Designed Synthesis and Crystallization of Isomorphic Molecular

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Designed synthesis and crystallization of isomorphic molecular gyroscopes with cell-like bilayer self-assemblies. Ma. Eugenia Ochoa, Pablo Labra-Vázquez, Norberto Farfan, and Rosa Santillan Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01542 • Publication Date (Web): 05 Apr 2018 Downloaded from http://pubs.acs.org on April 6, 2018

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Crystal Growth & Design

Designed synthesis and crystallization of isomorphic molecular gyroscopes with cell-like bilayer selfassemblies. Ma. Eugenia Ochoa,a Pablo Labra-Vázquez,b Norberto Farfán,b Rosa Santillan,a* a

b

Departamento de Química, Centro de Investigación y de Estudios Avanzados del IPN, Apdo. Postal 14-740, 07000, México

Facultad de Química, Departamento de Química Orgánica, Universidad Nacional Autónoma de México, 04510 México D.F., México.

ABSTRACT Two molecular rotors featuring pyridine and fluorobenzene rings as polar rotators and 9octylfluorenyl stators were synthesized. Their crystal structures were established through SXRD techniques, crystallizing in the monoclinic chiral P21 space group. The supramolecular assemblies of both isomorphs showed an orientation of static dipoles through the crystal lattice and the formation of intriguing 2D layers that resemble cell membranes, a typical example of an amphidynamic system. Small activation energies and the modulation of the first-order hyperpolarizabilities of these compounds as a function of rotational dynamics were revealed through DFT computations at the CAM-B3LYP/M06-2X/cc-pVDZ level of theory and correlated with a potential use of these materials as photonic switches. 1. INTRODUCTION Amphidynamic systems, defined as 3D molecular arrays that despite their long range phase order also display fast molecular dynamics, have the potential to act as molecular rotors, i.e. crystalline molecular machines that, when properly engineered, may display efficient rotational work such

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as that of macroscopic gyroscopes or compasses.1 These crystal systems, expected to display interesting properties like dichroism and birefringence,2 usually incorporate a 1,4diethynylphenylene ring linked at both ends to rigid frameworks that provide the supramolecular information that dictates the crystal packing. Our group of work has put intense collaborative efforts in studying the rotational dynamics of molecular rotors possessing diverse rigid steroidal frameworks linked through acetylenic axles to 1,4-phenylene stators.3-5 During the course of these investigations, the possibility of controlling the rotational dynamics of these molecular compasses like systems using a magnetic stimulus became appealing. Herein we describe the synthesis and supramolecular structure of molecular rotors featuring 9,9-dioctylfluorene groups as stators and pyridine and fluorobenzene rings as polar rotators. The static dipolar moments present in these molecules are intended to facilitate control of rotational dynamics upon application of an external magnetic field. Through SXRD analysis of these systems, we found that they present peculiar solid-state self-assemblies that resemble closely cell membranes, as well as other supramolecular features that make these crystalline isomorphs interesting systems to be studied as molecular gyroscopes. 2. EXPERIMENTAL General experimental considerations Materials


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All starting materials were purchased from Sigma-Aldrich and used without further purification. Solvents were purified by distillation over appropriate drying agents. Compounds 2–4 were obtained following reported methodologies;6-8 spectroscopic data is in good agreement. Instrumentation NMR spectra were recorded on Jeol ECA 500 and Jeol 270 MHz spectrometers using deuterated solvents; chemical shifts for 1H and

13

C NMR data are relative to the residual

nondeuterated solvent signal, fixed at δ = 7.26 ppm for 1H-NMR and δ = 77.00 ppm for

13

C-

NMR. Infrared spectra were registered on a FT-IR Varian ATR spectrometer. HRMS data was acquired using an Agilent G1969A MS TOF spectrometer. Elemental Analysis for MR2 was determined on a Thermofinnigan Flash 1112 equipment (CHONS). Absorption spectra were obtained in chloroform solutions using a Perkin Elmer Lambda 2S UV/Vis spectrophotometer. Synthetic procedures Synthesis of molecular rotors 2,5-bis((9,9-dioctyl-fluoren-2-yl)ethynyl)pyridine (MR1). 2-ethynyl-9,9-dioctylfluorene (4) (0.20 g, 0.48 mmol), 2,5-dibromopyridine (5) (0.06 g, 0.25 mmol), Pd(PPh3)2Cl2 (0.02 g, 0.028 mmol) and CuI (0.009 g, 0.03 mmol) were placed in a dried round bottom flask followed by the addition of diisopropylamine (1 mL) and freshly distilled THF (25 mL) under nitrogen atmosphere. The mixture was refluxed 8 h, allowed to cool to room temperature, quenched with a saturated solution of ammonium chloride (25 mL), extracted with ethyl acetate (3x25 mL), dried over Na2SO4, evaporated to dryness and chromatographed on silica gel (70-230 Mesh) using hexanes. The brown solid thus obtained was recrystallized from deuterated chloroform and


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acetonitrile to yield 80 mg (37 %) of MR1 as a yellow solid. M.P. 78-79 °C. FTIR (ATR cm-1): ν 3268, 2923, 2875, 2316, 1467, 843, 827, 737, 571, 555. 1H-NMR (CDCl3, 500 MHz) δ 8.82 (d, J = 1.9 Hz, 1H, H-24), 7.84 (dd, J = 8.1, 1.9 Hz, 1H, H-22), 7.72 – 7.68 (m, 4H, H-4/H-4’/H5/H-5’), 7.62 – 7.58 (m, 2H, H-1/H-3), 7.56 – 7.52 (m, 3H, H-1’/H-2’/H-3’), 7.36 – 7.32 (m, 6H, H-6/H-6’/H-7/H-7’/H-8/H-8’), 1.97 (q, J = 7.7 Hz, 8H, H-10/H-10’), 1.23 – 1.17 (m, 8H, H16/H-16’), 1.15 – 1.02 (m, 32H, H-12/H-12’/H-13/H-13’/H-15/H-15’), 0.82 (t, J = 7.2 Hz, 12H, H-17/H-17’), 0.61 (br s, 8H, H-11/H-11’). 13C-NMR (CDCl3,125 MHz) δ 152.47 (C-24), 151.18 (C-9a’), 151.07 (C-9a), 150.88 (C9b’), 150.78 (C-9b), 142.32 (C-4a), 142.13 (C-4a’), 141.92 (C20), 140.19 (C-4b/C-4b’), 138.30 (C-22), 130.73 (C-3’), 127.76 (C-7), 127.74 (C-7’), 126.92 (C6’), 126.90 (C-6), 126.71 (C-1), 126.22 (C-1’), 126.02 (C-21), 122.90 (C-8/C-8’), 120.43 (C-2’), 120.14 (C-5), 120.11 (C-5’), 120.07 (C-2), 119.74 (C-4), 119.68 (C-4’), 119.44 (C-23), 95.58 (C18’), 92.52 (C-18), 88.70 (C-19), 86.05 (C-19’), 55.17 (C-9’), 55.14 (C-9), 40.31 (C-10/C-10’), 31.76 (C-15/C-15’), 30.00 (C-14), 29.98 (C-14’), 29.21 (C-13/C-13’), 29.20 (C-12/C-12’), 23.72 (C-11’), 23.69 (C-11), 22.57 (C-16/C-16’), 14.06 (C-17/C-17’). HRMS (APCI-TOF+) m/z: [M+H]+ Observed: 904.6755, required for C67H86N: 904.6754, error: 0.23 ppm 2,5-bis(9,9-dioctyl-2-ethynyl-fluoren)fluorobenzene (MR2). Synthesized as described above for MR1, from 2-ethynyl-9,9-dioctylfluorene (4) (0.15 g, 0.36 mmol) and 1,4-dibromo-2fluorobenzene (6) (0.05 g, 0.18 mmol). The procedure yielded 92 mg (56 %) of MR2 as a pale yellow solid (92 mg, 56 %). M.P. 116 - 117 °C. FTIR (ATR, cm-1) 3063, 2925, 2853, 1467, 1451, 828, 737, 721. 1H-NMR (CDCl3, 500 MHz) δ 7.72 – 7.69 (m, 4H, H-4/H-4’/H-5/H-5’), 7.57 – 7.51 (m, 5H, H-1/H-1’/H-3/H-3’/H-21), 7.37 – 7.32 (m, 8H, H-6/H-6’/H-7/H-7’/H-8/H8’/H-22/H-24), 1.98 (t, J = 8.3 Hz, 8H, H-10/H-10’), 1.26 – 1.19 (m, 8H, H-16/H-16’), 1.15 – 1.05 (m, 32H, H-12/H-12’/H-13/H-13’/H-14/H-14’/H-15/H-15’), 0.82 (t, J = 7.2 Hz, 12H, H-


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17/H-17’), 0.65 – 0.59 (m, 8H, H-11/H-11’).

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C-NMR (CDCl3, 125 MHz) δ 162.09 (d, 1JC-F =

251.9 Hz, C-25), 151.07 (C-9a), 151.05 (C-9a’), 150.84 (C-9b’), 150.80 (C-9b), 141.96 (C-4a’), 141.95 (C-4a), 140.29 (C-4b), 140.25 (C-4b’), 133.18 (d, 3JC-F = 2.2 Hz, C-21), 130.78 (C-3’), 130.71 (C-3), 127.68 (C-7’), 127.64 (C-7), 127.30 (d, 4JC-F = 3.4 Hz, C-22), 126.90 (C-6’), 126.88 (C-6), 126.03 (C-1), 126.01 (C-1’), 124.88 (C-20), 123.01 (C-8), 123.01 (C-8’), 120.80 (C-2), 120.66 (C-2’), 120.07 (C-5/C-5’), 119.68 (d, 3JC-F = 5.0 Hz, C-24), 118.39 (C-4), 118.30 (d, 3JC-F = 22.6 Hz, C-23), 118.21 (C-4’), 97.36 (C-18), 93.34 (C-18’), 88.10 (C-19’), 82.57 (C19), 55.16 (C-9’), 55.14 (C-9), 40.33 (C-10/C-10’), 31.78 (C-15/C-15’), 30.00 (C-14/C-14’), 29.22 (C-12/C-12’/C-13/C-13’), 23.70 (C-11/C-11’), 22.59 (C-16/C-16’) 14.07 (C-17/C-17’). Anal. Calcd. for C68H85F: C, 88.64; H, 9.30. Found: C, 88.36; H, 9.62. Single X-Ray Diffraction studies Crystals of MR1 and MR2 suitable for single-crystal X-ray diffraction studies were obtained by room temperature evaporation of a saturated solution of the analytes in a chloroform/acetonitrile mixture. The intensity data were collected on EnrafNonius Kappa diffractometer with a CCD area detector (λMoKα = 0.71073 A˚, monochromator: graphite) and Bruker D8 Venture CMOS diffractometer at 173 K. The crystals were mounted on conventional MicroLoops.™ All heavier atoms were found by Fourier map difference and refined anisotropically. All reflection data set were corrected for Lorentz and polarization effects. The first structure solution was obtained using the SHELXS-2017 program and the SHELXL-2017 was applied for refinement and output data.9 All software manipulations were done under the WinGX environment program set. The programs Mercury 3.7 and ORTEP-3 were used to prepare artwork representations.10,

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Free


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volumes around the rotators were approximated to the space confined by a hypothetical prism with two edges defined with the distances shown in Figure 3 (~8 and ~10 Å) and the third edge taken as the C2-C2’ distance (~11 Å). CCDC 1582960 and CCDC 1582963 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via https://summary.ccdc.cam.ac.uk/structuresummary-form (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033). Theoretical methods Gas phase geometries were computed within the framework of the Density Functional Theory using the Gaussian 09W software package,12 from the crystal structures of MR1 and MR2. Both equilibrium geometries were obtained using the double-ζ 6-31G(d,p) polarized basis set with different hybrid functionals: B3LYP, PBE0 and the long-range corrected CAM-B3LYP and M06-2X. In either case a former optimization at the HF/6-31G(d,p) was required. These relaxed conformers were confirmed as energetic local minima through analytical inspection of their vibrational frequencies and were further optimized using the integral equation formalism of the Polarizable Continuum Model (IFPCM) with chloroform as solvent. The resulting geometries were subjected to a TD-DFT computation of the vertical excitation energies for MR1 and MR2, which were compared to the experimental electronic spectra. From all the methods, only the CAM-B3LYP and M06-2X functional were found to predict these energies accurately, with differences between the experimental and computed excitations below the usually accepted threshold of 0.3 eV (Figure S8). These functionals were consequently employed with Dunning’s cc-pVDZ basis set in the computation of first-order hyperpolarizabilities (β), as the use of such


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larger atomic functions has proved effective in predicting this optical parameter.13 For consistency reasons, the same method was applied in the computation of rotational activation energies (Ea) and static molecular dipoles (µ0). It should be remarked that throughout the entire set of computations, the CAM-B3LYP/cc-pVDZ was unambiguously found as the better DFT method both in terms of accuracy and computational expense. Also, the use of a smaller basis set such as the double-ζ 6-31G** basis set behaved catastrophically, particularly at predicting electronic parameters such as electronic excitation energies and should therefore be avoided when computing NLO parameters of molecules with similar quadrupolar architectures as the rotors herein studied. Evaluation of the modulation of µ0 and β as functions of the orientation of the pyridine and fluorobenzene rotators, was carried out by varying their relative positions from the co-planar (equilibrium) geometries. Single-point energy calculations were then performed for conformers with successive 15° increments in the dihedral angle formed between the plane containing the rotator and that of the 9-octyl-fluorenyl stators, i.e. the [C1-C2-C20-C21] torsion angle. In order to confirm or correct the NMR assignments, a Gaussian Invariant Atomic Orbital (GIAO) computation of the isotropic magnetic shieldings of

13

C nuclei was performed. These

calculations were conducted at the PBE0/6-31G(d,p) level of theory rather than with the M062X/cc-pVDZ approach, as it was unexpectedly found that the former predicted the desired

13

C

chemical shifts with better accuracy and performance, as reflected in lower Mean Unsigned Errors (MUE, Table S1) obtained for both MR1 and MR2. Although the methodology employed provides absolute chemical shifts, for clarity they are herein reported as scaled with respect to an


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external tetramethylsilane (TMS) reference computed at the B3LYP/6-311+G(2d,p) level of theory. 3. RESULTS AND DISCUSSION 3.1. Synthesis and Molecular Characterization Sonogashira double cross-coupling reaction between 2-ethynyl-9,9-dioctylfluorene (4) and either 2,5-dibromopyridine (5) or 1,4-dibromo-2-fluorobenzene (6) allowed access to unsymmetrical molecular rotors MR1 and MR2 in moderate yields (Scheme 1).

Scheme 1. Synthesis and numbering for molecular rotors MR1 and MR2. Displayed in color code are the 9-octylfluorenyl stators (blue), the polarizable rotators (red) and the acetylenic axles (black). Reagents and conditions: i) 1-Bromooctane, KOH, DMSO, ii) Ethynyltrimethylsilane,


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Pd(PPh3)2Cl2, CuI, DIPA, reflux, iii) K2CO3, MeOH/ether. rt. iv) Pd(PPh3)2Cl2, CuI, DIPA, THF, reflux. In the 1H-NMR spectrum of MR1, two spin systems could be readily identified through homonuclear COSY (1H-1H) 2D-NMR spectroscopy. The first coupled nuclei was ascribed to the pyridine ring, with the characteristic resonance of H-24 appearing as a highly unshielded doublet at δ = 8.82 ppm. Aromatic hydrogens from the fluorene submolecular fragments appeared as multiplets, with magnetic inequivalences between the H-1/H-1’ and H-3/H-3’ nuclei from both fluorenyl stators. Nonetheless, their signals were successfully assigned owing to their marked long-range scalar couplings. Most

13

C nuclei were also found to be magnetically non-equivalent in solution, which was

observed as a ubiquitous splitting of the signals in the 13C-NMR spectra of both MR1 and MR2, with chemical shift differences even below 0.05 ppm. Ambiguity found in the assignment of these resonances was thus circumvented employing APT and 2D (1H-1H, 1H-13C) NMR techniques and a Gaussian Invariant Atomic Orbital (GIAO) calculation of the isotropic

13

C

chemical shifts for both molecular rotors at the PBE0/6-31G(d,p) level of theory (For details, see Supplementary Table S1 and Figures S9 and S10). 3.2. X-Ray Diffraction Studies Crystals of compounds MR1 and MR2 suitable for SXRD experiments were grown in chloroform/acetonitrile at room temperature (Figure 1). Interestingly, despite the replacement of the pyridine nitrogen in MR1 for a C-F moiety in MR2, both compounds crystalized without solvent in the monoclinic P 21 space group, with 2 molecules per unit cell and with nearly the same cell constants (Table 1). Also, both showed an anti conformation for the two fluorenyl


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stators, which lie in close orthogonality to the plane containing the rotator and the acetylenic axles as evidenced by the values for the [C1-C2-C20-C21] dihedral angle of 84.6 and 86.2 ° for MR1 and MR2, respectively. To further evidence the likenesses of these isomorphic crystals, a comparison between their calculated PXRD patterns is depicted in Figure 2.

Table 1. Crystal structure and refinement data. Compound

MR1

MR2

Empirical formula

C67H85N

C68H85F

Formula weight

904.42

921.36

Temperature

173 K

173 K

Crystal system

Monoclinic

Monoclinic

Space group

P21

P21

a (Å)

12.610(16)

12.48(5)

b (Å)

14.291(18)

14.11(4)

c (Å)

16.78 (2)

16.49(5)

α (°)

90

90

β (°)

104.07 (4)

103.2(2)

γ (°)

90

90

Volume (Å)3

2933(6)

2825(16)

Z

2

2

Density (g⋅cm-3)

1.024

1.083

Crystal size (mm)

0.1 x 0.1 x 0.2

0.32 x 0.069 x 0.04

θ Range (°)

2.19 to 28.28

2.70 to 26.71

Index ranges

-16 ≤ h ≤ 16, -19 ≤ k ≤ 19, -22≤ l ≤ 22

-15 ≤ h ≤ 15, -17 ≤ k ≤ 17, -20≤ l ≤ 20

Nref

14556

10915

R (reflections)

0.0537 (8423)

0.0575 (7766)


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wR2 (reflections)

0.1385 (14374)

0.1167 (10915)


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Figure 1. Crystal structure of (a) MR1 and (b) MR2 with the thermal ellipsoids drawn at 50% probability for every atom other than hydrogen.

Figure 2. Calculated PXRD patterns for MR1 (top) and MR2 (bottom). Remarkably, in spite of the presence of the polar pyridine and fluorobenzene rotators, the crystal packing of both MR1 and MR2 is dominated exclusively by weak non-polar contacts among the alkyl chains of the fluorene stators (For details, see Supplementary Figure S10). It is worth noting that such absence of intermolecular interactions involving the rotators may facilitate their rotational dynamics.


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Further analysis of the crystalline structure of these compounds revealed that a given rotator has 3.8-5.2 Å of distance from the ring centroid to the closest atoms within the crystal lattice of MR1 and MR2 (see Figure 3), conferring a free volume of ca. 796 and 854 Å3 around the pyridine and fluorobenzene rotators, respectively, which may be enough to allow the rotational dynamics of these molecules.3 However, the distances between the centroids of two adjacent rotators are of 11.0 and 10.8 Å for MR1 and MR2 respectively, which may limit correlation of internal rotational motion. Nonetheless, cooperative effects arising by the alignment of molecular dipoles throughout the crystallographic c axis (vide infra) of these crystal arrays may help overcome this limitation.


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Figure 3. Distances (Å) from a rotator centroid to the closest atoms within the crystal packing of (a) MR1 and (b) MR2.


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Both MR1 and MR2 crystal structures exhibited intriguing 2D layers that resemble the lipidic bilayers that constitute cell membranes (Figure 4). This peculiar self-assembly, which spreads along the crystallographic c axis, is of relevance within the study of solid-state molecular rotors due to the need (of that particular type of molecular machines) of having an amphidynamic character, i.e. to display fast molecular dynamics while maintaining a highly ordered phase, such as that offered by a crystalline array. The similarity of the crystal packing of these studied molecular rotors with the lipidic bilayers in cell membranes is thus of importance due to the fact that cell membranes constitute an archetypical example of an amphidynamic system, given that they are well-known to present both phase order and fast molecular dynamics.14


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Figure 4. Schematic representation of 2D layers formed within the crystal packing of compounds MR1 (top) and MR2 (bottom). The crystal structures are viewed along the crystallographic b axis and the rotators are in space fill representation for clarity. As depicted in Figure 5, both MR1 and MR2 showed a peculiar orientation of molecular dipoles throughout the crystallographic b axis. We expect that given the free space that the rotators display and the low rotational energetic barriers provided by the acetylenic axles, the orientation of these microscopic molecular dipoles might change with the presence of an applied magnetic field, yielding these crystalline systems as molecular analogues of macroscopic compasses.

Figure 5. Orientation of static molecular dipoles along the crystallographic b axis for MR1 (left) and MR2 (right).


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It is worth noting that because of their chiral self-assembly, these isomorphic crystals are also of particular interest for nonlinear optical (NLO) applications, where non-centrosymmetric arrangements are required for solid-state applications, such as the second (or higher order) harmonic generation. Moreover, due to the alignment of molecular dipoles within the crystals, these materials are expected to display enhanced NLO properties due to cooperative effects among the static dipoles aligned throughout the crystal lattice. 3.3. Theoretical studies The molecular structure of rotors MR1 and MR2 was studied through DFT computations using the cc-pVDZ basis set with the hybrid long-range corrected M06-2X and CAM-B3LYP functionals. Negligible geometrical differences between the gas-phase geometries were found between both methods. When compared to the solid-state conformers, the main geometrical difference is that the former are predicted to have co-planarity among the pyridine (MR1) or fluorobenzene (MR2) rotators and the fluorenyl stators (Figure 6). Such conformations may reasonably be preferred due to a better stabilization of the static molecular dipoles (µ0) in these planar geometries. This was confirmed through single-point energy computation at the same level of theory for both conformers, showing changes in molecular dipoles (∆µ0) of ca. 0.25 and 0.05 Debye for MR1 and MR2, respectively, favoring the planar conformations.


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Figure 6. Superposed equilibrium (M06-2X/cc-pVDZ) and solid-state (blue) geometries for MR1 (top) and MR2 (bottom). In order to estimate how free the pyridine and fluorobenzene rotators are to perform the rotational work, the energy profiles of both MR1 and MR2 were computed for conformers with incremental 15 ° deviations from the equilibrium (planar) geometries, allowing us to predict very small activation energies (Ea) for both rotors (2.5 - 4.5 kcal/mol), mainly accounted for the loss of π-conjugation and aromaticity of these molecules as they deviate from co-planarity (Figure 7). It is worth noting that even considering that the non-planar solid-state conformers should have been observed due to potential C-H···π contacts involving the rotators, these weak (~1.5 kcal/mol)15 interactions should not constrain rotational motion in the solid state, as efficient dynamics have been observed with even higher energetic thresholds.5


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Figure 7. Energy profiles for the [C1-C2-C20-C21] dihedral angle in MR1 (top) and MR2 (bottom), respectively, using the cc-pVDZ basis set with the CAM-B3LYP (blue) and M06-2X (red) functionals. As mentioned earlier, prospective applications of molecular rotors include their use as microscopic analogues of compasses, with conformational changes effectively induced by a magnetic stimulus. Due to the absence of inversion symmetry in the crystal packing of MR1 and MR2, a pulsed laser could be used to induce not only the conformational change but also an associated even-order nonlinear optical (NLO) response, such as the Second Harmonic


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Generation (SHG). These optical phenomena may arise from a crystal as its molecules polarize due to the appliance of an external magnetic field, following:16 = + + ⋯

(1)

Where P(E) is the molecular polarization induced by the electric field (E), α and β are the molecular polarizability and first-order hyperpolarizability, respectively, and i,j,k refer to the molecular coordinate system. The SHG response in crystals is restricted to non-centrosymmetric space groups, as the presence of inversion symmetry within the molecule yields β = 0 and a cancelation of resonant dipoles is expected in centrosymmetric supramolecular arrangements. In non-centrosymmetric media, the magnitude of β greatly influences the strength of the SHG signal. This leads to the possibility of developing photonic switches from MR1 and MR2 where, as consequence of an induced reorientation of the rotator, a modulation of β and thus of the SHG response could be achieved. It should be noted that because of the quasi-centrosymmetric quadrupolar (D-π-A-π-D) topology of these molecules, small (but non-zero) values of β are expected in the coplanar conformations. Nonetheless, β should be modified as the rotators deviate from coplanarity, as this process interrupts π-conjugation and consequently alters the molecular topology, leading to an “OFF” state (i.e. the conformer with the lowest β) which could be switched to an “ON” state as molecular dipoles are re-oriented due to the application of an external magnetic field, as successfully described by our group for analogous systems.17 The results of a DFT computation of the modulation of µ0 and β as functions of the orientation of the fluorobenzene and pyridine rotators are depicted in Figure 8. Surprisingly, while the variation


20

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of µ0 followed the same trend in both rotors (increasing as the molecule deviates from coplanarity and thus of centrosymmetry), opposite behaviors were found for the modulation of β. In the case of MR1, β has a minimum value at a dihedral angle of 0 ° (i.e. in the coplanar conformer) and a maximum near 90 °, as expected for the change in topology previously explained. Strikingly, both DFT methods employed predicted the exact opposite trend for MR2 with a maximum value of β at coplanarity, which markedly decreases as the dihedral angle approaches 90 °.

Figure 8. Modulation of the static dipole moment (µ0, dotted lines) and the first-order hyperpolarizability (β, solid lines) as a function of the [C1-C2-C20-C21] dihedral angle for MR1


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(top) and MR2 (bottom) using the cc-pVDZ basis set with the CAM-B3LYP (blue) and M06-2X (red) functionals. The rationale behind this divergent behavior is far from trivial and out of the scope of this paper. Further studies should deal with computing the involved excited states, since the value of β, commonly assumed by a two-state model,18 depends not only on the herein studied ground state dipole (µg) but also on the change in dipole moment between the involved eigenstates (∆µge) and on the energy (Ege) and oscillator strength (fge) for the transition, following:

∝

∆

(2)

Despite the different behaviors of both molecular rotors, ON/OFF ratios of 2.6-2.9 for MR1 and 1.3-1.4 for MR2 are expected, according to DFT computations. While the computed values are not particularly appealing at the molecular level, an enhancement in the NLO response in these crystals may occur due to cooperative effects caused by the crystal packing as mentioned earlier (vide supra). 4. CONCLUSIONS Two molecular rotors featuring 9-octylfluorenyl stators with pyridine or fluorobenzene rings as rotators were successfully obtained employing a Sonogashira double cross-coupling reaction as the key synthetic step. The crystal structure of these compounds formed 2D layers that highly resemble cell membranes, which suggests these crystals may also display a dynamic behavior within a highly ordered phase. Free volumes of ca. 796 - 854 Å3 were estimated around the rotators, potentially allowing their rotational dynamics in the solid state, which may occur at room temperature according to DFT computations. Moreover, both their chiral crystal packing


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and the modulation of the first-order hyperpolarizabilites (β) with the orientation of the rotators, make these compounds particularly appealing as photonic switches, which may reorientate upon the appliance of a magnetic field, potentially displaying rotational work such as that of a gyroscope. In silico evaluation of the modulation of β as a function of the rotational dynamics showed ON/OFF ratios of ~2.7 and ~1.3, further confirming a prospective application as photonic switches. The experimental evaluation of the solid state rotational dynamics of deuterated isotopologues, as well as the study of the nonlinear optical behavior of these materials is ongoing. AUTHOR INFORMATION * Corresponding author: [email protected] ACKNOWLEDGEMENTS The authors acknowledge support from PAPIIT (IN-216616) and CONACYT for a doctoral scholarship for P.L.-V (337958). Marco A. Leyva, Geiser Cuellar and Teresa Cortez-Picasso (CINVESTAV-IPN) are gratefully acknowledged for X-ray structure determinations, HRMS analyses and NMR experiments, correspondingly. We would like to express our gratitude to Prof. Pascal G. Lacroix (CNRS) for his valuable comments on this work. ASSOCIATED CONTENT Supporting Information Available: Synthesis of starting materials, 1H,

13

C NMR and HRMS

spectra, elemental analyses, crystal packing, optimized geometries and Gaussian Invariant Atomic Orbital

13

C chemical shifts plots (DFT), as well as crystal packing of MR1 and MR2.

This material is available free of charge via the Internet at http://pubs.acs.org.


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“For Table of Contents Use Only”

Designed synthesis and crystallization of isomorphic molecular gyroscopes with cell-like bilayer self-assemblies. Ma. Eugenia Ochoa,a Pablo Labra-Vázquez,b Norberto Farfán,b Rosa Santillan,a*

Two molecular rotors featuring pyridine and fluorobenzene rings as polar rotators and 9octylfluorenyl stators were synthesized. Their crystal structures were established through SXRD techniques, crystallizing in the monoclinic chiral P21 space group. The supramolecular assemblies of both isomorphs showed an orientation of static dipoles through the crystal lattice and the formation of intriguing 2D layers that resemble cell membranes, a typical example of an amphidynamic system. Small activation energies and the modulation of the first-order hyperpolarizabilities of these compounds as a function of rotational dynamics were revealed through DFT computations at the CAM-B3LYP/M06-2X/cc-pVDZ level of theory and correlated with a potential use of these materials as photonic switches.


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Designed Synthesis and Crystallization of Isomorphic Molecular

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