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.