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.