3(L)2.5(4-PTZ)(DMF)3, labeled as NBU-9), which is generated based on a dual-functional ligand containing a tetrazolate ligand of 4-HPTZ with multiple N-donor atoms and the carboxylate ligand of H2L with heteroatomic (S) ring and carboxylate groups, as well as Cd(NO3)2·4H2O by solvothermal reaction. In particular, the NBU-9 compound can be also easily achieved based on the raw materials for the synthesis of 4HPTZ to the present mixtures through in situ [2 + 3]

INTRODUCTION In the past two decades, metal−organic frameworks as a new family of porous materials have exhibited wide applications containing sensors,1 gas storage and separation,2 heterogeneous catalysis,3 drug delivery,4 etc., due to their structural and chemical diversities. Metal−organic frameworks (MOFs) are constructed from the metal ions or metal-oxo units coordinating with electron-donating organic ligands, so they can be designedly synthesized with high porosities, large specific surface areas, uniform but tunable pore sizes, and functional structures. These structural features make them promising luminescent sensing candidates. Since, the functional pore surface as chemical sensors can improve molecular recognition through host−guest interactions including coordination bonds, π−π interactions, or hydrogen bonding; high surface areas as preconcentrator can concentrate analytes to high levels, enhancing the detective limit. Besides, such optical sensors also possess obvious advantages over traditional sniffer dogs and sophisticated analytical instruments like quickresponse, high simplicity, sensitive, selectivity, portability, inexpensive, and can be used in both solid state and solution.5 Over the past few years, literature reports have witnessed that various MOFs have been successfully used as luminescent sensors and showed good performance with high selectivity and sensitivity toward anions, cations, explosives, small molecules, © XXXX American Chemical Society

Received: December 4, 2017

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DOI: 10.1021/acs.inorgchem.7b03053 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry cycloaddition reaction between organonitriles and azido. Such an in situ approach provides not only the fascinating synthetic method for novel MOF construction but also an important clue to understand the self-assembly of MOFs, further optimizing the crystallinity of the target compound. The target NBU-9 MOF with oxygen-rich hollow and open coordinated sites realized highly selective detection toward the phenylmethanol molecule even if in the presence of other alcohol molecules with similar functions, as well as can sensitize Tb3+ emitting as an antenna.

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Table 1. Crystal Data for NBU-9 1813188 for in situ synthesis C30H30Cd3N8O13S2.5 Cd3(L)2.5(4-PTZ)(DMF)3 1127.97 296(2) 0.71073 0.37 × 0.34 × 0.29 orthorhombic Pbcn 20.912(12) 30.041(19) 15.519(9) 90 90 90 9749(10) 8 1.537 4432 52.46 1.462 43696/9523 [Rint = 0.0572] 98.0% 1.002 a R1 = 0.0579, bwR2 = 0.1855 a R1 = 0.0837, bwR2 = 0.2010

CCDC empirical formula structural formula formula weight temperature λ, Å crystal size, mm3 crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z calculated density, mg/m3 F(000) 2θ max (deg) adsorption coefficient, mm−1 reflections collected/unique completeness, % goodness-of-fit on F2 final R indexes [I > 2σ(I)] final R indexes (all data)

EXPERIMENTAL SECTION

Materials and Measurements. All chemicals were commercially obtained and used without further purification. H2L and 4-HPTZ were purchased from Jinan Henghua Sci. & Tec. Co., Lt. Elemental analyses of C, H, and N in the solid samples were performed with a VarioEL analyzer. Thermogravimetric and differential thermal analysis (TGDTA) data were recorded on a Thermal Analysis Instrument (SII TG/ DTA7300 apparatus) from room temperature to 800 °C with a heating rate of 10 °C/min under a N2 atmosphere. Powder X-ray power diffraction (XRD) patterns were performed on a D8 Focus (Bruker) diffractometer with Cu Kα radiation field-emission (λ = 0.15405 nm, continuous, 40 kV, 40 mA, increment = 0.02°). The infrared (IR) spectra were recorded on a Shimadzu FTIR-8900 spectrometer on a KBr disk within the 4000−500 cm−1 region. UV− vis spectra studies were performed at room temperature on a PerkinElmer Lambda 900 UV/vis/NIR spectrophotometer equipped with an integrating sphere in the wavelength range of 200−1200 nm. The luminescence decay curve for Tb3+@NBU-9 was obtained using a Lecroy Wave Runner 6100 digital oscilloscope (1 GHz) with a tunable laser (pulse width = 4 ns, gate = 50 ns) as the excitation source (Continuum Sunlite OPO). The luminescence lifetimes were calculated by the Origin 7.5 software package. Inductively Coupled Plasma (ICP) test for Tb3+ in Tb3+@NBU-9 is performed on an ICPOES (SPECTRO ARCOS) instrument based on a solid sample of NBU-9 after treatment in Tb3+ aqueous solution (0.1 mol/L). Synthesis of Cd3(L)2.5(4-PTZ)(DMF)3 (NBU-9). A mixture of Cd(NO3)2·4H2O (0.1 mmol, 30.8 mg), H2L (0.05 mmol, 8.6 mg), 4HPTZ (0.05 mmol, 7.4 mg), and DMF (6 mL) was placed in a Teflonlined stainless steel vessel (20 mL) and heated to 100 °C, maintained at this temperature for 3 days, and then cooled to room-temperature. The resulting colorless brick crystals were obtained; after they were washed with DMF solvent, the yield was 18.5 mg (82% based on H2L). Their purity was confirmed by X-ray power diffraction (XRD). Anal. Calcd (%) for NBU-9 C30H30Cd3N8O13S2.5 (Mr = 1127.97): C, 31.94; H, 2.68; N, 9.93. Found: C, 31.85; H, 2.67; N, 9.90. FT-IR (cm−1, see the Supporting Information): 3318 (w), 3085 (w), 2934 (w), 2161 (w), 1641 (s), 1562 (s), 1525 (m), 1435 (w), 1369 (vs), 1253 (m), 1111 (m), 1063 (w), 1018 (w), 849 (w), 814 (m), 773 (m), 715 (w), 677 (m), 538 (w). While 4-cyanopyridine (0.05 mmol, 5.2 mg) and NaN3 (0.01 mmol, 6.5 mg) as raw materials were used instead of 4-HPTZ, the mixture was heated at 140 °C for 3 days under DMF (6 mL). The same crystal was obtained with better crystallinity (Table 1). X-ray Crystal Structure Determination. Single-crystal X-ray diffraction data for NBU-9 from two synthetic methods were recorded on a Bruker Apex CCD II area-detector diffractometer with graphitemonochromated Mo−Kα radiation (λ = 0.71073 Å) at 296 K. Data processing was accomplished with the SAINT program.7 Absorption corrections were applied based on the intensities of equivalent reflections with the use of the SADABS program.8 Their structures were solved by direct methods using the SHELXS-97 program of the SHELXTL package and refined by using the full-matrix least-squares method on F2 with the SHELXTL-97 program.9 The anisotropic displacement parameters were applied to all non-hydrogen atoms. All hydrogen atoms of the organic molecule were placed by geometrical considerations and were added to the structure factor calculation. Because the remaining solvent molecules in the channels of NBU-9

R1 = ∑||Fo| − |Fc||/∑|Fo|. ∑w[(Fo)2]2}1/2.

a

b

wR2 = {∑w[(Fo)2 − (Fc)2]2/

were highly disordered and could not be modeled properly, the SQUEEZE routine in PLATON was applied to remove the contribution of their electron density.10 The formula for NBU-9 was determined by combining single-crystal structure, elemental microanalysis, and TGA. A summary of two crystallographic data for the complex is listed in Table 1 (based on in situ synthesis) and Table S1 (based on 4-HPTZ). Selected bond distances and angles are given in Table S2. Fluorescence Measurements. The photoluminescent (PL) spectra were recorded on a Hitachi F-4500 fluorescence spectrophotometer. For the experiment of sensing small organic molecules, the NBU-9−solvent emulsions were prepared by introducing 5 mg of NBU-9 powder into 3.00 mL of N,N-dimethylacetamide (DMA), N,N-dimethylformamide (DMF), 1-propanol, benzene, toluene, tetrahydrofuran (THF), ethanol (EtOH), dichloromethane (CH2Cl2), methanol (CH3OH), acetonitrile (CH3CN), 2-propanol, trichloromethane (CHCl3), and phenylmethanol, respectively. Different amounts of phenylmethanol were added into a standard NBU-9 emulsion in pure DMF, while the concentration of Cd2+ was kept constant. For the experiment of sensing metal ions, the NBU-9− solvent emulsions were prepared by introducing 5 mg of NBU-9 powder into 3.00 mL of M(NO3)x (0.1 mol/L, M = Na+, K+, Ag+, Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Al3+, Tb3+, Eu3+) DMF solution. All of the PL spectra for NBU-9-solvent emulsions were treated by ultrasonication for 30 min and then aged for 2 days to form a stable emulsion before fluorescence study. The Tb3+ DMF solution with different concentrations from 10−8 M to 10−1 M were prepared and used for monitoring the detection limit of NBU-9 (5 mg) for each one. Meantime, 30 mg samples for NBU-9 were immersed into a DMF solution containing an excess of Tb(NO3)3 (0.01 mol L−1). The resulting solids were collected and washed with pure DMF solvent (3 mL) for 4 times, and dried in air. PL spectra and luminescence decay curve of Tb3+@NBU-9 in the solid state were measured at room temperature. B

DOI: 10.1021/acs.inorgchem.7b03053 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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RESULTS AND DISCUSSION Structural Description of Cd3(L)2.5(4-PTZ)(DMF)3 (NBU9). Compound NBU-9 crystallizes in the orthorhombic Pbcn space group based on the analysis of single crystal and powder X-ray diffraction. As depicted in Figure 1, its asymmetric unit

ligand, respectively, and two oxygen atoms in the axis direction from another L2− ligand and one DMF molecule (O13). Cd3 adopts a typical {CdO6} octahedral coordination mode, in which two carboxylate oxygen atoms from two L2− ligands and two coordinated DMF molecules (O11 and O12) were in the equatorial plane with cis-formation, and two nitrogen atoms from two 4-PTZ ligands were in the axis direction. Three Cd centers were connected into a trinuclear secondary building unit (SBU) (CdII3) via carboxylate groups and tetrazolate group. In the coordination mode of three Cd centers, the relatively long Cd−O bonds can be easily found. In order to confirm the coordinated geometries of Cd metal centers, bond valence sum (BVS) calculation was applied. On the basis of the results of BVS, Cd1 is 1.994 with a long Cd1−O3A bond of 2.566(6) Å (symmetric code A x, −y + 1, z − 1/2); Cd2 is 2.282 with two long Cd−O bonds of 2.562(6) Å for Cd2−O2 and 2.576(5) Å for Cd2−O9 (Table S2); Cd3 is 2.296. The valence state of the related oxygen atoms with long Cd−O bonds was also calculated. All detailed BVS results can be found in Table S3, which further confirm that all Cd centers are seven-fold coordination. The other bond lengths of Cd−O and Cd−N in the range of 2.166(7)−2.411(7) Å and 2.330(9)− 2.392(11) Å, respectively, are comparable to those of reported values.11 In the asymmetric unit for NBU-9, four carboxylate ligands are fully deprotonated {L2−}, in which only {L2−} containing S1, labeled as {L(S1)2−}, is completely crystallographically independent; the other three carboxylate ligands {L(S2)2−}, {L(S3)2−}, and {L(S4)2−} are a half because three S atoms (S2, S3, and S4) lie on the glide plane and thus have 1/2 occupancy, and the other half were generated through symmetrical operation. As shown in Figure S1, each of {L(S1)2−} and {L(S4)2−} is hexadentate and links four Cd atoms by two μ2-η1 η2 carboxylate groups with two Cd-O-C-O four-membered rings. {L(S2)2−} chelated two Cd centers by two μ1-η1 η1 carboxylate groups with two Cd-O-C-O fourmembered rings. {L(S3)2−} connected four Cd centers by two μ2-η1 η1 carboxylate groups. All {L2−} and 4-PTZ ligands connected CdII3 SBUs into a three-dimensional (3D) framework of NBU-9 with a high-solvent-accessible volume of 3320.5 Å3 out of the 9749 Å3 unit cell volume (34.1% of the total crystal volume) based on PLATON calculation.10 In this 3D

Figure 1. An ORTEP representation of the asymmetric unit of NBU-9 with the atomic numbering scheme. Thermal ellipsoids are drawn at the 50% probability level. Three DMF molecules have been omitted for clarity, just leaving three coordinated oxygen atoms (O11, O12, and O13). Symmetry transformations used to generate equivalent atoms: A x, −y + 1, z − 1/2; B x − 1/2, −y + 3/2, −z + 1; C −x + 2, y, −z + 3/2; D −x + 1, y, −z + 3/2; E −x + 1, y, −z + 1/2; F x + 1/2, −y + 3/2, −z + 1; G x, −y + 1, z + 1/2.

contains three crystallographically independent Cd2+ ions, four carboxylate ligands, one deprotonated 4-PTZ ligand, and three coordinated DMF molecules (O11, O12, and O13). Cd1 and Cd2 are in distorted {CdNO6} pentagonal bipyramid geometries with seven-fold coordination (Figure S1), including four oxygen atoms and a nitrogen atom in the equatorial plane from two carboxylate groups of two L2− ligands and one 4-PTZ

Figure 2. Step-by-step fabricating processes for 3D framework of NBU-9. (a and b) Two kinds of 1D Cd-L chains. (c) 1D channel along c-axis. (d) 2D layer structure along the ac sheet. Important bridged atoms were labeled. (e) View the 3D network structure of NBU-9 with cubic 1D channels along the [001] drection, in which S, O, Cd, and N atoms are represented as black, red, green, and blue balls. 4-PTZ ligands are drawn in blue atom and bonds. In L2− ligands, C atoms and all bonds are shown as orange sticks. C

DOI: 10.1021/acs.inorgchem.7b03053 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry structure, Cd1 atoms were linked into a 1D chain by {L(S1)2−} carboxylate ligand along the c-axis (Figure 2a), namely, Cd-L chain A. Cd2 and Cd3 atoms were linked alternatively by {L(S3)2−} and {L(S4)2−} ligands into a 1D chain, labeled as Cd-L chain B (Figure 2b). Cd-L chains A and B linked each other alternatively into the 1D cubic channel of 9.59 × 10.24 Å2 (measured between two atoms) along the c-axis (Figure 2c). The exciting finding is that the internal surface of the cubic channel was covered by a large number of open carboxylate oxygen atoms and open S-sites. These 1D channels were connected into a 2D layer by linked {L(S2)2−} ligands on the ac sheet (Figure 2d), in which the CdII3 SBUs are located on the outside surface of the channels in the fashion of a half arc, orienting four directions in the way back to back, furthermore extending into the target 3D framework by 4-PTZ ligands (Figure 2e). Each unique 4-PTZ as a tetradentate ligand connected four Cd centers including two Cd3, one Cd1 and one Cd2 by one N atom from the pyridine group and three N atoms from a tetrazolate group, with one deprotonated noncoordinated N atom (Figure S2a). The whole porous 3D skeleton can be also regarded that those channels were extended by 4-PTZ ligands through coordinating CdII3 SBUs (Figure S2b), in which L(S2)2− ligands as 2-connected nodes do not participate in the formation of whole 3D skeleton. TOPOS analysis reveals that the 3D framework of NBU-9 can be simplified into a 2,2,2,2,2,7-connected net with a point symbol of {815.126}2{8}7 (Figure S2c), where each CdII3 SBU, L2− and 4-PTZ ligands are considered as a 7-connected node, and 2-connected nodes (Figure S2c). When 2-connected nodes were transformed into edges between metal nodes during simplification, a 7-connected uninodal hxg-d net with its point symbol of {415.66} was obtained (Figure S2d). Since the 2connected L(S2)2− ligands do not participate in the formation of the whole 3D skeleton, the overall 3D topology of NBU-9 can furthermore be simplified into an 8-connected uninodal eca net with point symbol of {39.412.57}, after omitting the L(S2)2− ligand (Figure S2e). Such a highly connected net is very limited up to now.12 Considering the structural features of the NBU-9 compound with high-solvent-accessible volume and 1D cubic channels, its gas adsorption isotherm for N2 at 77 K was performed on ASAP 2050 V1.01 E and Autosorb MP-1 apparatuses at 1 atm. As shown in Figure S3, NBU-9 shows a type-III sorption behavior, in which adsorption amount gradually increases along with increasing pressure. It exhibits a very small N2 gas sorption amount (8.16 cm3 g−1). Probable reasons for nonadsorption of N2 at 77 K might be (1) low thermal energy of the adsorbate relative to the large diffusional resistances, (2) the solvents in the channel are not completely removed, and (3) the pore surface of NBU-9 is not effective enough for adsorption of N2, which is a common phenomenon in MOF compounds and has been discussed in the previously reported MOFs.13 Its pore size distribution (∼2.6 Å and 4.7− 6.1 Å) based on N2 adsorption isotherms matches well with the crystal structure model. PXRD and Thermal Stability Analysis. In order to check the phase purity of the bulk sample of NBU-9, powder X-ray diffraction (PXRD) analysis was carried out. As shown in Figure S4a, all of the peaks displayed in the experimental patterns closely match those in the simulated patterns generated from single-crystal diffraction data, which indicates that the analyzed compounds represent a single phase. It is obvious that the intensity of the peaks for samples synthesized via the in situ method is much stronger than the one obtained

based on 4-HPTZ ligands under the same test condition (Figure S4b), demonstrating better crystallinity based on the in situ synthetic method. This is consistent with the result of single-crystal X-ray diffraction analyses for two kinds of crystals (Table 1 and Table S1). Therefore, the in situ synthetic method provides a promising strategy, which not only reduces the cost, simplifing the synthetic process, but also improves the process of crystal growth, helping the chemists to obtain more suitable single crystals for single-crystal X-ray diffraction analyses. Therefore, all other experiments are based on the samples via the in situ synthetic method. The thermogravimetric analysis of NBU-9 under a N2 atmosphere with a heating rate of 10 °C/min was investigated in the temperature range of 25−800 °C, and the TG-DSC curve is shown in Figure S5. The first weight loss of 18.8% at 92−220 °C corresponds to the loss of three coordinated DMF molecules per unit cell (calcd 19.4%). The decomposition of the desolvated framework takes place in the next step in a temperature range of 264−434 °C, which should correspond to the combustion of three coordinated L2− ligands (found, 38.4%; calcd, 38.2%). The final CdII3-PTZ chain is still stable up to 620 °C. The total weight loss at 692 °C is 68.7%. This result is very interesting and exciting, which not only verifies that the desolvated framework with open metal sites is thermally stable but also demonstrates that the 1D chain by 4-PTZ chelating CdII3 exhibited extremely high stability of ∼620 °C. As shown in Figure S4c, the FT-IR spectrum of the sample of NBU-9 after treatment at 600 °C for 30 min shows clear characteristic peaks at 3435 (s), 2924 (w), 1641 (w), 1599 (s), 1380 (m) and 1106 (w) due to vibration of heterocyclic aromatic with C−N and CN bonds, demonstrating the stability of Cd-PTZ. This also facilitates to understand the selfassembly of the MOF under black boxes condition. Multi-N sites can rapidly chelate metal centers into 1D intermediate of the Cd-PTZ unit, then being connected into a 3D net through H2L carboxylate ligands. This also provide a new insight for the construction of novel MOF materials with specific performance based on multistep processes. Luminescence Properties. The solid luminescence spectra of NBU-9, and free ligands H2L, and 4-HPTZ at room temperature are shown in Figure S6. The free ligand H2L displays two main fluorescent emission bands at λmax = 396 and 295 nm and one weak shoulder peak at λmax = 338 nm under excitation at 247 nm. The free 4-HPTZ exhibits strong photoluminescence with one emission maximum at 350 nm (λex = 321 nm). NBU-9 exhibits a broad luminescence emission centered at 412 nm excitation at 339 nm, which covers the whole range of emission bands for H2L and 4-HPTZ ligands, concomitant with a 16−62 nm red shift; hence, the emission of NBU-9 can be attributed to ligand-to-ligand change transfer (LLCT), admixing metal-to-ligand change transfer (MLCT) for such Cd(II) coordination complexes.13e,14 For MOFs, such ligand-based emission is more desirable, since the band gaps of host material can be altered along with exchanged guest molecules diffusing into the channels based on host−guest interaction, then leading to the different fluorescent response based on the change of luminescent intensity or position.15 Therefore, it is anticipated that NBU-9 can be utilized as fluorescence sensor with its intrinsic structural property containing large 1D cubic functional channels modified by a large number of open coordinated sites. Besides, NBU-9 is insoluble in common solvents and aqueous solutions, so its selective sensing ability was investigated for small solvent D

DOI: 10.1021/acs.inorgchem.7b03053 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. PL emission spectra (a) and related PL intensities (b) (λmax = 335 nm) for NBU-9 introduced into various solvents at an excitation of 292 nm.

Figure 4. (a) The reduction of photoluminescence intensities for NBU-9 by gradually increasing the ratio of phenylmethanol in DMF solutions, keeping in total 3 mL (excited at 290 nm). (b) The bar chart representation for corresponding emission intensities in different concentrations at λmax = 341 nm.

Figure 5. (a) Stern−Volmer plot and (b) the relative luminescence intensity (I/I0) of the NBU-9 DMF suspension as a function of phenylmethanol content (vol %).

different solvent emulsions, labeled as solvent@NBU-9. Then the luminescent properties of these different solvent emulsions were investigated. As shown in Figure 3, the PL intensities of NBU-9 are strongly influenced by the solvent molecules, especially in the case of phenylmethanol, which exhibits the most significant turn-off quenching effects, whereas other solvents show negligible or limited effect on the luminescence

molecules combining its robustness and permanent micropore feature. Sensing of Phenylmethanol Molecule. First, the assynthesized sample of NBU-9 was ground and immersed in different organic solvents (3 mL) including DMA, DMF, 1propanol, benzene, toluene, THF, EtOH, CH2Cl2, CH3OH, CH3CN, 2-propanol, CHCl3, and phenylmethanol to form E

DOI: 10.1021/acs.inorgchem.7b03053 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

luminescence materials is currently of interest in the fields of light display, lasers, and optoelectronic devices.18 Typically, freshly prepared samples of NBU-9 (5 mg) were ground and immersed in the DMF solutions (0.1 M = mol/L, 3 mL) containing different metal ions (Na+, K+, Ag+, Mg2+, Ca2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Pb2+, Al3+, Eu3+, and Tb3+) for 3 days to form a metal ion-incorporated NBU-9 suspension, namely, M@NBU-9 for luminescence studies. The related photoluminescence (PL) spectra of M@NBU-9 are shown in Figure 6 and Figure S8. It is obvious that the Tb3+-

intensities for NBU-9. To further examine the sensing sensitivity of NBU-9 toward phenylmethanol in detail, a series of concentration-dependent luminescence measurements were carried out. Figure 4 shows the emission spectra of NBU-9 dispersed in a mixed phenylmethanol−DMF solvent with different volume ratios (Vphenylmethanol/VDMF) under the total volume of 3 mL. It is obvious that the fluorescence intensity of phenylmethanol@NBU-9 decreases rapidly with an increase in the content of phenylmethanol and has become a half at a low phenylmethanol content of 1 vol %, and almost completely disappears at a concentration of 4.0 vol %; thus the compound can be seen as a candidate for selectively sensing the phenylmethanol molecule, which is also the first example to selectively recognize the phenylmethanol molecule in the presence of other alcohol molecules. In order to further quantify the quenching efficiency, the Stern−Volmer plots of relative luminescent intensity (I0/I) versus the phenylmethanol concentration are shown in Figure 5a. It is obvious that the I0/I versus phenylmethanol concentration plots were nearly proportional under low concentration, where I0/I = 1 + Ksv[M] (M = molar concentration of the target molecule, Ksv = the quenching effect of the target molecule, I0 and I = the fluorescence intensity of the emulsion in the absence and presence of the analyte, respectively). The quenching constant Ksv is 1.37% (41 μL each 3 mL). In other words, NBU-9 can recognize a phenylmethanol molecule in a very low concentration range. The corresponding visualized quenching percentage was calculated in order to build the relationship between the fluorescence quench percentage and quencher concentrations. As shown in Figure 5b, the decreasing trend of the fluorescence intensity at 341 nm versus the volume ratio of phenylmethanol (phenylmethanol content as abscissa) could be well fitted with a second-order exponential decay (R2 = 99.97%), indicating that fluorescence quenching of NBU-9 by phenylmethanol is diffusion-controlled.16 These PXRD patterns verify that NBU-9 still remains intact and the luminescence quenching was not caused by collapse of its framework after sensing experiments (Figure S4d). To deeply investigate the mechanism of this quenching phenomenon by phenylmethanol molecules, the UV−vis spectroscopy of the emulsions for NBU-9 in hexane, free 4-HPTZ, and H2L ligands in hexane as well as related pure organic solvents were performed (Figure S7). Two main adsorption bands in the range of 201−228 nm and 231−276 nm can be observed for phenylmethanol molecules, and benzene molecules show a broad range of 205−260 nm. Both of them cover the whole absorption band (220−240 nm) of NBU-9. This means that there is competition of absorption of the excited energy between target molecule and host framework upon excitation. For the phenylmethanol molecule with an electron-withdrawing −OH group, energy transfer occurs from the host framework to target solvent molecule upon excitation, resulting into a weakening or even quenching of fluorescence intensity, but for a benzene molecule as an electron-donating group, the opposite energytransfer effect occurs. The mechanism has been demonstrated by us and other reports based on experimental data and theoretical calculation.17 Sensing of Tb3+ Ion. The above luminescence sensing experiments have demonstrated high photoactivity and selectively recognized ability of NBU-9 based on open coordinated sites. Such porosity of NBU-9 encouraged us to use it to encapsulate lanthanide cations for tunable fluorescence. The preparation of such Ln3+-loaded NBU-9

Figure 6. Suspension-state fluorescence spectra for NBU-9 in DMF solutions of various metal cations (3 mL, 0.1 mol/L) when excited at 281 nm.

and Eu3+-doped NBU-9 exhibit their respective characteristic sharp emission bands, where Tb3+@NBU-9 emits four typical lines of Tb3+ emission in the visible spectrum, corresponding to transitions from the 5D4 state: 5D4 → 7F6 (491 nm), 5D4 → 7F5 (546 nm), 5D4 → 7F4 (586 nm), 5D4 → 7F3 (624 nm); among these transitions, 5D4 → 7F5 is the strongest. Eu3+@NBU-9 shows very weak characteristic emissions of the Eu3+ ion at 594, 619, and 698 nm. In the emission range of 300−450 nm (Figure S8), which is assigned to the absorption of organic ligands in the NBU-9 framework, alkaline metal ions (Na+ and K+) have a clearly luminescent enhancement compared with the one for NBU-9 in DMF solvent, alkaline-earth metal ions (Mg2+ and Ca2+) have a negligible effect on luminescent intensity of NBU-9, and other transition metal ions display different levels of quenching effects, especially Fe2+, Al3+, and Cd2+ with the most significant quenching effects. These results indicate that the photoluminescence properties of M-loaded NBU-9 are greatly influenced by the metal cations under the excitation of 281 nm, and the microporous NBU-9 is a suitable sensitizer for Tb3+ ion and Eu3+ ion, and the former is superior to the latter. It is well-known that the luminescence of Tb3+ is much easier than Eu3+ ions in the same matrixes, since the lower excited resonance level of Tb3+, locating at 20 500 cm−1 for 5D4, displays an obvious superior match with the energy state of the ligand through the ligand-to-metal energy transfer process, comparing with the one of Eu3+ at 19 000 cm−1 for 5 D1. Therefore, under the same excited energy (λex = 281 nm), the green light from Tb3+ will be more easily observed over the red light from Eu3+ for NBU-9. Its CIE coordinate for Tb3+@ NBU-9 also demonstrates that it falls in the green region (0.288, 0.598) (Figure 7b, H dot). These results are in accordance with the PL spectra of a solid sample for Tb3+@ F

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Figure 7. Emission spectra (a) and corresponding CIE chromaticity diagram (b) of Tb3+@NBU-9 in DMF solution of various concentrations of Tb3+ concentration from 10−8 M to 10−1 M (M = mol/L) (λex = 281 nm). Inset in (a): the enlarged PL spectra in the range of 300−460 nm, labeled as a black dotted line. In (b), A for 10−8 M (0.207, 0.142), B for 10−7 M (0.209, 0.144), C for 10−6 M (0.212, 0.148), D for 10−5 M (0.244, 0.403), E for 10−4 M (0.319, 0.518), F for 10−3 M (0.316, 0.564), G for 10−2 M (0.288, 0.598), H for 10−1 M (0.291, 0.606).

color changes as the increasing of concentrations of Tb3+ from 10−8 to 10−1 mol L−1 (Figure 7b).

NBU-9 in which the crystals after soaking in aqueous solution of Tb3+ for 1 day were washed with pure DMF solution to remove the residual TbIII cations on the surface of the sample (Figure S9). ICP measurement for Tb3+ in Tb3+@NBU-9 shows that the doped amount of Tb3+ is 1.02%. The results fully demonstrate that the NBU-9 not only as a host can trap lanthanide cations into its the pores but also as an antenna can successfully realize the sensitization and detection of Tb3+. The lifetime of the Tb3+@NBU-9 solid sample exhibits an ∼106-fold increase (1.38 ms) from ∼6.56 ns of NBU-9 (Figure S10). Both IR profiles and PXRD result verify that the basic framework of NBU-9 is still maintained after experiments (Figures S4d, S11, S12). Except of selectivity, sensitivity is also an important factor to evaluate the performance of sensing material. In order to evaluate the detection limit of NBU-9 for Tb3+cation in DMF solution, a fluorescence titration of NBU-9 (5 mg) with gradually increasing concentration (10−8 to 10−1 mol L−1) of Tb3+ in DMF solvent was observed (Figure 7a, Figure S13). The result indicated that the characteristic peaks of Tb3+ cation can be observed at low concentration of 10−5 mol L−1. The typical sharp emission bands of Tb3+ became clear under the concentration of 10−3 mol L−1; then the emission intensity increases rapidly to the maximum at 10−2 mol L−1. It is wellknown that lanthanide cations display a low molar absorptivity since the f−f transitions are spin- and parity-forbidden. Herein, the NBU-9 framework as a host effectively protects and sensitizes Tb3+-centered emission by energy transfer from the ligand to Tb3+ ions (“antenna effect”). Therefore, the intraligand emission in the NBU-9 framework may be suppressed for Tb3+-incorportaed NBU-9, which can be observed through the trend of emission bands in 300−400 nm for Tb3+@NBU-9 (Figure 7a, inserted image). As the concentration of Tb3+ changes from 10−8 to 10−1 mol L−1, the luminescent intensity from the ligand center rapidly decreases and the maximum of emission band shows an ∼30 nm red shift from 332 to 362 nm. This furthermore demonstrates that NBU-9 as an antenna can effectively sensitize the visibleemitting Tb3+ cation by efficient energy transfer. The CIE coordinates of Tb3+@NBU-9 visually exhibit the trajectory of

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CONCLUSIONS A novel 3D porous NBU-9 has been designed and successfully constructed based on dual-functional ligands of tetrazolate 4HPTZ ligand and carboxylate H2L ligand under solvothermal condition. Remarkably, the NBU-9 was also obtained based on an in situ synthetic process from raw materials of 4-HPTZ. Such an in situ strategy provides a promising way for understanding the self-assembly process of MOFs under airtight condition and improves the crystallinity of the compound with a simplified synthesis step and cheap material. The more important finding is that the NBU-9 contains a 1D cubic channel in its 3D structure, covered by a large number of open oxygen atoms and S atoms in its inner surface. As luminescent sensor, it is capable of selectively recognizing phenylmethanol. As a sensitizer, it can sensitize Tb3+ luminescence with high sensitivity in the concentration of 10−5 mol L−1, so the Tb3+-loaded NBU-9 tunable luminescent material has been successfully prepared. This work not only provides a new synthetic strategy but also confirms that luminescent MOFs may be rationally designed to serve as practical multifunctional materials combining sensor and sensitizer.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03053. Crystal data for NBU-9 based on 4-HPTZ ligand, selected bond bonds and angles, simulated and measured XRD patterns, FT-IR spectrum, TGA curve, BVS results, some structural images, solid-state emission spectra, and UV−vis spectra, PL spectra, and luminescence decay curve for Tb3+@NBU-9 (PDF) Accession Codes

CCDC 1813188 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_ G

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[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (F.-Y.Y.). *E-mail: [email protected] (L.H.). ORCID

Fei-Yan Yi: 0000-0003-0733-9712 Lei Han: 0000-0002-2433-9290 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21471086; No. 51572272), the Zhejiang Provincial Natural Science Foundation of China under Grant No. LY18B010004, the Natural Science Foundation of Ningbo (No. 2017A610062; No. 2017A610065), and the K.C. Wong Magna Fund in Ningbo University.

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DOI: 10.1021/acs.inorgchem.7b03053 Inorg. Chem. XXXX, XXX, XXX−XXX