Guest Controlled
Rotational Dynamics of Terephthalate Phenylenes in Metal–Organic
Framework MIL-53(Al): Effect of Different Xylene Loadings
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Abstract
MIL-53 is an interesting metal–organic
framework (MOF) with a “breathing” framework which is
envisioned for a number of potential applications. It is suggested
that the processes of hydrocarbon adsorption, diffusion, and separation
by this material are strongly influenced by flexibility of the framework
and fastly moving terephthalate phenylene fragments, representing
the array of molecular rotors. To govern the mentioned processes we
need to learn how to monitor the flexibility of the framework and
identify the specific effects of particular hydrocarbons on the rotational
motion of phenylene fragments. Here we demonstrate that flexibility,
i.e., large pore (LP) and narrow pore (NP) crystalline state interconversions
of the framework, can be monitored by following the evolution of phenylene
fragments dynamics with temperature by <sup>2</sup>H solid-state nuclear
magnetic resonance. We have established that the dynamics of phenylene
fragments is very sensitive to the loading of xylene guests and the
MOF structural state. The rotation rate is higher and the activation
barrier lower for the LP state of the guest-free or loosely loaded
material, whereas the NP and LP states with high loadings and dense
guest packing show a decrease of the rotation rates and increase of
the rotation energetic barrier