3There is great current interest in the use of porous materials for substrate storage and separations, catalysis, drug delivery, and sensing. [1][2][3][4][5] Understanding the host-guest interactions in these systems is vital if new generations of improved materials are to be developed. However, gaining such insight is challenging and often not feasible in large void materials due to lack of order in porous systems such as carbon, mesoporous silica and zeolites, 5 and also because the nature of these host-guest interactions is often based upon very weak and dynamic supramolecular contacts such as hydrogen-bond, π···π stacking, van der Waals, electrostatic or dipole interactions. Such supramolecular binding usually involves many hydrogen atoms and undergoes dynamic processes that are difficult to probe directly by experiment. 2,6 Recent studies on porous metal-organic frameworks (MOFs) for hydrogen storage, 7,8 carbon capture, 1 and hydrocarbon separations 9,10 have shown, in exceptional cases, location of guest molecules within the host via advanced crystallography studies, providing invaluable structural rationale for their function and properties. Most of these successes have been achieved within host systems that display strong confinement effects on the guest molecules and/or have specific relatively strong binding sites such as open metal centers. 7-12 However, host-guest systems involving primarily soft supramolecular interactions usually lead to serious positional disorder of the guest molecules, and hydrogen atoms involved in these binding processes are not readily seen or defined from crystallography studies, 13,14 leading to problems in defining the dynamics and motions of such host-guest systems.As a result, information on molecular binding dynamics and the motion of guests within the confined space of MOF hosts is largely lacking. Herein, we report the application of combined inelastic, quasi-elastic and elastic neutron scattering and synchrotron X-ray diffraction coupled to density functional theory (DFT) calculations to directly visualise the dynamics of host-guest interactions between adsorbed C 2 -hydrocarbons and the hydroxylfunctionalised porous MOF NOTT-300, which exhibits high selectivity and uptake capacity for unsaturated hydrocarbons. Moreover, direct observation of the mobility and diffusion for these adsorbed hydrocarbons within NOTT-300 has been achieved, representing important methodologies for their potential kinetic separations. These complementary experiments using dynamic, kinetic and static approaches lead to the same conclusion: four types of soft supramolecular interactions cooperatively bind guest molecules in these functionalised cavities, and these finetuned interactions lead to optimal uptake kinetics and binding dynamics affording excellent selectivities between these hydrocarbons.
Results and discussionMaterial and characterisation. and 5, respectively. These selectivities are, however, subject to uncertainties associated with isotherm measurement of the extremely low ...