Molecular rotors in periodic mesoporous organosilicas: modulation of the rotational frequency by external stimuli

Research towards the fabrication of materials containing molecular rotors mounted on surfaces or arranged in ordered 3D arrays is presently very intense. The organization of individual rotors into ordered arrays in solids can provide the necessary juxtaposition to make them operate as devices and to realize materials that could express useful functions in the fields of electronics, optoelectronics, and nanofluidics. Molecular rotors in bulk materials require both a large free volume and low energy barriers that allow rotation of the organic elements about their pivotal bonds. The fulfillment of these criteria has been accomplished in low-density organic crystals by reducing the interactions of the mobile elements and protecting them in closed molecular capsules. We present periodic mesoporous organosilicas (PMOs) containing large nanochannels and ordered arrays of molecular moieties covalently linked to a robust siloxane framework which behave as molecular rotors. Connection of the organic elements to the siloxane layers through a virtually barrierless C-Si bond dictates their regularity, separating them one from the other and enabling the realization of highly mobile organic moieties. The highly porous hybrid materials are endowed with extremely low density and high surface area, thus the molecular rotors have the advantage of being easily accessible from the surrounding space through the open pores. The dynamics of the p-phenylene moieties in p-phenyl, diphenylene and divinylbenzene hybrid materials was studied by 2H solid-state NMR spectroscopy on the deuterated compounds. The spin–echo spectra provide the mechanism of reorientation of the C-D vectors and the motional averaging for frequencies that overcome the MHz regime. Consequently, variable-temperature solid-state 2H spin–echo NMR spectra were recorded to determine the reorientation rate and the mechanism of motion of p-phenylene rings in the porous materials. The 2H NMR spectral profiles of the deuterated mesoporous p-phenylenesilica vary progressively with increasing temperature from 200 to 300 K exhibiting reorientation rates as fast as 108 Hz, even at room temperature. By Arrhenius analyses, energy activation barriers of about 10 kcal/mol were estimated. Control over the rotor dynamics was achieved by a variety of guest molecules diffused into the nanochannels from solution or from the melt. The screening of guests with varied polarity and molecular masses, such as n-eicosane (C20), tetraethylammonium chloride (TEA), and water, showed the modulated response of the rotor dynamics in the host framework, which resemble the active switching of molecular motion in engineered molecular machines.