Takejiro
Ogawa
ab,
Fumitaka
Ishiwari‡
*a,
Fatin
Hajjaj
a,
Yoshiaki
Shoji
abc,
Takashi
Kajitani
ade,
Koji
Yazawa
f,
Takahiro
Ohkubo
g and
Takanori
Fukushima
*abc
aLaboratory for Chemistry and Life Science, Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan. E-mail: ishiwari@chem.eng.osaka-u.ac.jp; fukushima@res.titech.ac.jp
bDepartment of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan
cResearch Center for Autonomous Systems Materialogy (ASMat), Institute of Innovative Research, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, Kanagawa 226-8501, Japan
dOpen Facility Development Office, Open Facility Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8501, Japan
eRIKEN SPring-8 Center, 1-1-1 Kouto, Sayo, Hyogo 679-5148, Japan
fJEOL Ltd, 3-1-2 Musashino, Akishima, Tokyo 196-8558, Japan
gDepartment of Applied Chemistry and Biotechnology, Graduate School of Engineering, Chiba University, 1–33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan
First published on 13th June 2024
Controlling the rotation of carbon–carbon bonds, which is ubiquitous in organic molecules, to create functionality has been a subject of interest for a long time. In this context, it would be interesting to explore whether cooperative and collective rotation could occur if dipolar molecular rotors were aligned close together while leaving adequate space for rotation. However, it is difficult to realize such structures as bulk molecular assemblies, since molecules generally tend to assemble into the closest packing structure to maximize intermolecular forces. To tackle this question, we examined an approach using a supramolecular scaffold composed of a tripodal triptycene, which has been demonstrated to strongly promote the assembly of various molecular and polymer units into regular “2D hexagonal packing + 1D layer” structures. We found that a molecule (1) consisting of a dipolar 1,2-difluorobenzene rotor sandwiched by two 10-ethynyl-1,8,13-tridodecyloxy triptycenes, successfully self-assembles into the desired structure, where the dipolar rotor units align two-dimensionally at a close interval of approximately 0.8 nm while having a degree of freedom for rotational motion. Here we describe the self-assembly behavior of 1 in comparison with the general trend in molecular self-assembly, as well as the motility of the two-dimensionally aligned molecular rotors investigated using solid-state 19F-MAS NMR spectroscopy.
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Fig. 1 (a–c) Chemical structures of previously reported molecular rotors by Garcia-Garibay et al. (A–C)1–3,8 and Michl, Kaleta et al. (D)14–17 and schematic illustrations of their assembly structures. (d) Chemical structure of the molecular rotor designed in this study, consisting of a 2,3-difluorobenzene and two tripodal triptycene units as dipolar rotor and stator parts, respectively, and its assembly structure formed via a heating/cooling process in the bulk state. |
In this study, we developed 1 (Fig. 1d), which combines the molecular rotor designs of Garcia-Garibay1,2,18 and Michl and Kaleta14–17 with our original finding that a particular type of triptycene can strongly induce 2D self-assembly. We have demonstrated that 1,8,13-substituted triptycene derivatives, which can be referred to as tripodal triptycenes, exhibit excellent self-assembling ability, forming robust “2D hexagonal + 1D lamellar” structures through nested packing of their phenylene blades.19–22 These tripodal triptycenes have also been shown to serve as a supramolecular scaffold23 to align functional molecular units such as C60 (ref. 20) and even high-molecular-weight polymers24–26 into 2D + 1D structures with hexagonal lattice parameters of approximately 0.8 nm. A detailed investigation of the self-assembling behavior of 1 revealed that the rotation of its rotor unit is inhibited in the thermodynamically stable structure. On the other hand, 1 can also form a kinetically metastable structure, which allows the rotor unit to rotate with a low activation barrier. This metastable structure is robust and long-lasting at room temperature.
When compound 1 was suspended in chloroform, heated under reflux to completely dissolve, and then allowed to cool to room temperature, a microcrystalline powdery material formed. 1H NMR spectroscopy in o-dichlorobenzene-d4, thermogravimetric analysis (TGA), and elemental analysis confirmed that the recrystallized powdery material did not contain the solvent molecules used in recrystallization. Unfortunately, the powder X-ray diffraction (PXRD) pattern of as-recrystallized 1 was different from that expected if a “2D hexagonal + 1D lamellar” structure had been formed (Fig. 2a). Presumably, similar to previously reported molecular rotor systems,1,2,8 self-assembly of 1 proceeding via thermodynamic equilibrium states would not result in the desired 2D + 1D structure that provides adequate space for the dipolar rotor unit to rotate.
Compound 1 exhibits phase transition behavior in the bulk state. In differential scanning calorimetry (DSC), upon heating at a rate of 10 °C min−1, as-recrystallized 1 melts at 232 °C after undergoing two endothermic processes at 167 and 219 °C (Fig. 3a).27 Upon subsequent cooling, two exothermic processes occur at 229 and 208 °C. When as-recrystallized 1 is heated once to melt, its original structural information is erased. Thus, in the second heating/cooling cycle, the two endothermic peaks in the first heating process disappear, and after melting, the DSC profile becomes identical to that observed in the first cooling process.
To ensure that 1 assembles into the desired 2D + 1D structure, we reduced the cooling rate from the isotropic melt to 2 °C min−1, as a slower cooling rate is usually associated with greater structural regularity. However, a more complex PXRD pattern resulted, where diffraction peaks arising from these unidentified structures became more pronounced (Fig. 2d). To our surprise, when the cooling rate was increased to 30 °C min−1, the diffraction peaks converged almost completely to those arising from the 2D + 1D structure, allowing us to fully index the observed diffraction (Fig. 2e). Furthermore, even when a sample of the isotropic melt was removed from the heater and cooled rapidly to room temperature, a well-defined PXRD pattern (Fig. 2f) was obtained, which was nearly identical to that shown in Fig. 2e. A closer look at Fig. 2e and f indicates that the PXRD pattern, measured after rapid cooling, features higher integrity in diffraction in the region overlapping the halo (q = 13–17 nm−1), and two new weak diffraction peaks are detected between q = 14–15 nm−1. The absence of diffraction peaks due to the unidentified structure (black reversed triangle in Fig. 2c) indicates that the structural order is likely to be better in the rapidly cooled sample. The 2D + 1D structure of 1 thus obtained was found to be maintained even upon cooling to −180 °C, as well as on standing at 25 °C for at least 6 months.
The cooling rate-dependent structuring behavior of 1 was confirmed by DSC analysis. As shown in Fig. 3, while the DSC profiles measured at three different scan rates (2, 10, and 30 °C min−1) are similar overall, the temperature of each corresponding peak is slightly shifted. In organic solids with low molecular mobility, phase-transition temperatures often change depending on the DSC scan rate. However, a closer look at the endothermic and exothermic peaks in the higher temperature range revealed that side peaks (indicated with asterisks) coexist in the profiles obtained at scan rates of 2 and 10 °C min−1, while such side peaks are absent at a scan rate of 30 °C min−1 (Fig. 3d–f). Based on these results and those from the PXRD experiments, the side peaks can be attributed to the unidentified structures that generate upon slow cooling from the isotropic melt of 1.
By tracing the phase transition of 1 using synchrotron PXRD, we found that the 2D + 1D structure of 1 is formed even after the second endothermic process in the first heating. Fig. 4 shows variable-temperature (VT) synchrotron PXRD patterns of 1, measured during rapid (>30 °C min−1) heating/cooling cycles. In a temperature range of 25–150 °C, which is lower than the first endothermic process (172 °C in DSC, Fig. 3c), multiple diffraction peaks characteristic of crystalline materials can be seen. After the first endothermic process, the number of peaks decreases, and the XRD pattern between q = 3–20 nm−1 changes, while the intense peak in the small-angle region (q = 1.63 nm−1) remains almost unchanged. When temperatures exceed the temperature of the second endothermic process (224 °C in DSC, Fig. 3c), a drastic change occurs, generating a typical structure, which is attributed to a 2D sheet-like triptycene assembly with nested hexagonal packing and a 1D layer of the 2D sheets. Notably, the PXRD pattern at this stage exhibits higher-order diffraction peaks even from the (008) planes. Upon further heating, 1 undergoes a phase transition into its isotropic melt, giving a featureless XRD pattern.
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Fig. 4 VT synchrotron PXRD profiles obtained for as-recrystallized 1 in a glass capillary with a diameter of 1.5 mm, when heated and cooled repeatedly at a rate of approximately 30 °C min−1. The detailed peak assignments are shown in Fig. S7 and Table S1 (ESI†). |
When the isotropic melt is cooled to a temperature (e.g., 225 °C) lower than the temperature of the first exothermic process (227 °C in DSC, Fig. 3c), the diffraction pattern characteristic of the 2D + 1D assembly of 1 again appears. Although a clear exothermic feature is seen at 204 °C in DSC, the pattern remains unchanged during the cooling process reaching e.g., 100 °C. We consider that the exotherm is due to a phase transition into a slightly different structure from the high-temperature phase, while essentially preserving the 2D + 1D order. For example, a structure in which the molecules are slightly tilted toward the 2D plane, as seen in the single-crystal structure of 1,8,13-tris(dodecyloxy)triptycene,19 can be assumed. Meanwhile, all of the peaks are slightly shifted to the wide-angle region, most likely due to thermal shrinkage of the material. Thus, the hexagonal parameter (a) of 0.82 nm and layer spacing (c) of 5.2 nm at 230 °C are changed to 0.80 nm and 4.9 nm at 25 °C, respectively (Fig. S7 and Table S1, ESI†). At 25 °C, while higher-order diffraction peaks arising from the (005)–(008) planes become unclear (see also Fig. 2f), the main diffraction peaks, including those from the 2D hexagonal triptycene assembly, are maintained. According to the DSC profile (Fig. 3c), 1 exhibits a glass-transition feature at around 49 °C under rapid cooling conditions. Presumably, the long alkyl chains vitrify to lower the structural regularity of the 1D layer. Upon heating again at 100 °C, the resulting PXRD pattern is identical to that observed at the same temperature in the first cooling process and is maintained until the melting point. The change in PXRD patterns in the second cooling process completely reproduces that of the first cooling process. The above observations clearly indicate that heating/cooling 1 in the solid state results in the formation of a 2D + 1D structure, featuring two-dimensionally aligned dipolar rotors with approximately 0.8 nm intervals, which is so robust that it is maintained unless the material melts.
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Fig. 6 Solid-state VT 19F-MAS NMR spectra of (a) as-recrystallized and (b) thermally treated 1 having a 2D + 1D structure. |
Fig. 6b shows the temperature dependence from −63 to 132 °C of the 19F NMR signal of a solid sample of 1 featuring the 2D + 1D structure (Fig. S9, ESI†). Even at −63 °C, the 19F NMR signal at −133 ppm was unimodal with a T1 value of 0.67 s (Fig. S10, ESI†). This value is considerably smaller than that observed for the as-recrystallized powder sample even at 25 °C (35.8 s), clearly indicating much greater motility based on rotational and/or flipping motion of the rotor moieties in the 2D + 1D assembly, compared with those in as-recrystallized 1. With increasing temperature, the 19F NMR signal became sharper (Fig. S9, ESI†), particularly above the glass-transition temperature (49 °C, Fig. 3c). Notably, unlike in the case of as-recrystallized 1, this spectral change was reversible during the heating and cooling processes, with T1 displaying complex behavior and values ranging from 0.67 s (at −63 °C) to 1.05 s (at 2 °C).
Fig. 7a illustrates Arrhenius-type plots of 1/T1. There are at least two peaks, suggesting the presence of multiple motional modes corresponding to the Larmor frequency (470 MHz) of the 19F nucleus of the rotor units. Fitting the profile with two Kubo–Tomita functions17,28–30 results in relatively small activation energies (Ea) of 2.0 kcal mol−1 and 3.3 kcal mol−1 for the lower (blue curve) and higher (red curve) temperature regions, respectively.31 However, compared with examples of phenylene rotor dynamics in a metal–organic framework (MOF-5, Ea = 11.3 ± 2 kcal mol−1)29 and a porous organic crystal (Ea = 6.7 kcal mol−1),30 the values of activation energy are relatively low, and considering also that the change in T1 is small, the Larmor frequency of the 19F nucleus may not precisely capture the motion of the rotor units. The linewidth of the 19F NMR spectrum is mainly due to 1H–19F or 19F–19F dipole interactions, which affect motion in the order of several tens of kHz. Thus, T1ρ was measured using a spin-lock frequency of 50 kHz at various temperatures (Fig. S9 and S11, ESI†), and Arrhenius-type plotting was carried out (Fig. 7b). The obtained T1ρ values change between 6.8 ms (at 132 °C) and 0.84 ms (at 44 °C), and a unimodal peak is observed at around 45 °C with a shoulder at around 10 °C and a tailing feature in the lower temperature region (<−30 °C). Fitting the profile with two Kubo–Tomita functions gives Ea values of 5.5 kcal mol−1 and 5.7 kcal mol−1 for the lower (blue curve) and higher (red curve) temperature regions, respectively.31 Note that these Ea values are low enough to allow for motion of the rotor units at ambient to moderate temperatures.
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Fig. 7 Plots of (a) 1/T1 and (b) 1/T1ρversus reciprocal of temperature of thermally treated 1 having a 2D + 1D structure (white circles with a black border) fitted with a linear combination (black curves) of two Kubo–Tomita functions28 (red and blue curves). Fitting parameters are shown in Tables S2 and S3 (ESI).† |
Although it is difficult to completely assign the motional modes of the dipolar rotors from NMR data alone, we presume that the Ea values obtained from the analysis of T1 and T1ρ reflect rapid motions including vibration and slow rotor dynamics such as rotation, respectively. Of particular note is the fact that a clear temperature dependence is observed for the linewidth and T1ρ of the 19F NMR signal, which suggests a kHz-order rotational motion of the two-dimensionally close-aligned difluorobenzene rotors, in which dipole–dipole interactions possibly operate. Importantly, the experimental results, including the structural analysis based on XRD and solid-state 19F-MAS NMR spectroscopy, are all consistent and indicate that the desired 2D dipolar rotor assembly illustrated in Fig. 1d has been achieved using the tripodal triptycene scaffold.
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Fig. 8 (a) Structure of a model (c-axis projection) with a 2 × 2 × 1 super-cell of 1′ (Z = 4) for DFT calculations. (b) Plots of relative energy versus the rotational angle (θ in molecule d, Fig. 8a) of the rotor units of 1′ located at the center of the 2D hexagonal lattice (molecule d), calculated under periodic boundary conditions. Insets: the arrangement of the rotors at selected rotational angles (see also Movie S1, ESI†). |
As shown in Fig. 8b, all the rotor units in the most stable structure (θ = 0°) are aligned in the same direction (inset, left). The energy profile contains three maxima at θ = 70, 180 and 290° and three saddle points at θ = 0, 110 and 245°. The largest barrier was determined to be 2.85 kcal mol−1 at θ = 180°, where the fluorine atoms of adjacent rotor units are in close proximity to each other (inset, right). This value roughly agrees with the Ea values obtained from the solid-state 19F NMR measurements (Fig. 7). The rather small rotation barrier estimated from these calculations indicates that the two-dimensionally arranged rotor unit has a large degree of motility that is enough to allow even a 360° rotation. It is interesting to note that there is a clear angular dependence in the rotational barriers (Fig. 8b); the most stable and unstable structures are found in the geometries where the orientation of the dipole moment of the rotating unit is parallel and antiparallel, respectively, to those of the other rotor units. This suggests the possibility that cooperative rotational motion can occur, and we have begun to address this issue.
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4sc02750g |
‡ Present address: Department of Applied Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan. |
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