Non-stackable molecules assemble into porous crystals displaying concerted cavity-changing motions

Abstract

With small molecules, it is not easy to create large void spaces. Flat aromatics stack tightly, while flexible chains fold to fill the cavities. As an intuitive design to make open channels inside molecularly constructed solids, we employed propeller-shaped bicyclic triazoles to prepare a series of aromatic-rich three-dimensional (3D) building blocks. This modular approach has no previous example, but is readily applicable to build linear, bent, and branched arrays of non-stackable architectural motifs from existing flat aromatics by single-pot reactions. A letter H-shaped molecule thus prepared self-assembles into porous crystals, the highly unusual stepwise gas sorption behaviour of which prompted in-depth studies. A combination of single-crystal and powder X-ray diffraction analysis revealed multiple polymorphs, and sterically allowed pathways for their reversible interconversions that open and close the pores in response to external stimuli.

Fig. 1. (a) Stacks of steel H-beams leaving non-collapsible open channels, which inspired (b) molecular-level construction of H-beams by lateral self-assembly. (c) Chemical structure of the building block 1, in which two iptycene units are appended to “lift up” the benzene core. In (b), the blue-coloured horizontal slab of the H-shaped 3D model corresponds to the electron-deficient triazole–phenyl–triazole triad within 1, the capped-stick representation of which is overlaid for clarity.

Scheme 1. Covalent modification of heteroaromatic-fused iptycenes.

Fig. 2. Molecular structures, Hirshfeld surfaces mapped with d_norm values, and single-crystal packing diagrams of (a) PhTI, (b) 1 (1A phase), (c) 2, and (d) 3, constructed with crystallographically determined atomic coordinates. Disordered solvent molecules are omitted for clarity. For each structure, the hydrogen bond length (d_C–H⋯N) denotes the interatomic distance between hydrogen and nitrogen atoms.

Scheme 2. Modular construction by one-pot C–N cross-coupling reactions.

Fig. 3. Schematic representation of the structural interconversions between the four polymorphs. The capped-stick packing diagrams are constructed with crystallographically determined atomic coordinates, and overlaid with space-filling models to show intermolecular contacts and pores.

Fig. 4. N₂ sorption isotherms and in situ PXRD pattern changes recorded during the sorption process. (a) N₂ adsorption (filled circles; red arrow) and desorption (empty circles; blue arrow) isotherms of activated 1B at T = 90 K showing double-step adsorption isotherm and desorption hysteresis. (b) Structural changes at the initial stage of N₂ uptake, as reflected on the two split (021) peaks (denoted by blue asterisks) merging into a single peak (denoted by a green asterisk), which indicates the recovery of the 1B phase by pore-opening by N₂ molecules. Red asterisks denote small amount of 1C phase formed during the activation process. (c) PXRD pattern at the final stage of N₂ desorption showing no noticeable changes of the (021) peak, which is consistent with the N₂ desorption hysteresis. PXRD pattern changes were recorded during the entire (d) adsorption, and (e) desorption processes.

Fig. 5. (a) Schematic representation of lateral association of 1 to form H-beam units, and their organization into the 3-D lattice. At each stage of this conceptual reconstruction, space-filling models based on the X-ray structures of 1B are shown next. The molecular electrostatic potential (MEP) map illustrates charge distribution that is responsible for intermolecular C–H⋯π interactions. (b) Synchronized sliding (either horizontal for 1A-to-1B, or vertical for 1A-to-1C) and rotating (for 1B-to-1Z) motions of the H-beam units in the solid state to drive phase transitions.