A Tetraphenylethene-Based Polymer Array Discriminates Nitroarenes

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

A Tetraphenylethene-Based Polymer Array Discriminates Nitroarenes Wei Huang,† Markus Bender,† Kai Seehafer,† Irene Wacker,‡,§ Rasmus R. Schröder,‡,§ and Uwe H. F. Bunz*,†,§ †

Organisch-Chemisches Institut, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany Cryo Electron Microscopy, Universitätsklinikum Heidelberg, BioQuant, Im Neuenheimer Feld 267, 69120 Heidelberg, Germany § CAM, Centre for Advanced Materials, Ruprecht-Karls-Universität Heidelberg, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany ‡

S Supporting Information *

ABSTRACT: Four tetraphenylethene (TPE)-based aryleneethynylene-type polymers (TPEPs) are reported in this work. All of them show aggregate-induced emission (AIE). Their optical properties have been investigated. The TPEPs are tested as a sensor array for 14 different nitroaromatic analytes and display fingerprint fluorescence quenching responses. The TPEPs demonstrate good sensitivity and discriminatory power in detecting explosives. The quenching efficiencies are dependent on the spectral overlap areas (absorbance of the analyte and the emission of the fluorescent polymer) and on the LUMO level of the analytes. The specific quenching responses are recorded and visualized after processing the data by linear discriminant analysis (LDA). Fourteen nitroarenes are discriminated by the four-element sensor array. Even five pairs of regioisomeric nitroarenes with similar physical and chemical properties were easily discriminated.

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fluorophores is quenched in their aggregated state (ACQ effect).9 In 2001, Tang discovered aggregation-induced emission (AIE).10 AIE fluorophores are nonfluorescent in solution but exhibit high fluorescence when aggregated.11 AIE and similar effects have been noticed for tetraphenylethene (TPE)-containing polymers; they emit weakly in good solvents but efficiently in poor solvents or in films.12,13 AIE is dependent upon polarity, viscosity, electrostatic and hydrophobic interactions, steric hindrance, coordination, and reactivityall exploited in sensing schemes.14,15 AIE sensors have been designed for ions, explosives, and biomolecules, to name a few.16−18,20−22 TPE’s AIE performance, facile synthesis, and its modular functionalization make it attractive.19 However, TPEbased sensor arrays with high sensitivity and discriminative power are not known. In hypothesis free sensor arrays, a limited number of receptors generate patterns that most efficiently discriminate analytes without any specif ic interactions. Here we have prepared four TPE-based polymers, constituting a fourelement sensor array, which discriminates 14 nitroarenes, including five pairs of regioisomers all in aqueous solution.

INTRODUCTION Nitroarenes are not only environmental health hazards but also of vital concern for national security.1 Thus, detection of nitrobased compounds draws significant attention in defense, security, and environment-based applications.2,3 Suitable sensing systems with high sensitivity and selectivity for highly nitrated arenes are attractive, and some powerful systems have already been constructed. In practical applications, both discrimination, i.e. selectivity, and sensitivity are critical to successful detection. Discrimination of strong electron acceptors (such as picric acid) from weak ones (such as nitrobenzene) is fairly easy.4 However, it is more challenging to discriminate different isomeric nitroarenes.5 Isomer classification is challenging for chemical identification, and analytical techniques such as GC-MS often struggle to discriminate isomers, so this is an acute question.6 Regioisomers display identical molecular formulas and same functional groups, but the substituents occupy different positions on a parent framework. The discrimination of positional isomers is difficult because of their similar physical and chemical properties. For nitroaromatics, regioisomers are common, as secondary substituents can be placed ortho, meta, or para to one or more NO2 group(s). Optical sensors provide an excellent method for explosives detection; they exhibit high sensitivity, selectivity, portability, and low cost compared to other methods, such as mass spectrometry or X-ray diffraction.7,8 Emission of most © XXXX American Chemical Society

Received: December 6, 2017 Revised: January 24, 2018

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DOI: 10.1021/acs.macromol.7b02590 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Structures of the TPEPs

Table 1. Photophysical Properties and GPC Data for TPEPs

a

polymer

λmax,absa [nm]

λmax,ema [nm]

ΦF,sola [%]

ΦF,aggb [%]

Mnc [g/mol]

Mw/Mnc

τsola [ns]

τaggb [ns]

TPEP-1 TPEP-2 TPEP-3 TPEP-4

385 376 377 362

515 522 517 520

4.0 1.3 2.4 1.6

26 35 18 16

17000 12000 17000 5000

1.5 1.8 1.8 1.3

0.08 0.08 0.07 0.05

1.26 1.24 0.62 0.68

Determined in THF. bDetermined in THF/H2O (5:95). cDetermined by GPC in THF.

Figure 1. (a) Emission spectra of TPEP-2 in different solvents. (b) Emission spectra of TPEP-2 in different ratios of H2O/THF (excited at 376 nm).

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RESULTS AND DISCUSSION

four polymers all appear around 520 nm, and the spectra share a similar emission shape with exception of the shoulder peaks around 475 nm. TPEP-1 featuring branched oligoethylene glycol side chains has a narrower Stokes shift (130 nm) compared to its analogues TPEP-2 (146 nm) with hexyl and TPEP-3 (140 nm) with hexyloxy side groups, while TPEP-4, containing pyridine, shows broad spectral features and the largest Stokes shift. The different side chains on the comonomers modulate the Stokes shift of the different TPEPs. Emission of the TPE-based compounds is enhanced in their aggregated state,23 and their emission spectra are recorded in different solvents to investigate their AIE performance. Absorption and emission spectra of TPEPs are

The TPEPs were synthesized through Sonogashira coupling of diethynyl-TPE and three diiodobenzene struts, carrying branched oligoethylene glycol (swallowtail, SW), hexyl, or hexyloxy side chains. One TPEP contains a pyridine comonomer (Scheme 1). The TPEPs form in 74−85% yield. The molecular weights (Mn) of TPEPs are 17 000, 12 000, 17 000, and 5000 g/mol (Table 1). The polydispersities range between 1.3 and 1.8. Table 1 shows the fundamental optical properties of TPEPs in THF and in water. The absorption and emission spectra of the polymers in THF are shown in Figure S1. TPEPs have different absorption maxima that range from 362 to 385 nm. However, the maximum emission peaks of the B


Article

Macromolecules Scheme 2. Structures of the Different Tested Nitroarenes

fairly solvent-independent; TPEP-2 shows different fluorescence intensities in different solvents (Figure 1a). In THF and chloroform, TPEP-2 is almost nonemissive. In water and methanol, the emission intensity increases. The AIE characteristics of TPEP-2 were further evaluated by incremental addition of water into its solution in THF (Figure 1b); above 70% water content, a sharp increase of fluorescence intensity is observed. The quantum yields of TPEPs in THF are 4.0%, 1.3%, 2.4%, and 1.6%, respectively. In a THF/water mixture (5:95) the quantum yields increase to 26%, 35%, 18%, and 16%6.5 to 27 times the values of the quantum yields measured in THF. All four TPEPs demonstrate AIE. The difference in the magnitude of the AIE effect for different polymers is not uncommon. The different side chains influence the AIE performances of TPEPs. TPEP-1 with hydrophilic side chains OSw shows a moderate quantum yield increase (6.5 times in THF/water mixture (5:95) compared to pure THF). TPEP-2 with the most hydrophobic hexyl chains has the best AIE performance (27 times increase of fluorescence intensity). Hydrophobic chains are therefore favorable in building AIE structures, as aggregation and nanoprecipitation are enforced. The morphology of TPEPs in the aggregated state in a THF/water mixture (5:95) was investigated through scanning electron microscope (SEM) and is shown in Figure S3. TPEP-1 forms homogeneous structures under drying, different from the structures of the other three polymers. TPEP-2, TPEP-3, and TPEP-4 apparently exist as nanoscale spheres in solution. We assume that his morphology was preserved. Therefore, the morphology of the polymers is mainly determined by the side chains of the polymers and forms in water as a consequence of their nanoprecipitation into stable dispersions. The four TPEPs detect and discriminate nitroarenes via fluorescent quenching in their aqueous phase. Some of these nitroarenes, including TNT and picric acid, are commercial explosives. Fourteen different analytes including structural isomers (Scheme 2) were investigated: nitrobenzene (NB), 4nitrophenol (pNP), 2-nitrophenol (oNP), 3-nitroaniline (mNA), 2-nitroaniline (oNA), 3-chloro-2-nitrobenzene (mCNB), 1-chloro-2-nitrobenzene (oCNB), 2,6-dinitrotoluene (2,6-DNT), 2,4-dinitrotoluene (2,4-DNT), 1,3-dinitrobenzene (mDNB), 1,2-dinitrobenzene (oDNB), 1-chloro-2,4-nitrobenzene (CDNB), trinitrotoluene (TNT), and picric acid (PA). By gradually adding the analytes, the fluorescence intensity of the polymer solutions decreases. We determined the Ksv

constants of TPEPs with all of the analytes in aqueous solution (H2O:THF, 95:5; Figure 2 and Table S2).24 The 14 analytes

Figure 2. Fluorescence quenching efficiencies of TPEPs for different analytes. The z-axis denotes the Stern−Volmer constant Ksv.

show distinct quenching abilities with the four TPEPs. NB is the weakest quencher, while the highly electron-deficient CDNB, TNT, and PA are good quenchers. Owing to the different side chains and comonomers, the four TPEPs demonstrate different optical (absorption and emission), electrical (HOMO and LUMO energy level), and physical properties and aggregated states. Thus, each of the four TPEPs has a unique quenching pattern for the 14 analytes. TPEP-1 and TPEP-3 display a higher quenching efficiency for most of the analytes than TPEP-2 and TPEP-4, which is a testament to their electron-rich character and also their improved spectral overlap with the absorption spectra of the analytes (Figure 3). The emission spectra of the polymers (Figure 3) are dominated by the tetraphenylethylene backbone. In the case of TPEP-1 and TPEP-3 an additional blue-shifted shoulder appears that might be due to planarization of the polymers. This shoulder increases the spectral overlap with some of the analytes’ absorption spectra and will therefore increase the interaction between nitroarene and polymer.25 The TPEPs are sensitive toward the analytes as their Ksvs are in excess of 104 M−1, especially for oDNB, CDNB, TNT, and PA. The limits of detection (LOD) of TPEPs for the 14 analytes were estimated and are listed in Table S3. The LOD for the explosives are C


Article

Macromolecules

Table 3. Calculated LUMO Level of the Analytes at the B3LYP/6-311++G** Level of Theorya nitroarenes

pNP

oCNB

oNA

mNA

mCNB

NB

oNP

LUMO (eV)

−2.42

−2.64

−2.67

−2.75

−2.90

−2.92

−3.19

2,4DNT

mDNB

2,6DNT

oDNB

CDNB

TNT

PA

−3.25

−3.31

−3.40

−3.45

−3.67

−3.92

−4.20

nitroarenes LUMO (eV) a

Analytes are ordered according to their calculated LUMO level.

between excited state electron transfer from the polymer to the nitroarene and Förster energy transfer. The TPEPs form a focused sensor array that discriminates all of our 14 analytes.28 Solutions of the TPEPs polymers (1 μM) in water were exposed to the analytes (0.1 mM) in six replicates, and the fluorescence changes were recorded. The average values of fluorescence changes form a characteristic response pattern (Figure 4). Discrimination is visualized after

Figure 3. Normalized absorption spectra of analytes and emission spectra of the TPEPs in water.

below ppm level, demonstrating the good sensitivity of the fluorescent polymers. The performance of TPEPs exceeds that of the earlier HCPs7 and PPEs,8 which also detect the nitroarenes in water (structures of the HCPs and PPEs are shown in Figure S2). We take PA as an example (Table 2). Table 2. Ksv and LOD Values of TPEPs Compared to That of HCPs and PPEs for Picric Acid TPEPs

TPEP-1

TPEP-2

TPEP-3

TPEP-4

Ksv LOD (M) HCPs

4.7 × 104 2.4 × 10−7 HCP-1-M

2.6 × 104 1.3 × 10−6 HCP-2-M

4.3 × 104 2.8 × 10−6 HCP-1

3.0 × 104 6.9 × 10−7 HCP-2

Ksv LOD (M) PPEs

1.4 × 104 2.8 × 10−7 P1

1.1 × 104 9.0 × 10−7 P2

6.9 × 103 1.2 × 10−6 P3

5.8 × 103 2.4 × 10−6 P4

Ksv LOD (M)

1.8 × 103 1.1 × 10−5

1.8 × 103 1.4 × 10−5

7.4 × 102 1.0 × 10−5

1.2 × 102 4.0 × 10−5

Figure 4. Fluorescent response pattern (I/I0 − 1) of the TPEP sensor array to the analytes (1−14 are NB, pNP, oNP, mNA, oNA, mCNB, oCNB, 2,6-DNT, 2,4-DNT, mDNB, oDNB, CDNB, TNT, and PA, respectively).

Compared to the water-soluble PPEs, the TPEPs show an order of magnitude higher Ksv for PA and a concomitantly lower LOD, also 1−2 orders of magnitude lower than for the PPEs. The HCPs are more similar in performance, but they do need adjuvants to work properly for nitroarene sensing, exploiting surfactochromic behavior. TPEPs directly detect nitroarenes in aqueous solution employing the AIE effect; therefore, this structure is an excellent choice for the detection of nitroarenes in water. The detection of the nitroarenes is composed of Förster resonance energy transfer (FRET) and photoinduced electron transfer (PET) from the polymers to the analytes, modulated by hydrophobic interactions.26 Analytes with lower LUMO energy display better quenching performance due to strong PET. The energy level of the 14 analytes were calculated through B3LYP/6-311++G** level of theory (Table 3). The HOMO levels of the TPEPs are determined by cyclic voltammetry to −4.76, −4.90, −4.93, and −4.83 eV. The LUMO values are −1.83, −2.00, −2.12, and −1.99 eV, respectively. The full electrochemical properties of TPEPs are listed in Table S4. The differing LUMO values suggest that the excited state energies of the AIE-aggregated TPEPs are different, allowing for quenching-based discrimination of the nitroarenes, as the quenching mechanism is probably a mix

processing the data by linear discriminant analysis (LDA), which converts the training matrix (4 polymers ∗ 14 analytes ∗ 6 replicates) into canonical scores according to their Mahalanobis distance.28 Four canonical factors (66%, 18%, 11%, and 5%) were generated from the data (Table S5). All of the 14 analytes are discriminated as 14 clusters on the 2-D score plot, based on the two larger canonical factors with 100% accuracy (Figure 5). The distribution of the clusters on the color map is influenced by the LUMO value of the analytes. The analytes with the lowest LUMO level (oDNB, CDNB, TNT, and PA) are located on the left side of the 2-D map (red circle), and the four analytes (oNP, 2,4-DNT, mDNB, and 2,6-DNT) with medium LUMO level (−3.00 to −3.40 eV) are located in the middle of the map (green circle). The five pairs of the regioisomers are marked with same shape, with solid or hollow symbols to distinguish them. They are separated from each other in our system. Our four-element sensor detects and discriminates 14 D


Article

Macromolecules

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voltammograms and electrochemical properties of TPEPs, training matrix of fluorescence response pattern, and the lifetime of the TPEPs under different concentration of PA (PDF)

AUTHOR INFORMATION

Corresponding Author

*Fax +49(6221)548401; e-mail [email protected]. de (U.H.F.B.). ORCID

Uwe H. F. Bunz: 0000-0002-9369-5387 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS W.H. thanks the CSC (Chinese Scholarship Council) for a fellowship.

Figure 5. 2-D canonical score plot of discriminant scores with 95% confidence ellipses for all obtained data points against different analytes.

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different nitroarene analytes without problem. The sensitivity of the TPEPs is better than other systems HCPs.7,8 The fluorescence lifetimes of the quenching system are shortened by dynamic quenching process but unaffected by static quenching process.29 Monitoring the lifetime changes allows to detect dynamic quenching. To investigate the quenching mechanisms of the analytes to our TPEP sensor array, emissive lifetimes of TPEPs were recorded with different concentrations of the representative quencher PA (Figure S5) present. Only small, irregular changes in emissive lifetimes were observed, suggesting that static quenching is the dominant mechanismas expected.

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CONCLUSION In conclusion, a sensor array formed from TPEPs AIE-gens in water discriminates nitroarenes by fluorescence quenching. The TPEPs display excellent sensitivities to the explosives and compare well with the reported PPE and HCP systems. Fourteen analytes were effectively detected and discriminated by this four-element sensor array, including five pairs of regioisomers. The dominant quenching mechanism is static in nature, as the fluorescent lifetimes of TPEPs do not change upon the addition of PA. Overall, the TPEPs open up an attractive vista for sensing as the TPE element can be freely engineered to display further electronic properties that should allow sense not only electron-deficient but also electron-rich analytes that plague the environment. Questions of discrimination but also of LOD are of great interest and should be tackled with our hypothesis free sensor arrays.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02590. General information, the protocols to prepare TPEPs, H NMR spectra of related materials, the absorption and emission spectra of TPEPs in THF, quantum yields of TPEPs under different conditions, SEM images of TPEPs, the Ksv constants of TPEPs to the analytes, the limits of detection toward the analytes, the cyclic E


Article

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A Tetraphenylethene-Based Polymer Array Discriminates Nitroarenes

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