Theoretical investigation of the structures and dynamics of crystalline molecular gyroscopes

Recently, molecular rotor systems have been emerging as a promising candidate of functional nanoscale devices. A macroscopic gyroscope like molecule in a crystalline solid is particularly unique owing to its variable physicochemical properties. Setaka et al. have achieved the synthesis of a novel crystalline molecular gyroscope characterized by a closed topology with a phenylene rotator encased in three long siloxaalkane spokes [Setaka, W.; et al. Chem. Lett.2007, 36, 1076]. We theoretically investigated the underlying mechanism of its rotational dynamics by utilizing the self-consistent-charge density-functional-based tight-binding (DFTB) method for crystal structures. We first found that the DFTB semiquantitatively reproduced the unit cell molecular geometries of all three stable X-ray structures under the periodic boundary condition. From the potential energy surface calculations, the activation barrier for phenylene rotation was estimated to be about 1.2 kcal/mol, which is much lower than those of other, previously synthesized gyroscopic compounds. In comparison to 1,4-bis(trimethylsilyl)benzene of a similar crystal structure but of an open topology, the siloxaalkane frame in the crystalline molecular gyroscope under consideration effectively blocks strong intermolecular steric interactions experienced by the phenylene rotator. The molecular dynamics simulations based on the DFTB exemplified facile phenylene flipping between the stable structures, especially at high temperature. The present results demonstrate the remarkable ability of the DFTB method to predict the crystal structures and rotational dynamics of this type of crystalline molecular gyroscopes.