Reversible Solid-State Isomerism of Azobenzene-Loaded Large-Pore Isoreticular Mg-CUK‑1

A large-pore version of Mg-CUK-1, a water-stable metal–organic framework (MOF) with 1-D channels, was synthesized in basic water. Mg-CUK-1L has a BET surface area of 2896 m² g^–1 and shows stark selectivity for CO₂ sorption over N₂, O₂, H₂, and CH₄. It displays reversible, multistep gated sorption of CO₂ below 0.33 atm. The dehydrated single-crystal structure of Mg-CUK-1L confirms retention of the open-channel structure. The MOF can be loaded with organic molecules by immersion in hot melts, providing single crystals suitable for X-ray diffraction. trans-Azobenzene fills the channels in a 2 × 2 arrangement. Solid-state UV–vis spectroscopy reveals that azobenzene molecules undergo reversible trans–cis isomerization, despite being close-packed; this surprising result is confirmed by DFT-simulated UV–vis spectra.