Tailoring porosity and rotational dynamics in a series of octacarboxylate metal-organic frameworks

Modulation and precise control of porosity of metal-organic frameworks (MOFs) is of critical importance to their materials function. Here we report modulation of porosity for a series of isoreticular octacarboxylate MOFs, denoted MFM-180 to MFM-185, via a strategy of selective elongation of metal-organic cages. Owing to the high ligand connectivity, these MOFs do not show interpenetration, and are robust structures that have permanent porosity. Interestingly, activated MFM-185a shows a high Brunauer-Emmett-Teller (BET) surface area of 4,734 m² g^-1 for an octacarboxylate MOF. These MOFs show remarkable CH₄ and CO₂ adsorption properties, notably with simultaneously high gravimetric and volumetric deliverable CH₄ capacities of 0.24 g g^-1 and 163 vol/vol (298 K, 5-65 bar) recorded for MFM-185a due to selective elongation of tubular cages. The dynamics of molecular rotors in deuterated MFM-180a-d₁₆ and MFM-181a-d₁₆ were investigated by variable-temperature ²H solid-state NMR spectroscopy to reveal the reorientation mechanisms within these materials. Analysis of the flipping modes of the mobile phenyl groups, their rotational rates, and transition temperatures paves the way to controlling and understanding the role of molecular rotors through design of organic linkers within porous MOF materials.