Dielectric Properties of Guest Molecule-loaded MOFs with Dipolar Rotors
Babak Farhadi Jahromi, Ruhr-Universität Bochum, Bochum, GermanyRochus Schmid, Ruhr-Universität Bochum, Bochum,
The application of metal-organic frameworks (MOFs) as chemical sensors is an ongoing research topic and various possible sensing mechanisms have been proposed, for instance measuring the MOFs dielectric response in an external electric field. Having previously shown in an extensive molecular dynamics (MD)-based study that the dielectric properties of MOFs can be systematically tuned by incorporating rotatable dipolar linker molecules, resulting in anisotropic responses to external electric fields that depend on the incorporated dipolar moiety [1], we have now extended this study by introducing guest molecules to two of the previously investigated MOFs, one with dipolar rotors, one without, and investigating the effect those have on the dielectric properties of the two MOF systems. Methanol was chosen as a representative polar guest molecule species, due to the recent development of a high-quality force field for bulk methanol [2], and methane was used as a non-polar reference species to gauge how sensitive the dielectric responses of these MOFs are to the overall polarity of the introduced guest molecules as well as to their total amount. Both investigated systems were found to be quite sensitive to the introduction of methanol guest molecules, showing anisotropic and non-linear increases of their relative permittivities as well as a strongly increasing component to the total system polarization induced by external electric fields for increasing numbers of guest molecules. In contrast to this, introducing non-polar methane guest molecules appeared to have almost no influence on the relative permittivities of the two systems, only introducing a slight non-linear decrease to the relative permittivity components associated to the rotor plane in the MOF containing dipolar rotors and only a negligible induced polarization component was observed compared to the empty MOFs. The potential of MOFs for the sensing of polar guest molecules could therefore be substantiated, while the observed low dielectric sensitivity of the investigated MOF with dipolar rotors for non-polar guest molecules necessitates further studies to identify dipolar moieties that allow for the detection of both polar and non-polar guest molecules. [1] J. Dürholt, B. Farhadi Jahromi, R. Schmid, ACS Cent. Sci. 2019, 5, 1440–1448. [2] S. Siwaipram, P. A. Bopp, J.-C. Soetens, R. Schmid, S. Bureekaew, J. Mol. Liq. 2019, 285, 526 –534