Fluorescent Porous Organic Frameworks Containing Molecular Rotors

Article pubs.acs.org/cm

Fluorescent Porous Organic Frameworks Containing Molecular Rotors for Size-Selective Recognition Jinqiao Dong,† Anil Kumar Tummanapelli,‡ Xu Li,‡ Shaoming Ying,† Hajime Hirao,*,‡ and Dan Zhao*,† †

Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585 Singapore Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore

‡

S Supporting Information *

ABSTRACT: Fluorescent porous materials have been under intensive investigation recently, because of their wide applications in molecular recognition and chemical sensing. However, it is a great challenge to achieve size selectivity and sensing linearity for molecular recognition. Herein, we report a series of porous organic frameworks (POFs) containing flexible tetraphenylethylene (TPE) moieties as molecular rotors with responsive fluorescent behavior. These fluorescent POFs exhibit size-selective turn-on fluorescence for the effective chemical sensing of volatile organic compounds (VOCs), which can be attributed to the different degrees of motion restriction of flexible TPE rotors by various VOCs, leading to the partially freezing of rotors in more fluorescent conformations. Significantly, a linear aggregation-induced emission (AIE) relationship is observed between the fluorescent POFs and the VOCs over a wide range of concentrations, which is highly beneficial for quantitative sensing applications. The gas-phase detection of arene vapors using POFs is also proven with unprecedentedly high sensitivity, selectively, and recyclability. The mechanism of responsive fluorescence in POFs is further investigated using molecular simulations and density functional theory (DFT) calculations.

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INTRODUCTION Volatile organic compounds (VOCs), such as aliphatic hydrocarbons and chlorocarbons, as well as benzene and its derivatives, have become a major source of air pollutants and can easily trigger a wide range of sensory irritation and chronic diseases (e.g., asthma, cystic fibrosis, renal failure, nervous system impairment, and cancer).1 Therefore, the efficient detection and identification of VOCs become greatly desirable. Currently, VOCs can be detected by portable electronic devices at parts per million (ppm) concentrations but with poor selectivities. For example, the discrimination of benzene/ toluene or o-xylene/m-xylene/p-xylene is still very challenging in VOC detection, because of the similar structural and physical properties of the molecules to be differentiated.2 Other moreaccurate techniques such as gas chromatography (GC) or GC coupled with mass spectroscopy (GC-MS) are too complicated and expensive for applications where on-site sampling is required.3,4 Therefore, it is highly desired to develop novel VOC sensors combining the merits of high sensitivity, broad selectivity, easy operation, and low cost that can be widely used. The recent decade has witnessed the rapid development of optical sensors coupled with various sensing materials including small organic molecules,5,6 metal−organic complexes,7,8 conjugate polymers,9,10 and crystalline porous materials.11,12 In particular, fluorescent sensors with aggregation-induced emission (AIE) mechanism have gained great attention in both © 2016 American Chemical Society

fundamental and applied studies, because they permit the use of dye solutions at different concentrations for chemical sensing and enable the development of turn-on sensors taking advantage of luminogenic aggregation.13,14 Moreover, the turn-on feature of AIE chemosensors offers higher sensitivity and accuracy than that achieved by the aggregation-caused quenching (ACQ) counterparts.15 Therefore, AIE has been receiving much attention over the past few years with demonstrations in fluorescent probes and chemical sensors,16,17 biological labels,18,19 solid-state emitters, key components of other electronic devices,20−22 etc. Because of their high surface areas, fluorescent porous materials such as metal−organic frameworks (MOFs) are capable of preconcentrating analytes within their porous frameworks, affording enhanced sensitivity, which is especially attractive in the context of gas-phase chemical sensing.23−26 Recently, we have reported a fluorescent MOF named NUS-1 containing flexible tetraphenylethylene (TPE) moieties as molecular rotors.27 TPE is a typical AIE fluorogen whose intramolecular motions can be restricted through molecular interactions with analytes, affording responsive turn-on fluorescence that can be exploited for the chemical sensing of Received: August 18, 2016 Revised: October 4, 2016 Published: October 18, 2016 7889

DOI: 10.1021/acs.chemmater.6b03376 Chem. Mater. 2016, 28, 7889−7897

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Chemistry of Materials VOCs. However, the free phenyl ring rotors in NUS-1 have been partially restricted via strong intermolecular π−π interactions between the adjacent packing layers leading to impaired molecular recognition. Besides, NUS-1 suffers from poor stability, which greatly limits its application in chemical sensing. Porous organic frameworks (POFs) are a group of emerging porous materials constructed from well-designed organic precursors through coupling or condensation reactions.28,29 Because of their attractive features such as high porosity, rich functionality, and excellent stability, POFs have been tested for wide applications such as gas storage and separation,30−33 heterogeneous catalysis,34−38 and light harvesting/emission.39 Although numerous POFs have been prepared from various functional building blocks, well-defined fluorescent POFs containing molecular rotors with tunable porosity and responsive fluorescent behavior for chemical sensing remain rather limited.10,40 In order to address these challenges, herein, we report the design and synthesis of a series of fluorescent POFs, labeled as NUS-20−NUS-23, that contain flexible TPE moieties as molecular rotors with responsive fluorescent behavior. These POFs can recognize various VOCs of different size by specific fluorescence emissions through the AIE mechanism. In addition, the fluorescence emission intensity bears a linear relationship to the concentration of analytes, which is beneficial for quantitative sensing applications. Besides liquid phase sensing, these POFs can also be used as effective gas sensors for the detection of VOC vapors via either turn-on or turn-off fluorescence emission with short response time and excellent reusability. Molecular simulations and HOMO−LUMO energy calculations provide further insights into the mechanism of versatile fluorescence in POFs. Our study has demonstrated the great potential of AIE-based fluorescent POFs as chemical sensors for VOC detections.

Figure 1. Synthetic routes of NUS-20−NUS-23 via Suzuki or Sonogashira coupling reactions. The phenyl rings with yellow color represent molecular rotors in the porous frameworks.

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RESULTS AND DISCUSSION Synthesis and Characterization of NUS-20−NUS-23. In this study, 1,2-diphenyl-1,2-bis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)ethane (TPE-1; see Figure S1 in the Supporting Information) and 1,2-bis(4-ethynylphenyl)-1,2diphenylethene (TPE-2; see Figure S1) were employed to synthesize fluorescent POFs. TPE-1 and TPE-2 have highly twisted molecular conformations that hamper the intermolecular π−π stacking interactions. More importantly, the central olefin stators of the two TPE molecules are surrounded by two free peripheral phenyl rings which can act as molecular rotors for turn-on fluorescent sensing. NUS-20 and NUS-21 were successfully synthesized via the Suzuki−Miyaura reaction (Figure 1), whereas NUS-22 and NUS-23 were obtained using the Sonogashira−Hagihara reaction (see the Supporting Information for details). The Fourier transform infrared spectroscopy (FT-IR) spectra indicate that the C−Br vibration bands of monomers (ca. 532 cm−1) have almost completely disappeared in the POFs, indicating the completion of the cross-coupling reactions (Figure 2a). The peaks located at 3275 and 2106 cm−1, which correspond to the ethynyl C−H vibration and the CC vibration of TPE-2, respectively, are also nonexistent in NUS-22 and NUS-23, further demonstrating the success of cross-coupling reactions. The X-ray photoelectron spectroscopy (XPS) spectra of NUS-20−NUS23 do not contain the Br 1s peak (70.2 eV) that is prominent in monomer-1 and monomer-2 (Figure 2b). In addition, the B 1s peak (191.0 eV) in TPE-1 has disappeared in the spectra of

NUS-20 and NUS-21. The solid-state 13C CP/MAS NMR spectra of NUS-20 and NUS-22 display one main peak at ∼63.50 ppm that is assignable to monomer-1, while the NMR spectra of NUS-21 and NUS-23 feature peaks at 38.10 and 46.50 ppm, which are attributable to monomer-2. Furthermore, one major peak at ∼88.70 ppm in NUS-22 and NUS-23 suggests the successful incorporation of TPE-2 into the POFs (Figure S2 in the Supporting Information). No diffraction peak can be observed in the powder X-ray diffraction (PXRD) patterns of the POFs, indicating that their amorphous nature is similar to that of other polymers obtained via cross-coupling reactions (see Figure S3 in the Supporting Information).36 Field-emission scanning electron microscopy (FE-SEM) images show that these POFs possess a spherical morphology that is caused by the agglomeration of smaller particles in the size range of 400−600 nm for NUS-20 and NUS-21 (Figures 2d and 2e), or 50−150 nm for NUS-22 and NUS-23 (Figures 2f and 2g). High-resolution transmission electron microscopy (HR-TEM) reveals amorphous yet porous textures of these POFs (Figures 2h−k).35 Thermogravimetric analysis (TGA) results show that these POFs are thermally stable up to 300 °C in a nitrogen atmosphere (Figure S4 in the Supporting Information). Excellent chemical stability of these POFs is also proven by soaking tests using water, hydrochloric acid (6 M), sulfuric acid (6 M), sodium hydroxide (8 M), and common organic solvents (Table S1 in the Supporting Information), 7890


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Figure 2. (a) FT-IR spectra of NUS-20−NUS-23. (b) XPS spectra of NUS-20−NUS-23. (c) Optical band gaps (Eg) of NUS-20−NUS-23. (d−g) FE-SEM images of NUS-20 (panel d), NUS-21 (panel e), NUS-22 (panel f), and NUS-23 (panel g). (h−k) HR-TEM images of NUS-20 (panel h), NUS-21 (panel i), NUS-22 (panel j), and NUS-23 (panel k). (l−o) Fluorescence photographs of NUS-20 (panel l), NUS-21 (panel m), NUS-22 (panel n), and NUS-23 (panel o) (λex = 365 nm; insets show optical photographs).

structures of NUS-20−NUS-23 have diamondoid or possibly interpenetrated diamondoid networks.47−49 Based on this hypothesis, crystalline structures of NUS-20−NUS-23 were built using the graphical user interface of Materials Studio software. Energy minimization was performed for the resultant models with the COMPASS II force field to remove geometric distortions. A molecular mechanics (MM) optimization approach was employed to determine the pore size of these structural models (Figures S5−S8 in the Supporting Information). The analysis showed that the pore widths for the diamondoid structures of NUS-20−NUS-23 were 20−33 Å (see Figure S9 and Table S2 in the Supporting Information); these values are much larger than the experimentally determined pore size. By contrast, the calculated pore size for the interpenetrated diamondoid structures was in the range of 6−19 Å (Table S3 in the Supporting Information), which agreed much better with the experimental data on pore size distribution. For example, the pore size along the b-axis of NUS-20 and NUS-22 is ∼8.2 and 11.9 Å, respectively (see Figures 3c and 3e), and the distances of two adjacent TPE rotors are 22.5 and 27.1 Å for NUS-20 and NUS-22, respectively, indicating that there is enough space for the rotation of TPE rotors. Furthermore, the pore size along the a-

which is consistent with their robust nature of pure organic polymers free from any sites susceptible to acid or base attack. Compared to other porous materials such as MOFs41 and covalent organic frameworks (COFs),42,43 the extremely high stability makes POFs especially attractive for applications, even under corrosive conditions. The permanent porosity of NUS-20−NUS-23 was demonstrated by their N2 sorption isotherms at 77 K, all of which exhibit type I sorption behavior with Brunauer−Emmett− Teller (BET) surface areas of 900, 835, 421, and 368 m2 g−1 and total pore volumes of 0.505, 0.640, 0.369, and 0.313 cm3 g−1 for NUS-20, NUS-21, NUS-22, and NUS-23, respectively (Figure 3a). The pore size distribution calculated using nonlocal density functional theory (NLDFT) reveals the average pore widths of ∼12.3−14.1 Å (Figure 3b), indicating mainly a microporous texture. Although these POFs are amorphous and devoid of long-range order, models in ideal situations (i.e., crystalline models) will help us verify the experimental porosity data and gain atomistic insight.44 This strategy has been adopted in the studies of other amorphous porous polymers such as PAF-11,28 PPN-4,45 and COP-5.46 Given the geometric restrictions that the monomers and their possible bonding patterns may impose, it is likely that the 7891


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Figure 2c, as well as Figure S14 in the Supporting Information).50 The TPE-1 and TPE-2 linkers emit blue fluorescence in the solid state with peaks at ∼443 and 456 nm, respectively. In sharp contrast, the POFs exhibit green fluorescence in the solid state with peaks at ∼510, 505, and 506 nm for NUS-20, NUS-21, and NUS-22, respectively (Figure S15 in the Supporting Information), suggesting the formation of extended porous structures. Notably, the fluorescence of NUS-23 is almost undetectable, possibly because of its highly conjugated structure facilitating nonradiative decay. As expected, TPE-1 and TPE-2 linkers are nonemissive when fully dissolved in THF solvent (good solvent) but exhibit strong fluorescence emission in a THF/ water (10/90) mixed solvent (bad solvent), which can be attributed to a typical AIE behavior in which the nonradiative decay via the active intramolecular rotations of phenyl rings is restricted in the solid precipitates (Figure S16 in the Supporting Information). Accordingly, locking TPE linkers into the rigid frameworks of POFs can effectively restrict their intramolecular motions, thereby leading to intensified fluorescence emission even in THF solutions, wherein NUS-20 shows an ∼1150-fold fluorescence enhancement over TPE-1 at a concentration of 0.3 mg mL−1. Similarly, an ∼12-fold fluorescence enhancement was observed for NUS-22 over TPE2 at the same concentration (Figure S17 in the Supporting Information). Solution-Based Chemical Sensing of VOCs. In this study, VOCs were selected as the analyte for chemical sensing tests, because their detection is of great importance in chemical assays,51 environmental monitoring,52 and even disease detection.1 The experiments were performed by soaking the POFs in various VOC solutions of different molecular size (see Figure S18 in the Supporting Information), followed by photoluminescence tests. As expected, all POFs exhibit different fluorescence emissions after soaking in various VOCs (Figures 4a−c). Using the fluorescence emission intensity of POFs being soaked in hexane (Ihexane) as the reference, we evaluated the relative intensity (IR = I/Ihexane) of the four POFs in different analytes (Figure 4e). Interestingly, there appears to be a positive correlation between the relative intensity and the molecular size of the analytes. Taking NUS-21 as an example, the IR values of NUS-21⊃benzene and NUS21⊃toluene are 1.67 and 2.05, respectively, and NUS21⊃mesitylene has an even larger IR value of 4.30. To our surprise, further increasing the molecular size of the analyte (e.g., 1,3,5-triisopropylbenzene) leads to a significantly reduced emission (IR = 1.32). Similar trends can be observed in the other three POFs (Figure 4e). We speculate that the size-dependent turn-on fluorescence of the POFs is due to the different restriction extents of flexible TPE rotors by VOC analytes with various sizes, leading to partially freezing of rotors in more fluorescent conformations. Larger analytes, as long as they can diffuse into the POFs, can interact tightly with molecular rotors, because of intensified steric hindrance, resulting in more rotor restriction and intensified turn-on fluorescence. The molecular size of mesitylene (5.8 Å × 6.7 Å) is relatively large among the VOC analytes (Figure S18), but is still smaller than the cavity size of POFs. Therefore, mesitylene can diffuse into the POFs and trigger rotor restriction. In the case of analytes with molecular sizes close to or even larger than the cavity size of POFs, such as 1,3,5-triisopropylbenzene (8.4 Å × 9.0 Å), their diffusion and incorporation into POFs would become difficult,

Figure 3. (a) N2 adsorption (filled symbols) and desorption (open symbols) isotherms of NUS-20−NUS-23 at 77 K. (b) Pore width distribution of NUS-20−NUS-23. (c) The simulated interpenetrated diamondoid structure of NUS-20 along the b-axis. The TPE rotors point toward into the cavity, which has a size of ∼8.2 Å, and the distance of two adjacent TPE rotors is 22.5 Å. (d) The simulated interpenetrated structure of NUS-20 along the c-axis by space-filling model with a pore size of 13.0 Å. (e) The simulated interpenetrated diamondoid structure of NUS-22 along the b-axis. The TPE rotors also point toward into the cavity, which has a size of ∼11.9 Å, and the distance of two adjacent TPE rotors is 27.1 Å. (f) The simulated interpenetrated structure of NUS-22 along the c-axis by a space-filling model with a pore size of 16.4 Å.

axis and c-axis of NUS-20 and NUS-22 is ∼ 6−19 Å, based on the interpenetrated diamondoid structures (see Figures 3d and 3f, as well as Figure S10 and Table S3 in the Supporting Information). The simulated interpenetrated pore size of NUS21 and NUS-23 is also similar to that of NUS-20 and NUS-22 (see Figures S11 and S12 in the Supporting Information). These comparisons between the experimental and computational data strongly indicate that NUS-20−NUS-23 should possess the interpenetrated diamondoid structures. The flexibility of dangling TPE molecular rotors was studied by cryogenic differential scanning calorimetry (DSC). A distinct endothermic peak at −65 °C was observed in NUS-22 during the heating scan (Figure S13 in the Supporting Information), indicating a phase transition in which the frozen TPE molecular rotors become rotatable. Similar phase transition has been observed in our previously reported MOF NUS-127 and is reminiscent of the glass transition in organic polymers, suggesting the presence of highly dynamic molecular rotors in these well-designed POFs. After confirming the chemical structure, porosity, and flexibility of synthesized POFs, we then focused on their photoluminescence properties. Optical band gaps (Eg) were estimated to be 2.83, 2.87, 2.66, and 2.70 eV for NUS-20, NUS21, NUS-22, and NUS-23, respectively, indicating their semiconductor nature capable of fluorescence emission (see 7892


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introduction of alkyne groups can greatly suppress the luminescence of the resultant POFs (NUS-22 and NUS-23), possibly because of reduced band gaps (2.66 eV for NUS-22 and 2.70 for NUS-23, Figure 2c) and/or larger framework voids that counteract the effect of rotor restriction. Nevertheless, the weak emission of the pristine POFs is not a drawback, as it provides a better background for the turn-on fluorescence during chemical sensing. For example, among the four POFs, NUS-22 exhibits the most distinct turn-on fluorescence (e.g., IR = 4.66 for NUS-22⊃benzene and IR = 6.10 for NUS22⊃toluene; see Figure 4e). Considering the fact that the differentiation of benzene over toluene is highly challenging in VOC detection, because of their similar structures, NUS-22 might be a promising chemosensor for the differentiation of these isomers. In addition, NUS-22⊃mesitylene shows the largest degree of fluorescence enhancement (IR = 11.5) among all the POFs examined in this study. In order to provide an insight into the rotor restriction by the loaded VOCs, simulated van der Waals interactions between mesitylene and NUS-22 were conducted using the Grand Canonical Monte Carlo (GCMC) method (Figure 4d), where it can be clearly seen that the TPE rotors are surrounded by congested mesitylene molecules, which effectively restrict the rotor motions. For NUS-23, although it is almost nonemissive in the pristine form, it can still demonstrate strong turn-on fluorescence in the presence of VOCs (see Figure 4e, as well as Figure S20 in the Supporting Information). By contrast, control experiments proved that the TPE-1 and TPE-2 linkers had no ability to detect these VOC analytes, because of their almost-nonemissive features in VOC solutions (see Figure S21 in the Supporting Information). Compared to the turn-on mode, the turn-off mode is frequently encountered in fluorescence-based chemical sensing applications.56 The turn-off mode is also observed in our POFs using an electron-withdrawing VOC, nitrobenzene, as the analyte, which has been proven by its strong ability to quench fluorescence (Figure 4). For example, it is interesting to observe a large blue shift (by ∼48 nm) of the fluorescence peak in NUS-21⊃nitrobenzene (446 nm), with respect to NUS-21 (494 nm, Figure 4b). The same trend was observed in the other POFs, indicating that the process of turn-off fluorescence should be mainly caused by the donor−acceptor electrontransfer mechanism, instead of the AIE mechanism that operates in the process of turn-on fluorescence.15 This result also presents an interesting example of fluorescent materials displaying two different luminescent mechanisms for chemical sensing.26 The relationship between fluorescence enhancement and analyte concentration was studied by monitoring the fluorescence emission of POFs being soaked in hexane/ mesitylene mixtures with various amounts of mesitylene (0− 100%). The fluorescence emission intensity of POFs increases as the concentration of mesitylene increases (see Figure 5, as well as Figure S22 in the Supporting Information). Surprisingly, the emission intensity was enhanced in an almost perfectly linear way (R2 > 0.99) across the entire concentration range (0−100%) when the concentration of mesitylene was increased. In order to further understand the linear turn-on fluorescence behavior of POFs at low concentrations of analytes, fluorescence titrations were performed by gradually adding trace amounts of benzene, toluene, or mesitylene to NUS-22 dispersed in hexane, wherein fluorescence enhancement was also observed (see Figures 5e and 5f, as well as Figure S23 in

Figure 4. Fluorescence emission spectra of (a) NUS-20, (b) NUS-21, and (c) NUS-22 suspended in various VOC solutions at 298 K (λex = 355 nm for NUS-20 and NUS-21 or 370 nm for NUS-22, c = 0.3 mg mL−1). (d) The simulated van der Waals interactions between mesitylene and NUS-22, using the Grand Canonical Monte Carlo (GCMC) method. Blue and red colors represent unoccupied and occupied mesitylene molecules. (e) Relative fluorescence intensity of NUS-20−NUS-23 in various VOC solutions (IR = I/Ihexane).

leading to insufficient rotor restriction and, thus, diminished fluorescence emission. In order to prove this point, we conducted TGA of NUS-20 after soaking into various VOC solutions (Figure S19 in the Supporting Information). The loading contents of toluene, pxylene, mesitylene, and 1,3,5-triisopropylbenzene in NUS-20 were determined to be 1.63, 1.96, 2.16, and 0.51 mmol mg−1, respectively, suggesting that mesitylene can easily diffuse into NUS-20, resulting in the most effective rotor restriction with the strongest turn-on fluorescence among the selected VOC analytes. By contrast, it is relatively difficult for 1,3,5triisopropylbenzene to diffuse into NUS-20, because of its larger size. Notably, there is no obvious fluorescence peak shift in the turn-on process for analytes ranging from hexane to mesitylene, suggesting that there is no strong π−π stacking or charge transfer between NUS-20−NUS-23 and the VOC analytes. This observation further substantiates our speculation that the turn-on fluorescence of the POFs, which occurs upon exposure to VOCs, should be mainly caused by the restriction of TPE rotors (AIE mechanism) rather than the charge-transfer mechanism.14,15 Although there are many reports on TPEbased fluorescent materials for chemical sensing,53−55 this finding represents a rare case of turn-on fluorescence dependency on the analyte size, which may be exploited for the further development of novel molecular chemosensors. Notably, the above-mentioned molecular restriction is brought about not only by the size of analytes, but also by the chemical structure of the POFs. In particular, the 7893


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Figure 6. Fluorescence emission spectra of NUS-20 before and after exposure to toluene vapor (a) or nitrobenzene vapor (b) for 2 min. (c) Percentage of fluorescence enhancement or quenching after exposing POFs to different VOC vapors for 2 min at 298 K. (d) Cycling test of NUS-20 for the chemical sensing of toluene vapor. (e) Fluorescence microscopy images of NUS-20 before (middle) and after exposure to nitrobenzene (left) or toluene (right) vapors (λex = 365 nm).

Figure 5. (a, b) Fluorescence emission spectra and plots of maximum emission intensity of NUS-20 versus mesitylene fraction in hexane/ mesitylene mixtures (λex = 355 nm, c = 0.3 mg mL−1). (c, d) Fluorescence emission spectra and plots of maximum emission intensity of NUS-22 versus mesitylene fraction in hexane/mesitylene mixtures (λex = 370 nm, c = 0.3 mg mL−1). (e) Fluorescence emission spectra of NUS-22 (c = 0.3 mg mL−1 in hexane) upon titration with mesitylene. (f) Plots of NUS-22 titration with benzene (purple), toluene (blue), and mesitylene (green), respectively.

exposure to saturated benzene, toluene, or chlorobenzene vapor, with rapid response time less than 30 s and quick saturation reached within 2 min. Among the three analytes, toluene triggers the most obvious fluorescence enhancement accompanied by a noticeable blue shift (∼15 nm) for NUS-20 (Figure 6a). Such a big fluorescence peak shift was not observed in liquid-based toluene sensing using NUS-20 (Figure 4a), indicating a different mechanism for turn-on fluorescence in the vapor phase. Considering the much lower VOC uptake of POFs in the vapor phase, the turn-on fluorescence may originate from π−π stacking or charge transfer between POFs and VOCs. The percentages of fluorescence enhancement are 152%, 117%, and 50% for NUS-20, NUS-21, and NUS-22, respectively (Figure 6c). Notably, the fluorescence enhancement of NUS-20 triggered by toluene vapor is higher than that for some of the reported MOFs62 and conjugated microporous polymers (CMPs).10 The selective fluorescence enhancement ratios {[(Itoluene/I0) − 1]/[(Ibenzene/I0) − 1]}63 of toluene over benzene are 1.85, 1.75, and 2.08 for NUS-20, NUS-21, and NUS-22, respectively, and are also higher than those for some of the reported MOFs11,62 and CMPs.10 In addition, our POFs exhibit excellent recyclability for repeatable usage. For example, NUS-20 can be regenerated by heating at 120 °C under vacuum for 30 min and reused for the sensing of toluene vapor without significant loss of the enhancement percentage (see Figure 6d, as well as Figure S27 in the Supporting Information). Control experiments verified that the fluorescence of the TPE-1 and TPE-2 linkers could not be enhanced by toluene vapor under the same conditions (Figure S28 in the Supporting Information). Instead, fluorescence quenching was observed because these AIE compounds can be gradually dissolved in the presence of toluene vapor, leading to enhanced nonradiative decay and, thus, reduced fluorescence emission. Besides the turn-on fluorescence, the POFs also exhibit remarkably quenched fluorescence upon exposure to nitro-

the Supporting Information). The rate of fluorescence enhancement is the fastest for mesitylene and decreases in the order mesitylene > toluene > benzene, which matches well with the above-mentioned size effect. The measured absorbance (I/I0) − 1 at 502 nm, where I0 is the original maximum peak intensity of NUS-22 and I is the maximum peak intensity after exposure to the analytes, varies linearly with analyte concentration (R2 > 0.99). Although AIE-based chemosensors have been extensively developed based on different materials such as small organic molecules,57,58 TPEbased supramolecular assemblies,54,55 TPE-based peptide bioprobes,59 and TPE-based polymers,18,60 most of them bear either a nonlinear relationship between fluorescence emission and analyte concentration or a linear relationship only at a low concentration range.61 To the best of our knowledge, this study represents a rare example of chemosensors featuring a perfectly linear relationship between turn-on fluorescence and analyte concentration over a full range of analyte concentration (0− 100%), which can be finely tuned as a “chemical nose” for quantitative chemical sensing applications. Vapor-Based Chemical Sensing of VOCs. VOC detection in real applications is always performed in the gas phase wherein a trace amount of VOC vapor needs to be detected. In order to prove the potency of the POFs for VOC sensing under such scenarios, fluorescence spectra were recorded on powder samples of POFs in a thin-layer form62,63 before and after exposing them to selected VOC vapors including benzene, toluene, chlorobenzene, and nitrobenzene (Figure 6 and Figure S24−S26). As expected, the fluorescence of the POFs can be dramatically enhanced upon 7894


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suggesting that ΔELUMO could be one of the factors determining the fluorescence enhancement in POFs, especially in the aforementioned vapor-based chemical sensing of VOCs. However, fluorescence emissions involving donor−acceptor electron transfer are often accompanied by fluorescence peak shifts. Considering the small fluorescence peak shift observed in the solution-based chemical sensing of electron-rich analytes, the rotor restriction in the POFs caused by the steric hindrance of incorporated analyte molecules should be the major reason (AIE mechanism) that accounts for the fluorescence enhancement.

benzene vapor for 15 s, with a significant blue shift (∼62 nm) for NUS-20 indicating an electron-transfer mechanism (Figure 6b). The percentages of fluorescence quenching caused by nitrobenzene vapor are 83%, 78%, and 30% for NUS-20, NUS21, and NUS-22, respectively (Figure 6c). The turn-on fluorescence caused by toluene vapor and the turn-off fluorescence triggered by nitrobenzene vapor in NUS-20 can be clearly seen from fluorescence microscopy images (Figure 6e), implying the potential applications of the POFs in nakedeye detection of VOCs. HOMO−LUMO Energy Calculations. Molecular simulations and density functional theory (DFT) calculations were performed on NUS-20 and selected analytes to better understand the different fluorescence emission behavior of the POFs in the presence of various analytes such as toluene (turn-on) and nitrobenzene (turn-off). DFT calculations (Figure 7) show that the lowest unoccupied molecular orbital

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CONCLUSION In summary, we have designed and synthesized four POFs containing partially flexible TPE moieties as molecular rotors for fluorescence-based chemical sensing of VOCs. These POFs respond noticeably in the form of intensified (turn-on) or attenuated (turn-off) fluorescence emission upon exposure to various VOCs. Size-selective fluorescence enhancement (turnon) was observed for solution-based chemical sensing of electron-rich VOCs, which is mainly due to the rotor restriction caused by the incorporation of bulky analytes (AIE mechanism). Linear relationships were obtained between fluorescence intensity and analyte concentration over a wide range of concentration (0−100%) indicating the versatile usage of POF-based chemosensors in quantitative sensing applications. Fluorescence quenching (turn-off) was also obtained for the detection of nitrobenzene, which can be attributed to the electron transfer from POFs to analytes (electron-transfer mechanism) and was supported by DFT calculations. Our study broadens the application of flexible AIE fluorogens as molecular rotors and paves the way for the tailored synthesis of fluorescent POFs used as chemosensors. We speculate that the next stage would be the functionalized sensor arrays with multiple POFs for the detection of VOC mixtures. By collecting the optical signals from POF arrays and deciphering them accordingly, the fluorescent-based “E-nose” for VOCs may be realized eventually.

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Figure 7. HOMO−LUMO energy profiles of mesitylene, toluene, benzene, chlorobenzene, NUS-20 fragment, and nitrobenzene (from left to right). The difference of LUMO energy state between NUS-20 fragment and various VOC analytes [ΔELUMO = ELUMO (analytes) − ELUMO (NUS-20)] is 1.49, 1.32, 1.26, 0.85, and −1.25 eV for mesitylene, toluene, benzene, chlorobenzene, and nitrobenzene (from left to right), respectively.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03376.

(LUMO) of an NUS-20 fragment (−2.29 eV) lies higher in energy than the LUMO of nitrobenzene (−3.54 eV). Therefore, efficient electron transfer may occur from NUS-20 to nitrobenzene, resulting in fluorescence quenching (electrontransfer mechanism), as observed in other porous materials such as MOFs62 and COFs.40 By contrast, the LUMOs of electron-rich VOC analytes are higher-lying than NUS-20 by 0.80 eV (mesitylene), 0.97 eV (toluene), 1.03 eV (benzene), and 1.44 eV (chlorobenzene). As a result, electrons may transfer from electron-rich analytes to NUS-20. Furthermore, the difference of LUMO energy state between NUS-20 fragment and VOC analytes [ΔELUMO = ELUMO (analytes) − ELUMO (NUS-20)] is 1.49, 1.32, 1.26, and 0.85 for mesitylene, toluene, benzene, and chlorobenzene, respectively, which parallel the degree of fluorescence enhancement in NUS-20,

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Experimental details on POFs synthesis, fluorescence sensing of VOCs, molecular simulations and DFT calculations (PDF) Crystallographic information for NUS-20−NUS-23 (ZIP)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (D. Zhao, for experiment). *E-mail: [email protected] (H. Hirao, for calculations). Notes

The authors declare the following competing financial interest(s): A Singapore non-provisional patent (No. 10201608475U) has been filed on October 10, 2016 based on the presented result. 7895


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mercury(II) cation and hydrogen sulfate anion. Chem. Commun. 2012, 48, 7504−7506. (17) Zhao, J.; Yang, D.; Zhao, Y.; Yang, X.-J.; Wang, Y.-Y.; Wu, B. Anion-Coordination-Induced Turn-On Fluorescence of an OligoureaFunctionalized Tetraphenylethene in a Wide Concentration Range. Angew. Chem., Int. Ed. 2014, 53, 6632−6636. (18) Wang, Z.; Chen, S.; Lam, J. W. Y.; Qin, W.; Kwok, R. T. K.; Xie, N.; Hu, Q.; Tang, B. Z. Long-Term Fluorescent Cellular Tracing by the Aggregates of AIE Bioconjugates. J. Am. Chem. Soc. 2013, 135, 8238−8245. (19) Yuan, Y.; Zhang, C.-J.; Xu, S.; Liu, B. A self-reporting AIE probe with a built-in singlet oxygen sensor for targeted photodynamic ablation of cancer cells. Chem. Sci. 2016, 7, 1862−1866. (20) Yuan, W. Z.; Lu, P.; Chen, S.; Lam, J. W. Y.; Wang, Z.; Liu, Y.; Kwok, H. S.; Ma, Y.; Tang, B. Z. Changing the Behavior of Chromophores from Aggregation-Caused Quenching to AggregationInduced Emission: Development of Highly Efficient Light Emitters in the Solid State. Adv. Mater. 2010, 22, 2159−2163. (21) Gu, C.; Huang, N.; Wu, Y.; Xu, H.; Jiang, D. Design of Highly Photofunctional Porous Polymer Films with Controlled Thickness and Prominent Microporosity. Angew. Chem., Int. Ed. 2015, 54, 11540− 11544. (22) Qian, J.; Zhu, Z.; Qin, A.; Qin, W.; Chu, L.; Cai, F.; Zhang, H.; Wu, Q.; Hu, R.; Tang, B. Z.; He, S. High-Order Non-Linear Optical Effects in Organic Luminogens with Aggregation-Induced Emission. Adv. Mater. 2015, 27, 2332−2339. (23) Allendorf, M. D.; Bauer, C. A.; Bhakta, R. K.; Houk, R. J. T. Luminescent metal-organic frameworks. Chem. Soc. Rev. 2009, 38, 1330−1352. (24) Cui, Y.; Yue, Y.; Qian, G.; Chen, B. Luminescent Functional Metal−Organic Frameworks. Chem. Rev. 2012, 112, 1126−1162. (25) Takashima, Y.; Martínez, V. M.; Furukawa, S.; Kondo, M.; Shimomura, S.; Uehara, H.; Nakahama, M.; Sugimoto, K.; Kitagawa, S. Molecular decoding using luminescence from an entangled porous framework. Nat. Commun. 2011, 2, 168. (26) Hu, Z.; Deibert, B. J.; Li, J. Luminescent metal-organic frameworks for chemical sensing and explosive detection. Chem. Soc. Rev. 2014, 43, 5815−5840. (27) Zhang, M.; Feng, G.; Song, Z.; Zhou, Y.-P.; Chao, H.-Y.; Yuan, D.; Tan, T. T. Y.; Guo, Z.; Hu, Z.; Tang, B. Z.; Liu, B.; Zhao, D. TwoDimensional Metal−Organic Framework with Wide Channels and Responsive Turn-On Fluorescence for the Chemical Sensing of Volatile Organic Compounds. J. Am. Chem. Soc. 2014, 136, 7241− 7244. (28) Ben, T.; Ren, H.; Ma, S.; Cao, D.; Lan, J.; Jing, X.; Wang, W.; Xu, J.; Deng, F.; Simmons, J. M.; Qiu, S.; Zhu, G. Targeted Synthesis of a Porous Aromatic Framework with High Stability and Exceptionally High Surface Area. Angew. Chem. 2009, 121, 9621−9624. (29) Zou, X.; Ren, H.; Zhu, G. Topology-directed design of porous organic frameworks and their advanced applications. Chem. Commun. 2013, 49, 3925−3936. (30) Furukawa, H.; Yaghi, O. M. Storage of Hydrogen, Methane, and Carbon Dioxide in Highly Porous Covalent Organic Frameworks for Clean Energy Applications. J. Am. Chem. Soc. 2009, 131, 8875−8883. (31) Lu, W.; Yuan, D.; Zhao, D.; Schilling, C. I.; Plietzsch, O.; Muller, T.; Bräse, S.; Guenther, J.; Blümel, J.; Krishna, R.; Li, Z.; Zhou, H.-C. Porous Polymer Networks: Synthesis, Porosity, and Applications in Gas Storage/Separation. Chem. Mater. 2010, 22, 5964−5972. (32) Li, B.; Zhang, Y.; Krishna, R.; Yao, K.; Han, Y.; Wu, Z.; Ma, D.; Shi, Z.; Pham, T.; Space, B.; Liu, J.; Thallapally, P. K.; Liu, J.; Chrzanowski, M.; Ma, S. Introduction of π-Complexation into Porous Aromatic Framework for Highly Selective Adsorption of Ethylene over Ethane. J. Am. Chem. Soc. 2014, 136, 8654−8660. (33) Lu, W.; Yuan, D.; Sculley, J.; Zhao, D.; Krishna, R.; Zhou, H.-C. Sulfonate-Grafted Porous Polymer Networks for Preferential CO2 Adsorption at Low Pressure. J. Am. Chem. Soc. 2011, 133, 18126− 18129. (34) Ding, S.-Y.; Gao, J.; Wang, Q.; Zhang, Y.; Song, W.-G.; Su, C.Y.; Wang, W. Construction of Covalent Organic Framework for

ACKNOWLEDGMENTS D.Z. acknowledges the financial support from National University of Singapore (No. CENGas R-261-508-001-646) and Singapore Ministry of Education (No. MOE AcRF Tier 1 R-279-000-472-112). H.H. is grateful for the financial support in the form of a JST-PRESTO grant and a Nanyang Assistant Professorship.

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REFERENCES

(1) Konvalina, G.; Haick, H. Sensors for Breath Testing: From Nanomaterials to Comprehensive Disease Detection. Acc. Chem. Res. 2014, 47, 66−76. (2) Wang, B.; Huynh, T.-P.; Wu, W.; Hayek, N.; Do, T. T.; Cancilla, J. C.; Torrecilla, J. S.; Nahid, M. M.; Colwell, J. M.; Gazit, O. M.; Puniredd, S. R.; McNeill, C. R.; Sonar, P.; Haick, H. A Highly Sensitive Diketopyrrolopyrrole-Based Ambipolar Transistor for Selective Detection and Discrimination of Xylene Isomers. Adv. Mater. 2016, 28, 4012−4018. (3) Bayn, A.; Nol, P.; Tisch, U.; Rhyan, J.; Ellis, C. K.; Haick, H. Detection of Volatile Organic Compounds in Brucella abortusSeropositive Bison. Anal. Chem. 2013, 85, 11146−11152. (4) Lin, H.; Jang, M.; Suslick, K. S. Preoxidation for Colorimetric Sensor Array Detection of VOCs. J. Am. Chem. Soc. 2011, 133, 16786− 16789. (5) Bencic-Nagale, S.; Sternfeld, T.; Walt, D. R. Microbead Chemical Switches: An Approach to Detection of Reactive Organophosphate Chemical Warfare Agent Vapors. J. Am. Chem. Soc. 2006, 128, 5041− 5048. (6) Liang, G.; Ren, F.; Gao, H.; Wu, Q.; Zhu, F.; Tang, B. Z. Bioinspired Fluorescent Nanosheets for Rapid and Sensitive Detection of Organic Pollutants in Water. ACS Sensors 2016, DOI: 10.1021/ acssensors.6b00530. (7) Moragues, M. E.; Toscani, A.; Sancenón, F.; Martínez-Máñez, R.; White, A. J. P.; Wilton-Ely, J. D. E. T. A Chromo-Fluorogenic Synthetic “Canary” for CO Detection Based on a Pyrenylvinyl Ruthenium(II) Complex. J. Am. Chem. Soc. 2014, 136, 11930−11933. (8) Moragues, M. E.; Esteban, J.; Ros-Lis, J. V.; Martínez-Máñez, R.; Marcos, M. D.; Martínez, M.; Soto, J.; Sancenón, F. Sensitive and Selective Chromogenic Sensing of Carbon Monoxide via Reversible Axial CO Coordination in Binuclear Rhodium Complexes. J. Am. Chem. Soc. 2011, 133, 15762−15772. (9) Yoon, J.; Chae, S. K.; Kim, J.-M. Colorimetric Sensors for Volatile Organic Compounds (VOCs) Based on Conjugated PolymerEmbedded Electrospun Fibers. J. Am. Chem. Soc. 2007, 129, 3038− 3039. (10) Liu, X.; Xu, Y.; Jiang, D. Conjugated Microporous Polymers as Molecular Sensing Devices: Microporous Architecture Enables Rapid Response and Enhances Sensitivity in Fluorescence-On and Fluorescence-Off Sensing. J. Am. Chem. Soc. 2012, 134, 8738−8741. (11) Campbell, M. G.; Liu, S. F.; Swager, T. M.; Dincă, M. Chemiresistive Sensor Arrays from Conductive 2D Metal−Organic Frameworks. J. Am. Chem. Soc. 2015, 137, 13780−13783. (12) Shustova, N. B.; Cozzolino, A. F.; Reineke, S.; Baldo, M.; Dincă, M. Selective Turn-On Ammonia Sensing Enabled by High-Temperature Fluorescence in Metal−Organic Frameworks with Open Metal Sites. J. Am. Chem. Soc. 2013, 135, 13326−13329. (13) Luo, J.; Xie, Z.; Lam, J. W. Y.; Cheng, L.; Chen, H.; Qiu, C.; Kwok, H. S.; Zhan, X.; Liu, Y.; Zhu, D.; Tang, B. Z. Aggregationinduced emission of 1-methyl-1,2,3,4,5-pentaphenylsilole. Chem. Commun. 2001, 1740−1741. (14) Liang, J.; Tang, B. Z.; Liu, B. Specific light-up bioprobes based on AIEgen conjugates. Chem. Soc. Rev. 2015, 44, 2798−2811. (15) Mei, J.; Leung, N. L. C.; Kwok, R. T. K.; Lam, J. W. Y.; Tang, B. Z. Aggregation-Induced Emission: Together We Shine, United We Soar! Chem. Rev. 2015, 115, 11718−11940. (16) Huang, G.; Zhang, G.; Zhang, D. Turn-on of the fluorescence of tetra(4-pyridylphenyl)ethylene by the synergistic interactions of 7896


Article

Chemistry of Materials Catalysis: Pd/COF-LZU1 in Suzuki−Miyaura Coupling Reaction. J. Am. Chem. Soc. 2011, 133, 19816−19822. (35) Xie, Z.; Wang, C.; deKrafft, K. E.; Lin, W. Highly Stable and Porous Cross-Linked Polymers for Efficient Photocatalysis. J. Am. Chem. Soc. 2011, 133, 2056−2059. (36) Dong, J.; Liu, Y.; Cui, Y. Chiral porous organic frameworks for asymmetric heterogeneous catalysis and gas chromatographic separation. Chem. Commun. 2014, 50, 14949−14952. (37) Sprick, R. S.; Jiang, J.-X.; Bonillo, B.; Ren, S.; Ratvijitvech, T.; Guiglion, P.; Zwijnenburg, M. A.; Adams, D. J.; Cooper, A. I. Tunable Organic Photocatalysts for Visible-Light-Driven Hydrogen Evolution. J. Am. Chem. Soc. 2015, 137, 3265−3270. (38) Xu, H.; Gao, J.; Jiang, D. Stable, crystalline, porous, covalent organic frameworks as a platform for chiral organocatalysts. Nat. Chem. 2015, 7, 905−912. (39) Xu, Y.; Chen, L.; Guo, Z.; Nagai, A.; Jiang, D. Light-Emitting Conjugated Polymers with Microporous Network Architecture: Interweaving Scaffold Promotes Electronic Conjugation, Facilitates Exciton Migration, and Improves Luminescence. J. Am. Chem. Soc. 2011, 133, 17622−17625. (40) Das, G.; Biswal, B. P.; Kandambeth, S.; Venkatesh, V.; Kaur, G.; Addicoat, M.; Heine, T.; Verma, S.; Banerjee, R. Chemical sensing in two dimensional porous covalent organic nanosheets. Chem. Sci. 2015, 6, 3931−3939. (41) Burtch, N. C.; Jasuja, H.; Walton, K. S. Water Stability and Adsorption in Metal−Organic Frameworks. Chem. Rev. 2014, 114, 10575−10612. (42) Côté, A. P.; Benin, A. I.; Ockwig, N. W.; O’Keeffe, M.; Matzger, A. J.; Yaghi, O. M. Porous, Crystalline, Covalent Organic Frameworks. Science 2005, 310, 1166−1170. (43) Waller, P. J.; Gándara, F.; Yaghi, O. M. Chemistry of Covalent Organic Frameworks. Acc. Chem. Res. 2015, 48, 3053−3063. (44) Martin, R. L.; Simon, C. M.; Smit, B.; Haranczyk, M. In silico Design of Porous Polymer Networks: High-Throughput Screening for Methane Storage Materials. J. Am. Chem. Soc. 2014, 136, 5006−5022. (45) Martin, R. L.; Shahrak, M. N.; Swisher, J. A.; Simon, C. M.; Sculley, J. P.; Zhou, H.-C.; Smit, B.; Haranczyk, M. Modeling Methane Adsorption in Interpenetrating Porous Polymer Networks. J. Phys. Chem. C 2013, 117, 20037−20042. (46) Xiang, Z.; Mercado, R.; Huck, J. M.; Wang, H.; Guo, Z.; Wang, W.; Cao, D.; Haranczyk, M.; Smit, B. Systematic Tuning and Multifunctionalization of Covalent Organic Polymers for Enhanced Carbon Capture. J. Am. Chem. Soc. 2015, 137, 13301−13307. (47) Aijaz, A.; Barea, E.; Bharadwaj, P. K. Diamondoid ThreeDimensional Metal-Organic Framework Showing Structural Transformation with Guest Molecules. Cryst. Growth Des. 2009, 9, 4480− 4486. (48) Gu, J.-M.; Kim, S.-J.; Kim, Y.; Huh, S. Structural isomerism of an anionic nanoporous In-MOF with interpenetrated diamond-like topology. CrystEngComm 2012, 14, 1819−1824. (49) Uribe-Romo, F. J.; Hunt, J. R.; Furukawa, H.; Klöck, C.; O’Keeffe, M.; Yaghi, O. M. A Crystalline Imine-Linked 3-D Porous Covalent Organic Framework. J. Am. Chem. Soc. 2009, 131, 4570− 4571. (50) Tai, G.; Hu, T.; Zhou, Y.; Wang, X.; Kong, J.; Zeng, T.; You, Y.; Wang, Q. Synthesis of Atomically Thin Boron Films on Copper Foils. Angew. Chem. 2015, 127, 15693−15697. (51) Wenger, O. S. Vapochromism in Organometallic and Coordination Complexes: Chemical Sensors for Volatile Organic Compounds. Chem. Rev. 2013, 113, 3686−3733. (52) Atkinson, R.; Arey, J. Atmospheric Degradation of Volatile Organic Compounds. Chem. Rev. 2003, 103, 4605−4638. (53) Liu, Y.; Deng, C.; Tang, L.; Qin, A.; Hu, R.; Sun, J. Z.; Tang, B. Z. Specific Detection of d-Glucose by a Tetraphenylethene-Based Fluorescent Sensor. J. Am. Chem. Soc. 2011, 133, 660−663. (54) Yan, X.; Cook, T. R.; Wang, P.; Huang, F.; Stang, P. J. Highly emissive platinum(II) metallacages. Nat. Chem. 2015, 7, 342−348. (55) Yan, X.; Wang, H.; Hauke, C. E.; Cook, T. R.; Wang, M.; Saha, M. L.; Zhou, Z.; Zhang, M.; Li, X.; Huang, F.; Stang, P. J. A Suite of

Tetraphenylethylene-Based Discrete Organoplatinum(II) Metallacycles: Controllable Structure and Stoichiometry, AggregationInduced Emission, and Nitroaromatics Sensing. J. Am. Chem. Soc. 2015, 137, 15276−15286. (56) Sun, X.; Wang, Y.; Lei, Y. Fluorescence based explosive detection: From mechanisms to sensory materials. Chem. Soc. Rev. 2015, 44, 8019−8061. (57) Xie, Z.; Chen, C.; Xu, S.; Li, J.; Zhang, Y.; Liu, S.; Xu, J.; Chi, Z. White-Light Emission Strategy of a Single Organic Compound with Aggregation-Induced Emission and Delayed Fluorescence Properties. Angew. Chem., Int. Ed. 2015, 54, 7181−7184. (58) Shao, A.; Xie, Y.; Zhu, S.; Guo, Z.; Zhu, S.; Guo, J.; Shi, P.; James, T. D.; Tian, H.; Zhu, W.-H. Far-Red and Near-IR AIE-Active Fluorescent Organic Nanoprobes with Enhanced Tumor-Targeting Efficacy: Shape-Specific Effects. Angew. Chem., Int. Ed. 2015, 54, 7275−7280. (59) Ding, Y.; Shi, L.; Wei, H. A “turn on” fluorescent probe for heparin and its oversulfated chondroitin sulfate contaminant. Chem. Sci. 2015, 6, 6361−6366. (60) Taniguchi, R.; Yamada, T.; Sada, K.; Kokado, K. StimuliResponsive Fluorescence of AIE Elastomer Based on PDMS and Tetraphenylethene. Macromolecules 2014, 47, 6382−6388. (61) Leung, C. W. T.; Hong, Y.; Hanske, J.; Zhao, E.; Chen, S.; Pletneva, E. V.; Tang, B. Z. Superior Fluorescent Probe for Detection of Cardiolipin. Anal. Chem. 2014, 86, 1263−1268. (62) Pramanik, S.; Zheng, C.; Zhang, X.; Emge, T. J.; Li, J. New Microporous Metal−Organic Framework Demonstrating Unique Selectivity for Detection of High Explosives and Aromatic Compounds. J. Am. Chem. Soc. 2011, 133, 4153−4155. (63) Dong, J.; Zhou, Y.; Zhang, F.; Cui, Y. A Highly Fluorescent Metallosalalen-Based Chiral Cage for Enantioselective Recognition and Sensing. Chem.Eur. J. 2014, 20, 6455−6461.

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