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.