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% (AzoMOF21%) in 30 min at the photostationary state and underwent backward isomerization into AzoMOF1% 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 AzoMOF20%⊃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 AzoMOF15% and AzoMOF1% 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.