Reversibly Stretching Cocrystals by the Application of a Magnetic Field

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Reversibly stretching the co-crystals by the application of magnetic field Yang-Hui Luo, Jing-Wen Wang, Chen Chen, Yao-Jia Li, and Bai-Wang Sun Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00116 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on March 26, 2017

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Crystal Growth & Design

Reversibly stretching the co-crystals by the application of magnetic field Yang-Hui Luo,* Jing-Wen Wang, Chen Chen, Yao-Jia Li and Bai-Wang Sun* †

School of Chemistry and Chemical Engineering, Southeast University, Nanjing, 211189, PR. China.

ACS Paragon Plus Environment


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Abstract: Carefully control of the non-covalent supramolecular interactions for the preparation of organic co-crystal materials with tailored properties is a challenging task. Herein, we report the reversibly stretching of the distances between co-formers (18-Crown-6/4, 5-dicyanoimidazole (co-crystal 1) and (18-Crown-6/1, 2, 4-triazole (co-crystal 2) in solution state, by the application of an external magnetic field with various strength (0.0, 0.5, 1.0 and 1.5 Tesla). As a consequence, the physical/chemical properties of the stretched co-crystals 1-0.5T, 1-1.0T and 2-0.5T have been remarkably altered to close to the individual co-formers, and finally, co-crystals 1 and 2 can be broken under 1.5 and 1.0 Tesla, respectively. Thus, the stretching mechanism of magnetic field can be proposed. This stretching effect, however, may therefore be of great interest in functional organic co-crystal materials that tuned by non-covalent supramolecular interactions.


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INTRODUCTION

Co-crystallization, one of the core of crystal engineering, is an extensively used strategy at the researcher’s disposal to acquire the unpredicted structures with versatile functionalities/chemico-physical properties for self-assembled molecules in solid state,

1-5

for their applications in such field as organic semiconductors,6

nonlinear optics (NLO),7-8 energetic materials,9,10 ferroelectricity,11-14 ambipolar charge transportation,15 long-distance proton transport,16-18 light-driven actuator,19 photo-physics,20,21

conductivity,22

room-temperature

phosphorescence,23

and

especially the pharmaceutical industry.24 The self-assembly of different co-formers can be directed by non-covalent supramolecular interactions,25-27 including hydrogen-bonding, halogen bonding, π-π interaction, electrostatic interaction and van der Waals interactions. As a consequence, it is these non-covalent interactions that play a dominate role in determine the recognition, aggregation, nucleation, growth as well as the stacking patterns of the co-crystals.28, 29 These information are necessary knowledge for a comprehensive understanding of the structure-property relationship, which in turn promote the rationally design of co-crystal materials. Unfortunately, comprehensive understanding of the above-mentioned features are still relatively scarce.30,

31

In this regard, careful control of the non-covalent

supramolecular interactions for the preparation of co-crystal materials with tailored properties remains a challenge.32, 33 It should be notated that the self-assembled molecular co-crystals are inherently dynamic,34-36 in this respect, the co-crystallizations process can be affected by the



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application of external physical-stimulus, such as light,36 pressure6 and magnetic field,38 which thus give rise to supramolecular structures and special properties that are not accessible in the absence of these stimulus. The effect of light or/and magnetic field on nanoscale materials,39 as well as the pressure and electric field on organic semiconductor6 and NLO materials7-8 have been rather well-established. While the application of magnetic field on the crystallizations process is much less investigated. Although, magnetic field have been successfully used to create previously unknown polymorphs of coronene,40 to grow high-quality protein crystals,41 to separate polymorphs,42 to preferentially nucleate the monoclinic form of terpyridine,43 to realize chiral aggregates,44 to align liquid crystal/block copolymer arrays,45 and to melt texturing alloys.46 However, the influences of magnetic field on the co-crystallization of molecules co-crystals still remains un-reported. The host-guest interaction between crown ethers and guest molecules is an important secondary interaction, which can be applied to mimics natural systems or constructs new materials.47 In our previously work, we have investigated in detail the influence of substituent on the secondary interaction in crown ether-ammonium salt.48 Herein, we report the influence of magnetic field with different strength (0.0, 0.5, 1.0 and 1.5 Tesla) on the co-crystallization of 18-crown-6 (18-C-6) with 4, 5-dicyanoimidazole (4, 5-dci), which result in different co-crystals 1-0.0T, 1-0.5T, 1-1.0T and 1-1.5T (Scheme 1), respectively. Structural analysis revealed that the presence of an external magnetic field can resulted in elongating of the distance within 18-C-6 and 4, 5-dci molecules, whom are connected by N-H…O hydrogen


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bonding contacts. Remarkably, thermal studies demonstrated the increase of thermal stability of these co-crystals upon elongating. Reciprocating change of the magnetic field strength shown that: On the one hand, the degree of elongating was increased with the magnetic field strength, as a consequence, the co-crystals can be stretched to broken when magnetic field reaching 1.5T. On the other hand, before broken, the stretched co-crystals can get back to their initial state when decrease the magnetic field strength. Thus, a multi-step reversible transformation between co-crystals in solution state can be achieved by properly adjusting the magnetic field strength, thereby, the stretching mechanism of magnetic field can be proposed. Furthermore, we have demonstrated the similar stretching effect on co-crystals of 18-C-6 with 1, 2, 4-triazole with the same magnetic field.

Scheme 1. Molecular structures of co-crystals 1-0.0T, 1-0.5T, 1-1.0T and 1-1.5T.



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RESULTS AND DISCUSSIONS

Co-crystallization Table 1. The crystal data and structure refinements for co-crystals 1-0.0T, 1-0.5T, 1-1.0T and 1-1.5T. Co-crystals

1-0.0T

1-0.5T

1-1.0T

1-1.5T

Formula Formula weight Crystal system Space group a/ Å b/ Å c/ Å α/° β/° γ/° V/Å3 T/K μ (mm-1) No. of reflns collected No. of unique reflns No. of params Goodness-of-fit on F2 R1, wR2 ((I>2σ(I)) R1, wR2 (all data) CCDC No

C22H28N8O6 500.52 Triclinic P-1 10.354(2) 10.530(2) 13.562(2) 77.99(3) 82.82(3) 65.80(3) 1314.3(5) 293(2) 0.094 5646 2064 325 1.02 0.094, 0.171 0.207, 0.225 1523345

C22H28N8O6 500.52 Triclinic P-1 10.369(2) 10.520(2) 13.502(3) 78.09(3) 83.03(3) 65.89(3) 1318.0(5) 293(2) 0.095 4835 1981 325 1.00 0.086, 0.169 0.195, 0.212 1523346

C22H28N8O6 500.52 Triclinic P-1 10.383(2) 10.549(2) 13.557(3) 78.09(3) 83.06(3) 65.91(3) 1325.3(5) 293(2) 0.094 4876 2229 325 1.00 0.070, 0.157 0.158, 0.190 1523347

C5H4N4O 136.12 orthorhombic Pna21 11.953(2) 4.8340(10) 11.514(2) 90 90 90 665.3(2) 293(2) 0.103 1515 1381 99 1.06 0.033, 0.072 0.039, 0.074 1523348

Co-crystallization of co-crystal 1-0.0T were performed by using slow evaporation technique of 0.2 mmol 18-C-6 with 0.2 mmol 4, 5-dci in a 10/1 (v/v) methanol and water mixture in 25 mL glass beaker at ambient condition. It crystallizes as prism crystals and deposit at the bottom of the beaker within two weeks. While the co-crystallization of co-crystals 1-0.5T, 1-1.0T and 1-1.5T were performed by place the corresponding beakers adhere to the one end of the permanent magnet with strength of 0.5, 1.0 and 1.5T, respectively. Big block-like single crystals, which were


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adhere to the wall of the beaker, were obtained within one week by using evaporation technique. The difference in crystal habit and crystal depositional position have provided a visual influence of external magnetic field on co-crystallization.

Figure 1. Summary of the transformations of co-crystal 1 (18-C-6/4, 5-dci) upon the application of an external magnetic field with strength of 0.0, 0.5, 1.0 and 1.5 Tesla. The crystal structures were shown in the whole asymmetric unit, and viewed from the crystallography c axis. Crystal Structure The crystal data and structure refinements for the four co-crystals were listed in Table 1. 1-0.0T is crystallizes in the triclinic space group P-1, with two entire molecules of 4, 5-dci and two halves of 18-C-6 in the asymmetric unit (Figure 1), the other two halves of 18-C-6 can be inversion generated (Figure S1). Interestingly, these two 18-C-6 exhibit distinct different conformation, one is the most commonly



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encountered “round-D3d like” conformation and the other is a scarce distorted one (Figure 2). Every two 4, 5-dci molecules are bind to one 18-C-6 centrosymmetricly, through N-H…O hydrogen bonding contacts (Table S1). Note that, for the distorted 18-C-6, short N…O contact between 4, 5-dci and 18-C-6 has been observed, and the N-H…O hydrogen bonding contact is also strengthened (decrease in N…O distance, Figure 2 and Table S2). These strong contacts presumably responsible for the scarce distorted conformation of 18-C-6. To the best of our knowledge, the existence of two distinct different conformations of 18-C-6 within one unit cell is unprecedented.

Figure 2. Representation emphasizing the dramatic influence of external magnetic field on the relative location of the two 18-C-6 molecules in different conformations, as well as the N-H…O hydrogen bonding and N…O contacts between 4, 5-dci and 18-C-6. For sake of clarity, hydrogen atoms have been omitted. When in the presence of an external magnetic field of 0.5 and 1.0 Tesla, the composition and space group of the corresponding co-crystals 1-0.5T and 1-1.0T are reserved (Table 1 and Figure 1). However, the intermolecular contacts between the co-formers have been changed (Figure 2). For 1-0.5T, the distance for N-H…O


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hydrogen bonding contact has increased to 2.818(6) and 2.743(6) Å, when compared with the distances of 2.795(5) and 2.733(5) Å for co-crystal 1. Meanwhile, the distance for N…O contact has also increased from 3.024(5) to 3.042(6) Å, suggesting a stretching effect between 18-C-6 and 4, 5-dci that created by the application of external magnetic field. Interestingly, when increase the magnetic field to 1.0T, the distances for N-H…O hydrogen bonding and N…O contacts have increased further on, demonstrating a continuous and quantitative stretching effect that created by the external magnetic field. This effect has further relieved the distortion degree of the 18-C-6 (Figure 3), and resulted in similar 3D stacking architecture with the two different conformations of 18-C-6 stacked alternatively along the crystallography c axis (Figure S1).

Figure 3. The variation tendency of the torsion angles for these two kinds of 18-C-6 induced by magnetic field. When the magnetic field was increased to 1.5 Tesla, the 4, 5-dci has failed to form co-crystal with 18-C-6, but generated a novel hydrate 1-1.5T. It crystallizes as



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bulk crystals in non-centrosymmetric space group with orthorhombic crystal system (Table 1), and the asymmetric unit is composed of an entire molecule of 4, 5-dci and H 2O

Figure 4. a) The basic connecting motif of 4, 5-dci with H2O for the double-interpenetrated architecture in crystal 1-1.5T; b) The connecting motif of the “10-membered” ([4, 5-dci]5[H2O]5) and “12-membered” ([4, 5-dci]6[H2O]6) cycles; c) The double-interpenetration pattern of these two different cycles in compound 1-1.5T; d) 2-D honeycomb-like sheet motif of the double-interpenetrated pattern. For all the graphs, hydrogen atoms are omitted for clarity. (Figure 1). Interestingly, every molecule of 4, 5-dci are connect to three H2O molecules through two O-H…N and one C-H…O hydrogen bonding contacts, and vise-versa for every H2O molecule (Figure 4a), leading to 2-D honeycomb-like sheet


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that is composed of “10-membered” ([4, 5-dci]5[H2O]5) and “12-membered” ([4, 5-dci]6[H2O]6) cycles (Figure 4b). Remarkably, the two different cycles are interweaved alternately (Figure 4c), generating a stable double-interpenetrated 2-D architecture (Figure 4d) that can maintained to 112°C (Figure S2). It should be notated that, in the absence of external magnetic field, the crystallization of 4, 5-dci in a methanol-water solution has failed to obtain the hydrate, suggesting that magnetic field is an essential condition for the formation of 4, 5-dci hydrate. What’s more, the failure to form co-crystal with 18-C-6 demonstrated that the stretching effect of 1.5 Tesla magnetic field have induced to the broken of the N-H…O hydrogen bonding and N…O contacts between 18-C-6 and 4, 5-dci. Transformation Between Co-crystals To investigate the transformation between the above mentioned four co-crystals, reciprocating change of the magnetic field strength during co-crystallization process have been performed (Experimental Section). As have been shown in Figure 1, co-crystal 1-0.0T can transfer to 1-0.5T, 1-1.0T and 1-1.5T, when in the presence of external magnetic field of 0.5, 1.0 and 1.5 Tesla, respectively. In addition, re-dissolution of co-crystals 1-0.5T (1-1.0T) in a methanol-water solution and then put it in an external magnetic field of 0.0, 1.0 and 1.5 Tesla (0.0, 0.5 and 1.5 Tesla), respectively, which can re-generate co-crystals 1-0.0T, 1-1.0T and 1-1.5T (1-0.0T, 1-0.5T and 1-1.5T) accordingly. Thus, a multi-step reversible transformation for co-crystal 1 in solution state has been achieved by properly adjusting the magnetic field strength.



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Stretching Effect To reify the stretching effect that induced by external magnetic field, we have compared the intermolecular distances within co-crystals 1-0.0T, 1-0.5T and 1-1.0T in

Figure 5. Summary of the compared ten items. d1, d1’: centroid…centroid distances between 4, 5-dci which bind to one 18-C-6; d2, d2’: plane…plane distances between 4, 5-dci which bind to one 18-C-6; d3, d4: centroid…centroid distances between adjacent 18-C-6 with different conformations; d5, d6: centroid…centroid distances between adjacent 4, 5-dci; d7, d8: plane…plane distances between adjacent 18-C-6 with the same conformation; d9 (d10): plane…plane distances between adjacent 4,


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5-dci along the crystallography b axis which bind to the “round-D3d like” (scarce distorted) 18-C-6.

Figure 6. The variation tendency of the ten intermolecular distances for co-crystal 1 upon the application of external magnetic field, together with the variation of unit cell parameters.

Figure 7. Representation of the stretching mechanism induced by external magnetic field, the shift directions of the different co-formers have been highlighted with arrows. detail. As have been shown in Figure 5 and Figure S3-S4, the compared ten items including centroid…centroid and plane…plane distances between adjacent 4, 5-dci and 18-C-6 molecules, respectively. The variation trends of the ten items were shown



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in Figure 6, together the unit cell parameters. Results shown that: the centroid…centroid and plane…plane distances between the adjacent 4, 5-dci molecules in all directions (d1, d1’, d2, d2’, d6, d6, d9 and d10), as well as the centroid…centroid distances between adjacent 18-C-6 molecules in all directions (d3 and d4) are increased remarkably with the strength of magnetic field, while the plane…plane distances between adjacent 18-C-6 molecules (d7 and d8) are decreased with the increasing of magnetic field strength, which lead to the enlargement of unit cell. Thus, the stretching mechanism can be proposed (Figure 7), that is: the 4, 5-dci molecules are stretched along the opposite direction which is vertical to the plane of 18-C-6, with a slight skewing. Meanwhile, the 18-C-6 molecules are stretched away from each other, along the opposite direction which is almost vertical to the shift direction of 4, 5-dci, with the plane distances become closer. As a consequence, the unit cell have been enlarged, and finally, the 4, 5-dci and 18-C-6 molecules can be separated completely, providing a magnetic field with enough strength.


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Figure 8. Raman spectra for co-crystals 1-0.0T, 1-0.5T, 1-1.0T and 4, 5-dci, the character peak of 4, 5-dci in the co-crystals are shift close to the individual 4, 5-dci. Vibrational Spectra and Hirshfeld surfaces analysis Based on the above discussion, we may concluded that stretching of the co-crystals can made the co-formers close to its individual one. In other words, the physical /chemical properties of the stretched co-crystal should be close to its co-formers, when compared with the un-stretched co-crystal. To verify this conclusion, vibrational spectra (IR and Raman), Hirshfeld surfaces analysis49 and thermal studies (TGA and DSC) for co-crystals 1-0.0T, 1-0.5T and 1-1.0T have been performed. The IR spectra have shown no discernable difference (Figure S5), but the Raman spectra did (Figure 8): that is, the more stretching effect on the co-crystal, the closer Raman spectra to the individual 4, 5-dci, which confirmed the above conclusion. Hirshfeld surfaces analysis (Figure S6-S7) revealed the increase of long-distances contacts with the magnetic field strength, and accompanied by the decrease of short contacts (Figure 9).

Figure 9. The variation tendency of the main intermolecular contacts that suffered by



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the two different kinds of 4, 5-dci (left) and the two different 18-C-6 (right) in the in co-crystals 1-0.0T, 1-0.5T and 1-1.0T. Which have demonstrated that the long-distances contacts were increased with the magnetic field strength, and accompanied by the decrease of short contacts. Thermal Studies Thermal analysis (Figure 10) shown that: 1-0.0T undergoes an endothermic process at around 70°C upon heating, and melts at 174.18°C. While there are no endothermic process for 1-0.5T and 1-1.0T, and the melting points of them are found to be 174.57 and 174.81°C, respectively, a value that close to the melting point of 4, 5-dci (176.05°C). What more, the decomposition temperature of 1-0.5T (277.56°C) and 1-1.0T (282.60°C) are larger than 1-0.0T (253.53°C), and close to 4, 5-dci (301.65°C). Which demonstrated that the stretching effect can lead to increase in the thermal stability for co-crystals, and confirmed again the above conclusion.

Figure 10. Comparison of the DSC profiles for co-crystals 1-0.0T, 1-0.5T and 1-1.0T (left), as well as the TGA profiles for co-crystals 1-0.0T, 1-0.5T, 1-1.0T and 4, 5-dci. Which were revealed that the stretching effect has increased the thermal stability of


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co-crystal 1. 18-C-6/1, 2, 4-Triazole System We further extend our research to 18-C-6/1, 2, 4-triazole system,50 and have obtained two new co-crystals 2-0.0T and 2-0.5T (Table S2). Surprisingly, co-crystal of 18-C-6/1, 2, 4-triazole can be completely broken when magnetic field strength is larger than 1.0 Tesla. The crystal structure analysis (Figure S8-S10), vibrational spectra (Figure S11-S12), Hirshfeld surfaces (Figure S13) and thermal studies (Figure S14) of co-crystals 2-0.0T and 2-0.5T, have made a similar conclusion with co-crystal 1. Hence, the stretching effect of magnetic field on co-crystals of 18-C-6 with aromatic N-heterocyclic compounds can be well-established.

CONCLUSIONS

In summary, by the application of external magnetic field with various strength, we have demonstrated that the distances between co-formers 18-C-6 and aromatic N-heterocyclic compounds (4, 5-dci and 1, 2, 4-triazole) can be stretched reversibly in solution state. As a consequence, the physical/chemical properties of the co-crystals have been remarkably altered, and finally, the co-formers can be completely separated. We believe that this stretching effect of magnetic field on co-crystals may therefore be of great interest in functional organic co-crystal materials which were tuned by non-covalent supramolecular interactions such as NLO, ferroelectricity, photo-physics and charge transportation.

EXPERIMENTAL SECTION



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Co-crystallization. The preparation of the co-crystals was conducted in solution crystallization experiments by using slow evaporation technique. Methanol–water (10:1 v/v) mixtures were used as solvents. Co-crystal 1-0.0T 1 was obtained by the following procedure: a 2:1 1 stoichiometric amount of 18-crown-6 (18-C-6, 0.02mmol) and 4, 5-dicyanoimidazole (4, 5-dci, 0.04mmol) was added to an 8 ml stirring methanol–water solution at a temperature of 40 °C, with a 10 ml beaker. The resulting solution was stirred for 10 min before being left to evaporate at room temperature. Colorless prism-like crystals of 1-0.0T, which were deposited at the bottom of the beaker were obtained within two weeks. Yield: ca. 80%. Mp, 174.18 °C. EA calcd (found) (%) for 1-0.0T (C22H28N8O6): C, 52.77 (52.58); N, 22.39 (22.41); H, 5.64 (5.49). IR (KBr pellet, cm-1): 3436, 3145, 2906, 2204, 1680, 1482, 1352, 1287, 1250, 1105, 960, 836, 637, 503. External magnetic field induced co-crystallization were performed by using the above prepared methanol–water mixtures of co-crystal 1-0.0T under 0.5, 1.0 and 1.5 Tesla, respectively. Through slow evaporation technique, co-crystals 1-0.5T, 1-1.0T and 1-1.5T were crystalize as big block crystals that adhere to the wall of the beaker within three weeks. Yield are all about 90%. EA calcd (found) (%) for 1-0.5T: C, 52.77 (52.60); N, 22.39 (22.43); H, 5.64 (5.51). IR (KBr pellet, cm-1): 3436, 3145, 2907, 2204, 1681, 1482, 1352, 1287, 1250, 1105, 961, 836, 637, 504. EA calcd (found) (%) for 1-1.0T: C, 52.77 (52.65); N, 22.39 (22.40); H, 5.64 (5.53). IR (KBr pellet, cm-1): 3436, 3145, 2907, 2204, 1681, 1482, 1352, 1287, 1251, 1105, 961, 836, 637, 504. EA calcd (found) (%) for 1-1.5T: C, 44.10 (44.15); N, 41.17 (41.26); H, 2.96 (2.73). IR (KBr pellet,


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cm-1): 3262, 3106, 2259, 2244, 1475, 1402, 1341, 1287, 1240, 1123, 1079, 939, 800, 634, 517, 505. Transformation between co-crystals 1-0.0T, 1-0.5T, 1-1.0T and 1-1.5T have been performed as follows: Re-dissolution of co-crystal 1-0.5T (0.02mmol) in 8 ml methanol-water solution with a 10 ml beaker, then put it under a magnetic field of 0.0, 1.0 and 1.5 Tesla, respectively. Co-crystals 1-0.0T, 1-1.0T and 1-1.5T can be re-obtained within three weeks. Similarly, re-dissolution of co-crystal 1-1.0T (0.02mmol) in methanol-water solution and then provided magnetic field of 0.0, 0.5 and 1.5 Tesla, respectively, co-crystals 1-0.0T, 1-0.5T and 1-1.5T can also be re-obtained. The co-crystallization of 18-crown-6 with 1, 2, 4-triazle, and the investigation of transformation between 2-0.0T and 2-0.5T were all performed by using the similar procedure with co-crystal 1. EA calcd (found) (%) for 2-0.0T (C8H15N3O3): C, 47.73 (47.65); N, 20.88 (28.56); H, 7.51 (7.42). IR (KBr pellet, cm-1): 3224, 3132, 2921, 2895, 2204, 1978, 1794, 1621, 1512, 1473, 1350, 1247, 1101, 1057, 965, 895, 841, 797, 678, 641, 526. EA calcd (found) (%) for 2-0.5T (C8H15N3O3): C, 47.73 (47.68); N, 20.88 (28.64); H, 7.51 (7.46). IR (KBr pellet, cm-1): 3224, 3132, 2921, 2895, 2204, 1978, 1794, 1621, 1512, 1473, 1350, 1247, 1101, 1057, 960, 835, 678, 641, 510. Chemicals, materials and instrumentation. All chemicals commercially available at AR grade and directly used without further purification. Infrared spectra (IR) were recorded on a SHIMADZU IR prestige-21 FTIR-8400S spectrometer in the spectral



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range 4000-500 cm-1, with the samples in the form of potassium bromide pellets. Elemental analyses were performed by a VARIO-EL III elemental analyzer for carbon, hydrogen, and nitrogen for these nine supramolecular self-assemblies. Thermal analysis were performed using a Mettler-Toledo TGA/DSC STARe System at a heating rate of 10 K min-1 under an atmosphere of dry N2 flowing at 20 cm3min-1. Raman spectra were recorded using a Raman microscope (Kaiser Optical Systems, Inc., Ann Arbor, MI, USA) with 785 nm laser excitation. The spectra were obtained for 2 min exposure of the CCD detector in the wave number range 50-3500 cm-1. Hirshfeld surfaces analysis were carried out by using CrystalExplorer software. The single-crystal X-ray diffraction data of the six co-crystals were collected at 293 K with graphite-monochromated Mo-Kα radiation (λ = 0.071073 nm) equipped with Bruker-AXS SMART APEX diffractometer. The lattice parameters were integrated using vector analysis and refined from the diffraction matrix, the absorption correction was carried out by using Bruker SADABS program with multi-scan method. The structures were solved by full-matrix least-squares methods on all F2 data, and used the SHELXS97 and SHELXL97 programs for structure solution and refinement respectively. The crystallographic data, data collection, and refinement parameters for the six co-crystals were given in Table 1 and S2. CCDC No 1523345-1523350 contains the supplementary crystallographic data for six co-crystals, respectively.

ASSOCIATED CONTENT

Supporting Information. Additional crystal data, crystal structures, vibrational


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spectra (IR Raman), Hirshfeld surfaces analysis and thermal studies (TGA and DSC) for the six co-crystals. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author *Phone: +86-025-52090614. Fax: +86-025-52090614. E-mail: [email protected] (Luo); [email protected] (Sun).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This research was based on work supported by the Scientific Research Foundation for New Scholars (No. 1107047111), Natural Science Foundation of China (Grant No. 21371031), International S&T Cooperation Program of China (No. 2015DFG42240) and PAPD of Jiangsu Higher Education Institutions.

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For Table of Contents Use Only

Reversibly stretching the co-crystals by the application of magnetic field Yang-Hui Luo,* Jing-Wen Wang, Chen Chen, Yao-Jia Li and Bai-Wang Sun*


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By the application of an external magnetic field with various strength, the distances between co-formers (18-Crown-6/4, 5-dicyanoimidazole and 18-Crown-6/1, 2, 4-triazole) can be stretched reversibly. As a consequence, the physical/chemical properties of the stretched co-crystals have been remarkably altered, and finally, the co-crystals can be broken. Thus, the stretching mechanism of magnetic field have been proposed.


Reversibly Stretching Cocrystals by the Application of a Magnetic Field

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