Highly Efficient DCL, UCL, and TPEF in Hybridized Ln-Complexes from Ir-Metalloligand

Open AccessCCS ChemistryRESEARCH ARTICLE1 Feb 2021Highly Efficient DCL, UCL, and TPEF in Hybridized Ln-Complexes from Ir-Metalloligand Jun-Ting Mo, Zheng Wang, Peng-Yan Fu, Lu-Yin Zhang, Ya-Nan Fan, Mei Pan and Cheng-Yong Su Jun-Ting Mo MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 , Zheng Wang MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 , Peng-Yan Fu MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 , Lu-Yin Zhang MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, School of Materials Science and Engineering, Lanzhou University of Technology, Lanzhou 730050 , Ya-Nan Fan MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 , Mei Pan *Corresponding author: E-mail Address: [email protected] MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 and Cheng-Yong Su MOE Laboratory of Bioinorganic and Synthetic Chemistry, Lehn Institute of Functional Materials, School of Chemistry, Sun Yat-Sen University, Guangzhou 510275 State Key Laboratory of Applied Organic Chemistry, Lanzhou University, Lanzhou 730000 https://doi.org/10.31635/ccschem.020.202000185 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail In molecular level lanthanide complexes, it is still challenging to achieve near-infrared (NIR) down-conversion luminescence (DCL) and visible up-conversion luminescence (UCL) efficiently under ambient conditions. By delicate design of hybridized iridium−europium−ytterbium (Ir–Eu–Yb) trimetallic complexes from an Ir-metalloligand, DCL and UCL emissions with opposite photon conversion pathways were achieved simultaneously in one integrated system. In detail, three different types of energy transfer (ET) and photoluminescence (PL) were detected, as follows: (1) Highly sensitized NIR-DCL from Yb(III) center via ETIr→Eu→Yb, which gave the longest decay lifetime (553 μs) at room temperature ever reported in solid-state Yb-complexes. (2) Unprecedented UCL red emission from Eu(III) center via ETYb→Eu using 980 nm continuous wavelength (CW) laser excitation, which required ultralow energy threshold (0.616 W/cm2). (3) Two-photon excited fluorescence (TPEF) via ETIr→Eu using femtosecond pulsed laser excitation. This exquisite module of "one stone, three birds" provides a new stimulus in the design and application of multifunctional UCL/DCL/TPEF optical materials under ambient conditions. Download figure Download PowerPoint Introduction Lanthanide complexes are studied extensively for applications in illuminating and displaying materials1, lasers2, optical amplifiers3, light convertors4, imaging probes in living tissue analysis, and others.5,6 Based on different assembling compositions and exciting conditions, an up-conversion luminescence (UCL) or down-conversion luminescence (DCL) could be engendered and utilized. Among these materials, highly efficient NIR emissions by lanthanide complexes using down-converted UV or visible light excitations are being pursued.7,8 Along this line, the following approaches have been put forward: (1) appropriate design of ligand system to match the triplet energy levels of NIR-emitting lanthanide ions, (2) protected coordination environment for lanthanide ions and/or replacement of H- by D- or F-groups to reduce the quenching effect of C–H vibration on the ligand or solvent, and (3) assembly of heteronuclear Ln-complexes to take advantage of multiple energy transfers (ETs) among the metal centers.9,10 However, so far, the ever reported Ln-complexes with efficient NIR emissions generally require unconventional conditions such as low temperature, deuterated reagents, and so on.10–13 It is an urgent need to develop alternative materials to achieve efficient NIR emissions with decay lifetimes of hundreds of microseconds or more under ambient conditions, especially, in solid-state and at room temperature (RT). On the other hand, UCL, in which low-energy photons are converted into higher-energy ones, has also been explored in lanthanide-based materials for labeling, imaging, photodynamic therapy, bioanalytical applications, and others.14–16 However, achieving UCL in molecular level lanthanide complexes is still a challenging task.17 Multiple factors need to be considered, which include the efficiency of long-wavelength absorption (usually in the visible to NIR region), protection of lanthanide emitters to stabilize the intermediate excited states, and appropriate ET pathways, established in the integrated molecular systems. Considering the integration of organic and inorganic components in the deliberate design of Ln-complexes, the first goal could be achieved by two different pathways. That is, by TPEF (two-photon excited fluorescence) pathway via the virtual state of organic ligand18,19 or by UCL pathway via the real oxidation state of Ln(III) ions. For the second issue, in order to stabilize the intermediate states of lanthanide emitters against C–H oscillating deactivation, the encapsulated coordination environment of lanthanide centers and the introduction of F-groups on the ligand might be effective. The third problem could be resolved through the assembly of hybridized metal species (including f-block and d-block types) into one integrated coordination system, which might result in multiple ET and photon conversion pathways in different directions.17,20–25 This is likely to provoke unique UCL/DCL combinations in one integrated molecular system. Regarding the considerations mentioned above, we designed an iridium (Ir)-metalloligand with F-group attachment, and then a series of homo- or hybridized lanthanide (Ln)-complexes were assembled. Excitingly, both efficient DCL-NIR emission and UCL–visible emissions were achieved in the molecular systems under ambient conditions (RT, 1 atm). Detailed photoluminescence studies disclosed three distinct ET pathways among the Ir-metalloligand and/or hybridized Ln(III) ions. Experimental Methods Experimental methods are available in the Supporting Information. Results and Discussion Construction of Ln-complexes from Ir-metalloligand The series of d–f heteronuclear complexes were prepared by using preconstructed mononuclear Ir(III) complex, [Ir(4,4'-dicarboxy-2,2'-bipyridine)(2-(4',6'-difluorophenyl)pyridine)2] (LIr) as the metalloligand ( Supporting Information Scheme S1). The LIr, the Ir(III) center was coordinated to two 2-(2,4-difluorophenyl)pyridine ligands and one bipyridine ligand possessing two dicarboxylic groups for further coordination with Ln(III) ions ( Supporting Information Figure S1). The bimetallic Ir–Ln (Ln = Eu, Gd, and Yb) complexes were obtained through hydrothermal synthesis at high temperature by dissolving the deprotonated LIr metalloligand in water with the aid of NaOH, followed by the addition of hydrated Ln(NO3)3 salts, offering crystalline samples, as characterized by infrared (IR) spectroscopy and thermal gravimetric (TG) analyses ( Supporting Information Figures S2 and S3). The trimetallic Ir–Eu–Yb systems were generated similarly by mixing two lanthanide salts in varied ratios together with LIr metalloligand. Single-crystal and powder X-ray diffraction (SC-PXRD; Supporting Information Figure S4 and Tables S1 and S2) showed Ir–Eu and Ir–Gd crystallized in P21/n space group and Ir–Yb crystallized in C2/c space group, while PXRD patterns of the trimetallic Ir–Eu–Yb samples proved that all were isostructural to Ir–Gd complex. It is to be noted that although Ir–Gd, and Ir–Yb were crystallized in different space group, the basic coordination environments for the metals and ligands were closely similar ( Supporting Information Figures S5 and S6). Therefore, we only analyzed the single-crystal structure of Ir–Gd complex as a representative. As shown in Supporting Information Figure S5, in Ir–Gd complex, the Ir(III)-center formed a 6-coordinated octahedral geometry in LIr, and the Gd(III)-center is 8-coordinated with six O atoms from six different LIr metalloligands together with two O atoms from NO3−. The bond distances and angles around the individual metal centers were in accordance with the usual Ir(III) and Gd(III) complexes, as listed in Supporting Information Table S2 [Ir–N: 1.93(4)–2.09(4) Å; Ir-C: 2.04(1) Å, Gd–O: 2.30(6)–2.57(2) Å], the closest Ir…Gd, Ir…Ir, and Gd…Gd separations were ∼8.1, 9.4, 5.8 Å. By the linkage of LIr metalloligands, the Ln centers aligned into a 1D {Ir2Ln}n zigzag chain, and the F-attached Ir-metalloligands aligned outside the chain. Similar structures were found in Ir-Eu and Ir–Yb complexes. In the trimetallic Ir–Eu–Gd complexes, Eu(III) and Gd(III) ions were distributed statistically in the coordination sites, which allowed for ET between each other (Scheme 1). Scheme 1 | Schematic structure (upper) and energy transfer mechanisms (lower) in hybridized Ir–Eu-Yb complexes with DCL, UCL, and TPEF (DCL, down-conversion luminescence; UCL, up-conversion luminescence; TPEF, two-photon excited fluorescence; ET, energy transfer). Download figure Download PowerPoint DCL via Ir→Ln energy transfer (Ln = Eu or Yb) The UV–vis absorption spectra ( Supporting Information Figure S7) of the LIr metalloligand and Ir–Ln complexes in the solid-state covered a wide range (200–600 nm) due to the good absorption property contributed by π–π* and metal-to-ligand charge transfer (MLCT) transition of LIr metalloligand. Excitation of LIr using 450 nm visible light produced a yellowish-green phosphorescence centered at ∼590 nm ( Supporting Information Figure S8), originating from the 3MLCT energy state. After coordination with Gd(III) into heteronuclear Ir–Gd complex, the LIr-originated green emission is blue-shifted to ∼540 nm ( Supporting Information Figure S9), for which the lifetime is 470 ns at RT and prolonged to 3.38 μs at 77 K (Table 1). This manifested further the triplet phosphorescence nature originated from 3MLCT state of the Ir-metalloligand. In Eu3+ and Yb3+ coordinated d–f heterometallic complexes, characteristic f→f emissions were observed (Figure 1a and b). The detected decay lifetimes for the visible red emission of Ir–Eu and NIR emission of Ir–Yb complex at RT were rather long, being 1153 and 17 μs, respectively (Figure 1; Supporting Information Figures S10 and S11; Table 1). The measured absolute quantum yield of the visible emission in Ir–Eu was 11%. According to τ/τ0 (τ0 represents the natural decay lifetime of 2F5/2 state of Yb3+ ∼2 ms), the quantum yield of the NIR emission in Ir–Yb was calculated to be 0.85% ( Supporting Information Table S3). Meanwhile, the decay lifetimes for the remaining Ir-centered MLCT emission from Ir-metalloligand were much shorter (252 ns and 490 ns), compared with pure LIr metalloligand (617 ns). This indicated that the Ir→Ln (d→f) ET occurred from the LIr metalloligand to the excited f-levels of Eu3+/Yb3+ centers. This is reasonable since the estimated triplet state of LIr from the phosphorescent data of Ir–Gd complex at 77 K of ∼18518 cm−1 was suitable for the accepting energy levels of Eu3+ and Yb3+. Figure 1 | DCL of (a) Ir–Eu, (b) Ir–Yb, and (c) Ir–Eu0.75Yb0.25 complexes at room temperature and 77 K. Download figure Download PowerPoint Table 1 | Down-DCL for LIr and Ir–Ln Complexes (Solid State) Compound Em-Ir(III) Em-Ln(III) λmax/nm τa/ns (RT) τa/ns (77 K) λmax/nm τb/μs (RT) τb/μs (77 K) LIr 590 617 3472 / / / Ir–Gd 540 470 3377 / / / Ir–Eu 550 252 689 613 1153 1437 Ir–Yb 550 490 951 980 17 34 aMeasured by 405 nm laser. bMeasured by μs pulsed flashlamp at the excitation of 450 nm. NIR-DCL emission via Ir→Eu→Yb energy transfer cascade Since Ln(III) ions have very abundant energy levels, the probability of finding matched energy gaps among different Ln(III) ions is high. Therefore, in a hybridized system, in which there were multiple Ln-centers, ET among different Ln centers is expected to occur. This prompted us to design trimetallic Ir–Ln1–Ln2 systems to achieve a cascade d→f→f ET process by incorporating both Eu3+ and Yb3+ ions into a coordination system together with Ir-metalloligand. Accordingly, we prepared a series of trimetallic Ir-Eu-Yb samples, as listed in Table 2, which were crystallized in isomorphous structure, as discussed earlier. The ratios of Eu3+ and Yb3+ ions integrated with the trimetallic samples were confirmed by energy-dispersive spectroscopy (EDS), which were, in essence, consistent with the feeding ratios. In the hybridized coordination complexes exemplified in Scheme 1, the Ir-metalloligands acted as both the UV–vis absorption antenna and ET donor. Since the Eu and Yb-centers were distributed statistically in the Ln-positions, it was possible to accomplish energy immigration from the Eu-center to the Yb-center. Table 2 | Decay Lifetimes for DCL (550 nm, Ir-Centered; 613 nm, Eu-Centered; 980 nm, Yb-Centered) and UCL (613 nm, Eu-Centered) Emissions in Ir–Eu–Yb Compared with Ir–Eu and Ir–Yb Complexes (RT, Solid-State) τaDCL-550/ns τbDCL-613/μs τbDCL-980/μs τcUCL-613/μs Ir-Eu 252 1153 / n.d. Ir-Eu0.40Yb0.60 281 462 200 501 Ir-Eu0.45Yb0.55 261 596 369 521 Ir-Eu0.50Yb0.50 256 607 394 551 Ir-Eu0.60Yb0.40 269 658 441 486 Ir-Eu0.75Yb0.25 299 911 553 845 Ir-Eu0.80Yb0.20 268 685 305 593 Ir-Yb 490 / 17 / aMeasured by 405 nm laser. bMeasured by μs pulsed flashlamp at the excitation of 450 nm. cMeasured by 980 nm CW laser. The decay lifetime study at RT provided more evidence regarding the ET process in these complexes. For Ir–Eu–Yb system with various Eu:Yb metal ratios, the lifetime of the Yb-based emission increased considerably to several hundred microseconds, compared with the 17 μs in Ir–Yb; while the lifetime of the Eu-based emission was much shortened accordingly ( Supporting Information Figure S12, Table 2). These results indicate that the ET from LIr to Eu3+ could immigrate further to Yb3+, leading to an obvious increase of Yb-centered NIR decay lifetime. The Eu→Yb ET efficiency (η) through space could be estimated using the following Equation (1)26: η = 1 − ( τ / τ 0 ) = 1 / [ 1 + ( R / R 0 ) 6 ] (1)where R0 is the distance required for 50% ET, R is the actual distance between donor and acceptor, and τ and τ0 are the lifetimes of the donor when acceptor exists or not. According to our decay testing results, τ0 is the lifetime of Eu3+ at 613 nm (1153 µs) in Ir–Eu complex and τ is the observed lifetime of Eu3+ at 613 nm in the series of Ir–Eu–Yb complexes with different metal ratios (462, 911 µs, Table 2). Using Equation 1, the Eu→Yb ET efficiency (η) was calculated to be in the range of 0.21–0.60 ( Supporting Information Figure S13). It was fantastic to find that, via the efficient Ir→Eu→Yb cascade ET process, the longest decay lifetime of 553 μs could be achieved for the Yb3+-based DCL-NIR in Ir-Eu0.75Yb0.25 complex, which is amongst the topmost in ever-reported Yb-complexes at such ambient conditions of RT and in the solid-state.27–30 TPEF–visible emission via Ir→Eu energy transfer TPEF from the TPA (two-photon absorption) the process of antenna chromophore has recently attracted widespread attention.31–36 This mechanism could be applied for extending the wavelengths required for the sensitization of lanthanide compounds to avoid the usage of short-wavelength irradiation, thereby, reducing damage in deep bioimaging applications. In our present scenarios, for LIr, Ir–Eu, and Ir–Eu–Yb compounds, TPA-induced emission spectra were acquired using the femtosecond laser over a broad excitation wavelength region from 820 to 920 nm (Figure 2 and Supporting Information Figure S14). As we could see, for LIr, broad Ir-centered emission with a maximum at ∼590 nm was produced. Meanwhile, for Ir–Eu and Ir–Eu–Yb complexes, the characteristic Eu-centered sharp-peaked with the strongest one at ∼613 nm, which were evident with some retained emission from Ir-metalloligand. Accordingly, these TPEF contours were consistent with the one-photon DCL emission. Furthermore, the TPEF intensities for the compounds were recorded at varying excitation powers, clearly showing a quadratic correlation slope of ∼2 on a log(I)–log(P) plot. This confirmed that the observed emission was generated by a nonlinear two-photon excitation pathway. Due to the equipment limitation, the TPA-involved NIR emissions from the Yb-based centers were not tested, which might be explored further in future studies. Figure 2 | Power-dependent TPEF spectra (λex =900 nm), and Log(I)–log(P) relationship of (a) Ir–Eu; (b) Ir–Eu0.45Yb0.55; and (c) LIr, respectively (Conditions: fs pulsed laser, solid-state, room temperature). Download figure Download PowerPoint UCL–visible emission via Yb→Eu energy transfer From the solid-state UV–vis–NIR absorption spectra of Ir–Eu–Yb complexes ( Supporting Information Figure S7b), we found a typical absorption peak ascribed to the 2F7/2→2F5/2 transition of Yb3+ at 980 nm. Therefore, we tested the UCL of the complexes from the point of Yb-absorption, followed by Yb→Eu ET. Using a continuous-wavelength (CW) laser excitation of 980 nm, and under mild conditions of solid-state at RT, the Ir–Eu–Yb samples with different metal ratios showed strong UCL (Figure 3, Supporting Information Figures S15 and S16). Decay lifetimes of the Eu-centered UCL for various samples are listed in Table 2. As could be seen, the decay lifetimes for the UCL-related 5D0 excited states of Eu3+ were mostly in accordance with the DCL-related ones (the 3 rd and 5th column of Table 2). The longest decay curve conformed well to a single-exponential function with a decay time of 845 μs in Ir-Eu0.75Yb0.25 complex. As far as we know, such sensitized Eu3+-centered UCL by Yb3+ as antenna irradiation using CW 980 nm laser has not yet been reported in Yb/Eu coordination system, different from the widely studied rare-earth-doped nanoparticles, which are easy to realize UCL.37 Figure 3 | (a) UCL spectra at different power, (b) Log(I)–Log(P) relationship of Ir–Eu0.75Yb0.25, and (c) Decay curves for Eu-centered UCL in Ir–Eu–Yb complexes with different metal ratio (Conditions: 980 nm continuous wavelength laser, solid-state, room temperature). Download figure Download PowerPoint To further investigate the UCL mechanism in hybridized Ir–Eu–Yb complexes, the relationship between pumping power and luminescent intensity was measured. Generally, for an unsaturated up-conversion process, the emission intensity strongly relies on its local environment and obeys an empirical relation that is given as38,39 I ∝ P n where n represents the number of photons absorbed in per up-converted process. As could be seen, the dependence of log(I) upon log(P) gives a slope of n = 1.95 for the characteristic UCL emission of Eu3+ in Ir-Eu0.75Yb0.25 complex. These facts suggested that the generation of the emission resulted from a nonlinear UCL process via the intermediate excited states and ET between Yb3+ and Eu3+ centers. For further clarification, we measured the UCL spectra of the Ir-Eu sample, with the results shown in Supporting Information Figure S17. We observed that there was no signal of the characteristic emission peaks of Eu3+ if without Yb3+. Also, it was necessary to identify whether the ET that happened in the ternary system was via intermolecular interaction between Ir–Eu and Ir–Yb or via intramolecular interaction between Eu3+ and Yb3+ centers. For precise comparison, we measured the luminescence of mechanically mixed powder samples by grinding crystals of Ir–Eu and Ir–Yb. The emission of the mechanical mixture of Ir–Yb under 980 nm CW laser was almost the same as that of Ir–Eu, and without the characteristic red UCL emission of Eu3+ ( Supporting Information Figure S18), confirming that efficient UCL could only occur in the molecular-based Ir–Eu–Yb ternary system via intermolecular ET. Based on the earlier analysis, the UCL observed in Ir–Eu–Yb complexes could be demonstrated further as an energy transfer up-conversion (ETU, Supporting Information Figure S19) in which the absorption at 980 nm of Yb3+ was followed by ET to the intermediate state of Eu3+. Subsequently, the second absorption of another Yb3+ led to the population at a higher energy level of 5D0. Then there was the release of the characteristic emission when going back to the 7FJ (J = 0∼6) states. Collectively, our results led us to put forward a general mechanism involving three different kinds of ET and DCL/UCL/TPEF processes (Scheme 1), as follows: First, the Ir-metalloligand absorbs single photons in the UV–visible region (200–500 nm). The electrons transit to the singlet excited state (1MLCT), and then the intersystem crosses to the triplet state (3MLCT). Since the 3MLCT energy of Ir-metalloligand is appropriate for the acceptable level of Eu3+ (ΔE (3MLCT−5D0) = 1018 cm−1), Eu-centered emission could be released when going back to the ground state. Alternatively, since the 5D0 level of Eu3+ is suitable for ET to the 2F5/2 state of Yb3+ (ΔE (5D0−2F5/2) = 7250 cm−1), part of the energy of Eu3+ could continue to be transmitted to Yb3+. That is, cascade Ir→Eu→Yb ET could happen in the ternary coordination system. Due to this additional ET progress, significant enhancement in the NIR-DCL emission of Yb3+ is achieved in Ir–Eu–Yb complexes. Thus, ultralong decay lifetimes are obtained under very mild conditions, different from the ever-reported cases usually needing low temperature or deuterated solvents. Second, based on the fact that the Ir-metalloligand could absorb two-photon excitation in a wide range of near-infrared wavelengths, we also get TPEF emission from the Eu3+ center via a two-photon related Ir→Eu ET process. Third, in the present Ir–Eu–Yb complex, Yb3+ could also serve as a suitable up-conversion sensitizer. Therefore, Yb3+ centers were utilized to absorb 980 nm photons emitted by a CW laser. Moreover, via efficient Yb→Eu ET, the UCL emission of Eu3+ center could be realized readily at an extremely low power of 0.616 W/cm2 under mild conditions. As far as we know, this is the first example of realizing DCL, UCL, and TPEF emissions in a single-molecular level coordination system. More importantly, both the DCL and UCL emissions were extremely efficient, considering that the ultralong decay lifetimes and the applied ultralow CW laser power were achieved under mild conditions of RT and solid-state. Analyzing the beneficial effect attributable in this ternary Ir–Eu–Yb system, the following factors might be accountable. First, the Ir-metalloligand afforded efficient absorption in long-wavelength and matched triplet energy states with the acceptable levels of lanthanide ions. Therefore, an efficient d→f ET could happen either through a one-photon or two-photon process. Second, the F-modification on the ligand led to a reduced nonradiative quenching, and hence, stabilized the intermediate excited states, which facilitated the emission of both DCL and UCL of the lanthanide ions. Last but not least, the Ln-hybridization in the system resulted in abundant ET pathways. Therefore, ET with opposite photon conversion directions (Eu→Yb or Yb→Eu) could happen under different excitation conditions, and both UCL and DCL were thus, achieved in this all-in-one system efficiently. Considering the good performance of this multimodal luminescent materials, we anticipated potential applications in high-level anticounterfeiting, bioimaging, and related fields, and corresponding studies are ongoing in our lab. Conclusion The hybridized coordination of Ln(III) ions with Ir-metalloligand results in the ternary Ir–Eu–Yb molecular system, leading to the co-benefit of three factors. That is, efficient long-wavelength absorption and matched donating states from Ir-metalloligand, reduced nonradiative quenching and stabilized intermediate states due to F-modification, and abundant ET pathways with opposite photon conversion directions by Ln-hybridization. As a result, three different kinds of ET and photoluminescence processes could be achieved under mild conditions of RT and solid-state. Namely, NIR down-conversion luminescence (NIR-DCL) of Yb3+ center with ultralong decay lifetime via cascade Ir→Eu→Yb ET, red UCL of Eu3+ center via Yb→Eu ET requiring ultralow CW laser power, and TPEF from Eu3+ center via Ir→Eu two-photon ET. 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