Crystalline Nanochannels with Pendant Azobenzene Groups: Steric or Polar Effects on Gas Adsorption and Diffusion?
2017
Huang, Hubiao | Sato, Hiroshi | Aida, Takuzo
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.
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