Electric field induced rotation of halogenated organic linkers in isoreticular metal-organic frameworks for nanofluidic applications

We present a systematic computational study which provides a plausible route to control the rotation of organic linkers in isoreticular metal-organic frameworks (IRMOF) by using an external electric field in order to manipulate the diffusion of molecules in nanopores. We achieve this by halogenating the organic linkers of IRMOF-1 and IRMOF-7 to create permanent dipole moments on the linkers, hence making them responsive to changes in the strength and direction of an electric field. More importantly we show that by varying the ligand size and the halogen type, number and substitution positions, the strength of the electric field required to control the rotation of linkers can be reduced significantly. Cl substitution is most effective in making the organic linkers electric field responsive since a greater dipole moment is created compared to those obtained by F or Br substitution. Cl substitution of a larger organic linker, i.e. 1,4-naphthalenedicarboxylate (IRMOF-7) rather than 1,4-benzenedicarboxylate (IRMOF-1), results in a greater dipole moment and reduces the electric field strength required for the rotation of the ligand. Furthermore, double Cl substitution and the optimization of the Cl substitution positions enable controlled rotation of the IRMOF-7 linkers with an electric field strength as low as 0.5 V nm(-1). Finally, using the electric field induced rotation of organic linkers we show that it is possible to enhance the diffusion of methane molecules in a chosen direction while limiting their mobility in other directions. Our study hints at the potential of using MOFs for flow control in nanofluidic systems.