Nanoscale Cu(II) MOFs Formed via Microemulsion: Vibrational Mode Characterization Performed using a Combined FTIR, Synchrotron Far-IR, and Periodic DFT Approach

Two Cu(II) metal–organic frameworks (MOFs) were prepared on the nanoscale at room temperature using a microemulsion method, namely, [Cu3(BTC)2(H2O)3] (BTC = benzene-1,3,5-tricarboxylate), known as HKUST-1 (1), and [Cu2(OH)(BTC)(H2O)]·2H2O (2). Thermochemical and gas sorption properties of the microporous topologies were characterized by mid- and far-infrared vibrational spectroscopy, supported by periodic density functional theory calculations. The mid-infrared profile of 1 appeared altered in response to gas sorption under variable temperature and pressure conditions. Vibrational mode analysis indicated the most sensitive infrared peaks were associated with the internal vibrations of organic linker moieties indirectly coupled to the Cu(II)–gas coordination site, activated by a lowered symmetry induced by guest interactions. Synchrotron far-infrared spectroscopy was shown to be a useful diagnostic for the microstructure of 1 and 2 where different temperature dependences were displayed in the low-frequency region. The loss of residual water during the activation of 2 at elevated temperature coincides with peaks indicative of free paddle-wheel moieties emerging in the far-IR spectra. As demonstrated for both materials 1 and 2, vibrational mode analysis was effective in screening MOF materials for their propensity toward gas uptake and, inversely, the diffusion of guest species such as adsorbed water from the microporous environments.