Luminescent Zinc Phosphonates for Ratiometric Sensing of 2,4,6

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Luminescent Zinc Phosphonates for Ratiometric Sensing of 2,4,6-Trinitrophenol and Temperature Rui-Biao Fu, Sheng-Min Hu, and Xin-Tao Wu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00669 • Publication Date (Web): 22 Jul 2016 Downloaded from http://pubs.acs.org on July 28, 2016

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Crystal Growth & Design

Table of Contents Luminescent Zinc Phosphonates for Ratiometric Sensing of 2,4,6-Trinitrophenol and Temperature Ruibiao Fu*, Shengmin Hu and Xintao Wu State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science, Fuzhou, Fujian, 350002, China *Corresponding author. E-mail: [email protected] Tel: +86-591-63179449

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A new luminescent zinc phosphonate is a promising luminescent probe for reusable, highly sensitive (as low as 143 ppb), and quantitative detection of 2,4,6-trinitrophenol (TNP).

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Luminescent Zinc Phosphonates for Ratiometric Sensing of 2,4,6-Trinitrophenol and Temperature Ruibiao Fu*, Shengmin Hu and Xintao Wu State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science, Fuzhou, Fujian, 350002 China *Corresponding author. E-mail: [email protected]. Tel: +86-591-63179449 ABSTRACT: Three new luminescent zinc phosphonates, [Zn(H2L)(H2O)2]·H2O (1), [Zn(H2L)] (2), and [Zn(H3L)2(H2O)2]·3H2O (3), have been hydrothermally synthesized based on 1-hydroxy-2-(imidazo(1,2-a)pyridin-3-yl)ethylidene-1,1-diphos phonic acid (H4L). Single-crystal X-ray diffraction reveal that compounds 1-3 are binuclear cluster, 2D polar sandwich-like framework, and mononuclear unit, respectively. Compounds 1-2 exhibit remarkable capabilities to detect the trace of TNP rapidly and quantitatively through luminescent quenching. It is worth to note that compounds 1-2 can be readily recovered and reused. On the other hand, compound 1 features dual emissions which facilitates it to be served as an useful self-calibrated ratiometric luminescent thermometer in temperature range of 77-243 K, as well as a more reliable and sensitive intensity-based luminescent thermometer in temperature range of 10-60 °C. Furthermore, compound 1 has good stability under simulated physiological condition, indicating that it can be potentially utilized as a luminescent thermometer for biological applications.

INTRODUCTION Simple, rapid, and sensitive detection of nitroaromatic explosives has become one of serious issues concerning the safety of people, national security, and environmental protection.1-9 Among these nitroaromatic explosives, 2,4,6-trinitrophenol (TNP) is an extremely hazardous chemical with high explosive power and low safety coefficient.10-11 In spite of this, TNP is widely used in the manufacture of rocket fuels, fireworks, matches, dyes, and pharmaceutical industries. Moreover, TNP has also been recognized as a toxic pollutant. During commercial production and use, the leakage of TNP can directly contaminate soil and aquatic biosystems, giving rise to the damage of respiratory system, anemia, skin irritation, male infertility, and abnormal liver functions. Therefore, it is desirable to develop sensitive and efficient analytical methods for the detection of TNP. In this regard, luminescence-based detection is gaining great attention owing to its high sensitivity, simplicity, short response time, low cost, and portability.8,12-19 These luminescent sensory materials are 2


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mainly focused on conjugated organic molecules, covalent-organic polymers, and metal-organic frameworks (MOFs). On the other hand, temperature is a fundamental thermodynamic parameter in numerous fields of science and technology, whose measurement plays a crucial role in scientific research and industrial production. During the past four years, self-calibrated ratiometric luminescent MOFs thermometers have attracted much attention because of their high sensitivity, fast response, and excellent spatial resolution.20-24 Furthermore, such thermometers are able to work in strong electro or magnetic fields and with quickly moving objects. However, these ratiometric luminescent MOFs thermometers are limited to Eu3+/Tb3+ mixed-lanthanide MOFs and a porous lanthanide MOF containing toxic perylene. Given the above consideration, it is necessary to explore new inexpensive luminescent sensors with simple component, reusability, and thermal stability. Such luminescent sensors can fulfill highly sensitive and quantitative detection of TNP, as well as the measurement of temperature. Herein, we report the synthesis of three new luminescent zinc phosphonates, [Zn(H2L)(H2O)2]·H2O (1), [Zn(H2L)] (2), and [Zn(H3L)2(H2O)2]·3H2O (3) (H4L = 1-hydroxy-2-(imidazo(1,2-a)pyridin-3-yl)ethyli dene-1,1-diphosphonic acid, Scheme 1). Solids 1-2 exhibit remarkable capabilities to detect the trace of TNP rapidly and quantitatively through luminescent quenching. Notably, solids 1-2 can be readily recovered and reused. On the other hand, solid 1 features dual emissions which facilitates it to be served as an useful self-calibrated ratiometric luminescent thermometer in temperature range of 77-243 K, as well as a more reliable and sensitive intensity-based luminescent thermometer in temperature range of 10-60 °C. Furthermore, solid 1 has good stability under simulated physiological condition, indicating that it can be potentially utilized as a luminescent thermometer for biological applications.

EXPERIMENTAL SECTION General. All reagents were purchased from commercial sources and used without further purification. Elemental analyses were conducted with a Vario EL III element analyzer. Infrared spectra were obtained on a VERTEX 70 FT-IR spectrometer. UV/Vis spectroscopy was carried out with a Lambda35 spectrophotometer and a Lambda900 spectrophotometer in the range of 250-600 nm. Luminescent emission and excitation spectra were recorded in solid state at room temperature with a model F-7000 fluorescence spectrophotometer. Temperature-dependent emission spectra were investigated in solid state with an Edinburgh model FLS980 fluorescence spectrometer. While the lifetime and the quantum yield were investigated in solid state with an Edinburgh model FLS920 fluorescence spectrometer. The quantum yield was measured according to the published method.25 Thermogravimetric analysis 3



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(TGA) was performed on a Netzsch STA449C instrument at a heating rate of 10 °C ·min-1 from room temperature to 1000 °C under an air gas flow. Powder X-ray diffraction (PXRD) patterns were acquired on a DMAX-2500 and a Ultima IV diffractometers using Cu-Kα radiation in an ambient environment. A SEM image was taken on a JSM-6700F scanning electron microscope. Synthesis of [Zn(H2L)(H2O)2]·H2O (1). A mixture of Zn(CH3COO)2·2H2O (0.0451 g, 0.205 mmol) and H4L·H2O (0.0905 g, 0.266 mmol) in 6.0 mL of distilled water with pH value adjusted to around 5.4, was sealed in a Parr Teflon-lined autoclave (23 mL) and heated at 110 °C for 20 h. After being slowly cooled to room temperature, colorless crystals were obtained as a homogeneous phase based on PXRD patterns. Yield: 0.0290 g (32 %). Anal. Calcd. for C9H16N2O10P2Zn: C 24.59, H 3.67, N 6.37 %. Found: C 24.66, H 3.76, N 6.48 %. IR (KBr pellet, cm-1): 3433s, 2967w, 2919w, 2850w, 2357w, 1653m, 1527m, 1464m, 1385m, 1260m, 1151m, 1125m, 1080m, 1017m, 883w, 802m. Synthesis of [Zn(H2L)] (2). A mixture of Zn(CH3COO)2·2H2O (0.0393 g, 0.179 mmol) and H4L·H2O (0.0344 g, 0.101 mmol) in 6.0 mL of distilled water with pH value adjusted to around 2.3, was sealed in a Parr Teflon-lined autoclave (23 mL) and heated at 140 °C for 30 h. After being slowly cooled to room temperature, colorless crystals were obtained as a homogeneous phase based on PXRD patterns. Yield: 0.0100 g (26 %). Anal. Calcd. for C9H10N2O7P2Zn: C 28.04, H 2.61, N 7.27 %. Found: C 27.75, H 2.80, N 7.26 %. IR (KBr pellet, cm-1): 3380m, 3110w, 2980w, 2928w, 2361w, 1654m, 1559w, 1528m, 1428m, 1340w, 1216m, 1151s, 1115s, 1070s, 1020m, 1006m, 897m, 808m, 756m, 686m, 598m. Synthesis of [Zn(H3L)2(H2O)2]·3H2O (3). A mixture of Zn(CH3COO)2·2H2O (0.0210 g, 0.0957 mmol) and H4L·H2O (0.0354 g, 0.104 mmol) in 6.0 mL of distilled water with pH value adjusted to around 1.5, was sealed in a Parr Teflon-lined autoclave (23 mL) and heated at 115 °C for 40 h. After being slowly cooled to room temperature, colorless crystals were obtained as a homogeneous phase based on PXRD patterns. Yield: 0.0296 g (71 %). Anal. Calcd. for C18H32N4O19P4Zn: C 27.10, H 4.04, N 7.02 %. Found: C 27.25, H 4.03, N 7.07 %. IR (KBr pellet, cm-1): 3376m, 3200m, 2960w, 2923w, 2859w, 2808w, 2338w, 1654m, 1559w, 1527m, 1466m, 1406m, 1263w, 1243w, 1141s, 1099s, 1066s, 990m, 906m, 751m, 694m, 583m, 548m, 518w, 459m. Heating Treatment: Solid 1-60 represents that polycrystalline of 1 is previously heated at 60 °C for 2 h under an air atmosphere, and then naturally cooled to room temperature. Caution! TNP is highly explosive and should be handled carefully in small amounts with necessary precautions.

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Luminescent quenching experiment. Solids 1-2 behave stable in ethanol. Moreover, ethanol has no significant absorption band around 275 nm. Therefore, ethanol is selected as the solvent. In typical experimental setup, a fine grinding sample of solid 1 (47.6 mg, Figure S1) and 2.00 mL ethanol were mixed in quartz cuvette to form a suspension. The luminescence in 300-550 nm range via excitation at 275 nm was measured in-situ after the incremental addition of freshly prepared TNP solution. The quartz cuvette was shaken thoroughly before each luminescent measurement. Each measurement was performed twice, and the average value was used. X-Ray crystallography. Single-crystal X-ray diffraction data for compounds 1-3 were collected at 293(2) K on a Rigaku Mercury CCD/AFC diffractometer using graphite-monochromated Mo Kα radiation (λ(Mo-Kα) = 0.71073 Å). Data of compounds 1-3 were reduced with CrystalClear version 1.3. Their structures were determined by direct methods and refined by full-matrix least-squares techniques on F2 using SHELXTL-97.26 All non-hydrogen atoms were treated anisotropically. Hydrogen atoms were generated geometrically. Crystallographic data for compounds 1-3 are summarized in Table 1. Selected bond lengths and angles for compounds 1-3 are listed in Tables 2-4, respectively. CCDC 1440034 (1), 1440035 (2) and 1440036 (3).

RESULSTS AND DISCUSSION Synthesis and Characterization. Compounds 1-3 were hydrothermally synthesized by the reaction of Zn(CH3COO)2·2H2O and H4L under different pH values. PXRD patterns of compounds 1-3 are all in agreement with those of simulated single-crystal X-ray data, respectively (Figures S2-4). Furthermore, elemental analyses of compounds 1-3 accord with respective calculated values. These results suggest that final products of solids 1-3 are all in the homogeneous phase. Structural descriptions. Single-crystal X-ray diffraction reveals that compound 1 crystallizes in monoclinic space group P2(1)/c (Figure 1a). The Zn1 atom is coordinated by three phosphonate oxygen atoms from two different H2L2- anions and two water molecules (O8, O9), resulting in a distorted trigonal-bipyramidal coordination geometry based on τ value (0.81). One H2L2- anion chelates to Zn1 through two phosphonate oxygen atoms (O1, O4) to form a six-member ring (Zn-O-P-C-P-O). The other H2L2- anion is interacted with Zn1 through one other phosphonate oxygen atom (O3a). The Zn-O bond lengths are in the range of 1.950(2)-2.242(3) Å, which are comparable with those of reported zinc phosphonates.27-29 On the other hand, the H2L2- anion exhibits a tridentate mode to combine two Zn(II) atoms through three phosphonate oxygen atoms. Neighboring two 5



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Zn atoms are bridged by two O-P-O groups into a Zn2 binuclear cluster (Figures S5a-b). Such Zn2 binuclear clusters are further packed into a two-dimensional structure through offset face-to-face π-π stacking interactions between H2L2- anions (Figures 1b, S5c).30 The separated distance is about 3.46 Å. As a result, H2L2- anions are arranged into columns through strong π-π stacking interactions. Furthermore, such Zn2 binuclear clusters are linked into a three-dimensional structure through hydrogen bondings (Figure S5d, Table S1). Compound 2 crystallizes in noncentral space group Pca2(1). The asymmetric unit contains one Zn(II) atom and one H2L2- anion (Figure 2a). The Zn1 atom is in a distorted [ZnO4] tetrahedral coordination geometry, which is defined by four phosphonate oxygen atoms from three different H2L2- anions. Same to that in compound 1, one H2L2- anion chelates to Zn1 through two phosphonate oxygen atoms (O3b, O4b) to form a six-member ring (Zn-O-P-C-P-O). The other two H2L2- anions are interacted with Zn1 through another two phosphonate oxygen atoms (O1a, O2), respectively. The Zn-O bond lengths are in the range of 1.911(4)-1.978(4) Å, which are also comparable with those of reported zinc phosphonates.27-29 On the other hand, the H2L2- anion exhibits a tetradentate mode to combine three Zn(II) atoms through four phosphonate oxygen atoms. Each [ZnO4] tetrahedron shares four corners with surrounding four [PCO3] tetrahedra, resulting in a Zn-O-P layer (Figures 2b, S6a). Organic groups of H2L2- anions hang on two sides of the Zn-O-P layer to form a 2D polar sandwich-like framework (Figure S6b). Such 2D sandwich-like frameworks are further linked into a three-dimensional structure through hydrogen bondings (Figure 2c). Different from compounds 1-2, compound 3 was obtained under strong acidic condition. Since only one hydrogen atom of H4L is deprotonated, the structure of compound 3 is rather simple. In compound 3, the Zn(II) atom is in a [ZnO6] octahedral coordination geometry (Figure 3), which is different from the distorted trigonal-bipyramidal geometry in compound 1 and the distorted tetrahedral coordination geometry in compound 2. The Zn1 atom is surrounded by two H3Lanions and two coordinated water molecules into a butterfly-like mononuclear unit. The interaction between neighboring mononuclear units is through offset face-to-face π-π stacking interactions between H3L- anions (Figure S7). The separated distance is about 3.54 Å, which is longer than that in compound 1. On the other hand, the H3Lanion exhibits a bidentate mode to chelate one Zn(II) atom through two phosphonate oxygen atoms. This mode is obviously different from the tridentate mode in compound 1 and the tetradentate mode in compound 2. In addition, the protonated nitrogen atom (N1), the protonated phosphonate oxygen atom (O2, O6), hydroxyl (O7), and water molecules (O8, O9, O10) provide hydrogen atoms to form hydrogen bondings with neighboring phosphonate oxygen atoms and water molecules (Table 6


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S2). Thermal Stabilities. TGA were carried out under an air atmosphere to examine thermal stabilities of solids 1-3 (Figure S8). TGA curve of solid 1 illustrates that little weight loss appears up to 100 °C. This result accords with the fact that the free water molecule participates in the formation of hydrogen bondings. When the sample is heated further, there is a stage in temperature range of 100-170 °C with a 11.9% weight loss, which is attributed to the loss of one free water molecule and two coordinated water molecules per formula (calculated value of 12.3%). This suggests that the decomposition of solid 1 starts at 100 °C. Furthermore, after solid 1 was heated at 60 °C for two hours under an air atmosphere, PXRD patterns are essentially in agreement with those of simulated from single-crystal X-ray diffraction data (Figure S9). This result indicates that solid 1 is stable up to 60 °C under an air atmosphere. TGA curve of solid 2 shows that there is little weight loss up to 150 °C. Upon further heating, weight loss stages appear corresponding to the decomposition of H2L2- anion. This denotes that the polar sandwich-like framework of solid 2 begins to collapse at 150 °C. Different from those of compounds 1-2, TGA curve of solid 3 reveals that weight loss stages start from 37 °C. This suggests that the degradation of solid 3 occurs at 37 °C. Luminescent Properties. Solid 1 displays bright purple luminescence with a broad emission profile via excitation at 276 nm. (Figures 4, S10). The broad profile contains a peak band at 352 nm and a shoulder peak at 428 nm. The excitation profile with emission fixed at 352 nm is similar to that fixed at 428 nm (Figure S11). The lifetime of λem = 428 nm is 3.8(1) ns (Figure S12). While both solids 2-3 display strong ultraviolet (UV) luminescence with narrow emission profiles (Figures S13-14). Their emission profiles contain maximal bands at 349 and 342 nm, respectively. Because UV emission and excitation profiles of solids 1-3 are similar to those of H4L·H2O (Figure S15), the strong UV emission of solids 1-3 can be assigned to ligand-centered π-π* transition. On the other hand, the emission of solid 1 in purple region may be attributed to stacked columns of H2L2- anions.31-32 External quantum yields of solids 1-3 are found to be 17.2, 14.0, and 16.0 %, respectively. This result makes these zinc phosphonates potential candidates as luminescence-based sensory materials. Luminescent Sensing of TNP. Luminescent properties of solid 1 dispersed in different solvents are investigated (Figure 5). The feature is that luminescent intensities are dependent on solvent molecules. Ethanol is selected as the solvent based on the following reasons. Firstly, solids 1-2 behave stable in ethanol. Secondly, ethanol has no significant absorption band around 275 nm. This is different from acetone which shows an absorption band range from 250 to 330 nm.33 As a result, the luminescence of solid 1 is almost quenched when solid 1 is dispersed in acetone. This 7



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is due to the fact that the excited light is almost absorbed by acetone molecules. The resonance energy transfer has been recognized in fluorescent quenching where the emission spectrum of sensory material overlaps with the absorption spectrum of analyte.3,13,34 As shown in Figure S16, the UV-Vis absorption spectrum of TNP shows a strong absorption band at 359 nm. It is worth to note that the absorption spectrum of TNP has an effective overlap with both luminescent emission spectra of solids 1-2. Moreover, both solids 1-2 behave thermal stable up to 60 °C under an air atmosphere. Therefore, in order to explore potential application of solids 1-2 in the detection of TNP, luminescence-quenching titrations are performed by gradual addition of TNP to a mixture of solid 1 or 2 dispersed in ethanol. As anticipated, a prominent quenching is observed upon the addition of TNP (Figure 6). For example, the quenching efficiency of λem = 370 nm for solid 1 reaches 35% when the concentration of TNP is only 2.90 µM (about 665 ppb). And the quenching efficiency of λem = 370 nm increases quickly up to 95% when the concentration of TNP reaches 87.7 µM. It is interesting that the color of the corresponding solution is turned into yellow, which can be easily seen by naked eyes. In addition, luminescent quenching can be clearly observed when the concentration of TNP is as low as 143 ppb (Figure S17). This indicates that solid 1 shows high sensitivity for the detection of TNP. Notably, the absorption spectrum of TNP covers from 290 to 460 nm, which has a large overlap with emission spectra of solids 1-2. Accordingly, profiles of luminescent quenching efficiency at different wavelengths for solids 1-2 are similar to the absorption spectrum of TNP (Figures S18-19). Since luminescent emission in the wavelength range of 480-550 nm has been little affected by TNP,34 peak bands of their emissions are shifted to around 480 nm upon the addition of TNP. And except for intensities, luminescent excitation spectra almost remain unchanged upon the addition of TNP (Figures S20-21). On the other hand, single-crystal X-ray diffraction reveal that there are no porosities in solids 1-2, which reduce their abilities to encapsulate TNP. These results indicate that the resonance energy transfer through light absorption by TNP is the main source for the observed luminescent quenching in our experiments. According to previous literature,35 the quenching effect can be calculated quantitatively by the Stern–Volmer equation (I0/I) = Ksv [M] + 1, where I0 and I are luminescent intensities before and after the addition of the analyte, [M] is the molar concentration of the analyte, and Ksv is the quenching coefficient (M-1). At low TNP concentrations, the linear Stern-Volmer (SV) relationship with Ksv values of 2.0 × 105 M-1 (for solid 1) and 9.9 × 104 M-1 (for solid 2) indicate static quenching through absorption (Figures S22-23).12-17 Notably, the Ksv value of solid 1 is comparable to 2.6 × 105 M-1 of a covalent-organic polymer sensor.12 With respect to the larger Ksv value of solid 1, there are two reasons. Firstly, the maximum emission band of solid 1 is 8


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closer to the peak absorption band of TNP. Secondly, the external quantum yield of solid 1 is larger than that of solid 2. Inspired by the fact that the intensity ratio of two emissions is usually used as a thermometric parameter in self-calibrated ratiometric luminescent temperature sensing, we suggest that the intensity ratio of two emissions can also be used as a parameter to measure the concentration of TNP. For solid 1, this parameter can be defined by the intensity ratio of λem=480 nm to λem=370 nm. In concentration range of 0-87.7 µM, there is a very good linear relationship between the I480/I370 ratio and the concentration of TNP (Figure 7a), which can be fitted as a function of [TNP] = -5.92 + 12.47 I480/I370, with correlation coefficient 0.9982, where I480 and I370 are luminescent intensities of λem=480 nm and λem=370 nm, respectively, [TNP] is the concentration of TNP (µM). And for solid 2, in concentration range of 0-73.6 µM, the emission intensity of λem=482 nm decreases slowly upon adding TNP, while that of λem=365 nm decreases quickly. As shown in Figure 7b, the I482/I365 ratio can be linearly correlated to the concentration of TNP as a function of [TNP] = -14.1 + 40.66 I482/I365, with correlation coefficient 0.9998, where I482 and I365 are luminescent intensities of λem=482 nm and λem=365 nm, respectively, [TNP] is the concentration of TNP (µM). It is worth to note that solids 1-2 can be easily recovered by filtration and washed with ethanol. PXRD patterns and luminescent spectra are same to those of as-synthesized 1-2 (Figures S24-28), respectively, indicating that solids 1-2 can be reused. Furthermore, quenching efficiencies of every cycle are almost unchanged above 95% through monitoring emission spectra of solid 1 dispersed in the presence of 93 ppm TNP in ethanol from cycle 1 to 4 (Figure S29). These results reveal that solids 1-2 are promising luminescent probes for reusable, highly sensitive, and quantitative detection of TNP at low concentration. To compare the luminescent sensing of solid 1 toward TNP, luminescence-quenching titrations are also performed by gradual addition of other nitroaromatic explosive compounds to a mixture of solid 1 dispersed in ethanol (Figures S30-35). These nitroaromatic explosive compounds include 2,4-dinitrophenol (DNP), p-nitrophenol (PNP), 2,4,6-trinitrotoluene (TNT), 2,6-dinitrotoluene (2,6-DNT), 4-nitrotoluene (PNT), and nitrobenzene (NB). Quenching coefficients of DNP, PNP, TNT, 2,6-DNT, PNT, and NB are found to be 3.5 × 104, 1.4 × 104, 3.2 × 103, 3.4 × 103, 3.8 × 103, and 1.7 × 103 M-1, respectively. It is well known that the resonance energy transfer can significantly improve the efficiency and sensitivity of fluorescence quenching. The higher quenching coefficient of DNP is due to the fact that the UV-Vis absorption spectrum of DNP has a large overlap with the luminescent emission spectrum of solid 1 (Figure S36). Accordingly, the peak band of emission is shifted to around 462 nm upon the addition 9



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of DNP. In contrast, UV-Vis absorption spectra of PNP, TNT, 2,6-DNT, PNT, and NB have little overlap with the luminescent emission spectrum of solid 1. As a result, quenching coefficients of PNP, TNT, 2,6-DNT, PNT, and NB are lower than that of DNP. And peak bands of these emissions are shifted to around 430 nm upon additions of PNP, TNT, 2,6-DNT, PNT, and NB. The selective detection of TNP is desirable for practical applications. As shown in Figure 8, the quenching coefficient of TNP is nearly 5, 13, 61, 58, 52, and 116 times greater than those of DNP, PNP, TNT, 2,6-DNT, PNT, and NB, respectively. Inspired by this result, the selective detection of TNP in the presence of other nitroaromatic explosive compounds is investigated. Firstly, the luminescent spectrum for solid 1 dispersed in ethanol is recorded, to this is added TNT, 2,6-DNT, PNT, and NB until concentrations of these four nitroaromatic explosive compounds are in a range of 2.70-4.91 µM, and then the corresponding emission spectrum is monitored. The addition of these four nitroaromatic explosive compounds shows slight effect on luminescent intensity (Figure 9). Secondly, to this solution is added TNP until the concentration of TNP is 3.45 µM, which is equal to those of four nitroaromatic explosive compounds. As a result, luminescent intensity is obviously weakened with 38.0% quenching efficiency. Finally, the quenching efficiency increases quickly upon further addition of TNP. Likewise, similar experiments are performed upon the addition of PNP or DNP followed by TNP to a mixture of solid 1 dispersed in ethanol (Figures S37-38). The addition of PNP or DNP shows little effect on luminescent intensity, whereas the addition of TNP give significant luminescent quenching. The above results suggest that the selective detection of TNP in the presence of other nitroaromatic explosive compounds makes solid 1 as a reliable sensor for TNP.10 Luminescent Sensing of Temperature. To assess solid 1 as a potential self-calibrated ratiometric thermometer, temperature-dependent luminescence are investigated. In temperature of 77-150 K, luminescent intensity of λem=347 nm decreases gradually with increasing temperature (Figure S39), which is normally due to the thermal of nonradiative-decay pathways.36,37 In contrast, luminescent intensity of λem=436 nm is almost constant from 77 to 150 K. That is to say, compound 1 exhibits a different temperature-dependent luminescent behaviour with respect to two emissions of λem=347 nm and λem=436 nm. Thus, the intensity ratio of λem=347 nm to λem=436 nm can be served as a thermometric parameter. In temperature range of 77-150 K, temperature-dependence of the I347/I436 ratio is depicted in Figure 10a. The I347/I436 ratio can be linearly correlated to absolute temperature as a function of T = 436.83 - 362.49 I347/I436, with correlation coefficient -0.9908, where I347 and I436 are luminescent intensities of λem=347 and λem=436 nm, respectively, and T is the temperature (K). This result demonstrates that solid 1 is a good self-calibrated ratiometric luminescent thermometer in temperature range of 77-150 K. According to 10


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previous definition,38-40 the maximum sensitivity is calculated to be 0.34 % K-1 at 150 K, which is higher than those of Tb0.99Eu0.01(BDC)1.5(H2O)2, [Eu0.7Tb0.3(cam) (Himdc)2(H2O)2]3, and [EuxTb1-x(L)2(COO)(H2O)2]·H2O (x = 0.1, 0.2, 0.3; H2L = 1,3-bis(4-carboxyphenyl)imidazolium).22,40,41 On the other hand, normalized intensity of λem=347 nm decreases linearly upon increasing temperature from 77 to 150 K (Figure 10b). The good linear relationship between normalized intensity of λem=347 nm and temperature can be fitted as a function of T = 470.78 - 391.79 I347, with correlation coefficient -0.9926, where I347 is normalized intensity of λem=347 nm, and T is the temperature (K). The maximum sensitivity is calculated to be 0.31 % K-1 at 150 K. The above results indicate that under cryogenic temperature (77-150 K), solid 1 can be served as not only a intensity-based luminescent thermometer, but also a self-calibrated ratiometric luminescent thermometer. To the best of our knowledge, this has not been reported in previous work. In temperature of 150-243 K, luminescent intensity of λem=347 nm still decreases gradually as the temperature increases (Figure S40). In contrast, luminescent intensity of λem=436 nm increases gradually with increasing temperature. That is to say, compound 1 exhibits a significantly different temperature-dependent luminescent behaviour with respect to two emissions of λem=347 nm and λem=436 nm. In temperature range of 150-243 K, temperature-dependence of the I347/I436 ratio is depicted in Figure 11. The I347/I436 ratio can also be linearly correlated to absolute temperature as a function of T = 774.71 - 621.12 I347/I436, with correlation coefficient -0.9916, where I347 and I436 are luminescent intensities of λem=347 nm and λem=436 nm, respectively, and T is the temperature (K). The maximum sensitivity is calculated to be 0.19 % K-1 at 243 K. This value is higher than those of [Eu0.7Tb0.3(cam)(Himdc)2(H2O)2]3, and [EuxTb1-x (L)2(COO)(H2O)2]·H2O (x = 0.1, 0.2, 0.3; H2L = 1,3-bis(4-carboxyphenyl)imidazolium).22,41 This result suggests that solid 1 is also a good self-calibrated ratiometric luminescent thermometer under low temperature (150-243 K). Encouraged by the above results, temperature-dependence of luminescent intensity for solid 1 is further investigated from 10 up to 60 oC (Figure 12). Both luminescent intensities of λem=347 nm and λem=436 nm decrease gradually upon increasing temperature, which are normally due to the thermal of nonradiative-decay pathways.36,37 It is interesting that both normalized intensities of λem=347 nm and λem=436 nm can be linearly correlated to absolute temperature as a functions of T = 195.75 - 186.92 I347 and T = 155.13 - 147.06 I436, respectively, with respective correlation coefficients -0.9957 and -0.9955, where I347 and I436 are normalized luminescent intensities of λem=347 nm and λem=436 nm, respectively, and T is the temperature (oC). The maximum sensitivities are calculated to be 0.74 % oC-1 for

11



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λem=347 nm and 1.04 % oC-1 for λem=436 nm at 60 oC. The sensitivity for λem=436 nm is close to 1.28% oC-1 for ZJU-88⊃ ⊃perylene,21 which is a complicated complex containing toxic perylene. Thus, solid 1 is shown to be excellent for sensing temperature based on temperature-dependent luminescent intensity changes of two emissions, which has rarely reported in previous work.25,30-32 This result indicates that solid 1 is a more reliable and sensitive intensity-based luminescent thermometer in temperature range of 10-60 °C. Since solid 1 is stable up to 60 °C under an air atmosphere, we further investigate its stability under simulated physiological condition in order to confirm its potential for biological applications. Solid 1 (15.2 mg) was immersed into the phosphate buffered saline (PBS) solution (20.0 mL) for 96 h. After soaked in PBS solution, PXRD patterns are in agreement with those of as-synthesized 1 (Figure S41). This result indicates that solid 1 behaves stable under simulated physiological condition. Different from ZJU-88⊃ ⊃perylene containing toxic perylene, solid 1 contains minodronic acid and Zn(II) atom. The minodronic acid is a third-generation biphosphonate which has been developed for the treatment of osteoporosis. While zinc is an important trace element in the body. These results suggest that solid 1 can be potentially utilized as a luminescent thermometer for biological applications.

CONCLUSION In summary, three luminescent zinc phosphonates with mononuclear unit, binuclear cluster, and 2D polar sandwich-like framework, have been described. [Zn(H2L)(H2O)2]·H2O and [Zn(H2L)] exhibit remarkable capabilities to detect the trace of TNP rapidly and quantitatively through luminescent quenching. It is worth to note that [Zn(H2L)(H2O)2]·H2O and [Zn(H2L)] can be readily recovered and reused. On the other hand, [Zn(H2L)(H2O)2]·H2O features dual emissions which facilitates it to be used as an useful self-calibrated ratiometric luminescent thermometer in temperature range of 77-243 K. Likewise, [Zn(H2L)(H2O)2]·H2O can be served as a more reliable and sensitive intensity-based luminescent thermometer in temperature range of 10-60 °C. Furthermore, [Zn(H2L)(H2O)2]·H2O behaves stable under simulated physiological condition, indicating that [Zn(H2L)(H2O)2]·H2O can be potentially utilized as a luminescent thermometer for biological applications. Future efforts will be focused on synthesizing new metal phosphonates with multiple emissions based on interesting phosphonic acids and exploring their practical application as sensory materials.

ASSOCIATED CONTENT 12


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Supporting Information Crystallographic data in CIF format (CCDC 1440034 (1), 1440035 (2) and 1440036 (3)), SEM image, PXRD patterns, TGA curves, UV-Vis absorption spectra, as well as additional tables, structural figures and luminescent plots. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (R. B. Fu) Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This research was supported by grants from the National Science Foundation of China (21373219 and 21233009), the National Basic Research Program of China (973 Program, 2014CB845603, and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20000000).

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(44) Sun, L. N.; Yu, J. B.; Peng, H. S.; Zhang, J. Z.; Shi, L. Y.; Wolfbeis, O. S. J. Phys. Chem. C 2010, 114, 12642-12648.

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For Table of Contents Use Only

Luminescent Zinc Phosphonates for Ratiometric Sensing of 2,4,6-Trinitrophenol and Temperature Ruibiao Fu*, Shengmin Hu and Xintao Wu State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Science, Fuzhou, Fujian, 350002, China *Corresponding author. E-mail: [email protected] Tel: +86-591-63179449

TNP

300

350

400

450

Wavelength (nm)

500

550

300

350

400

450

500

550

Wavelength (nm)

A new luminescent zinc phosphonate is a promising luminescent probe for reusable, highly sensitive (as low as 143 ppb), and quantitative detection of 2,4,6-trinitrophenol (TNP).

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Table 1. Crystal Data and Refinement Details for Compounds 1-3 Compounds Formula FW Space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z T(K) Measured/unique/obs erved reflections Dcalcd (g cm3) µ (mm-1) GOF on F2 Rint R1a [I>2σ(I)] wR2b [all data] a

1 C9H16N2O10P2Zn 439.55 P2(1)/c 7.773(9) 14.606(15) 12.971(18) 90 106.910(17) 90 1409(3) 4 293(2) 10609/3189/3012

2 C9H10N2O7P2Zn 385.50 Pca2(1) 25.482(6) 5.394(3) 8.992(5) 90 90 90 1236.0(11) 4 293(2) 8997/2808/2654

3 C18H32N4O19P4Zn 797.43 C2/c 19.314(15) 9.707(9) 16.675(13) 90 107.715(6) 90 2978(4) 4 293(2) 11488/3348/3064

2.072 2.032 1.052 0.0279 0.0301 0.0821

2.072 2.284 1.083 0.0319 0.0514 0.1303

1.779 1.131 1.102 0.0230 0.0431 0.1397

R1 = ∑(||Fo| - |Fc||) / ∑ |Fo|. b wR2 = {∑w [(F2o − F2c)] / ∑w [(F 2o ) 2]}0.5

Table 2. Selected Bond Lengths (Å) and Angles (deg) for 1 Zn(1)-O(1) Zn(1)-O(3)a Zn(1)-O(4)

1.950(2) 1.963(2) 2.086(2)

O(1)-Zn(1)-O(3)a 124.67(7) O(1)-Zn(1)-O(4) 94.30(12) O(1)-Zn(1)-O(8) 80.63(12) O(1)-Zn(1)-O(9) 122.21(10) a O(3) -Zn(1)-O(4) 98.43(7) Symmetry code: a - x, - y + 2, - z.

Zn(1)-O(8) Zn(1)-O(9)

2.242(3) 1.990(2)

O(3)a-Zn(1)-O(8) O(3)a-Zn(1)-O(9) O(4)-Zn(1)-O(8) O(4)-Zn(1)-O(9) O(8)-Zn(1)-O(9)

87.85(7) 111.05(10) 173.49(6) 91.51(11) 87.85(7)

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Table 3. Selected Bond Lengths (Å) and Angles (deg) for 2 Zn(1)-O(1)a Zn(1)-O(2)

1.946(4) 1.911(4)

Zn(1)-O(3)b Zn(1)-O(4)b

O(1)a-Zn(1)-O(2) 120.41(18) O(2)-Zn(1)-O(3)b a b 104.07(18) O(2)-Zn(1)-O(4)b O(1) -Zn(1)-O(3) O(1)a-Zn(1)-O(4)b 110.21(18) O(3)b-Zn(1)-O(4)b Symmetry codes: a x, y + 1, z; b - x + 1, - y, z + 1/2.

1.952(4) 1.978(4) 114.71(19) 106.52(19) 98.83(17)

Table 4. Selected Bond Lengths (Å) and Angles (deg) for 3 Zn(1)-O(1) Zn(1)-O(1)a Zn(1)-O(4)

2.092(3) 2.092(3) 2.110((2)

O(1)-Zn(1)-O(1)a 176.36(12) O(1)-Zn(1)-O(4) 89.03(10) O(1)-Zn(1)-O(4)a 93.62(10) O(1)-Zn(1)-O(8) 89.48(13) O(1)-Zn(1)-O(8)a 87.84(12) a O(1) -Zn(1)-O(4) 93.62(10) a a O(1) -Zn(1)-O(4) 89.03(10) O(1)a-Zn(1)-O(8) 87.84(12) Symmetry code: a - x, y, - z + 1/2.

Zn(1)-O(4)a Zn(1)-O(8) Zn(1)-O(8)a

2.110((2) 2.094(3) 2.094(3)

O(1)a-Zn(1)-O(8)a O(4)-Zn(1)-O(4)a O(4)-Zn(1)-O(8) O(4)-Zn(1)-O(8)a O(4)a-Zn(1)-O(8) O(4)a-Zn(1)-O(8)a O(8)-Zn(1)-O(8)a

89.48(13) 86.85(13) 94.10(14) 176.78(11) 176.78(11) 94.10(14) 85.1(2)

Scheme 1 Structure of H4L. 18


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(a)

(b)

Figure 1. (a) Ball-stick view of the coordination environment of Zn(II) atom and the coordination mode of H2L2- anion, (b) polyhedral view of the two-dimensional structure in 1. [ZnO5]: trigonal-bipyramid; [PCO3]: yellow tetrahedron. Blue-green lines represent hydrogen bondings. Symmetry codes: a - x, - y + 2, - z; b - x, - y + 1, z. c x + 1, y, z; d - x, y - 1/2, - z + 1/2; e - x, y + 1/2, - z - 1/2; f x, - y + 3/2, z + 1/2.

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(a)

(b)

(c)

Figure 2. (a) Ball-stick view of the coordination environment of Zn(II) atom and the coordination mode of H2L2- anion, polyhedral views of the Zn-O-P layer (b) and the three-dimensional structure (c) in 2. [ZnO4]: green tetrahedron; [PCO3]: yellow tetrahedron. Blue-green lines represent hydrogen bondings. Symmetry codes: a x, y + 1, z; b - x + 1, - y, - z + 1/2. c - x + 1, - y, z - 1/2 ; d x, y - 1, z; e - x + 3/2, y, z + 1/2.

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Figure 3. Ball-stick view of the coordination environment of Zn(II) atom and the coordination mode of H3L- anion in compound 3. Unrelated atoms are omitted for the sake of clarity. Symmetry codes: a - x, y, - z + 1/2; b x, y + 1, z. c - x, - y + 1, - z; d - x - 1/2, - y + 1/2, - z; e x, y - 1, z; f - x, - y, - z.

1 2 3 H4L.H2O

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


300

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500

Wavelength (nm) Figure 4. Luminescent emission spectra of solids 1-3 and H4L·H2O.

21



Ethanol Acetonitrile DMF Chloroform Tetrahydrofuran Acetone

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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500

Wavelength (nm) Figure 5. Luminescent emission spectra of solid 1 dispersed in ethanol, acetonitrile, DMF, chloroform, tetrahydrofuran and acetone via excitation at 275 nm..

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(a)

Intensity (a.u.)

0.00 µM 2.90 µM 5.69 µM 29.0 µM 60.7 µM 87.7 µM 203 µM 322 µM

300

350

400

450

500

550

Wavelength (nm)

(b)

Intensity (a.u.)

0.00 µM 3.67 µM 6.53 µM 12.7 µM 42.1 µM 73.6 µM 270 µM 434 µM

300

350

400

450

500

550

Wavelength (nm)

(c)

100

Quenching efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 40 20 1 2

0 -50

0

50

100 150 200 250 300 350 400 450

Concentration of TNP (µM)

Figure 6. Luminescent emission spectra of solids 1(a) and 2(b) dispersed in ethanol upon the incremental addition of TNP via excitation at 275 nm. c) The trends of quenching efficiency for solids 1-2 dispersed in ethanol upon the incremental addition of TNP via excitation at 275 nm.

23



(a)

8

I480 / I370

6

y = 0.48 + 0.080 x r = 0.9982

4

2

0 0

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80


(b) 2.0

1.5

I482 / I365

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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y = 0.347 + 0.0246 x r = 0.9998 1.0

0.5

0

10

20

30

40

50

60

70

80


Figure 7. (a) Concentration-dependent intensity ratio of λem = 480 nm to λem = 370 nm, and the fitted curve for solid 1 dispersed in ethanol in concentration range of 0-87.7 µM. (b) Concentration-dependent intensity ratio of λem = 482 nm to λem = 365 nm, and the fitted curve for solid 2 dispersed in ethanol in concentration range of 0-73.6 µM.

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TNP DNP PNP TNT 2,6-DNT PNT NB

40

(Io / I) - 1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30

20

10

0 0.0

-4

5.0x10

-3

1.0x10

-3

1.5x10

-3

2.0x10

Concentrations (M) Figure 8. Stern–Volmer plots of solid 1 dispersed in ethanol with the emission wavelength fixed at 370 nm upon incremental additions of different nitroaromatic explosive compounds via excitation at 275 nm.

25



(a)

Intensity (a.u.)

0.00 µM NB 4.91 µM, PNT 4.44 µM 2,6-DNT 3.34 µM, TNT 2.70 µM NB 4.91 µM, PNT 4.44 µM 2,6-DNT 3.34 µM, TNT 2.70 µM TNP 3.45 µM NB 4.91 µM, PNT 4.44 µM 2,6-DNT 3.34 µM, TNT 2.70 µM TNP 34.0 µM NB 4.91 µM, PNT 4.44 µM 2,6-DNT 3.34 µM, TNT 2.70 µM TNP 158 µM

300

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450

500

550

Wavelength (nm)

100

(b) Quenching efficiency (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80

60

40

20

0 NB 4.91 µM PNT 4.44 µM TNT 2.70 µM 2,6-DNT 3.34 µM

TNP 3.45 µM NB 4.91 µM PNT 4.44 µM TNT 2.70 µM 2,6-DNT 3.34 µM

TNP 34.0 µM NB 4.91 µM PNT 4.44 µM TNT 2.70 µM 2,6-DNT 3.34 µM

TNP 158 µM NB 4.91 µM PNT 4.44 µM TNT 2.70 µM 2,6-DNT 3.34 µM

Figure 9. Luminescent emission spectra (a) and the trend of quenching efficiency of

λem = 370 nm (b) for solid 1 dispersed in ethanol upon incremental additions of four nitroaromatic explosive compounds and TNP via excitation at 275 nm.

26


Page 27 of 29

(a)

1.00

y = 1.20 - 0.0027 x r = -0.9908

I347 / I436

0.95

0.90

0.85

0.80 80

90

100

110

120

130

140

150

Temperature (K)

(b)

y = 1.20 - 0.0025 x r = -0.9926

1.00

Normalalized intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.95

0.90

0.85

0.80 80

90

100

110

120

130

140

150

Temperature (K)

Figure 10. (a) Temperature-dependent intensity ratios of λem = 347 nm to λem = 436 nm and the fitted curve in temperature range of 77-150 K. (b) Temperature-dependent normalized intensities of λem=347 nm and the fitted curve in temperature range of 77-150 K.

27



y = 1.25 - 0.00161 x r = -0.9916

1.00

I347 / I436

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 29

0.95

0.90

0.85 140

160

180

200

220

240

Temperature (K) Figure 11. Temperature-dependent intensity ratios of λem = 347 nm to λem = 436 nm and the fitted curve in temperature range of 150-243 K.

28


Page 29 of 29

(a)

o

Intensity (a.u.)

10 C o 20 C o 30 C o 40 C o 50 C o 60 C

300

350

400

450

500

550

Wavelength (nm)

(b)

y = 1.05 - 0.0054 x r = -0.9957

Normalized intensity

1.0

0.9

0.8

0.7 10

20

30

40

50

60

o

Temperature ( C)

(c)

y = 1.05 - 0.0068 x r = -0.9955

1.0

Normalized intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.9

0.8

0.7

10

20

30

40

50

60

o

Temperature ( C)

Figure 12. (a) Luminescent emission spectra of solid 1 recorded from 10 to 60 oC. Temperature-dependent normalized intensities of λem=347 nm (b) and λem=436 nm (c), as well as fitted curves in temperature range of 10-60 oC.

29


Luminescent Zinc Phosphonates for Ratiometric Sensing of 2,4,6

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