An Efficient Blue-Emissive Metal–Organic Framework (MOF) for

Publication Date (Web): May 24, 2017. Copyright © 2017 American ... On the Better Understanding of the Surprisingly High Performance of Metal−Organ...
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An Efficient Blue-Emissive Metal−Organic Framework (MOF) for Lanthanide-Encapsulated Multicolor and Stimuli-Responsive Luminescence Wei Huang,† Feifei Pan,† Yang Liu,† Shuaidan Huang,† Yujie Li,† Juan Yong,† Yao Li,† Alexander M. Kirillov,‡ and Dayu Wu*,† †

Jiangsu Key Laboratory of Advanced Catalytic Materials and Technology, Advanced Catalysis and Green Manufacturing Collaborative Innovation Center, School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, China ‡ Centro de Quimica Estrutural, Complexo I, Instituto Superior Tecnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001, Lisbon, Portugal S Supporting Information *

ABSTRACT: A novel blue-emitting Zn(II) MOF featuring parallel 2D+2D interpenetrated layers and tubelike channels was generated and shown to efficiently accommodate lanthanide(III) cations (Ln3+ = Eu3+, Tb3+, or a mixture of Eu3+/Tb3+), resulting in the Ln3+-encapsulated functional materials with a tunable emission color, including red, green, and nearly pure white light. Furthermore, the thermalresponsive luminescence was investigated for the lanthanidecodoped MOF to exhibit the chromic transition from white at room temperature to blue around liquid nitrogen temperature.

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INTRODUCTION Metal−organic frameworks (MOFs) have been of tremendous attention for their recognized applications in many research areas, including luminescence, sensing, and ion exchange.1−4 In particular, lanthanide MOFs with their high and well-defined porosity are of great interest, because of intense, long-life, and sharp emissions in visible region.5−8 The emission behavior of MOFs and derived materials also is dependent on a channel microenvironment that can be tuned by changing the functional organic spacers, metal nodes, and sometimes counterions. White-light-emitting (WLE) materials have attracted much attention , because of their potential use in display technologies, fluorescent sensors, and other lighting applications.9 According to the chromaticity diagram of the Commission Internationale de l’Eclairage (CIE), an ideal WLE system must emit three primary RGB colors (red, green, and blue) or two complementary colors (e.g., blue and orange) with required intensities that cover the visible wavelength range from 400 nm to 700 nm. Compared to monochromatic and tetrachromatic counterparts, there is great interest in the dichromatic and trichromatic WLE materials, because they exhibit good color rendering properties and high emitting efficiency. As an important strategy, the Ln3+ doping can be used to fabricate a single-phase white-light emitter. The energy gaps of EuIII and TbIII [ΔE = 12 300 (5D0 → 7F6) and 14 800 (5D4 → 7F0) cm−1, respectively] are sizable,10 and they emit the relatively colorpure red/green colors without significant ligand field effects. Hence, the combination of EuIII and TbIII ions with a blue-light © 2017 American Chemical Society

emitter will render these ions to be attractive for the design of pure-phase WLE materials.11 Recently, Petoud and co-workers reported a zinc-adeninate metal−organic framework (bioMOF-1) for aqueous encapsulation and sensitization of nearinfrared and visible-light emitting lanthanide cations. Furthermore, the authors probed the oxygen sensing property using such lanthanide-encapsulated materials, [email protected] Dong’s group reported a porous heteroatom-rich Cd(II)polymeric framework with one-dimensional tubes (9−11 Å) for trapping cationic lanthanide hydrates such as Eu(H2O)83+, Tb(H2O)83+, and Nd(H2O)83+ under ambient conditions to generate Ln(H2O)83+-loaded materials.13 Just recently, HoltenAndersen et al. developed the light-emitting metallogels functionalized with lanthanide metal complexes.14 The optical properties of these highly luminescent polymer networks are readily modulated over a wide spectrum, including white-light emission, simply by tuning of the lanthanide metal ion stoichiometry. However, the fine-tuning of the emission color toward the desired white-light through the stoichiometry control of different lanthanide ions encapsulated in MOF has been rarely reported. In addition, the Eu/Tb-codoped lanthanide MOFs can be utilized to develop luminescent thermometers based on the temperature-dependence of emission intensity from lanthanide centers;15 however, the temperature-dependent energy transfer mechanism involving Received: February 19, 2017 Published: May 24, 2017 6362

DOI: 10.1021/acs.inorgchem.7b00457 Inorg. Chem. 2017, 56, 6362−6370

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Inorganic Chemistry

[Eu(NO3)3]0.87(H2O)6@C27H22ZnN2O5: C, 35.06; H, 4.03; N, 6.98. Found C, 35.02; H, 4.04; N, 7.10. WLmof-1. The sample was prepared by soaking mof-1 sample in the mixed solution of Eu(NO3)3 (2 mL, 45 μmol/L) and Tb(NO3)3 (2 mL, 235 μmol/L). After several minutes of soakage, the crystals were extensively washed several times with water to remove residual Tb3+ or Eu3+ cations on the surface, and then dried under vacuum prior to PXRD analysis. Anal. Calcd for [Eu0.17Tb0.83(NO3)3]0.05(H2O)1.4@C27H22ZnN2O5: C, 55.59; H, 5.15; N, 5.16. Found C, 55.58; H, 5.12; N, 5.06. X-ray Crystal Structure Analysis. The diffraction intensity data of mof-1 were collected at room temperature on a Bruker APEX-II CCD with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Data collection, data reduction, and cell refinement were performed by using the Bruker Instrument Service v4.2.2 and SAINT V8.34A software.16,17 Structure was solved using direct methods, via the SHELXS program, and refinement was performed using SHELXL based on F2 through full-matrix least-squares routine.18 Absorption corrections were applied using multiscan program SADABS.19 Hydrogen atoms of organic ligands were generated geometrically by the riding mode, and all the non-hydrogen atoms were refined anisotropically through full-matrix least-squares technique on F2 with the SHELXTL program package.20,21A summary of the crystallographic data and refinement parameters is shown in Table 1.

MOFs as host matrices is scarcely reported. Besides, this WLE luminescence arising from the lanthanide codoping with welldefined f → f transitions can be useful for display applications. However, new materials are still required to be developed to achieve true blackbody quality color rendering, phosphors with MOF’s broad emission spectra and, ideally, external stimulidriven color tunability. Hence, the present work aims to fill this gap and reports the synthesis and full characterization of a novel Zn(II) MOF and its application for the generation of white-light-emitting Ln3+-encapsulated materials, as well as their detailed photophysical properties.

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EXPERIMENTAL SECTIONS

Materials and General Procedures. All the reagents and solvents employed were commercially available and were used as received without further purification. Ln3+ nitrates (Eu(NO3)3·6H2O (99.99%) and Tb(NO3)3·5H2O (99.9%), from Aldrich) were used as purchased. All other chemicals were reagent grade and used as received without further purification. The IR spectra were recorded on a Bruker EQUINOX-55 Fourier transform infrared spectrometer (frequency ranged from 4000 cm−1 to 400 cm−1) using KBr pellets. Elemental analyses (C, H, N) were performed using a Vario EL elemental analyzer. Powder X-ray diffraction (PXRD) measurements were performed on a Bruker D8 Avance X-ray powder diffractometer with Cu Kα (1.5418 Å). Fluorescence spectra for the solid samples were recorded at room temperature on an Edinburgh Model FS5 instrument. Measurement of the Ln3+/Zn2+ molar ratios was performed on an X-7 series inductively coupled plasma−mass spectrometry (ICP-MS) system (Thermo Elemental, Cheshire, U.K.). Photoluminescence spectra and luminescence lifetimes (τ) were examined using an Edinburgh Model FS5 phosphorimeter. Quantum efficiency was directly measured using the integrating sphere method on an Edinburgh Model FS5 instrument employing an integrating sphere (150 mm diameter, BaSO4 coating) from the phosphorimeter. The quantum yield can be defined as the integrated intensity of the luminescence signal divided by the integrated intensity of the absorption signal. The absorption intensity was calculated by subtracting the integrated intensity of the light source with the sample in the integrating sphere from the integrated intensity of the light source with a blank sample in the integrating sphere. Synthesis and Analytical Data of [Zn(μ-L)(μ-1,3-dpp)] (mof1). A mixture containing 4,4′-oxybis(benzoic acid) (H2L; 0.0258 g, 0.1 mmol), 1,3-di(4-pyridyl)propane (1,3-dpp; 0.0198 g, 0.1 mmol), Zn(OAc)2·2H2O (0.0219 g, 0.1 mmol), water (6 mL), and DMF (2 mL) was sealed in a Teflon-lined stainless steel vessel (25 mL), which was then heated at 140 °C for 3 days under autogenous pressure. After cooling the reaction mixture to room temperature, colorless block crystals (including those of X-ray quality) were isolated in 43% yield, based on Zn. Infrared (IR) analysis (KBr pellet, cm−1): 449(w), 514(w), 625(w), 663(m), 780(w), 852(w), 878(w), 1095(w), 1135(w), 1159(w), 1300(w), 1364(w), 1497(w), 1569(m), 1616(m), 2929(m), 3068(m), 3423(s). Anal. Calcd for C27H22ZnN2O5: C, 62.38; H, 4.26; N, 5.39. Found C, 61.75; H, 4.21; N, 5.22. Preparation of Lanthanide-Doped Samples. GLmof-1. Crystalline sample of mof-1 (10 mg) was soaked in aqueous solution of Tb(NO3)3 (2 mL, 10 mmol/L). After several minutes of soakage, the crystals were extensively washed several times with water to remove residual Tb3+ cations on the surface, and then dried under vacuum prior to PXRD analysis. IR (KBr pellet, cm−1): 450(w), 515(w), 625(w), 665(m), 785(m), 815(m), 878(m), 1031(m), 1070(w), 1157(m), 1236(m), 1404(s), 1500(m), 1550(m), 1624(s), 2930(m), 3072(m), 3428(s). Anal. Calcd for [Tb(NO3)3]0.95(H2O)6.5@ C27H22ZnN2O5: C, 33.51; H, 3.96; N, 7.02. Found C, 33.45; H, 4.12; N, 7.23. RLmof-1. A process similar to that used for GLmof-1 was applied but Eu(NO3)3 solution was used instead of Tb(NO3)3. The infrared (IR) data are similar to that determined for GLmof-1. Anal. Calcd for

Table 1. Summary of Crystallographic Data for mof-1 parameter empirical formula formula weight crystal system space group unit-cell parameters a b c α β γ V Z F (000) goodness-of-fitb on F2 R1, wR2 (I > 2σ(I)a R1, wR2 (all data)a residuals (e Å−3)

value/remark C27H22N2O5Zn 519.83 orthorhombic Fddd 36.077(5) Å 13.9896(19) Å 19.745(3) Å 90.00° 90.00° 90.00° 9965(2) Å3 16 4228 1.076 0.1285, 0.2866 0.1776, 0.3110 0.697, −0.575

R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]]1/2. GOF = [∑[w(Fo2 − Fc2)2]/(Nobs − Nparams)]1/2, based on the data I > 2σ(I). a b

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RESULTS AND DISCUSSION We applied a solvothermal approach to prepare a new mixedlinker zinc(II)-based MOF (Scheme 1), [Zn(μ-L)(μ-1,3-dpp)] (mof-1), derived from H2L (4,4′-oxybis(benzoic acid)) and 1,3-dpp (1,3-di(4-pyridyl)propane) blocks.22 As shown in Figure 1a, compound [Zn(μ-L)(μ-1,3-dpp)] features a 2D MOF assembled from zinc(II) nodes and μ-L dicarboxylate and μ-1,3-di(4-pyridyl)propane linkers.23 The six-coordinate Zn1 centers possess a distorted octahedral {ZnN2O4} coordination environment, which is occupied by four O atoms from two bidentate COO groups of two μ-L moieties [Zn−O 2.080(9)− 2.310(1) Å] and two N atoms [Zn−N 2.078(1) Å] (the latter ones are in mutually cis position) from two 1,3-di(4pyridyl)propane ligands. Both the μ-L and μ-1,3-dpp blocks 6363

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cation can possibly interact with a free coordination site of the ligand in mof-1. Another possible site for lanthanide binding comes from the coordinated oxygen atom of carboxylate ligand. To gain further insight into the structure of 2D layers and their interpenetration pattern in mof-1, we analyzed the structure from a topological viewpoint, by following the concept of the simplified underlying net24 (see Figure 1b). Such a net was generated by reducing the μ-L and μ-1,3-di(4pyridyl)propane moieties to their centroids. The resulting underlying net is assembled from the 4-connected Zn1 nodes and the 2-connected μ-L and μ-1,3-di(4-pyridy)propane linkers. It can be topologically classified as an uninodal 4connected layer with the sql [Shubnikov tetragonal plane net] topology and the point symbol of (44.62). Besides, the 2D layers are arranged into interlaced pairs and exhibit a parallel 2D+2D → 2D interpenetration (see Figure 1c). The observed type of interpenetration does not lead to an overall network extension. Hence, the crystal structure of mof-1 consists of interlaced two-dimensional layers, which interpenetrate each other to form a parallel 2D+2D → 2D dense packing, leaving a hydrophilic tube-like channels (voids) ca. 4 Å × 3.5 Å in size along the crystallographic b-axis which is calculated with the Mercury software (Figure 1d). In addition, there are channels ca. 4.5 Å × 4 Å in size along the crystallographic c-axis. Given a rigid, channel-containing structure of mof-1, it acts as an efficient scaffold for hosting and sensitizing several Ln3+ cations, thus resulting in intense luminescence and long lifetimes of the derived LnIII@mof-1 products (LnIII = Eu, Tb, or a mixture of Eu/Tb in different molar ratios) (see the Electronic Supporting Information (ESI)). Subsequently, their multicolor luminescence can be tailored from red to green regions by modulating the molar ratio of Eu3+/Tb3+ cations in the material and thus allowing to adjust an emission color.25 More interestingly, an achieved white-light emission can be reversibly transformed to blue-light phosphor by applying thermal treatment. The tunable color luminescence of the obtained composite solids represents a promising strategy in the development of whitelight-emitting sensors and devices. Recently, several studies have demonstrated that the d10transiton metal luminescent MOFs (LMOFs) can be effective sensing platforms, because of their easy-to-functionalize surface and tunable channels that facilitate feasible host−guest interactions.6 Based on the current research on luminescent MOFs, the emission of d-block metal-containing components can be assigned to a ligand-to-metal charge transfer (LMCT),26 metal-to-ligand charge transfer (MLCT),27 or an intraligand π → π* transition. Generally speaking, the emission properties of the MOFs would be significantly influenced by metal coordination, when compared to the free organic ligands, which is an important property to consider when attempting to prepare new luminescent materials. Figure 2 shows the temperature dependence of the emission spectra of mof-1 in the solid state. As a typical example, the emission intensity of mof-1 (Figure 2a) increases on decreasing T from 300 K to 78 K, revealing a new red-shift peak centered at 435 nm. The CIE coordinates for the blue emission of mof-1 range from (0.221, 0.211) at room temperature to (0.176, 0.147) at 77 K, which is closer to the saturated blue emitter with the CIE coordinates of (0.14, 0.08).28 The quantum yield of mof-1 measured at λex = 300 nm increases from 2.5% at room temperature to 18.4% at 77 K. These data demonstrate that mof-1 is an efficient bluelight emitter.

Scheme 1. Structural Drawing of Mixed Linkers and Coordination of Zinc Ion in This Work

Figure 1. Structural fragments of mof-1: (a) One 2D metal−organic layer seen along the b-axis. (b, c) Topological representations showing the 2D+2D interpenetration of two adjacent sql layers seen along the b-axis (panel (b)) or the c-axis (panel (c)); each layer is shown by different color (cyan or gray), Zn nodes are represented as balls. (d) Crystal packing pattern along the b-axis; red ovals represent channels (voids) filled by lattice H2O molecules.

act as μ-linkers and interconnect the adjacent Zn1 centers into a plain 2D metal−organic layer. A notable structural feature of mof-1 consists of the dense packing and interpenetration of such metal−organic layers. The crystal packing pattern of mof1 along the b-axis reveals the tubelike channels (voids). There is an uncoordinated oxygen atom (O3) from the 4,4′-oxybis(benzoate) ligand on the wall of the channels with the separation of 6.736 Å (O3···O3) between the neighboring walls. This type of hydrophilic channels would provide the possible sites for lanthanide binding, wherein the lanthanide 6364

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Figure 2. (a) Temperature dependence of solid-state emission spectra of mof-1 with the excitation at 300 nm. (b) Corresponding variation of quantum yield at different temperatures.

Figure 3. Normalized emission spectra of mof-1 suspension (0.1 mM) with the increasing amount of (a) Tb3+ and (b) Eu3+, respectively, in aqueous solution; insets show the luminescence intensity at 616 and 545 nm, plotted against the molar concentration ratios C(Tb3+)/C(mof-1) and C(Eu3+)/C(mof-1), respectively. (c). Titration experiments showing emission spectra with the increasing Tb:Eu molar ratio (λex = 300 nm, the concentration of Eu3+ is fixed at 4.5 × 10−5 mol/L). Inset shows Job’s plot of the green/red (G/R) emission intensity ratio, as a function of the Tb:Eu molar ratio. (d) Tuning the chromaticity of mof-1 (dot 4) by doping it with Eu3+ ion (dot 1), Tb3+ ion (dot 2), or a mixture of Tb3+/Eu3+ (47:9) resulting in a tunable white-light emission (dot 3).

water leads to the characteristic emission peaks of lanthanide centers and, up to a Ln:MOF molar ratio of 1:1, gives rise to a green-light emissive suspension, GLmof-1 (τ = 0.9 ms, Φ = 32.1%, CIE coordinates (0.34, 0.47)). Low-concentration

Based on our previous work on luminescent small-moleculebased sensors,29 we attempted an incorporation of the Ln3+ cations into the MOF matrix. We found that an incremental addition of Tb(NO3)3 to a suspended solution of mof-1 in 6365

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suggesting that there is no change in crystallinity or deinterpenetration of the parent metal−organic network. Similar results were reported in the previous studies from other groups,12,13,30b wherein the lanthanide adsorption by MOF would not significantly influence the PXRD pattern, although lanthanide elements are strong X-ray scatterers. The loaded concentrations of Ln3+ were determined by ICP-MS and elemental analysis, revealing that a Ln3+/Zn2+ molar ratio is close to 1:1. Next, we investigated how to modulate an emission of the Ln3+-encapsulated MOFs by tuning the stoichiometry of the two lanthanide chromophores (green and red). Titration experiments aiming at modifying the Tb:Eu molar ratio lead to a series of Tb3+/Eu3+-codoped suspensions in water with a broad spectrum of emission (including white light) under the UV irradiation (Figure 3c).30 Two groups of characteristic emissions are developed according to a molar ratio of TbIII/ EuIII that changes from 1:9 to 47:9 while fixing the concentration of EuIII. As the amount of TbIII increases, the intensity of the TbIII/5D4 → 7F5 (545 nm) transition gradually increases; it is unexpected to observe that the peaks corresponding to the EuIII/5D0 → 7F2 (616 nm) transition also increase (see Figure S5 in the ESI). Moreover, the Ln3+encapsulated samples also emit their characteristic luminescence in the solid state (Figure S6 in the ESI), and the corresponding CIE coordinates are consistent with those of the suspensions in water. A particularly interesting feature concerns an intense white-light-emitting solid, WLmof-1 (CIE coordinates (0.33, 0.32)), which is established at the Tb:Eu molar ratio of 47:9. Hence, a straightforward luminescence control demonstrated here offers a simple design approach to achieve a broad-spectrum color tuning of light-emitting MOF materials.31 Luminescence lifetimes and quantum yields of the Ln3+encapsulated samples were investigated at room temperature in aqueous environment under the excitation wavelength of 300 nm and monitored by the most intense emission at 616 (Eu3+) and 545 nm (Tb3+); the results are summarized in Table 2 (see

titration spectra indicate that the signal is very sensitive, thus allowing mof-1 (as a lanthanide host) to detect a subnanomolar level of Tb3+ ion (see Figures S1 and S2 in the ESI) A similar procedure performed with Eu(NO3)3, instead of Tb(NO3)3, resulted in a red-light emissive Eu(III)-encapsulated suspension, RLmof-1 (τ = 0.26 ms, Φ = 8.8%, CIE coordinates (0.34, 0.20)). The changes of emission spectra upon addition of Ln3+ are shown in Figures 3a and 3b, the insets of which indicate the data points obtained from the titrations of mof-1 with hydrated Ln(NO3)3 and the fit to a 1:1 binding model. This is in agreement with the lanthanide cation being stoichiometrically incorporated in the hydrophilic channels rather than on the external surface of the MOF. Next, we checked the possibility of removing the encapsulated lanthanides from the channels via treatment with a strong donor such as Na2EDTA or NH4F. If the encapsulated lanthanides are removed from the channels as a consequence of the formation of Ln-EDTA complex or LnF3 precipitate, the addition of a relative excess of Na2EDTA or ammonium fluoride to Ln3+ ions would diminish the characteristic emission of lanthanide ions (see Figure S3 in the ESI). However, the emission intensity slightly increased upon the increment addition of the strong donors. This can be understood by the fact that the high-energy vibrations of the O−H groups of coordinated water molecules provide an efficient mechanism for the nonradiative de-excitation of the Ln(III) excited states. Hence, the addition of strong donors will substitute some weakly coordinated water molecules and the encapsulated lanthanides cannot be removed from the channels. As shown in Figure 4, a new strong peak at 1404 cm−1 and a moderate peak at 825 cm−1 appeared in the FT-IR spectra of

Table 2. Luminescence Lifetimes and Absolute Quantum Yields of LnIII@mof-1 (Ln = Eu, Tb, Eu/Tb) Luminescence Lifetimes (μs) material

τ1(H2O)a

τ2(H2O)

τ1(D2O)

τ2(D2O)

Φb (%)

λem

RLmof-1 GLmof-1 WLmof-1 WLmof-1

15.9 264.0 17.5 78.6

263 883 458 463

15.2 13.0 32.2 14.2

573 1121 745 876

8.8 32.1 2.2 2.8

616 545 616 545

a

λex = 300 nm; bMeasured in H2O.

Figures S7−S12 in the ESI). The best fit for each of the Ln3+encapsulated samples was systematically biexponential, revealing the presence of two distinct lanthanide environments within the MOF material. All lifetime values of these samples are rather high and comparable to the values for the corresponding homolanthanide samples. The quantum yields (8.8% for RLmof-1 and 32.1% for GLmof-1) are all reasonably high, considering an aqueous environment. They also indicate that the lanthanide cations are protected to a significant extent within the pores, and the energy transfer from the sensitizer embedded in the MOF to the lanthanide cations is efficient.32 Further evidence can be obtained by comparing the spectral data obtained in deuterated water, wherein the corresponding emission intensities are greatly lifted and the lifetimes are also

Figure 4. FT-IR spectra of mof-1 sample before and after lanthanide encapsulation in water.

complexes, RLmof-1, GLmof-1, and WLmof-1 corresponding to the characteristic vibration of nitrate anion, which indicates the effective lanthanide encapsulation. PXRD studies reveal that MOF maintains its crystalline integrity after the encapsulation of Ln3+ cations in water (see Figure S4 in the ESI). Although this encapsulation process is rather quick, a prolonged soaking of mof-1 in aqueous solutions of lanthanide(III) nitrates does not lead to appreciable changes in the PXRD patterns, 6366

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Figure 5. (a) Temperature-dependent PL spectra for the Ln3+-codoped sample WLmof-1 upon excitation at 300 nm in solid state; the inset shows the emission color change in liquid nitrogen and in air. (b) CIE coordinates for WLmof-1 sample at selected temperatures of 77, 160, 240, and 300 K.

Figure 6. (a) Temperature-dependent intensity of the EuIII/5D0 → 7F2 (616 nm) and TbIII/5D4 → 7F5 (545 nm) transitions, as well as of the ligandbased band at 430 nm for WLmof-1. (b) Temperature-dependent intensity ratio between Tb3+ (545 nm) and Eu3+ (615 nm) in WLmof-1. (c) The intensity ratio Δ [Δ = ITb /IEu, where ITb and IEu are the emission intensities of the 5D4 → 7F5 (Tb3+ at 545 nm) and 5D0 → 7F2 (Eu3+ at 616 nm) transitions, respectively] of WLmof-1, as a function of temperature in the range of 78−150 K and 270−350 K.

extended to 573 μs for RLmof-1, 1121 μs for GLmof-1, and 876 μs for WLmof-1, respectively. This behavior logically results from the water quenching by a partial loss of lanthanide

sensitization.33 The ability to be compatible with aqueous conditions in addition to the microporous structure character of these luminescent materials make them well suitable for 6367

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Inorganic Chemistry generating new fluorescent sensors in environmental and biological systems.34 To confirm this hypothesis, we can estimate the number of Eu- and Tb-bound water molecules using eq 1, which was previously developed and reported by Horrocks and Sudnick.35 Herein, n is the number of water molecules in the inner sphere of the Ln(III) ion, AEu = 1.05, ATb = 4.2, and the values of τ−1 are given in inverse milliseconds. n = Aln × (τH2O−1 − τD2O−1)

(τ given in ms)

In order to investigate the potential function as thermometer, the detailed PL spectra for WLmof-1 were studied in the temperature range of 77−350 K upon the excitation at 300 nm (see Figure 6, as well as Figure S13 in the ESI). Upon heating the sample, the blue-emissive band continues to decrease until ∼200 K, followed by a downward plateau ranging from 200 K to 350 K. Surprisingly, the characteristic peaks of lanthanide centers, i.e., at 616 nm (Eu3+) for WLmof-1, exhibit an unusual behavior, since their intensities continue to increase upon heating from 77 K to 150 K. The peak at 545 nm displays similar temperature-dependent luminescent behavior, but with a gentle downward trend from 150 K to 250 K. The intensity ratio of two different emissions (ITb/IEu) is widely accepted as a thermometric parameter in the ratiometric luminescent thermal sensor. As shown in Figure 5b, an intensity ratio Δ [Δ = I545/ I616, where I545 and I616 are the emission intensities of the 5D4 → 7F5 (Tb3+) and 5D0 → 7F2 (Eu3+) transitions, respectively] of WLmof-1, as a function of temperature, reveals two quasilinear relationships in the range of 77−170 K and 270−350 K, along with an intermediate plateau between 170 and 270 K (Figure 6(c)). The unusual PL properties of WLmof-1 are scarcely observed and can be attributed to an enhanced energy transfer from the MOF’s ligands to the Tb3+ and Eu3+ ions in the studied temperature range. The temperature-dependent emission intensity evidence a wider plateau, because of the equilibrium between the energy transfer from the MOF’s ligands to lanthanide ions and nonradiative thermal effect. After heating the sample above 250 K, the curves of the intensity for the EuIII/5D0 → 7F2 (616 nm) and TbIII/5D4 → 7F5 (545 nm) transitions decline on increasing the temperature, because of the thermal activation of the nonradiative energy transfer processes.38

(1)

The n-values correspond to a calculated number of water molecules coordinating to each lanthanide cation and were determined from the luminescence lifetimes in deuterated water (Table 2). Each component of the biexponential luminescence decay curve fitted for Ln3+-encapsulated mof-1 in H2O was paired with a corresponding value in D2O. The fact that there are two distinct Ln lifetimes indicated that there are at least two different coordination environments for these Ln complexes within the channels. Specifically, we coupled the two shortest components and the two longest components of the lifetimes generated from the iterative fitting process and then used these in the empirical expression given in eq 1. Both RLmof-1 and GL mol-1 exhibit the longer luminescence lifetimes for the longest component in D2O solvent; a calculated n-value of 2.2 ± 0.2 for RLmof-1 suggests that the mof-1 provides a limited protection from water in a channel environment. An n-value of 1.0 ± 0.5 calculated from the set of the two longest luminescence lifetimes for GL mol-1 provides a higher level of protection to Tb3+ ion. However, note that the above estimation cannot be used to calculate the Ln-bound water in the fully hydrated MOF material, because of the complex de-excitation pathways. The Ln-coordinated water molecules participate in a potential hydrogen bonding network with the oxygen atoms of the 4,4′-oxybis(benzoate) ligands in mof-1. Because of this consideration, n-values could not be calculated for the shortest component of the lifetimes in both cases. The results are again in excellent agreement with the previous studies,36 demonstrating that two different numbers of water molecules are bound to the lanthanide cations in the MOF channels. To further explore potential application of white-lightemitting MOF as stimuli-responsive materials, we examined the thermochromisc fluorescence of WLmof-1 in solid state.37 As illustrated in Figure 5a (inset), the changes in temperature indeed correlate with an emission color transition when WLmof-1 is cooled in a liquid nitrogen atmosphere. At room temperature, the white-light spectra consist of three primary RGB (red, green, and blue) emissions with the comparable intensity. However, at 77 K, the predominance of the emission centered at 430 nm over the entire spectrum with the CIE coordinates (0.180, 0.126) gave rise to the ligand-based blueband emission (Figure 5a). The emission color change is associated with the prior thermal sensitivity of the ligand-based luminescence. The ligand-based emission at 430 nm decreases more rapidly with increasing temperature than those of lanthanide-centered spectra. Consequently, upon heating the sample to room temperature, the CIE coordinates gradually move to an original white-light site (0.333, 0.328) and the thermochromic transition is reversible, even after heating the sample to 350 K (Figure 5b), indicating the thermal stability of the luminescence.

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CONCLUSIONS In summary, we synthesized and fully characterized a novel mixed-ligand two-dimensional (2D) MOF [Zn(μ-L)(μ-1,3dpp)] (mof-1). Apart from disclosing the sql topological network, its crystal structure features an interesting example of dense packing with channels (voids), wherein each two adjacent layers are interlaced into infinite arrays, resulting in the parallel 2D+2D → 2D interpenetration. The obtained results demonstrate that the selection of lengthy 4,4-oxybis(benzoate) and 1,3-di(4-pyridyl)propane linkers, along with zinc(II) nodes, can lead to novel mixed-ligand MOFs with interesting structural characteristics. We further explored the ability of mof-1 to host lanthanide ions and employed the encapsulation strategy for the design of various luminescent LnIII@mof-1 composite solids (LnIII = Eu, Tb, or Eu/Tb in different molar ratios). By tuning the ratio of red and green emissive sources, the Eu3+ and Tb3+ were simultaneously loaded into the pores of mof-1 to form a white-light-emissive solid. In particular, using the Eu3+/Tb3+ codoping material with appropriate proportion, the host framework exhibits the unusual thermal-induced emission transition from white-light emission to blue-light emission.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00457. 6368

DOI: 10.1021/acs.inorgchem.7b00457 Inorg. Chem. 2017, 56, 6362−6370

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Inorganic Chemistry Figures (Figures S1−S3) with fluorescence titration spectra, PXRD patterns, and additional photophysical data for obtained materials (PDF)

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Accession Codes

CCDC 1545602 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_ [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 Author

*E-mail: [email protected]. ORCID

Dayu Wu: 0000-0002-4132-4795 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for financial support by the Priority Academic Program Development of Jiangsu Higher Education Institutions. This experimental work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21471023 and 21671027) and sponsored by Jiangsu Provincial QingLan Project. A.M.K. acknowledges the FCT, Portugal (No. UID/QUI/00100/2013).

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