Molecular Rotors in Porous Supramolecular Architectures

A challenging issue is the dynamics of porous solids and the insertion of molecular rotors in their building blocks, promising access to the control of rotary motion by chemical stimuli. The combination of porosity with ultra-fast rotor dynamics was discovered in molecular crystals and covalent frameworks, by 2H spin-echo NMR spectroscopy and T1 relaxation times.[1-3] The rotors, as fast as 107 Hz at 200 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 organosiloxane frameworks allowed to realize periodic architectures of fast molecular rotors containing dynamic C-F dipoles in their structure.[4] The dipolar rotors showed not only the rapid dynamics of the aromatic rings (ca. 5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.[5] 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. Flexible molecular crystals were fabricated by a series of shape-persistent azobenzene tetramers that form porous molecular crystals in their trans configuration. The efficient trans→cis photoisomerization of the azobenzene units converts the crystals into a non-porous phase but crystallinity and porosity are restored upon Z→E isomerization promoted by visible light irradiation or heating. We demonstrated that the photoisomerization enables reversible on/off switching of optical properties as well as the capture of CO2 from the gas phase.[6] Control of molecular rotor dynamics by I2 molecule absorbed in the channels of a porous molecular crystal shown below.