Gate-Opening Gas Adsorption and Host–Guest Interacting Gas Trapping Behavior of Porous Coordination Polymers under Applied AC Electric Fields

The gate-opening adsorption behavior
of the one-dimensional chain
compound [Ru2(4-Cl-2-OMePhCO2)4(phz)]
(1; 4-Cl-2-OMePhCO2– = 4-chloro-o-anisate; phz = phenazine) for various gases (O2, NO, and CO2) was electronically monitored in
situ by applying ac electric fields to pelletized samples
attached to a cryostat, which was used to accurately control the temperature
and gas pressure. The gate-opening and -closing transitions induced
by gas adsorption/desorption, respectively, were accurately monitored
by a sudden change in the real part of permittivity (ε′).
The transition temperature (TGO) was also
found to be dependent on the applied temperature and gas pressure
according to the Clausius–Clapeyron equation. This behavior
was also observed in the isostructural compound [Rh2(4-Cl-2-OMePhCO2)4(phz)] (2), which exhibited similar
gate-opening adsorption properties, but was not detected in the nonporous
gate-inactive compound [Ru2(o-OMePhCO2)4(phz)] (3). Furthermore, the imaginary
part of permittivity (ε″) effectively captured the electronic
perturbations of the samples induced by the introduced guest molecules.
Only the introduction of NO resulted in the increase of the sample’s
electronic conductivity for 1 and 3, but
not for 2. This behavior indicates that electronic host–guest
interactions were present, albeit very weak, at the surface of sample 1 and 3, i.e., through grain boundaries of the
sample, which resulted in perturbation of the conduction band of this
material’s framework. This technique involving the in situ application of ac electric fields is useful not
only for rapidly monitoring gas sorption responses accompanied by
gate-opening/-closing structural transitions but also potentially
for the development of molecular framework materials as chemically
driven electronic devices.