posted on 2019-05-25, 00:00authored byRebecca Scatena, Yannick T. Guntern, Piero Macchi
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.