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

Abstract

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. <i>trans</i>-Azobenzene fills the channels in a 2 × 2 arrangement. Solid-state UV-vis spectroscopy reveals that azobenzene molecules undergo reversible <i>trans</i>-<i>cis</i> isomerization, despite being close-packed; this surprising result is confirmed by DFT-simulated UV-vis spectra.