Electron Density and Dielectric Properties of Highly Porous MOFs: Binding and Mobility of Guest Molecules in Cu3(BTC)2 and Zn3(BTC)2

Two isostructural highly porous metal-organic frameworks, the well-known Cu3(BTC)2 n (BTC = 1,3,5-benzenetricarboxylate), often appointed with the name HKUST-1, and Zn3(BTC)2 n, have been investigated as models for the buildup of dielectric properties, differentiating the role of chemi- and physisorbed guest molecules and that of specific intraframework and framework-guest linkages. For this purpose, electron charge density analysis, impedance spectroscopy, density functional theory simulations, and atomic partitioning of the polarizabilities have been exploited. These analyses at different degrees of pores filling enabled one to observe structural and electronic changes induced by guest molecules, especially when chemisorbed. The electrostatic potential inside the pores allows one to describe the absorption mechanism and to estimate the polarization of guests induced by the framework. The dielectric constant shows very diverse frequency dependence and magnitude of real and imaginary components as a consequence of (I) capture of guest molecules in the pores during synthesis, (II) MOF activation, and (III) water absorption from the atmosphere after activation. Comparison with calculated static-dielectric constant and atomic polarizabilities of the material has allowed for evaluating building blocks' contribution to the overall property, paving the way for reverse crystal engineering of these species.