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Highly Fluorescent Metal-Organic Frameworks Based on Benzene-Cored Tetraphenylethene Derivative with the Ability to Detection of 2,4,6-Trinitrophenol in Water Yangjun Deng, Nanjian Chen, Qiyang Li, Xiuju Wu, Xiaoli Huang, Zhihua Lin, and Yonggang Zhao Cryst. Growth Des., Just Accepted Manuscript • Publication Date (Web): 12 May 2017 Downloaded from http://pubs.acs.org on May 13, 2017

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Crystal Growth & Design

Highly Fluorescent Metal-Organic Frameworks Based on Benzene-Cored Tetraphenylethene Derivative with the Ability to Detection of 2,4,6Trinitrophenol in Water Yangjun Denga, Nanjian Chenb, Qiyang Lia, Xiuju Wua, Xiaoli Huanga, Zhihua Linb,*, Yonggang Zhaoa,* a

Key Laboratory of Flexible Electronics (KLOFE), Institute of Advanced Materials (IAM),

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing 211816, PR China b

College of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211816, PR China

KEYWORDS : Metal-Organic Frameworks, AIE, Fluorescence, Sensor, 2,4,6-Trinitrophenol

ACS Paragon Plus Environment

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ABSTRACT：：One tetracarboxylate functionalized benzene-cored TPE derivative was designed as linker for construction of AIE active metal-organic frameworks (MOFs), which exhibit the distinct aggregation-induced emission (AIE) behavior. Two two-dimensional (2D) MOFs materials, MOF-1 and MOF-2 have been successfully synthesized by using the organic linker precursor. Photophysical studies reveal that both MOFs exhibit remarkable fluorescent properties with the absolute quantum yield as high as 43%. The high PL efficiency, good water stability, as well as the electron-donating nature of prepared MOF materials, allow us to explore their potentials in sensing applications for electron-deficient nitro explosives in aqueous media. Especially, water-stable MOF-2 exhibits superior sensitivity and selectivity toward 2,4,6trinitrophenol (TNP) over other selected nitro analytes, including nitromethane (NM), 2,3dimethyl-dinitrobutane (DMNB), nitrobenzene (NB), 2,4,6-trinitrotoluene (TNT), 2,6dinitrotoluene (2,6-DNT), 2,4-dinitrotoluene (2,4-DNT), 1,3-dinitrobenzene (1,3-DNB), with the detection limit for TNP as low as 0.49 μM （≈110 ppb）. In virtue of its excellent detection efficiency, applicability in water and reusability, MOF-2 can be a promising fluorescent sensor material for the practical TNP sensing.

INTRODUCTION Aggregation-induced emission is a fascinating photophysical phenomenon with emission enhancement by aggregate formation, and has gained considerable interests both in fundamental and practical investigations due to the potential applications in optoelectronic systems, chemical sensing and biological sciences.1-3 AIE luminophores are usually non-emissive or weakly fluorescent in dilute solution, but turn to highly emissive upon aggregation, which can be


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rationalized to the elimination of the non-radiative energy dissipation of the singlet excited state by the restriction of intramolecular rotations and hence making the AIE luminophores highly emissive. Oppositely, many conventional organic luminophores are subjected to aggregationcaused quenching (ACQ) in condensed state, owing to the aggregation or the formation of excimers associated with strong π–π interaction. The ACQ effect not only presents a dilemma for conventional luminophores in their solid-state applications such as light-emitting motifs, but also limited their practical applications in sensing or biomedical imaging due to the tendency to aggregate in the polar media or in hydrophobic pockets of bio-systems, resulting in fluorescence quenching. From this point of view, luminophores with AIE characteristics are generally more appealing. Among these reported AIE-active luminophores, tetraphenylethene (TPE) and its derivatives are an important class of luminophores with typical AIE characteristics.4-9 TPE molecules adopt a propeller-shaped conformation, comprised of four phenyl rotors and one C=C bond as stator. In the dilute solutions, fast rotations of the aromatic rotors and partial twisting olefinic double bond dissipate the exciton energy through non-radiative pathways. Upon the aggregate formation, the intermolecular steric interactions restrict the rotational and twisting motions and hence turn on the fluorescence.10 Due to their accessible synthesis and facile modification, the TPE-based AIEactive materials have been under extensive studies and have already exhibited practical applications in various fields, such as OLEDs,11-13 chem-sensors,14-16 and bio-probes.17-19 Meanwhile, many benzene-cored TPE derivatives carrying multiple aromatic peripherals have been reported, and all show typical AIE behaviors: as distinct from their weak fluorescence in dilute solution, all these AIE luminogens are highly emissive in the condensed phase, with extremely high Φ (quantum yields).20-22


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As a new type of inorganic-organic hybrid materials, Metal-Organic Frameworks (MOFs), 23-27 provides an opportunity for combining the inorganic building blocks and organic AIE linkers to prepare high fluorescent materials, since the intromolecular motions could be further blocked within the rigid MOFs matrixes, and thus enhance the fluorescence.28-30 Some carboxylate functionalized TPE derivatives have been successfully designed as organic linkers to costruct rigid MOF materials with good luminescence performance,31-34 and some of which show great potential in sensing application.35-37 To enrich the research of such kinds of MOF materials, in this study, we employed a tetracarboxylate functionalized TPE derivative H4L as organic linker to construct AIE active fluorescent MOFs materials. Two compounds MOF-1 and MOF-2 were successfully prepared, and both exhibit good fluorescence performance. Our study also shows that MOF-2 would have great potential in selective and sensitive explosive sensing in aqueous media. EXPERIMENTAL SECTION Materials and Measurements. All of solvents, reagents and nitro explosives were commercial available and used as received . All of nitro explosives analytes used were purchased from Aldrich or TCI and used as received. Standard column chromatography methods were employed for chromatographic separation purpose. 13C NMR and 1H NMR spectra were recorded on a Bruker AV 400 spectrometer. Thermogravimetric analyses (TGA) were conducted on a Mettler Toledo analyzer with heating rate of 10 ºC per min under N2. Absorption spectra and fluorescent emission spectra were measured on a Shimadzu UV-1750 spectrophotometer and a F-4600 fluorescence spectrometer, respectively. Absolute quantum yield data were obtained using an Edinburgh Instruments F900 analytical spectrometer equipped with integrating sphere using BaSO4 as white standard. Fluorescence lifetime analyses were conducted on an Edinburgh


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FLS 980 fluorescence spectrometer. Powder X-ray diffraction patterns (PXRD) were collected on a Bruker D8 Advance X-ray diffractometer equipped with graphite monochromatized Cu Kα radiation (λ = 1.54 Å). Elemental analysis of C, H and N were carried out on a Vario EL Cube elemental analyzer. FT-IR spectra were obtained on an ALPHA PLATINUM-ATR Spectrometer. Synthesis of Zn2(L)(H2O)(DMA)·DMA (MOF-1): H4L (0.01 g, 0.013 mmol) and Zn(NO3)2·6H2O (0.029 g, 0.1 mmol) were placed in a 20ml Pyrex vial containing 4.2 mL N,N’dimethylacetamide (DMA). The mixture was first stirred under sonication for 10 min, then 4.2 mL water and 2 µL tetrafluoroboric acid were added. The sealed vial was then moved to an oven and heated at 80 °C for 48 hours. Colorless block crystals of MOF-1 were filtrated and then washed with DMA, and dried in air (yield: 48% based on H4L). Elemental analysis of MOF-1, Calculated (%): H 4.66, C 64.40, N 2.59; found: H 4.32, C 64.49, N 2.08. IR (KBr, cm-1): 3035 (w), 2916 (w), 1683 (s), 1603 (m), 1395 (s), 1306 (w), 1263 (m), 1174 (w), 1104 (w), 1016 (m), 857 (m), 770 (s), 698 (m), 570 (w). Synthesis of Zn2(H2L)2(Bpy)2(H2O)3·H2O (MOF-2):

H4L (0.01 g, 0.013 mmol),

Zn(NO3)2·6H2O (0.029 g, 0.1 mmol), and 4,4’-bipyridine (0.008 g, 0.051 mmol) were placed in a 20 mL Pyrex vial containing 4.5 mL DMA. The mixture was first stirred under sonication for 10 min, then 4.5 mL water and 2 µL tetrafluoroboric acid were added. The sealed vial was then moved to an oven and heated at 65 °C for 48 hours. Colorless block crystals of MOF-2 were filtrated and washed with DMA, and drying in air (yield: 67% based on H4L). Elemental analysis of MOF-2, Calculated: H 4.41, C 70.14, N 2.73; found: H 4.39, C 69.94, N 3.08. IR (KBr, cm-1): 3103 (w), 2949 (w), 2788 (w), 2613 (w), 2485 (w), 1681 (s), 1601 (m), 1586 (w), 1395 (s), 1263 (m), 1174 (w), 1104 (w), 1016 (w), 875 (w), 770 (m), 748 (m), 698 (m), 634 (w), 570 (w), 517 (m), 504(m).


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Crystal Structure Determination. The single crystal X-ray diffraction measurements were performed on a Bruker Apex2 CCD diffractometer with Mo-Kα radiation (λ =0.71073 Å), operating at 50 kV and 30 mA. The structures of MOF-1 and MOF-2 were solved with using direct methods and a full-matrix least-squares refinement was performed with the SHELXTL program.38 The diffraction data of MOF-2 were corrected for empirical absorption based on multi-scan. The reflection data were corrected by using SADABS program. For all non-hydrogen atoms anisotropic thermal parameters were applied, and all hydrogen atoms of organic ligands were calculated and added at ideal positions. The crystallographic data of MOF-1 and MOF-2 are given in Table 1, and selected bond lengths and angles are given in Table S6 and Table S7 (Supporting Information). CCDC numbers are 1528983 and 1528984 for MOF-1 and MOF-2, respectively. Table 1. Crystal Data and Structure Refinement of the Title Compounds compound

MOF-1

MOF-2

C58 H50 N2 O11 Zn2

C120 H90 N4 O21 Zn2

fw

1081.74

2054.74

cryst syst

Triclinic

Monoclinic

P-1

P21/m

a (Å)

10.802(6)

9.772(1)

b (Å)

14.229(9)

35.252(4)

c (Å)

16.886(9)

15.865(2)

α (deg)

75.616(16)

90

β (deg)

87.314(15)

101.719(2)

γ (deg)

78.720(16)

90

V (Å3)

2465(2)

5351.2(11)

temp (K)

296(2)

296(2)

2

2

Dcalcd (g·cm−3)

1.457

1.275

µ [mm-1]

1.040

0.520

chem formula

space group

Z


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F [000] θ [º] Reflections collected/unique Goodness-of-fit on F

2

R1 , wR2 [I > 2σ(I)] a

a

b

Rint R1 = Σ||F0| − |Fc||/Σ|F0|.

b

1120

2132

2.175-25.000

1.155-25.000

16718/8611

38468/9525

0.954

0.895

0.0937, 0.1971

0.701, 0.1772

0.1510 wR2 = [Σw(F0 − Fc ) /Σw(F02)2]1/2. 2

0.1028

2 2

Fluorescence Titration Experiments. MOF materials or H4L ligand (3 mg) were placed in water or methanol (3 mL), and treated by ultrasonication for one hour. Before the fluorescence titration experiments, the resulting suspensions were kept undisturbed for 24 hours to obtain stable suspensions. The fluorescence titration experiments were performed by the incremental addition of aqueous or methanol solution of various nitro explosives (1 mM, up to 400 μL) to a 2 mL aqueous suspension of MOF materials or methanol suspension of H4L. RESULTS AND DISCUSSION


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Figure 1. (a) Depiction of coordination mode of H4L ligand in MOF-1. (b) Depiction of the bilayer structure containing 1D channels in MOF-1 along the a-axis. (c) Topological 4,4connected 2D net of MOF-1. (d) Dense packed bilayers in MOF-1 along the b-axis. Structure description of MOF-1. MOF-1 crystallizes in the triclinic P-1 space group, with a porous 2D bilayer structure composed of unusual triangular-prismatic-like di-nuclear Zn(II) secondary building blocks (SBUs). Of the di-nuclear Zn(II) building blocks, there are two fivecoordinated, crystallographically independent Zn(II) centers: Zn1 is bridged by three L4- ligands from the layer A; the other one Zn2 is bridged by four L4- ligands, three of which from layer A and the last one from the neighboring layer B. Thus a double layer motif is formed (Figure 1). Each Ligand bridges four di-nuclear Zn(II) building blocks from two layers, in which two carboxylate groups adopt dimonodentate coordination to two Zn(II) centers of separate SBUs, one carboxylate group adopts both dimonodentate and monodentate coordination to two Zn(II) centers, and the last one is bound to one Zn(II) center (Zn2) from another layer in monodentate fashion. To finish the five-coordinated trigonal bipyramidal configuration, in the di-nuclear building block one zinc center (Zn2) is bound to a terminal DMA molecule, while the other one (Zn1) is bounded to a terminal water molecule. The asymmetry of the di-nuclear Zn(II) building block is quite pronounced with the Zn–O contact in the range 1.91–2.60 Å. From the viewpoint of topology, the 2D bilayer sheet could be simplified as a bi-nodal 4,4-connected net (Figure 1c). Moreover, by the aid of intermolecular H-bonding interactions involving the carboxylate groups and the coordinated DMA molecule, the close-packed 2D nets expand the structure to the third dimension. One-dimensional (1D) channels with the window sizes of ca. 5.7 × 4.5 Å2 (excluding van der Waals radii of the opposite atoms) is found in the hydrogen bonded supramolecular 3D framework contains along the a-axis, with space fulfilled by DMA and H2O solvent molecules.


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Figure 2. (a) Depiction of coordination environment around two independent metal centers and consequential two types of 1D zigzag chains in MOF-2. (b) Depiction of the wave-like rectangular grad layer in MOF-2. (c) Dense packed 2D nets of MOF-2. Structure description of MOF-2. MOF-2 crystallizes in the monoclinic P21/m space group, in which two crystallographically independent mononuclear Zn(II) centers are found: Zn1 adopts a trigonal bipyramid geometry, and is coordinated by two carboxylate oxygen atoms, two nitrogen atoms from 4,4'-Bipyridine (Bpy) and one terminal water molecule; Zn2 possesses an octahedral coordination geometry with bonding to two H2L2- ligands, two Bpy as well as two water molecules. It is interesting to note that unlike in MOF-1, the ligand in MOF-2 adopts a nearly linear bismonodentate mode to bridge two identical Zn(II) centers to form two different


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1D zigzag chains (Figure 2a), in which the other two carboxylate groups were not involved. The two different 1D zigzag chains are alternately arranged and further linked by co-ligand Bpy, to generate a wave-like 2D rectangular grad layer structure (Figure 2b). The adjacent layers are further closely stacked through O-H…O contacts into a 3D structure.

Figure 3. (a) Fluorescent emission spectra of H4L in DMF/H2O mixture at different volume ratios. The inset represents photographs of a solution/suspension of H4L in pure DMF (left) and a DMF–water mixture containing 99% volume fraction of water (right) taken under the irradiation of a handheld UV lamp. (b) Solid-state emission spectra of MOF-1, MOF-2 and H4L. Optical Properties of Compounds MOF-1 and MOF-2. To investigate the photophysical properties, the photoluminescence spectra of the H4L and its two coordination compounds MOF1 and MOF-2 were measured. As expected, the H4L ligand shows an obvious AIE behavior. The fluorescence emission spectra of H4L in a pure DMF solution shows no detectable signals, but upon the addition of water, the significant increase in emission intensity can be visually observed under UV illumination (Figure 3a), and the associated quantum yield of H4L in a DMF-H2O mixture containing 99% volume fraction of water measured by integrating sphere reaches 22%.


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Upon the excitation at 376 nm, the ligand emits strong fluorescence with the maximum emission peak being located at 503 nm in the solid state, with the absolute quantum yield to be 37%. The enhancement of light-harvesting efficiency can be contributed to the AIE process, in which the intramolecular motions have been greatly restricted in the dense state. The fluorescence spectra shows that in the solid state both MOF materials have strong emissions with the maximum emission peaks at 477 nm and 484 nm, respectively, slightly blue shift vs the H4L precursor (Figure 3b). The blue-shift emission of MOF-1 and MOF-2 could be contributed to the twist conformation of the H4L ligand in the frameworks.31 In both MOF materials, the average dihedral angles between the central plane of the double bonds and four conjoint phenyl rings are quite large, which can result in the breakdown of the conjugate system of the ligand, and consequently lead to the widened energy gap (Figure 4). The absolute fluorescence quantum yields are 37% for MOF-1 and 43% for MOF-2, which are comparable to that of H4L (37%) in its solid state (Figure 3b). The high quantum yield of MOF-1 and MOF-2 can be ascribed to the immobilization of TPE-based linkers within the rigid MOF matrixes via the formation of coordination bonds to the metal centers, as well as the closely packing observed in both structures.

Figure 4. The average dihedral angles between the central plane of the double bonds and four conjoint phenyl rings in MOF-1 and MOF-2.


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MOFs for nitro-explosives sensing. High photoluminescence (PL) efficiency of MOFs materials often provides excellent sensing performance, especially in the detection of electrondeficient nitro explosives. Fluorescent MOFs are usually excellent electron donors because of the existence of a degree of conjugation in the organic linkers. Their donor ability can be efficiently enhanced upon the excitation, which further increases the electrostatic interaction between MOFs and electron-deficient nitro explosives through the exciton migration.39 Also, their band gaps can be easily tuned by the changes to the connectivity between organic linkers and metal centers or inorganic building blocks in the framework, and such tunability is very important for sensing purpose as it directly relates to MOF materials’ optical properties. Taking these advantages, as well as the large functional surfaces and tunable porosity, fluorescent MOFs acting as chemosensors for nitro explosives have been extensively studied.40-47 However, the poor water stability for most MOFs limits their applications to organic solvents, and the detection in aqueous media is highly desirable.48-56 To evaluate the applicability of prepared MOF materials for the detection of electron-deficient nitro explosives in aqueous media, the PL properties of its dispersions in water and other common organic solvents, including acetone, dichloromethane, npropanol, acetonitrile, 1,4-dioxane, N,N-dimethylformamide, DMA, methanol, ethanol, tetrahydrofuran, were investigated (Figure S5). Spectra measurements show the significant solvent effect in fluorescence intensities, and from which MOF-2 exhibited strong fluorescence in water with the framework remains unaffected as revealed by the almost same PXRD patterns (Figure S2). Encouraged by the efficient solid-state emission as well as the good water stability of MOF-2, we start to examine its ability for sensing of nitro explosives, including 2,4,6trinitrophenol (TNP), nitromethane (NM), 2,3-dimethyl-dinitrobutane (DMNB), nitrobenzene (NB), 2,4,6-trinitrotoluene (TNT), 2,6-dinitrotoluene (2,6-DNT), 2,4-dinitrotoluene (2,4-DNT)


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and 1,3-dinitrobenzene (1,3-DNB). The titration experiments were carried out with the incremental addition of freshly prepared 1mM stock solutions of nitro analytes into the aqueous suspension of MOF-2, and changes in the fluorescence intensity were recorded. Upon the gradual addition of TNP, MOF-2 showed very efficient fluorescence quenching at 484 nm (Figure 5a), but only minor fluorescence quenching was detected for all of other nitro explosives (Figure 5b), which clearly suggests the highest selectivity of MOF-2 towards TNP in all selected nitro explosives. In addition to TNP, MOF-2 showed different response in presence of 2,4-DNT, 2,6-DNT and other analytes, and this can be ascribed to the difference in their electronwithdrawing ability.39,57 The nitro explosives sensing performance of MOF-1 and the ligand H4L were also studied for a comparison. Due to the fact that PXRD pattern of MOF-1 with water treatment suggests structural change of the framework (Figure S1), as an alternative, the titration experiments of MOF-1 were carried out in methanol. As shown in Figure S14, MOF-1 exhibited the similar selectivity towards TNP over other selected nitro explosives. Oppositely, considerable decrease in selectivity was observed for the H4L ligand itself as sensor in detection of TNP and other nitro explosives (Figure S16).


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Figure 5. (a) Fluorescence quenching of MOF-2 (λex = 395 nm) with addition of TNP (1 mM, from top 0 - 400 μL) in water. (b) Quenching efficiency plot for MOF-2 in the presence of selected nitro analytes (400 μL each). With using Stern–Völmer (S-V) equation (I0/I) = 1 + KSV[Q], we can calculate the quenching constant of MOF-2 to TNP, where I is the fluorescence intensity at TNP concentration of [Q], and I0 is the fluorescence intensity at [Q]=0, KSV is the quenching constant. By fitting the linear plot to the Stern–Völmer equation at lower concentrations of TNP, the KSV was calculated as 1.36×104 M-1, with the detection limit being calculated as low as 0.49 μM, approximately 110 ppb. The very low detection limit makes MOF-2 a good candidate as chemosensor to detect the existence of trace of TNP in water. Moreover, with using a handheld UV lamp (λmax= 365 nm), the TNP induced fluorescence quenching of MOF-2 can be easily distinguished by the naked eye (Figure 6), demonstrating the applicability of MOF-2 for real-time detection of TNP. As shown in Figure 7a and S17, in the SV plot the non-linear feature at higher concentration of TNP indicates the simultaneous attendance of both static and dynamic quenching mechanisms in sensing.58-60 To explore the mechanism, time-resolved fluorescence quenching experiments were performed by monitoring the fluorescence decays of MOF-2 before and after addition of TNP and other nitro explosives. The decay profiles show only slight changes in the lifetime values (Table S4) were recorded for all selected analytes, of which the mean lifetime of excited state were found to be 2.09 ns without TNP, and 1.85 ns with TNP (400 μL), respectively. These mean lifetime values indicate that the quenching process is essentially static.


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Figure 6. Upon the addition of various nitro explosives aqueous solution (400 µL each) in aqueous suspension of MOF-2 under UV light (λmax = 365nm). Encouraged by these results, we conducted competitive experiments to exclude the possibility of interference from other nitro explosives in TNP sensing (Figure 7b). Take TNT for example, upon the addition of aqueous TNT solution (1 mM, 80 µL) in the dispersion of MOF-2 in water, very little change in fluorescence intensity was monitored. Then an equal amount of aqueous TNP (1 mM) was added and induced significant fluorescence quenching. After that aqueous TNT and TNP solutions were added in sequence, the recorded changes in fluorescence intensity exhibit the quenching efficiency of TNP on MOF-2 remained unchanged. For all other nitro analytes, the similar trends were found, suggesting the exceptional selectivity of MOF-2 towards TNP.


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Figure 7. (a) Stern–Völmer plot of MOF-2 with incremental addition of different nitro analytes. (b) Fluorescence quenching of MOF-2 with the addition of different nitro analytes followed by TNP. To meet the requirements of real-life sensing applications, the reusability of a sensor material is very important. The reusability of the MOF-2 for sensing TNP is tested by recycling experiments (Figure 8). The test reveals that the used MOF-2 sample can be easily recovered by washing with organic solvents, and can be reused for many cycles of detection. After each cycle of repetition, the recorded initial fluorescence intensities of MOF-2 remained nearly constant, and the PXRD patterns indicate the crystallinity of MOF-2 samples was retained (Figure S20), demonstrating the high photostability and reusability of MOF-2 in TNP sensing, as well as showing the advantages and benefits of MOFs as sensor materials. Besides, to use the MOF-2 for in-field application, we took real sample (e.g. tap water, pond water, etc.) for the test. In all tests we done, MOF-2 still shows the high selectivity and sensitivity toward TNP over other selected explosives (Figure S21).


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Figure 8. The recyclability test of MOF-2 in water. The purplish red and grey bars represent the fluorescence intensities of MOF-2 before and after the addition of TNP in each cycle, respectively. Mechanisms for the TNP sensing. To obtain further understanding on the high selectivity of MOF-2 towards TNP, the sensing mechanisms were investigated. One possible quenching mechanism can be proposed to be the photo-induced electron transfer (PET). Generally, for electron-deficient nitro analytes, their lowest unoccupied MOs (LUMOs) lie somewhere in between the conduction band (CB) and valence band (VB) of the MOFs containing extended network (Table S5). The excited electrons from the CB of the MOFs can be transferred to the LUMO orbitals of nitro analytes upon excitation, consequently leading to a quenching effect. 41, 61-62

39-

Therefore, as electron acceptor, the smaller the LUMO energy of the nitro analyte, the

better the electron accepting ability and the better quenching efficiency. Accordingly, the highest selectivity for detection of TNP is consistent with its lowest LUMO energy level by comparison with all other selected nitro analytes. Nevertheless, the trend of LUMO energy values of all other


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nitro analytes is not fully consistent with the order of quenching efficiency, indicating that PET mechanism is not the sole mechanism governing the quenching behavior. The non-linear feature in the SV plot at higher concentration of TNP may also imply that another mechanism takes effect simultaneously, namely fluorescence resonance energy transfer (FRET).55-56,

63

For the

effective energy transfer between the fluorophore and the analyte, the efficient spectral overlap between the absorption spectrum of the analyte and the emission spectrum of the fluorophore is highly required. Further studies of the absorption spectra of selected nitro explosives show the greatest spectral overlap between the absorption spectrum of TNP and the emission spectrum of the MOF-2 (Figure S22), while negligible spectral overlap for all of the other nitro analytes. These results suggest that the efficient energy transfer from the organic linker in the framework of MOF-2 to TNP can occur upon the excitation, hence quench the fluorescence, therefore the FRET could be one major mechanism for the TNP induced highest quenching efficiency. It should be noted that, the blue-shift emission of MOFs in comparison with the ligand can effectively increase the spectral overlap with facilitating the energy transfer process and enhancing the quenching performance. The observed lower selectivity of H4L for TNP can be also explained by the smaller spectral overlap with the absorption spectrum of TNP. Besides, TNP is essentially an acid. The existence of electrostatic interactions between the Lewis basic pyridine moiety of Bpy and highly acidic hydroxyl group of TNP could also be one possible mechanism for the TNP induced highest quenching efficiency, since other selected explosive do not possess any acidic hydroxyl group.59,64 To verify the roles of electrostatic interactions in sensing, fluorescence titration experiments with 2,4-dinitrophenol (2,4-DNP) and 4-nitrophenol (NP), as well as p-nitrobenzoic acid (PNB) were performed. Considerable fluorescence quenching was found with incremental addition of 2,4-DNP or NP, but only minor


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quenching was observed for PNB. Furthermore, the fluorescence spectra of MOF-2 at different pH values (2-7) were tested. The results show that the acidic solution has little effect on the fluorescence of MOF-2 (Figure S27). Therefore, although the quenching performance of TNP, NP and 2,4-DNP is consisitent with the trend of their acidity, the high quenching efficiency is not mainly related to the electrostatic interactions, which is most likely due to the lack of free Lewis basic recognition sites in MOF-2. This can be also supported by the high selectivity of MOF-1 for detection of TNP, since the Lewis basic site is absent in MOF-1. The high quenching efficiency of TNP, 2,4-DNP and NP could be attributed to the massive spectral overlap between their absorption spectra and the emission spectrum of MOF-2. Based on the results above, the combined effects of PET and FRET mechanisms can be attributed to the highest sensitivity and selectivity of MOF-2 towards TNP. CONCLUSION In conclusion, two fluorescent 2D metal-organic frameworks MOF-1 and MOF-2 based on benzene-cored TPE derivative were successfully synthesized and investigated with their fluorescent properties, thus enriching the family of AIE active MOF materials. Photophysical studies reveal the good fluorescence performance of both MOFs when in solid state or being dispersed in different solvents, including water. Moreover, MOF-2 shows the potential for highly sensitive and selective detection of TNP in aqueous media, and can be reused after cycles of testing, indicating it’s a promising candidate for practical applications as chemical sensors. ASSOCIATED CONTENT The

Supporting

Information

is

available

free

of

charge

via

the

Internet

at

http://pubs.acs.org.on. Synthetic routes to the ligand, PXRD, TGA, FT-IR, Absorption and


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Emission spectra, Quenching-efficiency plot, Stern–Völmer plots, HOMO and LUMO energies, and Crystal data etc. AUTHOR INFORMATION Corresponding Author *Email: [email protected]. *Email: [email protected]. ACKNOWLEDGMENTS We are grateful to the National Natural Science Foundation of China (21471079 and 21501092), the NSF of Jiangsu Province for Youth (BK20140928), the NSF of Jiangsu Province for colleges and universities (13KJB150017), and Nanjing Tech University for their financial support to this work.

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For Table of Contents Use Only Highly Fluorescent Metal-Organic Frameworks Based on Benzene-Cored Tetraphenylethene Derivative with the Ability to Detection of 2,4,6Trinitrophenol in Water Yangjun Denga, Nanjian Chenb, Qiyang Lia, Xiuju Wua, Xiaoli Huang a, Zhihua Linb,*, Yonggang Zhaoa,*

Highly fluorescent metal-organic framework containing benzene-cored AIE luminogens demonstrates superior sensitivity and selectivity toward 2,4,6-Trinitrophenol.


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