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Dyes encapsulated in a novel flexible metal−organic framework show tunable and stimuli-responsive phosphorescence
Dyes and Pigments xxx (xxxx) xxx
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Dyes and Pigments journal homepage: http://www.elsevier.com/locate/dyepig
Dyes encapsulated in a novel flexible metal organic framework show tunable and stimuli-responsive phosphorescence Bin Li a, Weipeng Jiang a, Yingbo Xu b, Zhiqiang Xu b, **, Qingqing Yan a, Guoping Yong a, * a b
Department of Chemistry, University of Science and Technology of China, Hefei, 230026, PR China The USTC-Anhui Tobacco Joint Laboratory of Chemistry and Combustion, Hefei, 230066, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: 5-(2-chloroimidazo[1,2-a]pyridine-3carboxamido)isophthalic acid Metal-organic framework Dyes Encapsulation Phosphorescence Stimuli-responsive materials
A novel flexible metal-organic framework (MOF), [ZnL]∙H2O∙DMA (USTC-1), with interesting topology has been constructed based on a new T-shaped ligand, 5-(2-chloroimidazo[1,2-a]pyridine-3-carboxamido)iso phthalic acid (H2L). This MOF was characterized by IR spectroscopy, thermogravimetry, single-crystal, and powder X-ray diffraction methods. The single-crystal analysis reveals four carboxylate groups connect two zinc ions to generate a paddlewheel secondary building unit (SBU). Each paddlewheel SBU is six-connected to six L2 ligands, and each L2 is three-connected to three SBUs, thus, the framework can be described as a binodal (3,6)connected three-dimensional (3D) flexible network. The low N2 and CO2 gas uptake should be attributed to the flexible vibration of amide chains at ligands that close the pore window, however, the closed pore window can be opened by dye molecules, such as Rhodamine B (RB), Methyl Orange (MO) or Methylene Blue (MB). Two methods for dye encapsulation were investigated: soaking and in situ encapsulation. PXRD data showed that the framework in USTC-1 was unchanged after encapsulation of dyes. Interestingly, RB@USTC-1 simultaneously displayed the characteristic emissions of both the RB dye and the MOF, resulting in red/blue two-color lumi nescence with intensity ratio of 4.5 and tunable phosphorescence color. Moreover, RB@USTC-1 and MO@USTC1 could undergo protonation upon exposure to HCl vapor, and reveal reversible phosphorescent color switching in the response to acid-base vapor stimuli. The present work provides a promising approach for synthesizing novel flexible MOFs and a new access to develop the phosphorescent and stimuli-responsive MOF materials via encapsulation of various guests.
1. Introduction The luminescent materials of solid state have become an extensively studied area owing to their potential applications in lighting, display, sensing and optical devices [1–4]. In traditional, rare-earth ions are topics in the light emitting materials. However, recently, organic lumi nescent materials have a growing effect on the emissive materials [5–9], because of their many advantages such as low cost, diverse colors and tunability which can be used in fields of fluorescent sensors, probes, imaging agents, and organic light-emitting diodes. Moreover, the encapsulation of luminescent guest molecules into host matrices has attracted much attention on the basis of both fundamental studies and the applications [10,11]. According to the host–guest interactions and collective effects, the resulting composite materials can reveal unique functionalities (such as emission diversity and tunable luminescence)
which are not simply the sum of the individual host matrices or guests [12]. Metal–organic frameworks (MOFs), also known as porous coordi nation polymers (PCPs), are a kind of crystalline porous materials con structed from metal ions/clusters and organic linkers [13–16]. According to tunable structures, cavities and chemical characters, MOFs are promising candidates for gas storage/separation, catalysis, drug delivery and optical sensing/detection, etc [17–21]. Interestingly, MOFs have also important potential as multifunctional luminescent materials, in which light emitting can come from one or more of the following strategies: organic linkers, metal ions/clusters, encapsulation of emis sive guest molecules, defects or charge transfer processes [22–25]. Nevertheless, there are only a few of MOFs that exhibit tunable emission properties by encapsulation of guests. Gai et al. found out that the emission characteristics of MOFs can be adjusted by using various ratios
* Corresponding author., The USTC-Anhui Tobacco Joint Laboratory of Chemistry and Combustion, Hefei, 230066, PR China. ** Corresponding author. Department of Chemistry, University of Science and Technology of China, Hefei, 230026, PR China. E-mail addresses: [email protected] (Z. Xu), [email protected] (G. Yong). https://doi.org/10.1016/j.dyepig.2019.108017 Received 8 August 2019; Received in revised form 2 November 2019; Accepted 2 November 2019 Available online 9 November 2019 0143-7208/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Bin Li, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2019.108017
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of lanthanide metal ions in MOFs [26]. Other groups have also used trivalent lanthanide ions as guests to tune different emission properties [27,28]. The flaw of using lanthanide ions as guests is that their spectral profiles are narrow, and thus color rendering indices (CRIs) are also low. Therefore, the introduction of other guest molecules, such as metal complexes, fluorescent dyes and organic compounds, into MOF pores becomes significant and would avoid above drawbacks [29–35]. How ever, to date, compared with other host matrices (such as one-dimensional (1D) nanotubes, two-dimensional (2D) layered clays and three-dimensional (3D) zeolite materials), the encapsulation of guest molecules into 3D MOFs is still scarce. Hence, rational design of suitable host MOF materials and selective encapsulation of guest mol ecules are highly desired. Although the coordination polymers based on 2-position substituted imidazo [1,2-a]pyridine (IP) ligands have been reported, thus far, the complexes based on 3-position substituted IP ligands still are limited [36–40]. According to the potential of imidazo[1,2-a]pyridine de rivatives in the constructing coordination compounds, this work was to construct novel MOF by using 3-position substituted IP ligand. Herein, the first MOF ([ZnL]∙H2O∙DMA, named as USTC-1 ¼ University of Science and Technology of China) based on 3-position substituted IP ligand is presented. This new MOF material can encapsulate ionic dyes (Rhodamine B (RB), Methyl Orange (MO) and methylene blue (MB)) at room temperature to form MOF-based composites. These composites would overcome some weakness of MOFs themselves, and exhibit var iable and/or the enhanced luminescent emissions, even stimuli-responsive phosphorescent properties.
obtained. The existence of DMA and H2O in the channels of this MOF was demonstrated by elemental analysis. Anal. Calc.: C, 45.45; H, 3.60; N, 10.60%. Found: C, 46.71; H, 3.54; N, 11.11%. These solvent mole cules were further clarified by TGA analysis (Fig. 1a). Solvent mass % from TGA: 18.4 (corresponding to one H2O molecule (Calc.: 3.4%, Found: 2.8%) and one DMA molecule (Calc.: 16.5%, Found: 15.6%)). Thus, the formula of USTC-1 is [ZnL]∙H2O∙DMA (Yield: 19.0 mg, 0.036 mmol, 64.3% based on H2L). 2.3. In situ synthesis of dye@USTC-1 The same procedures used for synthesis of USTC-1 were used to synthesize dye@USTC-1, with the exception that 6.0 mg (0.014 mmol) of Rhodamine B (RB), 9.0 mg (0.027 mmol) of Methyl Orange (MO) or 9.0 mg (0.028 mmol) of Methylene Blue (MB) was included in each re action mixture. The crystals (RB@USTC-1, MO@USTC-1 or MB@USTC1) obtained from each mixture were washed extensively with fresh H2O and DMA in order to remove excess surface adsorbed dyes. TGA was also used to determine the solvent content. For example: solvent mass % from TGA: RB@USTC-1, 17.8 (Fig. 1c). 2.4. Dye encapsulation by activated USTC-1 After the as-synthesized USTC-1 was heated at 190 � C under vacuum for 10 h, the solvent molecules were removed to give the activated USTC-1, which was confirmed by TGA analysis (Fig. 1b). To a 80 mL aqueous solution of RB, MO or MB (5.0 mg/L, 0.40 mg), 10.0 mg of the activated USTC-1 sample was added, and stirred at 30 � C for a specific time interval, or until equilibrium was reached. At time t, or after adsorption equilibrium, the absorbance of the dyes in the supernatant was measured at their characteristic wavelengths and compared with those before the encapsulation. For RB, MO and MB, the absorbance peak is at 554, 464 and 664 nm, respectively. The encapsulation amount: qt (mg g 1) at time t (h), and qe (mg g 1) at the adsorption equilibrium were calculated as follows [41]:
2. Experimental 2.1. Materials and methods All of chemicals were AR reagents obtained from commercial sources and used without further purification. The H2L ligand (5-(2-chlor oimidazo[1,2-a]pyridine-3- carboxamido)isophthalic acid) was synthe sized according to previous procedure [38]. Microanalytical data (C, H, N) were collected on Vario ELIII elemental analyzer. FT-IR spectra were recorded using a Bruker EQUINOX55 FT-IR spectrophotometer. 1H solid-state magic angle spinning (MAS) NMR experiments have been acquired on a Bruker Avance 400 MHz spectrometer using the 4 mm zirconia rotors as sample holders spinning. UV/vis absorption spectra were measured using UV3100 spectrophotometer in aqueous solution at 298 K. The solution (10 μM H2O) and solid-state photoluminescence (PL) spectra, and the decay lifetime were determined at room temperature on a Fluorolog-3TAU fluorescence spectrophotometer. The solid-state quantum yield was measured also on a Fluorolog-3-TAU fluorescence spectrophotometer equipped with a BaSO4-coated integrating sphere. Powder X-ray diffraction (PXRD) patterns were collected on a Philips X’pert PRO SUPER diffractometer operating with nickel-filtered Cu-Kα radiation (λ ¼ 1.540598 Å) at 40 kV and 200 mA. Thermogravimetric analyses (TGA) were performed under nitrogen with a heating rate of 10 � C min 1 with a Shimadzu TGA-50H TG analyzer. Gas adsorption/desorption isotherms were recorded using an Omnisorp 100 CX instrument. The specific surface area value was calculated using the BET model. Both N2 and CO2 were of 99.999% purity.
qt ¼
qe ¼
ðC0
Ct ÞV m
(1)
ðC0
Ce ÞV m
(2)
where C0, Ct and Ce (mg/L) were the dye concentrations at the initial, any time t, and equilibrium in the solution, respectively. V (L) was the volume of the dye aqueous solution and m (g) was the mass of the activated USTC-1. C0 was the initial dye concentration. An adsorption
2.2. Solvothermal synthesis of MOF, [ZnL]∙H2O∙DMA (USTC-1) A mixture of Zn(NO3)2⋅6H2O (17.0 mg, 0.056 mmol), H2L ligand (20.0 mg, 0.056 mmol), H2O (1.0 mL), and DMA (7.0 mL) was placed into a 25 mL Teflon-lined stainless autoclave. The autoclave was sealed and heated at 100 � C for 24 h under autogenous pressure and static conditions. The resulting crystals were collected by filtration and washed with DMA and then dried in air. Colorless single crystals of USTC-1, suitable for X-ray single-crystal diffraction analysis was
Fig. 1. TGA curves of USTC-1(a), activated USTC-1(b), RB@USTC-1 (c) and RB@activated USTC-1(d). 2
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spectrum of the initial dye solution was recorded at wavelength 554, 464 and 664 nm, respectively. Ct was calculated by comparing the characteristic absorbance of dyes before and at time t after encapsula tion. Ce was the dye concentration in supernatant at equilibrium time determined by comparing the characteristic absorbance of the dyes before and after encapsulation. At last, RB@activated USTC-1 (pink), MO@activated USTC-1 (colorless) and MB@activated USTC-1 (blue) obtained from each experiment were filtered and washed extensively with fresh H2O in order to remove surface adsorbed dyes.
crystallographically independent Zn(II) ion. As shown in Fig. 2, the central Zn1(II) ion is five-coordinated by four carboxylate oxygen atoms (O2, O3a, O4d and O5c) and one imidazo[1,2-a]pyridine nitrogen atom (N1b) from five different L2 ligands, giving rise to a slightly distorted square-pyramidal coordination geometry. The Zn1–O bond distances are approximately equal [2.034(2), 2.040(2), 2.043(2) and 2.059(2) Å], and Zn1–N bond distance is 2.025(3) Å. The O2–Zn1–O3a and O3a–Zn1–O5c bond angles are 88.66(8) and 87.61(8)� (a: x, y, -zþ1/2; c ¼ x-1/2, y-1/ 2, -z-1/2), respectively (Table S1, in the Supporting Information). It is noticeable that interpenetration was not observed, ascribed to the long chain of the amide group [43]. However, the disordering amide chain only allowed better to locate the nitrogen atom from the electron density map. The remainder part of the amide chain is difficult to locate (Fig. S3, in the Supporting Information), resulting in the poor crystal data. Moreover, attempt to locate the guest molecules within the cavity was also unsuccessful, probably owing to their extensive disorder within the channels, exacerbated by the fact that the amide chain was disordered. Four carboxylate groups from four different L2 ligands connect two zinc ions (Zn⋯Zn distance of 3.089 Å) to generate a paddlewheel sec ondary building unit (SBU), in which the axial positions are occupied by two imidazo[1,2-a]pyridine nitrogen atoms from other two different L2 ligands (Fig. 3). Each paddlewheel SBU is six-connected to six L2 li gands, and each L2 is three-connected to three SBUs, therefore, from the topological point of view, each of the SBU acts as a six-connecting node and every L2 serves as a three-connecting node, thus the
2.5. Single-crystal X-ray crystallography The X-ray diffraction measurement was performed at 293(2) K on a Gemini S Ultra CCD diffractometer (Oxford diffraction Ltd.) using graphite monochromated Cu-Kα (λ ¼ 1.54184 Å). The structure was solved by direct method (SHELXL) and completed by difference Fourier method (SHELXL). Refinement was performed against F2 by weighted full-matrix least-squares (SHELXL), and empirical absorption correction (SCALE3 ABSPACK) was applied. All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were generated geometrically and refined by the riding mode. Weighted R factor (Rw) and all goodness of fit S are based on F2, conventional R factor (R) is based on F. The SQUEEZE option of PLATON [42] was used to calculate the solvents disordered area and remove their contribution to the overall intensity data. The final chemical formula of USTC-1 was obtained from crystal data combined with the results of elemental and thermogravimetric analyses. Selected bond lengths and bond angles are given in Table S1 (in the Supporting Information). 3. Results and discussion 3.1. Syntheses and crystal structure USTC-1 and dye@USTC-1 were solvothermally synthesized (Scheme 1). The complete deprotonation of the carboxylic groups of H2L ligand to give L2 in USTC-1 and dye@USTC-1 was clarified by IR spectral data, since no IR bands in the range of 1710–1680 cm 1 (carboxylic group characteric band of H2L ligand) were observed (Fig. S1, in the Sup porting Information). The MOF samples were identified by elemental analysis and TGA (see Experimental Section in details). The consistency of experimental and calculated PXRD patterns demonstrated the phase purity of USTC-1 (Fig. S2, in the Supporting Information). From Fig. S2 (in the Supporting Information), it also be found that USTC-1 and RB@USTC-1 are isostructural. The single crystal X-ray diffraction analysis indicates that USTC-1 crystallizes in the orthorhombic crystal system and Pbcn space group. The asymmetric unit consists of one L2 ligand and one
Fig. 2. ORTEP diagram showing coordination environment of Zn1 center in USTC-1 by thermal vibration ellipsoids with a 50% probability level. The H atoms are omitted for clarity. Symmetry code: a ¼ -x, y, -zþ1/2, b ¼ xþ1/2, -yþ1/2, -z, c ¼ x-1/2, y-1/2, -z-1/2, d ¼ -x-1/2, y-1/2, z.
Scheme 1. Syntheses of USTC-1 and dye@USTC-1. 3
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has a slightly low weight loss of 17.8% upon heating from room tem perature to 252 � C, which may be ascribed to the encapsulation of a few of RB molecules that results in slight decrease of guest H2O molecule (Fig. 1c). The nitrogen physisorption measurement was performed at 77 K in order to investigate the porosity of activated USTC-1. For this experi ment, the as-synthesized USTC-1 materials were vacuum dried at 190 � C for 10 h to remove the guest molecules. TGA shows that the solvents were removed completely for activated USTC-1 (Fig. 1b). The PXRD pattern of activated USTC-1 confirms the stability of the framework in the absence of guest molecules, characteristic of peak broadening at higher 2θ angles (Fig. S6b, in the Supporting Information), probably related to the flexible change of the amide group of ligand. The PXRD peaks could reversibly sharpen when the sample gets re-solvated in the mother liquor (H2O:DMA ¼ 1:7(v:v)) at room temperature for 24 h (Fig. S6c, in the Supporting Information), which further demonstrate the flexibility of the amide group. In principle, it was expected that activated USTC-1 should possess higher gas uptake owing to its larger channel size (~10.9 Å). Surprisingly, nitrogen physisorption measurement at 77 K reveals that the activated USTC-1 only has a low amount of N2 adsorp tion (total pore volume of 0.02 cm3 g 1, BET surface area of 4.6 m2 g 1, uptake value of 8.6 cm3 g 1 (1.09 wt %) at 77 K and P/P0 ¼ 1.0), and a broad adsorption/desorption hysteresis (Fig. 5). The CO2 adsorption of the activated USTC-1 at 273K was also studied, which reveals that the CO2 isotherm is linear [47] (Fig. 5), and a very low CO2 uptake value of 1.9 cm3 g 1 (0.38 wt %) at 273 K and P/P0 ¼ 1.0 was observed. Although the kinetic diameter of the N2 and CO2 gas is smaller than the effective pore window size (~1.1 nm), low gas uptake value should be attributed to the inherent surface blocking of activated USTC-1 after solvent removal. The reason should be the more flexible amide groups on the walls of the channels, which was proved by single-crystal X-ray data. The crystal structure showed that amide chain is disordered over two positions (Fig. S3, in the Supporting Information). Consequently, flex ible vibration of amide chains could close the pore window, resulting in low gas adsorption. A broad N2 adsorption/desorption hysteresis (Fig. 5) also demonstrates flexible channels in activated USTC-1, due to flexible amide groups on the walls of the channels. Therefore, flexible amide groups prevent the activated USTC-1 from taking up gas molecule, as confirmed by the gas adsorption experiments (Fig. 5).
Fig. 3. The paddlewheel SBU (six-connected node) in USTC-1.
framework can be described as a binodal (3,6)-connected three€fli) symbol for USTC-1 is dimensional (3D) network. The point (Schla (4∙62)242∙610∙83) calculated with TOPOS [44]. Such a novel neutral 3D framework with relatively high symmetry is rare [45]. There exist long channels along the b-axis, in which adjacent chan nels share the walls consisting of L2 ligands (Fig. S4, in the Supporting Information). The channels have a size of ~9.8 � 12.0 Å2 along the di agonals of the quasi-rectangle cross section. The pore length is ~10.9 Å if measured along the ligand channel walls, showing longer guest-free porous structure (Fig. 4). The channels are filled by disordered H2O and DMA solvent molecules in the as-prepared crystals of USTC-1. The solvent molecules are established to be one H2O and one DMA molecules per [ZnL] unit by elemental and thermogravimetric analyses (TGA). The accessible volume is 36.8% (1595.0 Å3) per unit cell (4330.3 Å3) esti mated by using PLATON [46]. 3.2. Thermogravimetric and gas adsorption analyses TGA reveals that USTC-1 is stable up to 380 � C. The weight loss of 18.4% from room temperature to 252 � C corresponds to the loss of one H2O and one DMA molecules per [ZnL] unit (calcd: 19.9%). There is no weight loss from 252 to 380 � C (Fig. 1a). Solid state 1H CP-MAS NMR spectrum of USTC-1 shows a signal around 1.7 ppm (Fig. S5, in the Supporting Information), which belongs to DMA remaining in channels of the MOF. However, the activated USTC-1 does not reveal this signal, meaning the removal of DMA. Comparison with USTC-1, RB@USTC-1
3.3. Encapsulation of dyes into MOF (USTC-1) There are two main strategies for the encapsulation of guests into MOFs: (i) soaking the MOF in a solution of the guest, and (ii) in situ encapsulation of the guest during the synthesis of MOF. For the in situ
Fig. 5. Adsorption isotherms of activated USTC-1 for N2 at 77 K and CO2 at 273 K.
Fig. 4. Space-filling representation of USTC-1, showing the guest-free channels. 4
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synthesis of dye@USTC-1 (dye ¼ RB, MO and MB), solvothermal re actions of the H2L ligand and Zn(NO3)2⋅6H2O were carried out together with a certain amount of RB, MO or MB. A color change from colorless USTC-1 crystal to pink RB@USTC-1 and blue MB@USTC-1 crystal was observed (Fig. S7, in the Supporting Information). However, no obvious color change (only pale pink) was observed for MO@USTC-1 crystal, meaning that only a tiny amount of MO is encapsulated. The encapsu lation of dyes does not affect the crystal structure of the MOF material, as confirmed by PXRD results (Fig. S2, in the Supporting Information). The loading amount of dyes was determined by soaking the activated USTC-1 into the aqueous solution of the dyes (see Experimental Section in details). By calculation using equation (2) (see Experimental Section), the equilibrium encapsulation amount (qe) for RB, MO and MB is 13.4 (1.34 mass %), 0.8 (0.08 mass %) and 26.6 (2.66 mass %) mg g 1, respectively (Fig. 6). The calculation details of the qt (see Experimental Section): Ct was obtained from absorbance values of UV–vis spectra of RB (Fig. S8, in the Supporting Information), MO (Fig. S9, in the Sup porting Information) and MB (Fig. S10, in the Supporting Information), respectively. The result also indicates very low loading amount of MO, consistent with in situ encapsulation method. Although TGA (Fig. 1d) and IR spectra (Fig. S1d, in the Supporting Information) show no sol vents (H2O) in the RB@activated USTC-1, attributable to its hydro phobic character after encapsulation of dyes, it is interestingly noted that the RB@activated USTC-1 exhibits the same PXRD pattern as that of the re-solvated sample (Fig. S11, in the Supporting Information), implying encapsulation of dyes could also reversibly recover the MOF structure, attributable to the flexible amide groups. The hydrophobic character after encapsulation of dyes is further evidenced from the decrease of the “O–H” stretching vibration of guest H2O molecule (Figs. S12b and S12d, in the Supporting Information). Conversely, very low loading amount of MO does not impart effectively hydrophobic character (Fig. S12c, in the Supporting Information). The above results indicate activated USTC-1 is more likely to selec tively adsorb the cationic dye RB and MB over the anionic dye MO. Although further studies are required to clarify this question, a possible explanation is the hydrogen-bonding interaction between the amide and dye [48]. The reports on the preferential adsorption of cationic dyes on neutral MOF frameworks are limited [49,50]. The bulky RB molecule (6.5 � 12.7 � 15.5 Å) can also be encapsulated by activated USTC-1, which should be ascribed to its flexible pore character. The pore size distribution indicates that the pore diameter of the activated USTC-1 is ca. 1.62 nm (Fig. S13, in the Supporting Information). In addition, compared to smaller size MB (4.2 � 5.0 � 13.4 Å), the loading amount and adsorption rate of cationic dye RB is low (Fig. 6), ascribed to its
larger molecule size. If the size of the dye molecules is matched well with the size of the channel of USTC-1 framework in two orientations, they will be captured more easily [47]. It is obvious that MB has more suit able size compared to RB.
Fig. 6. Encapsulation of RB (a), MO (b) and MB (c) into activated USTC-1 along with adsorption time.
Fig. 7. Solid-state PL spectra of H2L ligand, USTC-1 (a), RB@USTC-1(b), MO@USTC-1(c) and MB@USTC-1(d).
3.4. Photoluminescence (PL) spectra Solid-state photoluminescence (PL) spectra of H2L ligand, USTC-1 and dye@USTC-1 are presented in Fig. 7. The H2L and USTC-1 show emission peak around 433 nm, derived from π–π* electron transition of the ligand. MO@USTC-1 and MB@USTC-1 reveal similar blue emission band to USTC-1 (Fig. S14, in the Supporting Information), however, MB@USTC-1 exhibits weakened emission, attributable to very weak solid-state PL spectrum of MB itself (Fig. S15 (inset b), in the Supporting Information). Interestingly, RB@USTC-1 simultaneously displayed the characteristic emissions of both the RB dye and the MOF (USTC-1) after excitation at 365 nm in the solid state (Fig. 7b). As shown in Fig. S16 (in the Supporting Information), the RB aqueous solution (10 5 mol L 1) exhibited the characteristic emission at 602 nm when excitated at 365 nm, therefore, the red emission band around 602 nm for RB@USTC1 should originate from the RB dye. Such result indicates that the large RB dye molecules had been successfully encapsulated into the USTC-1 channels. No 677 nm emission peak in RB@USTC-1 also demonstrates that RB molecules are highly encapsulated into the inner pores, not the surface of the MOF (USTC-1) crystals, because the solid-state RB exhibits the characteristic emission at 677 nm when excitated at 365 nm (Fig. S15, in the Supporting Information), which should be attributed to the stacking-induced red-shifted emission (from 602 nm in solution to 677 nm in solid state) [51]. The RB@USTC-1 displays a pink light under 365 nm UV irradiation (Fig. S14b, in the Supporting Information) and even in daylight (It even can be excitated by 580 nm visible light (Fig. S17b, in the Supporting Information)). Compared with the pure USTC-1 with the emission position at ca. 433 nm, another new emission band at ca. 602 nm (Fig. 7) for the RB@USTC-1 originates from the emission of free isolated RB molecules. As a result, a dual-emitting RB@USTC-1 composite material with red/blue intensity ratio of 4.5 is achieved. It needs to be mentioned that the emission band of the RB in RB@USTC-1 is the same as that of RB in aqueous solution, but signifi cantly different from the RB in the solid state with very weak emission band at 677 nm, compared to USTC-1 (Fig. S18, in the Supporting In formation). These results demonstrate that the RB dye is uniformly encapsulated in the channels of USTC-1 as free isolated molecules. The microsecond lifetimes of USTC-1 (27.2 μs) and RB@USTC-1 (29.0 μs) in the solid state demonstrate their phosphorescent nature (Fig. S19A, in the Supporting Information). As a result, their phospho rescent quantum yield (Φph) is 4.0 (at 433 nm) and 6.6% (at 602 nm), respectively. Noticeably, although RB in aqueous solution is
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fluorescence with a ns level lifetime [52], the aggregation-induced phosphorescence [53,54] is observed for solid-state RB with lifetime of 19.2 μs (Fig. S19B, in the Supporting Information), regardless of its relative weak emission in the solid state (Fig. S18, in the Supporting Information). Moreover, phosphorescent character of RB in RB@USTC-1 (at 602 nm) might be due to MOF-to-RB energy-transfer [55]. Signifi cantly, RB@USTC-1 possesses enhanced emission band at 602 nm (Fig. 7), longer lifetime, higher Φph and even stronger solid-state exci tation spectrum (Fig. S17b, in the Supporting Information), compared to USTC-1. 3.5. Switchable color and phosphorescence via acid-base vapor stimuli The encapsulation (adsorption) of guest molecules into a MOF could help to expand the additional function of the MOFs. For example, the encapsulated dye molecules can be used as stimuli-responsive source, resulting in a color change of the MOF material in response to varying chemical environment [30]. When RB@USTC-1, MO@USTC-1 and MB@USTC-1 were exposed to HCl vapor in a closed container within 30 min, pink RB@USTC-1 and colorless MO@USTC-1 turned into pale-pink (Fig. 8a, middle) and weak pink (Fig. 8c, middle) respectively, whereas no color change was observed for MB@USTC-1 sample. Apart from solid-state color change, after HCl vapor stimulus, bright pink phosphorescence of RB@USTC-1, blue phosphorescence of MO@USTC-1 were transferred to dim-pink (Fig. 8b, middle) and dim-blue (Fig. 8d, middle), respectively. On the other hand, after sub sequent exposure of HCl vapor-disposed samples to NH3 vapor in a closed container within 30 min, their dim light can be back to bright pink and blue phosphorescence (Fig. 8b and d, right), respectively. The transformation between different phosphorescence colors is reversible in nature, that is, RB@USTC-1 and MO@USTC-1 turn into the corre sponding HCl vapor-disposed samples by using HCl vapor, and then they can be again back to RB@USTC-1 and MO@USTC-1 by using NH3 vapor (Fig. 9). As a result, the acid-base vapor can switch phosphorescent colors of the RB@USTC-1 and MO@USTC-1. From Fig. 9, it was found out that the protonation of RB@USTC-1 by HCl vapor led to obviously enhanced emission band at 433 nm, and weakened and blue-shifted emission band at 588 nm (from 602 to 588 nm). It should be noted that after protonation, RB@USTC-1 turned into blue/red two-color luminescence with intensity ratio of 1.3 (Fig. 9b), thus its pink phosphorescence was transferred to dim-pink (Fig. 8b, middle). However, NH3 vapor can recover its pink phospho rescence (Fig. 8b, right) with red/blue two-color intensity ratio of 2.2 (Fig. 9c). Moreover, the protonation of MO@USTC-1 by HCl vapor resulted in a new emission band at 578 nm (Fig. 9e), thus blue phos phorescence of MO@USTC-1 was changed into dim-blue (Fig. 8d, mid dle). Also, NH3 vapor can recover its blue phosphorescence (Figs. 9f and
Fig. 9. Solid-state PL spectra of RB@USTC-1 (a), RB@USTC-1 stimulated by HCl vapor (b) and then stimulated by NH3 vapor (c); solid-state PL spectra of MO@USTC-1 (d), MO@USTC-1 stimulated by HCl vapor (e) and then stimu lated by NH3 vapor (f).
8d, right). Such HCl vapor-induced emissive color transformation should be ascribed to the protonation effect of dyes in USTC-1 channels, because USTC-1 and solid-state dyes themselves do not reveal such switching behavior of acid-base vapor stimuli. According to molecular structure of dyes, RB and MO in USTC-1 channels were easily protonated by HCl vapor, whereas, MB is difficult (Fig. S20, in the Supporting Information). Such phosphorescent switching performance does actually result from the protonation-deprotonation switching of the free isolated dye mole cules in USTC-1 channels, which is further demonstrated by HCl-NH3 titration experiments. As shown in Fig. S21a (in the Supporting Infor mation), when titrated by HCl (pH: from 5.70 (10 5 mol L 1 RB aqueous solution) to 0.21), the emission intensity around 602 nm was gradually lessened, which is similar to weakened emission of the protonated RB in USTC-1 channels by HCl vapor (Fig. 9). Upon again titration with NH3 (pH: from 0.21 to 11.16), the emission intensity around 602 nm was gradually increased (Fig. S21b, in the Supporting Information), which is similar to enhanced emission of the deprotonated RB in USTC-1 chan nels by NH3 vapor (Fig. 9). It should be noted that upon protonation of RB@USTC-1 by HCl vapor, the blue-shifted emission band from 602 to 588 nm (Fig. 9) should be ascribed to the change of MOF-to-RB energytransfer process [55], owing to protonation of free isolated RB molecules in USTC-1 channels. This pH-sensitive behavior of a dye inside the MOF channels opens the potential application of the two-color ratiometric emission MOF (especial for RB@USTC-1) in sensors. The ratiometric luminescent sensor properties (such as for volatile organic compounds) of dye@USTC-1 and HCl vapor-disposed dye@USTC-1 samples will be performed in our laboratory. 4. Conclusion In conclusion, on the basis of a new T-shaped imidazo[1,2-a]pyridine dicarboxylate linker and zinc ion, a novel flexible MOF with interesting topology has been synthesized and characterized. It suggests that a combination of carboxylate and imidazo[1,2-a]pyridyl amide groups in a T-shaped ligand may construct flexible MOFs. Although the N2 and CO2 gas adsorption investigations exhibit low gas uptake, attributable to the flexible vibration of amide chains at linker that close the pore win dow, the closed pore window can be opened by dye molecules. Inter estingly, RB@USTC-1 simultaneously displayed the characteristic emissions of both the RB dye and the MOF, resulting in red/blue twocolor luminescence with intensity ratio of 4.5 and variable phospho rescence color. Moreover, RB@USTC-1 and MO@USTC-1 could undergo protonation upon exposure to HCl vapor, and reveal reversible
Fig. 8. Photographs of RB@USTC-1 taken under daylight (a) and 365 nm UV irradiation(b), and MO@USTC-1 taken under daylight (c) and 365 nm UV irradiation (d), showing reversible color and phosphorescence transformation stimulated by acid-base vapors. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 6
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phosphorescent color switching in the response to acid-base vapor stimuli. Consequently, the encapsulation of dyes into the MOF is an elegant way to create a multifunctional material for ratiometric lumi nescent sensor and stimuli-responsiveness. The present work provides a promising approach for synthesizing novel flexible MOFs and a new access to develop the phosphorescent and stimuli-responsive MOF ma terials via encapsulation of various guests.
[22] [23] [24] [25]
Declaration of competing interest
[26]
The authors declare no Conflict of Interest.
[27]
Acknowledgements
[28]
This work was supported by the Foundation of the USTC-Anhui To bacco Joint Laboratory of Chemistry and Combustion. We thank Dr. S. M. Zhou (HFNL, USTC) for the determination of the crystal structures.
[29]
[30]
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.dyepig.2019.108017.
[31] [32]
References
[33]
[1] Schubert EF, Kim JK. Solid-state light sources getting smart. Science 2005;308: 1274–8. [2] Veinot JGC, Marks TJ. Toward the ideal organic light-emitting diode. The versatility and utility of interfacial tailoring by cross-linked siloxane interlayers. Acc Chem Res 2005;38:632–43. [3] Binnemans K. Lanthanide-based luminescent hybrid materials. Chem Rev 2009; 109:4283–374. [4] Eliseeva SV, Bunzli JCG. Lanthanide luminescence for functional materials and biosciences. Chem Soc Rev 2010;39:189–227. [5] Grimsdale AC, Chan KL, Martin RE, Jokisz PG, Holmes AB. Synthesis of lightemitting conjugated polymers for applications in electroluminescent devices. Chem Rev 2009;109:897–1091. [6] Yang SK, Shi X, Park S, Ha T, Zimmerman SC. A dendritic single-molecule fluorescent probe that is monovalent, photostable and minimally blinking. Nat Chem 2013;5:692–7. [7] Stender AS, Marchuk K, Liu C, Sander S, Meyer MW, Smith EA. Single cell optical imaging and spectroscopy. Chem Rev 2013;113:2469–527. [8] Zhu M, Yang C. Blue fluorescent emitters: design tactics and applications in organic light-emitting diodes. Chem Soc Rev 2013;42:4963–76. [9] Yang Z, Cao J, He Y, Yang JH, Kim T, Peng X, Kim JS. Macro-/micro- environmentsensitive chemosensing and biological imaging. Chem Soc Rev 2014;43:4563–601. [10] Lee TW, Park OO, Yoon J, Kim JJ. Polymer-layered silicate nanocomposite lightemitting devices. Adv Mater 2001;13:211–3. [11] Agranovich VM, Gartstein YN, Litinskay M. Hybrid resonant organic inorganic nanostructures for optoelectronic applications. Chem Rev 2011;111:5179–214. [12] McCulloch I, Heeney M, Bailey C, Genevicius K, Macdonald I, Shkunov M, Sparrowe D, Tierney S, Wagner R, Zhang WM, Chabinyc ML, Kline RJ, Mcgehee MD, Toney MF. Liquid-crystalline semiconducting polymers with high charge- carrier mobility. Nat Mater 2006;5:328–33. [13] Long JR, Yaghi OM. The pervasive chemistry of metal-organic frameworks. Chem Soc Rev 2009;38:1213–4. [14] Murray LJ, Dinc� a M, Long JR. Hydrogen storage in metal-organic frameworks. Chem Soc Rev 2009;38:1294–314. [15] Wang XL, Qin C, Wu SX, Shao KZ, Lan YQ, Wang S, Zhu DX, Su ZM, Wang EB. Bottom-up synthesis of porous coordination frameworks: apical substitution of a pentanuclear tetrahedral precursor. Angew Chem Int Ed 2009;48:5291–5. [16] Pan M, Lin XM, Li GB, Su CY. Progress in the study of metal-organic materials applying naphthalene diimide (NDI) ligands. Coord Chem Rev 2011;255:1921–36. [17] Seo JS, Whang D, Lee H, Jun SI, Oh J, Jeon YJ, Kim K. A homochiral metal-organic porous material for enantioselective separation and catalysis. Nature 2000;404: 982–6. [18] Takashima Y, Martinez VM, Furukawa S, Kondo M, Shimomura S, Uehara H, Nakahama M, Sugimoto K, Kitagawa S. Molecular decoding using luminescence from an entangled porous framework. Nat Commun 2011;2:168. [19] Sun CY, Qin C, Wang CG, Su ZM, Wang S, Wang XL, Yang GS, Shao KZ, Lan YQ, Wang EB. Chiral nanoporous metal-organic frameworks with high porosity as materials for drug delivery. Adv Mater 2011;23:5629–32. [20] Huang Y, Lin Z, Cao R. Palladium nanoparticles encapsulated in a metal-organic framework as efficient heterogeneous catalysts for direct C2 arylation of indoles. Chem Eur J 2011;17:12706–12. [21] Lu G, Li SZ, Guo Z, Farha OK, Hauser BG, Wang Y, Wang X, Han SY, Liu XG, DuChene JS, Zhang H, Zhang QC, Chen XD, Ma J, Loo SCJ, Wei WD, Yang YH,
[34] [35] [36] [37] [38] [39] [40] [41] [42] [43]
[44] [45] [46] [47] [48] [49]
7
Hupp JT, Huo FW. Imparting functionality to a metal-organic framework material by controlled nanoparticle encapsulation. Nat Chem 2012;4:310–6. Ferey G. Hybrid porous solids: past, present, future. Chem Soc Rev 2008;37: 191–214. Allendorf MD, Bauer CA, Bhakta RK, Houk RJT. Luminescent metal-organic frameworks. Chem Soc Rev 2009;38:1330–52. Cui Y, Yue Y, Qian G, Chen B. Luminescent functional metal-organic frameworks. Chem Rev 2012;112:1126–62. Qi Y, Xu H, Li X, Tu B, Pang Q, Lin X, Ning E, Li Q. Structure transformation of a luminescent pillared-layer metal-organic framework caused by point defects accumulation. Chem Mater 2018;30:5478–84. Gai Y, Guo Q, Xiong K, Jiang F, Li C, Li X, Chen Y, Zhu C, Huang Q, Yao R, Hong MC. Mixed-lanthanide metal organic frameworks with tunable color and white light emission. Cryst Growth Des 2017;17:940–4. An J, Shade CM, Chengelis-Czegan DA, Petoud S, Rosi NL. Zinc-adeninate metalorganic framework for aqueous encapsulation and sensitization of near-infrared and visible emitting lanthanide cations. J Am Chem Soc 2011;133:1220–3. He HM, Sun FX, Borjigin T, Zhao NA, Zhu GS. Tunable colors and white-light emission based on a microporous luminescent Zn(II)-MOF. Dalton Trans 2014;43: 3716–21. Yanai N, Kitayama K, Hijikata Y, Sato H, Matsuda R, Kubota Y, Takata M, Mizuno M, Uemura T, Kitagawa S. Gas detection by structural variations of fluorescent guest molecules in a flexible porous coordination polymer. Nat Mater 2011;10:787–93. Grünker R, Bon V, Heerwig A, Klein N, Mueller P, Stoeck U, Baburin IA, Mueller U, Senkovska I, Kaskel S. Dye encapsulation inside a new mesoporous metal-organic framework for multifunctional solvatochromic-response function. Chem Eur J 2012;18:13299–303. Sun CY, Wang XL, Zhang X, Qin C, Li P, Su ZM, Zhu DX, Shan GG, Shao KZ, Wu H, Li J. Efficient and tunable white-light emission of metal-organic frameworks by iridium-complex encapsulation. Nat Commun 2013;4:2717. Yan DP, Tang YQ, Lin HY, Wang D. Tunable two-color luminescence and host-guest energy transfer of fluorescent chromophores encapsulated in metal-organic frameworks. Sci Rep 2014;4:4337. Yue Y, Binder AJ, Song R, Cui Y, Chen J, Hensley DK, Dai S. Encapsulation of large dye molecules in hierarchically superstructured metal-organic frameworks. Dalton Trans 2014;43:17893–8. Wen Y, Sheng T, Zhu X, Zhuo C, Su S, Li H, Hu S, Zhu QL, Wu X. Introduction of red-green-blue fluorescent dyes into a metal-organic framework for tunable white light emission. Adv Mater 2017;29:1700778. Dai Y, Zhang JJ, Liu SQ, Zhou H, Sun YJ, Pan YZ, Ni J, Yang JS. A trichromatic and white-light-emitting MOF composite for multi-dimensional and multi-response ratiometric luminescent sensing. Chem Eur J 2018;24:9555–64. Yong GP, Qiao S, Xie Y, Wang ZY. Cadmium(II) and zinc(II) coordination polymers with 1D ladder and 2D basket weave layer structures constructed from a new Tshaped ligand. Eur J Inorg Chem 2006:4483–8. Li B, Ni LS, Yong GP. Three new coordination compounds based on a new 3-po sition substituted imidazo[1,2-a]pyridine ligand: syntheses, crystal structures and photoluminescent properties. Polyhedron 2018;154:21–6. Jiang WP, Yong GP. One-dimensional coordination polymers based on a new 3position substituted imidazo[1,2-a]pyridine ligand: crystal structures, photoluminescent and magnetic properties. Polyhedron 2019;157:428–33. Liu SD, Kuo BC, Liu YW, Lee JY, Wong KY, Lee HM. Tuning of net architectures of Ni(II) and Zn(II) coordination polymers using isomeric organic linkers. CrystEngComm 2014;16:8874–84. Fu RB, Hu SM, Wu XT. Luminescent zinc phosphonates for ratiometric sensing of 2,4,6-trinitrophenol and temperature. Cryst Growth Des 2016;16:5074–83. Li HC, Cao XY, Zhang C, Yu Q, Zhao ZJ, Niu XD, Sun XD, Liu YL, Ma L, Li ZQ. Enhanced adsorptive removal of anionic and cationic dyes from single or mixed dye solutions using MOF PCN-222. RSC Adv 2017;7:16273–81. van der Sluis P, Spek AL. An effective method for the refinement of crystal structures containing disordered solvent regions. Acta Crystallogr Sect A Found Crystallogr 1990;46:194–201. Keceli E, Hemgesberg M, Grünker R, Bon V, Wilhelm C, Philippi T, Schoch R, Sun Y, Bauer M, Ernst S, Kaskel S, Thiel WR. A series of amide functionalized isoreticular metal organic frameworks. Microporous Mesoporous Mater 2014;194: 115–25. Blatov VA, Peaskov MV. A comparative crystallochemical analysis of binary compounds and simple anhydrous salts containing pyramidal anions LO3 (L ¼ S, Se, Te, Cl, Br, I). Acta Crystallogr B 2006;62:457–66. Xiang SL, Huang J, Li L, Zhang JY, Jiang L, Kuang XJ, Su CY. Nanotubular metalorganic frameworks with high porosity based on T-shaped pyridyl dicarboxylate ligands. Inorg Chem 2011;50:1743–8. Spek AL. Single-crystal structure validation with the program PLATON. J Appl Crystallogr 2003;36:7–13. Zhao Y, Wang L, Fan NN, Han ML, Yang GP, Ma LF. Porous Zn(II)-based metalorganic frameworks decorated with carboxylate groups exhibiting high gas adsorption and separation of organic dyes. Cryst Growth Des 2018;18:7114–21. Ji WJ, Hao RQ, Pei WW, Feng L, Zhai QG. Design of two isoreticular Cdbiphenyltetra-carboxylate frameworks for dye adsorption, separation and photocatalytic degradation. Dalton Trans 2018;47:700–7. Wang HN, Liu FH, Wang XL, Shao KZ, Su ZM. Three neutral metal-organic frameworks with micro- and meso-pores for adsorption and separation of dyes. J Mater Chem 2013;1:13060–3.
B. Li et al.
Dyes and Pigments xxx (xxxx) xxx
[50] Qin L, Zeng SY, Zuo WJ, Liu QH, Li J, Ni G, Wang YQ, Zhang MD. One neutral metal-organic framework with an unusual dmp topology for adsorption of dyes. Polyhedron 2017;121:231–5. [51] Yong GP, She WL, Zhang YM. Room-temperature phosphorescence in solution and in solid state from purely organic dyes. Dyes Pigments 2012;95:161–7. [52] Smith SN, Steer RP. The photophysics of Lissamine rhodamine-B sulphonyl chloride in aqueous solution: implications for fluorescent protein-dye conjugates. J Photochem Photobiol A Chem 2001;139:151–6.
[53] Yuan XWZ, Shen Y, Zhao H, Lam JWY, Tang L, Lu P, Wang CL, Liu Y, Wang ZM, Zheng Q, Sun JZ, Ma YG, Tang BZ. Crystallization-induced phosphorescence of pure organic luminogens at room temperature. J Phys Chem C 2010;114:6090–9. [54] Bolton O, Lee K, Kim HJ, Lin KY, Kim J. Activating efficient phosphorescence from purely organic materials by crystal design. Nat Chem 2011;3:205–10. [55] Chen DM, Zhang NN, Liu CS, Du M. Dual-emitting dye@MOF composite as a selfcalibrating sensor for 2,4,6-trinitrophenol. ACS Appl Mater Interfaces 2017;9: 24671–7.
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Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: http://www.elsevier.com/locate/dyepig
Dyes encapsulated in a novel flexible metal organic framework show tunable and stimuli-responsive phosphorescence Bin Li a, Weipeng Jiang a, Yingbo Xu b, Zhiqiang Xu b, **, Qingqing Yan a, Guoping Yong a, * a b
Department of Chemistry, University of Science and Technology of China, Hefei, 230026, PR China The USTC-Anhui Tobacco Joint Laboratory of Chemistry and Combustion, Hefei, 230066, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords: 5-(2-chloroimidazo[1,2-a]pyridine-3carboxamido)isophthalic acid Metal-organic framework Dyes Encapsulation Phosphorescence Stimuli-responsive materials
A novel flexible metal-organic framework (MOF), [ZnL]∙H2O∙DMA (USTC-1), with interesting topology has been constructed based on a new T-shaped ligand, 5-(2-chloroimidazo[1,2-a]pyridine-3-carboxamido)iso phthalic acid (H2L). This MOF was characterized by IR spectroscopy, thermogravimetry, single-crystal, and powder X-ray diffraction methods. The single-crystal analysis reveals four carboxylate groups connect two zinc ions to generate a paddlewheel secondary building unit (SBU). Each paddlewheel SBU is six-connected to six L2 ligands, and each L2 is three-connected to three SBUs, thus, the framework can be described as a binodal (3,6)connected three-dimensional (3D) flexible network. The low N2 and CO2 gas uptake should be attributed to the flexible vibration of amide chains at ligands that close the pore window, however, the closed pore window can be opened by dye molecules, such as Rhodamine B (RB), Methyl Orange (MO) or Methylene Blue (MB). Two methods for dye encapsulation were investigated: soaking and in situ encapsulation. PXRD data showed that the framework in USTC-1 was unchanged after encapsulation of dyes. Interestingly, RB@USTC-1 simultaneously displayed the characteristic emissions of both the RB dye and the MOF, resulting in red/blue two-color lumi nescence with intensity ratio of 4.5 and tunable phosphorescence color. Moreover, RB@USTC-1 and MO@USTC1 could undergo protonation upon exposure to HCl vapor, and reveal reversible phosphorescent color switching in the response to acid-base vapor stimuli. The present work provides a promising approach for synthesizing novel flexible MOFs and a new access to develop the phosphorescent and stimuli-responsive MOF materials via encapsulation of various guests.
1. Introduction The luminescent materials of solid state have become an extensively studied area owing to their potential applications in lighting, display, sensing and optical devices [1–4]. In traditional, rare-earth ions are topics in the light emitting materials. However, recently, organic lumi nescent materials have a growing effect on the emissive materials [5–9], because of their many advantages such as low cost, diverse colors and tunability which can be used in fields of fluorescent sensors, probes, imaging agents, and organic light-emitting diodes. Moreover, the encapsulation of luminescent guest molecules into host matrices has attracted much attention on the basis of both fundamental studies and the applications [10,11]. According to the host–guest interactions and collective effects, the resulting composite materials can reveal unique functionalities (such as emission diversity and tunable luminescence)
which are not simply the sum of the individual host matrices or guests [12]. Metal–organic frameworks (MOFs), also known as porous coordi nation polymers (PCPs), are a kind of crystalline porous materials con structed from metal ions/clusters and organic linkers [13–16]. According to tunable structures, cavities and chemical characters, MOFs are promising candidates for gas storage/separation, catalysis, drug delivery and optical sensing/detection, etc [17–21]. Interestingly, MOFs have also important potential as multifunctional luminescent materials, in which light emitting can come from one or more of the following strategies: organic linkers, metal ions/clusters, encapsulation of emis sive guest molecules, defects or charge transfer processes [22–25]. Nevertheless, there are only a few of MOFs that exhibit tunable emission properties by encapsulation of guests. Gai et al. found out that the emission characteristics of MOFs can be adjusted by using various ratios
* Corresponding author., The USTC-Anhui Tobacco Joint Laboratory of Chemistry and Combustion, Hefei, 230066, PR China. ** Corresponding author. Department of Chemistry, University of Science and Technology of China, Hefei, 230026, PR China. E-mail addresses: [email protected] (Z. Xu), [email protected] (G. Yong). https://doi.org/10.1016/j.dyepig.2019.108017 Received 8 August 2019; Received in revised form 2 November 2019; Accepted 2 November 2019 Available online 9 November 2019 0143-7208/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Bin Li, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2019.108017
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of lanthanide metal ions in MOFs [26]. Other groups have also used trivalent lanthanide ions as guests to tune different emission properties [27,28]. The flaw of using lanthanide ions as guests is that their spectral profiles are narrow, and thus color rendering indices (CRIs) are also low. Therefore, the introduction of other guest molecules, such as metal complexes, fluorescent dyes and organic compounds, into MOF pores becomes significant and would avoid above drawbacks [29–35]. How ever, to date, compared with other host matrices (such as one-dimensional (1D) nanotubes, two-dimensional (2D) layered clays and three-dimensional (3D) zeolite materials), the encapsulation of guest molecules into 3D MOFs is still scarce. Hence, rational design of suitable host MOF materials and selective encapsulation of guest mol ecules are highly desired. Although the coordination polymers based on 2-position substituted imidazo [1,2-a]pyridine (IP) ligands have been reported, thus far, the complexes based on 3-position substituted IP ligands still are limited [36–40]. According to the potential of imidazo[1,2-a]pyridine de rivatives in the constructing coordination compounds, this work was to construct novel MOF by using 3-position substituted IP ligand. Herein, the first MOF ([ZnL]∙H2O∙DMA, named as USTC-1 ¼ University of Science and Technology of China) based on 3-position substituted IP ligand is presented. This new MOF material can encapsulate ionic dyes (Rhodamine B (RB), Methyl Orange (MO) and methylene blue (MB)) at room temperature to form MOF-based composites. These composites would overcome some weakness of MOFs themselves, and exhibit var iable and/or the enhanced luminescent emissions, even stimuli-responsive phosphorescent properties.
obtained. The existence of DMA and H2O in the channels of this MOF was demonstrated by elemental analysis. Anal. Calc.: C, 45.45; H, 3.60; N, 10.60%. Found: C, 46.71; H, 3.54; N, 11.11%. These solvent mole cules were further clarified by TGA analysis (Fig. 1a). Solvent mass % from TGA: 18.4 (corresponding to one H2O molecule (Calc.: 3.4%, Found: 2.8%) and one DMA molecule (Calc.: 16.5%, Found: 15.6%)). Thus, the formula of USTC-1 is [ZnL]∙H2O∙DMA (Yield: 19.0 mg, 0.036 mmol, 64.3% based on H2L). 2.3. In situ synthesis of dye@USTC-1 The same procedures used for synthesis of USTC-1 were used to synthesize dye@USTC-1, with the exception that 6.0 mg (0.014 mmol) of Rhodamine B (RB), 9.0 mg (0.027 mmol) of Methyl Orange (MO) or 9.0 mg (0.028 mmol) of Methylene Blue (MB) was included in each re action mixture. The crystals (RB@USTC-1, MO@USTC-1 or MB@USTC1) obtained from each mixture were washed extensively with fresh H2O and DMA in order to remove excess surface adsorbed dyes. TGA was also used to determine the solvent content. For example: solvent mass % from TGA: RB@USTC-1, 17.8 (Fig. 1c). 2.4. Dye encapsulation by activated USTC-1 After the as-synthesized USTC-1 was heated at 190 � C under vacuum for 10 h, the solvent molecules were removed to give the activated USTC-1, which was confirmed by TGA analysis (Fig. 1b). To a 80 mL aqueous solution of RB, MO or MB (5.0 mg/L, 0.40 mg), 10.0 mg of the activated USTC-1 sample was added, and stirred at 30 � C for a specific time interval, or until equilibrium was reached. At time t, or after adsorption equilibrium, the absorbance of the dyes in the supernatant was measured at their characteristic wavelengths and compared with those before the encapsulation. For RB, MO and MB, the absorbance peak is at 554, 464 and 664 nm, respectively. The encapsulation amount: qt (mg g 1) at time t (h), and qe (mg g 1) at the adsorption equilibrium were calculated as follows [41]:
2. Experimental 2.1. Materials and methods All of chemicals were AR reagents obtained from commercial sources and used without further purification. The H2L ligand (5-(2-chlor oimidazo[1,2-a]pyridine-3- carboxamido)isophthalic acid) was synthe sized according to previous procedure [38]. Microanalytical data (C, H, N) were collected on Vario ELIII elemental analyzer. FT-IR spectra were recorded using a Bruker EQUINOX55 FT-IR spectrophotometer. 1H solid-state magic angle spinning (MAS) NMR experiments have been acquired on a Bruker Avance 400 MHz spectrometer using the 4 mm zirconia rotors as sample holders spinning. UV/vis absorption spectra were measured using UV3100 spectrophotometer in aqueous solution at 298 K. The solution (10 μM H2O) and solid-state photoluminescence (PL) spectra, and the decay lifetime were determined at room temperature on a Fluorolog-3TAU fluorescence spectrophotometer. The solid-state quantum yield was measured also on a Fluorolog-3-TAU fluorescence spectrophotometer equipped with a BaSO4-coated integrating sphere. Powder X-ray diffraction (PXRD) patterns were collected on a Philips X’pert PRO SUPER diffractometer operating with nickel-filtered Cu-Kα radiation (λ ¼ 1.540598 Å) at 40 kV and 200 mA. Thermogravimetric analyses (TGA) were performed under nitrogen with a heating rate of 10 � C min 1 with a Shimadzu TGA-50H TG analyzer. Gas adsorption/desorption isotherms were recorded using an Omnisorp 100 CX instrument. The specific surface area value was calculated using the BET model. Both N2 and CO2 were of 99.999% purity.
qt ¼
qe ¼
ðC0
Ct ÞV m
(1)
ðC0
Ce ÞV m
(2)
where C0, Ct and Ce (mg/L) were the dye concentrations at the initial, any time t, and equilibrium in the solution, respectively. V (L) was the volume of the dye aqueous solution and m (g) was the mass of the activated USTC-1. C0 was the initial dye concentration. An adsorption
2.2. Solvothermal synthesis of MOF, [ZnL]∙H2O∙DMA (USTC-1) A mixture of Zn(NO3)2⋅6H2O (17.0 mg, 0.056 mmol), H2L ligand (20.0 mg, 0.056 mmol), H2O (1.0 mL), and DMA (7.0 mL) was placed into a 25 mL Teflon-lined stainless autoclave. The autoclave was sealed and heated at 100 � C for 24 h under autogenous pressure and static conditions. The resulting crystals were collected by filtration and washed with DMA and then dried in air. Colorless single crystals of USTC-1, suitable for X-ray single-crystal diffraction analysis was
Fig. 1. TGA curves of USTC-1(a), activated USTC-1(b), RB@USTC-1 (c) and RB@activated USTC-1(d). 2
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spectrum of the initial dye solution was recorded at wavelength 554, 464 and 664 nm, respectively. Ct was calculated by comparing the characteristic absorbance of dyes before and at time t after encapsula tion. Ce was the dye concentration in supernatant at equilibrium time determined by comparing the characteristic absorbance of the dyes before and after encapsulation. At last, RB@activated USTC-1 (pink), MO@activated USTC-1 (colorless) and MB@activated USTC-1 (blue) obtained from each experiment were filtered and washed extensively with fresh H2O in order to remove surface adsorbed dyes.
crystallographically independent Zn(II) ion. As shown in Fig. 2, the central Zn1(II) ion is five-coordinated by four carboxylate oxygen atoms (O2, O3a, O4d and O5c) and one imidazo[1,2-a]pyridine nitrogen atom (N1b) from five different L2 ligands, giving rise to a slightly distorted square-pyramidal coordination geometry. The Zn1–O bond distances are approximately equal [2.034(2), 2.040(2), 2.043(2) and 2.059(2) Å], and Zn1–N bond distance is 2.025(3) Å. The O2–Zn1–O3a and O3a–Zn1–O5c bond angles are 88.66(8) and 87.61(8)� (a: x, y, -zþ1/2; c ¼ x-1/2, y-1/ 2, -z-1/2), respectively (Table S1, in the Supporting Information). It is noticeable that interpenetration was not observed, ascribed to the long chain of the amide group [43]. However, the disordering amide chain only allowed better to locate the nitrogen atom from the electron density map. The remainder part of the amide chain is difficult to locate (Fig. S3, in the Supporting Information), resulting in the poor crystal data. Moreover, attempt to locate the guest molecules within the cavity was also unsuccessful, probably owing to their extensive disorder within the channels, exacerbated by the fact that the amide chain was disordered. Four carboxylate groups from four different L2 ligands connect two zinc ions (Zn⋯Zn distance of 3.089 Å) to generate a paddlewheel sec ondary building unit (SBU), in which the axial positions are occupied by two imidazo[1,2-a]pyridine nitrogen atoms from other two different L2 ligands (Fig. 3). Each paddlewheel SBU is six-connected to six L2 li gands, and each L2 is three-connected to three SBUs, therefore, from the topological point of view, each of the SBU acts as a six-connecting node and every L2 serves as a three-connecting node, thus the
2.5. Single-crystal X-ray crystallography The X-ray diffraction measurement was performed at 293(2) K on a Gemini S Ultra CCD diffractometer (Oxford diffraction Ltd.) using graphite monochromated Cu-Kα (λ ¼ 1.54184 Å). The structure was solved by direct method (SHELXL) and completed by difference Fourier method (SHELXL). Refinement was performed against F2 by weighted full-matrix least-squares (SHELXL), and empirical absorption correction (SCALE3 ABSPACK) was applied. All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were generated geometrically and refined by the riding mode. Weighted R factor (Rw) and all goodness of fit S are based on F2, conventional R factor (R) is based on F. The SQUEEZE option of PLATON [42] was used to calculate the solvents disordered area and remove their contribution to the overall intensity data. The final chemical formula of USTC-1 was obtained from crystal data combined with the results of elemental and thermogravimetric analyses. Selected bond lengths and bond angles are given in Table S1 (in the Supporting Information). 3. Results and discussion 3.1. Syntheses and crystal structure USTC-1 and dye@USTC-1 were solvothermally synthesized (Scheme 1). The complete deprotonation of the carboxylic groups of H2L ligand to give L2 in USTC-1 and dye@USTC-1 was clarified by IR spectral data, since no IR bands in the range of 1710–1680 cm 1 (carboxylic group characteric band of H2L ligand) were observed (Fig. S1, in the Sup porting Information). The MOF samples were identified by elemental analysis and TGA (see Experimental Section in details). The consistency of experimental and calculated PXRD patterns demonstrated the phase purity of USTC-1 (Fig. S2, in the Supporting Information). From Fig. S2 (in the Supporting Information), it also be found that USTC-1 and RB@USTC-1 are isostructural. The single crystal X-ray diffraction analysis indicates that USTC-1 crystallizes in the orthorhombic crystal system and Pbcn space group. The asymmetric unit consists of one L2 ligand and one
Fig. 2. ORTEP diagram showing coordination environment of Zn1 center in USTC-1 by thermal vibration ellipsoids with a 50% probability level. The H atoms are omitted for clarity. Symmetry code: a ¼ -x, y, -zþ1/2, b ¼ xþ1/2, -yþ1/2, -z, c ¼ x-1/2, y-1/2, -z-1/2, d ¼ -x-1/2, y-1/2, z.
Scheme 1. Syntheses of USTC-1 and dye@USTC-1. 3
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has a slightly low weight loss of 17.8% upon heating from room tem perature to 252 � C, which may be ascribed to the encapsulation of a few of RB molecules that results in slight decrease of guest H2O molecule (Fig. 1c). The nitrogen physisorption measurement was performed at 77 K in order to investigate the porosity of activated USTC-1. For this experi ment, the as-synthesized USTC-1 materials were vacuum dried at 190 � C for 10 h to remove the guest molecules. TGA shows that the solvents were removed completely for activated USTC-1 (Fig. 1b). The PXRD pattern of activated USTC-1 confirms the stability of the framework in the absence of guest molecules, characteristic of peak broadening at higher 2θ angles (Fig. S6b, in the Supporting Information), probably related to the flexible change of the amide group of ligand. The PXRD peaks could reversibly sharpen when the sample gets re-solvated in the mother liquor (H2O:DMA ¼ 1:7(v:v)) at room temperature for 24 h (Fig. S6c, in the Supporting Information), which further demonstrate the flexibility of the amide group. In principle, it was expected that activated USTC-1 should possess higher gas uptake owing to its larger channel size (~10.9 Å). Surprisingly, nitrogen physisorption measurement at 77 K reveals that the activated USTC-1 only has a low amount of N2 adsorp tion (total pore volume of 0.02 cm3 g 1, BET surface area of 4.6 m2 g 1, uptake value of 8.6 cm3 g 1 (1.09 wt %) at 77 K and P/P0 ¼ 1.0), and a broad adsorption/desorption hysteresis (Fig. 5). The CO2 adsorption of the activated USTC-1 at 273K was also studied, which reveals that the CO2 isotherm is linear [47] (Fig. 5), and a very low CO2 uptake value of 1.9 cm3 g 1 (0.38 wt %) at 273 K and P/P0 ¼ 1.0 was observed. Although the kinetic diameter of the N2 and CO2 gas is smaller than the effective pore window size (~1.1 nm), low gas uptake value should be attributed to the inherent surface blocking of activated USTC-1 after solvent removal. The reason should be the more flexible amide groups on the walls of the channels, which was proved by single-crystal X-ray data. The crystal structure showed that amide chain is disordered over two positions (Fig. S3, in the Supporting Information). Consequently, flex ible vibration of amide chains could close the pore window, resulting in low gas adsorption. A broad N2 adsorption/desorption hysteresis (Fig. 5) also demonstrates flexible channels in activated USTC-1, due to flexible amide groups on the walls of the channels. Therefore, flexible amide groups prevent the activated USTC-1 from taking up gas molecule, as confirmed by the gas adsorption experiments (Fig. 5).
Fig. 3. The paddlewheel SBU (six-connected node) in USTC-1.
framework can be described as a binodal (3,6)-connected three€fli) symbol for USTC-1 is dimensional (3D) network. The point (Schla (4∙62)242∙610∙83) calculated with TOPOS [44]. Such a novel neutral 3D framework with relatively high symmetry is rare [45]. There exist long channels along the b-axis, in which adjacent chan nels share the walls consisting of L2 ligands (Fig. S4, in the Supporting Information). The channels have a size of ~9.8 � 12.0 Å2 along the di agonals of the quasi-rectangle cross section. The pore length is ~10.9 Å if measured along the ligand channel walls, showing longer guest-free porous structure (Fig. 4). The channels are filled by disordered H2O and DMA solvent molecules in the as-prepared crystals of USTC-1. The solvent molecules are established to be one H2O and one DMA molecules per [ZnL] unit by elemental and thermogravimetric analyses (TGA). The accessible volume is 36.8% (1595.0 Å3) per unit cell (4330.3 Å3) esti mated by using PLATON [46]. 3.2. Thermogravimetric and gas adsorption analyses TGA reveals that USTC-1 is stable up to 380 � C. The weight loss of 18.4% from room temperature to 252 � C corresponds to the loss of one H2O and one DMA molecules per [ZnL] unit (calcd: 19.9%). There is no weight loss from 252 to 380 � C (Fig. 1a). Solid state 1H CP-MAS NMR spectrum of USTC-1 shows a signal around 1.7 ppm (Fig. S5, in the Supporting Information), which belongs to DMA remaining in channels of the MOF. However, the activated USTC-1 does not reveal this signal, meaning the removal of DMA. Comparison with USTC-1, RB@USTC-1
3.3. Encapsulation of dyes into MOF (USTC-1) There are two main strategies for the encapsulation of guests into MOFs: (i) soaking the MOF in a solution of the guest, and (ii) in situ encapsulation of the guest during the synthesis of MOF. For the in situ
Fig. 5. Adsorption isotherms of activated USTC-1 for N2 at 77 K and CO2 at 273 K.
Fig. 4. Space-filling representation of USTC-1, showing the guest-free channels. 4
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synthesis of dye@USTC-1 (dye ¼ RB, MO and MB), solvothermal re actions of the H2L ligand and Zn(NO3)2⋅6H2O were carried out together with a certain amount of RB, MO or MB. A color change from colorless USTC-1 crystal to pink RB@USTC-1 and blue MB@USTC-1 crystal was observed (Fig. S7, in the Supporting Information). However, no obvious color change (only pale pink) was observed for MO@USTC-1 crystal, meaning that only a tiny amount of MO is encapsulated. The encapsu lation of dyes does not affect the crystal structure of the MOF material, as confirmed by PXRD results (Fig. S2, in the Supporting Information). The loading amount of dyes was determined by soaking the activated USTC-1 into the aqueous solution of the dyes (see Experimental Section in details). By calculation using equation (2) (see Experimental Section), the equilibrium encapsulation amount (qe) for RB, MO and MB is 13.4 (1.34 mass %), 0.8 (0.08 mass %) and 26.6 (2.66 mass %) mg g 1, respectively (Fig. 6). The calculation details of the qt (see Experimental Section): Ct was obtained from absorbance values of UV–vis spectra of RB (Fig. S8, in the Supporting Information), MO (Fig. S9, in the Sup porting Information) and MB (Fig. S10, in the Supporting Information), respectively. The result also indicates very low loading amount of MO, consistent with in situ encapsulation method. Although TGA (Fig. 1d) and IR spectra (Fig. S1d, in the Supporting Information) show no sol vents (H2O) in the RB@activated USTC-1, attributable to its hydro phobic character after encapsulation of dyes, it is interestingly noted that the RB@activated USTC-1 exhibits the same PXRD pattern as that of the re-solvated sample (Fig. S11, in the Supporting Information), implying encapsulation of dyes could also reversibly recover the MOF structure, attributable to the flexible amide groups. The hydrophobic character after encapsulation of dyes is further evidenced from the decrease of the “O–H” stretching vibration of guest H2O molecule (Figs. S12b and S12d, in the Supporting Information). Conversely, very low loading amount of MO does not impart effectively hydrophobic character (Fig. S12c, in the Supporting Information). The above results indicate activated USTC-1 is more likely to selec tively adsorb the cationic dye RB and MB over the anionic dye MO. Although further studies are required to clarify this question, a possible explanation is the hydrogen-bonding interaction between the amide and dye [48]. The reports on the preferential adsorption of cationic dyes on neutral MOF frameworks are limited [49,50]. The bulky RB molecule (6.5 � 12.7 � 15.5 Å) can also be encapsulated by activated USTC-1, which should be ascribed to its flexible pore character. The pore size distribution indicates that the pore diameter of the activated USTC-1 is ca. 1.62 nm (Fig. S13, in the Supporting Information). In addition, compared to smaller size MB (4.2 � 5.0 � 13.4 Å), the loading amount and adsorption rate of cationic dye RB is low (Fig. 6), ascribed to its
larger molecule size. If the size of the dye molecules is matched well with the size of the channel of USTC-1 framework in two orientations, they will be captured more easily [47]. It is obvious that MB has more suit able size compared to RB.
Fig. 6. Encapsulation of RB (a), MO (b) and MB (c) into activated USTC-1 along with adsorption time.
Fig. 7. Solid-state PL spectra of H2L ligand, USTC-1 (a), RB@USTC-1(b), MO@USTC-1(c) and MB@USTC-1(d).
3.4. Photoluminescence (PL) spectra Solid-state photoluminescence (PL) spectra of H2L ligand, USTC-1 and dye@USTC-1 are presented in Fig. 7. The H2L and USTC-1 show emission peak around 433 nm, derived from π–π* electron transition of the ligand. MO@USTC-1 and MB@USTC-1 reveal similar blue emission band to USTC-1 (Fig. S14, in the Supporting Information), however, MB@USTC-1 exhibits weakened emission, attributable to very weak solid-state PL spectrum of MB itself (Fig. S15 (inset b), in the Supporting Information). Interestingly, RB@USTC-1 simultaneously displayed the characteristic emissions of both the RB dye and the MOF (USTC-1) after excitation at 365 nm in the solid state (Fig. 7b). As shown in Fig. S16 (in the Supporting Information), the RB aqueous solution (10 5 mol L 1) exhibited the characteristic emission at 602 nm when excitated at 365 nm, therefore, the red emission band around 602 nm for RB@USTC1 should originate from the RB dye. Such result indicates that the large RB dye molecules had been successfully encapsulated into the USTC-1 channels. No 677 nm emission peak in RB@USTC-1 also demonstrates that RB molecules are highly encapsulated into the inner pores, not the surface of the MOF (USTC-1) crystals, because the solid-state RB exhibits the characteristic emission at 677 nm when excitated at 365 nm (Fig. S15, in the Supporting Information), which should be attributed to the stacking-induced red-shifted emission (from 602 nm in solution to 677 nm in solid state) [51]. The RB@USTC-1 displays a pink light under 365 nm UV irradiation (Fig. S14b, in the Supporting Information) and even in daylight (It even can be excitated by 580 nm visible light (Fig. S17b, in the Supporting Information)). Compared with the pure USTC-1 with the emission position at ca. 433 nm, another new emission band at ca. 602 nm (Fig. 7) for the RB@USTC-1 originates from the emission of free isolated RB molecules. As a result, a dual-emitting RB@USTC-1 composite material with red/blue intensity ratio of 4.5 is achieved. It needs to be mentioned that the emission band of the RB in RB@USTC-1 is the same as that of RB in aqueous solution, but signifi cantly different from the RB in the solid state with very weak emission band at 677 nm, compared to USTC-1 (Fig. S18, in the Supporting In formation). These results demonstrate that the RB dye is uniformly encapsulated in the channels of USTC-1 as free isolated molecules. The microsecond lifetimes of USTC-1 (27.2 μs) and RB@USTC-1 (29.0 μs) in the solid state demonstrate their phosphorescent nature (Fig. S19A, in the Supporting Information). As a result, their phospho rescent quantum yield (Φph) is 4.0 (at 433 nm) and 6.6% (at 602 nm), respectively. Noticeably, although RB in aqueous solution is
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fluorescence with a ns level lifetime [52], the aggregation-induced phosphorescence [53,54] is observed for solid-state RB with lifetime of 19.2 μs (Fig. S19B, in the Supporting Information), regardless of its relative weak emission in the solid state (Fig. S18, in the Supporting Information). Moreover, phosphorescent character of RB in RB@USTC-1 (at 602 nm) might be due to MOF-to-RB energy-transfer [55]. Signifi cantly, RB@USTC-1 possesses enhanced emission band at 602 nm (Fig. 7), longer lifetime, higher Φph and even stronger solid-state exci tation spectrum (Fig. S17b, in the Supporting Information), compared to USTC-1. 3.5. Switchable color and phosphorescence via acid-base vapor stimuli The encapsulation (adsorption) of guest molecules into a MOF could help to expand the additional function of the MOFs. For example, the encapsulated dye molecules can be used as stimuli-responsive source, resulting in a color change of the MOF material in response to varying chemical environment [30]. When RB@USTC-1, MO@USTC-1 and MB@USTC-1 were exposed to HCl vapor in a closed container within 30 min, pink RB@USTC-1 and colorless MO@USTC-1 turned into pale-pink (Fig. 8a, middle) and weak pink (Fig. 8c, middle) respectively, whereas no color change was observed for MB@USTC-1 sample. Apart from solid-state color change, after HCl vapor stimulus, bright pink phosphorescence of RB@USTC-1, blue phosphorescence of MO@USTC-1 were transferred to dim-pink (Fig. 8b, middle) and dim-blue (Fig. 8d, middle), respectively. On the other hand, after sub sequent exposure of HCl vapor-disposed samples to NH3 vapor in a closed container within 30 min, their dim light can be back to bright pink and blue phosphorescence (Fig. 8b and d, right), respectively. The transformation between different phosphorescence colors is reversible in nature, that is, RB@USTC-1 and MO@USTC-1 turn into the corre sponding HCl vapor-disposed samples by using HCl vapor, and then they can be again back to RB@USTC-1 and MO@USTC-1 by using NH3 vapor (Fig. 9). As a result, the acid-base vapor can switch phosphorescent colors of the RB@USTC-1 and MO@USTC-1. From Fig. 9, it was found out that the protonation of RB@USTC-1 by HCl vapor led to obviously enhanced emission band at 433 nm, and weakened and blue-shifted emission band at 588 nm (from 602 to 588 nm). It should be noted that after protonation, RB@USTC-1 turned into blue/red two-color luminescence with intensity ratio of 1.3 (Fig. 9b), thus its pink phosphorescence was transferred to dim-pink (Fig. 8b, middle). However, NH3 vapor can recover its pink phospho rescence (Fig. 8b, right) with red/blue two-color intensity ratio of 2.2 (Fig. 9c). Moreover, the protonation of MO@USTC-1 by HCl vapor resulted in a new emission band at 578 nm (Fig. 9e), thus blue phos phorescence of MO@USTC-1 was changed into dim-blue (Fig. 8d, mid dle). Also, NH3 vapor can recover its blue phosphorescence (Figs. 9f and
Fig. 9. Solid-state PL spectra of RB@USTC-1 (a), RB@USTC-1 stimulated by HCl vapor (b) and then stimulated by NH3 vapor (c); solid-state PL spectra of MO@USTC-1 (d), MO@USTC-1 stimulated by HCl vapor (e) and then stimu lated by NH3 vapor (f).
8d, right). Such HCl vapor-induced emissive color transformation should be ascribed to the protonation effect of dyes in USTC-1 channels, because USTC-1 and solid-state dyes themselves do not reveal such switching behavior of acid-base vapor stimuli. According to molecular structure of dyes, RB and MO in USTC-1 channels were easily protonated by HCl vapor, whereas, MB is difficult (Fig. S20, in the Supporting Information). Such phosphorescent switching performance does actually result from the protonation-deprotonation switching of the free isolated dye mole cules in USTC-1 channels, which is further demonstrated by HCl-NH3 titration experiments. As shown in Fig. S21a (in the Supporting Infor mation), when titrated by HCl (pH: from 5.70 (10 5 mol L 1 RB aqueous solution) to 0.21), the emission intensity around 602 nm was gradually lessened, which is similar to weakened emission of the protonated RB in USTC-1 channels by HCl vapor (Fig. 9). Upon again titration with NH3 (pH: from 0.21 to 11.16), the emission intensity around 602 nm was gradually increased (Fig. S21b, in the Supporting Information), which is similar to enhanced emission of the deprotonated RB in USTC-1 chan nels by NH3 vapor (Fig. 9). It should be noted that upon protonation of RB@USTC-1 by HCl vapor, the blue-shifted emission band from 602 to 588 nm (Fig. 9) should be ascribed to the change of MOF-to-RB energytransfer process [55], owing to protonation of free isolated RB molecules in USTC-1 channels. This pH-sensitive behavior of a dye inside the MOF channels opens the potential application of the two-color ratiometric emission MOF (especial for RB@USTC-1) in sensors. The ratiometric luminescent sensor properties (such as for volatile organic compounds) of dye@USTC-1 and HCl vapor-disposed dye@USTC-1 samples will be performed in our laboratory. 4. Conclusion In conclusion, on the basis of a new T-shaped imidazo[1,2-a]pyridine dicarboxylate linker and zinc ion, a novel flexible MOF with interesting topology has been synthesized and characterized. It suggests that a combination of carboxylate and imidazo[1,2-a]pyridyl amide groups in a T-shaped ligand may construct flexible MOFs. Although the N2 and CO2 gas adsorption investigations exhibit low gas uptake, attributable to the flexible vibration of amide chains at linker that close the pore win dow, the closed pore window can be opened by dye molecules. Inter estingly, RB@USTC-1 simultaneously displayed the characteristic emissions of both the RB dye and the MOF, resulting in red/blue twocolor luminescence with intensity ratio of 4.5 and variable phospho rescence color. Moreover, RB@USTC-1 and MO@USTC-1 could undergo protonation upon exposure to HCl vapor, and reveal reversible
Fig. 8. Photographs of RB@USTC-1 taken under daylight (a) and 365 nm UV irradiation(b), and MO@USTC-1 taken under daylight (c) and 365 nm UV irradiation (d), showing reversible color and phosphorescence transformation stimulated by acid-base vapors. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 6
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phosphorescent color switching in the response to acid-base vapor stimuli. Consequently, the encapsulation of dyes into the MOF is an elegant way to create a multifunctional material for ratiometric lumi nescent sensor and stimuli-responsiveness. The present work provides a promising approach for synthesizing novel flexible MOFs and a new access to develop the phosphorescent and stimuli-responsive MOF ma terials via encapsulation of various guests.
[22] [23] [24] [25]
Declaration of competing interest
[26]
The authors declare no Conflict of Interest.
[27]
Acknowledgements
[28]
This work was supported by the Foundation of the USTC-Anhui To bacco Joint Laboratory of Chemistry and Combustion. We thank Dr. S. M. Zhou (HFNL, USTC) for the determination of the crystal structures.
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Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.dyepig.2019.108017.
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References
[33]
[1] Schubert EF, Kim JK. Solid-state light sources getting smart. Science 2005;308: 1274–8. [2] Veinot JGC, Marks TJ. Toward the ideal organic light-emitting diode. The versatility and utility of interfacial tailoring by cross-linked siloxane interlayers. Acc Chem Res 2005;38:632–43. [3] Binnemans K. Lanthanide-based luminescent hybrid materials. Chem Rev 2009; 109:4283–374. [4] Eliseeva SV, Bunzli JCG. Lanthanide luminescence for functional materials and biosciences. Chem Soc Rev 2010;39:189–227. [5] Grimsdale AC, Chan KL, Martin RE, Jokisz PG, Holmes AB. Synthesis of lightemitting conjugated polymers for applications in electroluminescent devices. Chem Rev 2009;109:897–1091. [6] Yang SK, Shi X, Park S, Ha T, Zimmerman SC. A dendritic single-molecule fluorescent probe that is monovalent, photostable and minimally blinking. Nat Chem 2013;5:692–7. [7] Stender AS, Marchuk K, Liu C, Sander S, Meyer MW, Smith EA. Single cell optical imaging and spectroscopy. Chem Rev 2013;113:2469–527. [8] Zhu M, Yang C. Blue fluorescent emitters: design tactics and applications in organic light-emitting diodes. Chem Soc Rev 2013;42:4963–76. [9] Yang Z, Cao J, He Y, Yang JH, Kim T, Peng X, Kim JS. Macro-/micro- environmentsensitive chemosensing and biological imaging. Chem Soc Rev 2014;43:4563–601. [10] Lee TW, Park OO, Yoon J, Kim JJ. Polymer-layered silicate nanocomposite lightemitting devices. Adv Mater 2001;13:211–3. [11] Agranovich VM, Gartstein YN, Litinskay M. Hybrid resonant organic inorganic nanostructures for optoelectronic applications. Chem Rev 2011;111:5179–214. [12] McCulloch I, Heeney M, Bailey C, Genevicius K, Macdonald I, Shkunov M, Sparrowe D, Tierney S, Wagner R, Zhang WM, Chabinyc ML, Kline RJ, Mcgehee MD, Toney MF. Liquid-crystalline semiconducting polymers with high charge- carrier mobility. Nat Mater 2006;5:328–33. [13] Long JR, Yaghi OM. The pervasive chemistry of metal-organic frameworks. Chem Soc Rev 2009;38:1213–4. [14] Murray LJ, Dinc� a M, Long JR. Hydrogen storage in metal-organic frameworks. Chem Soc Rev 2009;38:1294–314. [15] Wang XL, Qin C, Wu SX, Shao KZ, Lan YQ, Wang S, Zhu DX, Su ZM, Wang EB. Bottom-up synthesis of porous coordination frameworks: apical substitution of a pentanuclear tetrahedral precursor. Angew Chem Int Ed 2009;48:5291–5. [16] Pan M, Lin XM, Li GB, Su CY. Progress in the study of metal-organic materials applying naphthalene diimide (NDI) ligands. Coord Chem Rev 2011;255:1921–36. [17] Seo JS, Whang D, Lee H, Jun SI, Oh J, Jeon YJ, Kim K. A homochiral metal-organic porous material for enantioselective separation and catalysis. Nature 2000;404: 982–6. [18] Takashima Y, Martinez VM, Furukawa S, Kondo M, Shimomura S, Uehara H, Nakahama M, Sugimoto K, Kitagawa S. Molecular decoding using luminescence from an entangled porous framework. Nat Commun 2011;2:168. [19] Sun CY, Qin C, Wang CG, Su ZM, Wang S, Wang XL, Yang GS, Shao KZ, Lan YQ, Wang EB. Chiral nanoporous metal-organic frameworks with high porosity as materials for drug delivery. Adv Mater 2011;23:5629–32. [20] Huang Y, Lin Z, Cao R. Palladium nanoparticles encapsulated in a metal-organic framework as efficient heterogeneous catalysts for direct C2 arylation of indoles. Chem Eur J 2011;17:12706–12. [21] Lu G, Li SZ, Guo Z, Farha OK, Hauser BG, Wang Y, Wang X, Han SY, Liu XG, DuChene JS, Zhang H, Zhang QC, Chen XD, Ma J, Loo SCJ, Wei WD, Yang YH,
[34] [35] [36] [37] [38] [39] [40] [41] [42] [43]
[44] [45] [46] [47] [48] [49]
7
Hupp JT, Huo FW. Imparting functionality to a metal-organic framework material by controlled nanoparticle encapsulation. Nat Chem 2012;4:310–6. Ferey G. Hybrid porous solids: past, present, future. Chem Soc Rev 2008;37: 191–214. Allendorf MD, Bauer CA, Bhakta RK, Houk RJT. Luminescent metal-organic frameworks. Chem Soc Rev 2009;38:1330–52. Cui Y, Yue Y, Qian G, Chen B. Luminescent functional metal-organic frameworks. Chem Rev 2012;112:1126–62. Qi Y, Xu H, Li X, Tu B, Pang Q, Lin X, Ning E, Li Q. Structure transformation of a luminescent pillared-layer metal-organic framework caused by point defects accumulation. Chem Mater 2018;30:5478–84. Gai Y, Guo Q, Xiong K, Jiang F, Li C, Li X, Chen Y, Zhu C, Huang Q, Yao R, Hong MC. Mixed-lanthanide metal organic frameworks with tunable color and white light emission. Cryst Growth Des 2017;17:940–4. An J, Shade CM, Chengelis-Czegan DA, Petoud S, Rosi NL. Zinc-adeninate metalorganic framework for aqueous encapsulation and sensitization of near-infrared and visible emitting lanthanide cations. J Am Chem Soc 2011;133:1220–3. He HM, Sun FX, Borjigin T, Zhao NA, Zhu GS. Tunable colors and white-light emission based on a microporous luminescent Zn(II)-MOF. Dalton Trans 2014;43: 3716–21. Yanai N, Kitayama K, Hijikata Y, Sato H, Matsuda R, Kubota Y, Takata M, Mizuno M, Uemura T, Kitagawa S. Gas detection by structural variations of fluorescent guest molecules in a flexible porous coordination polymer. Nat Mater 2011;10:787–93. Grünker R, Bon V, Heerwig A, Klein N, Mueller P, Stoeck U, Baburin IA, Mueller U, Senkovska I, Kaskel S. Dye encapsulation inside a new mesoporous metal-organic framework for multifunctional solvatochromic-response function. Chem Eur J 2012;18:13299–303. Sun CY, Wang XL, Zhang X, Qin C, Li P, Su ZM, Zhu DX, Shan GG, Shao KZ, Wu H, Li J. Efficient and tunable white-light emission of metal-organic frameworks by iridium-complex encapsulation. Nat Commun 2013;4:2717. Yan DP, Tang YQ, Lin HY, Wang D. Tunable two-color luminescence and host-guest energy transfer of fluorescent chromophores encapsulated in metal-organic frameworks. Sci Rep 2014;4:4337. Yue Y, Binder AJ, Song R, Cui Y, Chen J, Hensley DK, Dai S. Encapsulation of large dye molecules in hierarchically superstructured metal-organic frameworks. Dalton Trans 2014;43:17893–8. Wen Y, Sheng T, Zhu X, Zhuo C, Su S, Li H, Hu S, Zhu QL, Wu X. Introduction of red-green-blue fluorescent dyes into a metal-organic framework for tunable white light emission. Adv Mater 2017;29:1700778. Dai Y, Zhang JJ, Liu SQ, Zhou H, Sun YJ, Pan YZ, Ni J, Yang JS. A trichromatic and white-light-emitting MOF composite for multi-dimensional and multi-response ratiometric luminescent sensing. Chem Eur J 2018;24:9555–64. Yong GP, Qiao S, Xie Y, Wang ZY. Cadmium(II) and zinc(II) coordination polymers with 1D ladder and 2D basket weave layer structures constructed from a new Tshaped ligand. Eur J Inorg Chem 2006:4483–8. Li B, Ni LS, Yong GP. Three new coordination compounds based on a new 3-po sition substituted imidazo[1,2-a]pyridine ligand: syntheses, crystal structures and photoluminescent properties. Polyhedron 2018;154:21–6. Jiang WP, Yong GP. One-dimensional coordination polymers based on a new 3position substituted imidazo[1,2-a]pyridine ligand: crystal structures, photoluminescent and magnetic properties. Polyhedron 2019;157:428–33. Liu SD, Kuo BC, Liu YW, Lee JY, Wong KY, Lee HM. Tuning of net architectures of Ni(II) and Zn(II) coordination polymers using isomeric organic linkers. CrystEngComm 2014;16:8874–84. Fu RB, Hu SM, Wu XT. Luminescent zinc phosphonates for ratiometric sensing of 2,4,6-trinitrophenol and temperature. Cryst Growth Des 2016;16:5074–83. Li HC, Cao XY, Zhang C, Yu Q, Zhao ZJ, Niu XD, Sun XD, Liu YL, Ma L, Li ZQ. Enhanced adsorptive removal of anionic and cationic dyes from single or mixed dye solutions using MOF PCN-222. RSC Adv 2017;7:16273–81. van der Sluis P, Spek AL. An effective method for the refinement of crystal structures containing disordered solvent regions. Acta Crystallogr Sect A Found Crystallogr 1990;46:194–201. Keceli E, Hemgesberg M, Grünker R, Bon V, Wilhelm C, Philippi T, Schoch R, Sun Y, Bauer M, Ernst S, Kaskel S, Thiel WR. A series of amide functionalized isoreticular metal organic frameworks. Microporous Mesoporous Mater 2014;194: 115–25. Blatov VA, Peaskov MV. A comparative crystallochemical analysis of binary compounds and simple anhydrous salts containing pyramidal anions LO3 (L ¼ S, Se, Te, Cl, Br, I). Acta Crystallogr B 2006;62:457–66. Xiang SL, Huang J, Li L, Zhang JY, Jiang L, Kuang XJ, Su CY. Nanotubular metalorganic frameworks with high porosity based on T-shaped pyridyl dicarboxylate ligands. Inorg Chem 2011;50:1743–8. Spek AL. Single-crystal structure validation with the program PLATON. J Appl Crystallogr 2003;36:7–13. Zhao Y, Wang L, Fan NN, Han ML, Yang GP, Ma LF. Porous Zn(II)-based metalorganic frameworks decorated with carboxylate groups exhibiting high gas adsorption and separation of organic dyes. Cryst Growth Des 2018;18:7114–21. Ji WJ, Hao RQ, Pei WW, Feng L, Zhai QG. Design of two isoreticular Cdbiphenyltetra-carboxylate frameworks for dye adsorption, separation and photocatalytic degradation. Dalton Trans 2018;47:700–7. Wang HN, Liu FH, Wang XL, Shao KZ, Su ZM. Three neutral metal-organic frameworks with micro- and meso-pores for adsorption and separation of dyes. J Mater Chem 2013;1:13060–3.
B. Li et al.
Dyes and Pigments xxx (xxxx) xxx
[50] Qin L, Zeng SY, Zuo WJ, Liu QH, Li J, Ni G, Wang YQ, Zhang MD. One neutral metal-organic framework with an unusual dmp topology for adsorption of dyes. Polyhedron 2017;121:231–5. [51] Yong GP, She WL, Zhang YM. Room-temperature phosphorescence in solution and in solid state from purely organic dyes. Dyes Pigments 2012;95:161–7. [52] Smith SN, Steer RP. The photophysics of Lissamine rhodamine-B sulphonyl chloride in aqueous solution: implications for fluorescent protein-dye conjugates. J Photochem Photobiol A Chem 2001;139:151–6.
[53] Yuan XWZ, Shen Y, Zhao H, Lam JWY, Tang L, Lu P, Wang CL, Liu Y, Wang ZM, Zheng Q, Sun JZ, Ma YG, Tang BZ. Crystallization-induced phosphorescence of pure organic luminogens at room temperature. J Phys Chem C 2010;114:6090–9. [54] Bolton O, Lee K, Kim HJ, Lin KY, Kim J. Activating efficient phosphorescence from purely organic materials by crystal design. Nat Chem 2011;3:205–10. [55] Chen DM, Zhang NN, Liu CS, Du M. Dual-emitting dye@MOF composite as a selfcalibrating sensor for 2,4,6-trinitrophenol. ACS Appl Mater Interfaces 2017;9: 24671–7.
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