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Strong blue emissive supramolecular self-assembly system based on naphthalimide derivatives and its ability of detection and removal of 2,4,6-trinitrophenol Xinhua Cao, Na Zhao, Haiting Lv, Qianqian Ding, Aiping Gao, Qiangshan Jing, and Tao Yi Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01927 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017

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Graphical abstract:

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Strong blue emissive supramolecular self-assembly system based on naphthalimide derivatives and its ability of detection and removal of 2,4,6-trinitrophenol Xinhua Cao a*, Na Zhao a, Haiting Lv a, Qianqian Ding a, Aiping Gao a, Qiangshan Jin a, Tao Yi b* a

College of Chemistry and Chemical Engineering＆Henan Province Key laboratory of Utilization

of Non-metallic Mineral in the South of Henan, Institute for Conservation and Utilization of Agro-bioresources in Dabie Mountains，Xinyang Normal University, Xinyang 464000, China b

Department of Chemistry and Collaborative Innovation Center of Energy Materials, Fudan

University, Shanghai 200433, China. E-mail: [email protected]; [email protected] Abstract: Two simple and novel gelators (G-P with pyridine and G-B with benzene) with different C-4 substitution groups on naphthalimide derivative have been designed and characterized. Two gelators could form organogels in some solvents or mixed solvents. The self-assembly processes of G-P in mixed solvent of acetonitrile / H2O (1/1, v/v) and G-B in acetonitrile were studied by means of electron microscopy and spectroscopy. The organogel of G-P in mixed solvent of acetonitrile / H2O (1/1, v/v) formed intertwined fiber network, and its emission spectrum had obvious blue shift comparing with that of solution. In contrast, the organogel of G-B in acetonitrile formed straight fiber, and its emission had obvious red shift comparing with that of solution. G-P and G-B were employed in detecting nitroaromatic compounds (NACs) due to their electron-rich property. G-P is more sensitive and selective towards 2,4,6-trinitrophenol (TNP) comparing with that of G-B. The sensing mechanisms were investigated by 1HNMR, spectroscopic experiment and theoretical calculation. From these experimental results, it is proposed that electron transfer happens from the electron-rich G-P molecule to the electron deficient TNP due to the possibility of complex formation between G-P and TNP. G-P molecule could detect TNP in water, organic solvent media as well as using test strips. It is worth mentioning that organogel G-P can not only detect TNP, but also remove TNP from solution into organogel system. Keywords: supramolecular, self-assembly, organogel, detection, nitroaromatic compounds


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Introduction Nitroaromatic compounds (NACs) are well-known primary constituents of many explosives and are also widely used in the agrochemical industry and military fields. The excessive usage of NACs would pollute soil and water in war zone and around military facilities, and causing severe health problems such as severe irritation, skin allergy, anemia, carcinogenicity, dizziness, nausea, damage of liver, and kidney1-4. Therefore, it is an urgent desire to develop probes to sensitively detect NACs. Up to now, some materials and methods have been used to detect NACs such as MOFs5-8, small organic fluorescence molecules9-13, carbon nanoparticles14, quantum dots15-18, polymer nanoparticles19-21, supramolecular self-assembly system22-25, metal nanoclusters26, fluorescent silicon nanoparticles27 and so on. Most of those sensor are towards 2,4,6-trinitrotolene (TNT)28. However, among the common nitro-explosives, 2,4,6-trinitrophenol (TNP) is a superior explosive than 2,4,6- trinitrotolene (TNT), but is paid less attention compared with that of TNT29. Therefore, the development of efficient, selective, portable, fast and sensitive method for sensing TNP has very great significance. Low molecule weight organogels as a kind of supramolecular self-assembly system have been widely developed in the past two decades due to their potential application in many fields such as pollutant removal, light harvesting, cell cultures, tissue engineering, shape memory devices, drug delivery, photovoltaics, reaction vessels and reusable catalysts, sensor, controlling of crystal growth, separation.30-38 Those organogels can be formed and immobilize the solvents via intermolecular non-covalent interactions, including hydrogen-bonding, π-π stacking, hydrophobic and hydrophilic interactions, van der Waals forces, electrostatic interactions and coordination interaction.39, 40 At the same time, a various nanostructure can be obtained via the self-assembly process of organic molecules, including nanorings, tubes, helices, disks, rod and fibres.41 The properties of organogels such as color, emission, adsorption, magnetism, rheology, and redox behaviour can be effectively controlled due to weak intermolecular non-covalent interactions. The three-dimensional network in organogel can provide the chance of analytes contacting with gelator in gel system internal. Therefore, organogels can respond to external stimulus including light, temperature, sound, mechanical stimuli, anions, metal ions, gases, redox, proton/pH, and small molecules for detection.42, 43 And more importantly, it is possible that gel not only can detect toxic analytes, but also realize toxic analytes removal. However, very few supramolecular organogel


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systems with the ability of detecting TNP have been designed and synthesized, either in solution or gel state. Fluorescence detection technology is especially suitable for detecting NACs due to their many advantages such as fast, real-time responses and high sensitivity.44, 45 1,8-naphthalimide was an electron-rich organic fluorescent molecule system with the ability of detecting TNP. The spectroscopic properties of 1,8-naphthalimide can be tuned from red to blue by modifying the C-4 substitution with a high fluorescence quantum yield.46 1,8-naphthalimide is widely applied for fabrication of fluorescent chemodosimeter and fluorescent organogel. For example, Tian and coworkers have reported the first 1,8-naphthalimide derivative based ratiometric fluorescent chemodosimeter for selective detection of mercuric ion.47 We have reported a series of 1,8-naphthalimide derivatives as gelators, which are sensitive to amine and toxic nitrite anion in gel state.48, 49 NACs have an obvious characteristic of electron deficiency which is prone to interact with electron-rich organic fluorescent molecules. Herein, we have incorporated a 4-pyridinol and phenol into 1,8-naphthalimide-based gel molecule (G-P and G-B in Scheme 1) to generate two novel fluorescent gelators with the ability of visual detection of TNP in solution and gel state. To the best of our knowledge, this is the first example of a 1,8-naphthalimide-based organogel system displaying visual detection and removal of explosives in aqueous solution with the characteristic of efficient, selective, portable, fast and sensitive. O N O O N

G-P O N

O

O

G-B

Scheme 1 Molecular structures of G-P and G-B.

Experimental Reagents and materials Dodecylamine, 4-bromo-1,8-naphthalimide, 4-pyridinol and phenol were all purchased from Sinopharm Chemical Reagent Co., Ltd. NACs including NB (Nitrobenzene), DNB (1,4-Dinitrobenzene), DNP (2,4-Dinitrophenol), DNT (2,4-Dinitrotoluene), NP (4-Nitrophenol),


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NT (4-Nitrotoluene), NBA (4-Nitrobenzoic acid), TNP (2,4,6-trinitrophenol) were provided by Sinopharm Chemical Reagent Co., Ltd. All other reagents were analytically pure. Water used throughout was deionized and then three distilled. The gelation test The gelator and solvent were put in a septum-capped test tube and heated (>80 °C) until the solid was dissolved. The sample vial was then cooled to 25 °C (room temperature). Qualitatively, gelation was considered successful if no sample flow was observed upon inversion of the container at room temperature (the inverse flow method).50 Xerogels were obtained by evaporation of solvent from the gel via freeze drying. Instrumentation conditions 1

H NMR and

13

C NMR spectra were recorded on a Bruker-Avance (Bruker, Ltd.,

Switzerland), at 400 and 100 MHz, respectively. Proton chemical shifts are reported in parts per million downfield from tetramethylsilane. HRMS was recorded on a LTQ-Orbitrap mass spectrometer (ThermoFisher, San Jose, CA, USA). 1H NMR titration of G-P was performed upon dissolving G-P (21.8 mM) in CDCl3/CD3OD (5:1 v/v). TNP (0 to 1.4 equiv.) was added to G-P solution and the spectra were recorded using identical parameters. Field emission scanning electron microscope (FESEM) images were obtained using a FE-SEM S-4800 instrument (Hitachi, Ltd., Tokyo, Japan). Samples were prepared by spinning the samples on glass slices and coating with Pt. Powder X-ray diffractions were generated by using a Philips PW3830 (Philips, Ltd., Eindhoven, Holland) with a power of 40 kV at 40 mA (Cu target, λ = 0.1542 nm). UV–vis absorption spectra were recorded on a UV-vis 2550 spectroscope (Shimadzu, Ltd., Tokyo, Japan). Rheology experiments were performed on a MCR 302 Anton Paar (Austria) rheometer, with a Couette cell and a temperature control unit. The measurements were carried out on freshly prepared gels by using a controlled-stress rheometer. Parallel plate geometry of 25 mm diameter and 1 mm gap was employed throughout the dynamic oscillatory tests. The following tests were performed: increasing the amplitude of oscillation up to 100% apparent strain shear (kept a frequency of 6.28 rad s-1) and frequency sweeps at 20ºC (from 100 to 0.1 rad s-1, 0.1% strain). The structure of the HOMO and LUMO states of G-P and G-B with simplification were determined with the help of theoretical calculations, in the framework of density functional theory (DFT)


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calculations at the level of B3LYP/6-31G* in a suite of Gaussian 09 programs.51

RESULTS AND DISUSSION The synthesis detail of G-P and G-B is given in the Supporting Information (SI, NMR spectra as showed in Figure S1 and S2). The gelation tests of G-P and G-B were carried out in 15 different solvents including H2O using a test tube inversion method. Compound and solvent were added to the test tube and heated until the solid sample was completely dissolved. Then the test tube was slowly cooled to room temperature (25°C). The gelation ability of G-P and G-B was showed in Table S1. The compound G-P could form stable gels in DMF, acetone and DMSO with critical gelation concentration (CGC) of 2.64, 3.17 and 0.57 % wt/v, respectively. The partial gel G-P was obtained in acetonitrile under the concentration of 3.16 % wt/v. Interestingly, a stable gel G-P was obtained in mixed solvent of acetonitrile / H2O (1/1, v/v) with the CGC of 0.69 % wt/v. The gel G-P from mixed solvent of acetonitrile / H2O (1/1, v/v) was tolerant to some solvents such as ethyl acetate, toluene, methanol and H2O. This would provide convenience for detection and removal of NACs in aqueous solution via self-assembly organogel. Compound G-B exhibited good gelation ability and formed organogels in methanol, ethanol, acetonitrile, n-hexane, toluene and DMSO with the corresponding CGC of 0.53, 0.79, 0.45, 3.78, 0.57 and 0.38 % wt/v, respectively. The mixed solvent of acetonitrile / H2O (1/1, v/v) was also used to test gelation ability of G-B, but not successful. The images of gels of G-P in mixed solvent of acetonitrile / H2O (1/1, v/v) and G-B in acetonitrile were showed in Figure 1. Both gels show blue emission under the irradiation of 365 nm light. Gels of G-P in mixed solvent of acetonitrile / H2O (1/1, v/v) and G-B in acetonitrile were selected as typical samples and studied in detail.

Figure 1 Images of (a, a′) gels of G-P in mixed solvent of acetonitrile / H2O (1/1, v/v) and (b, b′) G-B in acetonitrile with the concentration of 0.69 and 0.45 % wt/v. (a′) and (b′) were in the dark with the irradiation of 365 nm light.


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A various microstructures were formed in the sol-gel transition process. The gel was diluted and dispersed onto mica plate, and dried via freeze drying technology. Herein, a nanofibre structure was observed with the width of 200-400 nm and length of several microns in xerogel G-P (Figure 2a). The nanofibres were intertwined and further built into three-dimensional network with the ability of gelation solvent. Microbelts with the width of 0.5-1.0 micrometer and the length of tens of microns were formed in gel G-B (Figure 2b). The microbelts were very straight and piled on top of each other. Circular Dichroism (CD) experiments of G-P and G-B in solution and gel state were carried out. The experimental results are listed in Figure S3. No CD signal was detected for both the G-P and G-B solution (both are 10-5 M) because there is no chiral functional group in either of the molecular structures of G-P and G-B. In the case of gel G-P from CH3CN / H2O (1/1, v/v), no obvious absorption bands was observed in Figure S3. For gel G-B from CH3CN, the negative bands at 307 and 391 nm were appeared in CD spectrum. An intertwined and helical structure could be observed in SEM image of G-B gel in Figure 2b.

Figure 2 SEM images of xerogels G-P in mixed solvent of acetonitrile / H2O (v/v, 1/1) and G-B in acetonitrile under the CGC condition. a) for xerogel G-P; b) for xerogel G-B. The scale bars for a) and b) are 2 and 20 µm.

The rheology behavior of organogels G-P and G-B in their corresponding solvents was studied and showed in Figure S4. To understand the mechanical properties of the gels G-P and G-B, the viscoelastic properties of organogels of G-P in mixed solvents of CH3CN/H2O (1/1, v/v) and G-B in acetontrile were studied by using dynamic oscillatory measurements. Initially, the linear viscoelastic region (LVR) of organogels G-P and G-B at the corresponding CGC was examined with an amplitude sweep at a constant angular frequency of 6.28 rad s-1 (Figure S4a). G´ was obviously larger than G´´ for organogels G-P and G-B under strain values 66.2% and 10.0%,


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respectively which indicated that these gels could maintain their elasticity. As the strain values were increased, G´ gradually decreased and G´´ gradually dominated over G´, which indicated that the organogel gradually became broken up and finally completely collapsed. At the same time, a frequency sweep of between 0.1 and 100 rad s-1 at the same temperature and within the LVR was performed (Figure S4b). In the frequency sweep experiment, the G´ and G´´ values of gels G-P and G-B showed some fluctuations as angular frequency was increased, but G´ were always greater than G´´, which suggested that these organogels have a good tolerance for external forces exerted on them. 52 The G´ and G´´ values of gels G-P and G-B were about 100, 700, and 10, 90 Pa, respectively. These reuslts indicated that an energy-storage process occurred without energy dissipation, which also confirmed that the three gels had a good elastic character.53 UV-Vis absorption and fluorescence spectrum can provide important information about the self-assembly process.54-57 The UV-Vis absorption and fluorescence spectra of G-P and G-B in solution (10-5 M) and gel states (0.69 and 0.45 % wt/v) were investigated (Figure 3). The absorption spectrum of G-P solution in mixed solvent of acetonitrile / H2O (v/v, 1/1) showed a series of peaks at 237, 270, 347 nm, which arises due to a π-π* transition along the long axis of the chromophore in the monomeric state.58,

59

The molar absorption coefficient (ε) of G-P in

acetonitrile and H2O solution at their corresponding maximum absorption of 347 nm was 4.47 × 103 and 7.08 × 104 dm3 mol−1 cm−1, respectively. This absorption had no shift but broadened in the gel sate. However, the absorption at 270 nm disappeared in the gel state. The fluorescence emission of G-P in acetonitrile solution was located at 493 nm with a quantum yield (φf) of 47.88 % and a lifetime of 2.0 ns. The fluorescence emission band of G-P gel was at 485 nm with a blue shift of 16 nm, comparing with that of G-P solution at 501 nm which was possible that the TICT excited state of G-P molecule in gel state might be suppressed and produced the blue-shift fluorescence emission.60 Comparing with compound G-P, G-B had the different UV-vis absorption and fluorescence emission behavior in solution and gel state. G-B in acetonitrile solution had two absorption bands at 240 and 359 nm (ε: 2.04 × 104 dm3 mol−1 cm−1), which showed 2 nm red shift in the acetonitrile gel. The fluorescence emission of G-B in solution was at 428 nm with a quantum yield of 27.73 %, and a lifetime of 3.04 ns. This emission was red-shifted to 444 nm in the corresponding gel state, suggesting π-π interaction of G-B molecules in the gel system. G-P and G-B solid even emitted strong fluorescence at 432, 469 nm and 469 nm with


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solid quantum yields up to 15.95 % and 25.08 %, respectively. The fluorescence lifetime of G-P in solid was extended to 4.71 ns (Figure S5). G-B solid had two fluorescence decays, with 3.01 and 30.19 ns lifetimes respectively. This high fluorescence quantum yield of G-P and G-B in solution

1.0

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and solid state made it very suitable for sensor application. G-P solution G-P gel in mixed solvent

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Figure 3 Normalized absorption and fluorescence emission spectra of G-P and G-B in solution (10-5M L-1) and gel state with the CGC under the irradiation of 347 nm light.

The temperature-varying (TV) 1H NMR experiments of gel G-P and G-B in their corresponding solvents were carried out and showed in Figure S6 .All proton singals of aromatic protons of H1-H7 in G-P molecule structure did not have any shift in the temperature range of 20-40℃. With the temperature ascending to 60℃, the proton signals of H1-H7 were obvious shifted to upfield. Considering that the temperature of 60℃ is just the gel to sol transition point of G-P, the 1H NMR spectra suggested the presence of π-π stacking in the gel state. For gel G-B formed in acetontrile, the gel to sol transition point was only 42℃. With the NMR experiment temperature increasing from 20℃ to 40℃, the chemical shifts of the proton singal of H1, H2 and H5 were shifted upflied from 8.69 to 8.67 ppm, 8.58 to 8.48 ppm and 8.86 to 7.72 ppm which indicated the presence of π-π stacking in the gel state. When the temperature was up to 50℃, the


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chemical shifts of all aromatic protons in molecule G-B did not shift with the temperature acsending. X-ray powder diffraction of xerogel can provide important information on the molecular aggregation mechanism. The XRD pattern of G-P xerogel showed two peaks at 2.46 and 1.63 nm (Figure S7). The peak at 2.46 nm was close to the molecule G-P length of 2.12 nm. The XRD pattern of G-B xerogel obtained from acetonitrile showed a series of peaks with d space values of 2.12, 1.07, 0.78 and 0.58 nm, which were exactly in the ratio of d/1:d/2:d/3:d/4, indicating a lamellar structure with an interlayer distance of 2.12 nm (Figure S7).

61, 62

At the same time, a

diffraction peak at 2θ = 24.8 ° could be observed in the diffraction pattern of the xerogel G-B, which corresponded to a d spacing of 0.36 nm and could be assigned to the interplanar space between the aromatic rings of G-B molecules based on a π-π interaction.63, 64 Pyridine group has been introduced into supramoelcular self-assembly system and constructed muti-functional materials through acid-base interaction and metal coordination.65-69 In the present work, we explored that pyridine group of G-P interacted with NACs, and triggered a change in its photophysical property or gel state transition which would provide a new type of NACs-sensing system. The structures of the NACs used in the present study were listed in Figure 4c. When 1.0 equiv. of several NACs were added to the G-P and G-B acetonitrile solution (10-5 M) at room temperature, the color of G-P solution with addition of DNP and TNP was changed from colorless into yellow due to their own color of DNP and TNP (Figure 4). But the emission of G-P solution with addition of TNP was obviously weakened comparing with the blank solution and addition of other NACs. The similar change trend was existed in G-B solution. The difference is that the fluorescence emission of G-B solution with addition of TNP (1.0 equiv.) was not quenched so much like that of G-P solution.


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C

NO2

NO2

NO2

NO2

OH

DNB

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DNT

NO2

O2N OH

DNP

NO2

NO2

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NB

NO2

NO2

NP

COOH

NT

NBA

NO2 OH

TNP

Figure 4 (a) Visual changes in color for G-P acetonitrile solution (10-5 M) in presence of different NACs (1.0 equiv.); (b) Visual changes in color for G-B acetonitrile solution (10-5 M) in presence of different NACs (1.0 equiv.); (c) Structure of the Nitro Aromatics (NACs) used in the present study. The upper and lower were under daylight and 365 nm UV lamp, respectively.

In order to further prove the selective and detective ability of G-P and G-B to TNP, the fluorescence emission of G-P and G-B toward NACs were first carried out in acetonitrile solution. The fluorescence of G-P had a rapid and obvious quenching in presence of TNP (1.0 equiv.) with G-P concentration of 10-5 M in acetonitrile solution (Figure 5a and 5b). G-B also showed the selective detection ability for TNP in acetonitrile solution, but fluorescence emission quenching was only about 58.4% which was smaller comparing with 82.6 % of G-P (Figure 5c and 5d). At the same time, the fluorescence emission of G-B solution had some red-shift when 1.0 eq DNP or TNP added to G-B solution. The fluorescence emission spectra of G-B were measured in different solvents so as to understand the red-shift of fluorescence emission of G-B solution due to addition of DNP or TNP (Figure S8). From the experiment results, the maximum fluorescence emission band of G-B had obvious red-shift with the polarity of solvent increasing. The maximum fluorescence emission band at 403 nm of hexane was red-shifted to the maximum fluorescence emission band at 438 nm of methanol. When 1.0 equiv. of TNP or DNP was added to G-B acetonitrile solution, the polarity of solvent was increased, and the fluorescence emission of G-B solution was red-shifted to some extent. In view of the excellent detection performance of G-P toward NACs, the detection process of G-P was studied in detail.


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G-P TNP DB DNP DNT NP NT DNB NBA

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Figure 5 (a) Fluorescence emission change of G-P (10-5 M, λex = 365 nm) toward different NACs (1.0 equiv.) in acetonitrile solution; (b) Plots of fluorescence intensity change of G-P at 493 nm upon addition of different NACs (1.0 equiv.) in acetonitrile solution; (c) Fluorescence emission change of G-B (10-5 M, λex = 365 nm) toward different NACs (1.0 equiv.) in acetonitrile solution (d) Plots of fluorescence intensity change of G-B at their corresponding maximum emission upon addition of different NACs (1.0 equiv.) in acetonitrile solution.

In order to further selectivity of G-P towards to TNP, fluorescence tritition experiments of G-P soluiton in acetonitrile with different nitroaromatic compounds were carried out and the Stern−Volmer constant (Ksv) was calculated (Figure S9). The Stern−Volmer equation is expressed as F0/F = 1 + KSV[NAC], where F0 and F are the initial and final fluorescence intensities after addition of analyte and [NAC] is the concentration of nitroaromatic compounds9. Compound G-P have very high quenching efficiency towards TNP through comparing with the Stern-Volmer constants towards different nitroaromatic compounds. The constants Ksv towards TNP, DNP, NP, DNB, DNT, NB, NBA and NT calculated from Stern–Volmer equation were 2.18×105 M-1, 7.12

×103 M-1, 5.94×103 M-1, 3.95×103 M-1, 3.48×103 M-1, 3.63×103 M-1, 3.44×103 M-1 and 3.51 ×103 M-1, respectively. The results show that the fluorescence of G-P is selectively quenched by TNP, which showed 30-63 folds higher in quenching efficiency compared to those other NACs tested. In order to further study the interaction between G-P and TNP in solution, the fluorescence


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emission and UV-vis absorption spectra of G-P solution were carried out under the addition of TNP (0-2.0 equiv.) to G-P acetonitrile solution (10-5 M) (Figure 6). The absorbance of G-P solution at 350 nm was gradually increased with addition of TNP (Figure S10a). Especially, the shoulder absorption in the range of 400-460 nm was appeared and also gradually increased with the increase of TNP concentration which well explained the color change of G-P solution from colorless to yellow with addition of TNP.66 At the same time, the concentration-dependent absorption spectra of TNP was done and added to Figure S10b. From the Figure S10b, the absorbance of TNP solution was gradually enhanced with the concentration of TNP solution increasing, and the color of TNP solution was gradually deepened with the concentration increasing (Figure S10c). The absorbance of TNP did not obviously increase in the addition amount range of from10-6 to 10-5 M in the fluorescence titration experiments of G-P with TNP. Fluorescence intensity at 493 nm was notably decreased by 96 % with the addition of 2.0 equiv. of TNP. Plotting the fluorescence intensity change of G-P at 493 nm as a function of 1/ [TNP] (1/ [I0-I] vs 1/ [TNP]) gave a linear curve. At the same time, Job's plot for the wavelength of emission at 493 nm was established to evaluate the stoichiometry of G-P / TNP complex under the total G-P and TNP concentration of 10-5 M in acetonitrile. Nonlinear fitting of the fluorescence titration curve exhibited a 1:1 stoichiometry for G-P and TNP, with the association constant (K) of 3.45 × 105 M−1 (Figure S11).70 The detection limit was 6.44×10-7 M (Table S2). In view of G-P with certain solubility in water, the TNP detection in G-P aqueous solution (10-5 M) was also studied (Figure S12). The fluorescence emission of G-P in aqueous solution was blue-shifted to 460 nm due to solvent effect. With gradual addition of TNP, the fluorescence intensity of G-P aqueous solution at 460 nm was gradually decreased. The fitting of this data indicated a strong association between G-P and TNP, with association constant of 1.96 × 105 M−1. The limit of detection of G-P for TNP was 5.86×10-7 M, even lower than that in organic solvent (Table S3 in the SI). To get an insight into the detection mechanism, 1H NMR titration of G-P with TNP was performed in mixed solvent of CDCl3/CD3OD (5:1 v/v) (Figure 6d). With addition of TNP to G-P solution, H6 and H7 protons of pyridine group in G-P molecule were downfield shifted from 7.71 and 6.65 ppm to 8.26 and 7.25 ppm, respectively. When the addition amount of TNP was up to 1.0 equiv., the chemical shift change of H6 and H7 was greater than that of H3 and H4 protons of naphthalimide. When the addition amount of TNP exceeded 1.0 equiv., the chemical shift change


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of H3, H4, H5, H6 and H7 protons was no longer large. These experimental results clearly indicated the strong interaction between pyridine group of G-P and OH of TNP and that the nitrogen of the pyridine group of G-P molecule was partially protonated in the interaction process of G-P and TNP. This conclusion was further verified via theoretically calculation. The HOMOs and LUMOs of G-P and G-B molecule were calculated using Gauss 09 at the level of B3LYP/6-31G* (Table S5). The long alkyl chains in G-P and G-B molecules were simplified as methyl group. The calculated HOMO and LUMO energy of G-P and G-B were -6.355, -2.478, -6.081 and -2.184 eV, respectively. For G-P, the HOMO primarily resided on the pyridine and naphthalene imide ring and the LUMO primarily resided on naphthalene imide ring. However, the HOMO and LUMO of G-B all primarily resided on naphthalene imide ring. This calculation results well explained the green light emission of G-P and blue light emission of G-B in acetonitrile solution. More importantly, the low-occupying LUMO of G-P and G-B were much higher than that of corresponding TNP (-3.90 eV) which was a criterion for electron transfer to occur from the probes to NACs68. It could be well explained why G-P and G-B could selectively detect TNP via comparing with the pKa of G-P, G-B and TNP. Theoretically calculation of pKa of protonation pyridine in G-P was 4.06 which clearly demonstrate the possibility of complex formation between G-P and TNP (pKa for TNP = 0.38, NP = 7.16, DNP = 4.00, NBA = 3.40,71 NT = 20.4,72 DNT = 17.12,73 NB = 36.2,74 the pKa value of DNB should be large and not be listed here.). The electron transfer from G-P and G-B to TNP could be further proofed via time-resolved emission spectral experiment (Figure S13). The average lifetime of G-P in aqueous solution was about 3.73 ns. With the addition of TNP, fluorescence lifetime was gradually decreased from 3.73 ns to 1.12 ns (0.5 equiv. TNP) and 0.94 ns (1.0 equiv. TNP). Fluorescence lifetime of G-B acetonitrile solution was also decreased from 3.04 ns to 2.95 ns (0.5 equiv. TNP) and 2.92 ns (1.0 equiv. TNP) with addition of TNP.


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Figure 6 (a) Fluorescence titrations of G-P in dilute solution (10-5 M) upon addition of TNP; (b) Fluorescence titrations of G-P in dilute aqueous solution (10-5 M) upon the addition of TNP; (c) Job's plot for the wavelength of emission at 493 nm was established to evaluate the stoichiometry of G-P-TNP complex. The total G-P and TNP concentration was 10-5 M in acetonitrile; (d) 1HNMR titration of G-P upon gradual addition of TNP (0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4 equiv.) in CDCl3/CD3OD (5:1 v/v). Inset showing assigned protons of G-P for convenience.

To unveil the structure-property relationship, absorption, fluorescence as well as HNMR characterizations should also be carried on G-B in the absence or presence of TNP. The UV-Vis absorption and fluorescence emission spectra of G-B solution (10-5 M) in acetonitrile under addition of TNP were measured and showed in Figure S14. With the addition of TNP, the UV-Vis absorbance of G-B solution was gradually increased. Especially, the absorbance of G-B solution at the range of 400-460 nm was developed from zero with the addition of TNP which showed the increasement in absorbance of G-B solution was due to addition of TNP. The fluorescence emission intensity of G-B solution was gradually decreased with the addition of TNP. Comparing with the absorption spectrum of TNP and fluorescence spectra of G-B solution, the emission spectrum of G-B solution had a good overlap with the absorption bands of TNP at the range of 400-460 nm. From these results, the fluorescence quenching of the G-P and G-B solution with


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addition of TNP had different quenching mechanism. In order to further understand the process, the 1HNMR titration experiment of G-B upon gradual addition of TNP was carried out in Figure S15. The proton singals of H2 and H5 belonging to naphthalimide were obviously shifted to downfield with the addition of TNP. Comparing with the 1HNMR experiment results of G-P solution upon addition of TNP, the proton signals of the benzene ring at C-4 position of G-B molecule did not have shift, not like the proton signals of pyridine of G-P molecule with a large shift. To investigate the ICT transition characteristic of G-P, the fluorescence emission spectra of G-P were measured in different solvents. As shown in Figure S16, significant solvatochromic effect could be observed with the increase of environmental polarity. The fluorescence emission spectra of G-P shift from the peak of 400 nm in nonpolar hexane or petroleum ether to the peak of 465 nm in higher polar ethyl acetate and the peak of 518 nm in methanol with highest polarity. From this experiment results, it validated the presence of ICT process in G-P.75 The general mechanism of excited state intermolecular proton transfer (ESIPT) is that a fast excited-state proton transfer from a proton donor (usually hydroxyl or amino group) to an acceptor group (often either oxygen or nitrogen) mediated by an intermolecular or intramolecular hydrogen bond (H-bond). For G-P, it was easy for intermolecular hydrogen bond between G-P and TNP, and the presence of excited state intermolecular proton transfer (ESIPT) was logical. In order to further prove the presence of ESIPT process, the UV-Vis absorption and fluorescence properties of protonated G-P were investigated in detail (Figure S17). The G-P solution in acetonitrile with the concentration of 10-5 M was protonated with CF3COOH solution in acetonitrile. With the addition of CF3COOH, the absorbance of G-P solution didn't have obvious change. The fluorescence emission intensity was only decreased by 9.97% even if 5.0 eq. CF3COOH was added to G-P solution. From this experiment result, it clearly showed that the ESIPT processes were involved in the fluorescence quenching procedures of G-P toward TNP. From the above experiment results, the possible detection mechanism of TNP based on G-P at molecule level was proposed and showed in Figure 7.


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Figuer 7 The propose detection mechanism of TNP based on G-P.

It is well known that gel can be formed and immobilize the solvents in three-dimensional network via intermolecular non-covalent interactions. These intermolecular non-covalent interactions endowed gel system with the advantage of being highly solvated and the rheological properties of solid-phase materials on analytical timescales. So, gel system could be potentially manipulated like solids, and gel system itself could intimately contact with environmental liquid phases due to their high surface areas and rapid internal diffusion kinetics. We had reported several gel systems with the ability of detecting cysteine and amine in gel state.46, 76 Herein, gel G-P was used as smart materials for TNP removal from water. The gel G-P was prepared using 5 mg of G-P gelator and 400 µL of mixed solvent of acetonitrile / H2O (1/1, v/v). The concentration of TNP aqueous solution for test was 5×10-4 M. 2 mL of TNP solution was carefully added to the gel surface. The removable amount of TNP was monitored by UV-Vis spectrum as shown in Figure 8a. The removable rate was fast at first few hours. For example, 22.63 % of TNP was removed via absorption into gel G-P system in one hour. Along with the prolonged time, the removable rate was gradually slowed. The whole removable efficiency was reached to 56.86 % in 24 hours. The TNP removal process via gel G-P absorption was also monitored by fluorescence spectrum. The fluorescence emission intensity of gel G-P after adsorption TNP for 24 hours was obviously weakened about 75.81% compared to the original one. The fluorescence emission band of gel G-P was blue-shifted form 482 nm to 464 nm before and after removal TNP. At the same time, the color of gel G-P was changed from white to yellow (the inset in Figure 8b). In the whole experiment process, the gel G-P was completely intact. In addition, the TNP removal experiment through G-B was also carried out for comparison in Figure S18. The absorbance of TNP solution onto G-B gel did not have any obvious change with the time going. From the experiment result, it showed that the G-B gel did not have the removal ability of TNP like the G-P gel. The above


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results showed that gel G-P could not only detect TNP in gel state, but also had the ability of removal TNP from aqueous solution. a

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Figure 8 (a) Time-dependent UV-Vis absorption change of TNP aqueous solution onto gel G-P surface; (b) Fluorescence emission change of gel G-P before and after absorption of TNP.

To further check whether the solid state of G-P can detect TNP, we used test strips as a solid support to prepare a solid probe based on G-P. Filter paper was cut into 1 cm2 pieces and dipped into THF solution of G-P for 1.0 min and then dried naturally at room temperature. The paper strip emitted strong blue light under 365 nm UV light. First, we investigated the detection ability of solid TNP using a solid probe which was prepared through filter paper dipping into THF solution of G-P with the concentration of 10-3M (Figure 9a). The blue emission of the paper strip was obviously quenched under the cover of TNP solid. In order to verify the detection sensitivity of the paper strip, the filter paper probe was prepared through dipping into 10-4 M THF solution of G-P for application in detection TNP aqueous solution. The different concentrations of TNP were dropped onto the test strips. The fluorescence emission intensity was great decreased until the TNP solution concentration was up to 10-8 M as showed in Figure 9b. These results well demonstrated that G-P has the potential to be used as solid-state probe to detect TNP solid or solution.


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Figure 9 Photographs of the test strips under a UV lamp (irradiation of 365 nm) for TNP visual detection; (a) TNP solid, (b) dropping TNP solution s with different concentrations (M L-1).

The selectivity for TNP of gelator G-P using test strips was carried out. As showed in Figure S19, the gelator G-P in the format of test strip also exhibited a good selectivity for TNP. The fluorescence emission intensity of G-P test strip with dropping of TNP solution was almost decreased. For DNP solution, the fluorescence emission intensity of G-P test strip also had a certain amount of weakening. The fluorescence emission intensity of G-P test strips had no obvious change with dropping of other nitroaromatic compounds. From these experiment results, the gelator G-P also featured a good selectivity for TNP detection in test strips. Conclusion In summary, we have fabricated two simple gelators (G-P and G-B) based on naphthalimide which can form organogel in some solvent or mixed solvent. The two molecules had different photophysical property, pKa and self-assembly behavior due to their different substituent groups. However, both of the gelators can selectively detect TNP in organic solvent. Interestingly, G-P can also detect TNP in pure water as well as in organic solvent. To the best of our knowledge, this is the first report of naphthalimide-based compound with the ability of TNP detection in pure water. The electron transfer phenomenon from an electron-rich chromophoric compound (G-P) to electron deficient TNP was observed through 1HNMR titration and fluorescence lifetime experiments. The selectivity in sensing TNP depended on the pKa of compound. The portable test strips were made successfully for rapid on-sight detection of TNP solid and TNP solution, providing a new strategy for the construction of NACs sensors. In addition, the gel G-P exhibited outstanding sensing and removal ability for TNP in solution due to high surface areas and rapid internal diffusion kinetics of gel networks. The present study will provide a new insight into the development of effective naphthalimide-based explosive sensors in the future.


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Acknowledgments Cao et al. gratefully acknowledge Prof. Junwei Zhao of Henan University for partial experimental support and research fellowships. The authors thanks for the financial support by the National Natural Science Foundation of China (No.51373039, 21401159), Program for University Innovative Research Team of Henan (2012IRTSHN017), the Science and Technology Key Project of Henan Education Department (13A150760), Institute for Conservation and Utilization of Agro-bioresources in Dabie Mountains, Funding scheme for the young backbone teachers of higher education institutions in Henan Province (2015GGJS-141), the Science & Technology Innovation Talents in Universities of Henan Province (No. 17HASTIT005) and the Nanhu Scholars Program for Young Scholars of XYNU. REFERENCES 1.

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