Tuning the Swing Effect by Chemical Functionalization of Zeolitic Imidazolate Frameworks

Many zeolitic imidazolate frameworks
(ZIFs) are promising candidates
for use in separation technologies. Comprising large cavities interconnected
by small windows they can be used, at least in principle, as molecular
sieves where molecules smaller than the window size are able to diffuse
into the material while larger molecules are rejected. However, “swing
effect” or “gate opening” phenomena resulting
in an enlargement of the windows have proven to be detrimental. Here,
we present the first systematic experimental and computational study
of the effect of chemical functionalization of the imidazole linker
on the framework dynamics. Using high-pressure (HP) single-crystal
X-ray diffraction, density functional theory, and grand canonical
Monte Carlo simulations, we show that in the isostructural ZIF-8,
ZIF-90, and ZIF-65 functional groups of increasing polarity (−CH3, −CHO, and −NO2) on the imidazole
linkers provide control over the degree of rotation and thus the critical
window diameter. On application of pressure, the substituted imidazolate
rings rotate, resulting in an increase in both pore volume and content.
Our results show that the interplay between the guest molecules and
the chemical function of the imidazole linker is essential for directing
the swing effect in ZIF frameworks and therefore the adsorption performance.