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

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