Metal-organic and organic frameworks: porosity, gas adsorption and fast dynamics

Crystalline porous materials are excellent candidates for the fabrication of molecular rotors in the solid state.1 The combination of remarkable porosity with ultra-fast rotor dynamics was discovered in molecular crystals and metal-organic frameworks (MOFs) by 2H spin-echo NMR spectroscopy and T1 relaxation times. Molecular rotors are exposed to the crystalline channels, which absorb CO2 and I2 vapors even at low pressure. Interestingly, dynamics could be controlled by I2 absorption/desorption, showing a remarkable change of material dynamics and suggesting the use of porous crystals in pollutant management. 2,3 A microporous MOF engineered to contain in its scaffold rod-like struts [1,4-bis(1H-pyrazol-4-ylethynyl)benzene] showed extremely rapid 180° flip reorientation of the central p-phenylene unit with rotational rates of 1011 Hz at 150 K. The permanent porosity of the crystals is demonstrated by CO2 adsorption isotherms at various temperatures and the selectivity of the MOF toward CO2/N2 binary mixtures is associated with the interaction energy, estimated to be 25 kJ mol-1, indicating a good interaction of CO2 with the channel walls. Crystal-pore accessibility of the MOF allowed the CO2 molecules to enter the cavities and control the molecular rotor spinning speed down to 105 Hz at 150 K (Fig. 1). 4 This strategy enabled the regulation of rotary motion by the diffusion of the gas within the channels and the determination of the energetics of rotary dynamics in the presence of CO2. This unique response of the materials to CO2 is of great importance for the environment, enlarging perspectives in the field of sensors and gas detection.