Macrocycle-Based Metal–Organic Frameworks with NO₂‑Driven On/Off Switch of Conductivity

Conductive
metal–organic frameworks (MOFs) have a wide range
of applications in supercapacitors, electrocatalysts, and fuel cells,
while gas-driven conductive MOFs have not yet been synthesized so
far. Herein, we report a gas-driven conductive MOF (A) constructed from calix[4]­resorcinarene macrocycle and Co­(II) cations,
which shows the conductivity enhancement by about eight orders of
magnitude through NO2 adsorption. The conductivities of
MOF A before and after the adsorption of NO2 were calculated to be about 1.3 × 10–11 and
8.4 × 10–4 S/cm, respectively. MOF A realizes the conversion from an insulator to a conductor by adsorbing
NO2. When NO2 is evacuated, MOF A quickly changes from a conductor back to an insulator in 42 s. In
the crystal structure of NO2-adsorbed MOF (termed as A-NO2), NO2 molecule connects
Co­(II) and uncoordinated carboxylate groups through hydrogen-bonding
interactions to form a conductive pathway, greatly reducing the electron
transmission distance between each two metal clusters. In addition,
NO2 molecule and H3O+ may also form
a conductive pathway by hydrogen-bonding interactions. This work presents
an interesting macrocycle-based MOF with a NO2-driven on/off
conductivity switch, proving the possibility for designing advanced
gas-driven conductive systems.