Anisotropic Swelling of a Single-Crystalline Hydrogen-Bonded Organic Framework Induced by Iodine Vapor Uptake

In this paper, we report that iodine vapor adsorption induces anisotropic swelling of a porous hydrogen-bonded framework (HOF) composed of hexacarboxylic acid with hexaazatrinaphthylene (CPHATN). Single crystals of HOF CPHATN-1a exhibited swelling along the stacking direction upon exposure to iodine after an induction period, achieving a maximum elongation ratio of 60% and iodine uptake up to 30% at 22°C by weight. Single-crystal X-ray diffraction analysis, microscopic observation, micro-CT, Raman spectroscopy, as well as molecular dynamics calculations, revealed that iodine adsorption onto the HOF proceeds in the following two steps: (1) adsorption into void channels and (2) intercalation into the layers. The latter process causes anisotropic elongation of the crystals and perturbs crystalline periodicity. Intercalation of iodine into the interlayer was facilitated by attractive halogen bonds between the σ-holes of iodine and the pyrazine nitrogen atoms of CPHATN. This is the first proposal of the swelling mechanism of organic porous crystals by iodine based on single-crystal structures, and the results provide new insights into the effects of inclusion of heavy-element molecules on porous frameworks, contributing the development of porous materials.