Crystalline Nanochannels with Pendant Azobenzene Groups: Steric or Polar Effects on Gas Adsorption and Diffusion?

An azobenzene-containing, zirconium-based metal-organic framework (^AzoMOF), upon irradiation with ultraviolet (UV) light at 365 ± 10 nm, underwent trans-to-cis isomerization of its azobenzene pendants to furnish the cis-isomer content of 21% (^AzoMOF^21%) in 30 min at the photostationary state and underwent backward isomerization into ^AzoMOF^1% upon either irradiation with visible light (420-480 nm) or heating. When the cis-isomer content increased, the diffusion rate and amount of CO₂ adsorbed into the nanochannels of ^AzoMOF decreased considerably. When erythrosine B, a polarity-probing guest, was used, it showed a red shift upon exposure of ^AzoMOF^20%⊃EB to visible light, indicating that the interior environment of ^AzoMOF turns less polar as the trans-isomer content becomes higher. In sharp contrast, the adsorption profiles of ^AzoMOF^15% and ^AzoMOF^1% for Ar having an analogous kinetic diameter to CO₂ but no quadrupole moment and a smaller polarizability were virtually identical to one another. Therefore, it is likely that CO₂ experiences a dominant effect of a polar effect rather than a steric effect in the crystalline nanochannels.