Peristaltic Motion in Structurally Adaptive Molecular Crystals Enables Selective Propyne Capture.

Cyclotetrabenzoin and its tetraacetate, two macrocyclic porous molecular crystals, were examined as adsorbents for light hydrocarbons, with a focus on C3 hydrocarbons: propane, propene, and propyne. While both materials exhibit a preference for propyne, only the tetraacetate─owing to its higher surface area (570 vs. 42 m2 g-1), enhanced uptake capacity (1.99 vs. 1.19 mmol g-1), and faster kinetics─achieves dynamic binary separation of propyne from propylene under ambient conditions and various influent ratios (1/1, 1/2, and 2/1, v/v). The high propyne selectivity and separation trends were explained by using a combination of in situ synchrotron powder X-ray diffraction and molecular dynamics. These techniques suggested that the more rigid, extensively hydrogen-bonded structure of cyclotetrabenzoin transports propyne chiefly through pore enlargement. In cyclotetrabenzoin acetate, the absence of hydrogen bonding and larger void volume (25.9 vs. 9.6% in cyclotetrabenzoin) allows extensive structural adaptation that facilitates the capture and transport of propyne through the crystal. Rotation of cyclotetrabenzoin acetate's benzene aromatic panels by ≈19° allows adjustment to the propyne structure, maximizing interactions with the C≡C triple bond and the acetylenic hydrogen. Beyond the molecule, extensive fluxionality allows for peristaltic transport of guests through the material but can also result in transient closure of one-dimensional channels observed in the single-crystal X-ray structure. These results highlight the importance of subtle structural adaptations in sorbent structures to the bulk separation performance and offer a new design strategy for gas sorption in transiently porous and ultramicroporous molecules.