Guest Controlled Rotational Dynamics of Terephthalate Phenylenes in Metal-Organic Framework MIL-53(AI): Effect of Different Xylene Loadings

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 lastly 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 H-2 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.