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 (T_GO) 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 samples 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 materials 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.