Racemates Have Much Higher Solid-State Fluorescence Efficiency

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Racemates Have Much Higher Solid-State Fluorescence Efficiency than Their Levo- and Dextrorotary Enantiomers Qiuhua Zhu, Chenshu Dai, Cuihong Huang, Sichao Zheng, and Yuanxin Tian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09170 • Publication Date (Web): 19 Oct 2017 Downloaded from http://pubs.acs.org on November 3, 2017

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The Journal of Physical Chemistry

Racemates Have Much Higher Solid-State Fluorescence Efficiency than Their Levo- and Dextrorotary Enantiomers Qiuhua Zhu,* Chenshu Dai, Cuihong Huang, Sichao Zheng and Yuanxin Tian* School of Pharmaceutical Sciences, Southern Medical University, 1838 Guangzhou Avenue North Guangzhou 510515, China. E‐mail: [email protected], [email protected]

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ABSTRACT

C6-unsubstituted tetrahydropyrimidines (THPs) are compounds with a chiral carbon and strong aggregation-induced emission. The fluorescent properties of their racemates have been studied in detail, but those of their enantiomers have not. Here, the solid-state fluorescent properties of the racemates and enantiomers of four chiral tetrahydropyrimidines (THPs 1–4) have been investigated by the steady-state and time-resolved fluorescence, single-crystal X-ray structures, HOMOs and LUMOs of their 7 racemic (three of them are polymorphs), 4 R- and 3 S-enantiomeric crystals. It was found that the R- and S-enantiomers of 1–4 can self-assemble as RS-paired, RS- or RR/SS-overlapped mode in their racemates, and as the same RR/SS-overlapped mode in their R- and S-enantiomers. Unexpectedly, the solid-state fluorescence quantum yields (ΦSF) of racemic 1–4 could increase to 93, 48, 80 and 100%, respectively, via a suitable heteroenantiomeric self-assembly; but the ΦSF values of their seven enantiomers are only 25–46% owing to much larger non-radiative rate constants than those of their racemates. This means that heteroenantiomeric self-assembly can be used as a new efficient method enhancing ΦSF values. The advantage of racemates is firstly reported and expected to encourage the development and application of racemates as a new kind of fluorescent materials.


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INTRODUCTION Organic fluorophores have wide applications such as organic light-emitting diode,1,2 fluorescent chemical probes3 and bioimaging.4 Conventional fluorophores usually have an aggregation-caused quenching (ACQ) effect, that is, highly emissive in dilute solutions but weaker or even no emissive upon aggregation owing to formation of excimers,5 which greatly limits their practical application. Although many methods, such as introducing bulky substituents to prevent from π- packing6-8 and donor-accepter moiety to form Jaggregation,9-11 and designing molecule forming cross dipole packing mode,12 have been developed to prevent ACQ effect, it is still a difficult problem to be solved. In 2001, unusual aggregation-induced emission (AIE) phenomenon, that is, no emission in solution but high emission upon aggregation, was found.13 The AIE characteristics overcome the ACQ effect fundamentally and have showed great advantages in many areas.14 However, not all AIE compounds have high solid-state fluorescence quantum yields (ΦSF). The method of enhancing ΦSF values is still needed in practical application. In 2011, we developed a convenient five-component reaction for the synthesis of novel racemic C6-unsubstituted tetrahydropyrimidines (THPs) with strong AIE characteristics.15,16 In addition to AIE characteristics, THPs show sensitive mechano-fluorochromism,17 unusual thermo-stimuli fluorescence response18 and no emission in surfactant micelles,19,20 and can be used as specific endoplasmic reticulum imaging dye.21 What about the optical properties of enantiomeric THPs? We noticed that there are lots of researches on the preparation of pure enantiomers22, 23 and the determination of chiral purity,24,25 but the comparison of the fluorescent properties of racemates and their homoenantiomers has not been reported. Therefore, the research on the fluorescence properties of racemic and enantiomeric THPs is very important for understanding the influence of chiral structure on optical properties. However, the research could not be conducted until a chiral resolution company could



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successfully separate racemic THPs. The optical properties of the racemates and enantiomers of four THPs (THPs 1–4) were investigated via their 7 racemic crystals (the single-crystal X-ray diffraction data later prove these crystals belonging to crystalline racemates in which the two enantiomers are present in equal amounts in a unit cell26), 4 Renantiomeric and 3 S-enantiomeric crystals (the molecular structures of chiral THPs 14 and their 14 crystals are shown in Figure 1). To our surprise, all these R- and S-enantiomeric crystals have much lower ΦSF values than those of their corresponding RS-racemates. Based on the obtained experimental results, the mechanism of the much higher ΦSF values of racemic THPs is discussed. Here, we report these results.

Figure 1. Molecular structures of chiral THPs 14 and their racemic, R- and S-enantiomeric single crystals under UV light (365 nm). Marker bars are 1 mm. Symbols (RS), (R) and (S), respectively, represent racemate, R-enantiomer and S-enantiomer, and symbols b and c, respectively, represent blue and cyan fluorescence. EXPERIMENTAL SECTION 1. General


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All the chemicals used in this paper were obtained from commercial suppliers and used without further purification. All melting points were taken on a XT-4 micro melting point apparatus and were uncorrected. 1H NMR (400 MHz) and 13C NMR (100.6 MHz) spectra were recorded by a Bruker Avance 400 MHz NMR spectrometer and using CDCl3 as solvent and TMS as internal standard. Photos were taken by Canon PC1356. Absorption as well as fluorescence excitation and emission properties were analyzed by UV-5500PC and Shimadzu RF5301PC spectrofluorophotometer, respectively. The absolute fluorescence quantum yields were measured using calibrated integrating sphere (Hamamatsu, Quantaurus-QY). Enantioseparation was conducted by Guangzhou Research and Creativity Biotechnology Co., Ltd (http://www.chiralse.com/en/about1.html) (Waters ACQUITY UPC2, ODA5007 chiral column, CO2 /MeOH= 90/10 mobile phase, Column specification 4.6*150 mm, flow rate 2.5 mL /min, back pressure 2000 Psi, pressure drop 751 Psi, check wavelength 241 nm, 40 C). Optical rotation was determined by WZZ-2B automatic polarimeter (SGW-1, error  0.02°). 2. Synthesis and enantioseparation of racemic THPs 1–4. Racemic THPs 1–4 were prepared by the 5CR that we previously reported.16 The R- and Senantiomeric THPs 1–4 were obtained through the chiral resolution of their racemates conducted by Guangzhou Research and Creativity Biotechnology Co., Ltd.. 3. Preparation of the single crystals of racemic, R- and S-enantiomeric THPs 14 The single crystals of racemic, R- and S-enantiomeric THPs 14 except (R)-1 (R-enantiomeric 1 is viscous oil) were prepared by recrystallization from dichloromethane-n-hexane (racemic 1–4, R- and S-enantiomeric 1, 2 and 4) or ethyl acetate-n-hexane solutions (R- and S-enantiomeric 3) at about 4ºC for different times. The detailed procedures are: dissolving THPs using

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dichloromethane or ethyl acetate as little as possible, and then adding n-hexane in the THP solution until almost saturation and keeping at about 4 ºC for different time. RESULTS AND DISCUSSION1. Physical and optical properties of racemic, R- and Senantiomeric THPs 1–4 in crystals. The physical and optical properties of these racemic, Rand S-enantiomeric 14 in solutions and crystals were firstly investigated. Enantiomeric THPs 1–4 have the same H1 and C13NMR spectra as those of their racemates16 (See copies of their H1 and C13NMR spectra in SI), which means that the structure characteristics of the R- and Senantiomers of THPs 14 in racemate and enantimer CDCl3 solutions are the same. All Renantiomeric 14 in solutions are levorotary, and their S-enantiomers in solutions are dextrorotary. These racemic, levo- and dextrorotary 1–4 in cyclohexane solutions have the same energy-lower absorption peak (317 nm) (The absorption spectra of racemic 1–4 and enantiomeric 4 shown in Figure S1A and B) and are practically no emissive (see the emission spectra and photos of the n-hexane solutions of racemates and enantiomers 3 and 4 in Figure S1C and D), which means that the R- and S-enantiomers of THPs 1–4 have the same optical properties in monomers no matter they are in racemate solutions or in pure enantiomer solutions. The melting points (MP), the energy-lower excitation peaks (ex), emission peaks (em), ΦSF, fluorescence lifetimes (), radiative rate constants (kr) (kr=ФF/) and non-radiative rate constants (knr) (knr=(1ФF)/) of racemic, levo- and dextrorotary 1–4 are listed in Table 1. As shown in Figure 2, levoand dextrorotary 24 have almost the same excitation and emission spectra with their em values longer than those of their racemates. The emission spectra of racemic, levo- and dextrorotary crystals 1–4 excited at different wavelengths (Figure S2) indicate that only the emission spectrum of (S)-1 correlates with excitation wavelengths, which proves to arise from two very different conformations (see the single-crystal X-ray diffraction data later), and those of others


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are independent of excitation wavelengths. Their fluorescence decay profiles are shown in Figure S3. All the fluorescence decay profiles can be well-fitted by a single exponential decay. The  values of enantiomeric 14 are different from those of their racemates, but those of levo- and dextrorotary 2–4 are almost the same. Compared with their corresponding racemates with high ФSF values, enantiomeric 1, 3 and 4 have much larger knr but relative smaller kr values. The melting points of all enantiomeric 14 are much lower than those of their racemates. The specific optical rotations of (-)- and (+)-14 in acetonitrile are shown in Table S1. The H1NMR spectra of racemic and enantiomeric 1–4 are the same and in accordance with those of their racemates reported in our previous work.16 1.0

409 355 386 434 484 502

()-1b ()-1c (+)-1

370 390 469 484

()-2c ()-2c' (-)-2 (+)-2

375 388 472 485

()-3 (-)-3 (+)-3

0.5 0.0 Normalized intensity

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1.0 0.5 0.0 1.0 0.5 0.0 1.0

368 385 459 488 494

0.5 0.0

320

400 480 560 Wavelength / nm

()-4c ()-4c (-)-4 (+)-4

640

Figure 2. Excitation (left) and emission (right) spectra of racemic, levo- and dextrorotary 1–4



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Table 1. Melting points, fluorescent properties of crystalline racemic, levo- and dextrorotary THPs 14. MP /C

exa

em b

d

kr e

knr e

/nm 434

ФF c /% 72

(RS)‐1b

130.0－130.5

/nm 355

/ns 7.2f

1.00

0.39

(RS)‐1c

119.5－120.0

409

484

93

13.2

0.70

0.05

(R)‐(‐)‐1

Viscous oil

(S)‐(+)‐1

74.9－75.1

386

502

28

7.9

0.35

0.91

(RS)‐2c

149.0－149.5

370

469

48

11.2

0.43

0.46

(RS) ‐2c'

148.5－149.0

390

484

28

6.2

0.45

1.16

(R)‐(‐)‐2

137.4－137.6

390

484

28

5.0

0.56

1.44

(S)‐(+)‐2

137.8－138.5

390

484

25

5.2

0.48

1.44

(RS)‐3

161.4－162.2

375

472

80

13.6

0.59

0.15

(R)‐(‐)‐3

138.6－138.9

388

485

46

10.6

0.43

0.51

(S)‐(+)‐3

136.9－137.2

388

485

39

10.5

0.37

0.58

(RS)‐4c

169.1‐169.7

368

459

100

10.4

0.96

0.00

(RS)‐4c'

157.7－158.3

385

488

79

13.9

0.57

0.15

(R)‐(‐)‐4

128.3－128.9

385

494

33

11.1

0.30

0.60

(S)‐(+)‐4

128.7－129.2

385

494

28

11.0

0.25

0.65

THP

a

Energy-lower peak excitation wavelength. bPeak emission wavelength. cAbsolute quantum yield

determined via calibrated integrating sphere, excited at 350 nm. dExcited at 405 nm. eUnit: s– 1

108. fExcited at 320 nm.


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2. Molecular packing modes of racemic, R- and S-enantiomeric THPs 1–4. The molecular packing modes of these racemic and enantiomeric THPs 1–4 in crystals were studied by single-crystal X-ray diffraction. Figure 3AD shows the front view of the molecular packing modes. It can be seen that the R- and S-enantiomers in the racemates can self-assemble in RS-paired mode (1b and 2c), RS-overlapped mode (4c) or RR/SSoverlapped mode (R- and R- or S- and S-enantiomers, respectively, are overlapped) (1c, 2c, 3 and 4c). Although enantiomers 1–4 are packed in the same RR/SS-overlapped mode as their racemates, their general molecular alignments are different from their racemates: the RR- or SS-overlapped geometry has two different orientations in enantiomers 1–4 (the different orientations of purple and purple or cyan and cyan circles) but only one orientation in their racemates (the same orientation of purple and cyan circles); and the molecules next to the RR- or SS-overlapped molecules are aligned as zigzag lines in levo and dextrorotary 4 owing to their R- or S-enantiomers unusually aligned along two different axes (Figure 3D and Figure S4). The main crystallographic data of racemic and enantiomeric 1–4 are listed in Table S2 and S3.



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Figure 3. Front view of the molecular stacking modes of racemic, levo and dextrorotary 1–4. Hydrogen atoms have been omitted for clarity. Cyan and purples balls, respectively, represent the R- and S-configurations of chiral carbon. CCDC 1560057–1560065 and 1565648 for (S)-(+)1, (R)-(-)-2, (S)-(+)-2, (R)-(-)-3, (S)-(+)-3, (RS)-3, (R)-(-)-4, (S)-(+)-4, (RS)-4c' and (RS)-4c, respectively; CCDC 818026 and 27 for racemic 1c and 1b, respectively;16 CCDC 850811 and 1011038 for racemic 2c′ and 2c, respectively.27


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3. Intermolecular interactions in the single crystals of racemic, R- and Senantiomeric THPs 1–4. Figure S5 displays the top view of the RR/SS-overlapped molecules. It can be seen that the R- and S-molecules, respectively, are arranged in zigzag lines and connected by the same chiral C-H...O (marked as a) as we previously reported.27 In addition a, there are C–H…π and/or ary C–H…O among some of these RR/SSoverlapped molecules. Besides, between the adjacent RR- or SS-overlapped molecules, there are aryl C–H…O in enantiomeric 2 (Figure S6A); strong intermolecular ring interactions in enantiomeric 4 (Figure S6B). Short-range ring interactions exist among all the zigzag-arranged molecules (Figure S7). The top view of the RS-paired molecules27 is shown in Figure S8A and B. The new found RS-overlapped mode has similar molecular alignment (Figure S8C) to that of RR/SS-overlapped molecules (arranged in a single line and connected by chiral C-H...O) (Figure S7), and similar intermolecular ring interactions (Figure S9C) to those in the RS-paired mode (Figure S9A and B). The detailed intermolecular interactions are listed in Table S4. 4. Conformations, HOMOs and LUMOs of racemic, R- and S-enantiomeric THPs 1–4 in single crystals. As shown in Figure 4 and S10, all the conformations of Rand S-enantiomers in racemic and enantiomeric 1–4 are highly stereo with three phenyls in completely different spatial directions (up, down and front the central ring) and have an intramolecular weak bond aryl C–H…N and a short-range ring interaction between the centers of phenyl A and C (Table S4). As we previously reported,27 their highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) show the electron conjugations through bond and through different spaces (marked with red, green and yellow circles) with HOMOs mainly located on C=C5-C=O, phenyls A and C,



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LUMOs on O=C-C=C5-C=O and only phenyl C (Figure 5 and S1114). The band gaps (E/ev or /bg/nm) between these HOMOs and LUMOs and the dihedral angles () between phenyl C and -C=C- plane (important factor influencing bg) are shown in Table S5. The shorter bg values (278–313nm) in crystals than the ab value (317 nm) in cyclohexane solution indicate the conformations in crystals are more twist than those in cyclohexane solution. It is worth mentioning that all racemic and enantiomeric 1–4 have only one conformation in crystals except enantiomeric 1 and 3 have two different conformations. With the same molecular structure and packing mode, the bg values of Rand S-enantiomers in the racemate are the same and similar to those in their enantiomeric counterparts except one of the conformations in (+)-1 is very different from another one.

Figure 4. Molecular conformations in racemic and enantiomeric 4. r and  in (RS)-4c: 5.299 Å, 32.93º; in (RS)-4c′: 5.299 Å, 32.93º; (R)-(-)-4: 5.348 Å, 34.49º; (S)-(+)-4: 5.334 Å, 34.24º.


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Figure 5. Conformations, HOMOs and LUMOs of racemic, levo and dextrorotary 4 in single crystals. The through-space conjugations in different areas are marked in different circles. 5. Origin of the fluorescence efficiency of racemic THPs much higher than those of their enantiomeric counterparts Our previous work27 indicated that the em values of racemic THPs mainly depend on the packing modes of their R- and S-enantiomers, and then are influenced by intramolecular dihedral angles () between phenyl C and -C=C- plane and through-space electronic conjugation to some extent. This case is true for the em values of racemic and enantiomeric HPs 14: the em values of the polymorphs formed by different R- and S-packing modes of racemic THPs 1 and 4 have very different em values (em = 50 and 29 nm, respectively), but the racemates and enentiomers with the same RR/SS-overlapped packing mode of all THPs 14 have the same or slightly different em values (em =018 nm). However, the ΦSF values (28–46%) of enantiomeric 1, 3 and 4 are much lower than those (79–100%) of their raecmates 1c, 3 and 4c' and only the ΦSF



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values (28 and 25%) of enantiomeric 2 are the same and slightly lower than that (28%) of their racemate 2c′. Since the fluorescence quantum yield can be calculated by knr and kr, that is, ΦSF =kr/(knr+kr), it can be deduced that the much lower ΦSF values of enantiomeric 1, 3 and 4 originate mainly from their much larger knr values because the knr and kr values of enantiomeric 1, 3 and 4, respectively, are 3.918.2 and 0.40.6 times those of their racemates. For AIE compounds, the restriction extent of intramolecular motions is an important factor influencing knr values.28,29 Our previous work27 indicates the restriction of intramolecular motions caused by the intramolecular ring interaction between phenyl A and C (shorter distance r between the centers of phenyl A and C) can greatly decrease the knr values of racemic THPs and hence increase ΦSF values, which is further proved by the higher ΦSF value (100%) of recemic 4c with shorter r (4.718 Å) than that (79%) of its polymorph 4c′ with longer r (5.188 Å) . The larger knr values of enantiomeric 13 are expected to be mainly caused by the weaker restriction (longer r) of their intramolecular motion than those of their racemates, but it is not so for those of levo and dextrorotary 4 because their r are shorter than that of their corresponding racemate 4c'. The larger knr values of enantiomeric 4 are expected to be caused by the strong ring interactions with the distance between ring centers shorter than 4Å (3.805 and 3.859 Å, in Figure S6B), which will lead to the formation of excimers5,30,31 and hence promote non-radiative decay process. Since molecular conformations, intra- and intermolecular interactions correlate with general molecular alignments, the difference in the intra- and intermolecular interaction of enantiomeric and racemic THPs should originate from the differences in the general molecular alignments (the different orientations of RR/SS-overlapped geometries in Figure 3) and space group (Table S2 and 3) of racemic and enantiomeric THPs caused by chiral structure. CONCLUSION


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We have studied the solid-state fluorescent properties of the racemates, R- and Senantiomers of chiral THPs 1–4 via their seven racemic crystals (three of the crystals are new) and seven new enantiomeric crystals. It was found that the R- and S-enantiomers in racemates 1–4 can self-assemble in RR/SS-overlapped, RS-paired or RS-overlapped mode with the ΦSF values of racemic THPs 1–4 up to 93, 48, 80 and 100%. Enantiomeric 1–4, packed in the same RR/SS-overlapped mode as those in their racemates, have unexpectedly much lower ΦSF values (25–46%). This is mainly because the heteroenantiomeric self-assembly can efficiently suppress non-radiative decay process (much smaller knr values), but the homoenantiomeric self-assembly promotes nonradiative radiative decay process (much larger knr values) owing to their differences in the general molecular alignments and space group caused by chiral structure. The efficient method of enhancing ΦSF values by simple heteroenantiomeric self-assembly is first reported and expected to be suitable for other organic compounds rather than only THPs. Since organic racemates can be easily prepared by chemical synthetic methods, this work is expected to encourage the development and application of racemates as a new kind of fluorescent materials. The investigations into the structure characteristics essential for such efficient heteroenantiomeric self-assembly and the applications of racemic THPs as fluorescent probes are under way in our group. ASSOCIATED CONTENT Supporting information Electronic Supplementary Information (ESI) available: Determination of optical rotation and calculation of specific rotation, 1H NMR spectra of racemic and enantiomeric



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THPs 14,

13

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C NMR spectra of racemic and enantiomeric THPs 3 and 4, Table S1S5,

Figures S1S14, and ten cif files of CCDC 15600571560065 and 1565648. ACKNOWLEDGMENTS We thank professor Hui Zhang for her recommendation to the chiral separation of racemic THPs. This work was supported by the Science and Technology Program of Guangdong Province (2015A010105015) and the National Natural Science Foundation of China (21272111). REFERENCES (1) Yao, L.; Zhang, S. T.; Wang, R.; Li, W. J.; Shen, F. Z.; Yang, B.; Ma, Y. G. Highly Efficient near-Infrared Organic Light-Emitting Diode Based on a Butterfly-Shaped Donor-Acceptor Chromophore with Strong Solid-State Fluorescence and a Large Proportion of Radiative Excitons. Angew. Chem. Int. Ed. 2014, 53, 2119-2123. (2) Reineke, S.; Lindner, F.; Schwartz, G.; Seidler, N.; Walzer, K.; Lussem, B.; Leo, K. White Organic Light-Emitting Diodes with Fluorescent Tube Efficiency. Nature 2009, 459, 234-238. (3) Yin, J.; Hu, Y.; Yoon, J. Fluorescent Probes and Bioimaging: Alkali Metals, Alkaline Earth Metals and Ph. Chem. Soc. Rev. 2015, 44, 4619-4644. (4) Stich1, M. I. J.; Fischer1, L. H.; Wolfbeis1, O. S. Multiple Fluorescent Chemical Sensing and Imaging. Chem. Soc. Rev. 2010, 3102-3114. (5) Jenekhe, S. A.; Osaheni, J. A. Excimers and Exciplexes of Conjugated Polymers. Science 1994, 265, 765-768. (6) Ozdemir, T.; Atilgan, S.; Kutuk, I.; Yildirim, L. T.; Tulek, A.; Bayindir, M.; Akkaya, E. U. Solid-State Emissive Bodipy Dyes with Bulky Substituents as Spacers. Org. Lett. 2009, 11, 2105-2107.


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Table of Contents Graphic


Racemates Have Much Higher Solid-State Fluorescence Efficiency

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