Understanding Charge-Transfer Interaction Mode in Cocrystals and

Article pubs.acs.org/crystal

Understanding Charge-Transfer Interaction Mode in Cocrystals and Solvates of 1‑Phenyl-3-(pyren-1-yl) Prop-2-en-1-one and TCNQ Hao Sun,† Mingliang Wang,*,† Xiang Wei,† Ruimin Zhang,† Shengzhi Wang,† Arshad Khan,† Rabia Usman,† Qi Feng,† Man Du,† Fangfang Yu,† Wei Zhang,† 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: Three binary charge-transfer cocrystals (I, II, and III) and ten cocrystal solvates (IV(1)−IV(10)) were prepared in different solvents, involving 1-phenyl-3-(pyren-1yl) prop-2-en-1-one (PPPO) as electron donor (D) and 7,7,8,8-tetracyanoquinodimethane (TCNQ) as electron acceptor (A). X-ray diffraction data, vibrational spectroscopy, thermal behaviors, and diffuse reflectance absorption spectroscopy were exploited to investigate their structures. The different charge-transfer interaction mode and molar ratio between donor and acceptor in these crystals can be affected by the change of solvents. In addition, PPPO and TCNQ can selectively accommodate aromatic solvents relative to other solvents except methanol to form an isostructural microporous framework with solvent residing in the channels. These results demonstrate that these crystals can have potential applications in purification and separation of aromatic hydrocarbons.

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INTRODUCTION Charge-transfer (CT) interactions are comparable to hydrogen bonding because of its alternating placement of electron-rich donor (D) and electron-deficiency acceptor (A) units, directional nature, and wider solvent tolerance.1 In the last decades, they have been applied to various domains, such as organic conductive material,2 organic luminescent material,3 energetic material,4 and photovoltaic devices.5 Thus, research of charge-transfer complex materials becomes a fast-growing field in crystal engineering in recent times. Among many other electron acceptors studied, TCNQ has been widely used in chemical research of CT interaction, and it exhibits excellent efficiency and stability of assembling cocrystals with electron donors via charge-transfer interactions, as well as other intermolecular interactions.6 Hence, exploring novel cocrystals involving TCNQ is an important method to realize CT interaction modes. The most common and efficient stratagem to obtain organic cocrystals is slow evaporation of solvents, which makes solvents to be regarded as a complicated and significant role in determining the crystal parameters and properties.7 Different solvents not only yield cocrystals with different crystalline structure but also had noticeable effects on molar ratio.8 Furthermore, the properties of the CT complexes can be regulated by controlling the stoichiometric ratio of the starting materials. In a recent study, a fact has been indicated that the binary CT cocrystal materials could be self-assembled in a certain molar ratio related to stoichiometry of the starting material.9 However, reports on regulation of CT interaction mode and the molar ratio by solvent effects are rather rare.10 © XXXX American Chemical Society

The formation of these cocrystals is usually impeded because the type of cocrystals generally contains amounts of cavities or channels but has insufficient intermolecular interactions to support the frameworks. The introduction of solvent molecules can settle such issues by assembling cocrystals as solvates, and fortunately some of them could still maintain the structure after solvent removal.11 Considering such prominent influence of solvents, assembling cocrystals with different CT interaction mode to make novel complexes can be realized. In this study, we obtained three binary CT cocrystals (I, II, and III) and ten cocrystal solvates (IV(1)−IV(10)) involving 1-phenyl-3-(pyren-1-yl) prop-2-en-1-one (PPPO) and TCNQ (Figure 1). Three binary CT cocrystals were assembled with different molar ratio 2:3 (cocrystal I), 2:1 (cocrystal II), and 1:1 (cocrystal III), respectively. Ten solvates are isostructural with PPPO and TCNQ in molar ratio of 1:1 and contain cavities selectively occupied by aromatic solvents relative to other solvents except methanol. It implies that solvents play a significant role in CT interaction mode and molar ratio between donor and acceptor.

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EXPERIMENTAL SECTION

Crystal Preparation. PPPO was synthesized according to the procedure previously reported in our laboratory.12 TCNQ (CAS 1518−16−7) was purchased from Alfa and used without further purification. Analytical-grade solvents were used. Received: May 12, 2015 Revised: July 15, 2015

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Cocrystal III. PPPO (0.1 mmol) and TCNQ (0.1 mmol) were dissolved in acetone/acetonitrile (20 mL, v:v = 1:1) mixed solvents in glass vials. The black acicular crystals yielded after slow evaporation at room temperature for 2 days. Cocrystal solvates IV(1)−IV(10). PPPO (0.1 mmol) and TCNQ (0.1 mmol) were dissolved in a solvent mixture (20 mL, v:v = 1:1). For preparation of cocrystal solvates IV(1)−IV(9), one solvent is acetone and another is aromatic solvent including p-xylene (1), pdichlorobenzene (2), toluene (3), chlorobenzene (4), o-xylene (5), mxylene (6), o-chlorotoluene (7), m-chlorotoluene (8), and pchlorotoluene (9), respectively. The mixture of solvents used for cocrystal solvate IV(10) is acetone and methanol. The black needle crystals yielded after slow evaporation at room temperature for 24 h. X-ray Diffraction. The PXRD patterns for the 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. 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. FT-IR/Raman/NMR. Infrared spectra were obtained with a Bruker Tensor 27 FT-IR spectrometer. Raman spectra were acquired with a Thermo Fisher Laser Micro-Raman Spectrometer. 1H NMR spectra were recorded at 303 K on a Bruker Avance 500 MHz NMR spectrometer using CDCl3 as a solvent and TMS as an internal standard.

Figure 1. Structures of PPPO and TCNQ. Cocrystal I. PPPO (0.1 mmol) and TCNQ (0.1 mmol) were dissolved in acetone (20 mL) in glass vials. The black cubic crystals yielded after slow evaporation at room temperature for 2 days (Figure 2).

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Figure 2. Micrograph of selective cocrystals at 1000 times magnification.

RESULTS AND DISCUSSION Crystal Structures. Two binary CT cocrystals and three CT solvates are suitable for single-crystal X-ray diffraction analysis. Their crystallographic data are presented in Table 1. Cocrystal I crystallized in the triclinic system and space group P1̅ with one molecule of PPPO and one and a half

Cocrystal II. PPPO (0.1 mmol) and TCNQ (0.1 mmol) were dissolved in acetonitrile (20 mL) in glass vials. The black needle crystals yielded after slow evaporation at room temperature for 2 days.

Table 1. Crystal Data and Structure Refinement crystal

I

II

IV(1)

IV(2)

IV(10)

formula formula weight 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) (Mgm−3) μ(MoKα) (mm−1) θ range for data collection (deg) F(000) R1, wR2 (I > 2σ(I)) R1, wR2(all data) goodness-of-fit, S CCDC

C43H22N6O 638.67 293 0.30 × 0.20 × 0.10 black block triclinic P1̅ 9.0640(18) 11.721(2) 16.445(3) 99.43(3) 105.49(3) 91.66(3) 1656.0(6) 2 1.281 0.71073 1.31−25.37 660 0.1043, 0.1856 0.2235, 0.2257 1.115 1056072

C31H18N2O 434.47 293 0.20 × 0.10 × 0.10 black needle monoclinic P21/c 12.335(3) 10.607(2) 17.974(4) 90.00 106.33(3) 90.00 2256.8(8) 4 1.279 0.71073 1.72−25.40 904 0.0701, 0.1303 0.1837, 0.1708 1.003 1056073

C41H25N4O 589.65 293 0.30 × 0.10 × 0.10 black needle triclinic P1̅ 11.345(2) 11.415(2) 13.446(3) 72.56(3) 76.89(3) 70.07(3) 1546.6(5) 2 1.266 0.71073 1.60−25.38 614 0.0956, 0.1820 0.2090, 0.2252 1.042 1056074

C40H22ClN4O 610.07 293 0.20 × 0.10 × 0.10 black needle triclinic P1̅ 11.306(2) 11.395(2) 13.377(3) 72.38(3) 76.89(3) 69.92(3) 1528.1(5) 2 1.326 0.71073 1.61−25.37 630 0.4570, 0.2230 0.1619, 0.3138 1.083 1056076

C38H24N4O2 568.61 293 0.20 × 0.10 × 0.10 black needle triclinic P1̅ 11.176(2) 11.394(2) 13.466(3) 72.83(3) 78.68(3) 69.39(3) 1525.3(6) 2 1.238 0.71073 1.59−25.38 592 0.0955, 0.1585 0.2120, 0.1948 1.059 1056075

B

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molecules of TCNQ in the asymmetric unit. In crystal I, two pyrene rings and three TCNQ molecules form the framework of five layers via strong charge-transfer interactions and adopt a face-to-face π-stacked arrangement (Figure 3). In addition, the

Table 2. Dihedral Angles of PPPO Molecules in I, II, IV(1), IV(2), and IV(10) crystal

angles (deg)a

I II IV(1) IV(2) IV(10)

2.79 27.02 63.9 63.77 67.19

a

The dihedral angles between the benzene rings and pyrene rings of the PPPO molecules.

pyrene rings of two independent PPPO molecules adopting a face-to-face π-stacked arrangement with the closest distance of 3.44 Å and forming a face-to-face sandwich-like mode via strong charge-transfer interactions along b axis (Figure 3). Furthermore, two neighboring PPPO molecules are connected via a weak face-to-face π-stacked interaction, which is generated by two adjacent pyrene rings. Two types of hydrogen bonds, C−H···O and C−H···N hydrogen bond, contribute to maintain the framework and twist the dihedral angle between the pyrene ring and benzene ring of the PPPO molecule (Table 3). Solvates IV(1), IV(2), and IV(10) are isostructural with a triclinic system and space group P1,̅ which contains porous framework and guest inclusions (aromatic solvent or methanol) encapsulated in the channels (Figure 3). Structure determination shows that solvates IV(1), IV(2), and IV(10) have the similar structure and that TCNQ adopts a face-to-face sandwich-like mode with the pyrene moiety of PPPO via charge-transfer interactions, which can be confirmed by the interplanar separation (dπ−π) and the closest centroid distance (dc−c) (Table 4). The molar ratio for IV(1) and IV(2) is 2:2:1 (PPPO/TCNQ/aromatic solvent) and the molar ratio for IV(10) is 1:1:1 (PPPO/TCNQ/methanol). The smaller size of the methanol molecule causes a greater amount of captured methanol molecules in every hole and C−H···O hydrogen bonds between methanol molecules with PPPO and TCNQ molecules, which can explain the noticeable variance of molar ratio between two types of solvates. The aromatic solvent molecules were stabilized in the channels via C−H···π

Figure 3. Crystal structures of I, II, IV(1), IV(2), and IV(10) (violet balls represent one aromatic solvent for IV(1) and IV(2) or two methanol molecules for IV(10)).

lateral TCNQ molecule partly overlaps with the adjacent axialdirection benzene rings on account of tension caused by C− H···N hydrogen bonds (Figure 4). This twisted structure leads to two different centroid−centroid distances (dc‑c) of TCNQ molecule to pyrene ring in the five layer framework, which is 3.61 Å for the center TCNQ molecule and 3.45 Å for the lateral TCNQ molecule. The C−H···N hydrogen bonds also induce the torsion of the dihedral angle between the pyrene ring and benzene ring in single PPPO molecule, which almost obligates these rings in the same plane (Table 2). Crystal II crystallized in the monoclinic system and space group P21/c with one molecule of PPPO and half molecule of TCNQ in the asymmetric unit. Other than crystal I, cyclohexadiene rings of TCNQ molecules locate close to two

Figure 4. Hydrogen bond interactions in these crystals. C

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the channels via C−H···O hydrogen bonds and weak van der Waals interactions in IV(10) (Figure 5 and Table 5).

Table 3. Intermolecular Hydrogen Bonds Parameters in Crystals I, II, IV(1), IV(2), and IV(10) crystal

D-H (Å)

C22−H22A···N2 C7−H7A···N2 PPPO5−H15A···N3 C40−H40A···N1 C25−H25A···N3 C36−H36A···N5

0.930(2) 0.930(2) 0.930(2) 0.931(2) 0.930(2) 0.930(2)

H···A (Å)

D···A (Å)

∠D-H···A (deg)

2.637(2) 2.655(2) 2.718(3) 2.654(2) 2.624(2) 2.727(3)

3.508(2) 3.431(3) 3.615(3) 3.337(3) 3.523(2) 3.396(2)

156(3) 141(3) 162(3) 130(3) 163(3) 129(3)

The distances were measured from hydrogen to the center of the aromatic ring (for C−H···π). bThe angles were measured between C− H−C (for C−H···π).

3.258(2) 3.508(2)

131(3) 146(3)

Powder X-ray diffraction (PXRD) of all crystals were performed, as shown in Figure 6. It is noticeable that the

3.313(2) 3.292(2)

135(3) 155(3)

3.311(2) 3.283(2)

134(3) 154(3)

3.261(2) 3.273(2) 3.834(3) 3.615(2) 2.200(3)

140(3) 156(3) 150(3) 165(3) 153(3)

Table 5. C−H···π Interactions in Crystals IV(1) and IV(10)

I

C27−H27A···O C21−H21A···N1 PPPO5−H15A···N2 C34−H34A···O IV(2) PPPO5−H15A···N4 C40−H40A···O PPPO5−H15A···N3 C36−H36A···O1 C30−H30A···O2 PPPO1−H11A···O2 O2−H2C···O2

II 0.931(2) 2.572(2) 0.930(2) 2.696(2) IV(1) 0.930(2) 2.590(2) 0.929(2) 2.430(2) 0.930(2) 2.600(2) 0.930(2) 2.420(2) IV(10) 0.930(2) 2.500(2) 0.929(2) 2.400(2) 0.930(2) 2.998(3) 0.931(2) 2.707(3) 0.851(3) 1.408(3)

crystal

interaction

distance (Å)a

angle (deg)b

IV(1) IV(10)

PPPO1−H11A···p-xylene PPPO1−H11A···p-dichlorobenzene

2.865 2.850

144 143

a

Table 4. Face-to-Face π-Stacking Interactions between TCNQ and Pyrene Moieties crystal I II IV(1) IV(2) IV(10)

interaction TCNQ···pyrene TCNQ···pyrene TCNQ···pyrene pyrene···pyrene TCNQ···pyrene TCNQ···pyrene TCNQ···pyrene TCNQ···pyrene TCNQ···pyrene TCNQ···pyrene

dπ−π, dc‑c (Å)a 3.508, 3.432, 3.417, 3.467, 3.371, 3.353, 3.339, 3.350, 3.372, 3.366,

3.615 3.457 3.447 5.082 3.384 3.370 3.355 3.367 3.389 3.376

angle (deg)b

Figure 6. PXRD patterns of PPPO, TCNQ, I, II, III, and IV series.

5.74 4.43 3.27 0.43 0.34 2.77 1.31 1.07 1.49 2.22

PXRD patterns are different from that of PPPO and TCNQ, and different space stacking mode and intermolecular interactions are reflected in the PXRD patterns of cocrystals I, II, and III. PXRD patterns of I, II, III, and IV series have distinct differences, which provide unambiguous proof to explain the discrepancy of these crystals. Index of X-ray powder diffraction data was accomplished to analyze PXRD in an acceptable manner. For five single crystals, the results of index indicate that the experimental PXRD data are in good agreement with the simulated data obtained from the singlecrystal diffraction (Tables S1 and S2), indicating a good phase purity of the crystals. Together with the thermal analysis data, the index of PXRD data clearly explain the similarity of the structure for IV(1)−IV(10).

a The interplanar separation (dπ−π) and the closest centroid distance (dc‑c). bAngles were measured between the mean planes of TCNQ (pyrene) and pyrene rings (for π−π interaction).

interactions and weak van der Waals interactions in IV(1) and IV(2), while the methanol solvent molecules were stabilized in

Figure 5. π···π and C−H···π interactions in I, II, IV(1), IV(2), and IV(10) (violet balls represent one aromatic solvent for IV(1) and IV(2) or two methanol molecules for IV(10)). D

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Due to their strong absorption that extends across the visible and into the near-infrared, these crystals appear black, which is different from PPPO (orange) and TCNQ (yellow). The CT absorption band and the color variance of the crystals are the prominent characteristics of CT interaction cocrystals. Thermal Behavior. Thermogravimetric analysis and differential scanning calorimetry (TGA/DSC) were applied for investigating the thermal behaviors of all of these crystals. Figure 8 illustrates that cocrystals I and II exhibit one endothermic peak at 204 and 186 °C between two original compounds, respectively. However, for cocrystal III there are two endothermic peaks at 109 and 201 °C, respectively. This phenomenon is possibly induced by crystal transformation through the thermal treatment process,15 which indicates that the first peak is contributed to the endothermic process of crystal transformation, and the formation of second peak is because of the eutectic point; it verifies the possibility for binary cocrystals of TCNQ and PPPO. In addition, the ten solvates exhibit three endothermic peaks. The first peak corresponds to the processes of removing solvents, and the second and third peaks, similar to cocrystal III, are contributed to endothermic process of crystal transformation and melting process of the remnant solid, which also reveals the similarities of the structure between the cocrystal III and IV series. As shown in TGA profiles of IV series, a solvent weight loss occurs from 100 to 200 °C. According to SCXRD and 1H NMR analysis, the actual weight loss coincides well with the theoretical weight loss of type A solvates with the molar ratio of PPPO/TCNQ/solvents = 2:2:1, and type B solvates with a molar ratio of 1:1:1 (Table 6). FT-IR and Raman Spectroscopy. FT-IR and Raman spectra were used to identify the noncovalent interactions within the crystals. The main bands of FT-IR spectra can be assigned to the respective vibrational modes of PPPO and TCNQ, as shown in Figures S6 and S7. Although changes of the FT-IR and Raman spectra of the cocrystals in many regions are not remarkable compared with the individual components, there are several significant differences that emphasize the presence of distinct interactions between PPPO and TCNQ. Compared with those of cocrystal III, the absorption peaks of CN at 2218 cm−1 and CO at 1660 cm−1 in I and II undergo a slight shift to lower frequency of 1−3 cm−1 because of the CT effect, which could explain the different π−π distance in these crystals. Additionally, cocrystals I, II, and III have four characteristics peaks at 1626, 1603, 1586, 1576 cm−1; 1627, 1601, 1586, 1576 cm−1; and 1625, 1598, 1584, 1574 cm−1 in Raman spectra, respectively. The slight downshift by 1−2 cm−1 attributes to the change in the hydrogen bonds and π−π stacking interactions. FT-IR and Raman spectra for solvates IV(1)−IV(10) maintain consistency on the account of similar frameworks.

As can be seen from the structural analysis above, cocrystals of different molar ratios between PPPO and TCNQ in CT crystals have potential relation to space stacking mode and intermolecular interactions, which can be mediated by solvents. An equilibrium point is assumed to make the process of crystallization comprehensible.13 The equilibrium point refers to the relative balanced condition for two original components in solution, which make the intermolecular interaction between two components greater than between original components and solvent molecules to form a stable steric configuration when it reaches beyond the equilibrium point. Most obviously, solubility of PPPO and TCNQ varies with the alteration of solvents, which induces the fluctuation of the relative concentration for two original components during the process of solvent evaporation. As the saturability of PPPO and TCNQ rises, a stable steric configuration forms via strong CT interactions when the system attains the equilibrium point, and it will dissolve out as a crystal in a certain molar ratio. For PPPO and TCNQ, a stable steric configuration of molar ratio as 1:1 is easily formed in solvent, which can be observed through the crystallization period. However, this type of cocrystal solvate contains amounts of cavities or channels supported by solvent molecules, and the framework will collapse after solvent removal. The phenomenon also occurs throughout the formation of two binary cocrystals. A stable steric configuration of molar ratio 1:1 dissolves out and collapses, and the solvent resolves the components until the system reaches another equilibrium point. The structures of I and II, which seems like the main framework of the solvates after torsion and translation, provide an explicit evidence to support the assumption. Thus, it is possible to assemble cocrystals of tunable molar ratio via solvent-mediated method. Solid-State Adsorption Spectra. Diffuse reflectance absorption spectroscopy (DRS) was exploited to investigate the CT interaction of these cocrystals.14 As shown in Figure 7,

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CONCLUSION In summary, we have successfully constructed a series of novel CT crystals from 1-phenyl-3-(pyren-1-yl) prop-2-en-1-one (PPPO) and TCNQ with tunable molar ratio, which could be controlled by solvent-mediated method. X-ray analysis revealed that all three binary cocrystals have different CT interaction modes, while solvate IV series exhibits a similar structure with closed channels selectively filled with aromatic solvent molecules and surrounded by PPPO and TCNQ (1:1). The concept of equilibrium point was assumed to make the variance of molar ratio comprehensible. In fact, the structure of

Figure 7. Absorption spectra of the solids.

all these cocrystals exhibit a new broad absorption band from 600 to 750 nm. Meanwhile, these cocrystals retain the absorption band of PPPO and TCNQ, which evidently illuminates the formation of CT complexes. Compared with cocrystal III, a red-shift occurs in the absorption band of cocrystals I and II, which suggests a stronger degree of the CT interaction. In addition, similar structure of ten cocrystal solvates induces the similar absorption bands as we expected. E

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Figure 8. TGA/DSC profiles of PPPO, TCNQ, I, II, III, and IV series. (a) TGA profiles of PPPO, TCNQ, I, II, and III. (b) DSC profiles of PPPO, TCNQ, I, II, and III. (c) TGA profiles of IV series. (d) DSC profiles of IV series.

*E-mail: [email protected].

Table 6. Thermal Data (DSC/TGA) for IV Series crystal

calcd weight loss (%)

obsd weight loss in TGA (%)

guest loss Tpeak in DSC (°C)

solvent boiling point (°C)

IV(1) IV(2) IV(3) IV(4) IV(5) IV(6) IV(7) IV(8) IV(9) IV(10)

8.99 12.06 7.90 9.50 8.99 8.99 10.55 10.55 10.55 5.63

8.51 11.21 7.23 9.63 9.11 8.05 11.41 11.06 10.38 6.01

168.37 139.85 143.18 130.82 139.78 125.43 127.40 120.67 132.50 111.82

138.5 173.4 110.6 132.2 144.4 139.0 158.5 159.8 162.0 64.7

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).

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two binary cocrystals, which seems like the main framework of the solvates after torsion and translation, provides a explicit evidence to support the assumption. These results imply that these CT crystals may provide an innovative approach in supramolecular design and potential applications in purification and separation of aromatic hydrocarbons.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR for crystals I, II, III, IV(1), and IV(2), IR/Raman spectra for I, II, III, and IV series, and X-ray crystallographic information files (CIF) for I, II, IV(1), IV(2), and IV(10). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b00656.

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REFERENCES

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DOI: 10.1021/acs.cgd.5b00656 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

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DOI: 10.1021/acs.cgd.5b00656 Cryst. Growth Des. XXXX, XXX, XXX−XXX