Facile electrospinning preparation and superior luminescence properties of BODIPY composite nanofibers

Abstract

In this work, two kinds of 4, 4-difluoro-4-bora-3a, 4a-diaza-s-indacene (BODIPY) composite nanofibers with excellent emitting properties are reported. These contain 5,5-difluoro-1,3,7,9-tetramethyl-10-phenyl-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-4-ium-5-uide (DBDP) and 10-(4-(diphenylamino)phenyl)-5,5-difluoro-1,3,7,9-tetramethyl-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-4-ium-5-uide (TBDP). DBDP and TBDP, which were obtained using a one-pot process, were respectively incorporated into the poly(methylmethacrylate) (PMMA) matrix with different concentrations and thus formed two series of BODIPY luminescent nanofibers via electrospinning technology. The composite nanofibers were further characterized by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA) and luminescence. The SEM images show that the composite nanofibers have a smooth and uniform surface with a diameter about 0.4 µm. FT-IR spectra analysis indicates that the BODIPY compounds were successfully doped into the PMMA matrices. The TGA results show that decomposition temperature of BODIPY nanofibers was increased to about 113℃ and 88℃ for TBDP and DBDP, respectively, compared with that of BODIPY compounds. Furthermore, the BODIPY nanofibers also exhibit excellent optical fluorescence properties compared with the BODIPY compounds in dichloromethane (BODIPY/DCM) or in a solid state.

Keywords

BODIPY composites optical properties electrospinning

In the past few decades, fluorescent dyes have attracted much attention not only in the fields of photochemistry and photophysics, but also for optoelectronic devices and dye-sensitized solar cell (DSSC).^1–5 Among these dyes, BODIPY dyes with good chemical stability, high fluorescence quantum yields and a large photoabsorption coefficient in the visible and red/near-IR (NIR) region were considered as novel fluorophores.^6,7 However, most BODIPY dyes exhibited weak fluorescence in solid state due to self-absorption, which is closely associated with their narrow Stokes shift, and thus limited their further application as optoelectronic devices.⁸ Simultaneously, they showed strong fluorescence in dilute solution. But the problems of liquid leakage, low-processing capability, weak mechanical strength and low thermal stability have restricted their possible applications in optoelectronic devices.⁹ In order to overcome these deficiencies, it would be very significant to develop BODIPY dyes with appropriate properties. This paper shows how BODIPY dyes were doped into polymer matrices. It is outstanding that PMMA, one of the most ideal candidates, proved to be excellent for the development of molecular materials, with thermal and chemical stability, flexibility, versatility and biocompatibility, which can also influence the optical properties of BODIPY dyes.^10,11

In addition, one-dimensional (1D) nanostructures are of interest in both scientific and industrial areas.^12–14 1D nanostructures can be prepared by many methods such as template-directed methods,¹⁵ vapor-phase methods,¹⁶ self-assembly¹⁷ and electrospinning,¹⁸ etc. Among these methods, electrospinning is a versatile, convenient and inexpensive strategy for producing 1D nanostructures from a wide range of materials, with tunable diameters and morphologies.^19–22 Hence, a new compound/polymer composite material formed with BODIPY, electrospun to form 1D nanofibers, will enhance the thermal stability, flexibility and luminescent properties of BODIPY.²³

In this paper, two BODIPY dyes were synthesized: 5,5-difluoro-1,3,7,9-tetramethyl-10-phenyl-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-4-ium-5-uide (DBDP) and 10-(4-(diphenylamino)phenyl)-5,5-difluoro-1,3,7,9-tetramethyl-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-4-ium-5-uide (TBDP). These two dyes, at different concentrations, were incorporated into the PMMA matrix, and generated two series of luminescent nanofibers, via electrospinning technology. The effects of the two series of composite nanofibers on the luminescence, morphology and thermal properties were investigated in detail by numerous methods. As expected, the BODIPY nanofibers showed higher thermal stability and stronger fluorescence in comparison with the precursor BODIPY.

Experimental

Materials

Poly(methyl methacrylate) (PMMA, Mw = 120,000) was obtained from Tianjin Chemical Reagent Factory Damao (China). N,N-dimethylformamide (DMF) was purchased from Tianjin Chemical Reagent Factory (China) and used as a solvent to prepare the electrospinning solution. 4 -(diphenylamino)benzaldehyde (98%), 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (98%) and 2,4-dimethylpyrrole (98%) were purchased from Aladdin (China). Benzoyl chloride, petroleum ether (PE) and dichloromethane (DCM) were purchased from Tianjin Chemical Reagent Factory (China).

BODIPY synthesis

DBDP and TBDP were prepared according to the literature.²⁴ The detailed synthesis procedures for the compounds are shown in Figure 1.

Figure 1.

Synthesis of the BODIPY compounds (DBDP and TBDP).

Synthesis of compound DBDP

2,4-dimethylpyrrole (1.40 g, 14.74 mmol) and benzoyl chloride (0.92 g, 6.58 mmol) were added to a 500 mL round-bottomed flask containing 300 mL nitrogen-degassed DCM. The mixture was then stirred overnight at room temperature. After complete consumption of the aldehyde (monitored via TLC), 8 mL dry triethylamine (TEA) was added to the mixture and after stirring for 30 min, BF₃▪OEt₂ (8 mL) was added dropwise at 0℃. The mixture was stirred continuously for 12 h and then the reaction mixture was washed with water and extracted with DCM. The organic phase was dried over Na₂SO₄. The solvent was evaporated and the residue was purified by column chromatography over silica gel (PE/DCM = 20:1 v/v) to obtain an orange solid 1.46 g (68%). ¹H NMR (400 MHz, DMSO) δ 7.57 = (d, J = 5.8 Hz, 3H), 7.40–7.35 (m, 2H), 6.18 (s, 2H), 2.45 (s, 6H), 1.34 (s, 6H). ¹³C NMR (100 MHz, CDCl₃) δ = 155.57 (s), 143.28 (s), 141.87 (s), 135.14 (s), 131.57 (s), 129.16 (d, J = 18.6), 128.09 (s), 121.34 (s), 77.48 (s), 77.16 (s), 76.84 (s, 18H), 14.70 (s), 14.44 (s). HRMS (ESI, m/z): [M + H]⁺ calculated for C₁₉H₁₉BF₂N₂, 325.1687; found, 325.1692. Analysis calculated for C₁₉H₁₉BF₂N₂: C 70.40; H 5.91; B 3.33; F 11.72; N 8.64; found: C 70.53; H 5.88; B 3.35; F 11.65; N 8.59.

Synthesis of compound TBDP

2,4-dimethylpyrrole (1.20 g, 12.61 mmol) and 4 -(diphenylamino)benzaldehyde (1.53 g, 5.61 mmol) were added to a 500 mL round-bottomed flask containing 300 mL nitrogen-degassed DCM. One drop of trifluoroacetic acid (TFA) was added while stirring at room temperature overnight. After complete consumption of the aldehyde (monitored via TLC), a solution of DDQ (1.27 g, 5.61 mmol) in dry toluene was added. The mixture was then stirred continuously for another 3 h. Then 8 mL dry TEA was added to the mixture and, after stirring for 30 min, BF₃▪OEt₂ (8 mL) was added dropwise at 0℃. The mixture was stirred continuously for 12 h and then the reaction mixture was washed with water several times and extracted with DCM. The organic phase was dried over Na₂SO₄. The solvent was evaporated and the residue was purified by column chromatography over silica gel (PE/DCM = 10:1 v/v) to obtain an orange solid 1.73 g (28%). ¹H NMR (400 MHz, DMSO) δ = 7.36 (t, J = 7.9, 4H), 7.23 (d, J = 8.5, 2H), 7.10 (m, 8H), 6.19 (s, 4H), 2.45 (s, 6H), 1.55 (s, 6H). ¹³C NMR (100 MHz, CDCl₃) δ = 155.36 (s), 148.66 (s), 147.44 (s), 129.59 (s), 128.98 (s), 128.28 (s), 124.84 (s), 123.50 (s), 121.28 (s), 14.74 (s). HRMS ESI, m/z: [M + H]⁺ calculated for C₃₁H₂₈BF₂N₃, 492.2422; found, 492.2437. Analysis calculated for C₃₁H₂₈BF₂N₃: C 75.77; H 5.74; B 2.20; F 7.73; N 8.55; found, C 75.69; H 5.76; B 2.15; F 7.79; N 8.60.

Preparation of luminescent nanofibers

The preparation procedures of luminescent nanofibers are shown in Figure 2. The electrospinning solutions were prepared by first dissolving PMMA (1 g) in 4 g DMF solution at a concentration of 20 wt% at room temperature (5 g electrospinning solution in each vial). The DBDP and TBDP were then added (DBDP and TBDP to PMMA equal to 7, 9, 11, 13 and 15 wt%, respectively) into the previous mixture solution vials and magnetically stirred for 24 h at 45℃. The uniform solutions were placed into 1 mL plastic syringes with a 90deg angle blunt stainless steel needle with an inner diameter of 0.37 mm. During electrospinning the polymer solution was extruded from the needle with a flow rate of 0.7 mL·h⁻¹, controlled by a digital syringe pump. The electrospinning setup with a DC high-voltage generator was purchased from Beijing Machinery & Electricity Institute.

Figure 2.

Schematic illustration for the fabrication process of BODIPY/PMMA electrospun composite nanofibers.

All of the electrospinning processes were carried out under ambient conditions (25℃ with relative humidity 30% ± 5%). During the preparation of DBDP/PMMA composite nanofibers (DBDP/PMMA) and TBDP/PMMA composite nanofibers (TBDP/PMMA), a positive high voltage of 13 kV was applied to the needle and the distance between the collector and the tip of the needle was 20 cm. The nanofibers were collected as randomly overlaid mats on an electrically grounded aluminum foil. After electrospinning the composite nanofiber mats were dried in a vacuum oven at room temperature for 12 h before characterization.

Measurement of properties

A Hitachi, Ltd. (Japan) S-4800 scanning electron microscope (SEM) from Japan was employed to obtain the SEM images of the electrospun nanofibers. Prior to SEM examination, the specimens from Germany were sputter-coated with gold to avoid charge accumulation. A Leica Microsystems (Germany) DM400 M fluorescence microscope with a Leica DFC 425 C camera were used to study the luminescence of the composite nanofibers. FT-IR spectral data were obtained on a PerkinElmer Inc. (American) Spectrum One spectrophotometer using KBr disks in the range of 4000–400 cm⁻¹. Thermal analyses were conducted on a Perkin-Elmer STA 6000 with a heat in rate of 10℃ min⁻¹ with a temperature range of 30℃ to 800℃ under an O₂ atmosphere. Excitation and emission spectra were measured with an Edinburgh Instruments Ltd. (England) FLS 920 fluorescence spectrophotometer. The data were analyzed by software supplied by Edinburgh Instruments. Both the slit widths for excitation and emission were set at 2.0 nm. The fluorescence dynamics of the samples were measured with an FLS 920 instrument (Edinburgh Instruemnts). During the measurements, an oscillograph was used to record the decay dynamics, and the 367 nm incident light generated from a microsecond flashlamp, which was used as the excitation source. The NMR spectra were recorded on a Bruker Corporation (Swiss) Avance (III) 400 MHz (¹H NMR) and 100 MHz (¹³C NMR) spectrometer in CDCl₃ and DMSO solutions.

Computational details

The DFT-B3LYP (density functional method)^25–27 using a 6-31 G(d) basis set were employed to optimize the geometry structures in the ground and excited states, respectively. On the basis of the optimized geometry structures in the ground and excited states, the absorption and emission properties can be calculated by time-dependent density functional theory (TDDFT).^28,29 All of the calculations were carried out using a suite of Gaussian 03 programs.³⁰

Results and discussion

Computational studies

As shown in Table 1, by comparison of the Highest Occupied Molecular (HOMO) and Lowest Unoccupied Molecular (LUMO) electronic cloud distributions in the ground, the HOMO and LUMO electronic cloud distributions in the singlet excited state of DBDP and TBDP are unsymmetrical. It is noted that the electronic cloud distribution of HOMO in the ground state of DBDP is localized at the BODIPY unit, while that of TBDP is localized at the triphenylamine group as electron donor, which suggests that more electron transformation has happened in the stronger donor–acceptor (D–A) conjugated structure of the molecule.

Table 1.

Spatial plots of the selected frontier molecular orbits of the ground (up) and excited (down) states of DBDP and TBDP

	The ground state of DBDP	The singlet excited state of DBDP	The ground state of TBDP	The singlet excited state of TBDP
LUMO+1
LUMO
HOMO
HOMO-1

DBDP: 5,5-difluoro-1,3,7,9-tetramethyl-10-phenyl-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-4-ium-5-uide; TBDP: 10 -(4-(diphenylamino)phenyl)-5,5-difluoro-1,3,7,9-tetramethyl-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]diazaborinin-4-ium-5-uide; HOMO: Highest Occupied Molecular Orbital; LUMO: Lowest Unoccupied Molecular Orbital.

The HOMO and LUMO levels for the ground and singlet excited states for DBDP were found to be −5.33 eV and −2.31 eV, and −6.83 eV and 0.84 eV, respectively. The HOMO and LUMO levels for the ground and singlet excited states of TBDP were found to be −5.22 eV and −2.29 eV, and −6.80 eV and 0.90 eV, respectively.

Effects of complex concentration on the luminescence of nanofibers

The luminescent properties of BODIPY/PMMA, BODIPY and BODIPY/DCM were recorded at room temperature. The emission spectra of BODIPY/PMMA excited at 495 nm are shown in Figure 3. The emission spectra for TBDP/PMMA (Figure 3(a)) at different concentrations show the emission intensity enhanced with increasing content of the TBDP and reached its maximum value at 11 wt%. Similarly, the emission intensity of DBDP/PMMA reached its maximum value at 13 wt% (Figure 3(b)). This was mainly because the nanoparticles of the BODIPY in the PMMA matrices start to aggregate slightly and the dyes predominantly existed as molecular clusters and/or nanoparticles, which led to fluorescence quenching when the content of the complex was high enough.³¹ During electrospinning the solutions contain uniformly dispersed dye molecules, and the rapid evaporation of the solvent concomitant with the fast solidification of the filaments (within tens of milliseconds) hindered the aggregation of the dyes.³²

Figure 3.

Emission spectra (λ_ex = 495 nm) of (a) TBDP and (b) DBDP in the composite nanofibers with contents of, 7, 9, 11, 13 and 15 wt%.

The emission spectra of BODIPY/PMMA excited at 495 nm, BODIPY excited at 451 nm and BODIPY/DCM excited at 366 nm are shown in Figure 4. The emission maximum peak at 577 nm in 11% (w/w) TBDP/PMMA (Figure 4(a)) rendered large redshifts (52 nm) in TBDP/DCM. The emission maximum peak at 535 nm in 13% (w/w) DBDP/PMMA rendered redshifts (18 nm) in DBDP/DCM (Figure 4(b)). As seen from emission spectra, BODIPY/PMMA shows larger red shifts than that of BODIPY/DCM. The redshifts were mainly due to the hydrogen bond that formed between the F atoms in BODIPY and PMMA. The hydrogen bonding affected the BODIPY/PMMA result at a different emission maximum peak. The redshifts of TBDP/PMMA are greater than those of DBDP/PMMA. Because the HOMO electronic cloud distribution in the ground state of TBDP is localized at the triphenylamine group, this led to the conjugation extension of the TBDP/PMMA.³³

Figure 4.

Emission spectra of (a) TBDP; and (b) DBDP for the state of composite nanofibers (λ_ex = 495 nm), solid state (λ_ex = 451 nm) and liquid state (λ_ex = 366 nm).

The excitation spectra for various samples of TBDP and DBDP are shown in Figure 5. The excitation maximum peak at 496 nm for TBDP/PMMA with concentrations of 11% (w/w) (Figure 5(a)) rendered large redshifts (132 nm) compared with TBDP/DCM. The excitation maximum peak at 494 nm for DBDP/PMMA with concentrations of 13% (w/w) (Figure 5(b)) rendered large redshifts (123 nm) compared with DBDP/DCM. As seen from excitation spectra, BODIPY/PMMA shows larger red shifts than that of BODIPY/DCM. The redshifts were mainly due to the hydrogen bond that formed between the F atoms in BODIPY and PMMA. The hydrogen bonding affected the BODIPY/PMMA result in different excitation maximum peaks. The excitation spectra for BODIPY exhibited a strong absorbance band peak at about 500 nm assigned to the S₀–S₁ transition.³⁴ A second and higher energy absorption maximum peak is shown broadly at 350–400 nm assigned to the S₀–S₂ transition.³⁵

Figure 5.

Excitation spectra of (a) TBDP and (b) DBDP in the state of composite nanofibers, solid state (λ_ex = 451 nm) and liquid state (λ_ex = 366 nm).

Furthermore, the fluorescence quantum efficiencies of TBDP/PMMA and DBDP/PMMA were higher than those of the pure solid powder and in dichloromethane solution (Tables 2 and 3). Both emission spectra and excitation spectra exhibited larger bathochromic shifts and higher fluorescence quantum efficiency of TBDP/PMMA compared with TBDP and TBDP/DCM. However, the luminescence effect of DBDP/PMMA was worse than that of TBDP/PMMA, which may be caused by the electron-withdrawing effect of BODIPY and its enhanced symmetry. In addition, the carbonyl of the PMMA may strengthen the electron-withdrawing effect of the triphenylamine group in TBDP, which formed a stronger D–A conjugated structure, and this interaction enhanced the charge transfer in the TBDP internal structure. The phenyl group of DBDP has a weak electron-withdrawing effect and a conjugated system. So the luminescence properties of TBDP/PMMA were higher than DBDP/PMMA and the whole spectrum moves to the red region (Tables 2 and 3). As expected, the new composite nanofibers exhibited excellent photophysical properties compared with the dye in a solid state and in a dichloromethane solution.

Table 2.

The fluorescence quantum efficiency (Φ) and the maximum emission peak (λ_em) of TBDP in solid state (λ_ex = 451 nm), liquid state (λ_ex = 366 nm) and in composite nanofibers (λ_ex = 495 nm)

TBDP	Solid powder	In DCM	Composite nanofibers with different concentrations of TBDP (wt%)
TBDP	Solid powder	In DCM	7	9	11	13	15
λ_em max (nm)	589	525	570	587	588	577	577
Φ (%)	11	15	17	26	30	27	16

DCM: dichloromethane.

Table 3.

The fluorescence quantum efficiency (Φ) and the maximum emission peak (λ_em) of DBDP in solid state (λ_ex = 451 nm), liquid state (λ_ex = 366 nm) and in composite nanofibers (λ_ex = 495 nm)

DBDP	Solid powder	In DCM	Composite nanofibers with different concentrations of DBDP (wt%)
DBDP	Solid powder	In DCM	7	9	11	13	15
λ_em max (nm)	590	511	531	539	534	529	531
Φ (%)	8	11	12	14	15	17	13

Morphology of composite nanofibers and dispersion of BODIPY

Figure 6(a) to (d) shows the SEM images and the fiber diameter distributions of the electrospun PMMA-based composites nanofibers with 11 wt% TBDP and 13 wt% DBDP, respectively. An optical microscope (Leica Microsystems (Germany), DM2500M) was used to measure the diameters of 100 nanofibers that were randomly chosen from the SEM photos, and their mean value and standard deviation were calculated. As can be seen from Figure 6, the nanofiber average diameter of BODIPY/PMMA composite was about 0.4 µm. It was also observed in the images that the surface of these BODIPY/PMMA composite nanofibers was smooth and beadless, with no BODIPY nanoparticles being seen on the surface of these nanofibers (Figure 6(a) and (b)).

Figure 6.

SEM images and fiber diameter distributions. (a) and (c) TBDP/PMMA; (b) and (d) DBDP/PMMA. (e) TBDP/PMMA, (f) DBDP/PMMA, corresponding fluorescence microscope images. The samples are the composite nanofibers with 11 wt% TBDP and 13 wt% DBDP, respectively.

The fluorescence microscopy was used to confirm the incorporation of the BODIPY nanoparticles into the composite nanofibers. The fluorescence microscope images (Figure 6(e) and (f)) show a luminescent intensity of 11% (w/w) TBDP/PMMA and 13% (w/w) DBDP/PMMA, which both gave extraordinary orange light under ultraviolet irradiation. The luminescent intensity of TBDP/PMMA clearly demonstrates an enhanced performance compared with DBDP/PMMA. That is because the triphenylamine group forms a stronger D–A conjugated structure that can increase conjugated system emission. The computational studies also show that more electron transfer occurred in the TBDP molecule.

FT-IR measurement analysis

Figure 7 showed the results for FTIR spectra obtained for DBDP, TBDP, PMMA nanofibers, DBDP/PMMA with a concentration of 13% (w/w) and TBDP/PMMA with a concentration of 11% (w/w). It shows two weak absorption peaks at 1508 cm⁻¹ and 1540 cm⁻¹ in DBDP/PMMA, which were assigned to the overtone of the C = C stretching mode in DBDP. However, the absence of the two peaks in PMMA nanofibers suggests that DBDP was successfully doped into PMMA. The same situation also occurred in TBDP and TBDP/PMMA, with two peaks at 1511 cm⁻¹ and 1546 cm⁻¹.

Figure 7.

FT-IR spectra of DBDP, TBDP, PMMA, 13% (w/w) DBDP/PMMA and 11% (w/w) TBDP/PMMA.

Thermal properties of the nanofibers

Thermogravimetric analysis (TGA) experiments were carried out to prove the thermal stability of the composite nanofibers samples. The results are shown in Figure 8. The thermal decomposition of 11% (w/w) TBDP/PMMA and 13% (w/w) DBDP/PMMA began at around 321℃ and 289℃, which revealed obvious increments of 113℃ and 88℃ for the decomposition temperature (Td) in comparison with the corresponding TBDP and DBDP, respectively. It is also revealed that the degradation of undoped PMMA nanofibers starts at 259℃. The weight loss of TBDP/PMMA (321℃–423℃) and DBDP/PMMA (304℃–419℃) occurs over a wide temperature range, which demonstrated enhancements at about 64℃ and 60℃ compared with the undoped PMMA nanofibers (259℃–397℃). The improved thermal stability for the BODIPY/PMMA fibers can be attributed to the hydrogen bond between the F atoms of BODIPY and PMMA. Owing to the strong hydrogen bonding effect, the structure of BODIPY/PMMA is steadier than that of PMMA. The result suggested that the thermal stabilities of the composite nanofibers are better than that of the dyes, which also proved that the PMMA as a polymer matrix can provide an excellent and stable chemical environment for the dyes.

Figure 8.

Thermogravimetric analysis curves of TBDP, DBDP, PMMA, 11% (w/w) TBDP/PMMA and 13% (w/w) DBDP/PMMA.

Conclusions

In summary, TBDP/PMMA and DBDP/PMMA with good emitting properties have been fabricated successfully by electrospinning. The microstructures of nanofibers obtained by SEM show that continuous nanofibers with a homogeneous morphology have been prepared successfully. In addition, the thermal stability of the composite nanofibers is much better than that of the precursor dyes because of the addition of the polymer matrices. The new composite nanofibers exhibit higher fluorescence quantum efficiency than dyes in a dichloromethane solution or in a solid state. The fluorescence quantum efficiency of TBDP/PMMA is also higher than that of DBDP/PMMA; and the fluorescence spectrum of TBDP/PMMA moves into the red region relatively easily. This study provide a new way to designate and fabricate a novel composite nanomaterial containing luminescent BODIPY dyes, and the development of 1D nanomaterial could find potential important applications, particularly in organic light-emitting diodes, organic photovoltaics and fluorescent probes. Considering the simple synthetic protocol, we are confident that this strategy could be successfully exploited for the development of efficient emitting materials and devices.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by National Natural Science Foundation of China (grant number 51303045), the Education Department of Heilongjiang Province of China (grant number 12521413) and School of Chemical Engineering and Biological Engineering Donghua University (grant number PhC11590501-517).

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