Compression of a Flapping Mechanophore Accompanied by Thermal

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Compression of a Flapping Mechanophore Accompanied by Thermal Void Collapse in a Crystalline Phase Takuya Yamakado, Kazuya Otsubo, Atsuhiro Osuka, and Shohei Saito J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b03833 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 13, 2018

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Journal of the American Chemical Society

Compression of a Flapping Mechanophore Accompanied by Thermal Void Collapse in a Crystalline Phase Takuya Yamakado, Kazuya Otsubo, Atsuhiro Osuka, and Shohei Saito* Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan

Supporting Information Placeholder ABSTRACT: Mechanical control of molecular energy

landscape is an important issue in modern materials science. Mechanophores play a unique role in that the mechanical responses are induced against activation barrier for intramolecular transformation by the aid of external forces. Here we report an unprecedented activation process of a flexible flapping mechanophore. Namely, thermal void collapse in a crystalline phase triggers mechanophore compression in a definite proportion. Unfavored conformational planarization of the flapping mechanophore is compulsory induced by packing force, leading to total energy gain in crystal packing. Fluorescence chromism indicates extended π conjugation by the mechanophore compression, giving rise to an energy transfer from the unpressed to compressed conformers.

Controlling molecular energy profile by mechanical force offers productive mechanochemistry.1 Recent upsurge on synthetic mechanophores, molecular structures that show mechanical responses by intramolecular transformation, has rapidly developed fruitful polymer mechanochemistry in this decade. Originally, the mechanophore activation is an energetically uphill process, but it can be realized by the aid of external forces.2 Not only by stretching, pressing, shearing/grinding, swelling, or freezing polymer materials,3 mechanophore activation has been demonstrated by picking up a single polymer chain using AFM or optical tweezers,4 by employing sonication/flow2a,5 or shockwave, 6 by surface pressure at the air/liquid interface,7 and very recently by hydrostatic pressure on crystals.8 Here we report a new conformationally flexible flapping mechanophore, different from twisted flippers,9 rotaxane,10 and universal joint system,11 which undergoes unprecedented activation process, mechanophore compression triggered by thermal void collapse in a crystalline phase (Figure 1).

Figure 1. Compression of flapping mechanophores in a crystalline phase, and an energy profile for the conformational planarization of the isolated mechanophore. Voids in the crystal packing thermally collapse during the crystal phase transition.

The flapping mechanophore is designed by attaching two anthracene moieties on a central cyclooctatetraene (COT) ring in a fused manner. On the basis of the flapping design, a series of photofunctional molecules and materials,12,13 such as visco/thermoprobes,12b,13c a liquid crystal showing excited-state aromaticity,12c a light-melt adhesive,12d and singlet-fission chromophores,12e have emerged, in which the excited-state dynamics are focused. However, a mechanophore function of the flapping molecular system has not been unveiled. To avoid interchromophore interaction, here we introduced bulky triisopropylsilyl(TIPS)ethynyl groups on the anthracene moieties (Figure 2a). The molecule takes a V-shaped conformation as the most stable form, in which π-conjugation of each anthracene moiety is practically isolated due to the bent structure of COT (Figure S8.4). By contrast, effective π delocalization over the whole molecule is expected for a planar conformation, although a small but significant energy barrier (5–9 kcal/mol, See Table S8.1) is estimated for the conformational planarization in the ground state (Figure 2b).

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gravimetry, indicating the preserved material balance (Figure 4b). Owing to the slow transition process, clear enthalpy change was not detected in differential scanning calorimetry (Figure S6.2). Phase B microcrystals showed a characteristic XRD pattern, but severe cracks of the annealed crystals inhibited singlecrystal X-ray structure analysis.

Figure 2. (a) Conformational flexibility of the flapping mechanophore bearing bulky silyl groups (TIPS = triisopropylsilyl). (b) Calculated energy profile and electronic perturbation of the TIPS-ethynyl substituted mechanophore as a function of the COT bent angle θ, in which (TD-)DFT calculations were performed at the TDPBE0/6-31+G(d)//PBE0/6-31G(d) level of theory.

The energy for the electronic transition becomes significantly smaller as the COT bent angle θ decreases (Figure 2b). Although a rapid flapping motion is expected in solution, the conformational flexibility is suppressed in a crystalline phase. Single crystals obtained from a CH2Cl2/MeOH solution contained the Vshaped mechanophores and CH2Cl2 molecules (Figure S4.1). After removal of the solvent molecules under vacuum, the crystalline lattice was still preserved to allow single-crystal X-ray structure analysis,14 in which the solvent molecules were replaced by large void space15 with negligible electron density, occupying ~670 Å3 with 1.75 Å probe radius (2.3%volume) (Figure 3, called Phase A). The mechanophores retain the V-shaped conformation in Phase A, whose bent angles θ are 38–46°.

Figure 3. (a) X-ray crystal packing consisting of the Vshaped mechanophores and (b) large void space (yellow) in Phase A.

Annealing the vacuum-treated crystal on a hot stage at 180 °C induced an irreversible phase transition within 6 h, accompanied by a color change from yellow to orange (Figure 4a). Concomitantly, remarkable fluorescence (FL) chromism was observed under UV irradiation; initial Phase A showed green FL and final Phase B showed orange FL. During the phase transition, no weight loss was observed in thermo-

Figure 4. (a) Microscopic observation of the crystal phase transition on a hot stage at 180 °C. Color change from Phase A into Phase B in bright-field images and FL chromism under UV irradiation. (b) Thermogravimetry of the Phase A microcrystals, scanning at the rate of 1 °C/min. (c) Powder XRD of the annealed Phase B microcrystals (red, room temperature) and simulated XRD pattern of the crystal packing displayed in Figure 5 (black, –183 °C, scaling factor: 0.96).

Fortunately, the packing structure of Phase B was determined from single crystals obtained from a different solvent system (Et2O/EtOH)16 that displayed the corresponding XRD pattern to the annealed crystals (Figure 4c). Le Bail XRD analysis of the annealed microcrystals also showed good agreement in the lattice parameters (Figure S4.11), and optical properties of the crystals further supported this correspondence (Figure S5.8). To our surprise, the crystal packing contained a molecule with an almost planar conformation (θ = 13–17°) along with those having V-shaped conformations (θ = 42–49°) (Figure 5a). The planar molecule was sandwiched by bulky TIPS substituents of the neighboring V-shaped molecules, suggesting that the planar one was mechanically compressed, reflecting the conformational flexibility of the flapping mechanophore (Figure 5b). In contrast to the polymer mechanochemistry, the ratio of the activated and unactivated mechanophores are accurately determined by crystallography to be 1:2. On the structure of the planar conformer, weak contacts between the anthracene centers and the TIPS groups were indicated by Hirshfeld surface analysis17 (Figure 5c). Importantly, the large void observed in Phase A disappeared by the thermal phase transition into Phase B, and accordingly, the crystal density was significantly increased from 1.030 to 1.076 g cm–3.


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Total energy gain by the void collapse would allow the unfavored conformational planarization of the flapping mechanophore. DFT calculation using the GGA-PBE functional supported this assumption quantitatively, in which Phase B is more stable by 4.6 kcal/mol than Phase A (Table S8.3). It is worth noting that, although there are many examples of conformational polymorphs where molecules often take unfavorable conformations in crystals,18 only a few examples are reported for the crystal structure of originally unstable conformation far from a local minimum, rather close to a transition state.19

Figure 5. (a) X-ray crystal packing consisting of the planar and V-shaped mechanophores in a 1:2 ratio, (b) side view of the planar conformer compressed between bulky TIPS groups of the unpressed conformers, and (c) Hirshfeld surface of the planar conformer mapped with the contact distance over the range from –0.03 to 1.80.

Optical properties of the microcrystals in Phase A and Phase B are shown in Figure 6. Diffuse reflectance spectrum of Phase A had a vibronic structure peaked at 460, 436 and 414 nm, well corresponding to the absorption spectrum in a CH2Cl2 solution (Figure S5.1a). The agreement supports negligible electronic interaction among the anthracene-based chromophores owing to the bulky TIPS groups; otherwise π stacking structures were formed with smaller silyl groups (Figure S4.4–S4.9). Diffuse reflectance spectrum of Phase B showed a characteristic peak at 542 nm, marked in red in Figure 6b, assigned to the absorption of the compressed planar conformer, which is distinguished from the structured band around 460 nm originating from the other unpressed V-shaped conformers. The remarkable redshift is strong evidence for the mechanophore function of the flapping molecule, whose excitation energy becomes smaller as the molecule is compressed to be flat (Figure 2b). Green FL of the Phase A microcrystals was observed with the maximum wavelength of 513 nm emitted from the V-shaped conformers (Figure 6a), providing the quantum yield of 0.08 (Table S5.2). By contrast, orange FL of the Phase B micro-

crystals no longer showed the 513-nm emissive band, and three sharp bands at 544, 588 and 640 nm were observed with ΦFL = 0.10 (Figure 6b), whose FL lifetime of 34 ns was constant at each wavelength (Figure S5.15, Table S5.2). Similar spectral tendency was also confirmed in microspectroscopy of the fluorescent single crystal (Figure S5.8). The lack of the green FL despite the presence of the unpressed V-shaped conformers suggested an energy transfer from the Vshaped to planar conformers in the Phase B crystal packing, although a contribution of reabsorption cannot be ruled out. The mechanism of the FL chromism is clearly different from the reported FL mechanochromism20 and FL chrosmism induced by crystalto-crystal transitions,21 in that the π extention of the compressed mechanophore gave rise to the energy transfer between different conformers in a singlecomponent crystal.

Figure 6. Diffuse reflectance spectra using the KubelkaMunk function (black) and FL spectra (green, orange) of the Phase A microcrystals (a) and the Phase B microcrystals (b).

In conclusion, we report a mechanophore function of a COT-based flapping anthracene dimer and unprecedented mechanophore activation process. By using the flapping molecule bearing the bulky substituents, unique mechanophore compression is demonstrated in a thermally induced crystal phase transition via dynamic void collapse. Unfavored conformational planarization of the V-shaped mechanophore has been realized by packing force, leading to an extended π conjugation with smaller excitation energy. Fluorescence chromism with an energy transfer from the unpressed to the compressed conformers offers a new opportunity to investigate quantitative energetics of mechanochemistry by crystallography (both in energies of a single molecule and a crystal packing), which should be deeply related to the rapidly grown science of dynamic crystals.22 ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website, containing synthesis,



optical properties, crystallographic data, and theoretical calculations. Crystallographic data are provided free of charge by The Cambridge Crystallographic Data Centre.

AUTHOR INFORMATION Corresponding Author

* [email protected]. Funding Sources

This work was supported by JST, PRESTO Grant Number JPMJPR16P6, by JSPS, KAKENHI Grant Numbers JP15H05482 and JP15H01083, and by Inoue Foundation for Science, Inoue Science Research Award to S.S. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank Prof. Dr. Naoki Aratani at NAIST for his help in single-crystal X-ray structure analysis.

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Compression of a Flapping Mechanophore Accompanied by Thermal

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