Robust assembly of aromatic molecules Organic materials can exhibit high porosity, but the structures often collapse or decompose at high temperatures. Yamagishi et al. synthesized an aromatic molecule that bears a symmetrical outer shell of three dipyridylphenyl wedges and crystallized it from highly dielectric solvents. Porous crystals formed with complex pore-wall structures that resulted from labile C–H⋯N bonds and van der Waals forces. Despite the weakness of these interactions, the porous structure was stable up to 202°C and could be recovered after collapse by exposure to solvent vapor. Science, this issue p. 1242

Abstract Here we report an anomalous porous molecular crystal built of C–H···N-bonded double-layered roof-floor components and wall components of a segregatively interdigitated architecture. This complicated porous structure consists of only one type of fully aromatic multijoint molecule carrying three identical dipyridylphenyl wedges. Despite its high symmetry, this molecule accomplishes difficult tasks by using two of its three wedges for roof-floor formation and using its other wedge for wall formation. Although a C–H···N bond is extremely labile, the porous crystal maintains its porosity until thermal breakdown of the C–H···N bonds at 202°C occurs, affording a nonporous polymorph. Though this nonporous crystal survives even at 325°C, it can retrieve the parent porosity under acetonitrile vapor. These findings show how one can translate simplicity into ultrahigh complexity.

The research field of crystal engineering was initiated by Desiraju and co-workers, who established its basic concept in the late 1980s by using small organic molecules of geometrical simplicity (1). Since then, organic molecules of further structural complexity have begun to be used for crystal engineering in conjunction with coordinating metal ions to obtain more-complex crystals that have tailored functions (2–4). During our curiosity-driven study on the solution behaviors of hyperbranched, multijoint, fully aromatic molecules, we made a serendipitous finding that may be antithetic to the current trend. Like dendrimers (5–7), such aromatic molecules are characterized by their nonplanar morphology and conformational flexibility, so that they are noncrystalline in the solid state, except for a few examples (8, 9). Py 6 Mes is a newly designed D 3 h -symmetric molecule that consists exclusively of aromatic rings that are connected via single bonds (Fig. 1A). The nonplanarity of this multijoint aromatic molecule stems from a large steric repulsion between the methyl groups of the mesitylene (Mes) core and connected phenylene rings (Fig. 1B). Py 6 Mes has a propeller shape with a symmetry group of D 3 h , which carries a polar outer shell composed of six pyridyl (Py) units for sterically protecting the nonpolar aromatic core (Fig. 1, A to C). We were curious about whether Py 6 Mes, which has a nonpolar solvophobic core, is soluble in highly dielectric media as a discrete spherical amphiphile because of its solvophilic shell. However, despite this characteristic, Py 6 Mes readily precipitated in acetonitrile (MeCN) and 2-propanol (isoPrOH), affording the porous crystal Pyopen, which included crystallographically disordered solvent molecules in its nanopores (Fig. 1D). Although Pyopen comprises only one type of geometrically simple molecule, it accomplishes very complicated tasks (Fig. 2B). As shown in Figs. 1D and 2D, Py 6 Mes uses two of its three dipyridylphenyl wedges for the construction of C–H···N-bonded double-layered roof-floor components, adopting a cofacial orientation. In an orthogonal direction to the roof-floor components, Py 6 Mes uses its residual one dipyridylphenyl wedge together with the mesitylenyl core for the construction of wall components formed by a solvophobically interdigitated architecture (Fig. 2E). Such difficult tasks, as accomplished by Py 6 Mes, illustrate how ultrahigh structural complexity can be achieved through self-assembly of a single molecule of ultrahigh symmetry. Furthermore, even though Pyopen has C–H···N-bonded parts, it has an exceptionally high thermal stability (Fig. 3B) and, at the same time, has an excellent self-healing ability (Fig. 3C). These findings also allow us to tackle a current issue in materials science: how thermally robust materials are made to self-heal (10–15).

Fig. 1 Molecular structures of Py 6 Mes and crystal packing diagrams of Pyopen⊃MeCN, Pyopen⊃isoPrOH, PyVDW⊃CHCl 3 , and PyVDW⊃THF. (A to C) Molecular structure (A), wireframe (B), and CPK (C) representations of Py 6 Mes. The pyridyl rings are colored in blue and green, whereas the mesitylenyl and phenyl rings are colored in black and white. (D to G) Wireframe representations of the crystal-packing diagrams of Pyopen⊃MeCN (D), Pyopen⊃isoPrOH (E), PyVDW⊃CHCl 3 (F), and PyVDW⊃THF (G).

Fig. 2 Modes of C–H···N bonds in Pyopen⊃MeCN and crystal-packing diagrams of Pyopen⊃MeCN. (A) Drawings representing the modes of C–H···N bonds in Pyopen⊃MeCN. (B to E) CPK representations of the crystal-packing diagrams of Pyopen⊃MeCN. Orange outlines indicate the C–H···N-bonded roofs and floors in Pyopen⊃MeCN [(B) and (C)], and dashed purple outlines indicate the wall connected to the roof and floor [(B), (C), and (E)].

Fig. 3 Transformations among Pyopen⊃MeCN, Pyopen, and Pyclose, and their N 2 adsorption isotherms. (A) Wireframe representations of the crystal-packing diagrams of Pyopen⊃MeCN and Pyclose. (B) PXRD profiles of Pyopen measured at different temperatures upon heating [wavelength (λ) = 1.07965 Å]. θ, angle of scattering. (C) Time-dependent PXRD profiles of Pyclose incubated in a cuvette filled with MeCN vapor (λ = 1.54 Å). The black line represents a PXRD profile simulated from the single-crystal structure of Pyopen⊃MeCN. (D) N 2 adsorption isotherms of Pyopen (red circles) and Pyclose (blue circles) at 77 K.

C–H···N bonds are far more labile than conventional hydrogen bonds (16–20). However, in contrast to hydrogen bonds, C–H···N bonds can survive even in highly dielectric media because their attraction force is given essentially by a dispersion force (21–23). To the best of our knowledge, Pyopen reported herein is the most thermostable porous crystal among those comprising nonclassical C–H···X bonds (24–26) and among all healable porous organic and metal-organic crystals reported thus far (10–12). Py 6 Mes, the component of Pyopen, carries three dipyridylphenyl wedges at its mesitylene core (Fig. 1A; details for the synthesis of Py 6 Mes are available in the supplementary materials). In MeCN that has an exceptionally high relative permittivity (ε r = 37.5), Py 6 Mes self-assembles into single crystals that are suitable for x-ray crystallography. The as-received crystal adopts a space group of P2 1 /c and includes crystallographically disordered MeCN molecules in its zigzag-shaped one-dimensional (1D) nanopores, which have diameters of 6 Å (Pyopen⊃MeCN; Fig. 1D and fig. S13). As expected, each dipyridylphenyl wedge tilts almost perpendicularly to the connected mesitylenyl core, adopting a dihedral angle in the range of 70 to 90°, but none of the three dipyridylphenyl wedges are crystallographically identical. Despite a large number of aromatic rings, only one pair of pyridyl rings that contain N3 atoms likely forms a cofacial π-stack with an interplanar distance of 3.36 Å (fig. S14B). Among the five pairs of geometrically close H and C atoms, only two pairs (H8A···C18 and H18···C13; fig. S14A and table S1) form a C–H···π bond. Five out of the six pyridyl rings in Py 6 Mes are involved in the formation of the C–H···N bonds (Fig. 2, A to D, and table S1). Overall, the back-to-back connected double-layered roof-floor components, supported by C–H···N bonds, are formed along the crystallographic bc plane (Fig. 2, B to D), whereas the wall components that are formed by the interdigitation of the dipyridylphenylmesitylenyl parts are constructed along the crystallographic b axis (Fig. 2E). Thermogravimetric analysis (TGA) of Pyopen⊃MeCN showed that the included MeCN molecules began to be released from the nanopores even at room temperature, affording guest-free Pyopen at 50°C, which is a much lower temperature than the boiling point (82°C) of MeCN (black curve in fig. S7). These observations indicate that the guest MeCN molecules are only weakly trapped in the nanopores of Pyopen. As expected, the powdery sample of guest-free Pyopen displayed a typical type I sorption isotherm for N 2 , with a steep slope in the low relative pressure (P/P 0 ) region and a Brunauer-Emmett-Teller (BET) surface area of 219 m2 g–1 (red circles in Fig. 3D). The pore size distribution, as estimated by micropore analysis (27), was unimodal with a maximum peak at 0.6 nm (fig. S8), which is in excellent agreement with the diameter of the 1D nanopore observed in Fig. 1D.

As has been discussed for porous crystals in the literature (28), the relative permittivity of the crystallization solvent largely affects the mode of solute-solvent interactions and therefore affects the crystal structures. When Py 6 Mes was allowed to assemble in highly dielectric isoPrOH (ε r = 18.3), the formation of solvent-including porous crystal Pyopen⊃isoPrOH (Fig. 1E) occurred, which is isomorphic to Pyopen⊃MeCN (Fig. 1D), and its nanopores, again, included crystallographically disordered isoPrOH molecules. Considering also that the TGA profile of Pyopen⊃isoPrOH with respect to the release of isoPrOH (red curve in fig. S7) is analogous to that observed for Pyopen⊃MeCN, we conclude that Pyopen weakly traps isoPrOH, similarly to when MeCN is used, in its nanopores. In sharp contrast to the above cases in highly dielectric media, when Py 6 Mes was allowed to assemble in moderately dielectric media such as chloroform (CHCl 3, ε r = 4.8) and tetrahydrofuran (THF, ε r = 7.6), classical van der Waals inclusion crystals denoted as PyVDW⊃CHCl 3 and PyVDW⊃THF formed, respectively (Fig. 1, F and G; for their crystal structures, see the supplementary materials). As summarized in table S2, we observed a clear correlation between the relative permittivities of the crystallization solvents and space groups of the resulting polymorphs (29). Inclusion crystals PyVDW⊃CHCl 3 (Fig. 1F) and PyVDW⊃THF (Fig. 1G) are isomorphic to each other and have a space group of P2 1 /n, but they are geometrically different from Pyopen⊃MeCN (Fig. 1D) and Pyopen⊃isoPrOH (Fig. 1E), both of which adopt a space group of P2 1 /c. It is likely that Py 6 Mes is no longer amphiphilic in moderately dielectric CHCl 3 and THF; that is, these solvents are affinitive toward both the shell and core parts of Py 6 Mes. Consequently, Py 6 Mes does not form a solvophobically interdigitated structure, as observed for the wall components in Pyopen⊃MeCN and Pyopen⊃isoPrOH (Fig. 1, D and E). In PyVDW⊃CHCl 3 (Fig. 1F) and PyVDW⊃THF (Fig. 1G), the included solvent molecules are essential constituents for supporting the crystal structures. Hence, as confirmed by TGA (fig. S7) and x-ray diffraction (XRD) (fig. S10), the crystals were easily demolished after heating at 100°C for 3 hours to remove the included solvent molecules.

Porous crystals, particularly when guest-free, are intrinsically unstable unless they use metal coordination bonds or dynamic covalent bonds. In the literature (24–26), none of the reported porous crystals that are composed of nonclassical C–H···X bonds can survive above 130°C. As described earlier, Pyopen turned out to be far more thermally robust than the reported examples. In a differential scanning calorimetry (DSC) profile over a wide temperature range from 40° to 300°C, guest-free Pyopen in the first heating process displayed a single exothermic peak at 202°C due to a crystalline phase transition (red curve in fig. S5). Upon subsequent cooling, neither an exothermic peak nor an endothermic peak appeared (blue curve in fig. S5), indicating that the phase transition at 202°C is irreversible. Accordingly, its powder XRD (PXRD) profiles did not change up to 202°C (red, yellow, and green curves in Fig. 3B) and remained essentially identical to that simulated from the single-crystal structure of Pyopen⊃MeCN (fig. S12). Upon further heating to induce the phase transition, the PXRD profile changed abruptly (blue and purple curves in Fig. 3B) and irreversibly (fig. S11), affording a new crystalline phase. Although the crystals thus formed were heavily cracked and no longer eligible for single-crystal x-ray structural analysis, we successfully identified the crystalline structure by means of Rietveld analysis for the PXRD profile measured at 50°C (see supplementary materials). In contrast to Pyopen, the product, denoted nonporous Pyclose adopting a space group of , has no solvent-accessible 1D channels (Fig. 3A and fig. S15, A to D). Consistently, Pyclose showed a higher density (1.192 g cm–3) than porous Pyopen (1.022 g cm–3) and barely adsorbed N 2 (blue circles in Fig. 3D). The Rietveld analysis indicated that none of the C–H···N bonding pairs that were originally present in Pyopen were shorter than sum of the van der Waals radii of the H and N atoms (table S3), suggesting that the crystalline phase transition at 202°C was triggered by thermal breakdown of the C–H···N bonds in the double-layered roof-floor components (fig. S15, A to D).

Although self-healing is one of the attractive features in materials science, an essential challenge, in the case of solid materials, is to address the general issue that high thermal robustness and excellent healing ability are mutually exclusive (13–15). Pyclose is thermally more robust than Pyopen and can survive even at 325°C (Fig. 3A). Nevertheless, under MeCN vapor, Pyclose self-healed to retrieve its parent porosity at ambient temperatures. Typically, crystalline Pyclose was put in a glass vial that was filled with MeCN vapor and incubated at 20°C. As shown in Fig. 3C, a set of new PXRD peaks assignable to Pyopen⊃MeCN gradually emerged at the expense of the original diffractions due to Pyclose, where the complete recovery of the original diffractions required 7 hours (Fig. 3C). The total volume of MeCN vapor eventually adsorbed by Pyclose on its recovery was nearly equal to the original pore volume of Pyopen (fig. S17). Guest-free Pyopen obtained from Pyopen⊃MeCN thus recovered again showed a N 2 adsorption behavior characteristic of microporous materials (fig. S16). The observed recovery is not the consequence of a trivial process involving partial dissolution of Pyclose in MeCN followed by recrystallization of Py 6 Mes, because a finely ground amorphous powder sample of Pyopen, when likewise treated under MeCN vapor at 20°C, did not transform back into Pyopen but gradually exerted a totally different XRD pattern (fig. S9) and MeCN adsorption profile (fig. S18). Namely, the reversible transformation occurs only between Pyopen⊃MeCN and Pyclose.

Here we demonstrate that Pyopen⊃MeCN consecutively transforms into guest-free Pyopen and then nonporous Pyclose, which transforms back into Pyopen⊃MeCN under MeCN vapor. Figure 4 shows a possible energy diagram for the overall crystalline transformation based on the DSC and TGA profiles. As described above, the release of guest MeCN molecules from Pyopen⊃MeCN to afford Pyopen likely requires only a negligibly small activation energy, as evidenced by the TGA profile of Pyopen⊃MeCN (fig. S7). By contrast, the transformation of Pyopen into Pyclose requires considerable heating and features a single exothermic peak in DSC at 202°C (fig. S5), for which the change in enthalpy (ΔH) value was evaluated to be 15.3 kJ mol–1. By means of the Kissinger method (30), the activation energy for this process was evaluated to be as high as 320 kJ mol–1 (fig. S6). On the other hand, the regeneration of Pyopen⊃MeCN from Pyclose under MeCN vapor occurs autonomously (Fig. 3C), suggesting that this transformation is energetically downhill and has a small activation energy to overcome, even at ambient temperatures. As is apparent in Fig. 3A, this process requires only a moderate reorientation of the interdigitating dipyridylphenylmesitylenyl units to effectively cancel their dipoles. We suppose that MeCN vapor would play an intermediating role in this reorientation event.

Fig. 4 An energy diagram for the consecutive transformation cycle involving Pyopen⊃MeCN, Pyopen, and Pyclose. Energy levels of Pyopen⊃MeCN, Pyopen, and Pyclose were estimated on the basis of the DSC and TGA profiles. The arrows indicate the direction of the crystal transformations. E a , activation energy.

As exemplified by the formation of PyVDW in moderately dielectric media (Fig. 1, F and G), assembled structures usually reflect the symmetry of constituent molecules. In this sense, it may be difficult to imagine that high-symmetry Py 6 Mes is the sole constituent for porous Pyopen, whose roof-floor components are structurally and orientationally different from its wall component (Fig. 1, D and E). Apart from the exciting physical properties of Pyopen that may change the preconception of healable solid materials (Fig. 4), the most intriguing result presented here is that Py 6 Mes, in highly dielectric media for constructing Pyopen, uses its three equivalent wedges for totally different tasks (Fig. 2B). This example demonstrates that even a single molecule of geometrical simplicity could assemble into a highly complicated exotic material of low symmetry.

Supplementary Materials www.sciencemag.org/content/361/6408/1242/suppl/DC1 Materials and Methods Figs. S1 to S18 Tables S1 to S3 References (31–38)

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Acknowledgments: H.Y. thanks A. I. Cooper and M. A. Little in the Department of Chemistry and Materials Innovation Factory, University of Liverpool, for x-ray diffraction measurements. Synchrotron radiation experiments were performed at BL44B2 in SPring-8 with the approval of the RIKEN SPring-8 Center (proposal 20160024). Funding: This work was supported by the Japan Society for the Promotion of Science (JSPS) through its Grant-in-Aid for Scientific Research (S) (18H05260) on “Innovative functional materials based on multi-scale interfacial molecular science.” H.Y. thanks the JSPS for a Young Scientist Fellowship and Leading Graduate Schools (MERIT) and the Leverhulme Trust (Leverhulme Research Centre for Functional Materials Design) for providing funding during a research placement at the University of Liverpool. Author contributions: H.Y. performed and interpreted all the experiments associated with molecular synthesis, crystal growth, and structural characterization. H.S. performed the sorption experiments. K.K. conducted synchrotron x-ray studies. A.H., Y.S., and R.M. carried out crystallographic studies. All authors contributed to the writing and editing of the manuscript. H.Y. and T.A. conceived the project, designed experiments, and directed the research. Competing interests: None declared. Data and materials availability: Crystallographic data reported in this paper are listed in the supplementary materials and archived at the Cambridge Crystallographic Data Centre under reference numbers CCDC 1857527 to 1857531. All other data needed to evaluate the conclusions in the paper are present in the paper or the supplementary materials.