Dendritic AIE-active luminogens with a POSS core: synthesis, characterization, and application as chemosensors

Abstract

Two nano-hybrid dendrimers were synthesized by grafting tetraphenylethene (TPE) units onto a polyhedral oligomeric silsesquioxane (POSS) core through a one-step hydrosilylation reaction. The two dendrimers demonstrated typical aggregation-induced emission (AIE), and they exhibited outstanding thermal stability and high fluorescence quantum yields. The emissions of their nanoaggregates can be quenched by picric acid or selectively by Ru³⁺ ions with a superamplification effect. The quenching constants of the dendrimers for picric acid and Ru³⁺ ions are as high as 560 000 and 473 640 M⁻¹, respectively, suggesting that they are highly sensitive chemosensors for explosives and metal ions.

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Introduction

Since the pioneering work of Tang and co-workers,¹ molecules with the property of aggregation-induced emission (AIE) have received increased attention both in academic research and industrial development.^2–7 The phenomenon of the AIE effect is opposite to the general belief of aggregation-caused quenching (ACQ)⁸ of luminescence processes, and opens a new avenue for the development of novel luminescent materials with highly emissive aggregation states. These materials have displayed high potential in the areas of optoelectronics,^9–12 chemical sensors,^13–15 and bioprobes.^16–19 Attracted by the fascinating phenomenon and its promising applications, many research groups have enthusiastically engaged in AIE studies. Synthesis of AIE-active luminogens (AIEgens) with various topological structures and exploring their new applications are very important themes in the field of AIE research. Though AIEgens with various topological structures such as linear, zigzag, star-shaped, hyperbranched, and crosslinked structures were accessed,²⁰ dendritic AIEgens have rarely been reported in part due to a dearth of appropriate scaffolds and the difficulty of precise synthesis of well-defined and perfect 3D structures.^21–24

On the other hand, due to its unique nanometer-sized structure, high thermal stability, and facile chemical modification, the cage-like polyhedral oligomeric silsesquioxane (POSS),²⁵ especially the highly symmetrical and topologically ideal cube octasilsesquioxae (T₈-POSS) with a cage size of approximately 0.5–0.7 nm,²⁶ has drawn intense attention as a constructive unit for the fabrication of various functional organic–inorganic hybrid materials. The design of new hybrid optoelectronic materials containing chromophore units covalently bonded to POSS has been an active research topic.²⁷ A large variety of POSS-containing fluorescent materials have been developed by the combination of POSS and various chromophores in different modes, for example, mono-functionalized POSS as pendant groups attached onto conjugated polymers,²⁸ multi-functionalized POSS incorporated as a core in conjugated dendrimers and hyperbranched polymers.^29–31 These materials generally displayed improved quantum efficiencies, thermal properties, and color stabilities compared with their parent molecules without a POSS unit. Recently, Tang and coworkers have introduced mono-functionalized POSS and tetraphenylethene (TPE) as pendant units onto polymer main chains and prepared AIE-active films. They further used the films to detect explosive vapors.³² However, to the best of our knowledge, there is no report on dendritic AIEgens based on POSS precursors, in part because of the synthesis difficulty caused by the large steric hindrance of the POSS core and the introduction of multi-functional groups simultaneously, as well as the isolation of the well defined dendrimer.

In this work, we synthesized two organic–inorganic hybrid dendrimers, POSS2 and POSS4, by decorating TPE units onto eight corners of a T₈-POSS core through a simple hydrosilylation reaction. The nano-hybrid materials have dramatically increased thermal stability compared with their parent TPE derivatives, and exhibited obvious AIE characteristics. The new molecules showed improved luminescence quantum efficiencies in both dilute solution and solid states. Inspired by their AIE characteristics and 3D topological structure with a rigid scaffold, the two AIEgens were further used as chemosensors, which showed good sensitivity to picric acid (PA) and high selectivity to Ru³⁺ ions.

Results and discussion

Design and synthesis

TPEs are nonplanar propeller-shaped luminogens. In a dilute solution, four phenyl rotors in a TPE molecule undergo dynamic intramolecular rotations against its stator, which induces larger steric hindrance than planar molecules in the synthesis of a perfect dendrimer with a TPEgen on each corner of an inorganic T₈-POSS core. We designed the route of the hydrosilylation reaction to achieve dendritic AIEgen with a POSS core. The synthesis routes to POSS2 and POSS4 are described in Scheme 1. POSS2 was successfully synthesized via hydrosilylation reaction of 1 and POSS1 in the presence of Karstedt’s catalyst in 83% isolated yield.³³ Similarly, POSS4 was obtained from the reaction of 2 and POSS3 in 32% isolated yield. The ¹H NMR spectrum of POSS4 indicates that the ratio of E/Z-vinylene isomers is 1.7/1. The two isomers could not be isolated. POSS2 and POSS4 were easily purified using column chromatography. They were characterized using multinuclear (¹H, ¹³C and ²⁹Si) NMR spectroscopy, FT-IR, and MALDI-TOF mass spectrometry. Both POSS2 and POSS4 possess good solubility in common organic solvents, such as hexane, dichloromethane, tetrahydrofuran, and toluene, but are insoluble in water and methanol.

Scheme 1 Synthesis routes to POSS2 and POSS4.

Thermal properties and morphology

Thermal stability of organic optoelectronic materials is one of the most important factors which affects device lifetime and reliability. The new organic–inorganic hybrid dendrimers exhibit outstanding thermal stability because of the existence of a POSS core. The thermogravimetric analysis (TGA) curves of POSS2 and POSS4 show that the temperatures of 5% weight loss (T_d) are around 450 °C in N₂ and above 355 °C in air (Fig. 1). The T_d is increased by about 200 °C in N₂ and 100 °C in air for the dendrimers compared to those of their molecular counterparts. Solids 1 and 2 are crystalline, but after grafting onto a POSS cage, the corresponding POSS2 and POSS4 show amorphous states. X-ray diffraction (XRD) spectra of POSS2 and POSS4 show a broad peak at about 2θ ≈ 19° (Fig. 2). The differential scanning calorimetry (DSC) study also confirms the amorphous characteristics. Fig. S1† shows the DSC curves for 1, 2, POSS2 and POSS4. The DSC curves of 1 and 2 only show a sharp melting endothermal peak at 115 °C and 157 °C, respectively (Fig. S1A†). A glass transition (T_g) peak at 62 °C appears for POSS2 (Fig. S1B†). No evidence of a glass transition for POSS4 was observed up to 400 °C (Fig. S1B†).

Fig. 1 TGA curves of 1, 2, POSS1, POSS2, POSS3 and POSS4 under N₂ (A) or air (B) at a heating rate of 10 °C min⁻¹.

Fig. 2 XRD patterns of POSS2 and POSS4.

Photophysical properties

Fig. 3 shows the UV-vis spectra of POSS2 and POSS4 in their dilute THF solutions, together with the data of 1 and 2, for comparison. The dendrimers exhibit absorption peaks inherent to their parent molecular counterparts. POSS2 shows the same absorption maximum at 313 nm as that of 1. The absorption maximum of POSS4 is located at 324 nm, which slightly red-shifts 4 nm to 320 nm for 2.

Fig. 3 Absorption spectra of THF solutions of 1, 2, POSS2 and POSS4. Solution concentration: 10 μM.

To check whether the dendrimers were AIEgens, their fluorescence behavior in THF and a THF–water mixture with different water fractions was studied (Fig. 4). The photoluminescence (PL) spectrum of POSS2 in THF is basically a flat line parallel to the abscissa, indicating its nonemissive character in the solution state (Fig. 4A and C). However, when large amounts of water, a poor solvent, were added into its THF solution, intense PL spectra with an emission peak at 486 nm were observed. This fact suggests the formation of the aggregates in mixed solvents. From the molecular solution in THF to the aggregate state in 90% aqueous solution, the emission intensity of POSS2 was increased 322-fold. Similar emission behavior was also observed for POSS4. The emission spectra of POSS4 in THF–water mixtures are displayed in Fig. 4B. Its emission peak emerges at 492 nm as the water fraction increases up to 50%. At 90% water content, the emission intensity of POSS4 was increased 458-fold with a 6 nm blue-shift in the emission maximum. These results confirm that POSS-cored luminogens POSS2 and POSS4 are AIE-active.

Fig. 4 Emission spectra of POSS2 (A) and POSS4 (B) in THF–water mixtures. (C) and (D) Plot of (I/I₀) values versus the compositions of the aqueous mixtures. I₀ = emission intensity in pure THF solution. Solution concentration: 10 μM; excitation wavelength: 315 nm for POSS2, and 320 nm for POSS4. Inset: photographs of POSS2 and POSS4 in THF and THF–water (1 : 9 v/v) mixture taken under illumination of a handheld UV lamp.

To confirm the formation of nanoparticles in the THF–H₂O mixtures with high water content, particle size analysis was carried out (Fig. 5). The average sizes for the nanoparticles of POSS2 formed in 60, 70, 80, and 90% aqueous mixtures are 285.1, 266.6, 247.7, and 151.8 nm, respectively (Fig. 5A). With the increase of the amount of water, the sizes of the particles decreased. This result may be explained as follows: when a small amount of water was added to the THF solution of POSS2, only limited POSS2 formed aggregates, which served as cluster centers for other dissolved molecules and slowly formed larger nanoaggregates. When a large amount of water was added, more POSS2 aggregated quickly, forming nanoparticles of smaller sizes. A similar result was also found for POSS4 (Fig. 5B).

Fig. 5 Particle size distributions of aggregates of POSS2 (A) and POSS4 (B) suspended in THF–water mixtures with water fractions (f_w) of 60, 70, 80 and 90 vol%. Abbreviation: d = Z-average diameter, PDI = polydispersity.

The nano-size POSS core can improve the fluorescence quantum yield (Φ_F) of AIEgens both in solution and solid state. The Φ_F values of the thin solid films of POSS2 and POSS4 are 44.3% and 55.6%, respectively, which are higher than the data of 1 and 2 by around 6.6% and 9.5%, respectively. The Φ_F values of POSS2 and POSS4 in the solution state are still low (0.46% and 0.48%, respectively, Table 1), though the growth rates of which are 64% and 41%, respectively, based on the data of their parent TPE derivatives (0.28% and 0.34%, respectively, Table 1). The possible reason is: in solution, though the nano-sized POSS core contributes to aggregation, the relatively long linkage between the TPE units and POSS core can provide enough space for intramolecular rotation (IMR) of the aromatic rotors in the AIEgens. IMR consumes the excited state energy of the AIEgens and results in the weak fluorescence in the solution. In the solid state, the IMR is restricted, which is the main cause of the AIE effect. Furthermore, the rigid and bulky POSS core aligns the TPE units in a radial fashion relative to the core and isolates them from each other, which reduces intermolecular interactions and increases the emission efficiency. These two reasons result in higher quantum yields of POSS2 and POSS4 in the solid state.

Table 1 Optical and thermal properties of 1, 2, POSS2 and POSS4

	λabs^a/nm	λem^a/nm	ΦF (%)		Td^d (°C)
			(Soln)^b	(Film)^c	N2	Air
a In THF solution (soln, 10 μM).b Determined using quinine sulfate (ΦF = 55% in 0.1 M sulfuric acid) as standard.c Film drop-cast on a quartz plate. Determined using an integrating sphere.d Determined using TGA at 5% mass loss, 10 °C min^−1 in N2 or air.
1	313	485	0.28	37.7	234	243
POSS2	313	486	0.46	44.3	451	355
2	320	487	0.34	46.1	251	258
POSS4	324	493	0.48	55.6	447	367

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Explosive detection

The detection of explosives such as 2,4,6-trinitrotoluene (TNT), 2,4-dinitrotoluene (DNT), 1,3,5-trinitro-perhydro-1,3,5-triazine (RDX), and PA, has become an increasingly important and urgent issue for environmental protection and homeland security. Among detection techniques, fluorescence sensing of explosives has attracted the most attention because it’s more simple, sensitive, and selective. TPE derivatives have displayed their potential in the detection of explosives,^34,35 so we expected the new POSS-cored dendrimers would have higher sensitivity due to the existence of eight TPE units in one molecule. The application of the new AIEgens as chemosensors for the detection of explosives was then examined. The nanoaggregates of the compounds in THF–water mixtures with 90% water content were utilized as probes. Commercially available PA was employed as a model compound. As shown in Fig. 6A and B, the emission intensities progressively decreased when PA was gradually added to the nanoaggregates of POSS2 or POSS4 in aqueous mixtures with a solution concentration as low as 1 μM. The PL quenching of POSS2 and POSS4 was discerned at a PA concentration as low as 5.7 μM or 1.3 ppm. When the PA concentration was increased to 0.86 mM, no emission was observed from the aqueous mixtures. The Stern–Volmer plots of I₀/I − 1 of POSS2 and POSS4 versus PA concentration are shown in Fig. 6C and D to further quantify the quenching efficiency. The plots showed upward-bending curves and were composed of three stages, indicating that the PL quenching became more efficient with increasing quencher concentration.^36,37 The largest quenching constants of POSS2 and POSS4 were up to 560 000 M⁻¹ and 376 000 M⁻¹, respectively. Notably, their quenching constants for PA are much higher than that of TPE-based monomers (34 000 M⁻¹),³⁸ as well as that of linear polysiloles and polygermoles reported in the literature (6710–11 000 M⁻¹).³⁹ The dendritic POSS2 and POSS4 possess 3D topological structures with a rigid POSS scaffold, which contain more molecular cavities to capture PA molecules by electron and/or energy transfer complexation.^40,41 As expected, the dendritic POSS2 and POSS4 can be used as highly sensitive fluorescent chemosensors for explosive detection.

Fig. 6 Emission spectra of POSS2 (A) and POSS4 (B) in THF–water mixtures with 90% water content containing different amounts of PA. Solution concentration: 1 μM; excitation wavelength: 315 nm for POSS2, 320 nm for POSS4. Stern–Volmer plot of I₀/I − 1 of POSS2 (C) and POSS4 (D) versus PA concentration in THF–water mixtures with 90% water content and K_sv values in different concentration regions. I₀ = PL intensity at [PA] = 0 mM.

Metal-ion sensors

Ruthenium(III) complexes are catalysts in olefin metathesis reactions, but they are corrosive and destructive to the respiratory tract, eyes, skin, and digestive tract. The development of highly sensitive and selective sensors to detect Ru³⁺ ions is in great demand in view of environmental protection. The reported method to detect Ru³⁺ ions is mainly focused on inductively coupled plasma mass spectrometry (ICP-MS), which requires sophisticated operation and time-consuming sample preparation, and has limited sensitivity.⁴² Fluorescent chemosensors could be an alternative method for Ru³⁺ ion detection, but fluorescent probes for Ru³⁺ ions are quite rare. To the best of our knowledge, there are only four reports concerning fluorescent probes for Ru³⁺ ions, one is based on a fluorescein derivative⁴³ and the others are AIEgens.^11,44,45 POSS2 and POSS4 show typical AIE characteristics and high fluorescence quantum yields; we guessed that the dendritic AIEgens may be good sensitive and selective fluorescent chemosensors for Ru³⁺ ion detection with lower detection limits. To confirm this, relative investigations were performed.

The nanoaggregates of the compounds in THF–water mixtures with 90% water content were utilized as probes. With the gradual addition of Ru³⁺ ions to the nanoparticle suspensions, the emissions of POSS2 and POSS4 decreased progressively (Fig. 7). The PL quenching can be clearly recognized at a low Ru³⁺ ion concentration of 0.2 μM or 20.2 ppb. Compared with the reported results for AIEgens-based fluorescent probes for Ru³⁺ ions (around 1 ppm), this is an exciting result, as it is the lowest detection concentration for known AIE chemosensors.^11,44,45 At [Ru³⁺] = 20 μM, the PL intensity of POSS2 is merely 37% of its original value. At the same ion concentration, the emissions of POSS4 are almost quenched completely, showing a 5-fold higher sensitivity than POSS2. Electrostatic interaction or charge-transfer complexation between electron-rich POSS2 (POSS4) and electron-deficient Ru³⁺ may account for the PL annihilation. The Stern–Volmer plot of I₀/I − 1 of POSS2 and POSS4 versus Ru³⁺ ion concentration gave an upward bending curve instead of a linear line, indicating a superamplified quenching effect. The Stern–Volmer curves of POSS2 and POSS4 can be well fitted to the exponential equations of I₀/I = 1.75e^41898[Ru3+] −0.91 and I₀/I = 11.3e^41915[Ru3+] −11.6, with quenching constants of about 73 322 M⁻¹ and 473 640 M⁻¹, respectively.

Fig. 7 (A) Emission spectra of POSS2 (A) and POSS4 (B) in THF–water mixtures with 90% water content containing different amounts of Ru³⁺ ions. Solution concentration: 10 μM; excitation wavelength: 315 nm for POSS2, 320 nm for POSS4. Stern–Volmer plot of I₀/I − 1 of POSS2 (C) and POSS4 (D) versus Ru³⁺ ion concentration in THF–water mixtures with 90% water content and K_sv values. I₀ = PL intensity at [Ru³⁺] = 0 μM.

To evaluate the selectivity of POSS2 and POSS4 toward Ru³⁺ detection, we studied the PL change of the dendrimers in the presence of other metal ions. As shown in Fig. 8, the addition of other metal ions, including Mg²⁺, Fe²⁺, Fe³⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Rh³⁺, Cd²⁺, Pd²⁺ and Hg²⁺, exerts little change on the PL of POSS2 and POSS4, which indicates the high selectivity of the dendrimers to Ru³⁺. Although the reason for such selectivity remains unclear at present, we think that the higher standard reduction potential of the Ru(III)/Ru(0) couple relative to other cation/metal systems may be responsible for such selective sensing.^11,44,45

Fig. 8 Changes in relative emission intensities (I₀/I − 1) of POSS2 (A) and POSS4 (B) in THF–H₂O mixtures (1 : 9 by volume; concentration = 10 μM) with various metal ions (100 μM). I₀ = intensity in the absence of metal ions.

Conclusions

In summary, we have successfully synthesized two novel nano-hybrid dendrimers, POSS2 and POSS4, with eight TPE units bonded to a POSS core by a convenient one-step hydrosilylation reaction. The nano-hybrid materials exhibit AIE properties with many attractive merits for application as organic optoelectronic materials. The 3D topological structures of POSS2 and POSS4 allowed them to possess more molecular cavities or voids than organic small molecules or linear polymers for analyte capturing and diffusion pathways for exciton migration, making them attractive chemosensors. The emissions of the nanoaggregates of the new AIEgens could be quenched efficiently by PA and Ru³⁺ ions with a superamplification effect, suggesting that they have promise for application in the detection of explosives and metal ions.

Experimental

Unless otherwise mentioned, materials were obtained from commercial suppliers and used without further purification. The key starting compounds 1,⁴⁶ 2,⁴⁶ POSS1,⁴⁷ and POSS3 (ref. 48) were synthesized according to previously published procedures. Karstedt’s catalyst was received from Aldrich. Tetrahydrofuran (THF) was dried by distillation over sodium metal prior to use.

The UV-vis and fluorescence spectra were recorded with a TU-1900 spectrophotometer and Hitachi F-4500 spectrofluorophotometer, respectively. The FT-IR spectra were recorded with a Bruker Tensor 27 FT-IR spectrometer. The ¹H and ¹³C NMR spectra were recorded on a Bruker Avance III 400 HD spectrophotometer using CDCl₃ as solvent. The ²⁹Si NMR spectra were recorded on a Bruker DMX 300 spectrophotometer. Chemical shifts are referenced to the solvent peak (for CDCl₃, ¹H NMR: 7.26 ppm, ¹³C NMR: 77.0 ppm) and TMS (²⁹Si); data are reported as follows: chemical shifts in ppm (δ), multiplicity (s = singlet, d = doublet, quint = quintet, m = multiplet), integration. The absolute fluorescence (PL) quantum yield was tested on a Hamamatsu C11347 spectrometer. TGA was carried out on a Hitachi STA 7300 under N₂ or air at a heating rate of 10 °C min⁻¹. DSC was carried out on a Seiko EXSTAR DSC6220 under N₂ at a heating rate of 10 °C min⁻¹. The high-resolution mass spectra (HRMS) were recorded with a Bruker SolariX 9.4T spectrometer operating in MALDI-FT mode. The particle sizes of the aggregates were measured on a Zetasizer Nano ZS ZEN 3600. The XRD patterns were recorded on a PANalytical Empyrean.

Preparation of compound POSS2

To a THF (1 mL) solution of POSS1 (30.7 mg, 0.025 mmol, 1.0 eq.) and 1 (127 mg, 0.325 mmol, 13 eq.) was added Karstedt’s catalyst. The reaction mixture was stirred at room temperature for 30 min and warmed to 80 °C. After refluxing for 24 h, the reaction mixture was concentrated under vacuum to get a residue. The residue was purified using silica gel column chromatography using petroleum ether/dichloromethane = 8 : 1 as eluent to give 90.5 mg (83.2%) of compound POSS2 as a white solid. ¹H NMR (400 MHz, CDCl₃), δ (ppm): 7.21 (d, 16H, Ar–H), 7.09–6.98 (m, 136H, Ar–H), 0.63–0.58 (m, 16H, CH₂), 0.47–0.43 (m, 16H, CH₂), 0.19 (s, 48H, CH₃), 0.09 (s, 48H, CH₃). ¹³C NMR (100 MHz, CDCl₃), δ (ppm): 143.0, 143.8, 143.7, 141.0, 137.2, 132.9, 131.3, 130.5, 127.6, 126.4, 126.3, 9.6, 6.8, −0.9, −3.5. ²⁹Si NMR (CDCl₃, 60 MHz), δ (ppm): δ 6.6, −8.4, −115.6. IR (KBr), ν (cm⁻¹): 3054, 3019, 2956, 2908, 1612, 1410, 1252 (Si–CH₃ bending), 1090 (Si–O–Si stretching), 830 (Si–CH₃ stretching), 699, 554. HRMS (MALDI-TOF): m/z ([M + Na]⁺) = 4368.55254 (calcd for C₂₅₆H₂₈₀O₂₀Si₂₄Na = 4368.52467).

Preparation of compound POSS4

The product was synthesized according to the procedure as described above for the synthesis of POSS2, giving a yellow solid of the product POSS4 in 32.4% yield. ¹H NMR (400 MHz, CDCl₃), δ (ppm): 7.16–7.07 (m, 36H, Ar–H), 7.07–6.96 (m, 95H, Ar–H), 6.96–6.89 (m, 21H, Ar–H), 6.83 (d, 6H, E-ArCH= and Ar–H), 6.25 (dd, 5H, E-SiCH=), 5.78 (s, 3H, Z-ArCH=), 5.68 (s, 3H, Z-SiCH=), 0.17 (d, 48H). ¹³C NMR (100 MHz, CDCl₃), δ (ppm): 150.1, 144.6, 143.8, 143.7, 143.6, 143.6, 141.9, 141.1, 140.8, 140.8, 140.6, 135.9, 131.5, 131.4, 131.3, 131.2, 127.7, 127.6, 126.5, 126.4, 126.4, 126.1, 0.37, 0.25. ²⁹Si NMR (CDCl₃, 60 MHz), δ (ppm): −5.4, −6.2, −116.0. IR (KBr), ν (cm⁻¹): 3061, 2960, 1609, 1256 (Si–CH₃ bending), 1167, 1089 (Si–O–Si stretching), 849 (Si–CH₃ stretching), 764, 700, 561. HRMS (MALDI-TOF): m/z ([M + Na]⁺) = 3888.20435 (calcd for C₂₄₀H₂₁₆O₂₀Si₁₆Na = 3888.20854).

Preparation of metal-ion solutions

Inorganic salts (magnesium chloride, iron(II) chloride, iron(III) chloride, cobalt(II) chloride, nickel(II) chloride hexahydrate, copper(II) chloride, zinc chloride, ruthenium(III) chloride, rhodium(III) chloride, cadmium chloride, mercury(II) nitrate hydrate) were dissolved in distilled water (5 mL) to afford 50 mM aqueous solutions. The stock solutions were diluted to the desired concentrations with distilled water for further experiments.

Acknowledgements

The authors gratefully acknowledge the National Natural Science Foundation of China (NSFC, No. 50673094 and 20774102) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Preparation of compound 1 and 2. ¹H, ¹³C and ²⁹Si NMR spectra of synthetic compounds. See DOI: 10.1039/c5ra18152f

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