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Elucidating the Effects of Fluoro and Nitro Substituents on Halogen Bond Driven Assemblies of Pyridyl-capped #-Conjugated Molecules Suong T Nguyen, Arnold L. Rheingold, Gregory S. Tschumper, and Davita L. Watkins Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b01321 • Publication Date (Web): 11 Oct 2016 Downloaded from http://pubs.acs.org on October 15, 2016

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Crystal Growth & Design

Elucidating the Effects of Fluoro and Nitro Substituents on Halogen Bond Driven Assemblies of Pyridyl-capped -Conjugated Molecules Suong T. Nguyen a, Arnold L. Rheingold b, Gregory S. Tschumper a, and Davita L. Watkins a* a

Department of Chemistry and Biochemistry, University of Mississippi, University, MS 38677-

1848, USA b

Department of Chemistry, University of California, San Diego, La Jolla, CA 92093-0358, USA

ABSTRACT. Among recent advances towards efficient semiconducting materials, rational design guidelines have emerged focusing on the synergy between various intermolecular interactions to improve the solid-state order of -conjugated molecules in organic electronic devices. Herein, we focus our attention on halogen bonding (XB) interactions and the crucial role of electron withdrawing substituents (e.g., nitro and fluoro) towards influencing solid-state properties via secondary interactions. Employing iodoethynyl benzene derivatives (F2BAI and (NO2)2BAI) and thiophene/furan-based building blocks equipped with pyridyl groups as self-assembling domains (PyrTF and PyrT2), co-crystals driven by XB and -stacking interactions were formed and studied. Spectroscopic and thermal analysis of 1:1 mixtures provide initial evidence of cocrystallization. X-ray crystallography affords the inherent solid state packing motifs within each

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assembly. Computational studies support experimental observations, revealing the dominant interactions and contribution of each substituent group towards increasing the stability of the resulting assemblies.

INTRODUCTION Progression in the field of organic electronic devices has been stimulated by the development of novel π-conjugated molecules as applicable optoelectronic materials. Devices such as organic light-emitting diodes (OLEDs), organic field-effect transistors (OFETs), and organic photovoltaics (OPVs) rely on the optoelectronic properties of molecules possessing rigid π-conjugated backbones, HOMO-LUMO gaps ranging from 1.5 to 3 eV, and solid-state architectures to generate and/or transport charges.1-3 Rational design guidelines have emerged focusing on the effects of solubilizing groups,4 acceptors,5 and molecular structure6 as they influence intermolecular orbital overlap and effective -conjugation. Particular attention is given to both inter- and intramolecular interactions as they largely impact the dimensionality of the electronic packing structure.7,8 The synergy between these interactions has proven to be crucial to solid-state properties; and, thus, performance of the organic device. Among recent advances towards efficient semiconducting materials, general strategies towards nanoscale architectures and enhanced performance have deployed noncovalent interactions.9-14 For instance, Yagai et al., employ barbituric acid hydrogen bonding (HB) units and π-π interactions to self-assemble oligothiophene derivatives into highly ordered nanorods.15,16 Upon mixing with PC61BM, the resulting nanostructures exhibit power conversion efficiencies comparable to those of the most commonly employed polymeric material, poly(3-hexylthiophene) (P3HT). Similarly, Würthner et al., account for the construction of various complex supramolecular architectures via π-scaffolds and highlight their applications in organic electronics and photonics as well as


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photovoltaics.17 Noteworthy are applications of halogen bonds, analogues of hydrogen bonds, in which highly directional, multidimensional assemblies can be fabricated from the cocrystallization of two or more different organic π-conjugated molecules.18,19 Halogen bonding (XB) is a type of sigma-hole (-hole) interaction.20-22 It is best described as an electrostatic interaction between an electron deficient area on the outer p orbital of a halogen (i.e., XB donor) and the electron rich site of a counter atom (i.e., XB acceptor). Typical atoms that serve as XB acceptors are oxygen, sulfur, and/or nitrogen. The strength of a XB interaction with a given electron rich site correlates to the degree of electron deficiency on a localized area of the halogen atom.23 The magnitude of the -hole interaction is dependent upon the polarizability of the halogen atom (XB donor) and the electron releasing ability of the XB acceptor.21,24 Likewise, the electron withdrawing and releasing properties of substituents on the remainder of the molecule affect the XB interaction. The charge distribution within the entire molecule plays a crucial role in the overall stability of the system.

Figure 1. Top image: Molecular structure of XB acceptors, PyrTF and PyrT2, and donors, F2BAI and (NO2)2BAI. Bottom image: X-ray crystal structure of the complex of halogen bond donor and acceptor (contacts indicated by grey dotted line).


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Such trends are relevant towards identifying suitable XB donors and acceptors that can be integrated into π-conjugated backbones as self-assembling units. As a result, by exploiting the XB interactions, well-defined assemblies can be engineered and further used to elucidate the impact of substituents on packing patterns and identify competing interactions. Herein we report the analyses of single crystal structural data for a series of XB driven co-crystals comprised of optoelectronic building blocks and iodoethynyl benzene derivatives. Figure 1 shows the chemical structure of the four molecules of study. Prepared as oligomeric moieties for the construction of single crystal organic semiconductor devices, 4-(5-(furan-2-yl)thiophen-2-yl)pyridine (PyrTF) and 4-([2,2'-bithiophen]-5-yl)pyridine (PyrT2), were co-crystallized with 1-(iodoethynyl)-3,5difluorobenzene (F2BAI) and 1-(iodoethynyl)-3,5-dinitrobenzene ((NO2)2BAI). The XB acceptors, PyrTF and PyrT2, represent derivatives of the most common building blocks used in the development of semiconducting materials.25 The backbone of repeating heterocycles— thiophene and furan—possesses high planarity and rigidity, necessary for intermolecular overlap of -orbitals.26,27 Incorporation of the pyridyl group as the halogen bond moiety guides the selfassembly of the molecules to yield highly directional XB induced assemblies which stack via 𝜋-𝜋 interactions. Due to the polarizability of the atom, iodine-based molecules F2BAI and (NO2)2BAI were selected as XB donors.28 The inductive effect provided by the fluoro and nitro substituents increases the magnitude of -hole bonding. In addition, these electron withdrawing groups introduce other attractive intermolecular contacts (e.g., CH···O/F). These secondary interactions are far weaker than the dominant XB and -stacking contacts;29 however, they contribute to the overall stability of the resulting assembly. A total of four co-crystals were formed using vapor diffusion and validated via thermal and spectroscopic means: PyrTF-F2BAI, PyrT2-F2BAI, PyrTF-(NO2)2BAI, and PyrT2-(NO2)2BAI (Fig. 1). The presence of XB and – stacking was


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confirmed experimentally via X-ray crystallography. Further analysis via density functional theory (DFT) computations supports experimental observations and reveals the dominant energetics within the observed assemblies. Attention has been directed towards the inter- and intramolecular interactions present in each of these molecules to help improve our understanding of secondary interactions and the intricate molecular/solid-state structure property relations crucial for enhancing the performance of organic electronic devices.

EXPERIMENTAL SECTION Reagents and solvents were purchased from commercial sources and used without further purification unless otherwise specified. Infrared spectra were recorded with an Agilent Cary 660 ATR-FTIR. A Fisher-Johns melting point apparatus was used to determine melting points. Additional synthetic details, summary of theoretical calculations, structural figures, TG/DTA plots, and X-ray crystallographic tables containing bond distances and angles can be found in the SI. Thermogravimetric Analysis. Measurements were performed on Seiko Instruments TG/DTA 6200 (platinum pan, room temperature to 600 C, ramp rate of 20 Cmin-1 under nitrogen atmosphere) and analyzed on TG/DTA Highway Conversion Software. X-ray Crystallography. Crystal evaluation and data collection were performed on a Bruker Kappa diffractometer with Mo Kα (λ = 0.71073 Å) radiation. Reflections were indexed by an automated indexing routine built in the APEXII program suite. The solution and refinement were carried out in Olex2 version 1.2 using the program SHELXTL.30,31 Non-hydrogen atoms were refined with anisotropic thermal parameters while hydrogen atoms were introduced at calculated positions based on their carrier/parent atoms. Crystal data and structure refinement parameters for


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all compounds are given in the SI. The CCDC numbers for the single crystal X-ray structures of each co-crystal is as follows: 1502497, 1502498, 1502499, and 1502500. Theoretical Methods. The M06-2X32 global hybrid density functional was employed in conjunction with a triple- correlation consistent basis set augmented with diffuse functions on all atoms and a relativistic pseudopotential for the iodine centers (aug-cc-pVTZ-PP)33-36 to compute the electronic interaction energies (Eint) of the various pairwise contacts observed in the crystal structures. The interaction energies were calculated by comparing the electronic energy of a pair of fragments from the crystal structure to the energies of the corresponding isolated fragments. All computations were performed with the Gaussian 0937 software package (Rev: D.01) and used atomic coordinates obtained from the crystal structures.

RESULTS AND DISCUSSION Preliminary Investigations and Crystal Growth. XB acceptors and donors were synthesized according to modified literature procedures (Supporting Information, SI).38-40 Co-crystals were prepared in duplicate at a 1:1 ratio by dissolving each acceptor separately in a chlorinated solvent (dichloromethane or chloroform) and adding it dropwise to a borosilicate glass vial containing the donor. The resulting mixtures were ultrasonicated for 10 minutes. The open vial was placed in a secondary vial containing n-hexane. Using vapor diffusion methods, crystals were allowed to form at -5 oC over 8 days. Confirmation of co-crystallization was observed through a ~10 oC difference in melting point between the co-crystals and the XB acceptor.41,42 For each set of samples, the solvent was allowed to evaporate, completely. The co-crystals obtained from these vials were analyzed using IR spectroscopy. Successful XB interactions between the acceptor and donor were identified using the triple bond peaks of the mixture compared to those of the starting iodoethynyl


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benzene derivative. An outcome of co-crystallization is indicated by a substantial red shift (≥ 12 cm-1) in the band associated with the triple bond of the XB donor when comparing the neat substance to the crystal spectrum. This decrease in frequency is due to a weakening in the triple bond with the formation of the C≡C-I···N halogen bond.42 Comparative analysis of co-crystals containing F2BAI as the XB donor to those consisting of (NO2)2BAI reveals considerable differences between the spectroscopic features and thermal stability of the co-crystals. Red shifts in IR stretching modes for the triple bond in co-crystals containing (NO2)2BAI are relatively larger (≥ 5 cm-1) than those containing F2BAI. The increase in red shift is attributed to the nature of the nitro substituent in comparison to that of fluorine in increasing the -hole on iodine; in turn, strengthening the I···N interaction and weakening the triple bond. Further analysis of (NO2)2BAI exposes higher melting and decomposition temperatures (≥ 100 oC) correlating to the overall strength of the assembly which is warranted by the total number of contacts and energies associated with the various intermolecular interactions present. Additional details of this analysis are provided in the SI (Table S1). These experimental studies utilized to identify signatures of XB provide indirect evidence of successful cocrystallization. Table 1. Crystallographic Information and Selected Structural Featuresa Co-crystal Formula M (g/mol) Temperature (K) Space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg)

PyrTF-F2BAI C21H12F2INOS 491.28 100.0 P21/c 10.681(2) 4.221(2) 40.107(8) 90.00 92.85 90.00

PyrT2-F2BAI C21H12F2INS2 507.34 100.0 P1̅ 4.0288(9) 11.599(3) 20.709(4) 102.07 92.78 93.49

PyrTF-(NO2)2BAI C21H12IN3O5S 545.30 100.0 P21/c 19.084(4) 30.968(6) 14.021(3) 90.00 94.66 90.00

PyrT2-(NO2)2BAI C21H12IN3O4S2 561.36 100.0 P1̅ 8.3335(4) 11.1648(6) 12.4716(6) 101.70 102.30 107.25


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V (Å3) 1805.7 942.7 8259 Z 4 2 16 R factor (%) 2.96 3.51 4.86 a Additional data related to the experimental details can be found in the SI. Structural Properties.

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1037.8 2 2.40

A summary of the crystallographic data is provided in Table 1.

Crystallographic data was needed to elucidate the exact nature of the XB interactions as well as assess the stoichiometry and metrics of the resulting co-crystals (Table 2). Table 2. Geometries of XB Interactions and Reduction Comparisona X···N C−X···N van der Waals (Å) angle (°) reduction (%) PyrTF-F2BAI 2.78 178.7 21.2 PyrT2-F2BAI 2.76 178.1 21.8 PyrTF-(NO2)2BAI 2.67 174.9 24.4 2.76 178.9 21.8 2.74 177.6 22.4 2.72 177.9 22.9 PyrT2-(NO2)2BAI 2.70 174.5 23.5 a Geometries obtained from crystal analysis are in good agreement with those reported in the literature28,39 Co-crystal

Single crystal X-ray analysis provides detail about the 1:1 assembly of PyrTF-F2BAI (Fig. 2). The co-crystal exhibits a monoclinic structure with the P21/c space group. XB interactions afford dimers in which the N∙∙∙I–C angle is 178.7 and the N∙∙∙I distance is 2.78 Å corresponding to a 21.2% shortening relative to the total van der Waals radii of nitrogen and iodine (Table 2). The dimers pack antiparallel to each other along the a-axis participating in slipped-stack arrangement along the b-axis with minimal distances of 4.22 Å. The dihedral angle within the thiophene-furan building block is 174.4, a planarity that is essential to applications in electronic devices.1,2 Acidic protons on the furan moiety afford adjacent columns of dimers that associate via F∙∙∙H (2.47 Å,


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97.9, 156.4) interactions with neighboring fluorines on the XB donor. Such interactions contribute to the isotropic packing behavior of the co-crystal.

Figure 2. (a) Packing diagram conveying the presence of both XB and -stacking in PyrTFF2BAI; (b) Secondary interactions of F∙∙∙H afford a 3D assembly within the co-crystal Alternatively, a 1:1 assembly of PyrT2-F2BAI (Fig. 3) yields crystallization in the triclinic space group P1̅. These dimers pack parallel to each other into two-dimensional arrays along the a-axis participating in co-facial - interactions with minimal distances of 4.03 Å. XB between the pyridyl nitrogen and iodine is characterized by linear N∙∙∙I–C angles of 178.1 and N∙∙∙I distances approximately 21.8% less than the sum of their van der Waals radii. The dihedral angle within the bithiophene building block is 179.6, making it more rigid than the thiophene-furan derivative. Partial site occupancy is observed in which 75% anti conformation for the two thiophene rings is favored. Conventionally, the rotational barrier within bithiophene is diminished affording coplanarity in the solid state.


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Figure 3. Packing diagram in PyrT2-F2BAI showing both XB and -stacking interactions Probing the effects of substitution, the XB donor F2BAI was replaced with (NO2)2BAI. In doing so, short contacts involving the nitro oxygen atoms lead to a series of unique secondary interactions. Co-crystals of PyrTF-(NO2)2BAI (Fig. 4) exhibit a monoclinic structure with the P21/c space group. Crystallographic disorder is evident by the presence of four distinct dimers. One XB pair displays a N∙∙∙I–C angle of 174.9 with a N∙∙∙I distance of 2.67 Å; while a second dimer possesses a XB distance of 2.76 Å with an angle of 178.9 (SI, Table S4). Two additional dimers possess similar XB distances (2.74 Å and 2.72 Å) and angles (177.6 and 177.9); however, the XB acceptor of one of the dimers lacks planarity. The torsion angle between the pyridyl and thiophene ring is 159.0 divergent from the co-planarity observed in PyrTF-F2BAI and PyrT2F2BAI. These variations are due to the nitro groups participating in three different types of short contacts: (1) -system of neighboring acetylene linkages, 3.06 Å; (2) acidic hydrogen atoms, 2.55 Å on the XB acceptor; and (3) lone pairs on the oxygen atom in the furan moiety, 3.08 Å.43,44 As a result, the planarity and order needed for adequate - stacking interactions are reduced limiting the applicability of PyrTF-(NO2)2BAI co-crystals.45


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Figure 4. Disorder in the packing arrangement of PyrTF-(NO2)2BAI co-crystals in which four different XB interactions (3 shown, grey dotted line) alongside secondary contacts are observed Similar secondary interactions are observed for co-crystals of PyrT2-(NO2)2BAI, however changing the XB acceptor molecule gives rise to a more ordered solid-state assembly. Co-crystals of PyrT2-(NO2)2BAI exhibit a triclinic structure with the P1̅ space group. In this case, the sulfur atom in thiophene is less electronegative than oxygen reducing the electropositivity of hydrogens on the ring as well as its affinity to complex with the nitro substituent. The lone pairs on oxygen atoms in (NO2)2BAI interact with the acetylene  system of neighboring XB donors (3.33 Å); however, they act in a more stabilizing manner than that which was surveyed in PyrTF(NO2)2BAI. In PyrT2-(NO2)2BAI, -stacking interactions are 3.87 Å in distance along the b-axis. XB dimers exhibit N∙∙∙I distances of 2.70 Å with N∙∙∙I–C angles of 174.5. These smaller contact metrics are indicative of a more stable XB assembly compared to those of PyrTF-F2BAI, PyrT2F2BAI, and PyrTF-(NO2)2BAI. Thermal data showing that PyrT2-(NO2)2BAI has both higher melting and decomposition temperatures correlate well to support the stabilizing contacts observed (Table S1).


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Figure 5. Fragment of the packing diagram for PyrT2-(NO2)2BAI in which secondary contacts contribute to the stability of the assembly (a); combined effects of XB interactions and -stacking interactions (b) Theoretical Calculations. DFT computations were performed to investigate the relative strength of the interactions forming the assembly and the contributions provided by the XB donor fluoro and nitro substituents. The procedures employed here have been carefully calibrated and shown to reliably reproduce benchmark geometries and dissociation energies for a large set of XB dimers.46 Total contacts within the crystal structure varied amongst each co-crystal with additional consideration of syn and anti configurations observed in XB donor of PyrT-F2BAI and PyrTF(NO2)2BAI. The M06-2X interaction energies of the nearest-neighbor pairwise contacts observed in the crystal structure are summarized in Table 3. Table 3. Summary of Electronic Interaction Energies for Nearest-neighbor Pairwise Contacts Observed in the Crystal Structures

Co-crystal

XB

Average Einta (kcal/mol) Other c

Total Contacts

PyrTF-F2BAI

4

-7.5

-8.7

-0.9 (CH…F)

PyrT2-F2BAI

7b

-7.6

-9.4

—e

PyrTF-(NO2)2BAI

8b

-8.5

-8.0d -2.1 (NO2…H) -2.3 (NO2…)

PyrT2-(NO2)2BAI 5 -8.7 -8.7 -1.8 (NO2…) a Average of M06-2X/aug-cc-pVTZ-PP interaction energies computed with and without BoysBernardi counterpoise procedure. bSyn and anti-configurations of the thiophene-thiophene and thiophene-furan rings were observed; only anti is reported based on partial site occupancy value. c Substantial pi-stacking interactions afforded from the XB acceptor; dStacking interactions deviate substantially from a parallel face-to-face arrangement and exhibits a tilt angle between the rings. e No contributing secondary interactions observed.


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In analyzing the XB-driven solid-state assembly, the distance and angle between the XB donor and acceptor atoms correlate to the strength of the halogen bond. Any additional intermolecular interactions such as those originating from secondary contacts contribute to the thermal stability of the assembly (Table S1). Computational results indicate that the XB interactions and - stacking between the XB donor and acceptor molecules are quite significant and of similar magnitude. Co-crystals containing (NO2)2BAI have higher XB interaction energies due to the inductive effect of the nitro group increasing the -hole on the iodine atom. In addition, these cocrystals exhibit competing interactions (Table S4 and S5) where - stacking between pairs of donor molecules and donor-acceptor pairs are 1-2 kcal/mol stronger than that of XB. The more disordered PyrTF-(NO2)2BAI crystal structure exhibits an appreciably tilted stacking interaction between the PyrTF and (NO2)2BAI molecules (40.5° angle between benzene and thiophene ring planes), rather than a nearly parallel face-to-face configuration (≈ 0° angle between ring planes). Nevertheless, the corresponding Eint is only slightly weaker (-8.0 kcal/mol) than the other - contacts in Table 3 (-8.7 to -9.4 kcal/mol) which is entirely consistent with -type interactions observed in other aromatic systems.47-50 Interestingly, PyrT2-(NO2)2BAI is the only system to exhibit - stacking between the XB donor and acceptor, and it corresponds to the largest interaction energy (-11.0 kcal/mol) of all the contacts examined (Tables S2 – S5). The PyrT2(NO2)2BAI crystal structure also has strong homogeneous stacking interactions between PyrT2 pairs (-8.7 kcal/mol) and (NO2)2BAI pairs (-9.6 kcal/mol). The abundance and collective strength of these - stacking interactions strongly suggest that co-crystals of PyrT2-(NO2)2BAI possess applicable solid-state assemblies.

CONCLUSION


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In summary, we describe the use of common -conjugated building blocks and variations in substituent effects to achieve co-crystals possessing solid-state properties relevant towards applications in organic electronic devices. Spectroscopic and thermal analysis provides indirect evidence of XB interactions. X-ray crystallography and theoretical data of the co-crystals indicates that XB and - stacking provide the dominant forces driving the assemblies; while secondary interactions from substituent groups act in either complimentary or disruptive means. The results of the study speak to strategic application and rational design of XB molecules for organic electronic devices. Device fabrication and conductivity measurements are currently underway. ASSOCIATED CONTENT Supporting Information Supporting materials contain experimental details regarding crystallographic data, details on thermal analysis, theoretical calculations, 1H NMR spectra and supplementary results. Additionally, all crystallographic information files (CIFs) have been attached. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic information files are also available from the Cambridge Crystallographic Data Center (CCDC) upon request (http://www.ccdc.cam.ac.uk, CCDC deposition numbers 1502497-1502500). AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT D. L. W. appreciates financial support of this work from Oak Ridge Associated Universities through the Ralph E. Powe Award. The computational work is supported by the Mississippi Center for Supercomputing Research and the National Science Foundation under Grant Numbers CHE-1338056 (G. S. T.). D. L. W. thanks the University of Mississippi for Laboratory Start-up Funds. REFERENCES (1)

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"For Table of Contents Use Only"

Elucidating the Effects of Fluoro and Nitro Substituents on Halogen Bond Driven Assemblies of Pyridyl-capped -Conjugated Molecules Suong T. Nguyen , Arnold L. Rheingold , Gregory S. Tschumper , and Davita L. Watkins *

Herein, we summarize the results of crystallographic and computational analysis examining the role of electron withdrawing substituents (e.g., nitro and fluoro) on halogen bond driven assemblies of semiconducting building blocks for materials application. Studies indicate that cocrystals of PyrT2-(NO2)2BAI possess solid-state assemblies applicable for organic electronic device applications.


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