Structure, Dynamics and Gas Uptake of Porous Architectures: a Solid State NMR Perspective

Porous architectures, thanks to their functional properties, offer great opportunities in several applicative fields. Our scientific activity is devoted to the fabrication of porous materials and spectroscopic observation of gases diffused into the channels as well as of polymers obtained in situ inside the cavities. The matrices range from fully-organic covalent frameworks and metal-organic frameworks to porous molecular crystals of both synthetic and biological origin.[1-6] The diffusion and localization of the gas molecules within the cavities were investigated by 2D 1H-13C PMLG HETCOR solid state NMR on samples loaded with naturally abundant and enriched 13CO2, showing the accessibility of cavities to guest molecules from the gas phase. In the case of confined polymerization stereochemistry, chain alignment and morphology of the final products were addressed. 2D 1H-13C and fast-1H MAS NMR spectroscopy played a key role in determining the host-guest interactions at the interfaces and the intimacy between the two components, providing extremely valuable information ever detected by other techniques.Another challenging issue is the dynamics of porous solids and the insertion of molecular rotors in their very building blocks, thus promising access to switchable organic dielectrics. The combination of porosity with ultra-fast rotor dynamics was discovered in molecular crystals and covalent frameworks, by 2H spin-echo NMR spectroscopy. [7,8] The rotors, as fast as 108 Hz at 240 K, are exposed to the crystalline channels, which absorb CO2 and I2 vapors even at low pressure. Interestingly, the rotor dynamics can be switched on and off by vapor absorption/desorption, showing a remarkable change of material dynamics. Novel mesoporous organosilica frameworks allowed to realize periodic architectures of fast molecular rotors containing dynamic C-F dipoles in their structure.[9] The mobile elements showed not only the rapid dynamics of the aromatic rings (ca. 108 Hz at 325 K), as detected by solid-state NMR spectroscopy, but also a dielectric response typical of a fast dipole reorientation under the stimuli of an applied electric field. Crystals with permanent porosity were exploited in an unusual way to decorate crystal surfaces with regular arrays of dipolar rotors. The inserted molecules carry alkyl chains which are included as guests into the channel-ends.[10] The rotors stay at the surface due to a bulky molecular stopper which prevents the rotors from entering the channels. The host-guest relationships were established by 2D solid-state NMR and GIAO HF ab initio calculations.