A fluorescent “turn on” chemosensor based on Bodipy–anthraquinone for Al(III) ions: synthesis and complexation/spectroscopic studies

Abstract

A novel triazole-linked Bodipy–anthraquinone compound (Bodipy-A) was easily prepared through one step click chemistry. Bodipy-A displayed a remarkable increase in fluorescence intensity in the presence of trace amounts of Al(III). These changes could be attributed to intramolecular charge transfer (ICT), and Bodipy-A also showed high selectivity over a series of other metal ions in methanol/water (9/1). The results obtained from an absorption spectroscopy titration (Job’s plot) suggested a 2 : 1 ligand-to-metal complex, which was further demonstrated by FT-IR spectroscopy. The binding constant K of the target complex was then calculated using the Benesi–Hildebrand equation. All fluorescence experiments have shown that the compound Bodipy-A has a good selectivity for Al(III) ions. The results show that this system could be used in semi-aqueous systems.

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1. Introduction

Aluminum metal is generally found as an ion in the Al(III) form in a lot of biological tissues and waste/natural water. An excess of aluminum is harmful for plants/humans and can cause serious problems such as Alzheimer’s or Parkinson’s disease, nervous system disorders etc.^1–3 Thus, the detection and removal of Al(III) came into prominence in most biological assays and environmental processes. The fluorometric method is one of the most effective in the detection of aluminum, just as for other metal ions. The method exhibits a tangible result in the fluorescence spectrum depending on the intensity or wavelength associated with the conversion to complex form. This is the advantage of the fluorometric method dispatched to chemists for the synthesis of selective/sensitive fluorescent chemosensors.^4–14 Among the fluorescent sensors, Bodipy dyes have been given great attention owing to their excellent properties, such as easy solubility, sharp absorption profile, high quantum yield, large extinction coefficient etc.^15–20 Bodipy dyes have been used for the detection of a lot of s, p, d and f block metal ions, in which the sensitivity was explained with an energy transfer mechanism.^21–23 In comparison with d-block ions, chemists have reported fewer fluorescent chemosensors for Al(III). Despite the poor complexation ability of aluminum, researchers who study sensors are still investigating selective/sensitive fluorescent chemosensors for Al(III) ions.^24–33

Anthraquinone derivatives derived from hydroxyl terminals have been widely used in chemistry applications as good chelating agents, which convert to complex forms with metal ions.^34–36 Especially, the anthraquinones prepared using click chemistry are useful due to a rich nitrogen–oxygen coordination environment and are often used for the fluorometric detection of metal ions.^27,37,38

The combination of Bodipy and anthraquinone as a fluorescent chemosensor for the detection of metal ions was not found in the sensor literature. In the literature, the Bodipy unit has been preferred due to its high fluorescence character and molar absorption coefficient. Herein, we present Bodipy-A as a novel fluorescent chemosensor which could give a highly selective/sensitive response to Al(III) ions in MeOH/H₂O (9/1) solution.

2. Experimental section

2.1. Instruments and materials

2,4-Dimethyl-3-ethylpyrrole, propargyl bromide, 1,3-dibromopropane, sodium azide, 4-hydroxybenzaldehyde, borontrifluoride diethyl etherate, triethylamine, trifluoroacetic acid, 1,8-dihydroxyanthraquinone and potassium carbonate were purchased from Sigma-Aldrich Chemicals Pvt. Ltd., USA and were of high purity. Solvents (AR Grade) were obtained from Fischer Chemicals Pvt. Ltd., India, in high purity and were used without any purification. Dry dichloromethane was obtained by using CaH₂ according to standard methods. The metal ions in aqueous solution were prepared from their nitrate salts. Column chromatography was performed on silica gel (32–70 mesh). The NMR spectra (¹H- and ¹³C-NMR) were recorded on a Varian 400 MHz instrument in CDCl₃. The chemical shifts were referenced to internal TMS. The absorption studies were carried out on a Perkin Elmer Lambda 25 UV-vis spectrophotometer using quartz cells of 1.0 cm path length. The fluorescence spectra were recorded using a Perkin Elmer LS 55 fluorescence spectrometer upon excitation at 470 nm wavelength. The percentages of carbon, hydrogen and nitrogen in all compounds were determined using a TruSpec elemental analyzer. The mass measurements were recorded using Bruker Compass Data Analysis 4.0 (ESI-TOF-MS) (the mass spectrometers used were a microTOFQ and a maXis quadrupole time-of-flight mass spectrometer).

2.2. Synthesis of 4-(3-bromopropyloxy)benzaldehyde (1)

4-(3-Bromopropyloxy)benzaldehyde was prepared according to a known procedure^20–23,39 and purified by some purification techniques. 1,3-Dibromopropane (3.02 g, 30 mmol, 1.5 equiv.), 4-hydroxybenzaldehyde (2.44 g, 20 mmol) and K₂CO₃ (5.52 g, 40 mmol, 2.0 equiv.) were refluxed in acetonitrile (100 mL) for 24 h. Then, the mixture was cooled to r.t. and filtered through a funnel. The residue was purified by column chromatography (silica gel, ratio: 1 : 10, 1 : 9 and 1 : 8, ethyl acetate–hexanes) to afford 2.86 g (59%) of 4-(3-bromopropyloxy)benzaldehyde. ¹H-NMR [400 MHz, CDCl₃]: 9.90 (s, 1H, CHO), 7.82 (d, 2H, ArH), 7.01 (d, 2H, ArH), 4.19 (t, 2H, CH), 3.56 (t, 2H, CH), 2.33 (m, 2H, CH). ¹³C-NMR [100 MHz, CDCl₃]: 190.84, 163.60, 131.85, 130.05, 114.75, 65.67, 31.90, 28.42. Anal. calc. (%) for C₁₀H₁₁BrO₂: C, 49.41; H, 4.56; found: C, 49.57; H, 4.34.

2.3. Synthesis of 4-(3-azidopropyloxy)benzaldehyde (2)

4-(3-Azidopropyloxy)benzaldehyde was prepared according to a known procedure^20–23,39 and purified by some purification techniques. A mixture of 4-(3-bromopropyloxy)benzaldehyde (2.43 g, 10 mmol) and NaN₃ (0.845 g, 13 mmol) in 30 mL DMF was refluxed at 100 °C overnight. The mixture was poured onto ice and extracted with DCM. The pure compound was obtained by evaporation of the solvent, as a colorless oil (2.01 g, 99%). ¹H NMR [400 MHz, CDCl₃]: δ (ppm) 9.89 (s, 1H, CHO), 7.81 (d, 2H, ArH), 6.99 (d, 2H, ArH), 4.19 (t, 2H, CH), 3.53 (t, 2H, CH), 2.07 (m, 2H, CH). ¹³C NMR [100 MHz, CDCl₃]: δ (ppm) 190.84, 163.60, 131.85, 130.05, 114.75, 65.00, 48.12, 28.52. Anal. calc. (%) for C₁₀H₁₁N₃O₂: C, 58.53; H, 5.40; N, 20.48; found: C, 58.46; H, 5.69; N, 20.13.

2.4. Synthesis of 4,4-difluoro-8-(4-(3-azidopropoxy))phenyl-1,3,5,7-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (3)

4-(3-Azidopropyloxy)benzaldehyde (1.03 g, 5 mmol) and 3-ethyl-2,4-dimethylpyrrole (2 equiv.) were stirred in dry DCM (200 mL) and degassed with nitrogen for 20 min. After the addition of TFA (0.1 mL), the mixture was stirred at r.t. under nitrogen for 30 min. The DCM was removed under vacuum; the dark oily residue was purified using an eluent of DCM, giving a red-orange solution in the column. To a solution of the resulting oil in diethylether (30 mL), DDQ (470 mg, 2.00 mmol) in ether/methanol (6 mL : 4 mL) was added and the reaction was allowed to proceed at room temperature/dark ambient conditions under an inert atmosphere for 30 minutes. The claret-red residue was purified by column chromatography (by a difficult/tiring procedure). Then, a solution of the intermediate product in toluene (100 mL) was degassed in a three-necked flask for 30 minutes under a nitrogen atmosphere. 10 equivalents of triethylamine were added. The solution was heated to 70 °C for 30 minutes after 5 equiv. of boron trifluoride etherate was added in four portions. A white fume appeared and the reaction was stirred overnight. After evaporation, the black residue was re-dissolved in a minimal amount of dichloromethane and purified by column chromatography (DCM : petroleum ether 40–60%). A red-orange solid was obtained.²² Yield; 22% (0.527 g). ¹H-NMR [400 MHz, CDCl₃]: 7.26 (d, 2H, ArH), 7.07 (d, 2H, ArH), 4.13 (t, 2H, CH₂), 3.57 (t, 2H, CH₂), 2.52 (s, 6H, CH₃), 2.30 (q, 4H, CH₂), 2.12 (m, 2H, CH₂), 1.35 (s, 6H, CH₃), 0.98 (t, 6H, CH₃). ¹¹B-NMR [128.3 MHz, CDCl₃]: 0.00 (s, 1B); ¹⁹F NMR [376.83 MHz, CDCl₃]: −145.8224, −145.908, −145.9943, −146.0864 (dd, 2F); ¹³C-NMR [100 MHz CDCl₃]: 159.2, 152.4, 139.8, 133.1, 132.9, 130.0, 128.8, 115.7, 64.5, 48.1, 28.8, 17.0, 14.6, 12.5, 11.8. Anal. calc. (%) for C₂₆H₃₂BF₂N₅O; C, 65.14; H, 6.73; N, 14.61; found; C, 65.19; H, 6.88; N, 14.44.

2.5. Synthesis of 1,8-bis(prop-2-ynyloxy)anthracene-9,10-dione (4)

A mixture of 1,8-dihydroxyanthraquinone (3.16 g, 10 mmol), K₂CO₃ (3 equiv.) and 3.54 g (80% w/w solution in toluene) (30 mmol) of propargyl bromide in 50 mL of DMF was refluxed at 100 °C for 24 h. The contents were poured into ice-salt, the solvent was removed by filtration, and the residue was dried and washed with hexane. A yellow solid was obtained. Yield: 91%, 2.88 g. ¹H NMR [400 MHz, CDCl₃]: δ (ppm) 7.85 (t, 2H, ArH), 7.61 (d, 2H, ArH), 7.44 (d, 2H, ArH), 7.10 (d, 1H, ArH), 4.85 (s, 4H, CH₂), 2.49 (s, 2H, CH). ¹³C NMR [100 MHz, CDCl₃]: δ (ppm); 184.2, 182.4, 157.5, 135.3, 133.1, 125.4, 121.6, 120.9, 78.3, 76.2, 57.1. Anal. calc. (%) for C₂₀H₁₂O₄: C, 75.94; H, 3.82; found: C, 75.42; H, 3.99.

2.6. Synthesis of Bodipy-A

2.2 equiv. of 4,4-difluoro-8-(4-(3-azidopropoxy))phenyl-1,3,5,7-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (0.263 g, 0.55 mmol) was added to a solution of 1,8-bis(prop-2-ynyloxy)anthracene-9,10-dione (0.79 g, 0.25 mmol) in CHCl₃ : EtOH : H₂O (ratio; 10 : 1 : 1). Then, 0.3 equiv. of sodium ascorbate was added to this mixture and stirred for 15 min, followed by 0.15 equiv. of CuSO₄. The heterogeneous mixture was vigorously stirred for 72 h at r.t. (until TLC analysis indicated consumption of the starting material). After the completion of the reaction, the solvents were evaporated and the residue was extracted with ethylacetate–water (3 times). The concentrated organic phase was purified by column chromatography (ethylacetate as the eluent) (0.42 g, 66% yield) (Scheme 1).

Scheme 1 The synthesis of Bodipy-A.

¹H NMR [400 MHz, CDCl₃]: 8.05 (t, 2H, ArH), 7.92 (d, 2H, ArH), 7.59 (s, 2H, CH), 7.42 (d, 2H, ArH), 7.18 (d, 4H, ArH), 6.98 (d, 4H, ArH), 5.45 (s, 4H, CH₂), 4.45 (t, 4H, CH₂), 4.06 (t, 4H, CH₂), 2.56 (s, 12H, CH₃), 2.43 (q, 8H, CH₂), 2.19 (m, 4H, CH₂), 1.33 (s, 12H, CH₃), 1.05 (t, 12H, CH₃). ¹³C NMR [100 MHz, CDCl₃]: 183.3, 158.1, 151.3, 141.8, 140.6, 139.3, 137.1, 133.3, 130.9, 130.4, 128.2, 127.8, 123.0, 122.1, 119.6, 115.3, 110.4, 111.2, 73.9, 73.1, 65.9, 65.3, 49.1, 29.2, 27.9, 17.0, 14.3, 12.7, 10.6. Anal. calc. (%) for C₇₂H₇₆B₂F₄N₁₀O₆; C, 67.82; H, 6.01; N, 10.99. Found: C, 68.01; H, 6.23; N, 10.57. ESI-TOF-MS [+H⁺]; mz: 1275.6.

3. Results and discussion

Fig. 1 shows the absorption spectra of Bodipy-A and Bodipy-A in the presence of 20 equiv. of metal ions, available as nitrate salts, in MeOH–H₂O (v/v = 9 : 1). Bodipy-A showed four main absorption bands around 212, 290, 400 and 513 nm, and one shoulder at 480 nm in the absence of metal ions. The absorption band around 400 nm was assigned to π–π* transitions of aromatic groups in the anthraquinone core and the two Bodipy units.³⁷ While compound 3 gave a Bodipy band at 526 nm,²² the absorption transition at 513 nm was ascribed to the classic band of Bodipy that occurs as a long-wavelength band between 490–700 nm in other studies.^17,18 This shift to blue, at around 520 nm, confirmed the synthesis of Bodipy-A. The transition (shoulder) around 480 nm in the spectrum could be attributed to the electromagnetic interaction of a lot of chromophoric groups in the anthraquinone ring and the pyrrole and benzene units of Bodipy. Almost no changes in the absorption spectra for Bodipy-A were observed upon addition of 20 equiv. of Zn(II), Ga(III), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Ag(I), Cr(III), Hg(II), La(III), Er(III), Yb(III), Tb(III) or Eu(III) metal ions, however, obvious changes occurred after addition of 20 equiv. of Al(III). As Fig. 1 shows, the absorption bands of Bodipy-A broadened or shifted (to blue or red) around 290 and 400 nm upon the 20 equiv. of Al(III) addition. Thus, Bodipy-A could serve as a sensitive fluorescent chemosensor for Al(III) cations. The changes in the absorption curve of Bodipy-A could be assigned to a charge transfer transition occurring between the ligand and metal ion with the coordination of Al(III) by two triazole rings. This results show that this system could be used in a semi-aqueous system. The facts suggest that the presence of Al(III) influenced the charge-transfer from the electron-rich Bodipy’s, as the antenna groups, to the anthraquinone-triazole moieties.

Fig. 1 The absorption spectra of Bodipy-A (1 × 10⁻⁶ M) and Bodipy-A (1 × 10⁻⁶ M)–metal ion (20 × 10⁻⁶ M) mixtures in MeOH/H₂O (9/1).

The fluorescence sensing of Bodipy-A was investigated for the same metal cations [Zn(II), Ga(III), Mn(II), Al(III), Fe(II), Co(II), Ni(II), Cu(II), Ag(I), Cr(III), Hg(II), La(III), Er(III), Yb(III), Tb(III), Eu(III)]. The mixtures were excited at 470 nm and the emission slits were set at 5. As shown in Fig. 2, all tested metal ions caused minor changes in the emission band at 545 nm (characteristic Bodipy emission) except for Al(III). The addition of Al(III) gave a higher emission band centered at 545 nm. Moreover, a small shoulder band appeared at 510 nm, which could be attributed to the electronic interaction between the anthraquinone and Bodipy moieties. The Bodipy unit acted as an antenna group in photon-induced electron transfer (PET) mechanism. Bodipy-A can be used for the selective detection of Al(III) in MeOH–H₂O solution.

Fig. 2 The emission spectra of Bodipy-A (1 × 10⁻⁷ M) and the mixtures of Bodipy-A (1 × 10⁻⁷ M)–metal ions (20 × 10⁻⁷ M) (λ_exc: 470 nm, slit: 5) in MeOH/H₂O (9/1).

The excitation data were recorded at 545 nm emission and the excitation slits were set at 5. Fig. 1 in the shows the changes of the excitation intensity of Bodipy-A in the absence and presence of metal ions (λ_em = 545 nm). The other studied cations gave a similar excitation spectrum to Bodipy-A, while it gave a broader band with the addition of Al(III) in the excitation curve. The observation of the strong complexation of Bodipy-A with Al(III) can be ascribed to the electron donating substituents (triazole rings) playing an important role in the reorganization of Bodipy-A in response to Al(III). This remarkable broadening indicated an energy transfer efficiency because of broader spectral overlaps between the donor and acceptor when the aluminum ion is bound to the triazole ring nitrogens and anthraquinone oxygen.⁴⁰ The excitation spectrum of Bodipy-A in the presence of Al(III) presented a more effective energy transfer in the target complex compound.

Furthermore, competition ion studies were also performed for Bodipy-A in the presence of Al(III) at 2 μM mixed with 2 μM of the tested metal cations such as Zn(II), Ga(III), Mn(II), (III), Fe(II), Co(II), Ni(II), Cu(II), Ag(I), Cr(III), Hg(II), La(III), Er(III), Yb(III), Tb(III) or Eu(III) (Fig. 3). In Fig. 3, I₀ is the emission of Bodipy-A; I is the emission of Bodipy-A in the presence of the metal cation. In the presence of other metal cations, no important change in the fluorescence intensity was observed in the sensing of Al(III). Although the emission of the Bodipy-A/Al(III) mixture was slightly increased by the addition of Hg(II), the presence of other cations did not affect the selectivity of Bodipy-A for Al(III) ions. The complex may show the “hard and soft acids and bases” correlation.¹⁹ The aluminum ion belongs to the hard acids group, which has a strong affinity for electron-donor groups such as triazoles and carbonyls, and the formation of a large π conjugate system is useful for increasing the planar structure of the molecule. Moreover, it could be argued that the radius of aluminum is suitable for two triazole cages binding to anthraquinone. These competing ion results proved that Bodipy-A could be presented as a potential chemosensor for the detection of Al(III) ions with very high selectivity.

Fig. 3 The concentration of Bodipy-A was set at 1 × 10⁻⁷ M and the concentration of Al(III) and the other competing ions was 20 × 10⁻⁷ M (emission_max: 545 nm).

The complex stoichiometry of Bodipy-A/Al(III) was determined by the fluorescence intensity’s dependence on an increased concentration of metal ion. A Job’s plot of Bodipy-A and Al(III) indicated that the target complex exhibited a 2 : 1 ligand–metal ratio with a good linear relationship. The fluorescence enhancement of Bodipy-A on addition of Al(III) could be attributed to triazole–anthraquinone → Al(III) π-cation interactions. The complexation effect could be considered as the holding of Al(III) ions between the nitrogen atoms of the triazole rings and the oxygens of anthraquinone (Fig. 4).

Fig. 4 Job’s plot for Bodipy-A (1.0 × 10⁻⁷ M) and Al(III) complexation (on emission intensities) in a mixture of MeOH/H₂O (9 : 1, v/v).

As shown in Fig. 5, the binding constant K of the target complex was then calculated to be 9.44 × 10⁷ L² mol⁻² using the Benesi–Hildebrand equation:

where I, I_max are the fluorescence intensities of Bodipy-A in the presence of Al(III) ions with intermediate and infinite concentrations, respectively and and I₀ presents the fluorescence intensity of the Bodipy-A solution in the absence of Al(III) ions.

Fig. 5 Benesi–Hildebrand plot (at 545 nm) for the complexation of chemosensor Bodipy-A with Al(III).

A short response time is preferred for many fluorescent chemosensor compounds. Therefore, the effect of the reaction time on the binding process of Al(III) ions to Bodipy-A was investigated, as shown in Fig. 6. The change in the fluorescence intensity of Bodipy-A was specified depending on the response times of 1, 2, 3, 4, 5, 10, 15 and 20 minutes. Fig. 6 indicates that the enhancement effect of the fluorescence intensity almost stopped for the chemosensor after the first 5 min. Thus, a reaction time of 5 min may be used for this system and the data show that the target chemosensor has a reasonable response time.

Fig. 6 The fluorescence turn on profile of addition of Al(III) (20.0 equiv.) to Bodipy-A (1.0 × 10⁻⁷ M) in methanol–water (v/v, 9/1) from 1 to 20 min, em: 545 nm.

For practical purposes, a pH titration of Bodipy-A was conducted to investigate a suitable pH range for Al(III) ion sensing between pH 2.0 and 12.0. In the presence of Al(III) ions, the complex form was stable at this wide pH range. Upon addition of Al(III) ions, the emission intensity at 545 nm increased gradually when the pH value was between 2.0 and 7.0 and decreased at pH > 7.0, which shows a poorer stability of the Bodipy-A/Al(III) complex at higher pH values (Fig. 7). At higher pH (after 7.0), the fluorescence intensity of Bodipy-A strictly decreased. Thus, Bodipy-A can detect the Al(III) ion within a wide pH range (4–7).

Fig. 7 The variation in fluorescence intensity with the pH of Bodipy-A (1.0 × 10⁻⁷ M) in the presence of Al(III) (20 equiv.), em: 545 nm.

The detection limit (LOD) was investigated using a fluorescence titration. An increasing amount of Al(III) was added to Bodipy-A (1 μM). Representation of the fluorescence at the appropriate wavelength vs. the concentration of Al(III) allowed the limit of detection to be calculated.⁴⁰

where σ is the standard deviation of the blank solution and k is the slope of the calibration curve. The detection limit of Bodipy-A towards Al(III) was found to be (1.8 ± 0.2) × 10⁻⁸ M. The value is lower than or comparable to that of most previously reported highly sensitive sensors.⁴¹ Thus, the low detection limit makes Bodipy-A suitable for Al(III) detection in environmental or biological processes.

The FT-IR spectra of the synthesized compounds and target complex are also given in Fig. 8. Firstly, the structure of the compound 4 was confirmed by that the C=O stretching band of 1,8-dihydroxyanthraquinone at 1678 cm⁻¹ shifted⁴² to a higher area and appeared at 1668 cm⁻¹ in the infrared spectrum of compound 4. While the aromatic C–H stretching bands of 1,8-dihydroxyanthraquinone were around 2900 cm⁻¹; in addition to them, compound 4 exhibited two new bands around 3200 cm⁻¹, which can be attributed to the binding of alkyne fragments. Moreover, the C–O broad forked band at 1282–1273 cm⁻¹ was observed at 1240–1221 cm⁻¹, owing to the conversion of the alcohol unit to an etheric bond.

Fig. 8 The infrared spectra of compound 4, Bodipy-A and the target Bodipy-A–Al(III) complex.

The broad band at 1682 cm⁻¹ indicates both the shifted C=O band of compound 4 and the C=N stretching of the Bodipy core/triazole ring, in which multi-bands overlapped in the same area. The etheric bond (C–O–C) after the reaction between compound 3 and compound 4 broadened and appeared at 1250 and 1221 cm⁻¹. The bands around 2900 cm⁻¹ were ascribed to various C–H stretching vibrations (aliphatic or aromatic) of Bodipy-A. After the complexation between Bodipy-A and Al(III); the band at 1682 cm⁻¹ shifted to 1673 cm⁻¹. Moreover, other stretching vibrations (C=C, C–O–C, C–H etc.) slightly changed to a lower or higher area due to the complexation effect. The chelation of Bodipy-A with Al(III) could be described by the hard–soft acid–base principle.³⁸ Hence, Al(III) is known already as a hard acid and Bodipy-A can be claimed to be a soft base due to a lot of donor atoms (N and O).

4. Conclusion

In summary, a fluorescent chemosensor based on Bodipy–anthraquinone with two triazole rings was prepared and studied for its complexation and spectroscopic properties. The chelation effect of Bodipy-A toward Al(III) ions exhibited an increase in the fluorescence band at 545 nm, owing to CHEF and ICT effects. The complexation could be dependent on both the triazole-nitrogens and carbonyl-oxygen, with a metal–ligand ratio of 1 : 2. Bodipy-A can be used as a potential chemosensor for Al(III) ions. The low detection limit makes Bodipy-A also suitable for Al(III) detection in environmental or biological processes.

Acknowledgements

We thank Tübitak (114Z095) and Selcuk University (BAP) for financial support of this work. The authors express their appreciation to Prof. Ross W. Boyle for helpful discussions.

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