MOLECULAR ROTORS IN POROUS MATERIALS

Introduction: Molecular rotors in the solid state constitute an interesting area within the class of dynamic materials. They can be applied in several fields, including molecular machinery and tunable dielectric response for optical and electric devices. Rotary motion is usually hampered when the molecules are condensed in the crystalline state. However, by the exploitation of porous materials rotor dynamics at the incredible speed of 108 Hz not only at room temperature but in some cases even below 200K can be achieved. Materials and methods: We have prepared a series of micro- and mesoporous materials endowed with high surface areas up to 5000 m2/g that contain molecular rotors in their walls. In particular, porous molecular crystals held together by charge-assisted hydrogen bonds, hybrid mesoporous organosilicas (PMOs) and porous aromatic frameworks (PAFs) have been studied. The molecular rotors are constituted by p-phenylene moieties, typically pivoted on Csp2-Csp and Csp2-Si bonds. The key method to study the dynamics was 2H NMR spectroscopy, which is sensitive to the motional reorientation of the C-D bonds from 103 to 108 Hz. 2H NMR spectra were collected as function of temperature from 150 to 400 K and the energy barrier for rotation were determined by an Arrhenius plot. For this sake, porous materials containing deuterated p-phenylene moieities have been prepared. Results: 2H NMR spectra of the porous materials change systematically as a function of temperature and their profiles were simulated by a two-site 180° flip reorientation mechanism. Extremely fast reorentational rates in the order of 107-108 Hz already at 200K were detected. The results suggest that such rotors are among the fastest ever described in the literature. At high temperatures large amplitude librations up to 36° have been observed, in addition to the 180° flip reorentation. Activation energies as low as 4-6 kcal/ mol were determined. When C-F dipoles are asymmetrically mounted on the p-phenylene units, rotor reorientation brings together a dipole reorientation, which is sensitive to an applied electric field. The porosity of the materials offers a great advantage: guests diffusing in cavities can modulate at will the motional dynamics of the molecular rotors exposed in the channels. Actually, iodine molecules entered the pores and could regulate reversibly rotor dynamics. Discussion: The present measurements demonstrate that the rotors decorating nanochannel walls gain a high degree of mobility when the channels are empty. When the materials are exposed to a vapor, even at a low pressure, molecular rotors extensively interact with the guest molecules and their dynamics is controlled