An AIE+TICT activated colorimetric and ratiometric fluorescent sensor for portable, rapid, and selective detection of phosgene

Dyes and Pigments 176 (2020) 108229

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An AIEþTICT activated colorimetric and ratiometric fluorescent sensor for portable, rapid, and selective detection of phosgene Qinghua Hu **, Qiuxiang Huang , Kexin Liang , Yuyuan Wang , Yu Mao , Qiang Yin , Hongqing Wang * School of Chemistry and Chemical Engineering, Hunan Key Laboratory for the Design and Application of Actinide Complexes, University of South China, 28 Changsheng West Road, Hengyang, Hunan, 421001, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Fluorescent sensors AIEþTICT Phosgene Colorimetric Ratiometric

Colorless and highly toxic phosgene has long been considered to be a great menace to human health and national security. Excellent AIE þ TICT characteristic ratiometric and colorimetric fluorescent sensor TPE-CI was con­ structed with the combination of the TICT based 3-benzo[d]imidazole-chromen-2-imine unit and AIE based tetraphenylethene (TPE) unit. TPE-CI can detect phosgene not only in organic solvent through a change from the TICT process to the AIE process, but also in solid-state through a new change from the AIE þ TICT process to the AIE process. TPE-CI exhibited convenient to use as a test strip, rapid response (less than 6 s in solvent and 2 min in air atmosphere), excellent selectivity and fair sensitivity for visual inspection of trace phosgene in the organic solvent and in the gas phase. The limit of detection (LOD) was calculated and derived as 0.36 μM in solvent and 0.27 ppm in the air atmosphere, which was far below the acute phosgene exposure level of human response. The satisfactory results indicate that this AIE þ TICT strategy may offer valuable thinking to develop convenient solid-state optical sensors with dual ratiometric and colorimetric fluorescence response for gaseous phosgene identification.

1. Introduction Phosgene, also known as carbonyl chloride (COCl2) is a dangerous colorless highly toxic gaseous pollutant [1]. Phosgene is seriously haz­ arded to the respiratory system of human beings. Exposure to phosgene gas may lead to pulmonary emphysema, noncardiogenic pulmonary edema, asphyxia, and even death [2–4]. As an insidious poison, even at lethal concentration, the poisoning reaction occurs a few hours after inhaled [5]. Unfortunately, during World War I, phosgene was intro­ duced as a biochemical weapon and caused tens of thousands of deaths [6,7]. Unlike other strictly regulated and prohibited biological weapon agents (such as tabun, soman, and sarin), phosgene is widely used as an essential material in various industrial processes, such as in the pro­ duction of pharmaceuticals, dyes, pesticides, isocyanate-based resins and many other useful intermediates [8,9]. Therefore, considering the high toxicity and its potential security threats from unexpected indus­ trial accidents or intentional nefarious releases. Accordingly, it’s an urgent requirement to develop rapid, convenient, portable, efficient and

reliable ways for the monitoring of phosgene. Recently, several conventional detection methods including elec­ trochemical methods [10], GC-MS [11] and HPLC-MS [12] have been constructed for testing phosgene. Unfortunately, these methods are often restricted for field detection because of their high cost, complex operational processes, and lack of convenient. In comparison, fluores­ cence detecting technology, as a reliable detection method, has been investigated extensively, due to its operational simplicity, high sensi­ tivity and selectivity, easy for field use, cost-effective, and capabilities of real-time detection [13–24]. In particular, fluorescent probes with a dual ratiometric and colorimetric expression not only provided conve­ nient visual detection but also can avoid outside disturbance by intrinsic calibration of environmental and instrumental impacts based on two separated absorbance and emission bands [25–29]. Therefore, this type of probes is more appropriate for phosgene detection in actual condi­ tions than that of the intensity-based fluorescent probes. During past years, over twenty phosgene testing fluorescent chemi­ cal sensors have been published so far, which were designed utilizing

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Q. Hu), [email protected] (H. Wang). https://doi.org/10.1016/j.dyepig.2020.108229 Received 23 October 2019; Received in revised form 17 January 2020; Accepted 19 January 2020 Available online 27 January 2020 0143-7208/© 2020 Elsevier Ltd. All rights reserved.

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monitoring, which shows very good potential application prospects as a portable paper-based test strip for naked eye identification and fluo­ rescent testing of gaseous phosgene in the air.

organic small molecules fluorophores [30–57]. Almost all of them likely to undergo intensity decreased or even fully quenched in aggregation state based on aggregation caused quenching (ACQ) effect, such as coumarins [30–33], Rhodamines [34–38], 1,8-naphthalimide [39–44], and BODIPYs [45–49]. Thereby, ACQ based fluorescent probes are not the best choice for developing portable solid-state detection systems, such as test strips or membrane probes for phosgene detection. On the contrary, aggregation-induced emission (AIE) based probes exhibit almost no fluorescence in the dissolved state but give off strong fluorescence emission under aggregate or in solid-state [58–66]. As a result, AIE-activate fluorophores could be more conducive to the con­ struction of easy-to-use test-strip or membrane sensors compared to the conventional ACQ-activated sensors. Up to now, only one AIE activate fluorescent probe (OPD-TPE-Py-2CN) has been reported [57]. This probe was constructed by incorporating a malononitrile moiety and an o-phenylenediamine group into a classical AIE fluorophore tetraphe­ nylethene. Although this probe can detect phosgene in a ratiometric way, the largest emission wavelength just moved from 475 nm(blue-­ green) to 496 nm (green), which will inevitably be suffered undesired spectral crosstalk. Finally, the accuracy of phosgene detection will be affected, especially at low concentrations. At the same time, the probe cannot be used for colorimetric detection. Therefore, it’s highly desir­ able to developing ratiometric and colorimetric phosgene probes that allow detection in solid-state without the ACQ effect. With these requirements, we herein design a promising easy-to-get fluorescent probe (probe TPE-CI) for dual ratiometric and colorimetric visual testing of gaseous phosgene based on AIE þ TICT activate pro­ cesses. As shown in Scheme 1, this probe was designed by combined a 3benzo[d]imidazole-chromen-2-imine unit and a tetraphenylethene (TPE) derivative. Owning to the 3-benzo[d]imidazole-chromen-2-imine unit, the probe possesses TICT characteristics. Meanwhile, the probe also exhibits the AIE feature via its TPE derivative moiety. Since the probe TPE-CI has two active NH groups, which could react with phos­ gene through twice carbamylation reactions to generate a rigid hexa­ tomic cyclic carbamate product TPE-CI-phos. More interestingly, when TPE-CI dissolved in dilute solution, phosgene could induce the color variance from colourless to deep yellow, accompanied by a fluorescence decrease at 503 nm due to the change from TICT process to AIE process. While in the solid-state as a test strip, with the existing of phosgene in air atmosphere, a remarkable ratio fluorescent response from 495 nm to 570 nm along with the color changes from creamy white to yellow, caused by a novel change from AIE þ TICT process to AIE process, which can effectively avoid the shortcomings of the ACQ effect. Moreover, TPE-CI exhibit excellent sensitivity and selectivity, the rapid response time (within 6 s in solvent and 2 min in the test strip). Based on this unique advantage, TPE-CI would serve as an excellent dual-mode ratiometric and colorimetric fluorescence sensor for phosgene

2. Experimental process 2.1. Material and measurements Unless otherwise mentioned, all reagents or solvents used for syn­ thesis and purification were purchased from some common commercial reagent companies (Aladdin, J&K Chemicals or TCI) were analytically pure and used directly without the further purifications. Optical spectra (UV–vis spectra and Fluorescence spectra) were tested by an ultra­ violet–visible absorption spectrometer (Model: Hitachi U-3900) pro­ duced in Japan and a fluorescence spectrophotometer (Model: Hitachi F7000) also produced in Japan. 1H NMRs were determined by a Bruker NMR Spectrometer (500 MHz, produced in Germany). ESI-HRMS were determined through a Waters Alliance e2695 ACQUITY Qda highresolution mass spectrometer. 2.2. Synthesis and characterization of the sensor Compounds 2 and 3 were prepared on the basis of one previous literature published by our group [27]. The probe TPE-CI can be readily synthesized through two steps as described in Scheme 2. 2.2.1. Synthesis of compound 3 1.12 g Compound 2 (4.5 mmol) mixture with 1.0 g bromo­ triphenylethylene (3.0 mmol) were dissolved in 12 mL toluene and transferred to a 50 mL two-neck flask, followed by addition of the pre­ pared K2CO3 aqueous solution (2 M, 6 ml), 14 mg Pd(0)(PPh3)4 (0.012 mmol) and 0.10 g TBTA(0.3 mmol). The reactant liquid was stirred at least 0.5 h at room temperature, then heated to 100 � C and stirred for another 20 h under nitrogen protection. After cooled, 20 ml of distilled water was slowly added to the reactant liquid and the organic phase was extracted three times with CH2Cl2. Then combined the organic phase and dried with anhydrous Na2SO4. Concentrated by using a rotary evaporator under reduced pressure, the crude product was further pu­ rified via column chromatography (200–300 mesh silica gel) using PE/ CH2Cl2 (6/1, v/v) as eluant, a yellow solid is obtained. (730 mg, 65% yield). 1H NMR (DMSO‑d6, 500 MHz): δ ¼ 10.71 (s, 1H), 10.13 (s, 1H), 7.25 (s, 1H), 7.17 (m, 10H), 7.01 (t, 6H), 6.76 (d, J ¼ 10 Hz, 1H). HRMS (ESI) [MþNa]þ: m/z ¼ 397.1197; Calculated: C27H20O2 ¼ 376.1463. 2.2.2. Synthesis of probe TPE-CI 47 mg (2-benzimidazolyl) acetonitrile (0.3 mmol) was dissolved with 12 mL methanol and transferred to a two-necked round-bottom flask, then 60 μL of piperidine was dripped to the reaction liquid after stirred for 0.5 h at 25 � C. Then 113 mg Compound 3 (0.3 mmol) was placed into the reaction liquid and stirred for another 6 h at 25 � C, the product was

Scheme 1. The proposed testing mechanism of TPE-CI for phosgene in solvent and in solid-state.

Scheme 2. The synthetic routes of TPE-CI. 2

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precipitated out. The sediment was filtered and washed three times with absolute methanol, a yellowish solid powder was obtained to give the final product probe TPE-CI (127 mg, yield 82%). 1H NMR (CDCl3, 500 MHz): δ ¼ 12.59 (s, 1H), 8.43 (s, 1H), 7.75 (d, J ¼ 10 Hz, 1H), 7.64 (s, 1H), 7.49 (d, J ¼ 10 Hz, 1H), 7.28 (d, J ¼ 5 Hz, 2H), 7.12 (m, 11H), 7.04 (m, 6H), 6.93 (d, J ¼ 15 Hz, 1H). HRMS (ESI) [MþH]þ: m/z ¼ 516.2074; Calculated: C36H25N3O ¼ 515.1998.

3. Results and discussion 3.1. Sensor preparation and characterization TPE-CI can be conveniently synthesized in two steps from commer­ cially available 1-Bromo-1,2,2-triphenylethylene and (2-benzimida­ zolyl)acetonitrile (As illustrated in Scheme 2). Compounds 2 and 3 were synthesized refer to the previous literature [27]. Compound 3 and (2-benzimidazolyl)acetonitrile were stirred with piperidine in methanol for 5 h to obtain the probe TPE-CI in a yield of 82%. A reference com­ pound TPE-CI-phos was also synthesized by the reaction of TPE-CI with triphosgene in MeCN including excessive TEA in a 77% yield. The structures of compounds 3, TPE-CI, and TPE-CI-phos were completely confirmed by 1H NMR and HRMS(ESI) analyses (The characterization data are shown in Figs. S1–S6).

2.2.3. Synthesis of TPE-CI-phos 103 mg probe TPE-CI (0.2 mmol) was put in a 100 mL two-neck flask and dissolved in a mixed solvent with 15 mL in dry acetonitrile and 15 mL dichloromethane, then 404 mg Et3N (4 mmol) was added, cooling down with an ice bath. Stirred for 10 min under nitrogen, then 10 mL of acetonitrile solution with 119 mg of triphosgene (0.4 mmol) dissolved was slowly added into the reaction liquid with an injector. The mixture solution was stirred overnight at 25 � C. After concentrated via reduced pressure distillation, the crude solid product was then purified via col­ umn separation (200–300 mesh silica gel) using CH2Cl2/PE (3/1, v/v) as the eluant to obtained an orange-yellow powder TPE-CI-phos (83 mg, 77% yield). 1H NMR (CDCl3, 500 MHz): δ ¼ 8.75 (s, 1H), 8.47 (m, 1H), 7.82 (m, 1H), 7.47 (m, 5H), 7.13 (m, 9H), 7.06 (m, 6H). HRMS (ESI) [MþH]þ: m/z ¼ 542.2504; Calculated: C37H23N3O2 ¼ 541.1790.

3.2. Optical properties measurement Optical properties of the compounds TPE-CI and TPE-CI-phos were tested by ultraviolet–visible absorption spectrophotometer and fluores­ cent spectrophotometer. Because of the ICT based portion of TPE-CI possesses triphenylethylene act as an electron donor and 3-benzimid­ azole is considered to be the electron acceptor, which are connected by a single bond that can be rotated, so the ICT process is likely to form a TICT state caused by a molecular configuration change. To evaluate the TICT process of the TPE-CI, the change in the absorption and maximum fluorescence spectroscopy with the solvent polarity were investigated. As shown in Fig. S7, they exhibit quite a similar absorption peak at 340 nm, which changed faintly with the polarity of solvents increased. The maximum emission wavelength varied gradually from 484 nm to 509 nm (Fig. 1A). The change in the maximum emission wavelength with the solvent polarity empirical parameter ET(30) is plotted in Fig. 1B and summarized in Table S1. A linear line with a good linear relationship of R2 ¼ 0.961 and a small slope of 2.38 was obtained. The results indicate that TPE-CI shows a weak solvatochromism effect from the triphenyl­ ethylene to the 3-benzimidazole unit. To further verify the TICT char­ acteristics of the probe TPE-CI. We investigated the effect of solvent polarity on the fluorescent emission, which can be further evaluated by the Lippert Mataga equation: Eq. (1) [67]. The related experimental data are summarized in Table S1 and Fig. S8.

2.3. Assay experiments TPE-CI and TPE-CI-phos were dissolved in HPLC grade THF as a stock solution (1.0 mM), while other analytes stock solutions were prepared by dissolving in dichloromethane respectively. UV–vis ab­ sorption and fluorescent spectrum were measured in pure HPLC grade THF, CH3CN or mixture of water solution at 25 � C, all the data were measured in a quartz fluorescent dish (10.0 � 10.0 mm). Phosgene was generated by a less toxic and nonvolatile triphosgene with the existence of 0.1% trimethylamine (TEA) in THF or CH3CN solvents. 2.4. Preparation of TPE-CI loaded test strips The portable test strip for phosgene detection was obtained as fol­ lows: some 1.5 � 6 cm cut filter papers were immersed in a THF stock solution of TPE-CI (8 � 10 4 M) and dried in air. The test strips were stored in a cool and dry place after preparation. paper strips. 2.5. Detection of phosgene in solvents

Δν ¼

Spectrum testing of phosgene in a solvent, the sensing process was implemented by added various concentrations of triphosgene in dichloromethane into the sensing system TPE-CI (40 μM) in THF with 0.1% TEA added at room temperature. After 1 min, the changes in the emission spectrum and absorption of TPE-CI were then monitored.

Δf ¼

2Δf ðμ hcr3 e

ε 1 2ε þ 1

μg Þ2 þ const n2 1 2n2 þ 1

(1) (2)

Δν represent the Stokes shift of the absorption and emission maxima, c represents the speed of light, h represents Planck’s constant, and r represent the radius of the Onsager cavity. Δf represents the solvent polarity parameter and can be calculated by Eq. (2), ε and n represent the dielectric constant the refractive index of the solvent respectively. A plot of the Δν versus Δf fitted out a slope as high as 5571.24 (Fig. S8), which yields the significant difference (Δμ) between the dipole moment of the ground state (μg) and the excited state (μe). Furthermore, the Δν of the probe increases with the increasing polarity of the solvent. This solvatochromic effect indicates there was charge separation in the TPECI structure and further confirmed the TICT theory. In order to deeply understand the AIE þ TICT effect, we then tested the emission spectra of TPE-CI in the THF/water mixture systems as well as TPE-CI-phos in the MeCN/water mixtures systems with increased water fractions (fw). As shown in Fig. 2A, C, E, as the water fraction increases from 10 to 70%, the emission of TPE-CI decreased, along with a slight emission wavelength red-shift. This phenomenon could be explained by the increased fraction of higher polar water enhance the TICT effect of TPE-CI in the THF solution. Once the fw was increased from 80% to 95%, there was an obviously enhanced intensity at about

2.6. Phosgene detection with test strips Gaseous phosgene detection was tested by TPE-CI loaded test strips. Firstly, 10 μL of different concentrations of triphosgene solutions in dichloromethane were placed at the bottom of a sealed conical flask (250 mL), then the test strips were hanged and fixed in each sealed conical flask, finally, 10 μL of dichloromethane solution with 0.1% TEA added was injected to each sealed conical flask. The spectra of all test strips exposure to phosgene vapors with various concentrations for 2 min were recorded on a fluorimeter (λex ¼ 470 nm). Photos were collected by a digital camera under daylight and 365 nm UV lamp irradiation.

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Fig. 1. (A) Fluorescent emission spectra of TPE-CI in various solvents. (B) The plot of the maximum emission wavelength of TPE-CI in different solvents versus each solvent polarity empirical parameter ET(30). Concentration: 40 μM; excitation at: 365 nm. Fig. 2. Fluorescence emission spectra of (A) TPE-CI in THF/water and (B) TPE-CI-phos in MeCN/water mixture systems with increased water fractions (fw); The plot of peak intensity vs increased water fraction (fw) of (C) TPE-CI (at 503 nm) in the THF/water and (D) TPE-CI-phos (at 570 nm) in the MeCN-water mixture systems; Fluores­ cent photographs of (E) TPE-CI in THF/ water and (F) TPE-CI-phos in MeCN/ water mixture systems with increased water fractions (fw). Concentration: 40 μM; excitation wavelength: 365 nm.

510 nm, since the probe TPE-CI is insoluble in water, it must have been aggregated in a large amount of water fraction, demonstrating the AIE activity. It is strange that the emission slightly reduced when the fw increased from 90% to 95%. This may be owing to the water-induced crystalline aggregates feature of TPE-CI, and different fw will affect the aggregate size [68]. When the TPE-CI added to the 95% fw mixture solution, the rapid aggregation will be resulting in smaller emissions. In comparison, as shown in Fig. 2B, D, F, TPE-CI-phos is almost non­ emissive in acetonitrile (MeCN) solution. However, when the fw was increased from 60% to 95%, the fluorescence intensity at 570 nm

gradually increases. The results indicated only the AIE effect of TPE-­ CI-phos can be observed. The TICT behavior of TPE-CI-phos could be restricted efficiently, due to the twice carbamylation reactions with phosgene to form a cyclic carbamate structure. In order to have a better insight into the optical properties, the frontier orbital distributions of TPE-CI and the expected reaction product TPE-CI-phos were optimized with density functional theory (DFT) under Gaussian 16 at the B3LYP/6-31G(d) basis set. The opti­ mized geometries and electron clouds on HOMO and LUMO were illustrated in Fig. 3. Both of the electron clouds on the HOMO of TPE-CI 4

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which restricted the TICT behavior. Moreover, both of the triphenyl­ ethylene group in TPE-CI and TPE-CI-phos were optimized as highly distorted configuration, which is of great benefit to active intra­ molecular rotations so as to exhibit AIE properties. Meanwhile, the TPECI-phos has a smaller LUMO-HOMO energy gap (0.106 eV) than that of the probe TPE-CI (0.123 eV), which is consistent with the redshift of maximum absorption wavelength from 340 nm to 374 nm and maximum emission peak from 495 nm to 570 nm. 3.3. Sensing performance of probe TPE-CI in the organic solvent First of all, the optical sensing performance of TPE-CI toward phosgene in THF was explored. As mentioned before, phosgene was produced from triphosgene via catalytic reaction by triethylamine. The experiment was done by adding different concentrations of triphosgene stock solution into 2 mL of TPE-CI (40 μM) in THF with 0.1% TEA added. The UV–Vis absorption together with emission wavelength changes was monitored. As described in Fig. 4A, TPE-CI shows a large absorption band at around 340 nm in THF. After added various concentrations of triphosgene, the absorption bands at around 340 nm gradually reduced and another two new absorption bands around 283 nm and 374 nm appeared. As a result, the TPE-CI solution showed noticeable color changes from colorless to deep yellow. Meanwhile, we also tested the fluorescence spectra of the probe TPE-CI toward phosgene. As shown in Fig. 4B, a remarkable on-off type fluorescence change can be observed, the emission intensity decreased dramatically at 503 nm with various concentrations phosgene (0–600 μM) added. When the phosgene con­ centration rises to 520 μM, the quenching efficiency up to 97.37%. The relationship between phosgene concentrations and the emission in­ tensity are fitted well with the nonlinear Stern-Volmer curve equation of I0/I ¼ 0.0389 � exp(0.0131 � [phos])þ1.81 (R2 ¼ 0.979, Fig. S9(A)), I and I0 represent the emission intensity of solutions added different concentrations of phosgene and the blank sample respectively. This curve equation indicated that the probe TPE-CI follows simultaneous static and dynamic quenching with phosgene in the solvent system [69].

Fig. 3. Comparison of the HOMO and LUMO orbital surfaces of TPE-CI and TPE-CI-phos using the DFT B3LYP/6-31G(d) method.

and TPE-CI-phos were well delocalized along the whole π-conjugated chains. While excited, on the LUMO of TPE-CI and TPE-CI-phos, almost all electron clouds were transferred to the 3-benzo[d]imidazole-chro­ men-2-imine unit, which could be interpreted as the obvious ICT ef­ fect from the electron-donating triphenylethylene to the 3benzimidazole unit. In addition, through careful comparison, we found that the HOMO and LUMO of TPE-CI exhibited partial twist caused by the molecular configuration change via a freely rotatable single bond between 3-benzimidazole and the chromen-2-imine unit. It is further proved that this is a certain TICT effect. However, the TPE-CIphos shows a rigid planar structure duo to the cyclization reaction,

Fig. 4. (A) UV–Visible absorption spectrum and (B) fluorescence spectrum of probe TPE-CI (40 μM) in THF with an increasing range of phosgene added (0–600 μM). (C) Linear relationships of emission intensity at 503 nm for probe TPE-CI (40 μM) versus concentrations of phosgene in THF. (D) Time-dependent emission response of probe TPE-CI (40 μM) to phosgene (600 μM) in THF. λex ¼ 365 nm, slits: 10/10 nm. Each spectrum was recorded after 1 min at 25 � C. 5

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In order to have a deeper understanding of the quenching mechanism, which can be further investigated by using the Stern-Volmer Equation: Eq. (3). Where KS is the Stern-Volmer static quenching constant. I = I0 ¼ Ksv ½phos� þ 1

discussed the feasibility of the sensor TPE-CI used to prepare a phosgene rapid test paper strip. The test paper strips can be easily prepared by immersed some cut filter papers into the TPE-CI stock solution and dry naturally. Various concentrations of phosgene can be obtained by injecting different concentrations of triphosgene into a cork-sealed conical flask containing a proper amount of TEA. Then the test strips were suspended in the flask, exposed to phosgene vapors prepared by the previous experimental design for 2 min. As can be seen in Fig. 5A, when no phosgene exists, the test paper strip showing a blue-green characteristic emission peak at around 495 nm. However, with the phosgene level increased, the emission intensity at around 495 nm gradually reduced accompanied by a new rapidly raised peak at around 570 nm. As shown in Fig. 5B, the plot of the emission intensity ratio I570/ I495 against the phosgene concentration range of 0–44 ppm, which showed a good linear relationship (R2 ¼ 0.987). The LOD was then calculated as low as 0.27 ppm, which was far below the dangerous range (20 ppm) that can cause serious respiratory system injuries [52]. Furthermore, in order to demonstrate the visual detection feasibility, we took photographs for the TPE-CI loaded test paper strips exposure to various quantitative concentrations of phosgene contained air atmo­ sphere, under natural light and a portable UV lamp (365 nm). From Fig. 5C, with the phosgene level increases, a gradual color changed can be observed from creamy white to deep yellow accompanied by obvious fluorescence changed from blue-green to brown by the naked eye. This AIE þ TICT activate fluorescence sensor for phosgene testing with visible color and fluorescence response. It is of great significance to the development of portable on-site detection sensors. The above experiments have confirmed that TPE-CI is a rapid, sen­ sitive detection process for phosgene. We investigated the selectivity of the TPE-CI loaded test strips for phosgene among other potential gaseous detection contaminants. As shown in Fig. 6A and B, the fluo­ rescence emission and intensity ratio I570/I495 of the TPE-CI loaded test strips remained unchanged or only slightly changed. From Fig. 6C, we can see, only phosgene can cause a noticeable color variation from creamy white to deep yellow along with a fluorescent color variation from blue-green to brown under a portable ultraviolet lamp (365 nm) radiation, while the other potential gaseous interfering materials could not induce obvious variations in fluorescence or color of the test paper strips. The above exciting results indicated that TPE-CI loaded test paper strip may serve as a portable, fast, and highly sensitive on-site detector

(3)

As shown in Fig. S9 (B), the Stern-Volmer plot shows good linear relationship behavior in the range of 0–120 μM and 180–420 μM with a good correlation coefficient (R2) equal to 0.924 and 0.975. As inferred, in the range of 0–120 μM and 180–420 μM the probe TPE-CI follows static quenching with phosgene in the THF solution. The corresponding Ksv values are calculated to be 2.98 � 103 M 1 and 3.19 � 104 M 1 based on the Stern-Volmer equation respectively. Generally, the higher values for Ksv indicate higher reaction efficiency between phosgene and the probe. Obviously, the blue-green fluorescence solution turned to nonfluorescent under a 365 nm light. These dramatic color and fluo­ rescence changes might be ascribed to the twice carbamylation reactions via electrophilic phosgene coupled with two active NH groups, as well as form a cyclic carbamate structure, which restricted the TICT behavior and leading to a change from TICT character to AIE character in THF solution. As shown in Fig. 4C and the inset, the plot displayed excellent linear relationship (R2 ¼ 0.995) based on the recorded emission in­ tensity at 503 nm vs the concentration of phosgene within the scope of 60–240 μM. Based on the empirical equation LOD ¼ 3σ/k, the limit of detection (LOD) was calculated to be 0.36 μM in THF. In addition, the time-dependent fluorescence change at 503 nm of TPE-CI with phosgene was recorded. As shown in Fig. 4D, the emission intensity of the sensing system of TPE-CI in THF with 0.1% TEA decreases rapidly and reaches a plateau no more than 6 s start with the addition of triphosgene. The above results suggest that TPE-CI can be exploited as a colorimetric and intensity based on-off type fluorescent senor for phosgene monitoring in the solvent. 3.4. Sensing properties of probe TPE-CI in solid state Because phosgene exists as a gas in its usual state, there is not much practical significance in detecting phosgene in the solvent. Therefore, solid-state based optical sensors including rapid test paper or polymer membrane sensors are more useful for locating phosgene leaks inside or around phosgene processing facilities. Especially, this is a unique advantage of AIE based probes compared to ACQ ones. Then, we

Fig. 5. (A) Fluorescent emission spectra of TPE-CI loaded test paper strips upon expo­ sure to different phosgene content for 2 min. (B) The emission intensity ratio of I570/I495 as a function of phosgene content, Insert Linear relationships of emission ratio I570/ I495 for probe TPE-CI versus concentrations of phosgene. Excitation wavelength: 470 nm. (C) Photos of color (a) and fluorescence response (b) of TPE-CI loaded test strips upon exposure to an increasing range of phosgene vapor for 2 min at 25 � C. Under a hand-held ultraviolet lamp (365 nm). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

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Fig. 6. (A) Fluorescence spectrum and (B) emission intensity ratios (I570/I495) of TPECI loaded test paper strip to phosgene (80 ppm) and equal amounts of other gaseous analytes for 2 min (1) blank(air), (2) phos­ gene, (3) POCl3, (4) CH3COCl, (5) C2Cl2O2, (6) SOCl2, (7) toluenesulfonyl chloride (TsCl), (8) diethyl chlorophosphate (DCP), (9) SO2Cl2, (10) HCl, (11) TEA, (12) NH3. (C) The color (top) and fluorescence response (bottom) of the test paper strips in the presence of above gaseous analytes. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

undergo twice carbamylation reactions in the presence of phosgene, 1H NMR and ESI-HRMS experiments were simultaneous implemented for the sensor TPE-CI and the product from the reaction between TPE-CI and phosgene. As shown in Fig. 7A, the proton resonance peak at 8.43 ppm and 12.59 ppm were attributed to the two NH protons of H4 and H5 in TPE-CI molecule, the chemical shifts at 7.49, 7.64 and 7.75 ppm were

for visual monitor low doses of phosgene gas. 3.5. Exploration of testing mechanism In order to further confirm our thoughts, the testing mechanism was investigated. Firstly, to ascertain whether the two active NH groups

Fig. 7. Partial 1H NMR spectrum of TPE-CI (A) in CDCl3 and its reaction product with phosgene (B) in CDCl3. 7

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attributed to the corresponding proton resonance signals of H1, H2, and H3 in the chromen-2-imine unit. The proton peaks of H4 and H5 dis­ appeared after TPE-CI reacted with phosgene, corresponding to the generation of a cyclic carbamate, the proton peaks of H1, H2 and H3 exhibited an obvious downfield-shift (Fig. 7B) to H1’ (7.82 ppm), H2’ (8.47 ppm) and H3’ (8.75 ppm) due to the deshielding effect caused by the cyclization. Furthermore, the generation of the hexatomic ring product was identified by ESI-HRMS. As shown in Fig. S10, two domi­ nant peaks were observed at 542.1853 (M þ Hþ) and 564.1677(M þ Naþ), which corresponded to the expected reaction product (calculated 541.1790) of probe TPE-CI and phosgene. Combined with the 1H NMR and HR-MS and the previous DFT calculation results, it is sufficient to confirm the sensing mechanism of the sensor TPE-CI for gaseous phos­ gene as proposed in Fig. S11.

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4. Conclusion In conclusion, by taking the advantage of AIE þ TICT characteristics, we have successfully constructed an AIE þ TICT activated excellent dual-mode ratiometric and colorimetric solid fluorescent chemosensor TPE-CI for visual sensing of phosgene in solvent and in the air atmo­ sphere. The experimental results showed that the TPE-CI loaded portable test strip exhibits excellent selectivity, rapid response time and very convenient to use. The LOD of TPE-CI for phosgene sensing was calculated no more than 0.36 μM in solvent and 0.27 ppm in the gas phase, which was far below the dangerous range that can cause lung injuries. Moreover, based on the unique advantage of AIE and TICT character, which can effectively avoid the drawbacks of the ACQ based fluorescent sensors. Overall, compared with other phosgene type sen­ sors, AIE þ TICT characteristics solid-state sensors are a more suitable candidate for portable on-site detection of phosgene. Declaration of competing interest There are no competing financial interests to declare. CRediT authorship contribution statement Qinghua Hu: Conceptualization, Methodology, Software, Formal analysis, Writing - review & editing. Qiuxiang Huang: Investigation, Formal analysis. Kexin Liang: Investigation, Formal analysis. Yuyuan Wang: Resources. Yu Mao: Software, Investigation, Formal analysis. Qiang Yin: Software, Formal analysis. Hongqing Wang: Methodology, Supervision, Validation, Writing - review & editing. Acknowledgments Our work was financially supported by NSFC (11804146 and 11875161); NSF of Hunan Province (2016JJ5003); College Students’ Innovation & Entrepreneurship Training Project of University of South China (No. 2018XJXZ332); the Scientific Research Fund of Hunan Ed­ ucation Department (Project No. 17B223). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.dyepig.2020.108229. References [1] Borak J, Diller WF. Phosgene exposure: mechanisms of injury and treatment strategies. J Occup Environ Med 2001;43:110–9. [2] Staub NC. Pulmonary edema. Physiol Rev 1974;54:678–811. [3] Holmes WW, Keyser BM, Paradiso DC, Ray R, Andres DK, Benton BJ, Rothwell CC, Hoard-Fruchey HM, Dillman JF, Sciuto AM, Anderson DR. Conceptual approaches for treatment of phosgene inhalation-induced lung injury. Toxicol Lett 2016;244: 8–20.

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