Continuous Structural Evolution upon Iodine Adsorption in Adaptive Hydrogen-Bonded Networks Constructed by Organic Ammonium Halides

A simple crystalline material system (1,8-octanediammonium dihalides, ODAHX, X = Cl, Br, I) is capable of dynamically capturing molecular iodine through unique “adaptive hydrogen-bonded networks (HBNs).” The system achieves an exceptional I2 uptake capacity up to 8.0 mol·mol–1 (ODAHI) with >95% efficiency after 5 cycles, while time-dependent X-ray diffraction characterization identifies as many as four distinct intermediate phases for ODAHCl during iodine incorporation. Combining crystallographic analysis, adsorption kinetics, and DFT calculations, we reveal how multiple halogen-bonded motifs between halide and I2 molecules, conformational flexibility of the organic cation, and tunable NH···X– hydrogen bonds enable continuous structural evolution during iodine adsorption. Notably, X–···I2 halogen bonding initiates the adsorption and further drives the penetration of I2 molecules, along with the synergistic adjustment of the HBNs and adaptive change of the ODAH2+ conformations. This work provides atomic-level evidence for spontaneous void-gas creation in originally nonporous networks and thus redefines the engineering principles for next-generation smart adsorbents targeting pollutant molecules.