Tuning the Solid-State White Light Emission of Postsynthetic

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Tuning the Solid-State White Light Emission of Postsynthetic Lanthanide-Encapsulated Double-Layer MOFs for Three-Color Luminescent Thermometry Applications Liya Qiu,† Chengfeng Yu,† Xiaoling Wang,† Yangbin Xie,† Alexander M. Kirillov,‡,§ Wei Huang,*,† Jipeng Li,† Peng Gao,† Ting Wu,† Xiangwei Gu,† Qi Nie,† and Dayu Wu*,†

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†

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, People’s Republic of China ‡ Centro de Quimica Estrutural, Instituto Superior Tecnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisbon, Portugal § Peoples’ Friendship University of Russia (RUDN University), 6 Miklukho-Maklaya st., Moscow 117198, Russian Federation S Supporting Information *

ABSTRACT: Postsynthetic modification represents an efficient strategy for the fabrication of tunable metal−organic frameworks (MOFs) and derived highperformance functional materials. Herein, we report the synthesis of a mixed-linker zinc(II)-based double-layered MOF (dlMOF) with dual-emissive luminescence, which was further applied as a host matrix to fabricate highly tunable Ln@dlMOF materials (Ln = Eu, Tb, Eu/Tb). The emission characteristics of these materials can be readily modulated over a wide spectrum, including white light emission, by simply tuning the Eu3+/Tb3+ molar ratio in EuTb@dlMOF. Furthermore, by virtue of the difference in thermal sensitivity between triple-emissive sources, the Eu3+/ Tb3+-codoped thermometer EuTb@dlMOF exhibits real-time successive chromogenic switches from red (room temperature) to white (intermediate temperature) to blue/green (cryogenic temperature) emission in a wide temperature region. The versatile performance and the facile assembly from easily available linkers suggest that postsynthetic lanthanide encapsulation represents an efficient strategy for the future engineering of advanced photoluminescent materials with stimuli-responsive and thermochromic properties.

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INTRODUCTION The postsynthetic modification of metal−organic frameworks or porous coordination polymers (MOFs or CPs) has proved efficient for promoting the reversible dissociation/association between the MOF host and guest molecules, which is a desirable feature for developing functional molecular solids with potential application in electronic, fluorescent, and catalytic materials.1−6 In the past few decades, an increasing number of structurally tunable MOF and MOF-derived materials with improved performance has been prepared through postsynthetic methods.7−13 Among these materials, postsynthetic fluorescent MOFs have attracted considerable attention, owing to their advantageous structure designability that facilitates the fine tuning of the luminescent properties.14−21 In particular, more than one emitting species is necessary in order to obtain white-light-emitting (WLE) materials, because the fluorescence of a single emitting source does not usually cover the entire visible spectrum.22−28 Lanthanide codoping is an important strategy for the fabrication of WLE lanthanide MOFs, by integrating red (Eu3+, Pr3+, Sm3+)-, green (Tb3+, Er3+)-, and blue (Tm3+, Ce3+)-emitting lanthanide ions in an appropriate ratio. However, it is difficult to precisely control the stoichiometry © XXXX American Chemical Society

of different lanthanide ions in a material via direct synthesis. Thus, the postsynthetic doping strategy may offer some advantages (especially in the design of WLE materials) over the direct synthesis, such as the simpler fabrication process, reduced cost, enhanced accuracy, and improved structural predictability. From the viewpoint of applications, the luminescence of postsynthetic hybrid materials can be tailored to cover the full visible region by the reversible encapsulation of heterolanthanides within the MOF, enabling the design of a multicolor thermometer.29−33 To date, most luminescence-based thermometers rely on single or dual emission, whereas the synthesis of emissive materials exhibiting a thermal-induced switch among more than three colors remains challenging.34,35 Postsynthetic WLE materials could potentially serve as chromogenic thermometers switching between high-contrast white/red or white/blue light in different temperature ranges. Most importantly, multiple-color red ↔ white ↔ blue transitions could take place successively, which is highly desirable because the transition from white to colored light can Received: January 10, 2019

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DOI: 10.1021/acs.inorgchem.9b00084 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Scheme 1. Postsynthetic Modification of Double-Layer MOF and Design of PL Chromogenic Thermometer in This Work

FT infrared spectrometer in the 4000−400 cm−1 range, using KBr pellets. UV−vis studies of sample film on quartz were performed using PerkinElmer Lambda 950 UV−vis instrument. Elemental analyses (C, H, N) were undertaken on a Vario EL elemental analyzer. ICP/MS was performed on an X-7 series inductively coupled plasma−mass spectrometry system (Thermo Elemental, Cheshire, U.K.). Powder XRD (PXRD) patterns were recorded on a RINT2000 vertical goniometer with a Cu Kα X-ray source (operated at 40 kV and 100 mA). Simulated PXRD patterns were calculated with the CrystalDiffract package (CrystalMaker Software, Ltd.), using the single-crystal diffraction data. Fluorescence spectra were recorded on an Edinburgh FS5 instrument coupled to a liquid N2 closed-cycle cryostat for lowtemperature data collection. The slit width was fixed at 1 nm for both excitation and emission light, and an excitation wavelength of 270 nm was used in all measurements. Emission and excitation spectra were also corrected for the spectral response of the monochromator and the detector, using typical correction spectra provided by the manufacturer. The absolute emission quantum yield (QY) values were measured at room temperature using an Edinburgh quantum yield measurement system, equipped with a 150 W xenon lamp coupled to a monochromator for wavelength discrimination, an integrating sphere as the sample chamber, and a multichannel analyzer for signal detection. Photoluminescence (PL) decays were recorded using an Edinburgh FLS980 steady-state fluorimeter coupled to a time-correlated single photon counting (TCSPC) spectrometer and a pulsed xenon lamp as the excitation source. N2 adsorption and desorption isotherms of the activated materials were measured on an Autosorb-IQ (Quantachrome, v5.2) instrument at 77 K. Approximately 60 mg of sample was used in each experiment, and the specific surface areas were analyzed using the Brunauer−Emmett−Teller model, on the basis of the N2 sorption data in the 0.12−0.28 P/P0 range. Pore size distributions were obtained with the Barrett−Joyner− Halenda method, using a carbon slit pore model with a N2 kernel. Synthesis of [Zn(μ-L)(μ-bidpe)]n. A mixture containing Zn(NO3)2·6H2O (29.7 mg, 0.1 mmol), bidpe (28.4 mg, 0.1 mmol), and H2L (24.0 mg, 0.1 mmol) dissolved in 8 mL of water was placed in a Parr Teflon-lined stainless steel vessel (15 mL) and heated at 170 °C for 3 days under autogenous pressure. A large quantity of colorless block crystals, suitable for X-ray diffraction, was obtained after cooling the vessel to room temperature. The crystals were isolated, washed with water and ethanol, and dried under vacuum to give dlMOF (yield: 46%). Anal. Calcd for ZnC32H22O5N4: C, 63.22, H, 3.62, N, 9.22. Found: C, 63.18, H, 3.65, N, 9.24. IR (KBr, cm−1): 3400 (w),

be easily recognized by the naked eye. However, the use of postsynthetic lanthanide doping based emissive materials in thermometric applications has never been reported to date, due to the difficulty in stabilizing the lanthanide−framework interaction in the solid state or to the quenching effect of the surrounding environment. In this work, we selected 4,4′-bis(imidazol-1-yl)diphenyl ether (bidpe) and biphenyl-3,5-dicarboxylic acid (H2L) as mixed organic linkers (Scheme 1) for the preparation of layered MOFs and investigated the specific interaction between the resulting framework and the lanthanide cations. Single-crystal X-ray diffraction (XRD) analysis revealed a double-layered zinc(II) MOF (Zn(μ-L)(μ-bidpe), dlMOF) structure. Due to its enhanced surface area, the double-layered MOF is expected to exhibit a superior performance in the encapsulation of lanthanide cations in solution, in comparison to that reported for a monolayer MOF in our previous work.36 The present double-layered metal−organic framework was successfully applied as a host matrix to trap a low concentration of lanthanide cations, such as Eu3+, Tb3+, and a Eu3+/Tb3+ mixture, with a controllable stoichiometry. Furthermore, highly tunable EuTb@dlMOF materials were fabricated as WLE sources. Taking advantage of the different thermal sensitivities of the multiple emitting sources, we show that the white-light-emitting EuTb@dlMOF solid can serve as a rapid and real-time chromogenic luminescent thermometer (red ↔ white ↔ blue) in a wide temperature range. The physical and chemical tunability of the postsynthetic luminescent solid can support a highly promising strategy for the development of white-light-emitting sensors and stimuliresponsive devices.

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EXPERIMENTAL SECTION

Materials and General Procedures. All chemicals such as Eu(NO3)3·6H2O and Tb(NO3)3·6H2O were purchased from Aldrich and used as received. H2L37,38 and bidpe39 were prepared according to the methods reported in the literature. NMR spectra were recorded on a Bruker Advance 400 MHz Fourier transform (FT) NMR spectrometer. IR spectra were recorded on a Nicolet Magna-IR 750 B

DOI: 10.1021/acs.inorgchem.9b00084 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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3124 (w), 1636 (s), 1568 (s), 1387 (s), 1278 (s), 1126 (s), 1021 (m), 965 (w), 948 (w), 931 (w), 875 (m), 835 (s), 764 (s), 722 (m), 656 (m), 528 (m), 493 (w). Preparation of Lanthanide-Doped Sample. A well-ground sample of crystalline dlMOF solid (10 mg) was immersed in a 2 mL methanol solution containing Tb(NO3)3 (20 μmol/L). After 5 min of soaking, the suspension was washed several times and then centrifuged, followed by drying the sample under vacuum to obtain the Tb@dlMOF material. It should be noted that the MOFs cannot uptake solvent in the lattice and the residual solvent on the surface of the material can be easily dried under vacuum before measuring the spectra. IR (KBr pellet, cm−1): 3409 (w), 3109 (w), 1627 (m), 1587 (s), 1400 (m), 1283 (m), 1134 (s), 1034 (m), 970 (w), 962 (m), 921 (m), 864 (m), 837 (m), 762 (m), 703 (w), 652 (w), 536 (w), 490 (w). A similar process was applied to prepare Eu@dlMOF, except that an Eu(NO3)3 solution was used instead of the Tb(NO3)3 solution. The IR data are identical with those of Tb@dlMOF (for details, see the Supporting Information). For the preparation of Eu3+and Tb3+-codoped EuTb@dlMOF samples, we used a twocomponent solution containing a mixture of Eu(NO3)3 and Tb(NO3)3 with an Eu:Tb molar ratio varying from 5:0 to 5:30. The IR data are identical with those obtained for Tb@dlMOF (see the Supporting Information for details). The Ln3+ concentrations loaded into EuTb@dlMOF were determined by inductively coupled plasma-mass spectrometry (ICP-MS). A Eu:Tb ratio of 5:13 was measured for the white-light-emitting material, which is close to the 5:15 value corresponding to the component ratio in solution. X-ray Analysis of Crystal Structure. A Bruker APEX-II CCD diffractometer was used to collect the diffraction data of dlMOF at 173 K with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The software Bruker Instrument Service v4.2.2 and SAINT v8.34A were used for data collection, data reduction, and cell refinement.40,41 The SHELXS software was used to refine the structures by using direct methods.42 The SADABS program was used for absorption corrections.43 All non-hydrogen atoms were anisotropically refined by using full-matrix least-squares techniques based on F2, while the hydrogen atoms were generated geometrically and refined by the riding mode.44,45 For the crystallographic data, see Table 1.

param

value/remark ZnC32H22O5N4 607.90 173 0.71073 monoclinic P21/n 11.7007(7) 11.7063(6) 20.8001(11) 90 96.011(2) 90 2833.4(3) 4 1248 1.038 0.0461, 0.1117 0.0810, 0.0982 1.980, −0.588

RESULTS AND DISCUSSION

In this work, a zinc(II)-based double-layered MOF, [Zn(μL)(μ-bidpe)]n, was prepared from H2L and bidpe blocks under solvothermal conditions. The asymmetric unit of [Zn(μ-L)(μbidpe)]n is composed of the Zn1 center, a bridging μ-L2− dicarboxylate ligand, and a flexible μ-bidpe linker. The fourcoordinated Zn1 center adopts a distorted-tetrahedral {ZnN2O2} coordination geometry involving two O donors from two μ-L2− moieties (Zn1−O2, 1.957(2) Å; Zn1−O3, 1.929(2) Å) and two N atoms from two μ-bidpe ligands (Zn1−N1, 2.000(3) Å; Zn1−N4, 2.002(3) Å) (Figure 1a). In the μ-L2− moiety, both carboxylate groups act as monodentate ligands and, along with the μ-bidpe N,N-linkers, provide a multiple interlink between adjacent Zn1 centers, generating an intricate 2D metal−organic double layer (Figure 1b,c). Within this layer, the Zn1−Zn1 separations via the μ-L2− and μ-bidpe linkers are 7.939 and 16.551 Å, respectively. To obtain further details of the structure of the 2D double layers in dlMOF, we generated a simplified underlying network (Figure S1 in the Supporting Information) and analyzed it from a topological perspective.46−49 The underlying net was built from 4-connected Zn1 nodes and 2-connected μ-L2− and μ-bidpe linkers. The topological analysis revealed a uninodal 4connected net with 4L2 topology and (65.8) point symbol. Furthermore, the adjacent 2D double layers are interdigitated (Figure 1d) and extend into a 3D supramolecular assembly via weak interactions such as intermolecular C−H···O hydrogen bonds, π−π interactions between aromatic rings, and van der Waals forces. Therefore, the crystal structure of dlMOF consists of an interlaced 2D double-layer network with 4L2 topology and abundant O donors on the surface; these features are expected to provide a platform for hosting Ln3+ cations. The Zn(II) dlMOF showed two emission peaks upon 270 nm excitation at low temperature (Figure 2a): a first band at ca. 340 nm and a second low-energy broad band centered at ca. 500 nm. As shown in Figure 2a (inset), after the sample was cooled the intensities of the UV band around 340 nm increased more rapidly, due to the excited-state deactivation pathways. The luminescence decay times at 80 K were monitored for both emission bands at 340 and 500 nm, and the corresponding luminescence decay curves are shown in Figure 2b. The photoluminescence lifetimes τ for the different emitting peaks at low temperature were determined using the exponential decay function in eq 1

Table 1. Summary of Crystallographic Data for dlMOF empirical formula formula wt temp (K) wavelength (Å) cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z F(000) goodness of fita on F2 R1, wR2 (I > 2σ(I))b R1, wR2 (all data) residuals (e Å−3)

Article

I(t ) = I0 + A exp( − t /τ )

(1)

where I0 and A are constants. The best fits yielded τ = 4.22 ± 0.02 ns at 340 nm and 5.47 ± 0.07 ns at 500 nm, highlighting the dual-fluorescence character of the present materials. Following our previous work on the lanthanide encapsulation in monolayer MOFs,36 here we introduced the doublelayered metal−organic framework as a platform for the incorporation of Ln3+ cations into the dlMOF matrix. The emission spectrum of a dlMOF suspension in solution exhibited a small red shift to 353 nm in comparison to that of the solid. This solvent effect could be due to several factors, such as solvent attachment or different emissive traps on the surface of dlMOF.50 The incremental addition of Tb3+ to a suspension of dlMOF in methanol produced a decrease in the luminescence intensity of the ligand-centered emission band (Figure 3a), along with the appearance of new emission bands

GOF = [∑[w(Fo2 − Fc2)2]/(Nobs − Nparams)]1/2, based on the data with I > 2σ(I). bR1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = [∑[w(Fo2 − Fc2)2]/∑[w(Fo2)2]]1/2. a

C

DOI: 10.1021/acs.inorgchem.9b00084 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. Structural fragments of dlMOF: (a) coordination environment around the Zn1 atom and connectivity of μ-L2− and μ-bidpe ligands; (b, c) side and front views of 2D metal−organic double layer (along the b and c axes, respectively); (d) four interdigitated 2D double layers shown by different colors. H atoms are omitted for clarity. Color codes: Zn (cyan balls), O (red), N (blue), C (gray).

Figure 2. (a) Variable-temperature emission spectra of Zn(II) dlMOF in the solid state excited at 270 nm. The inset gives the temperaturedependent emission counts at the different emission peaks. (b) Decay curves of the dual emission of Zn(II) dlMOF in the solid state at 80 K monitored at 340 and 500 nm, respectively.

assigned to the 5D4 → 7F6, 5D4 → 7F5, 5D4 → 7F4, and 5D4 → 7 F3 transitions of Tb3+, which indicated an efficient energy transfer from MOF to the lanthanide center. The addition of Tb3+ resulted in a green-light-emissive suspension, Tb@ dlMOF (τ = 0.014 ms, Φ = 18.3%, Commission Internationale de l’É clairage (CIE) coordinates (0.311, 0.497)). A similar phenomenon was observed upon Eu3+ addition to a dlMOF suspension, with the occurrence of five characteristic Eu(III)based emission bands centered at 581 nm (5D0 → 7F0), 596 nm (5D0 → 7F1), 617 nm (5D0 → 7F2), 651 nm (5D0 → 7F3), and 687 nm (5D0 → 7F4) (Figure 3b), giving rise to a red-lightemissive suspension, Eu@dlMOF (τ = 0.022 ms, Φ = 18.2%, CIE coordinates (0.440, 0.256)). Titration spectra with subnanomolar concentrations of lanthanide cations indicated that the spectra are highly sensitive to the encapsulated

lanthanide concentration (Figure S2 in the Supporting Information). Next, we checked whether the encapsulated lanthanides can be removed from the channels of dlMOF. If a strong donor such as Na2EDTA, NH4F, etc. can remove the encapsulated lanthanides, the addition of excess Na2EDTA or NH4F relative to the Ln3+ ions present in the suspensions would reduce the characteristic emission of lanthanide ions (see Figures S3 and S4 in the Supporting Information). However, no obvious changes were observed in the emission intensity, which indicates a strong binding interaction between the lanthanide ions and the dlMOF channels. Through controlling the ratio of the two lanthanide chromophores, i.e. green and red, fine tuning of the emission of the Ln3+-encapsulated dlMOF (Ln@dlMOF) materials was achieved in the titration experiments. Different Eu3+/Tb3+D

DOI: 10.1021/acs.inorgchem.9b00084 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. Emission spectra of dlMOF suspension (8 mM) in MeOH with increasing amounts of (a) Tb3+ and (b) Eu3+. The insets show the luminescence color change before and after addition of the saturated concentration of Ln3+, respectively. (c) Titration experiments showing the formation progress of white light emission on varying the Eu:Tb molar ratio from 5:1 to 5:18 (λex = 270 nm). The inset shows the Job plot of the green:red (G:R) emission intensity ratio, as a function of the Tb:Eu molar ratio. (d) Tuning the chromaticity of dlMOF (dot 1) by doping it with Eu3+ ion (dot 2), Tb3+ ion (dot 3), or a Tb3+/Eu3+ mixture resulting in a tunable white-light emission (dot 4). (e) Emission color changes and white-light tuning process upon addition of increasing amounts of Eu3+ and Tb3+ in the MOF suspension under UV light (254 nm).

(III) nitrates did not lead to significant changes in the PXRD patterns. This result indicated that the structure of the metal− organic double layer is stable. The thermogravimetric analyses showed that the Ln@dlMOF and dlMOF solids were thermally stable and their frameworks began to collapse at ca. 400 °C (Figure S8 in the Supporting Information) The UV−vis spectra of dlMOF in the solid state showed two wellresolved peaks centered at 205 and 290 nm, which could be ascribed to ligand-based electronic transitions (Figure S9 in the Supporting Information). However, the absorption peaks exhibited marked red shifts to 245 and 305 nm, respectively, in the spectra of the EuTb@dlMOF samples, denoting electronic perturbations of the ligand due to the lanthanide binding. In addition, the absorption band at 1400 cm−1, corresponding to the characteristic vibration of the nitrate anion (originating from the Ln3+ nitrate), appeared in the FTIR spectra of the encapsulated samples (Figure S10 in the Supporting Information), thus confirming the effective lanthanide encapsulation into dlMOF. In addition, the Ln3+-encapsulated solids also displayed their characteristic luminescence emission (Figure S11 in the Supporting Information). The temperature dependences of the emission spectra of Eu@dlMOF and EuTb@dlMOF from 80 to 350 K are plotted in Figure 4a−d, whereas the integrated

codoped dlMOF suspensions with a wide spectrum of emissions, including white light, under UV light were obtained (Figure 3c). When the Eu3+:Tb3+ ratio was changed, two groups of characteristic emissions changed from 5:0 to 5:30 (Figure S5 in the Supporting Information). Remarkably, an intense white-light-emitting suspension, EuTb@dlMOF (CIE coordinates (0.33, 0.32); Φ = 25.7%) could be obtained by simply adjusting the Eu3+:Tb3+ molar ratio in solution to 5:15. Hence, the luminescence color of the Ln3+-encapsulated samples covers the full visible region (Figure 3d,e), including the white light range. The isotherm measurements and the analysis of the pore size distributions indicated the absence of large amounts of micropores for both dlMOF and EuTb@dlMOF (Figure S6 in the Supporting Information). We next attempted to identify the precise chemical environment of Ln3+ ions in the framework through the single-crystal XRD technique; however, the single crystals broke into microcrystals after soaking in a solution containing lanthanide nitrate and became unsuitable for X-ray diffraction. The powder XRD measurements revealed that dlMOF maintained its crystalline integrity after the encapsulation of Ln3+ cations (Figure S7 in the Supporting Information). Despite the rather rapid encapsulation process, prolonged immersion of dlMOF in a solution of lanthanideE

DOI: 10.1021/acs.inorgchem.9b00084 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a, b) Variable-temperature emission spectra of Eu@dlMOF in the solid state between 80 and 160 K and between 200 and 350 K. The inset shows temperature-dependent photon counts at different emission peaks. (c, d) Variable-temperature emission spectra of EuTb@dlMOF in the solid state at different temperature regimes with excitation at 270 nm. (e, f) Temperature-dependent integrated intensity of the corresponding transitions.

intensities of the 5D4 → 7F5 (Tb3+, 545 nm) and 5D0 → 7F2 (Eu3+, 616 nm) transitions, along with those of the MOFbased fluorescence at 335 and 487 nm, are shown in Figure 4e,f. Below 180 K, the fluorescence of Eu@dlMOF at 487 nm showed a continuous decrease; however, the intensities at 335 and 616 nm exhibited an unusual gradual increase with increasing temperature. The increase in the intensity of the Eu3+ peak at 616 nm with the temperature could be explained by a thermally activated energy transfer from a ligand-based excited state. The thermally induced increase in luminescent intensity at 335 nm is unusual, indicating the possible occurrence of reversible intersystem crossing in this system. As the temperature was further increased above 180 K, both 335 and 616 nm emissions of Eu@dlMOF showed a gradual decrease, due to the thermal activation of a nonradiative decay pathway. Hence, the temperature dependence of the fluorescence is likely to be the result of some competitive

processes, such as thermally activated energy transfer and thermal quenching. The codoped EuTb@dlMOF exhibited a temperature-dependent luminescent behavior slightly different from that of Eu@dlMOF. The emission intensity at 487 nm of EuTb@dlMOF decreased upon heating the sample. At 80 K, the emission bands at 335 and 487 nm virtually dominated the whole emission spectrum, whereas the Eu3+ emission became dominant at room temperature, despite the very low Eu3+ concentration in EuTb@dlMOF (ca. 0.1% MOF). These results were different from those obtained by Qian et al. for the mixed lanthanide Eu0.0069Tb0.9931-DMBDC MOF (DMBDC = 2,5-dimethoxy-1,4-benzenedicarboxylate),14 for which the temperature-dependent emissions and luminescence colors involved a temperature-dependent energy transfer from the Tb3+ to Eu3+ ions. In this case, the temperature-dependent emission spectra of Eu@dlMOF and EuTb@dlMOF demonF

DOI: 10.1021/acs.inorgchem.9b00084 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a, b) Variable-temperature emission spectra of EuTb@dlMOF in the solid state between 80 and 160 K (a) and 200 and 350 K (b) excited at 270 nm. (c) Thermally induced chromism transition under UV light and tuning of the the chromaticity of EuTb@dlMOF in the solid state by changing the temperature in the 80−298 K range. Selected temperatures: 80 K (dot 1), 120 K (dot 2) 180 K (dot 3), 298 K (dot 4).

strate that the MOF to Ln3+ energy transfer pathway may be operative in these systems. Next, we investigated the temperature dependence of PL in terms of both intensity and fluorochromism, so as to apply the species as luminescent chromogenic thermometers. The different thermal sensitivities of the luminescent emissions at 335, 487, 545, and 616 nm by EuTb@dlMOF make it an excellent candidate for self-referencing luminescent thermometers; therefore, this material was examined in further detail. In Figure 5a,b, the temperature showed a clear correlation with the emission intensity ratio. In particular, T exhibited a linear relationship with η (I335/I487) from 80 to 160 K (eq 2) and with γ (I335/I616 from 180 to 350 K (eq 3). T = 28.2 + 45.45η

(2)

T = 400 + 250γ

(3)

material, because external stimuli may facilitate the color switch between multiple emissive sources: i.e., red, green, blue, and/or their mixture. To further explore the potential applications of EuTb@dlMOF as a naked-eye temperature detector, we examined its emission in the solid state. The thermal-induced emission color changes are shown in Figure 5c. To illustrate the real-time detection of temperature changes, the dynamic chromogenic response was recorded in a video displaying successive red ↔ white ↔ blue changes with the temperature upon heating (for details, see the video SV1 in the Supporting Information). At room temperature, the emission centered at 616 nm was dominant over the entire spectrum, with CIE coordinates of (0.39, 0.31) characteristic of a red emission. When the sample was cooled, the entire spectrum, with CIE coordinates of (0.33, 0.32), gave rise to a white-light emission in the 160−180 K temperature interval. However, below 120 K, the dlMOF dual emission became dominant, with CIE coordinates of (0.271, 0.327), characteristic of blue/green emissions. Hence, the PL color changes mainly arise from the thermally activated energy transfer between the MOF and lanthanide ions. Notably, the reversible thermochromic transition, even after thermal treatment at 350 K, highlighted the thermal stability of the luminescence. To further probe the level of protection provided by the dlMOF material and to investigate the environment

These results indicate that EuTb@dlMOF is an excellent luminescent thermometer in the two different temperature regimes reported above (Figure 5). This type of Eu3+/Tb3+codoped lanthanide MOF thermometer is unique because it not only involves a MOF-based dual emission but also has a higher sensitivity based on the self-calibrated lanthanide transition at 616 nm (5D0 → 7F2), as displayed by its larger slope. A feature of particular interest is the direct observation of the fluorochromic change of the Eu3+/Tb3+-codoped WLE G

DOI: 10.1021/acs.inorgchem.9b00084 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Accession Codes

surrounding the encapsulated lanthanide cations, we measured all phosphorescence lifetimes at 78 and 298 K (Figures S12− S15 in the Supporting Information). The luminescence lifetimes were investigated in the solid state and monitored by the most intense emissions at 616 nm (Eu3+) and 545 nm (Tb3+); the results are summarized in Table 2. The best-fitting

CCDC 1874799 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 [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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Table 2. Luminescence Lifetimes and Absolute Quantum Yields of Ln@dlMOFs (Ln = Eu, Tb, Eu/Tb) material

temp

Eu@MOF

room temp room temp room temp room temp 78 K 78 K

Tb@MOF EuTb@ MOFc

Corresponding Authors

τ1 (μs)a

τ2 (μs)

Φb (%)

λem (nm)

2.3 ± 0.06

22.4 ± 1.8

18.2

616

*E-mail for W.H.: [email protected]. *E-mail for D.W.: [email protected].

2.0 ± 0.06

13.7 ± 0.9

18.3

545

ORCID

2.2 ± 0.1

11.0 ± 2.3

25.7

616

2.3 ± 0.06

10.9 ± 1.0

545

2.5 ± 0.2 2.0 ± 0.1

42.6 ± 9.6 30.9 ± 3.4

616 545

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 (W.H. and D.W.). A.M.K. acknowledges the FCT and Portugal 2020 (LISBOA-01-0145-FEDER-029697) and the RUDN University (the publication was prepared with the support of the RUDN University Program 5-100).

a

Excited at 270 nm. bMeasured by using direct method. cThe ratio Eu:Tb = 5:13 was determined by ICP-MS.

model for each Ln@dlMOF sample was always biexponential, suggesting the presence of two distinct lanthanide environments within the metal−organic framework. At room temperature, the lifetimes of Eu@dlMOF and Tb@dlMOF were 22.4 ± 1.8 and 13.7 ± 0.9 μs, respectively (as determined from the set of longer lifetime values). The luminescence lifetimes at 78 K increased to 42.6 ± 9.6 and 30.9 ± 3.4 μs for Eu@dlMOF and Tb@dlMOF, respectively. All observed quantum yields (18.2% for Eu@dlMOF, 18.3% for Tb@dlMOF, and 25.7% for EuTb@dlMOF) were reasonably high, indicating that the lanthanide cations are well protected by the dlMOF layer. In conclusion, we have successfully fabricated a doublelayered MOF with multicolor luminescence properties (Ln@ dlMOF) via postsynthetic lanthanide encapsulation. The dlMOF material can incorporate different lanthanide cations with precisely controllable stoichiometry in solution, thus allowing the preparation of highly tunable emissive materials. Furthermore, we investigated the thermochromic behavior of the lanthanide-codoped material EuTb@dlMOF, which exhibits multiple emissions, including red, blue/green, and white light, in different temperature ranges. To the best of our knowledge, this is the first example of a luminescence-based thermometer that exhibits multicolor emission, i.e., red ↔ white ↔ blue/green, in a wide temperature region. We believe that the versatile and facile strategy for generating advanced luminescent solid materials described here will provide new insights toward the development of solution-processable light emitters in optoelectronics.

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AUTHOR INFORMATION

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00084. Topological analysis, fluorescence titration spectra,, nitrogen isotherms, PXRD patterns, TGA, UV and FTIR spectra, solid-state excitation and emission spectra, and additional photophysical data for dlMOF and derived Ln@dlMOF hybrid materials (PDF) Thermometry chromogenic experiment (MOV) H

DOI: 10.1021/acs.inorgchem.9b00084 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.9b00084 Inorg. Chem. XXXX, XXX, XXX−XXX