An ultra-dynamic anion-cluster-based organic framework

Porous organic frameworks that can dynamically expand their voids to guest uptake remain formidable to design. The challenge comes from finding the balance between rigidity and flexibility. Rigidity is required to retain the framework, and the flexibility is needed for reversible expansion/contraction. Herein, we introduced bisulfate anion clusters as soft joints to construct a single-crystalline hydrogen-bonded cross-linked organic framework H_COF-6. Upon the uptake of guest molecules, the anion clusters of H_COF-6 were disrupted, allowing the crystal to expand rapidly (<40 min) to more than twice its original length. Removing these guests restored the anion clusters and the crystallinity of H_COF-6 quickly (<10 min). These guest-induced material size expansions and contractions are highly reversible, and the dynamic anion cluster dissociation-reassociation is confirmed by molecular dynamics simulations and solid-state NMR experiments. The guest-induced size change of H_COF-6 highlights a new approach to synthesize dynamic framework materials by introducing anion clusters.