Reversible Accommodation and Desorption of Aromatics on a Charge

Article pubs.acs.org/crystal

Reversible Accommodation and Desorption of Aromatics on a Charge Transfer Cocrystal Involving an Anthracene Derivative and TCNQ Xiaojuan Wu,† Mingliang Wang,*,† Man Du,† Jiao Lu,† Jinxi Chen,† Arshad Khan,† Rabia Usman,† Xiang Wei,† Qi Feng,† and Chunxiang Xu*,‡ †

School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, P. R. China State Key Laboratory of Bioelectronics, Southeast University, Nanjing 210096, P. R. China

‡

S Supporting Information *

ABSTRACT: A charge transfer organic cocrystal 1 was constructed from an anthracene derivative, 1-(5-(anthracen-9-yl)-3-(4-methoxyphen-yl)-4,5-dihydropyrazol-1-yl)ethanone (AMPE) and TCNQ. Eight cocrystal solvates which can be placed into types I and II were obtained from solution. Type I solvates include 1·toluene, 1· chlorobenzene, 1·o-chlorotoluene, 1·p-chlorotoluene, 1·o-xylene, 1·m-xylene, and 1·pxylene-1, while 1·p-xylene-2 belongs to type II solvate. Crystal structure analysis revealed that cocrystal 1 was an isomorphic desolvate of type I solvates. Slitlike channels were created in 1 through charge transfer, hydrogen bond, and π−π interactions, which are expanded in corresponding type I solvates, while the type II solvate shows a different structure from that of type I solvates. The neighboring racemic chains were connected by one kind of p-xylene molecules through C−H···π interactions and formed square-grid channels. Cocrystal 1 selectively accommodates aromatic solvents over other polar and nonpolar nonaromatic solvents such as acetone, CH2Cl2, and cyclohexane to form type I solvates. These solvates converted to 1 again upon desolvation, and the transformation occurs in a crystal-to-crystal manner. Type II solvate (1·p-xylene-2) can also be converted to 1 upon desolvation, but 1 cannot accommodate p-xylene to form the type II solvate again. This result can be used to certify the conversion from 1 to type I solvates by the absorption mechanism.

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INTRODUCTION Supramolecular chemistry is assembled from various noncovalent interaction, such as hydrogen bonds, C−H···π, π···π, and charge-transfer interactions by molecular recognition. It has attracted considerable attention in recent years because of their fascinating structural topologies and potential applications.1 For example, hydrogen-bonded organic frameworks, owing to their highly directional nature, structural diversity, was proposed for selective guest inclusions, gas storages in recent years.2 However, these molecular crystals containing cavities or channels are prone to be isolated from solution as solvates. The solvent guests play a vital role in stabilizing structure, so the frameworks will collapse as soon as the solvent guests are removed.3 Therefore, the examples of this kind of organic material with permanent porosity for adsorption of gases or aromatics are very rare.2,4 Thus, it is necessary to explore other strong supramolecular interaction or combination of weak interactions to stabilize the networks to avoid structure collapse after solvent removal. On the other hand, the charge transfer (CT) complexes, electron-rich (donor, D) and electron-deficienct (acceptor, A), are self-assembled through charge transfer interactions, as well as hydrogen bonds. Charge-transfer (CT) interactions are comparable to hydrogen bonding because of its directional © 2014 American Chemical Society

nature, alternating placement of the D and A units and wider solvent tolerance.1b Although CT cocrystals of various D−A pairs have been studied as solid-state organic materials in superconductivity,5 ferroelectricity,6 photoconductive properties,7 and so on,8 for a long time and have also been extensively used to produce many fascinating interlocked structures such as rotaxanes and catenanes in recent years,9 there are fewer examples of such CT cocrystals applied to accommodate aromatic molecules because infinite DADA chains in structure impede encapsulation of aromatic solvents. Thus, design CT cocrystals which can accommodate aromatic molecules to overcome this drawback by reinforcing the CT interactions with auxiliary noncovalent forces such as solvophobic forces and hydrogen bonding become challenging. In this work, we reported an organic cocrystal 1 involving electron-rich 1-[5-(Anthracen-9-yl)-3-(4-methoxyphenyl)-4,5dihydropyrazol-1-yl]ethanone (AMPE) and electron-deficient 7,7,8,8-tetracyanoquinodimethane (TCNQ) (Figure 1), which can include various aromatics in the slitlike channels to form a series of cocrystal solvates, namely, type I solvates: 1·toluene Received: October 10, 2014 Revised: November 23, 2014 Published: November 24, 2014 434

dx.doi.org/10.1021/cg501514x | Cryst. Growth Des. 2015, 15, 434−441

Crystal Growth & Design

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RESULT AND DISCUSSION

Crystal Structures. Single-crystal X-ray diffraction (SCD) analysis were performed for cocrystal 1 and 1a−1h. Structure determination shows that solvates 1a−1g have a similar structure to that of cocrystal 1, which is different from crystal 1h. The molar ratio in cocrystal 1 is 2:1 between AMPE and TCNQ. These solvates can be classified in two different types according to their stoichiometry ratios, framework arrangements, and cell parameters (Table 1). Solvates 1a−1g belong to type I solvates with a ratio of AMPE:TCNQ:solvents = 2:1:1, while 1h belongs to type II solvate with a ratio of 2:1:2. Crystal 1 and type I solvates 1a−1g, including two crystallographically independent AMPE molecules (types A and B are shown in pink and sky blue, respectively) in the asymmetric units (ASU), whereas there was only one kind of AMPE molecule in the ASU of type II solvates 1h, which exhibits a T-shaped twisted conformation with the dihedral angle between the pyrazoline ring and anthracene ring listed in Table 2. In all these crystals, TCNQ adopts a face-to-face sandwichlike mode with the anthracene moiety of AMPE via charge-transfer interactions, which can be confirmed by the interplanar separation (dπ−π) and the closest centroid distance (dc‑c) (Table 3). All crystallographic data are presented in Table 1. Cocrystal 1 crystallized in the triclinic system and space group P1̅ and includes one molecule of AMPE and a half molecule of TCNQ in the asymmetric unit. As shown in Figure 1, two different orientations of TCNQ act as a bridge to connect two different types of AMPE through charge-transfer interactions, C−H···O, C−H···N hydrogen bonds (Table 4), and C−N···π and C−H···π interactions (Table 5) to form two racemic chains, which linked together along the b axis through C−H···π interactions (Table 5) between the two neighboring homochiral AMPE molecules to form the slitlike channels. Type I solvates 1a−1g are isostructural with a triclinic system and space group P1.̅ Crystal structures show that they possess the same framework, which is similar to that of cocrystal 1 (Figure 2), except for cell parameters and the enlarged cell volumes in 1·solvents (Table 1). The aromatic solvent molecules were stabilized in the channels via weak van der Waals interactions for toluene in 1a, C−Cl···π interaction for chlorobenzene in 1b, C−H···O hydrogen bonds and C−H···π interactions for o-chlortoluene in 1c, C−H···π interactions for p-chlorotoluene in 1d, C−H···O hydrogen bonds and C−H···π interactions for o-xylene in 1f, weak van der Waals interactions for m-xylene in 1g, and C−H···π interactions for p-xylene in 1h (Figure 3). The single-crystal structural analysis indicates that the framework of type I solvates do not show significant guestindependent cell volume variation (Table 1). The different guest species in 1 only resulted in a 3.33% cell variation (from 2861.4 to 2956.7 Å3, Table 1), indicating that the framework of 1 is rigid. This also gives a sign that the type I solvates may convert to 1 without structure collapse, which will be discussed later. The p-xylene-containing type II solvate 1h also has a triclinic system and space group P1̅. In this system, the two neighboring heterochiral AMPE molecules stabilized by C−H···π interactions (Table 5 and Figure 4). TCNQ molecules sandwiched in the middle of anthracene moieties of AMPE molecules along the a axis and stabilized with AMPE molecules via C−H···O, C−H···N hydrogen bonds, C−N···π, and face-to-face π−π interaction (Table 3−5 and Figure 4). It also acted as bridges to

Figure 1. Preparation of 1. Pink (type A) and sky blue (type B) colored molecules are the two crystallographically independent AMPE molecules. The dotted lines show hydrogen bonds and C−H···π interactions, respectively. Hydrogen atoms not participating in the interactions have been omitted for clarity.

(1a), 1·chlorobenzene (1b), 1·o-chlorotoluene (1c), 1·pchlorotoluene (1d), 1·o-xylene (1e), 1·m-xylene (1f), 1·pxylene-1 (1g), and type II solvate, 1·p-xylene-2 (1h). Interestingly, cocrystal 1 exhibits a selective accommodation for aromatics to form type I solvates over other nonaromatics, which is verified as an adsorption mechanism. These solvates can convert to cocrystal 1 upon desolvation at 130−170 °C and retain its single crystallinity as a crystal-to-crystal transformation without structure collapse. This reversible transformation between 1 and its solvates may have potential applications in separation and purification. To the best of our knowledge, no such charge transfer organic cocrystals have been reported before.

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Article

EXPERIMENTAL SECTION

Crystal Preparation. AMPE was synthesized according to the procedure previously reported in our laboratory.10 7,7,8,8-Tetracyanoquinodimethane (TCNQ, CAS 1518-16-7) was purchased from Alfa and used without further purification. Analytical-grade solvents were used. Cocrystal 1. AMPE and TCNQ were dissolved in acetone/ dichloromethane (v:v = 1:1) mixed solvents in glass vials. Slow evaporation of the solvents at room temperature for 4−5 days yielded the black block crystals. Type I Solvates 1a−1g. AMPE and TCNQ were dissolved in an aromatics/CH 2Cl2 mixture (v:v = 1:1, aromatics = toluene, chlorobenzene, o-chlorotoluene, p-chlorotoluene, o-xylene, m-xylene, and p-xylene, respectively) in a glass vial. Slow evaporation of the solvents at room temperature for 4−5 days yielded black block crystals, namely, 1·toulene (1a), 1·chlorobenzene (1b), 1·o-chlorotoluene (1c), 1·p-chlorotoluene (1d), 1·o-xylene (1e), 1·m-xylene (1f), and 1·p-xylene-1 (1g), respectively. Type II Solvate 1h. 1·p-xylene-2 (1h) were formed in p-xylene/ CH2Cl2 mixture by the same method except at a lower temperature (about 15 °C). UV/vis Spectroscopy. UV−vis absorption spectra were recorded on a Shimadzu UV-3600 spectrometer. Thermogravimetric Analysis (TGA) and Differential Scanning Calorimetry (DSC). TGA/DSC patterns were recorded with a Mettler-Toledo TGA/DSC 1 thermogravimetric analyzer with the temperature scanned from 50 to 300 °C at 10 °C/min. X-ray Diffraction. The PXRD patterns for the three crystals were recorded using a 18 KW advance X-ray diffractometer with Cu Kα radiation (λ = 1.54056 Å). Single X-ray diffraction data for these crystals were collected on Nonius CAD4 diffractometer with Mo Kα radiation (λ = 0.71073 Å). The structures were solved with direct methods using the SHELXS-97 program and refined anisotropically using a full-matrix least-squares procedure. 435



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Table 1. Crystal Data and Structure Refinement crystal

1

1′

1a

1b

1c

formula temperature (K) crystal size (mm3) morphology crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρ (calcd)/Mg m−3 θ range for data collection (deg) F(000) ref collected/unique R1, wR2 [I >2σ(I)] R1, wR2 (all data) goodness-of-fit, S CCDC crystal

C32H24N4O2 293 0.30 × 0.20 × 0.10 block triclinic P1̅ 10.631(2) 16.476(3) 17.192(3) 63.23(3) 84.48(3) 82.36(3) 2662.5(9) 2 1.239 2.96−25 1040 22806/9372 0.0799, 0.1031 0.2062, 0.1659 0.975 1005877 1d

C32H24N4O2 293 0.30 × 0.24 × 0.10 block triclinic P1̅ 10.517(2) 16.373(3) 17.041(3) 63.35(3) 84.38(3) 82.28(3) 2596.5(11) 2 1.270 1.34−25.38 1040 10079/9514 0.0495, 0.1017 0.1985, 0.1568 1.011 1005878 1e

C71H56N8O4 293 0.32 × 0.30 × 0.25 block triclinic P1̅ 9.853(2) 17.314(4) 18.193(4) 111.45(3) 96.50(3) 91.75(3) 2861.4(10) 2 1.260 1.21−25.37 1140 11148/10409 0.0985, 0.1681 0.2151, 0.2113 1.001 1005879 1f

C70H53ClN8O4 293 0.20 × 0.20 × 0.10 block triclinic P1̅ 9.884(2) 17.500(4) 18.122(4) 111.29(3) 96.85(3) 92.11(3) 2888.9(10) 2 1.271 1.22−25.38 1156 11261/10597 0.0781, 0.1252 0.1896, 0.1543 1.000 1005880 1g

C71H55ClN8O4 293 0.30 × 0.20 × 0.10 block triclinic P1̅ 9.947(2) 17.958(4) 18.072(4) 110.88(3) 92.71(3) 98.97(3) 2960.7(13) 2 1.256 3.02−27.48 1172 25671/10397 0.0993, 0.2368 0.2623, 0.3140 1.019 1005881 1h

formula temperature (K) crystal size (mm3) morphology crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z ρ (calcd)/Mg m−3 θ range for data collection (deg) F(000) ref collected/unique R1, wR2 [I > 2σ(I)] R1, wR2 (all data) goodness-of-fit, S CCDC

C71H55ClN8O4 293 0.28 × 0.26 × 0.24 block triclinic P1̅ 9.853(2) 17.501(3) 17.934(4) 109.60(3) 96.52(3) 92.41(3) 2883.8(12) 2 1.289 3.09−27.48 1172 29542/13244 0.0797, 0.1920 0.1963, 0.2506 1.027 1005882

C72H58N8O4 293 0.30 × 0.20 × 0.10 block triclinic P1̅ 9.876(2) 17.403(4) 18.353(4) 111.25(3) 96.44(3) 91.53(3) 2913.6(10) 2 1.253 1.20−25.38 1156 11351/10681 0.0730, 0.1092 0.2087, 0.1555 1.007 1028459


C72H58N8O4 293 0.35 × 0.28 × 0.14 block triclinic P1̅ 9.959(2) 17.475(4) 18.237(4) 110.09(3) 95.95(3) 91.89(3) 2956.7(10) 2 1.235 3.04−25 1156 17189/10225 0.0887, 0.2267 0.2224, 0.1723 1.026 1028461


link AMPE molecules along the c axis and formed one racemic chain. The neighboring racemic chains were connected by one kind of p-xylene molecules through C−H···π interactions (Table 4) and formed square-grid channels along the a axis. The other p-xylene molecules existed outside of the squarelike channels by a weak hydrogen bond (Figure 4). Solid-State Adsorption Spectra. Diffuse reflectance absorption spectroscopy was exploited to investigate the charge transfer interaction of these CT cocrystals.11 As shown in Figure 5, compared with the two original components, all these crystals show a new broad band from 700 to 800 nm while significantly retaining the absorption peaks of the individual AMPE and TCNQ, suggesting the formation of charge transfer complexes. Due to their strong absorption that extends across

the visible and into the near-infrared, these crystals appear black. Meanwhile, no CT band can be observed in UV−vis spectra (Figure S7 of the Supporting Information), which indicates no CT complex formation in dilute solution. Thermal Properties. The thermal behaviors of these crystals were investigated with DSC and TGA. The DSC pattern (Figure 6a) shows that cocrystal 1 exhibits one endothermic peak at 221 °C, which is different from the two original compounds, suggesting the formation of a cocrystal. All solvates 1a−1h exhibit two endothermic peaks corresponding to the processes of removing solvents (toluene, chlorobenzene, o-chlorotoluene, p-chlorotoluene, o-xylene, m-xylene, p-xylene, and p-xylene) and the melting process of the remanent solid, respectively. The weight loss (Figure 6b) coincides well with 436



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Table 2. Dihedral Angles of AMPE Molecules in Each Crystal

Table 4. Intermolecular Hydrogen Bond Parameters in Crystal 1 and 1a−1h

angles (deg)a

crystal

77.36 77.36 88.65 88.01 86.92 85.19 84.28 85.31 83.05

1 1′ 1a 1b 1d 1e 1f 1g 1h

(A), (A), (A), (A), (A), (A), (A), (A),

crystal

86.62 (B) 86.72 (B) 85.32 (B) 87.29 (B) 87.79 (B) 85. 51 (B) 86.25 (B) 86.65 (B)

1 C24−H24B···N12 C55−H55···O3 1a C10−H10A···O4 C15−H15A···O1 C46−H46A···N6 C53−H53A···O2 C59−H59A···O4 1b C10−H10A···O1 C22−H22A···O4 C29−H29A···N8 C54−H54A···O4 C61−H61A···O2 1c C10−H10···O4 C15−H15···O1 C54−H54···O2 C55−H55···O2 C61−H61···O4 C67−H67···O1 1d C36−H36A···O2 C41−H41A···O3 C54−H54A···O4 C60−H60A···O2 1e C9−H9A···O3 C15−H15A···O2 C57−H57A···O3 C62−H62A···O1 C67−H67A···O2 1f C10−H10···O4 C15−H15···O1 C46−H46···N8 C54−H54···O4 C61−H61···O2 1g C10−H10A···O4 C15−H15A···O2 C57−H57A···O1 C63−H63A···O4 1h C11−H11···N4 C28−H28···O2 C33−H33···O2

a

The dihedral angles between the pyrazoline rings and anthracene rings of the two crystallographically independent AMPE molecules, type A and type B, respectively.

Table 3. Face-to-Face π-Stacking Interactions between TCNQ and Anthracene Moieties crystal

interaction

dπ−π, dc‑c(Å)a

angle (deg)b

1

TCNQ···anthracene(A) TCNQ···anthracene(B) TCNQ···anthracene(A) TCNQ···anthracene(B) TCNQ···anthracene(A) TCNQ···anthracene(B) TCNQ···anthracene(A) TCNQ···anthracene(B) TCNQ···anthracene(A) TCNQ···anthracene(B) TCNQ···anthracene (A) TCNQ···anthracene (B) TCNQ···anthracene (A) TCNQ···anthracene (B) TCNQ···anthracene (A) TCNQ···anthracene (B) TCNQ···anthracene

3.504, 4.020 3.510, 3.757 3.476, 4.108 3.446,3.680 3.475, 4.102 3.484,3.739 3.469, 4.092 3.589,3.956 3.394, 4.103 3.463, 3.741 3.464, 4.091 3.459, 3.709 3.484, 4.081 3.465, 3.704 3.432, 4.047 3.479, 3.740 3.384, 4.116

0.80 5.75 1.08 0.43 0.75 1.89 1.31 4.39 1.17 0.80 1.70 0.69 0.62 0.90 1.52 0.43 7.52

1a 1b 1c 1d 1e 1f 1g 1h a

The interplanar separation (dπ−π) and the closest centroid distance (dc‑c) and bThe angles were measured between the mean planes of TCNQ and anthracene rings (for π−π interaction) of types A and B, respectively.

the theoretical weight loss of type I solvates 1a−1g with the molar ratio of AMPE:TCNQ:solvents = 2:1:1, and type II solvates 1h with a molar ratio of 2:1:2 (Table 6). The second peak of these solvates is nearly the same with the melting point of cocrystal 1, which implies the desolvates may have the same structure with cocrystal 1. Crystal Transformation. As shown in Figure 7, PXRD patterns of these crystals are well in agreement with the simulated patterns from their single crystal data. Strikingly, crystal 1a (1·toluene) was changed to crystal 1′ when subjected to 170 °C at normal pressure. Although many of the crystals become opaque upon desolvation, several SCD-qulility crystals still remained intact, which is revealed by single crystal X-ray diffraction, indicating the desolvation transformation is a single crystal to single manner. Crystal analysis revealed that crystal 1′ is same with crystal 1 (Table 1). Similarly, 1′ could also be obtained by removing the solvents from other type I and type II solvates. The powder X-ray diffraction (PXRD) patterns of the relative desolvated crystals were found to be in agreement with those of crystal 1 (Figure 8). Obvious differences can be found between PXRD patterns of 1 and 1·solvents (Figure 7). Such a

D−H (Å)

H···A (Å)

D···A (Å)

∠D−H···A (deg)

0.960 0.930

2.542 2.587

3.490 3.458

169.33 156.04

0.930 0.980 0.931 0.930 0.930

2.576 2.679 2.561 2.229 2.393

3.315 3.459 3.445 2.958 3.234

136.65 136.73 158.61 134.82 150.39

0.981 0.930 0.930 0.929 0.930

2.585 2.573 2.610 2.397 2.208

3.430 3.356 3.477 3.236 2.946

144.34 142.16 155.38 150.03 135.70

0.929 0.981 0.929 0.930 0.930 0.929

2.614 2.705 2.445 2.406 2.340 2.585

3.400 3.654 3.030 3.014 3.170 3.399

142.66 162.72 121.01 122.88 148.44 146.61

0.930 0.878 0.931 0.930

2.611 2.560 2.212 2.427

3.397 3.364 2.918 3.237

142.63 152.47 132.00 145.54

0.930 0.980 0.930 0.931 0.929

2.584 2.695 2.398 2.233 2.432

3.356 3.431 3.233 2.932 3.205

140.76 132.24 149.23 131.30 140.68

0.931 0.980 0.931 0.931 0.930

2.615 2.645 2.611 2.399 2.235

3.381 3.410 3.507 3.235 2.955

140.00 135.08 161.68 149.37 133.73

0.931 0.981 0.930 0.930

2.613 2.543 2.216 2.435

3.388 3.344 2.936 3.256

141.04 138.79 133.71 147.08

0.929 0.931 0.930

2.605 2.445 2.647

3.489 3.290 3.565

158.99 151.00 169.22

change in PXRD patterns might be attributed to the solventinduced framework transformations because of the shifting of the frameworks, thus the pore space for the encapsulation in 1 is reduced compared with 1·solvents. In order to investigate the reversibility of the transformation from crystal 1′ to solvates, desolvated crystal 1′ (100 mg) were immersed in (2 mL) corresponding aromatics (toluene, chlorobenzene, o-chlorotoluene, p-chlorotoluene, o-xylene, mxylene, and p-xylene, respectively) at room temperature for 1 day then filtered and dried at room temperature, which is 437



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Table 5. C−H···π, C−Cl···π, and C−N···π Interactions in Crystal 1 and 1a−1h crystal

interaction

distance (Å)a

angle (deg)b

1

C6−H6···anthracene(B) C8−H8···anthracene(B) C10−H10···anthracene(B) C15−H15···benzene(A) C63−N10···anthracene(A) C32−H32A···anthracene(B) C34−H34A···anthracene(B) C36−H36A···anthracene(B) C58−N5···anthracene(B) C63−N7···anthracene(A) C44−H44A···anthracene(B) C46−H46A···anthracene(B) C48−H48A···anthracene(B) C57−N6···anthracene(A) C63−N7···anthracene(B) C70−Cl···anthracene(A) C32−H32···anthracene(B) C34−H34···anthracene(B) C36−H36···anthracene(B) C57−N5···anthracene(B) C64−N8···anthracene(A) C69−H69···anthracene(A) C6−H6A···anthracene(B) C8−H8A···anthracene(B) C10−H10A···anthracene(B) C58−N6···anthracene(B) C63−N7···anthracene(A) C69−H69A···anthracene(A) C71−H71B···anthracene(A) C53−N5···anthracene (A) C59−N8···anthracene (B) C31−H31A···anthracene (B) C33−H33A···anthracene (B) C35−H35A···anthracene (B) C42−H42B···o-xylene C65−H65A···anthracene (A) C57−N5···anthracene (A) C63−N7···anthracene (B) C32−H32···anthracene (B) C34−H34···anthracene (B) C36−H36···anthracene (B) C55−N5···anthracene (B) C59−N8···anthracene (A) C32−H32A···anthracene (B) C34−H34A···anthracene (B) C36−H36A···anthracene (B) C70−H70A···anthracene (A) C15−H15···benzene C31−N4···anthracene C37−H37···anthracene

3.087 2.906 2.631 2.866 3.839(5) 3.073 2.922 2.685 3.638(10) 3.658(5) 3.051 2.934 2.708 3.699 3.651 3.604 2.684 2.881 2.951 3.456(8) 3.847(6) 2.756 2.659 2.884 3.015 3.642(7) 3.670(5) 2.838 2.963 3.762(6) 3.580(9) 2.682 2.900 2.973 2.932 2.635 3.748(6) 3.635(10) 2.717 2.909 2.975 3.666(10) 3.694(6) 2.673 2.919 3.031 2.912 2.848 3.656(2) 2.982

165.95 157.50 156.90 143.92 77.14 155.88 164.99 174.78 77.14 76.48 157.68 168.74 176.16 76.91 74.46 101.58 175.81 168.17 159.31 81.53 74.49 138.45 171.87 159.91 151.38 75.69 76.13 130.58 133.10 77.17 163.31 175.89 166.51 158.20 131.58 153.97 76.73 73.68 175.94 165.09 157.30 74.61 77.18 172.08 160.01 152.08 130.73 144.23 73.49 139.09

1a

1b

1c

1d

1e

1f

1g

1h

Figure 2. Crystal structure of type I solvates. The circles represent toluene, chlorobenzene, o-chlorotoluene, p-chlorotoluene, o-xylene, mxylene, and p-xylene, respectively.

Figure 3. Interframe-solvent interactions in type I solvates.

a

The distances were measured from the hydrogen or nitrogen atom to the center of the aromatic ring (for C−H···π or C−N···π interactions). b The angles were measured between C−H−c or C−N−c (for C− H···π or C−N···π interactions).

Figure 4. Crystal structure of type II solvates.

verified by PXRD (Figure 9 and Figure S1−S6 of the Supporting Information). We found that crystal 1′ was changed to type I solvates 1a−1g. When the same experiment was done in p-xylene, only type I solvate 1g was obtained and no type II

solvates 1h were generated, even at a lower temperature. These results indicate that transformation from 1 to type I solvates 1a−1g was an adsorption process, not a dissolution of the cocrystal and recrystallization process. 438



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Table 6. Thermal Data (DSC/TGA) for Crystal 1 and 1a−1h crystal

calcd weight loss (%)

obsd weight loss in TGA (%)

guest loss Tpeak in DSC (°C)

solvent boiling point (°C)

1a 1b 1c 1d 1e 1f 1g 1h

8.49 10.18 11.31 11.31 9.65 9.65 9.65 17.61

7.9 9.8 10.81 10.54 9.52 8.81 9.40 16.87

168.37 168.44 144.33 139.01 147.60 140.05 139.85 138.48

111 131 158 162 143 139 138 138

Figure 5. Absorption spectra of the solids.

As an example, removing toluene molecules from 1·toluene was used to understand framework transformation between type I solvates and cocrystal 1. We observed that b- and c-axes length and unit cell volume reduced by 5.43%, 6.33% and 9.26%, respectively (Table 1) and the dihedral angles between the anthracene and the pyrazoline rings of type A AMPE molecules have changed from 88.65 to 77.36 deg, which result in the channel structure changed from an expanded one in type I solvates to a narrow compressed shape in 1. These results show that this supramolecular structure, constructed from noncovalent (charge transfer) interactions, is flexible so that the structure can remain intact after removing the solvent and can accommodate different kinds of aromatics. However, the mechanism for the conversion of the type II solvate 1h to crystal 1 is different. When the solvent p-xylene in 1h was removed, the two neighboring racemic chains rotated a certain degree, and the corresponding TCNQ molecules also turned a certain degree, effectively, extending the length of the b axis and resulting in the filling of the void spaces. Additionally, when the single crystals of 1 (100 mg) were immersed into nonaromatic solvents (3 mL), such as CH2Cl2, acetone, methanol, cyclohexane, and tetrachloroethylene, PXRD analysis of these products showed the same as that of cocrystal 1, revealing no adsorption of these nonaromatic solvents (Figure 10). Thus, it was indicated that crystal 1 showed a selective adsorption for aromatics. Such a result demonstrates the solvent property is the dominating factor for this selective sorption. It can be explained by the interactions between these aromatic solvents and AMPE molecules in the crystal (Figure 3). The two types of AMPE and TCNQ create a specific nonpolar interior that might differentiate solvent molecules on the basis of intermolecular interaction and shape.

Figure 7. Experimental (black) and simulated (red) PXRD patterns of crystals 1 and 1a−1h.

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CONCLUSION In summary, we have successfully constructed a novel organic CT cocrystal 1 based on 1-(5-(anthracen-9-yl)-3-(4-methoxyphen-yl)-4,5 -dihydropyrazol-1-yl)ethanone (AMPE) and TCNQ. Eight aromatic inclusion solvates which can be placed

Figure 6. TGA/DSC profiles of crystals 1 and 1a−1h. 439



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proved by a crystal-to-crystal transformation. Cocrystal 1 selectively accommodate aromatic solvents over other polar and nonpolar solvents such as acetone, CH2Cl2, and cyclohexane to form type 1 solvates and this conversion is verified to be an adsorption mechanism. This reversible transformation implies this novel CT cocrystal may have potential applications in purification and separation of aromatics.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR for crystal 1, 1′, and solvates 1a−1d, PXRD patterns of absorption for chlorobenzene, o-chlorotoluene, p-chlorotoluene, o-xylene, m-xylene, and p-xylene, UV−vis spectra of cocrystal 1 in cyclohexane solution, and X-ray crystallographic information files (CIF) for ten crystals. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

Figure 8. Comparing of PXRD patterns of solid obtained after desolvation of 1a−1h (1a′−1h′) with simulated 1.

*E-mail: [email protected]. Tel: +862585092237. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported by the National Basic Research Program of China (Grant 2011CB302004). REFERENCES

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Figure 9. PXRD patterns of crystal 1 (a) before and (b) after absorption of toluene with (c) simulated pattern of 1a.

Figure 10. PXRD patterns of 1 absorbed with (a) dichloromethane + acetone, (b) dichloromethane + methanol, (c) dichloromethane + cyclohexane, (d) tetrachloroethylene + acetone. (e) The simulated PXRD patterns of 1 from SCD.

into type 1 and 2 were obtained from solution. Single-crystal Xray analysis revealed that cocrystal 1 exhibited two racemic charge transfer chains with closed channels. Type I solvates (1a−1g) showed the similar frameworks to 1 with slitlike channels, whereas type II solvate (1h) exhibited a different structure arrangement with square channels. Type 1 solvates converted to 1 at 170 °C without loss of crystallinity, which is 440



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Reversible Accommodation and Desorption of Aromatics on a Charge

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