English

• Supramolecular gels as a kind of soft materials with three-dimensional network structure, which can be achieved by means of the noncovalent interactions such as hydrogen bonds, van der Waals forces, π-π stacking, dipole–dipole, ion−ion, charge-transfer, coordination interactions [1-10]. Supramolecular gels are reversible and sensitive to environmental changes including pH, temperature, ionic strength and other changes, and further brings significant alteration about their color, emission, gel state and self-assembly structures [11-16]. Supramolecular gels have been wide application in many fields such as pollutant removal, drug delivery, biosensors, catalysis, optoelectronics, enzyme immobilization, solar cells [17-28]. For example, the three-dimensional network in gel and exerogel sates facilitates the penetration of analytes, and large contact area will produce quick response [13].

  Oxalyl chloride ((COCl)₂) and phosgene are highly toxic gases even at low concentration although they are widely used in industrial chemical processes [29-32]. Exposure to oxalyl chloride and phosgene will irritate individual respiratory system and skin, cause severe damage to the lungs of humans, and even induce death [33-35]. Therefore, it is of great significance to develop reliable, sensitive, prompt and convenient methods for the detection of (COCl)₂ and phosgene. Some methods and materials for the detection of (COCl)₂ and phosgene have been reported in recent years [36-40]. However, these methods have numerous disadvantages such as long analysis time, complicated sample processing, poor portability and high cost. In comparison, fluorescent detection methods have attracted more attention because of their great convenience, operational simplicity, low cost and high sensitivity [41,42]. Li and Wang have designed a dual sensor based on a fluorescent benzothiazole dye for (COCl)₂ and phosgene with the high sensitivity and selectivity [43]. (COCl)₂ and phosgene could be detected by some sensors reported by us [44,45], but they were not be obviously distinguished. Therefore, it is desiderated to develop new probes with highly sensitive and selective detection ability.

  In this work, a novel fluorescent gelator (NN-3) containing two naphthalimide groups were designed and synthesized, which was used as a self-assemble gel sensor for simultaneously selective detecting of oxalyl chloride and phosgene (Scheme 1). In molecule NN-3, the naphthalimide group with a terminal phenol at 4-position was as energy donor, and another naphthalimide group with an amidogen at 3-position was used as energy acceptor, and efficient energy transfer could be occurred between them. As a control molecule, a similar gelator molecule NN-4 with an amidogen at 4-position of naphthalimide was produced, which only owned a little difference about the position of amidogen. Gelator NN-3 exhibited better detection performance towards oxaloyl choride and phosgene than that of NN-4, and oxaloyl choride and phosgene could be differentiated via their different response time.

  Scheme 1

  

  Scheme 1.  Molecule structures of gelators NN-3 and NN-4.

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  4-Amino-1,8-naphthalic anhydride, 3-amino-1,8-naphthalic anhydride, CH₃COCl, DCP, DECP, DMMP, POCl₃, tosyl chloride, oxalyl chloride, triphosgene and SOCl₂ were all purchased from shanghai Titan technology Co., Ltd. Other reagents were purchased from shanghai Darui finechemical Co., Ltd. All other reagents were analytical grade. To visually determine phosgene and its gas with this system, a predetermined amount of triphosgene/TEA (100 µmol/L) chloroform solution was placed in a centrifuge tube at room temperature for 5 min.

  The gel abilities of NN-3 and NN-4 were investigated by the inverse flow method [44]. Films NN-3 and NN-4 were obtained by evaporation of solvent naturally. The detailed instrumentation information was listed in Supporting information.

  Gelators NN-3 and NN-4 were synthesized according to our previous reported method (Scheme S1 in Supporting information) [46]. Solutions NN-3 exhibited two absorption bands covering the range of 342-347 nm and 423-437 nm, which were assigned to π-π* transitions of the naphthalimide group with a terminal phenol and naphthalimide group with an amino, respectively (Fig. S1a in Supporting information) [44]. Solutions NN-4 also exhibited two absorption bands in the scopes of 360-367 nm and 405-436 nm in Fig. S1b (Supporting information). Solutions NN-3 showed two emission peaks covering the range of 424-429 nm and 460-568 nm, which were assigned to the two naphthalimide groups (Fig. S1c in Supporting information). There was a specific different emission behavior for solution NN-3 in ethyl acetate and toluene, which showed solvatochromism behavior. Solutions NN-4 had a single emission peak located in the scope of 489-542 nm (Fig. S1d in Supporting information). Emission light of solutions NN-3 and NN-4 was from blue to green and yellow light with different intensity (Fig. S2 in Supporting information). Moreover, fluorescence quantum yield and lifetime of NN-3 and NN-4 were also investigated in different solvents (Table S2 in Supporting information). The absolute quantum yields of solutions NN-3 and NN-4 were in the range of 5.01%-32.87% and 12.13%-48.9% with the lifetime scope of 2.99-9.60 ns and 2.66-7.06 ns.

  To verify the intramolecular energy transfer (ET), UV-vis absorption and fluorescence emission spectra of precursor compounds 1-3 in CH₂Cl₂ were done (Fig. 1). Compounds 1 and 2 had an absorption band at 361 nm and 405 nm, respectively. There were two absorption bands at 338 and 408 nm for solution 3. Under the excitation of 361 nm, solution 1 emitted blue light at 428 nm, which was exactly overlapped by absorption band of compounds 2 and 3. It was concluded that ET from the naphthalimide group with a terminal phenol to the naphthalimide group with an amino was happened in molecules NN-3 and NN-4 with different extents [47].

  Figure 1

  

  Figure 1.  UV-vis absorption spectra and fluorescence spectra of solutions 1-3 in CH₂Cl₂ with the concentration of 10⁵ mol/L. The excitation wavelength for compounds 1, 2 and 3 are 361, 405 and 408 nm, respectively.

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  NN-3 could form gels only in DMSO/H₂O (v/v, 10/1) and DMF/H₂O (v/v, 5/1) with the critical gel concentration (CGC) of 5.68, 6.94 mg/mL (Table S2). Solutions and precipitates of NN-3 were obtained in other solvents. NN-4 could form gels in ethanol/H₂O (v/v, 10/1), acetonitrile/H₂O (v/v, 10/1), DMSO/H₂O (v/v, 10/1) and DMF/H₂O (v/v, 5/1) with the CGC of 9.09, 11.4, 5.68 and 6.94 mg/mL, respectively. Yellow gels NN-3 and NN-4 formed in these solvents emitted yellow-green light under 365 nm light (Fig. 2). NN-3 self-assembled into nanofibers in its gels (Figs. 3a and b). The similar nanofiber structures were observed in gels NN-4 from DMSO/H₂O (v/v, 10/1) and ethanol/H₂O (v/v, 10/1) (Figs. 3d and e). The bulky nanofibers were obtained in gels NN-4 from DMF/H₂O (v/v, 5/1) and acetonitrile/H₂O (v/v, 10/1) (Figs. 3c and f).

  Figure 2

  

  Figure 2.  Images of gels NN-3 and NN-4 in different solvent. (a, aˊ) NN-3 for DMF/H₂O (v/v, 5/1); (b, bˊ) NN-3 for DMSO/H₂O (v/v, 10/1); (c, cˊ) NN-4 for DMF/H₂O (v/v, 5/1); (d, dˊ) NN-4 for DMSO/H₂O (v/v, 10/1); (e, eˊ) NN-4 for ethanol/H₂O (v/v, 10/1); (f, fˊ) NN-4 for acetonitrile/H₂O (v/v, 10/1). The upper and lower were under daylight and 365 nm light, respectively.

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  Figure 3

  

  Figure 3.  SEM images of gels NN-3 and NN-4 in different solvents (a) NN-3 in DMF/H₂O (v/v, 5/1); (b) NN-3 in DMSO/H₂O (v/v, 10/1); (c) NN-4 in DMF/H₂O (v/v, 5/1); (d) NN-4 in DMSO/H₂O (v/v, 10/1); (e) NN-4 in ethanol/H₂O (v/v, 10/1); (f) NN-4 in acetonitrile/H₂O (v/v, 10/1). The scale bars of (a-f) are 1, 1, 1, 1, 1 and 2 µm.

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  To explore the self-assembly modes of NN-3 and NN-4 in gel, UV-vis absorption spectra of NN-3 and NN-4 in solution and gel were compared. Two absorption bands at 347 and 434 nm of solution NN-3 in DMF/H₂O (v/v, 5/1) were turned into 347, 370 and 453 nm for gel NN-3, which indicated the J-aggregate mode was existed in gel (Fig. S3a in Supporting information) [48]. Two absorption bands at 347 and 440 nm of gel NN-3 formed in DMSO/H₂O (v/v, 10/1) did not have any shift (Fig. S3b in Supporting information). In Fig. S3c (Supporting information), the absorption band at 431 nm of solution NN-4 was red-shifted to 437 nm in gel NN-4 in DMF/H₂O (v/v, 5/1). UV-vis absorption band at 350 nm of solution NN-4 was red-shifted to 365 nm for gel NN-4 in DMSO/H₂O (v/v, 10/1) (Fig. S3d in Supporting information). The absorption band of gels NN-4 from acetonitrile/H₂O (v/v, 10/1) and ethanol/H₂O (v/v, 10/1) was not shifted (Figs. S3e and f in Supporting information). These results showed that NN-3 and NN-4 had the different self-assembly modes in different solvents. Emission peaks at 433 and 536 nm of solution NN-3 in DMF/H₂O (v/v, 5/1) were red-shifted to 450 and 559 nm in gel state, and indicating π-π stacking in gel (Fig. S4a in Supporting information). In addition, the emission intensity at 450 nm was further decreased in gel state due to the enhanced energy transfer. The similar fluorescence change from solution to gel state was observed in Figs. S4b-f (Supporting information). Comparing with our previous reported naphthalimide-based gel system [49], the small red-shift from solution to gel indicated that amino in naphthalimide went against π-π stacking between aromatic structures to some extent [50].

  In FTIR spectra of xerogels NN-3, N-H stretching peaks of 3365, 3357 cm^–1 and the stretching peak of C=O at 1647 and 1648 cm^–1 indicated that hydrogen bonding played a significant role in the formation of gel (Fig. S5a in Supporting information). The similar N-H and C=O stretching peaks were observed in FTIR spectra of xerogels NN-4 (Fig. S5b in Supporting information). As shown in Figs. S6a, b, e and f (Supporting information), XRD patterns of xerogels NN-3 and NN-4 from DMF/H₂O (v/v, 5/1) exhibited a series of diffraction peaks, which demonstrated that self-assembly of NN-3 and NN-4 was highly ordered. Rare diffraction peaks in that of xerogels NN-3 and NN-4 from other solvents indicated that self-assemblies of NN-3 and NN-4 in these gels were in the long range disorder (Figs. S6c, d and g-l in Supporting information). The diffraction peaks with d-space values of 0.40, 0.41, 0.39, 0.42, 0.38 and 0.38 nm should be π-π stacking between naphthalimide group [51].

  (COCl) ₂, DCP, DECP, DMMP, POCl₃, phosgene, SOCl₂, TsCl and CH₃COCl were selected to investigate the selectivity of NN-3 and NN-4 towards acyl chloride (Fig. 4). Upon addition of 1.0 equiv. of (COCl)₂ and phosgene, the emission peak at 476 nm of solution NN-3 was blue-shifted to 426 and 437 nm with intensity enhancement, respectively (Fig. 4a). For other acyl chlorides, a weak fluctuation was observed. For solution NN-4, 5.0 equiv. of (COCl)₂ and phosgene were added, fluorescence emission was also blue-shifted from 494 nm to 437 and 432 nm along with the emission intensity significant quenching and enhancing, respectively (Fig. 4b). Upon the addition of (COCl)₂ and phosgene to solutions NN-3 and NN-4, absorption band in the visible region were obviously decreased (Figs. S7a and b in Supporting information). The photographs of solutions NN-3 and NN-4 well validated the fluorescence and absorption change (Fig. S7c in Supporting information).

  Figure 4

  

  Figure 4.  Fluorescence emission spectra change of solutions (a) NN-3 and (b) NN-4 in CH₂Cl₂ upon the addition of different acyl chloride (1.0 equiv. and 50.0 equiv. for NN-3 and NN-4). The concentrations of solutions NN-3 and NN-4 are 10 µmol/L.

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  In order to further study the detection performance of NN-3 and NN-4, the titration experiments of solutions NN-3 and NN-4 by phosgene and (COCl)₂ were done by fluorescence and UV-vis absorption spectra. With the addition of (COCl)₂, the fluorescence emission at 476 nm of solution NN-3 was weakly enhanced along with an appearance of new emission peak at 426 nm, which showed intramolecular ET was restrained (Fig. 5a). A good linear relationship between the ratio of emission intensity at 476 and 426 nm (F₄₂₆/F₄₇₆) was constructed with the LOD of 210 nmol/L on the base of LOD = 3σ/b (Fig. 5b) [52]. With the addition of (COCl)₂ to solution NN-4, fluorescence emission at 493 nm was gradually lessened (Fig. 5c). The linear equation was constructed in the range of 40-160 µmol/L with the LOD of 12.4 µmol/L (Fig. 5d). When solution NN-3 was titrated by phosgene, the emission intensity at 476 nm was slightly fluctuated, and the new emission peak was emerged at 429 nm with heightening by 3 times (Fig. 5e). A linear relationship was founded in scope of 0-10 µmol/L with the LOD of 90 nmol/L (Fig. 5f). In the presence of phosgene, the emission intensity at 431 nm of solution NN-4 was gradually enhanced accompanying with no change about 495 nm (Fig. 5g). The liner relationship between the ratio of emission at 432 and 496 nm and the concentration of phosgene was generated with the LOD of 64 µmol/L (Fig. 5h). Emission light of solutions NN-3 and NN-4 was changed from green to blue (the inset images in Fig. 5). In the titration process, the isobestic points at 386, 401, 375 and 377 nm in UV-vis absorption spectra indicated that new compounds were produced (Fig. S8 in Supporting information). Fluorescence kinetics of solutions NN-3 and NN-4 towards (COCl)₂ and phosgene exhibited the different response times (Fig. S9 in Supporting information). With the addition of (COCl)₂ and phosgene, solution NN-3 needed 50 s and 150 min for reaching to its destination and half of the maximum, respectively (Figs. S9a and b). For solution NN-4, it needed 3 min and 250 min to arrive at its half of the minimum and maximum, respectively (Figs. S9c and d). (COCl)₂ and phosgene could be differentiated by NN-3 and NN-4 via the response speed. In addition, the selectivity of NN-3 and NN-4 was investigated in THF (Fig. S10 in Supporting information). The experimental results showed that solution NN-3 in THF responded to SOCl₂ and phosgene, but solution NN-4 in THF did not obviously respond any kind of analytes. The different selectivity of NN-3 and NN-4 in CH₂Cl₂ and THF was possible due to the solvent effect on nucleophilic reaction.

  Figure 5

  

  Figure 5.  (a, c, e, g) Fluorescence spectra of solutions NN-3 and NN-4 (10 µmol/L) under the titration of (COCl)₂ and phosgene. (b, d, f, h) Linear relationship of the fluorescence emission intensity at respective versus the concentration of phosgene and (COCl)₂; The inset figures were images of solutions NN-3 and NN-4 before and after titration of phosgene and (COCl)₂ under 365 nm light lamp, µM is µmol/L.

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  In terms of three-dimensional network with large contacting area in gel systems, it endowed the convenience for analytes into the interior [53,54]. Solutions NN-3 and NN-4 in CH₂Cl₂ were coated onto quartz plate and prepared into film structure via the fast volatilization of CH₂Cl₂. Nanofibers structure and irregular nanospheres was observed in films NN-3 and NN-4, respectively (Fig. S11 in Supporting information). The selectivities of films NN-3 and NN-4 towards different chlorides were firstly investigated one by one (Fig. S12 in Supporting information). As shown in Fig. S12, films NN-3 and NN-4 exhibited the selectivity only towards (COCl)₂. When film NN-3 was contacted with (COCl)₂ gases with the concentration of 4.7 ppm, emission intensity at 530 nm was quenched by about 50% (Fig. 6a). With the concentration of (COCl)₂ gas increasing, a wide emission peak at 488 nm was appeared along with its intensity enhancement. LOD of film NN-3 towards (COCl)₂ gases was calculated as 2.0 ppm (Fig. 6b). Emission of film NN-3 was changed into blue light from yellow light (inset in Fig. 6a). Emission of film NN-4 at 573 nm was weakened by about 86.7% in the titration process (Fig. 6c). The emission change of film NN-4 was well proved by the inserted images in Fig. 6c. LOD of film NN-4 towards (COCl)₂ gases was calculated as 8.34 ppm according to the linear equation (Fig. 6d). The fluorescence kinetics experiments of films NN-3 and NN-4 towards (COCl)₂ gas exhibited the response time of 1 min and 5 min, respectively (Fig. S13 in Supporting information). Unfortunately, films NN-3 and NN-4 did not work as sensor for phosgene.

  Figure 6

  

  Figure 6.  (a, c) Fluorescence spectra of films NN-3 and NN-4 under the titration of (COCl)₂ gas. (b, d) The curved of the fluorescence emission intensity at 530 nm and 573 nm versus of films NN-3 and NN-4 with the concentration of (COCl)₂ respectively.

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  To reveal the detection mechanism of NN-3 and NN-4 toward phosgene and (COCl)₂, ¹H NMR and HRMS experiments were done. After reacting with phosgene and (COCl)₂, the chemical shift of NH₂- in molecule NN-3 was shifted from 5.92 ppm to 5.93 ppm and 5.85 ppm, respectively, then the integration of amine group was reduced from 1.71 to 1.0 and 0.75 (Fig. S14 in Supporting information). HRMS data of NN-3 with 691.25 was turned into 713.25 and 741.25 in Fig. S16 (Supporting information). The reaction products from NN-3 reaction with phosgene and (COCl)₂ were speculated in Figs. S14 and S16. The reaction of NN-3 and phosgene and (COCl)₂ produced an amide group along with another chlorine atom changing into hydroxyl group. The similar reaction was also occurred on NN-4. After NN-4 reacted with phosgene, the NMR peak at 7.33 ppm of NH₂– of molecule NN-4 was greatly minished and the wide NMR peak at 10.43 should be assigned to the carboxy group (Fig. S15 in Supporting information). NMR peak of –NH₂ of molecule NN-4 was significantly decreased, but the wide NMR peak belonged to the carboxy group was not appeared for the reaction of NN-4 and (COCl)₂. HRMS change about the reaction process of NN-4 with phosgene and (COCl)₂ was also observed in Fig. S17 (Supporting information), and the corresponding molecule structures of NN-4 was provide in Figs. S15 and S17. The simplified precursor compounds 1-3 and the corresponding products from compounds 2-3 reaction with (COCl)₂ and phosgene were subjected to theoretical calculation for further understanding the emission change of NN-3 and NN-4 (Fig. S18 in Supporting information). The energy gaps between HOMO and LUMO of the simplified compounds 1-3 was 3.9 eV, 3.56 eV and 3.66 eV, respectively. After compound 2 reacted with (COCl)₂ and phosgene, the energy gaps of its corresponding product were 3.82 eV and 3.83 eV, which indicated UV-vis absorption of the product was blue-shifted to low wavelength, and further decreased the intramolecular energy transfer efficiency. The semblable theoretical calculation results were also occurred on compound 3 in Fig. S18. Theoretical calculation results explained the photophysical properties change in the detection process.

  In conclusion, we designed and developed two gelators NN-3 and NN-4 based on naphthalimide derivatives with the gelation capability in some solvents. Difference about the position of the amino group in NN-3 and NN-4 could lead to their differences in gelation ability, photophysical properties, and self-assembly structures. Interestingly, NN-3 and NN-4 both exhibited high selectivity for (COCl)₂ and phosgene, and they could be distinguished by the response time. NN-3 and NN-4 exhibited the shorter response time towards (COCl)₂ than phosgene. While NN-3 has a lower LODs towards (COCl)₂ and phosgene than these of NN-4 due to the high reactivity of amino group at 3- position. (COCl)₂ and phosgene was detected in solution and gases states by NN-3 and NN-4. LODs of solutions NN-3 and NN-4 towards (COCl)₂ and phosgene were 0.21 µmol/L, 90 nmol/L, 12.4 µmol/L and 64 µmol/L, respectively. LODs of films NN-3 and NN-4 towards (COCl)₂ by were 2.0 ppm and 8.34 ppm. Based on these data, we expect that probes NN-4 and NN-3 to be an effective tool for detecting (COCl)₂ and phosgene. Overall, this work may offer a facile, visual and sensitive method for rapid detection of (COCl)₂ and phosgene in solution and gas phase.

  Declaration of competing interest

  The authors declare that there are no conflicts of interest.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (No. U1704164), the Basic Research Project of Henan Provincial Key Scientific Research Project (No. 22ZX002).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2022.06.067.

  
  
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  Scheme 1  Molecule structures of gelators NN-3 and NN-4.

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  Figure 1  UV-vis absorption spectra and fluorescence spectra of solutions 1-3 in CH₂Cl₂ with the concentration of 10⁵ mol/L. The excitation wavelength for compounds 1, 2 and 3 are 361, 405 and 408 nm, respectively.

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  Figure 2  Images of gels NN-3 and NN-4 in different solvent. (a, aˊ) NN-3 for DMF/H₂O (v/v, 5/1); (b, bˊ) NN-3 for DMSO/H₂O (v/v, 10/1); (c, cˊ) NN-4 for DMF/H₂O (v/v, 5/1); (d, dˊ) NN-4 for DMSO/H₂O (v/v, 10/1); (e, eˊ) NN-4 for ethanol/H₂O (v/v, 10/1); (f, fˊ) NN-4 for acetonitrile/H₂O (v/v, 10/1). The upper and lower were under daylight and 365 nm light, respectively.

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  Figure 3  SEM images of gels NN-3 and NN-4 in different solvents (a) NN-3 in DMF/H₂O (v/v, 5/1); (b) NN-3 in DMSO/H₂O (v/v, 10/1); (c) NN-4 in DMF/H₂O (v/v, 5/1); (d) NN-4 in DMSO/H₂O (v/v, 10/1); (e) NN-4 in ethanol/H₂O (v/v, 10/1); (f) NN-4 in acetonitrile/H₂O (v/v, 10/1). The scale bars of (a-f) are 1, 1, 1, 1, 1 and 2 µm.

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  Figure 4  Fluorescence emission spectra change of solutions (a) NN-3 and (b) NN-4 in CH₂Cl₂ upon the addition of different acyl chloride (1.0 equiv. and 50.0 equiv. for NN-3 and NN-4). The concentrations of solutions NN-3 and NN-4 are 10 µmol/L.

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  Figure 5  (a, c, e, g) Fluorescence spectra of solutions NN-3 and NN-4 (10 µmol/L) under the titration of (COCl)₂ and phosgene. (b, d, f, h) Linear relationship of the fluorescence emission intensity at respective versus the concentration of phosgene and (COCl)₂; The inset figures were images of solutions NN-3 and NN-4 before and after titration of phosgene and (COCl)₂ under 365 nm light lamp, µM is µmol/L.

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  Figure 6  (a, c) Fluorescence spectra of films NN-3 and NN-4 under the titration of (COCl)₂ gas. (b, d) The curved of the fluorescence emission intensity at 530 nm and 573 nm versus of films NN-3 and NN-4 with the concentration of (COCl)₂ respectively.

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