Niflumic Anion Intercalated Layered Double Hydroxides with Mechano

Article pubs.acs.org/IECR

Niflumic Anion Intercalated Layered Double Hydroxides with Mechano-Induced and Solvent-Responsive Luminescence Yibing Zhao,† Heyang Lin,† Mingxing Chen,‡ and Dongpeng Yan*,† † ‡

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, P.R. China College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P.R. China S Supporting Information *

ABSTRACT: Stimuli-responsive luminescent materials play an important role in fluorescent switches, optical storage devices and smart sensors. In this work, we report a mechano-induced and solvent stimuli-responsive luminescent change by the assembly of a typical aggregation-induced-emissive (AIE) molecule, niflumic acid (NFC), into the interlayer region of Zn−Allayered double hydroxides (LDHs) with heptanesulfonate (HPS) as a cointercalation guest. The structure, chemical composition, and thermostability of the as-prepared NFC-HPS/LDHs composites were characterized by X-ray diffraction, elemental analysis, and thermogravimetry and differential thermal analysis (TG-DTA). Fluorescence spectra demonstrate that the sample with 5% NFC initial molar percentage, with respect to the interlayer guests, exhibits the optimal luminescent intensity. The NFC-HPS/ LDH (5%) sample also exhibits the most obvious luminescent mechano-response with a 16 nm blue-shift and increase in the fluorescent intensity after grinding, while the pristine NFC solid shows little to no mechano-responsive behavior. Moreover, the NFC-HPS/LDH (5%) also presents reversible luminescent response to different volatile organic compounds (VOCs) (such as tetrahydrofuran, methanol, acetone, toluene, and chloroform). Therefore, this work not only gives a detailed description on the dual stimuli (mechanics and solvent)-responsive luminescence for future sensor applications but also supplies a deep understanding of the optical properties of the new AIE molecule within the confined LDH layers.

1. INTRODUCTION Stimuli-responsive luminescent materials have recently received much attention due to their potential applications in fluorescent switches, optical storage devices and smart sensors.1−5 Compared with photo-, thermo- and electrochromic counterparts, mechanochromic luminescent materialsthe fluorescence of which can change upon external grinding, pressure, shearing and/or other mechanical processesare still in their early stage, however.6−9 Such types of materials can be used as optical detection systems to monitor the change in external force and pressure especially under extreme conditions. Recent development in organic mechanochromic fluorescent crystals have indicated that the alternation of the molecular arrangement and packing mode became an effective strategy to tune the fluorescence due to the modification of the intermolecular interactions.10,11 For example, Dong et al.10 have found that the piezochromic behavior of (9,10-bis((E)-2-(pyrid-2-yl)vinyl)anthracene) is related to its molecular aggregation state and specifically to the strength of the π−π interaction between the anthracene rings of adjacent molecules; Mizuguchi et al.12 have found the crystalline powder of 4-[bis(4-methylphenyl)amino]benzaldehyde was changed from light blue to greenish yellow by grinding treatment. Although great efforts have recently been made on the mechanochromic fluorescence of pure organic fluorophores,10,13 how to rationally design and prepare such materials continues to be a considerable challenge. Recently, the compounds with aggregation-induced emission (AIE)the lack of fluorescence emission in solution but strong luminescence in solid stategradually became a promising direction by which to obtain a mechanofluorochromism property.14,15 © 2014 American Chemical Society

Layered materials stand for a large family of functional organizations, which involve tunable interlayer volumes and variable interlayer guests.16,17 Recently, much attention has been focused on two-dimensional (2D) clay−chromophore supramolecular hybrid materials for their novel functionalities which differ from those of their individual components.18−22 Importantly, the orientation and arrangement of the luminescent guest species can be tuned within the interlayer galleries of the 2D matrix, which facilitates the modulation of the luminescence properties. Layered double hydroxides (LDHs) are one type of layered compositions which exhibit a particularly large versatility by virtue of their tunable chemical composition and gallery space.23 LDHs can be generally expressed by the formula [M2+1−xM3+x (OH)2](An−)x/n·mH2O, where M2+and M3+ are divalent and trivalent metal cations, respectively, and An− is a functional anion. The host structure consists of brucite-like layers of edge-sharing M(OH)6 octahedra, and the partial substitution of M3+ for M2+ results in positively charged host layers, balanced by the interlayer anions. Recently, LDHs have drawn much attention for their potential applications in the fields of fluorescence,18−20 bioinorganic chemistry24 and catalysis.25 Several photoactive molecules have been intercalated into the LDH layers for the requirement of enhancing the luminous efficiency of dye.26−28 In addition, the intercalated chromophores often exhibit some additional photophysical behavior in a 2D confined environReceived: Revised: Accepted: Published: 3140

November 30, 2013 January 12, 2014 January 29, 2014 January 29, 2014 dx.doi.org/10.1021/ie404054v | Ind. Eng. Chem. Res. 2014, 53, 3140−3147

Industrial & Engineering Chemistry Research

Article

Figure 1. (a) comparison of PL spectra of the NFC in tetrahydrofuran (THF) solution and solid state, respectively. (Inset) Images of NFC solution and solid powder under UV light at 365 nm. (b) the optimized structure of niflumic acid (calculated by CFF91 force field in Materials Studio software)33−35 and its crystal structure36 (dashed lines show the H-bonding interactions); (c) the molecular structure of HPS.

ment that is absent in solution; the host−guest interaction system offers a higher mechanical, thermal, and chemical stability for the interlayer molecules.29,30 Moreover, on the basis of the fact that external pressure can deform over intercalated bulky guests (resulting in the slipping of LDH sheets) which can further tune the interlayer molecular configuration and influence the host−guest interactions, we have recently designed a piezochromic fluorescent material by assembly of a stilbene anions into LDH layer.31 However, whether this strategy can extend to other LDH systems and how to select suitable interlayer chromophore molecules for achieving mechanochromic fluorescence are still unresolved issues. Niflumic acid (NFC) is a well-known pharmaceutical compound, which has been used largely in the biological and medical chemistry fields.32 From a structural perspective, NFC is a flexible molecule containing a rotatable aromatic amine unit, and we noted that NFC presents AIE property (Figure 1a) with the PLQY values of 0.03% and 4.26% for solution and solid state respectively. This phenomenon can be attributed to molecular stacking in the solid state restricting free rotation of the NFC and thus increasing the conjugated degree and coplanarity of the NFC (Figure 1b). As a comparison, the NFC molecules are highly flexible in solution form. UV−vis spectra (Figure S1 in Supporting Information [SI]) show that the absorption peaks for the NFC solution are located at ∼286 and 336 nm, and new absorption band appears at ∼410 nm for the solid-state sample, indicating the molecular aggregates forms. The pure NFC compound cannot exhibit mechano-induced fluorescent change due to the highly ordered H-bonding network within the molecular solid, for which it is difficult to modify the intermolecular interactions (Figure 1b). In the present work, by introduction of NFC molecules and the cointercalated unit, heptanesulfonate (HPS), into the gallery of Zn−Al-LDHs, we show how the mechanochromic fluorescence can be obtained in the NFC-HPS/LDH composites. The selection of heptanesulfonate (HPS) as the cointercalation agent is based on the fact that the length of HPS (1.10 nm, Figure 1c) in the long-axis direction is close to that of NFC (1.06 nm), which facilitates the tunable interlayer compositions and the microenvironment. The results show that the NFCHPS/LDH composites have a higher thermal and chemical stability compared with those of the pure NFC; fluorescence spectra demonstrate that the sample with 5% NFC initial molar percentage, with respect to the total interlayer anions, exhibits

the optimal luminescent efficiency. It was also found that the NFC-HPS/LDH (5%) material exhibits luminescent change upon grinding accompanied with the increasing fluorescent intensity. Moreover, their solvent-responsive luminescence behavior was also detected. Since the pristine NFC solid shows lack of mechanochromism, the transformation of a mechanochromism-free fluorophore into a mechanochromism material by incorporation into LDH matrix is the most distinctive feature of this work. It can be expected that the design strategy by the incorporation of AIE molecules into the layered materials can be extended to other 2D organized array system for developing new types of luminescent-smart responsive materials.

2. EXPERIMENTAL SECTION 2.1. Materials. NaOH (AR), Zn(NO3)2·6H2O (AR), and Al (NO3)3·9H2O (AR) were purchased from Beijing Chemical Plant Limited. Niflumic acid (NFC) and sodium heptanesulfonate (HPS) (HPS, >97%) were purchased from Sigma Chemical. Co. Ltd. 2.2. Precursor Solutions. Solution A: Zn(NO3)2·6H2O (5 mmol), Al(NO3)3·9H2O (2.5 mmol), NFC (a mmol) and HPS (b mmol) (a + b = 2.5 mmol; b:a = 99:1; 95:5; 90:10; 3:1; 1:1; 1:3; 0:100. x% = a/(a + b)) were dissolved in 70 mL of deionized water and 35 mL of anhydrous ethanol. Solution B: NaOH (15 mmol) was dissolved in 105 mL of deionized water. 2.3. Synthesis of NFC-HPS/LDHs Powder. Solution A (105 mL) and solution B (105 mL) were simultaneously added to a colloid mill, rotating at 3000 rpm, and mixed for 1 min. The resulting slurry was removed from the colloid mill and was sealed into 90-mL Teflon-lined stainless steel autoclaves purged with nitrogen atmosphere for 5 min and heated at 100 °C for 24 h. The products were washed with hot, distilled water and anhydrous ethanol several times until the cleaning liquid was colorless and then were dried in a vacuum at 60 °C for 10 h. 2.4. Instrumentation. Powder XRD (PXRD) of the all samples were measured on a Bruker AXS diffractometer (D8 advance) using Cu Kα radiation (λ = 1.5406 Å), generator voltage 40 kV, and current 30 mA. Samples were scanned in the range of 2θ = 3−70° at the scan rate of 4 s/step with a step width of 0.02°. Thermogravimetric analysis (TGA) was done with a TA thermal analysis system at heating rate 10 °C/min. C, H, N analyses were carried out using a Perkin-Elmer Elementarvario elemental analysis instrument. The solid-state UV−vis absorption spectra were collected in the range 220− 3141

dx.doi.org/10.1021/ie404054v | Ind. Eng. Chem. Res. 2014, 53, 3140−3147


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Figure 2. (a) XPRD patterns and (b) interlayer spacing values of the series of NFC-HPS/LDHs.

Table 1. Chemical Compositions of the NFC-HPS/LDHs (5% and 100%) Composites initial ratios (%)

chemical composition

Zn/Al

final ratios (%)

5 100

Zn1.2831Al0.7961(OH)2(C13H9O2N2F3)0.0299(C7H15O3S1)0.7664·0.47H2O Zn1.3728Al0.8532(OH)2(C13H9O2N2F3)0.8532·0.53H2O

1.612 1.609

3.76 100

Figure 3. TG-DTA curves of both (a) the pristine NFC and (b) NFC/LDH product.

the range from 19.57 Å (x% = 0%) to the maximum (22.43 Å, x % = 100%). The variation of the interlayer spacing can be illustrated by the different arrangements of interlayer guest molecules with different ratios of HPS and NFC within the LDH layer. In addition, the chemical compositions of the asobtained composites were further analyzed, and the typical results with x% = 5% and 100% are shown in Table 1. It can be seen that the experimental ratio of NFC/HPS in the NFCHPS/LDHs is close to the initial nominal ratio as expected. Moreover, the molar ratios of Zn to Al deviate from the initial value of 2.0, which may be attributed to the difference in the dissolution equilibrium between Zn(OH)2 and Al(OH)3 in the LDH host layer during the hydrothermal process. This phenomenon has also been observed in other LDH systems.38 To compare the thermolysis behavior of NFC before and after intercalation, TG-DTA analyses were performed on both the pristine NFC and NFC/LDH product, which are shown in Figure 3. It can be observed that, for the pristine NFC, a sharp weight loss of NFC with an exothermic peak appears at ∼205.5 °C, corresponding to the fast melting process within the NFC organic solid; whereas for the NFC/LDH, the initial weight loss in the range from room temperature to 190 °C accompanying the weight loss of 11.5% is due to the removal of surface adsorbed and interlayer water molecules. The following weight loss of ∼45% with a broad exothermic range from 200 to 475 °C can be attributed to the decomposition and/or combustion

900 nm on a Shimadzu U-3000 spectrophotometer, with the slit width of 1.0 nm and BaSO4 as the reference. The solid-state fluorescence spectra were performed on RF-5301PC fluorospectrophotometer with 360 nm excitation light. The width of both the excitation and emission slit was 3 nm. The fluorescence decays were measured using a LifeSpec-ps spectrometer by 372 nm laser excitation, and the lifetimes were calculated with the F900 Edinburgh instruments software. Photoluminescence quantum yield (PLQY) was measured using an HORIBA Jobin-Yvon FluoroMax-4 spectrofluorimeter, equipped with an F-3018 integrating sphere.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Composition of the NFCHPS/LDH Powder. NFC and HPS cointercalated Zn−AlLDHs have been prepared by the separate nucleation and aging steps,37 followed by a hydrothermal treatment. Figure 2A shows the typical PXRD patterns of the NFC-HPS/LDH (x%) samples, in which x% stands for the initial molar percentage of NFC accounting for the summation of NFC and HPS. All the patterns of these samples can be indexed to a hexagonal lattice with R3̅m rhombohedral symmetry. The interlayer spacing can be calculated by averaging the positions of the three harmonics: d = 1/3(d003 + 2d006 + 3d009). It can be observed from Figure 2b that the interlayer spacing of NFC-HPS/LDH increases systematically with the increasing NFC contents, which is in 3142



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Figure 4. (a) Solid UV−vis absorption spectra for NFC-HPS/LDH with different molar ratios of NFC to HPS; (b) fluorescence emission spectra for NFC-HPS/LDH with different molar ratios of NFC to HPS.

of the interlayer NFC, suggesting that the noncrystalline state of the interlayer NFC and the thermal stability of NFC molecule can be enhanced when intercalated into the gallery of LDHs. The third one in the range 475−650 °C corresponds to the decomposition collapse of the layer, which features a strong exothermic peak at ∼525 °C in the DTA curve. 3.2. Tunable Solid-State Luminescence. To further compare the chemical environment of the interlayer NFC molecules with those in the pristine solid state, the spectroscopic characteristics were further studied for the NFC-HPS/LDH samples. The UV−vis spectra of the NFCHPS/LDH with different interlayer NFC concentrations are shown in Figure 4a, and it can be observed that two main absorption bands are located in the UV region, and the intensity below 300 nm increases upon the increase of the concentration of interlayer NFC. Furthermore, the absorption band has no obvious change in the position from 1% to 50%, but undergoes a blue-shift to lower wavelength when the concentration of NFC increases from 50% (297 nm) to 100% (274 nm). Compared with the UV−vis absorption spectra of the pure NFC in solution (10−5 mol/L in THF) and solid forms (Figure S1 in Supporting Information), the positions of the absorption band for NFC-HPS/LDH were totally alternated, suggesting the change in the interlayer aggregation state of the NFC relative to their pristine samples. Figure 4b shows the fluorescence emission spectra for NFC-HPS/LDH with different molar ratios of NFC to HPS, in which the fluorescence intensity increases at first to a maximum and then decreases with the ratio of NFC to HPS increases. The optimal luminescent intensity presents in the sample with 5% initial concentration of NFC and the emission peak appearing at 439 nm. The maximum emission wavelength shifts to 469 nm, and its intensity decreases quickly when the interlayer NFC content increases to 100%. The trend of the fluorescence intensity is also consistent with the change of the photoluminescent quantum yield (PLQY) values as shown in Table 2. This behavior can be attributed to the fact that NFC exhibits singlemolecule luminescence with low concentrations, giving rise to the increase in the luminous intensity first, and the less emissive excimers occur when the content of NFC increases to a certain concentration, resulting in the red-shift of emission as well as the decrease in the fluorescence intensity and PLQY value.

To better understand the excited-state information of fluorescence for the interlayer NFC, The samples were studied by detecting their fluorescence decays. The fluorescence lifetimes were obtained by fitting the decay profiles (Figure 5) with both one-exponential and double-exponential form

Figure 5. Fluorescence decay curves for NFC-HPS/LDH (1%, 5%, 10%, 50%, and 100%).

respectively, and the results are tabulated in Table 3. In the double-exponential case, the average lifetime, ⟨τ⟩, is also listed in Table 3. The fluorescence lifetime decreases first from 2.72 ns (1%) to 1.64 ns (5%) and then increases to 8.82 ns (100%) upon the increasing interlayer NFC content, which has a reversed trend compared with that of the luminescent intensity. This observation is also consistent with the formation of excimers since it was reported that such species can result in the increase of the fluorescent lifetime.39 Therefore, it can be concluded that the state of interlayer NFC is totally alternated compared with the pure solid-sate NFC. For the pure NFC sample, the molecular aggregation restricts the rotation and increases the coplane of the NFC molecule, and exhibits more efficient emission than in the dissolved form. While for the NFC-HPS/LDH, the LDH layers to some extent confine the NFC molecules on the basis of host−guest interactions (such as H-bond and electrostatic interaction), and the anionic state, molecular configuration and microenvironment of interlayer NFC may be totally different from the pure solid forms, which results in the aggregation-caused quenching phenomenon as mentioned above. The fluorescence and absorption spectra clearly demonstrate that the introduction of HPS as the second guest molecule in the gallery of LDH can tune and optimize the

Table 2. PLQY Values for NFC-HPS/LDHs (%) samples

1%

5%

10%

50%

100%

PLQY (%)

0.82

1.47

0.44

0.25

0.22 3143



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Table 3. Fluorescence Decay Data of the NFC-HPS/LDHsa samples (x%) 1

5

10

50

100

m

τi (ns)

Ai (%)

1 2

1.82 1.04 3.61 1.35 0.81 2.25 1.32 0.90 3.37 1.56 0.57 3.74 1.92 0.44 9.21

100.00 34.63 65.37 100.00 42.52 57.48 100 41.42 58.58 100.00 24.86 75.14 100.00 4.49 95.51

1 2 1 2 1 2 1 2

⟨τ⟩ (ns)

χ2

2.72

2.109 1.193

1.64

2.349 1.044

2.35

1.645 1.101

2.95

4.005 0.980

8.82

3.208 1.061

3.4. Solvent-Responsive Luminescence. Inspired by the lack of luminescence in the solution form of NFC, we further studied the solvent-responsive luminescence behaviors of the NFC-HPS/LDH. NFC-HPS/LDH (5%) thin film was fabricated by a solvent evaporation method.40 The typical measurement setup with glancing incidence and normal emission geometry was employed to determine the polarized fluorescence spectra and fluorescence anisotropic value r,27 and r can be expressed by the formula r = ((IVV − GIVH)/(IVV + 2GIVH)), where IVH stands for the photoluminescence intensity obtained with vertical polarized light excitation and horizontal polarization detection, and IVV, IHH, IHV are defined in a similar way; G = IHV/IHH, which is used for calibrating the instrument. Polarized fluorescence spectra (Figure 8) show that the NFCHPS/LDH (5%) thin film exhibits a polarization anisotropy of ca. 0.2. The polarization anisotropy of the film sample can be attributed to the preferred orientation and arrangement of fluorescent molecules in the gallery of LDH layers, and this can result in the uniform emission transition dipole of the chromophores within the films.41 The luminescence of the NFC-HPS/LDH (5%) was measured upon treatment with different volatile organic compound (VOC) solvents (such as tetrahydrofuran (THF), methanol, acetone, toluene, and chloroform). Graphs a and b in Figure 9 show the typical PL spectra of the NFC-HPS/LDH (5%) responding to acetone and THF, respectively. It can be observed that the fluorescent intensities of the NFC-HPS/LDH have different degrees decrease by various solvents, with the THF being the most significant (Figure 9d), and the fluorescent decay curves (Figure 9c) show that the fluorescent lifetime of the film increases from 1.64 to 3.51 ns upon simultaneous treatment with the THF. Figure 9d shows the normalized fluorescence intensity of NFC-HPS/LDH (5%) responding to different VOCs in a descending order, and this selectivity of the fluorescent change of the NFC-HPS/LDH to different VOCs can be attributed to the different quenching degree and interaction between the VOC molecules and NFC within the LDH layers. By identifying the quenching degree of the VOC solutions, it can be expected that selective fluorescent reorganization of the VOCs can be achieved. Moreover, the recovery of the fluorescence of NFC-HPS/LDH was also investigated. Upon evaporation of the VOC at room temperature, the fluorescence intensity NFC-HPS/LDH can nearly recover to its initial state as the time increases (a and b of Figure 9), suggesting that the luminescent properties of the NFC-HPS/LDH can be switched. In our opinions, the solventinduced luminescent change is related to the intermolecular interaction between the solvent molecules and the interlayer NFC, which can result in the flexible configuration and rotation of the NFC molecule. Such a mechanism is similar to the AIE behavior of pristine NFC solid as mentioned above.

m is the mono- or double-exponential fitting of the fluorescence decay curve; τi is the fluorescence lifetime, for m = 1, lifetime is τ1, and for m = 2, two lifetimes is τ1 and τ2; Ai is the percentage of τi . The goodness of fit is evaluated by the value of χ2. In the doubleexponential case, ⟨τ⟩ = A1τ1 + A2τ2; A1 + A2 = 1. a

emissive performance compared with the pure solid-state NFC and NFC/LDH systems. 3.3. Mechano-Responsive Luminescence of NFC-HPS/ LDH. Due to the slipping of LDH sheets and reorientation of the interlayer surfactant, the HPS anion may occur under external stimuli, which can further influence the host−guest interactions. In this case, the molecular conformation and intermolecular interaction of NFC anions can be tuned more easily by perturbations than that of pure solid state. One of the goals of this work is to detect the possibility of mechanoresponsive luminescence for the cointercalation NFC-HPS/ LDH systems. Figure 6a−e show that the NFC-HPS/LDHs have different degree of mechanofluorochromism with a concomitant increase in the emission intensity. However, the emission position of pristine solid-state NFC samples remains nearly unchanged under pressure (Figure 6f). The most obvious change in fluorescence on mechanical force was found for NFC-HPS/LDH (5%), in which the maximum wavelength shifted from 439 to 423 nm. As a comparison, the pure mixture composed by NFC (5%) and HPS (95%) present no change in luminescent wavelength upon grinding (Figure S2 in SI), suggesting the layered, confined environment plays a key role in grinding-induced luminescent change. To obtain insight into the influence of external stimuli on the supramolecular organization, XRD was carried out on the NFC-HPS/LDH (5%) system before and after grinding. It can be observed from Figure 7a that the series of 00l diffraction peaks were shifted to a lower angle after grinding compared with pristine powder, confirming the increasing interlayer spacing upon grinding treatment. The change in the gallery height can be related to the fact that the cointercalated HPS molecules are flexible, and thus their interlayer arrangement is relative easily influenced by external stimuli. This can also induce changes in the molecular configuration and aggregation state of interlayer NFC anions as well as the host−guest interactions, which further influence their optical properties. In addition, fluorescent decay curves show that the fluorescent lifetime of NFC-HPS/LDH (5%) increases from 1.64 to 4.25 ns upon grinding as shown in Figure 7b.

4. CONCLUSION In summary, niflumic acid (NFC, a compound with typical aggregation-induced emission in the solid-state) and heptanesulfonate (HPS) with different molar ratios have been coassembled into Zn−Al-layered double hydroxides (Zn−AlLDHs). The structure, chemical composition, and thermostability of the NFC-HPS/LDHs composites were characterized by X-ray diffraction, elemental analysis, and thermogravimetry and differential thermal analysis (TG-DTA). Fluorescence spectra and PLQY analyses demonstrate that the sample with 5% NFC initial molar percentage, with respect to the total 3144



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Figure 6. Fluorescence spectra of the NFC-HPS/LDHs (a) 1%; (b) 5%; (c) 20%; (d) 50%; (e) 100%); and (f) pure NFC before and after grinding.

Figure 7. (a) Powder XRD patterns and (b) fluorescence decay curves of NFC-HPS/LDH (5%) before and after grinding. The detection wavelength was 439 and 423 nm, respectively. Figure 8. Polarized fluorescence spectra of the NFC-HPS/LDH (5%) thin film.

interlayer anions, exhibits the optimal luminous efficiency. The variation tendency of luminous intensity and the UV−vis absorption spectra of the composites proclaimed the formation of the NFC excimers in the interlayer region. Furthermore, the 3145



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Figure 9. In situ time-dependent monitoring of the fluorescence of the NFC-HPS/LDH (5%) responding to the typical VOCs (a) acetone; (b) THF; (c) fluorescence decay curves of NFC-HPS/LDH (5%) before and after treatment with THF; (d) the decreasing percentage of PL intensity treated with VOCs.

Commission of Education (20111001002), the Fundamental Research Funds for the Central Universities and the 111 Project (Grant B07004).

luminescent mechanochromism of the NFC-HPS/LDHs with different NFC initial molar percentages was investigated by comparing the fluorescence spectra of the samples before and after grinding. The NFC-HPS/LDH(5%) presents obvious mechano-responsive emission with a 16 nm blue-shift upon grinding accompanied with the increasing fluorescent intensity. In addition, the NFC-HPS/LDH also exhibits solvent-sensitive luminescence with THF the most significant. It can be expected that the combination of grinding- and solvent-responsive luminescence of the NFC-HPS/LDH systems have potential applications in the fields of multiple stimuli-responsive intelligent materials and luminescent sensors.

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ASSOCIATED CONTENT

S Supporting Information *

UV−vis absorption spectra of the pure NFC in solution and the crystalline solid (Figure S1); fluorescent spectra of the NFCHPS (5%) mixture (without LDH) before and after grinding (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. cn. Fax: +86-10-64425385. Tel: +86-10-64412131. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 Program (Grant No. 2014CB932103), the National Natural Science Foundation of China (NSFC), the Scientific Fund from Beijing Municipal 3146



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Niflumic Anion Intercalated Layered Double Hydroxides with Mechano

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