StructureâProperty Relationship of Supramolecular Rotators of

Article pubs.acs.org/crystal

Structure−Property Relationship of Supramolecular Rotators of Coronene in Charge-Transfer Solids Yukihiro Yoshida,*,† Yoshihide Kumagai,‡ Motohiro Mizuno,‡ and Gunzi Saito† †

Faculty of Agriculture, Meijo University, Shiogamaguchi 1-501 Tempaku-ku, Nagoya 468-8502, Japan Department of Chemistry, Graduate School of Natural Science & Technology, Kanazawa University, Kakuma, Kanazawa, Ishikawa 920-1192, Japan

‡

S Supporting Information *

ABSTRACT: Single crystals of charge-transfer (CT) complexes composed of the polyaromatic hydrocarbon, coronene, as an electron donor (D) and 7,7,8,8tetracyanoquinodimethane (TCNQ) analogues as electron acceptor (A) were obtained. Elucidation of crystal structures of CT complexes enables a systematic investigation of dynamic properties of coronene molecules lying in different types of crystalline environments. Solid-state 2H NMR spectra of CT complexes formed with deuterated coronene confirmed the in-plane 6-fold flipping motion of the coronene molecules. The dihedral angle between adjacent coronene and TCNQ analogue within the DA-type alternating π-column is closely correlated with the dynamic properties, such as rotational rate and activation energy. Side-by-side intermolecular hydrogen-bonding also seems to have an effect in ways that lead to the suppressed rotation. These findings would provide an initial step toward the selection, design, and engineering of counter components of supramolecular rotators in the CT solids.

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INTRODUCTION Solid-state supramolecular rotators are a promising entry into the field of artificial molecular machines and have attracted much attention for applications in materials science and nanotechnology.1−4 In particular, these supramolecular systems enable chemical modification of their component molecules and diverse combinations of these molecules, facilitating the design and control of their dynamic properties. In a search for structures with low rotational barriers in the solid state, we have focused on charge-transfer (CT)-type supramolecular assemblies based on a highly symmetric (D6h) polycyclic aromatic hydrocarbon, coronene (Scheme 1), as an electron donor (D) because experimental5 and theoretical6−10 studies suggested that coronene is a promising platform for obtaining such supramolecular rotators. These CT-type supramolecular assemblies would open the way for a systematic structure− property relationship study leading to desirable functions

because of their easy preparation and diverse combinations with electron acceptors (A). Recently, we have reported the rotational behavior of coronene-based CT complexes with 1:1 and 3:1 stoichiometries with a typical electron acceptor, 7,7,8,8tetracyanoquinodimethane (TCNQ; Scheme 1),11 in which coronene acts as a rotator while TCNQ acts as a stator. While we found that these CT complexes undergo an in-plane molecular motion of coronene molecules in the gigahertz regime at room temperature, it is also desirable to identify the factors determining the dynamic properties, such as rotational rate and activation energy. Here, we report the first structure− property relationship study of supramolecular rotators of coronene in the CT solids with 1:1 stoichiometry involving DA-type alternating π-columns composed of coronene and TCNQ analogues with different electron accepting abilities.

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RESULTS AND DISCUSSION The synthesis of the neutral CT complexes with a 1:1 stoichiometry, (coronene)(F 4 TCNQ) (1), (coronene)(TCNQ) (2), 12−14 (coronene)(Me 2 TCNQ) (3), and (coronene)[(MeO)2TCNQ] (4), was accomplished by a diffusion or solvent evaporation method, as shown in our previous report.11 Single crystals of 1 suitable for X-ray diffraction study were obtained by diffusion in a dichloromethane/pentane (1:5 v/v) mixed solution instead of a

Scheme 1. Molecular Structures of (a) Coronene and (b) TCNQ Analoguesa

a

For F4TCNQ: R1 = R2 = R3 = R4 = F. For TCNQ: R1 = R2 = R3 = R4 = H. For Me2TCNQ: R1 = R3 = Me, R2 = R4 = H. For (MeO)2TCNQ: R1 = R3 = MeO, R2 = R4 = H. © XXXX American Chemical Society

Received: December 6, 2014 Revised: January 10, 2015

A

DOI: 10.1021/cg5017774 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Article

Crystal Growth & Design

Figure 1. Crystal structures of (a) 1 along the c axis and (b) 3 along the b axis. (c) Overlap pattern of adjacent coronene and F4TCNQ within a column viewed perpendicularly to the molecular plane of coronene in 1. (d) Schematic structure of a column. For parameters l and θ, see text. (e) Plot of overlap integral between the HOMO of coronene and the LUMO of the TCNQ analogue within the column against the first redox potential of the TCNQ analogue (E11/2(A)). The blue line is a guide for the eye. Coronene and TCNQ analogues appear in green and red, respectively.

Table 1. Typical Parameters for 1−4 complex acceptor A E11/2(A), V vs SCEa dRMS, Åb,i hRMS, Åc,i l, Åd,i θ, dege,i Sf,i krot at 173 K, kHzg,j Ea, kJ mol−1h

1

2

3

4

F4TCNQ 0.601 1.73 × 10−3 2.05 × 10−2 3.24 0.50 2.70 × 10−4 10 30

TCNQ 0.218 2.67 × 10−3 3.02 × 10−2 3.26 2.40 1.42 × 10−4 900 17

Me2TCNQ 0.147 2.48 × 10−3 3.56 × 10−3 3.38 2.14 4.41 × 10−5 100 24

(MeO)2TCNQ 0.054 1.63 × 10−3 1.16 × 10−2 3.37 1.44 5.35 × 10−5 70 23

pristine coronene

1.70 × 10−3k 1.37 × 10−2k

10 27

a

First redox potential of TCNQ analogues; SCE = saturated calomel electrode. bRoot mean square deviation of 12 rim carbon atoms from the average of the distances of the atoms from the molecular centroid. cRoot mean square deviation of 12 rim carbon atoms from a mean plane including 24 carbon atoms along the out-of-plane direction. dInterplanar distance between adjacent coronene and TCNQ analogue within a column, which is defined as half the distance between coronene molecules across a TCNQ analogue. eDihedral angle between adjacent coronene and TCNQ analogue within a column. fOverlap integral between adjacent coronene and TCNQ analogue within a column, calculated by the extended Hückel method. g Rotational rate at 173 K estimated from the spectral simulation. hActivation energy estimated from the temperature dependent T1 curve above 273 K. iX-ray diffraction data at 100 K were used. jValues were estimated on the assumption of a Cole−Cole distribution for rotational rates. kX-ray diffraction data at 200 K were used, because the structural refinement at 100 K was not successful because of a crack occurring upon cooling (even at a cooling rate of −0.5 K min−1).

lower than 0.1. Theoretical calculations have predicted that the coronene monocation undergoes an in-plane Jahn−Teller distortion because of the splitting of the degenerate of e2u HOMO level.18,19 The root-mean-square deviations of the distance between each rim carbon atom and the molecular centroid from their average of the distance (dRMS = (∑Δdi2/ 12)1/2; Δdi = di − ∑di/12, where di is the distance of the rim carbon atom with a number i from the molecular centroid) of 1−4 lie in the range between 1.63 × 10−3 and 2.67 × 10−3 Å (Table 1 and Figure S1 in the Supporting Information). These values are comparable to 1.70 × 10−3 Å in pristine coronene crystal and is much lower than 1.25 × 10−2 Å of coronene monocation optimized within D2h symmetry by density

dichloromethane solution as reported in ref 11 (see Experimental Section). Despite their distinct crystal lattices, all complexes consist of DA-type alternating π-columns (Figure 1a and Figure 1b for 1 and 3, respectively).15 Coronene and TCNQ analogues in 1−4 are located on an inversion center; namely, a half coronene and a half TCNQ analogue are crystallographically independent. The charge on F4TCNQ, which is the strongest electron acceptor of the TCNQ analogues considered, was estimated to be −0.13(4) in 1 on the basis of bond length analysis,16,17 in good agreement with the value expected from the vibrational spectra (ca. −0.1).11 The degrees of CT in 2−4 composed of weaker electron acceptors than F4TCNQ is expected to be B


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Figure 2. Short interatomic contacts (gray dotted lines) around one coronene molecule in (a) 1, (b) 2,11,14 (c) 3, and (d) 4 at 100 K. Coronene and TCNQ analogues appear in green and red, respectively.

extended Hückel method21 amount to (0.53−2.70) × 10−4 (Table 1). The order of S roughly coincides with that of E11/2(A) (Figure 1e), suggesting that the CT interactions dominate intermolecular interactions along the stacking direction. Along the side-by-side direction, the packing motifs strongly differ between 1−4. Coronene molecules are parallel to each other in triclinic 1 and 4 (Figure 1a for 1), but display two orientations with respect to the stacking direction in monoclinic 2 and 3 (Figure 1b for 3). As shown in Figure 2a, each coronene in 1 is surrounded by six F4TCNQ molecules in a hexagonal arrangement, involving C−H···N (C3−H···N1, 2.68 Å; C4−H···N2, 2.72 Å; sum of van der Waals radii, 2.75 Å22) hydrogen bonds to four F4TCNQ molecules and C−H···F (C6−H···F1, 2.54 Å; C7−H···F1, 2.53 Å; sum of van der Waals radii, 2.67 Å22) hydrogen bonds to the remaining ones. Each coronene forms short interatomic contacts with four TCNQ molecules through C−H···N (C3−H···N1, 2.61 Å) hydrogen bonds with two TCNQ molecules and C−H···H−C (C1−H··· H−C15, 2.38 Å; C9−H···H−C14, 2.36 Å; sum of van der Waals radii, 2.40 Å22) dihydrogen bonds23,24 to the others (Figure 2b) in 2.11,14 In the complex 3, each coronene forms short interatomic contacts with four Me2TCNQ molecules through C−H···N (C1−H···N2, 2.64 Å; C4−H···N2, 2.72 Å) hydrogen bonds (Figure 2c). On the other hand, each coronene in 4 only shows short interatomic contacts with two (MeO)2TCNQ molecules through C−H···N (C3−H···N1, 2.71 Å) hydrogen bonds in 4 (Figure 2d), which may stem from the presence of bulky methoxy groups. The

functional theory (DFT) method.20 The nearly isotropic structure of coronene strongly indicates the neutral ground state of 1−4. It is notable that coronene molecules in 1−4 are not completely planar but have a more or less undulating structure with a root-mean-square deviation from a mean plane (hRMS = (∑hi2/12)1/2, where hi is the out-of-plane deviation of the rim carbon atom with a number i from the mean plane) ranging between 3.56 × 10−3 and 3.02 × 10−2 Å (Table 1 and Figure S2 in the Supporting Information). Although the DFT calculation has predicted that the coronene molecule with C2h symmetry shows an out-of-plane deformation,20 its degree of deformation (hRMS = 2.98 × 10−4 Å) is 1−2 orders of magnitude smaller than those in 1−4. Thus, this molecular deformation may arise from the intermolecular interactions in the close-packed crystals, as shown below. As shown in Figure 1c for 1, in the column, adjacent coronene and TCNQ analogue exhibit a ring-over-bond overlap fashion; other complexes have essentially identical pattern) with an interplanar distance (l) of 3.24−3.38 Å (Table 1), which is defined as half the distance between coronene molecules across a TCNQ analogue (Figure 1d). The l value appears lower in TCNQ analogues with small substituents (1 and 2) than in ones with bulky substituents (3 and 4). It should be noted that adjacent coronene and TCNQ analogue within a column arrange in a nonparallel fashion with a dihedral angle (θ; Figure 1d) of 0.50−2.40° (Table 1). The overlap integrals between the HOMO of coronene and the LUMO of the TCNQ analogue within a column (S) calculated by the C


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indistinguishable from each other, we utilized a model involving 6-fold flipping motion about the 6-fold axis as in our previous report.11 Because the rotational rate (krot), namely jumping rate between adjacent 6-fold sites, falls within the fast exchange limit (ca. 10 MHz) at room temperature, the krot values were compared at 173 K. It decreases in the order 2d (900 kHz) > 3d (100 kHz) > 4d (70 kHz) > 1d (10 kHz), and therefore, all the present complexes exhibit a similar or faster rotation in compared to the herringbone-packed pristine deuterated coronene (10 kHz at 173 K). Whereas the krot values have no simple correlation with E11/2(A), hRMS, l, and S values (Table 1), they appear to increase with increasing deviation from the parallel arrangement of coronene and TCNQ analogue within a column (θ; Figure 4a), possibly as a consequence of the depressed face-to-face π···π interactions. Considering the fact that each coronene molecule in 1 with the lowest krot value is connected to surrounding six F4TCNQ molecules through C− H···N or C−H···F hydrogen bonds, the side-by-side intermolecular interactions may also exert a significant effect on the in-plane flipping motion. Figure 4b displays the logarithmic plot of the spin−lattice relaxation time (T1) at ν = 45.282 MHz as a function of the reciprocal temperature for 1d−4d, together with pristine deuterated coronene, above 273 K, because the magnetization recovery process in this temperature region could be analyzed by assuming a single component. All complexes exhibit a T1 minimum except 2d, implying that the rotational rate exceeds the 2H Larmor frequency (ω = 2πν = 285 MHz) above 273 K. The observed straight line for 2d suggests that the rotational rate reaches the frequency even below 273 K, and a remarkably fast rotation in 2d is as expected by comparison of krot at 173 K. The Ea values fall in the order 1d (30 kJ mol−1) > 3d (24 kJ mol−1) > 4d (23 kJ mol−1) > 2d (17 kJ mol−1). It appears that all complexes, except 1d, have lower rotational barriers than that of the pristine deuterated coronene (27 kJ mol−1), and the order roughly coincides with the reciprocal order of krot at 173 K. Therefore, the Ea value is also related to θ (Figure 4c) and the reason must be the same as given above for krot; namely the face-to-face π···π interactions along the stacking direction and the hydrogen-bonding interactions along the side-by-side direction suppress the in-plane flipping motion. These results strongly suggest that the dynamic properties of coronene molecules in neutral CT solids change independently of the redox potential (i.e., electron-withdrawing/donating ability of

(MeO)2TCNQ molecules are arranged to form an infinite ribbon through double Csp3−H···N (C19−H···N2, 2.72 Å) hydrogen bonds along the a + c direction. To investigate the dynamics of coronene molecules by solidstate 2H NMR spectroscopy, CT complexes of deuterated coronene (hereafter indicated as 1d−4d) were synthesized in the same manner as 1−4. X-ray diffraction measurements confirmed that deuterated complexes 1d−4d exhibit isomorphously substituted structures of corresponding 1−4. 2 H NMR spectra show a typical powder pattern for the firstorder nuclear quadrupole splitting expected for a nuclear spin of I = 1 (Figure 3a for spectra at 173 K and also see Figure S3

Figure 3. (a) Solid-state 2H NMR spectra of 1d−4d at 173 K and (b) simulated spectra using the 6-fold flipping model at the rotational rates (krot) shown. For the parameter δ, see the Supporting Information. Spectra of pristine deuterated coronene are also shown for comparison.

in the Supporting Information). Spectra were analyzed on the assumption of the Cole−Cole distribution of rotational rate (see the Supporting Information). The observed powder patterns confirmed that coronene molecules undergo a flipping motion, rather than a continuous rotation in 1d−4d. Although simulations assuming in-plane 6-fold (nearest neighbor jumps) and 3-fold (next-nearest neighbor jumps) flipping motions are

Figure 4. (a) Logarithmic plot of the rotational rate (krot) at 173 K against the dihedral angle between coronene and TCNQ analogue within a column (θ). (b) Spin−lattice relaxation time (T1) of 1d (pink), 2d (red), 3d (green), 4d (blue), and deuterated coronene (black) plotted as a function of the reciprocal temperature at 45.282 MHz. For the solid lines, see the Supporting Information. (c) Plot of the activation energy (Ea) against θ. Blue lines in (a) and (c) are guides for the eye. D


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width and τ were 2.0−2.5 and 20 μs, respectively. 2H NMR T1 was measured by the inversion recovery and saturation recovery methods. For details on analysis, see the Supporting Information. Simulated spectra were produced with FORTRAN original programs.27 Calculation of Overlap Integrals. The overlap integrals (S) between the HOMO of coronene and LUMO of TCNQ analogues were calculated by using the extended Hückel method.21 Parameters for Slater-type atomic orbitals were taken from refs 28 and 29.

substituents) and the size ((MeO)2TCNQ > Me2TCNQ > F4TCNQ > TCNQ) of the TCNQ analogues. It is notable that, although the contribution of the side-by-side C−H···F hydrogen-bonding interactions in 1 is unclear, the change of a few degrees in θ leads to the marked difference in the dynamic properties of π-planar coronene molecules in the CT solids.

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CONCLUSION By studying the relative properties of four CT complexes composed of coronene and TCNQ analogues, we have arrived at a clear picture of the structure−property relationship of supramolecular rotators of coronene in neutral CT solids. The difference of only a few degrees of the dihedral angle between adjacent coronene and TCNQ analogue within the alternating π-column is crucial in determining dynamical properties such as rotational rate and activation energy. Side-by-side intermolecular hydrogen-bonding also has a certain influence on the dynamical properties. A special fascination of the supramolecular rotators in neutral CT solids is that the selection and design of electron-accepting (or electron-donating) static molecules can tailor the dynamic properties of electrondonating (or electron-accepting) rotator molecules. The fact that there are numerous redox-active molecules that can be easily synthesized would facilitate the fine-tuning of the dynamic properties in the CT solids. Further work, particularly on the development of criteria for predicting dynamic properties that could be valuable for materials science and nanotechnology, is in progress.

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ASSOCIATED CONTENT

S Supporting Information *

Details of spectral analyses, in-plane (Figure S1) and out-ofplane (Figure S2) deformations of coronene in 1−4, temperature-dependent 2H NMR spectra for 1d−4d (Figure S3), and X-ray crystallographic data in CIF format for 1−4. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Y.Y.: Tel. +81-52-838-2552, fax +81-52-833-7200, E-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI Grant Nos. 23225005 and 25288041.

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EXPERIMENTAL SECTION

REFERENCES

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General. Solvents (dichloromethane and pentane) were used as received from Wako Chemical. Coronene (C24H12; Tokyo Kasei, 98%) and deuterated coronene (C24D12; Cambridge Isotope Laboratories, 97%) were recrystallized several times from dry benzene, and TCNQ analogues were purified by recrystallization and/or sublimation after being prepared by our research group or purchased commercially. Single crystals of CT complexes 2−4 were obtained using the same procedures as shown in ref 11. Preparation of (Coronene)(F4TCNQ) (1). CT complex 1 was obtained using a diffusion method. Typically, coronene (15 mg, 0.05 mmol) was added to one side of an H-shaped cell and F4TCNQ (14 mg, 0.05 mmol) was added to the other side of the cell. After the cell was vacuum-dried, ca. 50 mL of dichloromethane/pentane (1:5 v/v) mixed solvent was carefully poured into the cell. After 10 days, single crystals were obtained as dark green needles. Yield: 40%. X-ray Structural Analysis. Single-crystal X-ray diffraction data were collected at 100 K on a CCD-type diffractometer (Bruker SMART APEX II) with a graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). The crystal structures were solved by a direct method using SIR200425 and were refined by a full-matrix least-squares method on F2 using SHELXL.26 All non-hydrogen atoms were anisotropically refined. The positional parameters of the hydrogen atoms were calculated under the assumption of fixed C−H bond lengths of 0.93 and 0.96 Å with sp2 and sp3 configurations, respectively, of the parent atoms. In the refinement procedures, isotropic temperature factors with magnitudes 1.2-fold greater than those of the Ueq of the parent atoms were applied to the hydrogen atoms. Crystallographic data were deposited with the Cambridge Crystallographic Data Centre: deposition nos. CCDC 1024222 for 1, 1042339 for 2, 1024223 for 3, and 1024224 for 4. Solid-State 2H NMR Spectroscopy. The 2H NMR spectra and T1 were measured using a JEOL ECA-300 spectrometer at 45.282 MHz. The NMR spectra were observed using a quadrupole echo sequence (90°)x − τ − (90°)y − τ − tacq, where τ and tacq are the intervals of echo and acquisition time, respectively. The 90° pulse E


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