Letter pubs.acs.org/Langmuir
Remarkable Stimulation of Emission Quenching on a Clay Surface Keita Sato,† Kazuki Matsubara,† Satomi Hagiwara,‡ Kenji Saito,† Masayuki Yagi,† Shinsuke Takagi,‡ and Tatsuto Yui*,† †
Department of Material Science and Technology, Faculty of Engineering, and Center for Transdisciplinary Research, Niigata University, 8050 Ikarashi-2, Niigata 950-2181, Japan ‡ Department of Applied Chemistry, Graduate Course of Urban Environmental Sciences, Tokyo Metropolitan University, Minami-ohsawa 1-1, Hachiohji, Tokyo 192-0397, Japan S Supporting Information *
ABSTRACT: Tetra-cationic pyrene derivative (Py4+) and tris(bipyridine)ruthenium(II) (Ru2+) were hybridized onto the surface of a synthesized clay. We observed the remarkable stimulation of excited Py4+ emission quenching on the clay surface, with a very large apparent quenching rate constant (kq = 7.4 ± 0.7 × 1015 L mol−1 s−1).
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as the “size-matching effect”.20,21 In recent years, they have reported that other various cationic dyes,20 such as tetracationic pyrene (Py4+),22 exhibit this size-matching effect. In this Letter, we report the appearance of weak intermolecular interactions between the tetra-cationic pyrene derivative (Py4+)22 and tris(bipyridine)ruthenium(II) (Ru2+) when both are adsorbed onto the negatively charged SSA surface without aggregate or complex formation. Emission from excited Py 4+ molecules was remarkably quenched by coadsorbed Ru2+ because of the weak interaction on the SSA surface, and the estimated apparent quenching rate constant (kq) was 7.4 ± 0.7 × 1015 L mol−1 s−1. Generally, the intermolecular electron and energy transfer rate constants in homogeneous solutions do not exceed 1010 L mol−1 s−1 because of the diffusion-determining step involving solute molecules. Energy transfer reactions in homogeneous solutions beyond the diffusion limit have been reported:23 The energy transfer rate constants from anthracence to perylene or perylene to rubrene in benzene solutions were estimated to be 12 and 13 × 1010 L mol−1 s−1, respectively, and the values are still ca. 10 000 times slower than that for the present systems. In the present case, fixation of both Py4+ and Ru2+ onto the SSA surface might result in the following phenomena: shortening of intermolecular distance, the appearance of weak intermolecular interactions, and suppression of diffusion. Thus, such a high apparent quenching rate constant might be observed.
INTRODUCTION The rate constant of a photoreaction in a solution is strongly dependent on the distance between solute molecules. With respect to the intermolecular distance R, the photoinduced electron transfer efficiency decreases exponentially as a function of R, and the Förster energy transfer efficiency is inversely proportional to R6.1 These facts strongly indicate that intermolecular distance control is very important in achieving efficient photoreactions. To overcome these points, the fixation of photochemically active molecules onto various surfaces or interfaces has been widely investigated. In particular, ordered inorganic host materials2,3 such as zeolites,4,5 mesoporous silicas,6−8 and layered materials9−16 are some of the materials used for this purpose because of their rigid and ordered nanostructures. Typically, photoinduced electron and/or energy transfers between dye molecules on clay dispersions were well studied.9,10,15−20 However, an excessively high concentration of dye molecules induces aggregate formation on the solid surface of these materials,10,20 and the aggregates, in general, exhibit low photoreactivity because of their very short excited lifetimes. Thus, the development of materials compatible with high concentrations of dyes (shortening of distance) and the maintenance of nonaggregated state have been proven to be difficult. In the past decade, we have reported the highly concentrated adsorption of tetra-cationic porphyrins without aggregate formation onto synthesized clay (Sumecton SA, SSA).21 The matching of two parameters, i.e., the negative charges on the clay surface and the intercationic charge within the porphyrin, was very important for preventing aggregation. The authors referred to this unique phenomenon © 2014 American Chemical Society
Received: November 25, 2014 Revised: December 22, 2014 Published: December 25, 2014 27
DOI: 10.1021/la504597t Langmuir 2015, 31, 27−31
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EXPERIMENTAL SECTION
Tris(bipyridine)ruthenium(II) dichloride (Ru2+) was purchased from TCI and used without further purification. The tetracationic pyrene derivative (1,3,6,8-tetrakis(N-methylpyridinium-4-yl)pyrene 4PF6−; Py4+) was synthesized according to the literature method.22 Cation-exchangeable saponite clay (SSA) was provided by Kunimine Industries Co., Ltd. and was used as received. The SSA was hydrothermally synthesized, and it exhibited a very large surface area (750 m2 g−1) and a cation exchange capacity (CEC) of 0.997 mequiv g−1.20,21,24 A hybrid of Py4+ and Ru2+ adsorbed onto SSA (Py4+/Ru2+/SSA) was prepared by mixing an aqueous SSA dispersion (12 mg L−1, 0.012 mequiv L−1), an aqueous solution of Py4+ (1.5 μmol L−1), and the aqueous Ru2+ (0−1.5 μmol L−1) for 2 h at room temperature and under dark. Absorption and emission measurements were performed using Ar-saturated aqueous dispersions. Detailed experimental procedures are summarized in the Supporting Information (SI). The loading amounts of dyes are expressed as the percentage of the amount of cationic charge of dyes relative to the CEC of the SSA (% CEC is the unit henceforth).20,21,24 Under these conditions, 50% CEC of Py4+ was adsorbed onto SSA, and the Ru2+ loading was varied from 0% to 25% CEC. The adsorption of both Py4+ and Ru2+ onto SSA was confirmed by filtration, i.e., the two dyes were not detected in the filtrate under experimental conditions, as summarized in Figure S1 in the SI.
Figure 1. (a) Absorption spectra of Py4+/Ru2+/SSA in water with various amounts of Ru2+ loaded ([SSA] = 12 mg L−1, [Py4+] = 1.5 μmol L−1 (50% CEC), [Ru2+] = 0−1.5 μmol L−1 (0%−25% CEC)). (b) LB plot of Py4+/Ru2+/SSA (solid circles) and the aqueous mixture of Py4+ and Ru2+ without SSA (open circles). The intensities of the absorption maxima at 463 and 453 nm are plotted for Py4+/Ru2+/SSA and their mixture, respectively; [SSA] = 0 or 12 mg L−1, [Py4+] = 1.5 μmol L−1 (50% CEC), [Ru2+] = 0−1.5 μmol L−1 (0%−25% CEC).
shown in Figure 1b (solid circle). When the amount of Ru2+ loaded was less than approximately 0.8 μmol L−1 (below 13% CEC adsorption), the absorption intensities from Ru2+ linearly increased with the increasing adsorption of Ru2+. The estimated ε for Py4+/Ru2+/SSA (1.41 ± 0.15 × 105 L mol−1 cm−1) is quite similar to that for Ru2+/SSA (1.43 ± 0.05 × 105 L mol−1 cm−1). This result indicates that Ru2+ molecules did not interact with neighboring Py4+ or Ru2+ molecules on the SSA surface when the amount of Ru2+ adsorbed was less than 0.8 μmol L−1. Interestingly, however, the ε of Py4+/Ru2+/SSA, when [Ru2+] was more than 0.8 μmol L−1 (13% CEC), decreased to 0.97 ± 0.09 × 105 L mol−1 cm−1. Such gradual change of ε was not caused by desorption of Ru2+ for the following two reasons: (1) Ru2+ was not detected in the filtrate solution, (2) the ε of desorbed Ru2+ in aqueous solution at 463 nm (1.25 × 105 L mol−1 cm−1) is larger than that for the Py4+/Ru2+/SSA system with [Ru2+] > 0.8 μmol L−1 (13% CEC). In contrast, LB plots of an aqueous mixture of Py4+ and Ru2+ (without SSA) remained linear as a function of the concentration of Ru2+ (ε = 1.41 ± 0.11 × 105 L mol−1 cm−1 at 453 nm) within a homogeneous solution (Figure 1(b, open circles). This result indicates that Py4+ and Ru2+ molecules did not interact in the aqueous solution because of electronic repulsion between them. As previously described, the absorption spectral shapes of the Py4+/Ru2+/SSA did not change with the amount of Ru2+ loaded; however, the estimated ε value was immediately changed at [Ru2+] ≈ 0.8 μmol L−1 (13% CEC). This phenomenon, i.e., feeding of the color, is known as the hypochromic effect.25−30 The hypochromic effect can be caused by weak interactions between aromatic compounds. Thus, in the present case, Py4+ and Ru2+ on the SSA might weakly interact in their ground states. Emission spectra of Py4+/Ru2+/SSA with Py4+ = 50% CEC and Ru2+ = 0%−25% CEC using 347 nm excitation light are shown in Figure 2. Under the condition of Ru2+ = 25% CEC, i.e., maximum adsorption of Ru2+, the ratio of the absorption of photons between the Py4+ and Ru2+ on SSA surfaces was 81:19 at 347 nm. Thus, Py4+ molecules on SSA near-selectively absorbed incident photons, and emission intensities were practically collected by the absorbed photons of Py4+. In the absence of Ru2+, a strong emission band from Py4+ was observed, with λmax = 500 nm. The adsorption of Ru2+ onto the Py4+/SSA surface caused emission quenching from excited Py4+ on SSA, and a new broad emission band corresponding to the
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RESULTS AND DISCUSSION Before investigating the Py4+/Ru2+/SSA systems, we also prepared single-component Py4+ and Ru2+ with SSA (Py4+/ SSA22 and Ru2+/SSA) for comparison. The basic properties of the single-component systems are summarized in Supporting Information Figures S2−S3 and Table S1. The results indicate that loaded Py4+ or Ru2+ molecules were efficiently adsorbed onto SSA under experimental conditions. Lambert−Beer (LB) plots at the absorption maxima of Py4+/SSA (450 nm) and Ru2+/SSA (463 nm) are shown in Figure S4. The absorption intensities of both systems increased monotonically with increasing dye loadings, and the absorption coefficients (ε) of the Py4+/SSA and the Ru2+/SSA were estimated to be 4.03 ± 0.27 and 1.43 ± 0.05 × 105 L mol−1 cm−1, respectively. These results strongly indicate that Py4+ and Ru2+ exist as monomeric species on SSA surfaces under experimental conditions. Normalized emission and absorption spectra of Py4+/SSA and Ru2+/SSA are shown in Figure S5. The spectral overlapping integral (J)1 is estimated to be 9.1 × 1013 mol−1 L cm−1 nm4; this J value is expected to Förster energy transfer from excited Py4+ to Ru2+ on SSA. Absorption spectra of the Py4+/Ru2+/SSA mixed systems with 50% CEC of Py4+ adsorbed and various Ru2+ loadings were measured; the spectra are shown in Figure 1a. The absorption maxima at approximately 460 nm monotonically increased with the increasing amount of adsorbed Ru2+. Typical absorption spectra of Py4+/SSA, Ru2+/SSA, and Py4+/Ru2+/SSA and the calculated spectrum are compared in Figure S6. The experimentally observed absorption of Py4+/Ru2+/SSA agreed well with the calculated one (Figure S6b,c); i.e., adsorbed Ru2+ did not affect the absorption spectral shapes of Py4+/Ru2+/SSA. This result indicates that neither Py4+ nor Ru2+ formed aggregates or complexes at the ground state. LB plots at the absorption maximum of Ru2+ (463 nm) of Py4+/Ru2+/SSA with various amounts of Ru2+ loaded into Py4+/SSA mixtures are 28
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Figure 2. Emission spectra of Py4+/Ru2+/SSA with various loading amounts of Ru2+ in Ar-saturated water; [SSA] = 12 mg L−1, [Py4+] = 1.5 μmol L−1 (50% CEC), [Ru2+] = 0−1.5 μmol L−1 (0%−25% CEC). The excitation wavelength was 347 nm. The inset shows that the typical emission spectra of Py4+/Ru2+/SSA deconvoluted using the emission spectra of Py 4+ /SSA and Ru2+ /SSA as standards: Experimentally observed emission spectra with [Ru2+] = 1.5 μmol L−1 (25% CEC) (blue line); Py4+ component (green line); Ru2+ component (red line).
Figure 3. SV plot of Py4+ emission in Py4+/Ru2+/SSA in water: [SSA] = 12 mg L−1, [Py4+] = 1.5 μmol L−1 (50% CEC), [Ru2+] = 0−1.5 μmol L−1 (0%−25% CEC).
of the LB plot of Ru2+ and the SV plot of Py4+ within Py4+/ Ru2+/SSA system was the same (13% CEC). These results strongly suggest that the very weak interaction between Py4+ and Ru2+ causes the enhancement of emission quenching at Ru2+ loadings greater than ca. 13% CEC. The apparent emission quenching rate constant (kq) of Py4+ in the Py4+/Ru2+/SSA system was estimated using the emission lifetime of Py4+/SSA with Py4+ = 50% CEC adsorption (1.6 ns, see Table S2) and two different previously mentioned KSV values. The kq values estimated for Ru2+ loadings below and above 13% CEC were 8.1 ± 1.7 × 1014 and 7.4 ± 0.7 × 1015 L mol−1 s−1, respectively. In general case, typical kq values in homogeneous solutions are approximately 109−1010 L mol−1 s−1;1 however, extremely large kq values were observed in the present systems. In fact, no emission quenching from excited Py4+ was observed for homogeneous aqueous mixtures of [Py4+] = 1.5 μmol L−1 and [Ru2+] = 0−1.5 μmol L−1. Such large kq values might be caused by the fixation of the two dyes onto the SSA surface even under low loadings of Ru2+ (less than 0.8 μmol L−1). Interparticle quenching15,16 thought to be negligible because of following reasons: (1) Concentration of SSA was very low (12 mg L−1); (2) SSA sheets were completely exfoliated in the present conditions and obtained apparent kq values that exceeded the diffusion rate constant; (3) excited Py4+ may be efficiently quenched by neighboring Ru2+. Further adsorption of Ru2+ might induce the weak interaction between Py4+ and Ru2+ on SSA, thus resulting in a large apparent kq (7.4 ± 0.7 × 1015 L mol−1 s−1). SV analysis and lifetime measurements clearly indicate that both Py 4+ and Ru2+ molecules were homogeneously dispersed, that is, not segregated11,16 on the SSA surface or particles, because of synthesized clay (SSA) has homogeneous anionic charge distributions on its surface. Based on the surface area of SSA and the adsorbed amounts of Py4+ (50% CEC) and Ru2+ (25% CEC), estimated average center-to-center intramolecular distance24 was estimated to be 3.2 nm, and thus Py4+ and Ru2+ molecules were highly concentrated on the SSA surface.21 The two dyes might assume regular orientations on the SSA surface due to SSA having a regular distribution of negative surface charges. Thus, an additional possibility for the efficient quenching of Py4+ by Ru2+ on the SSA surface may be explained as follows: The regular adsorption leads to an adequate orientation of the dipole moment of Py4+ and Ru2+ molecules for Förster energy transfer.1
Ru2+ appeared at approximately 550−750 nm. These quenching behaviors were confirmed by the emission lifetime measurements of Py4+ (Figure S7 and Table S2). These results clearly suggest that the adsorbed Ru2+ efficiently quenched the coadsorbed excited Py4+ on the SSA surface. Emission spectra of Py4+/Ru2+/SSA were deconvoluted for all wavelength regions by the linear sum of spectra using Py4+/SSA and Ru2+/SSA as model spectra with the least-squares method. Typical emission spectra of Py4+/Ru2+/SSA with deconvoluted Py4+ and Ru2+ components are shown in the inset in Figure 2. The sum of the two components reproduced experimentally observed emission spectra of Py4+/Ru2+/SSA (Figure S8). Exact emission intensities from Py4+ and Ru2+ components within the Py4+/Ru2+/SSA were separated using the aforementioned spectral fitting procedures. Emission quantum yields were measured at three-fold concentrated systems due to weak emission intensity from Ru2+ under typical conditions: [SSA] = 36 mg L−1, [Py4+] = 0 or 4.5 μmol L−1 (0% or 50% CEC), [Ru2+] = 0 or 4.5 μmol L−1 (0% or 25% CEC); however, the same tendencies were confirmed under typical concentration. The emission quantum yields of Py4+ (ΦPy) in the absence of Ru2+ and in the presence of maximum adsorbed Ru2+ (25% CEC) were estimated as 34.3 ± 1.1% and 4.3 ± 0.4%. Moreover, amplification of the emission quantum yield of Ru2+ (ΦRu) on SSA was observed; ΦRu increased from 6.7 ± 0.8% to 20.2 ± 1.5% in the absence and presence of Py4+, respectively. These results may conclude that energy transfer from excited Py4+ to Ru2+ occurred on the SSA surface. Stern−Volmer (SV) analyses1 were applied for the emission quenching of Py4+ on the Py4+/Ru2+/SSA system, where I and I0 are the integrated emission intensities from Py4+ within Py4+/ Ru2+/SSA systems in the presence and absence of quencher (Ru2+), respectively. The SV plot for Py4+ quenching within the Py4+/Ru2+/SSA system is shown in Figure 3; the SV plot was classified as having two stages. Under the condition of [Ru2+] < 0.8 μmol L−1 (< 13% CEC), the I0/I ratio slowly and linearly increased. Based on the loaded concentration of Ru2+, the apparent SV constant (KSV) was estimated to be 1.33 ± 0.27 × 106 L mol−1. However, the slope of the SV plot drastically changed at [Ru2+] > 0.8 μmol L−1 (13% CEC), and the estimated KSV was 1.21 ± 0.12 × 107 L mol−1. Such a sudden KSV change might be caused by a new interaction between Py4+ and Ru2+ on the SSA surface. Interestingly, the inflection point 29
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Nalwa, H. S., Eds.; American Scientific Publishers: Valencia, CA, 2009; Vol. 5, pp 35−90. (11) Ghosh, P. K.; Bard, A. J. Photochemistry of Tris(2,2′bipyridyl)(ruthenium(II)) in Colloidal Clay Suspensions. J. Phys. Chem. 1984, 88, 5519−5526. (12) Yui, T.; Kameyama, T.; Sasaki, T.; Torimoto, T.; Takagi, K. Pyrene-to-Porphyrin Excited Singlet Energy Transfer in LBLDeposited LDH Nanosheets. J. Porphyrins Phthalocyanines 2007, 11, 428−433. (13) Park, D.-H.; Hwang, S.-J.; Oh, J.-M.; Yang, J.-H.; Choy, J.-H. Polymer−Inorganic Supramolecular Nanohybrids for Red, White, Green, and Blue Applications. Prog. Polym. Sci. 2013, 38, 1442−1486. (14) Ogawa, M.; Saito, K.; Sohmiya, M. A Controlled Spatial Distribution of Functional Units in the Two Dimensional Nanospace of Layered Silicates and Titanates. Dalton Trans. 2014, 43, 10340− 10354. (15) Miyamoto, N.; Yamada, Y.; Koizumi, S.; Nakato, T. Extremely Stable Photoinduced Charge Separation in a Colloidal System Composed of Semiconducting Niobate and Clay Nanosheets. Angew. Chem., Int. Ed. 2007, 46, 4123−4127. (16) Nakato, T.; Watanabe, S.; Kamijo, Y.; Nono, Y. Photoinduced Electron Transfer between Ruthenium−Bipyridyl Complex and Methylviologen in Suspensions of Smectite Clays. J. Phys. Chem. C 2012, 116, 8562−8570. (17) Lu, L.; Jones, R. M.; McBranch, D.; Whitten, D. SurfaceEnhanced Superquenching of Cyanine Dyes as J-Aggregates on Laponite Clay Nanoparticles. Langmuir 2002, 18, 7706−7713. (18) Bujdák, J.; Chorvát, D. a.; Iyi, N. Resonance Energy Transfer between Rhodamine Molecules Adsorbed on Layered Silicate Particles. J. Phys. Chem. C 2009, 114, 1246−1252. (19) Olivero, F.; Carniato, F.; Bisio, C.; Marchese, L. Promotion of Förster Resonance Energy Transfer in a Saponite Clay Containing Luminescent Polyhedral Oligomeric Silsesquioxane and Rhodamine Dye. Chem.Asian J. 2014, 9, 158−165. (20) Takagi, S.; Shimada, T.; Ishida, Y.; Fujimura, T.; Masui, D.; Tachibana, H.; Eguchi, M.; Inoue, H. Size-Matching Effect on Inorganic Nanosheets: Control of Distance, Alignment, and Orientation of Molecular Adsorption as a Bottom-Up Methodology for Nanomaterials. Langmuir 2013, 29, 2108−2119. (21) Takagi, S.; Shimada, T.; Eguchi, M.; Yui, T.; Yoshida, H.; Tryk, D. A.; Inoue, H. High-Density Adsorption of Cationic Porphyrins on Clay Layer Surfaces without Aggregation: The Size-Matching Effect. Langmuir 2002, 18, 2265−2272. (22) Hagiwara, S.; Ishida, Y.; Masui, D.; Shimada, T.; Takagi, S. Unique Photochemical Behavior of Novel Tetracationic Pyrene Derivative on the Clay Surface. Tetrahedron Lett. 2012, 53, 5800− 5802. (23) Barltrop, J. A.; Coyle, J. D. Excited States in Organic Chemistry; John Wiley & Sons, Ltd.: Bristol, U.K., 1975. (24) Yui, T.; Fujii, S.; Matsubara, K.; Sasai, R.; Tachibana, H.; Yoshida, H.; Takagi, K.; Inoue, H. Intercalation of a Surfactant with a Long Polyfluoroalkyl Chain into a Clay Mineral: Unique Orientation of Polyfluoroalkyl Groups in Clay Layers. Langmuir 2013, 29, 10705− 10712. (25) Hase, Y.; Nagai, K.; Iida, H.; Maeda, K.; Ochi, N.; Sawabe, K.; Sakajiri, K.; Okoshi, K.; Yashima, E. Mechanism of Helix Induction in Poly(4-carboxyphenyl isocyanide) with Chiral Amines and Memory of the Macromolecular Helicity and Its Helical Structures. J. Am. Chem. Soc. 2009, 131, 10719−10732. (26) Lange, A. W.; Herbert, J. M. Both Intra- and Interstrand ChargeTransfer Excited States in Aqueous B-DNA Are Present at Energies Comparable To, or Just Above, the 1ππ* Excitonic Bright States. J. Am. Chem. Soc. 2009, 131, 3913−3922. (27) Terenziani, F.; Parthasarathy, V.; Pla-Quintana, A.; Maishal, T.; Caminade, A.-M.; Majoral, J.-P.; Blanchard-Desce, M. Cooperative Two-Photon Absorption Enhancement by Through-Space Interactions in Multichromophoric Compounds. Angew. Chem., Int. Ed. 2009, 48, 8691−8694.
CONCLUSION Two different dye molecules, i.e., Py4+ and Ru2+, were hybridized on the surface of SSA. A hypochromic effect was observed under the condition of [Ru2+] > 0.8 μmol L−1 (< 13% CEC). Coadsorbed Ru2+ efficiently quenched excited Py4+ on clay, with kq = 8.1 ± 1.7 × 1014 and 7.4 ± 0.7 × 1015 L mol−1 s−1 when the amount of adsorbed Ru2+ was below and above 0.8 μmol L−1, respectively. Such a large apparent kq may be caused by the fixation and concentration of two dyes on the SSA surface. Further details, such as the microscopic orientation of the two dyes, the energy transfer phenomena origin of the weak intermolecular interactions, and the emission enhancement of Ru2+ on the SSA are currently underway in our laboratory.
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ASSOCIATED CONTENT
* Supporting Information S
Experimental details, absorption analysis of various dyes and SSA systems, emission lifetime of Py4+ systems, deconvoluted emission spectra. See DOI: 10.1039/c000000x/. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work has been partly supported by Nippon Sheet Glass Foundation for Materials Science and Engineering. REFERENCES
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