Molecular rotors in mesoporous materials

Molecular rotor systems have been one of most challenging topics in the last years. Inspiring to biological molecular machines based on rotor systems, such as ATP-synthase, chemists have tried to reproduce functional nanodevices based on molecular rotors. One of the main targets in this research field was the attainment of materials wherein molecular rotors are organized in such a way that the free space around them is maximized and the rotational barrier is as low as possible. In this work we present a first example of highly-ordered materials wherein molecular rotors are linked to the framework through robust covalent bonds, which confer both thermal and chemical stability to the final material. We have chosen Periodic Mesoporous Organosilica (PMO) systems with crystal-like pore walls, such as p-phenyl, diphenylene and p-divinylphenylensilica, to study the rotational properties of the organic linkers pivoted between two siloxane layers. In these materials, organic moieties are organized at molecular level in the pore walls with the adjacent organic linkers posed at a distance greater than the sum of van der Waals radii. We have explored the dynamics of the molecular rotors by 2H NMR spectroscopy on the deuterated aromatic moieties. In fact, 2H NMR spectroscopy is a method of choice to determine the mechanism of motion. Rotational frequencies as fast as 108 Hz were estimated at room temperature and energy activation barriers of about 10 kcal/mol were calculated. Interestingly, we demonstrated for the first time that the rotational motion could be controlled by the presence of guest molecules diffused-in the mesopores and the materials change from a 2D rotational glass to a 2D rotational fluid behavior by increasing the temperature. In conclusion, these materials could find applications as gas-sensor systems or carriers for controlled release of stored molecules.