Crystal Growth, Dynamic and Charge Transfer Properties of New

Nov 16, 2015 - The combination of X-ray diffraction and electrochemistry in the solid state and a time-resolved one in solution allowed us to clarify ...
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Crystal Growth, Dynamic and Charge Transfer Properties of New Coronene Charge Transfer Complexes Olga Kataeva,*,† Mikhail Khrizanforov,† Yulia Budnikova,† Daut Islamov,‡ Timur Burganov,† Alexander Vandyukov,† Konstantin Lyssenko,§ Benjamin Mahns,# Markus Nohr,# Silke Hampel,# and Martin Knupfer# †

A. E. Arbuzov Institute of Organic and Physical Chemistry, Russian Academy of Sciences, A.E. Arbuzov str. 8, Kazan 420088, Russia A. M. Butlerov Chemistry Institute, Kazan Federal University, Kremlevskaya str. 18, 420008, Kazan, Russia § Institute of Organoelement Compounds, Russian Academy of Sciences, Vavilova str. 28, 119334 Moscow, Russia # IFW Dresden, P.O. Box 270116, D-01171 Dresden, Germany ‡

S Supporting Information *

ABSTRACT: Two new coronene charge transfer complexes with F4TCNQ of 2:1 and 1:1:1 (solvate with acetonitrile, MeCN) stoichiometry were obtained using crystal growth procedures from the solution and vapor phase. It was shown that mobility of coronene molecules in crystals is more affected by the asymmetry of its surrounding than by the composition and degree of charge transfer and interstack interactions. The combination of X-ray diffraction and electrochemistry in the solid state and a time-resolved one in solution allowed us to clarify the nucleation in solution showing that the formation of 2:1 coronene/F4-TCNQ complexes is thermodynamically preferable. The X-ray single crystal data for pristine coronene showed the crystal structure to be the same as at ambient temperature, raising doubt about the previously reported phase transitions at 140−180 K.

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where coronene is used as a guest molecule.11,12 All these properties of the coronene as a part of multicomponent systems are to a large extent determined by its disk-like shape and high symmetry (D6h). The disk-like shape harbors an additional degree of freedom to coronene in crystalline materials and allows its rotation around the axis perpendicular to the molecular plane. Yoshida and co-workers13,14 reported the molecular motion of coronene in a series of charge-transfer complexes with TCNQ derivatives and nickel bis(maleonitriledithiolate). This motion was revealed by variable temperature X-ray single crystal diffraction and solid state NMR study. The presence of dynamic motion in molecular chargetransfer materials destroys or reduces the ordering of the crystal structure and inhibits effective charge transport. However, it can also trigger new applications of the material in the field of molecular machines. The decisive factor responsible for the low rotational barrier of the coronene molecule almost in any crystalline environment is its flat geometry with high axial symmetry. Only several weak interactions contribute to the barrier height.14,15 Solid NMR studies of the pristine coronene showed the rotational barrier to be equal to 27 kJ/mol, which is higher than the barrier in mixed crystals of coronene.14 Besides

INTRODUCTION A revival of interest to charge-transfer (CT) systems in the last few years is predominantly caused by their new applications in molecular electronics.1−4 A theoretical analysis of the charge transport performed for a series of CT mixed crystals5 revealed that the transfer integral can be much larger than in the best single-component semiconductors. A typical two-component charge-transfer complex consists of an electron-donor and electron-acceptor molecule, respectively. These partner molecules should have matching boundary orbitals and complementary spatial shapes to provide effective interaction and ordered structure. As a donor component of the complex, polyaromatic hydrocarbons (PAH) are of major advantage due to their extended electron-rich π-system and planar structure, which provides effective interaction with planar acceptor molecules and enhances the lateral ordering in the thin films, as well as close packing and order in single crystals. Among the PAHs, coronene has attracted special attention. Its co-crystal with hexanuclear copper(I) complexes exhibits photoluminescence,6 and the electron transfer from intercalated alkali metals to the coronene molecule resulted in the formation of its radical anions.7,8 A system based on coronene radicals with molybdenum chloride cluster was shown to be a 3D π-conductor.3 Also important are the studies of binding interactions of coronene to various surfaces: graphite,9 silver and gold,10 or host−guest interactions in crystalline compounds © 2015 American Chemical Society

Received: September 8, 2015 Revised: November 9, 2015 Published: November 16, 2015 331

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the dynamic motion, in many co-crystals, the coronene molecule exhibits static positional disorder.16,17 Our study is an attempt to acquire the control over the behavior of coronene in mixed crystals and to study the complex formation process in solution. This is illustrated using two new crystalline charge transfer complexes (Scheme 1) Scheme 1. Structural Formulas of Coronene (Left) with the Labelling Scheme, and F4-TCNQ (Right), Bond Lengths a, b, c, d Directly Correlate with Charge Transfera Figure 1. Quartz ampule after growth procedure. The insets show a detailed view of the obtained crystals of pure coronene (right), pure F4-TCNQ (left), and coronene/F4-TCNQ charge transfer crystals (2:1) (middle). counter electrode. 0.1 M Et4NBF4 was used as the supporting electrolyte for the determination of current−voltage characteristics. Acetonitrile was distilled over P2O5 and KMnO4, and then over molecular sieves. After purification, the solvent was stored under dry argon. Used as a base salt, Et4NBF4 was recrystallized from ethanol and dried in a vacuum chamber at 100 °C for 2 days. To study powder samples, a modified CPE working electrode was used. Its preparation was as follows: the carbon particles/ phosphonium salt (dodecyl(tri-tert-butyl)phosphonium tetrafluoroborate) composite electrode was prepared by grinding a mixture of graphite powder and phosphonium salt in a ratio of 90/10 (w/w) in a mortar to give the homogeneous mass.25,26 A modified electrode was made in a similar manner except that a part (ca. 5%) of graphite powder was replaced by the coronene/F4-TCNQ complex under investigation. A portion of the resulting paste was packed firmly into the cavity (3 mm in diameter) of a Teflon holder. Vibrational Spectroscopy. IR spectra of single crystals 1 were recorded using an IR microscope Hyperion 2000 attached to IR Fourier spectrometer Tensor-27 (Bruker). The number of scans was 128, and the resolution was 4 cm−1.

a

CT complex 1: Coronene/F4-TCNQ (ratio 2:1). CT complex 2: Coronene/F4-TCNQ/MeCN (ratio 1:1:1).

incorporating coronene and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) components, exhibiting drastically different behavior of the coronene molecules. The donor− acceptor interactions are characterized by IR spectroscopy and cyclic voltammetry in the crystalline phase and the solution; the crystal structure and coronene motion are studied by variable temperature X-ray single crystal diffraction.

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EXPERIMENTAL SECTION

Crystal Growth. Co-crystals 1 of coronene and F4-TCNQ (2:1) were grown using physical vapor transport in closed, evacuated quartz glass ampules (p ≈ 3 × 10−5 mbar) using a horizontal one-zone oven heated to 220 °C, as described in our previous paper.18 Several attempts were made to grow crystals of various stoichiometry using different starting ratios of the individual components; however, all experiments gave the same 2:1 co-crystals. Co-crystals 2 of coronene/F4-TCNQ/MeCN (1:1:1) were grown by slow evaporation of the solution of coronene (1.5 mg, 0.5 mmol) and F4-TCNQ (1.4 mg, 0.5 mmol) in acetonitrile. X-ray Crystallography. Variable temperature X-ray data for cocrystal 1 were collected at six temperature settings from 100 to 260 K with the step of 40 deg, as well as at ambient temperature of 293 K using Cu Kα radiation. For comparison, the same crystal was studied using Mo Kα radiation at 100 and 260 K. X-ray data for co-crystal 2 were collected at 100 and 293 K using Cu Kα radiation. Coronene single crystals obtained in the quartz ampule during the co-crystal growth were also studied at 100 K using Cu Kα radiation. All data sets were collected using a Bruker AXS Kappa Apex Duo diffractometer. The applied analysis programs are listed in the following: data collection APEX2,19 data reduction SAINT,20 absorption correction SADABS,21 structure solution SHELXS,22 and structure refinement by full-matrix least-squares against F2 using SHELXL.22 Hydrogen atoms were placed into calculated positions and refined as riding atoms. The figures were generated using Mercury 3.1 program23 and ORTEP.24 Details of the crystal data and refinement for the data collected at 100 K are given in Table 1, and the details of data collection carried out at other temperature settings are in Supporting Information. Electrochemistry. Electrochemical measurements were performed on a BASiEpsilonE2P electrochemical analyzer (USA). The program handles wave Epsilon-EC-USB-V200. A conventional three-electrode system was used with glassy carbon for solutions or carbon paste electrode (CPE) for powder samples as the working electrode, the Ag/ AgCl (0.01M) electrode as the reference electrode and a Pt wire as the

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RESULTS AND DISCUSSION Taking into account the molecular motion of the coronene molecules in various CT complexes, it was desirable to study the crystal structure at lower temperature to minimize the systematic errors due to possible disorder. Thus, we have performed the X-ray single diffraction experiments in the range 100−293 K for the CT complexes as well as for individual coronene. Crystal Structure of Coronene at 100 K. Although the crystal structure of individual coronene was determined several times,27−29 all studies were carried out at ambient temperature. The low temperature structure has not been determined so far, as coronene was reported to undergo two structural phase transitions at 50 K and in the range of 140−180 K, as determined from the analyses of the luminescence spectral shapes.30−32 Furthermore, its slow cooling resulted in the single crystal breakage.14 We have managed to carry out the X-ray single crystal diffraction study of the coronene crystals at 100 K in the following way. The crystals were washed with Fomblin, mounted on a cactus needle, and put directly into the cold nitrogen stream of the diffractometer. No crystal crack happened as it was observed upon slow cooling.14 The data collection at 100 K gave exactly the same space group and crystal structure (Table 1) as the one reported for ambient temperature.27−29 No signs of a structural phase transition or small symmetry distortions were revealed. Thus, some 332

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Table 1. Crystallographic Data for Coronene and Charge Transfer Complexes compound reference

coronene

complex 1

complex 2

temperature, K chemical formula formula mass radiation type crystal size, mm3 crystal system space group no. of formula units per unit cell, Z a, Å b, Å c, Å β, ° unit cell volume, Å3 no. of reflections used for unit cell measurement Dcalc, g·cm−3 absorption coefficient, μ/mm−1 2θmax, ° no. of reflections measured no. of independent reflections no. of observed reflections (I > 2σ(I)) Rint no. of parameters final R1 values (I > 2σ(I)) final wR(F2) values (I > 2σ(I)) final R1 values (all data) final wR(F2) values (all data) goodness of fit on F2 Δρmax, Δρmin (e Å−3) completeness

100(2) C24H12 300.34 Cu Kα 0.23 × 0.08 × 0.03 monoclinic P21/n 2 10.0086(4) 4.6651(2) 15.5437(6) 106.576(3) 695.59(5) 3517 1.434 0.623 134.72 14036 1233 976 0.0354 109 0.0454 0.1272 0.0573 0.1340 1.131 0.141, −0.183 0.992

100(2) 2C24H12·C12F4N4 876.83 Cu Kα 0.48 × 0.12 × 0.08 monoclinic P21/n 2 10.5429(5) 9.5360(4) 19.3078(9) 93.082(1) 1938.3(2) 9880 1.502 0.835 134 21119 3432 3297 0.0191 307 0.0463 0.1245 0.0475 0.1255 1.049 1.150, −0.263 0.993

100(2) C24H12·C12F4N4·C2H3N 617.55 Cu Kα 0.31 × 0.03 × 0.03 monoclinic C2/c 4 21.6173(6) 6.9678(2) 18.3594(5) 104.494(3) 2677.4(1) 2780 1.532 0.939 133 18373 2352 1964 0.0405 213 0.0341 0.1014 0.0441 0.1138 0.898 0.169, −0.218 0.992

the overlap of the neighboring coronene molecules in pristine coronene and in complex 1 is significantly different (Figure 2):

additional data are required to clarify the topic of potential coronene phase transitions at 140−160 K, as luminescence gives no direct evidence in favor of phase transition, and at the same time the latter cannot be excluded based on our X-ray single crystal data, as the sharp cooling and the coating of the crystal with Fomblin could result in the stabilization of the metastable phase. Crystal Structure of Coronene CT Complexes. The only CT complex of coronene with F4-TCNQ described in the literature13 was grown from dichloromethane/pentane solution and has the ratio of donor and acceptor molecules 1:1; the coronene/TCNQ systems with the ratio 1:1 and 3:1 are also reported.13,33 In crystal 1 of coronene/F4-TCNQ (2:1), which is the only CT phase obtained in multiple physical vapor growth experiments regardless of the ratio of individual components and growth conditions, the asymmetric unit contains only one coronene and half of the F4-TCNQ molecule. The molecules of coronene and F4-TCNQ are packed in face-to-face stacks with the alternation of two donor and one acceptor molecules. The donor and acceptor molecules in stacks are not completely parallel with the dihedral angle between the coronene plane and the six-membered ring plane of F4-TCNQ equal to 2.3°. The F4-TCNQ molecule is not ideally planar; two of the nitrile groups are out of plane by 0.17 Å. The quantum chemistry calculations34,35 of the coronene dimers revealed the graphite-like structure to correspond to an energy minimum, and the offset and interplanar distance to be interdependent.36,37 The barriers for going from one energy minimum to another one are very small 0.48 kcal/mol. Indeed,

Figure 2. Respective positions of coronene molecules in crystals of individual coronene (left) and in complex 1 (right), view perpendicular to the mean coronene plane.

at 100 K in coronene it is ring-over-bond stacking with an interplanar distance 3.40 Å and the slippage 3.2 Å, while in the complex 1 it is ring-over-atom overlap with a slightly larger distance 3.43 Å, but much smaller slippage of 1.7 Å. A very similar respective orientation of two coronene molecules with exactly the same interplanar distance is observed in the 3:1 complex of coronene with TCNQ.13 An example of the CT coronene complex (2) containing solvate molecules was grown by slow evaporation from solution in acetonitrile; it is a three component system, composed of coronene, F4-TCNQ, and acetonitrile (1:1:1). The coronene and F4-TCNQ are arranged in alternating stacks, while the acetonitrile molecules fill the channels between the stacks. The arrangement of the stacks is significantly different in coronene complexes of different stoichiometry (Figure 3): in complex 1 333

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Figure 3. Fragments of crystal packing (view along the stacks) of the charge transfer complexes 1 (left) and 2 (middle) and the coronene/F4-TCNQ (1:1) complex14 (right).

Figure 4. Fragments of crystal packing showing herringbone packing in crystals 1 and 2.

the stacks form an interpenetrating arrangement, while in complexes with a 1:1 ratio of donor and acceptor molecules the stacks are arranged in layers. The corresponding differences in crystal packing are clearly seen in the projection showing a herringbone packing motif (Figure 4). Interesting enough is that the coronene/F4-TCNQ (1:1) complex14 crystallizes in P1̅ space group and does not have a herringbone arrangement; the stacks are parallel to each other. Another important issue is the respective position of the donor and acceptor molecules which should provide efficient charge transfer and the close packing in crystal. In most of the coronene charge transfer salts the respective positions of the donor and acceptor molecules are similar to the ring-over-bond overlap (Figure 5). At 100 K the interplanar distance between the donor and acceptor molecules in the complexes is in the range between 3.245 and 3.269 Å, while the slippage varies in larger limits (Table 2). In spite of the different side-on interactions, the dihedral angles between the vectors passing through 1,4 carbon atoms of the central six-membered rings of

Table 2. Selected Geometrical Parameters of Complexes and the Evaluation of Charge Transfer Using Kirstenmacher Approach38 parameter

complex 1

complex 2

complex (1:1) coronene /F4-TCNQ14

T, K a, Å b, Å c, Å d, Å αCT δa, e Rb, Å slippage, Å

100(2) 1.346(2) 1.441(2) 1.377(2) 1.439(2) 0.478 0.05 3.258 1.076

100(2) 1.349(2) 1.440(2) 1.379(2) 1.435(2) 0.48 0.14 3.245 1.268

100(2) 1.345(2) 1.440(2) 1.380(2) 1.438(1) 0.479 0.10 3.243 1.322

complex picene/F4TCNQ18 150(2) 1.343(3) 1.441(3) 1.383(3) 1.429(3) 0.481 0.19 3.253 -

δ - value of the charge transfer, δ = αCT-α0/α−1-α0, where α=c/(b+d); α−1 - F4-TCNQ charged by one electron,39 α0 pure F4-TCNQ.40 bRdistance between centroid (F4-TCNQ ring) and the mean plane of the coronene. a

the donor and acceptor molecules vary within a narrow interval from 5 to 13°. The interplanar distance between the donor and acceptor molecules is generally accepted as a measure of the strength of the charge transfer interactions; on the other hand the amount of the transferred charge can be estimated via the vibrational frequency shift in the IR and Raman spectra, as well as from the bond lengths changes in the acceptor molecule upon complexation (Table 2). Charge Transfer Evaluation from the Single Crystal Data and IR Spectroscopy. Concerning the Kirstenmacher approach one should keep in mind that direct comparison of the transferred charge values (δ) must be preferably performed for the same temperature or comparable displacement parameters of the fragment. For example the δ values for

Figure 5. Respective positions of the donor and acceptor molecules in charge transfer complexes of coronene with F4-TCNQ, view perpendicular to the mean coronene plane. 334

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crystals 1 and 2 calculated for room temperature data are two times higher than those at 100 K; that is solely the consequence of the bond lengths’ shortening due to thermal motion. Thus, we have performed all comparisons for low-temperature data. As one can see from the Table 2 the observed variation of charge transfer degree (δ) cannot be simply rationalized as the function of the type of overlap (see Figure 5) and/or interplane distance (R). The absence of such correlation can be a reason for some deficiency of this approach or probably additional factors affecting the charge transfer, for example, the mobility of coronene molecule and mutual disposition of stacks. Furthermore, the usage of another approach based on vibrational spectroscopy leads to an almost equal or even slightly larger δ value for 1 in comparison to the picene/F4TCNQ complex.18 Unfortunately the same value for complex 2 cannot be estimated via the CN-frequencies due to the presence of CH3CN solvate molecule. In the complex coronene/F4-TCNQ 1:1, reported earlier13 the stretching vibration ν(CC) is observed at the same frequency 1591 cm−1, indicating a very similar charge transfer amount.

Figure 7. CV of F4-TCNQ (A), coronene (B) (10−3 M), complex 1 and 2 (C and D respectively). The colors indicate the assignment of the peaks: yellow − F4-TCNQ, green and blue − reduction and oxidation of coronene, respectively.

complex 2 the potential is (Epred = −0.71 V, Figure 7D) considerably larger than for the 1:2 complex (Epred = −1.80 V, Figure 7C). That is, the energy of LUMO of coronene decreases. Reduction potentials of F4-TCNQ in the complex are also shifted to smaller reduction potentials; i.e., its reduction is easier and its LUMO decreases upon complexation. Coronene oxidation potentials in the complex are increased as compared with the free coronene; i.e., the HOMO level is higher. The difference between the first reduction potentials of F4-TCNQ (in complex) and oxidation of coronene in CT salts is ΔE 0.70 V for complex 2 and ΔE = 0.62 V for the complex 1. This may indicate a decrease in the value of the complex electrochemical gap compared with the value for the individual components (0.86 V); i.e., an increase in the polarizability of the system upon complexation is observed. Thus, cyclic voltammetry shows the significant change in the electronic properties of the electroactive units F4-TCNQ and coronene in the complex. Moreover in our case cyclic voltammetry can give valuable information about the association of donor and acceptor molecules in solution and nucleation processes of crystal growth in solution. The CV of individual acceptor molecule F4-TCNQ was compared with the data for its equimolar solution with the donor coronene compound in acetonitrile. The CVs for the solution of the mixture of F4-TCNQ acceptor and coronene donor (1:1) were recorded with regular time intervals during 96 h. The corresponding plots show complicated processes of donor−acceptor interactions. The CV recorded immediately after the solution preparation shows the reduction peaks of the individual F4-TCNQ (Figure 8). Small shifts of the peaks and the deformation of the curves are clearly seen already in an hour (Figure 8). Significant changes are observed after the solution was stirred during 24 h in an inert atmosphere at ambient temperature. Reduction currents of the original nonbound F4-TCNQ solution gradually decrease (at 0.54 and 0.01 V), as the concentration of “free” acceptor decreases, and new peaks at more positive potentials (in the region of +1.0+0.8 V) appear, which can be attributed to the F4-TCNQ bound with coronene in the complexes of unclear variable composition. On the CV recorded after 24 h one can see the oxidation peaks corresponding to the complex 1:1 (with redox peaks 1.30/1.20 V) and to the complex of unknown composition (0.87/0.80 V). After 48 h the peaks which can be attributed to the complex with donor−acceptor ratio 2:1

Figure 6. IR spectra for the single crystals of complex 1 (blue) and of the complex picene/F4-TCNQ18 (red) in the CC (left) and CN (right) stretching mode regions.

Electrochemistry. A deeper insight into charge transfer interactions in the coronene complexes might be obtained by cyclic voltammetry, as electrochemistry is a sensitive probe for the analysis of the frontier molecular orbitals. For this purpose we have compared the redox properties of the individual components and CT salts in the crystalline phase. In addition the dynamics of interactions in solution was also analyzed by cyclic voltammetry. The redox properties of powder samples are summarized in Figure 7; all potentials were measured vs Ag/AgCl and recalculated vs the ferrocenium/ferrocene couple(Fc+/Fc)). The cyclic voltammetry performed for individual powder samples of F4-TCNQ (Figure 7A) and coronene (Figure 7B) shows that they undergo one-electron oxidation and reduction reversible steps. The cyclic voltammogram of individual acceptor F4-TCNQ reveals a two-wave reversible reduction with peak reduction potentials equal to 0.54 V and 0.01 versus the Ag/AgCl (0.13 V and −0.40 V ref Fc/Fc+), which corresponds to the formation of the radical anion F4-TCNQ•− and dianion F4-TCNQ2− species (Figure 7A). The donor (coronene) exhibits the reversible oxidation wave at +1.40 V and reversible reduction wave at −2.13 V peak potential (Figure 7B), related to the formation of the corresponding cation and anion radicals. In the charge-transfer complexes a shift of the reduction and oxidation potentials of the partners was observed (Figure 7C,D). The coronene reduction is facilitated, and for the 335

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Table 3. First Peak Potentials ref Ag/AgCl F4-TCNQ:coronene F4-TCNQ Epred Epox

coronene

1:1

1:2

1.40

1.20 1.90 0.70

1.30 1.92 0.62

0.54

ΔE = Epcoronene (ox) − EpF4‑TCNQ (red)

Molecular Rotation of Coronene. It is well-known that the motion features of a rigid group in a crystal can be interconnected with the strength and directionality of intermolecular interactions.14,41 Thus, it was intriguing to compare the anisotropic displacement parameters (ADP) in the studied crystal series in order to estimate an influence of different packing factors on the mobility of a system. As one can see from the simple representation of the nonhydrogen coronene atoms by thermal ellipsoids (50% probability) the motion of coronene molecule at 100 K is pronouncedly more anisotropic in the crystal 1 in comparison with crystals of individual coronene and complex 2 (Figures 9

Figure 9. ORTEP view of the coronene molecule at 100, 220, and 293 K in crystal 1.

Figure 10. ORTEP view of the coronene molecule at 100 and 293 K in crystal 2.

Figure 8. CVs of F4-TCNQ and coronene 1:1 mixture (10−3 M). Red - immediately after mixing, coincides with pure F4-TCNQ; others after stirring under inert atmosphere; at GC electrode in CH3CN ref Ag/AgCl.

and 10). We have studied the mobility of coronene in CT complexes using widely known TLS (translation, libration, and screw) formalism,42,43 which has revealed that at 100 K the coronene molecule can be considered as a rigid body in all crystals (the Rij value varies in the range of 0.05−0.06), the external libration with maximal mean-square amplitude always corresponding to the rotation around the C6 axis of the coronene molecule. The maximum eigenvalues of the libration tensors (L1) are rather small (0.0006−0.0007 rad2) for the pristine coronene and the coronene molecule in complex 2 but are significantly higher in the complex 1 (0.0033 rad2). It is noteworthy that the L1 value for the F4-TCNQ molecule in complexes is small and corresponds to the libration around the axis perpendicular to the C6 axis of coronene. In other words, the libration movement of two fragments in the CT complexes is independent from each other. It implies that the presence of stacking interaction between coronene and F4-TCNQ mole-

appear; however, the 1:1 complexes are also present (redox peaks 1.30/1.20 V and 1.05/0.8 V). When the solution of the mixture was kept for a longer time, the insoluble black precipitate drops. These precipitate shows the cyclic voltammogram coinciding with the CV for 2:1 complex, and its crystal parameters were found to correspond to crystal 1, which does not contain acetonitrile solvate molecule. Thus, one may suppose that though the solution contains the complexes of both stoichiometries 1:1 and 2:1, which can serve as nuclei for the growth of the corresponding crystals, the thermodynamically more stable is the phase of 2:1. This might explain the exclusive growth of the complexes 2:1 from the vapor phase. 336

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high libration of the coronene molecule in the stacks is observed. One should note that similar T-like CH··· π coronene−coronene interactions are sufficient to hinder its rotation at room temperature in the pristine coronene. However, there is one universal feature characteristic for crystals, where coronene libration is negligible. It is the symmetric crystal environment of the coronene molecule. In all the crystals with coronene being in the special position in the inversion center, i.e., when the inversion center of coronene coincides with the crystallographic inversion center, the amplitude of libration is small, and probably the contributions of various weak interactions in the symmetric environment are in balance (Table 5).

cules does not affect the mobility of the donor coronene system. Taking into account that intensive libration of coronene molecule in the 1:2 complex observed even at 100 K can be a consequence of static disorder, we carried out additional X-ray diffraction investigations for the 1:1 and 1:2 complexes at higher temperatures. While coronene moieties can be considered as rigid bodies in both systems at low temperatures, the Rij factor in the case of the 1:2 complex increases upon heating up to 12.6% at 293 K that can be caused either by dynamic disorder or by anharmonic effects of motion. The peculiarities of external motion in two systems also differ significantly. Thus, the L1 value for the coronene fragment in the complex 1 significantly increases upon heating from 0.0033 at 100 K up to 0.0171 rad2 at 293 K. On the contrary, even at room temperature the L1 value for the coronene molecule in the 1:1 complex is lower (0.0021 rad2) than that for the 1:2 complex at 100 K. It is noteworthy that the values of rotation barrier (Table 4) obtained for the coronene molecule in the 1:2

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CONCLUSIONS Two new coronene charge transfer complexes with F4-TCNQ with different stoichiometry were obtained using crystal growth procedures from solution and vapor phase. Variable temperature X-ray single crystal diffraction studies showed significant dynamic motion of the coronene molecule in complex 1 of coronene/F4-TCNQ (2:1) at ambient temperature. Complex 2 of coronene/F4-TCNQ/MeCN (1:1:1), with acetonitrile solvate molecules filling the channels between the DA stacks, exhibits no coronene rotation. The analysis of anisotropic displacement parameters revealed larger libration of coronene in complex 1 at 100 K than in complex 2 at 293 K. It was shown that mobility of coronene molecules in crystals is more affected by the asymmetry of its surrounding than by the composition and degree of charge transfer and interstack interactions. The combination of X-ray diffraction and electrochemistry in solid state and time-resolved one in solution allowed us to clarify the nucleation of CT complexes in solution showing the formation of 2:1 coronene/F4-TCNQ thermodynamically preferable, which might explain the exclusive growth of 2:1 complexes from the vapor phase regardless of the starting ratios of the individual components and crystal growth conditions.

Table 4. Results of TLS Analysisa and the Estimated Rotational Barrier Ebar crystal

T, K

L1, rad2

Rij

Ebar, kJ/mol

complex 1

100(2) 140(2) 180(2) 220(2) 260(2) 293(2) 100(2) 293(2) 100(2)

0.0033 0.0053 0.0076 0.0099 0.0131 0.0171 0.0006 0.0021 0.0007

0.0510 0.0667 0.0811 0.0887 0.1023 0.1263 0.0663 0.0446 0.0588

14 12 11 10.6 9.5 8.3 77 65 66

complex 2 coronene a

The XP program44 was used for TLS analysis.

complex using TLS model and cosine potential approximation (V6)45 is in agreement with the NMR data for other coronene complexes where the high anisotropy of ADP was observed at the same temperatures.13,14 The low barrier value in the case of the 1:2 complex together with some violation of rigid-body conditions at high temperatures demonstrates the absence of strong intermolecular interactions in this system. Although the interstack interactions in the crystals 1 and 2 are different, it is rather difficult to estimate in which crystal they are stronger. One could assume the crucial role of the acetonitrile solvate molecule hindering the molecular rotation; however, a similar situation is observed in the 3:1 complex13 in which the stacks are separated by the third coronene, but not the solvate molecule. On the contrary, in this particular crystal

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01301. Variable temperature crystallographic data and IR spectrum of the single crystal 1, temperature dependence of thermal displacement parameters (PDF) CIF files for complex 1 (CCDC 1422744−1422749), for complex 2 (CCDC 1422750, 1422751) and coronene (CCDC 1422752). (CIF1, CIF2, CIF3, CIF4, CIF5, CIF6, CIF7, CIF8, CIF9)

Table 5. Selected Crystal Parameters for Coronene and Its CT Complexes crystal

Ra, Å

Rb, Å

space group

Z′ of coronenec

Libration

complex 1 complex 2 complex 1:1, Yoshida14 coronene, T = 100(2) K coronene−TCNQ 3:113 coronene−TCNQ 1:133 coronene−[Ni] 2.5:113

3.258 3.245 3.243

3.434

P21/n C2/c P1̅ P21/n P1̅ P21/c P1̅

1 0.5 0.5 0.5 1.5 0.5 2.5

+ − − − +; − − +; −

3.360 3.255

3.403 3.426 3.458

a

Distance between centroid (TCNQ ring) and the plane of the coronene. bDistance between centroid of the coronene and the plane of the coronene. cZ′ − the number of independent molecules in the asymmetric part of the unit cell. 337

DOI: 10.1021/acs.cgd.5b01301 Cryst. Growth Des. 2016, 16, 331−338

Crystal Growth & Design

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Article

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the Russian Foundation for Basic Research (Projects 14-03-91343, 16-03-00195), German Research Foundation (DFG Grant KN 393/20-1) Russian Government Program of Competitive Growth of Kazan Federal University are gratefully acknowledged. K.A.L. is thankful to the Russian Scientific Foundation for financial support of thermal motion analysis studies (Project No. 14-13-00884).

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NOTE ADDED IN PROOF While this manuscript was under revision, a paper by Yu. Yoshida et al. appeared online (DOI: 10.1021/ acs.cgd.5b01138) reporting complex 1, studied by solid state NMR, which confirms the dynamic properties of the coronene molecule.

338

DOI: 10.1021/acs.cgd.5b01301 Cryst. Growth Des. 2016, 16, 331−338