A Rare L1D + R1D → 3D Luminescent Dense ... - ACS Publications

May 13, 2014 - Shandong Collegial Key Laboratory of Biotechnology and Utilization of Biological Resources, College of Life Science, Dezhou. University...
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A Rare L1D + R1D → 3D Luminescent Dense Polymer as Multifunctional Sensor to Nitro Aromatic Compounds, Cu2+, and Bases Ling-Yan Pang,† Guo-Ping Yang,† Jun-Cheng Jin,† Meng Kang,† Ai-Yun Fu,†,‡ Yao-Yu Wang,*,† and Qi-Zhen Shi† †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an 710069, China ‡ Shandong Collegial Key Laboratory of Biotechnology and Utilization of Biological Resources, College of Life Science, Dezhou University, Dezhou 253023, China S Supporting Information *

ABSTRACT: A rarely reported L1D (left-handed helical 1D chain) + R1D (right-handed helical 1D chain) → 3D polycatenated network, [Cd(H2ttac)bpp]n (1), constructed from the vertical interpenetration of triple-stranded homochiral helices, has been synthesized under hydrothermal condition by H4ttac (1,1′,2′,1″-terphenyl-4,4′,4″,5′-tetracarboxylic acid), 1,3-bis(4-pyridyl)propane (bpp), and Cd(NO3)2·4H2O. The luminescent properties of 1 and the ones immersed in various kinds of organic compounds and nitrate@DMF solutions have been investigated. Importantly, 1 shows highly sensitive response to nitro aromatic compounds and Cu2+ through luminescence quenching effects, making it a promising luminescent sensor for nitro aromatic compounds and Cu2+. Besides, experiments upon deprotonation by NaOH, KOH, methylamine, and triethylamine are conducted, respectively. As base increases, the luminescence spectra exhibit gradual blue shifts. The mechanisms of the sensing properties have been studied in detail.

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Hence, how to detect nitro aromatic compounds and Cu2+ quickly and quantitatively has become a hot topic for scientists. Though various techniques, such as X-ray dispersion, voltammetry, IMS, and atomic absorption spectroscopy, have been developed, these sophisticated methods are limited by their high cost and low portability, as well as the complicated pretreatment procedures and instruments.5 In contrast, however, fluorimetric methods based on suitable sensors for these analytes are of great interest because of their nondestructive detection, rapid performance, supernal sensitivity, and real-time detection.6 Moreover, the intuitive change of light can provide quick and qualitative analysis even with the naked eye. Among the various fluorimetric methods, luminescent coordination polymers (LCPs) have been proven to be excellent luminescent probes7 for their prominent optical properties, tunable structure, and relatively long emission wavelengths even in the visible region. Especially when active functional groups, such as −NH2, −OH, −CF3, and −COOH are incorporated in the polymers, their applications can be largely improved.7f,22a,b However, the inherently poor stability in aqueous and organic solvents has largely confined their sensing function.8 In consequence, the exploration and

INTRODUCTION

Exploring chemosensors as fast detectors of toxic organic matter or heavy metals is of great importance for public health and environmental protection.1 Nitro aromatic compounds, also called high explosives, have been well-known for their widespread use as significant industrial materials in chemical synthesis, pesticide production, and the manufacture of explosives for several decades, but simultaneously, they are also notorious environmental pollutants, several of which have been declared by the Environment Protection Agency (EPA) for carcinogenicity and high toxicity.2 Improper use of nitro aromatic compounds by inhalation, intake, or cutaneous contact will cause vomiting and coma, to which long-term exposure can even cause respiratory failure. Cu2+ ions, which have been widely applied in electric, light-industry, and machine manufacture and rank third in abundance among heavy metals in the human body, have long been considered important to humans.3 Moderate consumption of Cu2+ ions is indispensable for its participation in many important biological processes, as purine metabolism and promotion of absorption for iron. However, huge uptake or long exposure to Cu2+ will increase the burden of the liver and kidney; may cause diseases in nervous system, digestive system, and blood system; and can also catalyze the oxidation process, resulting in damage to protein, DNA, and organisms.4 Therefore, monitoring Cu2+ ions in an accurate way is of much concern. © 2014 American Chemical Society

Received: February 16, 2014 Revised: April 28, 2014 Published: May 13, 2014 2954

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72.2%. Calcd for C35H26O8Cd1N2 (714.97): C, 58.74; H, 3.64; N, 3.92. Found: C, 58.69; H, 3.58; N, 3.92. IR (KBr, cm−1) (SI Figure S1): 3893vw, 3748vw, 3428vw, 2933m, 2499m, 1934vw, 1716s, 1592vs, 1550vs, 1365vs, 1263s, 1180w, 1099w, 1016m, 921w, 850s, 769m, 715w, 665w, 613w, 520w, 470w.

synthesis of chemically stable, efficient, and sensitive LCP probes are in high demand. Here we report a highly stable LCP, [Cd(H2ttac)bpp]n (1), by reacting 1,1′,2′,1″-terphenyl-4,4′,4″,5′-tetracarboxylic acid (H4ttac), 1,3-bis(4-pyridyl)propane (bpp), and Cd(NO3)2· 4H2O hydrothermally. 1 consists of the vertical interpenetration of left-handed helices and right-handed helices, representing a rare example for the third category of L1D + R1D → 3D networks with polycatenane and interpenetration of triplestranded helical chains, simultaneously. As far as we know, this is the first work for one-fold L1D + R1D → 3D networks currently based on diamine bridging ligand. PXRD experiments prove that 1 possesses high chemical stability in various solvents and nitrate@DMF solutions; meanwhile, TGA experiment of 1 also proves its excellent thermal stability. Most importantly, there are two protonated carboxylic groups in H2ttac2− dangling outside the helices, and these possible active sites may allow the interaction between guest molecules/ions and the framework. Luminescent measurements for 1 show highly sensitive response to nitro aromatic compounds and Cu2+ through fluorescence quenching effects as well as significant blue shifts upon the introduction of bases, which makes it a promising luminescence sensor. It should be noted that LCPs reported for guest sensing often incorporate pores, channels, or large cages in the structure, while 1 takes on the dense stacking mode without solvent accessible volume. Consequently, this may represent a new approach to develop LCPs serving as luminescent sensor with high stability and less structure limits.

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RESULTS AND DISCUSSION Structural Description. Single crystal X-ray diffraction analysis reveals that 1 crystallizes in tetragonal space group of I4̅ 2d and in the asymmetric unit there exists one Cd2+ ion, one H2ttac2−, and one bpp ligand. Cd2+ ion resides in the center of a distorted octahedron defined by four oxygen atoms (Cd−O1 = 2.588 Å and Cd−O2 = 2.300 Å) from two H2ttac2− ligands and two nitrogen atoms (Cd−N1 = 2.299 Å) from two bpp (Figure 1). Each H2ttac2− bonds to two Cd1 ions using its two

Figure 1. Coordination environment of Cd1 ion in 1; all hydrogen atoms but H3A have been omitted for clarity.

EXPERIMENTAL SECTION carboxylate groups, both of which adopt a bidentate chelating coordination mode with the coordination symbol of η4μ2χ4 for the whole ligand. Uniquely, further study of the structure shows that Cd2+ ions are connected by H2ttac2− and bpp to form the 1D triplestranded left-handed helical chains along the a-axis and righthanded helical chains along the b-axis with both pitches of 12.222 Å. The two kinds of vertical triple-stranded helical chains (Figure 3a,b) with opposite chirality are interlocked with each other (Figure 2, Figure 3c, and Figure 4b) through Hopf rings based on two [Cd-H2ttac-Cd-bpp-Cd-H2ttac] groups (Figure 4a,c). Interpenetration only exists between the adjacent left-handed and right-handed helical chains. Every chain can interpenetrate with all the adjacent vertical chains on its two sides, but every pair of two locked chains can only interpenetrate once, where DOC = 1 (DOC = degree of catenation). Between the adjacent helical chains with the same chirality, there exist strong O3− H3A···O2 hydrogen bonds. The short O3···O2 distance 2.6929 Å indicates a remarkable interaction (Figure 12). Interestingly, the left-handed/right-handed helical chains are built on the same Cd1 ions, and the 1D sideline is composed of the Xshaped bracket H2ttac2− ligand and the flexible bpp. It is common neither for the homochiral triple-stranded helical chains built on the same metal nodes10 nor for 1D structure in which the X-shaped bracket ligand and bpp are involved, because they are inclined to produce higher-dimensional structures instead of 1D chains. From the viewpoint of topology, Cd2+ ion, H2ttac2−, and bpp can be respectively simplified as 4-, 2-, and 2-connected nodes; therefore, the network of 1 belongs to a 4-connected uninodal topology with the Schläfli symbol as (33.42.5). As an X-shaped bracket ligand,

Materials and Physical Measurements. Starting materials in this work were bought and employed as original. From 4000 to 400 cm−1, infrared spectrum was obtained through the Bruker EQUINOX55 FT-IR spectrometer together with KBr pellet. Equipment Netzsch TG209F3 was employed to carry out thermogravimetric measurements under nitrogen condition with the temperature increasing 5 °C per minute. PerkinElmer 2400C Elemental Analyzer was used to carry out elemental analyses for N, H, and C. Bruker D8 ADVANCE X-ray powder diffractometer was used to get PXRD patterns (Cu Kα, λ = 1.5418 Å). Hitachi F4500 fluorescence spectrophotometer was employed to study the luminescence properties of solid powder. Flsp920 steady state and time resolved fluorescence spectrometry was conducted for the luminescence lifetime experiments. ASAP 2020 M adsorption equipment was used to get the gas adsorption isotherms. Xray photoelectron spectroscopy was measured on a PHI-5400 spectrometer. Hitachi U-3310 UV−vis spectrometer was selected to perform diffusion reflectance measurement. Crystallographic Data Collection and Refinement. Bruker SMART APEX II CCD detector was employed to obtain the crystal data of 1 at 296(2) K using ω rotation scan with the width as 0.3° as well as Mo Kα radiation (λ = 0.71073 Å). Direct method was employed to determine the structure followed by SHELX-97 refinements with full-matrix least-squares method based on F2.9 Anisotropic refinement was applied to every non-hydrogen atom. SI Table S1 summarizes the crystal data for 1 and details of data refinements and collection. SI Table S2 gives the selected angles and bond lengths. CCDC reference number is 951774. Synthesis of [Cd(H2ttac)bpp]n (1). Cd(NO3)2·4H2O (0.1 mmol, 0.0308 g), H4ttac (0.05 mmol, 0.0203 g), and bpp (0.05 mmol, 0.0099 g) were added to 14 mL H2O and 0.1 mL triethylamine. The system was slightly turbid with a small amount of insoluble substance. Afterward, the mixture was encapsulated in the 25 mL Teflon-lined stainless reactor with 72 h heat treatment at 150 °C. When cooled down to room temperature with the decrease of 5 °C per minute, a large block of light yellow crystals were collected whose yield was 2955

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backbone, H2ttac2− ligands adopt bridging bidentate coordination mode and act as the twisted V-shaped linkage, forming the 1D helical chains along the a-axis and the b-axis; (2) on the arms around the helical chains, two terminal carboxylic groups remain protonated, which may be potential active sites for host and guest interaction, and this provides opportunities for 1 to become a candidate as the luminescent sensor. Based on the above structure analyses, we believe that 1 belongs to the rarely reported third category of one-fold L1D + R1D → 3D polycatenated networks,10 and as far as we know, is the first case built on diamine bridging ligand; among polymers of this structure type, 1 is also the first example containing two protonated carboxylic groups, and the removal of active protons may have an effect on the luminescence properties, which also deserve close attention. Phase Purity and Thermal Stability. PXRD pattern has been tested to verify the purity of the sample (SI Figure S6a), and it demonstrates good agreement with the simulated pattern by single crystal X-ray diffraction data. To study the thermal stability of 1, thermal gravimetric measurement from 30 to 800 °C has been employed in nitrogen atmosphere. 1 displays excellent thermal stability without weight loss until 350 °C. Then, an abrupt weight loss occurs indicating the collapse of the whole framework. I2 and H2 adsorption experiments show that the interaction between I2/H2 and 1 just occurred on the surface of the polymer (SI Figure S4 and S5). Luminescent Measurement and Discussion. When excited at 320 nm in solid state, 1 displays highly blue luminescence emission centered at 415 nm, which can be attributed to the π* → π or π* → n transition from L for their similar emission wavelengths (415 nm for 1 vs 404 nm for L, SI Figure S2), band shapes, and lifetimes (0.54 ns (71.78%) and 2.81 ns (28.22%) for 1 vs 0.75 ns (73.14%) and 3.58 ns (26.86%) for L, Figure S3).11 The emissions of both the ligand and 1 are in good agreement with the previous report for H4ttac.12 The maximum emission of 1 occurs at 415 nm with a red shift of 11 nm compared with the free ligand H4ttac. Generally, formation of the coordination complex is believed to increase rigidity, thus reducing the energy loss during the fluorescence emission process, and finally shows up as the redshift of emission wavelength on the macro level.13 Owing to its good luminescence properties, cheap cost, high thermal stability, as well as the potential active sites (−COOH) for host and guest interactions, this Cd-based luminescent L1D + R1D → 3D polycatenated network has stirred up great interest in its application as luminescent probes in sensing organic molecules and metal ions. Detection of Nitro Aromatic CompoundsPhotoinduced Electron Transfer. The luminescent response behaviors of 1 dispersed in various kinds of solvents (H2O, methanol, ethanol, 2-propanol, butanol, isoamylol, N,N-dimethylformamade, N,Ndimethylacetamide, acetonitrile, dichloromethane, chloroform, N-methyl pyrrolidone, tetrahydrofuran, and nitrobenzene) have been investigated as a suspension after immersion and ultrasonic agitation for 30 min. The excitations were conducted under the wavelength of 320 nm. As shown in Figure 5a and c, the luminescence intensities are greatly dependent on solvents, particularly for nitrobenzene, in which significant quenching effects appear. PXRD patterns collected for each 1@solvent are similar to that of 1, showing that the crystallinity is wellpreserved in all the solvents (SI Figure S6b). A smart sensor should not only detect the target qualitatively, but also have the abilities to measure quantitatively. To further

Figure 2. View of 1D → 3D polycatenation (blue for left-handed triple-stranded helices and purple for right-handed triple-stranded helices.

Figure 3. (a) Left-handed helical chain, (b) right-handed helical chain, and (c) their interlocking through hopf ring.

Figure 4. (a) Hopf ring and the schematic representations of (b) the interlocking of two kinds of helices, (c) Hopf ring, and (d) the 1D → 3D polycatenation.

H4ttac, which is half-deprotonated as H2ttac2−, displays two key characters in the formation of the network: (1) in the 2956

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Figure 5. (a) Comparison of the relative luminescence intensities of various 1@solvent suspensions. (b) Emission spectra of 1@nitrobenzene@ DMF suspensions with nitrobenzene concentration varying from 0 to 3050 ppm (excited at 320 nm); the insets show the visual change upon the addition of nitrobenzene (left) and the plot of relative intensity versus nitrobenzene concentration. (c) Visual change upon addition of various solvents.

Figure 6. Relative intensity of 1 in the solution of EDC@ethanol from Class 1; EDC = electron deficient compound.

Figure 7. Relative intensity of 1 in the solution of ERC@ethanol from Class 2; ERC = electron rich compound. 2957

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Figure 8. (a) Comparison of the relative luminescence intensities of various 1 − M(NO3)x@DMF suspensions. (b) Emission spectra of 1 − Cu(NO3)2@DMF suspensions with Cu2+ concentration varying from 0 to 200 ppm (excited at 320 nm). The inset shows the plot of relative intensity versus Cu2+ concentration. (c) Visual change upon the addition of various M(NO3)x.

dense stacking of 1 and the PXRD patterns after solvent immersion, it is impossible for the incorporation of analytes or the break of structure, but is more inclined to the surface interaction between the analytes and 1. The emission spectrum of 1 starts from 350 nm, so UV absorption spectra for nitro aromatic compounds have almost no effective overlap on the emission spectrum of 1 and this gives an exclusion of the energy transfer.16b Diffusion reflectance measurements of 1 show the optical band gap to be 3.2 eV (SI Figure S7), indicating the high reducibility for the excited state of 1.7a,b,17 Meanwhile, nitrobenzene, 2-nitrotoluene, 2,4-dinitrotoluene, and 1,4-dinitrobezene from Class 1 all show strong electron deficiency and their LUMOs (lowest unoccupied molecule orbital) locate at lower energies (SI Table S3);16a as a result, they can act as excellent electron acceptors. The high reducibility of 1 and the electron deficiency of the analytes together drive the electron to transfer from the conduction band of 1 to the LUMO of the electron deficient compound causing the quenching of luminescence. On the other hand, electron rich compounds from Class 2 possess much higher lying LUMO,16a and when exposed to the UV radiation, they may drive their excited electron to transfer from the analytes to 1, leading to the intensity enhancement of the system.18 Detection of Metal IonsPhotoinduced Electron Transfer. In order to investigate the luminescent responses of 1 to metal ions, the luminescent spectra of 1 dispersed in DMF solutions of different nitrate (Class 1: AgNO3, Al(NO3)3, Ca(NO3)2, NaNO3, KNO3, Zn(NO3)2, Cd(NO3)2, Pb(NO3)2; Class 2: Cu(NO3)2, Mn(NO3)2, Co(NO3)2, Ni(NO3)2) have also been recorded. As demonstrated in Figure 8a and c, the luminescence intensities of the various systems are largely dependent on the metal ions. Decreases in luminescence intensity could not be observed with the presence of metals ions from Class 1, while ions from Class 2 could generally induce luminescence quenching at various degrees, especially

research the quenching effect of nitrobenzene to 1, titration experiments were applied. A batch of 1@nitrobenzene@DMF suspensions was prepared by dispersing 5 mg of 1 into 3 mL DMF with the nitrobenzene concentration increasing from 0 to 3050 ppm to monitor the emission response. As shown in Figure 5b, with the increase of nitrobenzene concentration, the luminescence intensity of 1@nitrobenzene@DMF suspension decreases distinctly. The relationship between the luminescence intensity and the volume concentration of nitrobenzene follows the first-order exponential decay formula of I = 0.936 × exp(−cV/320.258) + 0.0582, showing that luminescence quenching is diffusion controlled,7f,14 as well as providing 1 the possibility to detect nitrobenzene quantitatively (I = luminescence intensity of 1@nitrobenzene@DMF suspensions/luminescence intensity of 1@DMF suspension; cV = nitrobenzene volume concentration, ppm; R2 = 0.997). It is noteworthy that the intensity decreases to 50% at the concentration of only 200 ppm, which is comparable to that reported by Sun’s group,15 allowing the sensor to detect the presence of even trace nitrobenzene. To better explore the quenching trend of 1, more electron deficient compounds (Class 1, including: nitrobenzene, 2nitrotoluene, 2,4-dinitrotoluene, 1,4-dinitrobezene) and electron rich compounds (Class 2, including: benzene, toluene, chlorobenzene, bromobenzene) dispersed in 3 mL ethanol at the concentration of 0.5 mM have been employed. Compared with the intensity of 1@ethanol, compounds from Class 1 can generally cause luminescent quenching phenomena in the order of 1,4-dinitrobezene > 2,4-dinitrotoluene > nitrobenzene > 2nitrotoluene (Figure 6), while compounds in class 2 can basically increase the original intensity in the order of toluene > benzene > chlorobenzene > bromobenzene (Figure 7). Generally, there are mainly two reasons for these sensing phenomena: (1) photoinduced electron transfer or (2) resonance energy transfer or both.7a,b,16 Considering the 2958

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transfer.16a XPS (X-ray photoelectron spectroscopy) experiment on 1 − Cu(NO3)2@DMF sample (SI Figure S9) shows that the typical energy of Cu 2p1 shifts to 954.5 eV, giving an increase of 2.3 eV compared with the standard value of free Cu2+, indicating an interaction between Cu2+ ions and the material.7f For the peak at 934.5 eV especially, it is almost exactly the same as the value of the Cu···(OH)2 interaction (934.6 eV), suggesting the existence of interaction between Cu2+ and the terminal −OH groups.23 In addition, the well recovered intensities after multiple cycles of Cu2+ sensing experiments may provide proof that Cu2+ ions are mainly absorbed on the surface. Similar to the previous reports, 21 Cu 2+ ion adopts unsaturated electron configuration (3d9), which possesses extreme electron affinity and is an excellent acceptor for electron. So, when Cu2+ ions interact with the −OH groups on the surface of 1, it provides a significant passageway for electrons to transfer from 1 to Cu2+, thus reducing the electron transfer efficiency within the ligand H2ttac2−, and leading to the luminescence quenching phenomenon.16b,23 For metal ions with the closed-shell configuration (p6 and 10 d ; see SI Table S4) in Class 1, the diamagnetism makes them nonideal electron acceptors, so their interference will hardly cause luminescence quenching to the system. Especially for Ag+, although it also belongs to the coinage metals as Cu2+, the fully filled configuration 4d10 makes it unable to accept electrons and thus has little effect on the intensity. As far as other paramagnetic ions from Class 2, they display much milder quenching effects compared with Cu2+, while this may be the comprehensive function of binding energy between ions and −OH, electron affinity of the ions, and so on. Luminescent Response to BasesChange of the Host Structure. As 1 keeps two protonated carboxylic groups in every H2ttac2− ligand, whether the −COOH groups will affect luminescence properties has aroused our great interest. To explore the luminescent responses of 1 under protonation and deprotonation conditions, the luminescence emission spectra of 5 mg 1 in 5 mL H2O were recorded with gradual addition of NaOH (0.03 M). As shown in Figure 10a and b, with the increase of NaOH (from 0 to 1.0 equiv), the maximum emission wavelength moved from 416 to 396 nm gradually. To further study the mechanism, titrations with KOH, methylamine and triethylamine have been employed. As expected, titrations by all selected bases can cause blue shifts. Significantly, the blue shifts of the maximum emission for 1 are largely dependent on the strength of the base at the same

for Cu2+, which created a significant quenching effect to the system. With Cu2+ as an example, the decrease can be easily distinguished by the naked eye, and this confirms that, compared with other metal ions, 1 displays high selectivity toward Cu2+. To gain more insight into the quenching effect caused by Cu2+, titration experiments were carried out. The emission responses were monitored by the gradual addition of Cu(NO3)2@DMF (0.1 mol/L) into the suspension of 1@ DMF. The increased amount of Cu2+ resulted in a gradual decrease of luminescence intensity at 415 nm (Figure 8b), and the intensity was reduced to 50% at a concentration of 40 ppm for Cu(NO3)2, which is comparable to those reported by Cao’s group19 and Zhang’s group.20a Importantly, the luminescence intensity of the suspension follows the first-order exponential decay formula I = 59.26 × exp(−cV/4.61) + 7.25, by which Cu2+ could be detected quantitatively (I = luminescence intensity of 1 − Cu(NO3)2@DMF suspensions, cV = Cu2+ volume concentration, R2 = 0.995). In addition, multiple cycles of Cu2+ sensing experiments have been conducted and the material could greatly regain its intensity after filtration and washing by DMF (Figure 9).

Figure 9. Multiple cycles for the quenching by Cu(NO3)2 and the recovery after filtration and ultrasonic washing by DMF for a few times. The red bars show the pristine intensity and regenerated ones after washing; while the blue bars show the intensities after addition of Cu(NO3)2@DMF solutions.

PXRD pattern performed after Cu(NO3)2 immersion (SI Figure S6c) proves the structure is well preserved. The UV absorption spectrum for Cu(NO3)2@DMF solution has also been recorded, while there is no obvious adsorption above 260 nm (SI Figure S8), giving no overlap on the emission spectrum of 1. Therefore, the quenching should origin from the electron transfer from 1 to Cu2+, instead of the resonance energy

Figure 10. (a) Emission spectra of 1 in H2O on (b) NaOH titration and (c) HNO3 titration. 2959

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could not be recovered. As a result, HNO3 titration proved nonreversible, and the reason may be the instability of 1 in acid phenomena where the structure may have been broken down. According to the experiments and analyses above, 1 is no doubt a competent multifunctional sensor to nitro aromatic compounds, Cu2+, and bases.

amount of titration. As shown in Figure 11, the blue shifts are in the order of KOH ≥ NaOH > triethylamine > methylamine, which agree well with the strength of the base.

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CONCLUSION In summary, a highly stable and luminescent L1D + R1D → 3D polycatenated polymer, [Cd(H2ttac)bpp]n (1), has been synthesized and characterized. Photoluminescence studies show that 1 exhibits significant luminescent sensitivity to nitro aromatic compounds, Cu2+, and bases; the possible mechanisms are ascribed to the photoinduced electron transfer for sensing nitro aromatic compounds and Cu2+, and the breaking of hydrogen bonds for the sensing of bases. Especially, the lowest detection limits for nitrobenzene and Cu2+ are down to 200 and 40 ppm, respectively, which are comparable to or even lower than the ones reported. The good selectivity and sensitivity provide 1 the possibility to be an efficient and economical luminescence sensor. Importantly, 1 possesses a dense structure without solvent accessible volume, instead of pores, channels, or large cages in the structure. As a result, this work not only enriches the rarely reported L1D + R1D → 3D polycatenation but also provides a new approach to develop LCPs serving as luminescent sensor with high stability and fewer structure limits.

Figure 11. Comparison of maximum emission wavelength of 1 upon deprotonation by NaOH, KOH, methylamine, and trithylamine.

The shifts should be related to the free carboxylic groups and their deprotonation process. Structure analysis shows that there exist significant hydrogen bonds, O3−H3A···O2, between the adjacent left-handed (right-handed) helical chains. The addition of bases into the system will gradually react with the free −COOH groups in 1, leading to the removal of H3A and the breaking of hydrogen bonds, accompanied by increasing instability and reduced conjugation in 1 (Figure 12). The instability and reduced conjugation of the system should be responsible for the blue shifts of the maximum emission wavelength. PXRD after NaOH titration (SI Figure S10) was measured and the patterns showed that the basic framework of 1 was kept. Then the sample was titrated with HNO3 (1:60) straight after NaOH titration to see whether the process is reversible. However, along with the increase of HNO3, the emission intensity decreased dramatically, and slight shifts of the maximum emission wavelength showed up, back to 404 nm finally, in good accord with the free H4ttac (Figure 10c). In addition, the suspension gradually turned clear. Afterward, NaOH titration was performed to the above system, but unfortunately the original emission wavelength and intensity

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ASSOCIATED CONTENT

* Supporting Information S

X-ray crystallographic file in CIF format; crystal data, selected bond lengths, angles, IR, solid luminescence spectra, luminescence decay curves, adsorption graph, PXRD, and UV spectra in PDF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel/Fax:+86-29-88308031. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the NSF of China (Grants 21371142, 20931005, 91022004, 21201139), the NSF

Figure 12. Hydrogen bonds between the adjacent right-handed helices and the schematic representation of the deprotonation process by base. 2960

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Crystal Growth & Design

Article

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of Shaanxi, China (grant 2013JQ2016) and NSF of China (Grant 21171031).

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dx.doi.org/10.1021/cg5002418 | Cryst. Growth Des. 2014, 14, 2954−2961